GB2564152A

GB2564152A - Offshore decommissioning vessel and methods of use

Info

Publication number: GB2564152A
Application number: GB1710828.3A
Authority: GB
Inventors: Arend Van Duyvenbode Edward
Original assignee: Oil & Gas Decommissioning Ltd
Current assignee: Oil & Gas Decommissioning Ltd
Priority date: 2017-07-05
Filing date: 2017-07-05
Publication date: 2019-01-09
Also published as: WO2019008357A1; GB201710828D0

Abstract

A system and corresponding method for decommissioning an offshore platform (fig 1, 10), the offshore platform comprising a topside (fig 1, 12) and a support structure (fig 1, 14). The system comprises a topside lifting unit adapted to lift the topside from the support structure, a support structure lifting unit adapted to lift the support structure from the water such that the support structure is supported by the vessel, and a vessel 100 configured to interchangeably receive the topside lifting unit and the support structure lifting unit. The vessel comprises a hull 110 which is adapted to accommodate an installed support structure, for example by incorporating a slot 112, and a ballasting system for lifting the topside. The vessel 100 may further comprise an energy recovery system comprising turbines (fig 2c, 118a) mounted in ducts (fig 2c, 118) below the waterline.

Description

Embodiments of the present invention relate generally to vessels, systems and methods for the installation, decommissioning and removal of offshore oil and gas platforms.

BACKGROUND

Offshore oil and gas platforms are used to extract oil and gas from subsea reserves around the world. However - as with all fossil fuel reserves - the supply of oil and gas hydrocarbons available in offshore fields is finite. Despite continued advancements which prolong the operating lives of oil and gas fields, a field’s reserves are eventually depleted to an extent that continued extraction becomes uneconomic, leaving behind obsolete and aging oil and gas facilities including wells, pipelines, subsea well protection systems and offshore platforms.

There are increasing regulatory, political and public opinion pressures on the owners and operators of offshore platforms to decommission old offshore platforms safely, cost-effectively and with a minimum of environmental and ecological effects.

Fixed offshore platforms commonly comprise two parts: an upper portion commonly termed the ‘topside’ which sits above the sea level and typically range in size and complexity from very large integrated treatment facilities and equipment including the drill rig, accommodation blocks, processing and ancillary systems to typically smaller relatively simple topsides consisting only wellheads or risers; and a lower, support structure which holds the topside above the water and away from the impact of wave forces. The form of the support structures may vary considerably between offshore platforms dependent upon typically the topsides size, weight and complexity, location with its consequential environmental impacts and water depth. In shallower water, support structures which are fixed to the seabed are most economic and include steel jacket structures (which are formed of tubular steel members typically supported on piles driven into the sea bed), gravity based structures (such as concrete caissons) and jack-up rigs. In deeper water, the lateral forces of currents and waves mean that such fixed supports and foundations are unfeasible. Instead, compliant or floating (e.g. semi-submersible, tension leg platform or spar) support structures are often used.

Decommissioning and removal of these complex installations is a slow and laborious process as the topsides can weigh over 40,000 tonnes (4,000,000 kg), whilst the weight of steel jacket support structures can be as high as 45,000 tonnes (45,000,000 kg) depending on the water depth.

Historically, the only means of installing or removing offshore platforms has been using heavy lift crane vessels. Currently these vessels, which may be ships, sheerlegs or semi-submersibles are fitted with a crane, or pair of cranes, socalled tandem lifts, suitable for lifting loads of up to approximately 15,000 tonnes (15,000,000 kg). However, a significant disadvantage of a heavy lift crane is that its capability to lift heavy loads diminishes with the amount of reach the jib has to travel in order for the hook to be over the centre of gravity of the lifted load, the amount of lift with reach is dictated by its lift curve. Thus, many existing heavy lift crane vessels are limited in their ability to lift the topside or the jacket of an offshore platform in a single lifting operation and as such are confined to lifting lower topside or jacket weights.

Additionally, many platform topsides are constructed in a modular fashion, and as such are not easily removed using a single lift operation by crane. Thus, in order to remove an offshore platform with the above limitations, the topside and the jacket structure must be broken apart in situ into smaller sections which a crane vessel is capable of lifting. This is especially true of platforms which were built in the early years of offshore oil and gas exploitation, when the lift capacity of the then heavy lift vessels was significantly less than current lifting capacity. These smaller sections may then be lifted from the platform and transferred to shore either by the crane vessel itself or using an ancillary barge or transfer vessel.

This process can take up to or even over a year to complete because of the need to carefully and safely separate the modules and systems within an offshore platform and because the multiple lifting operations can be significantly delayed by periods of adverse weather. Hence, these traditional methods of decommissioning are slow, expensive and dangerous to those working on the platform during the preparatory stage and during lift operations.

In response to this issue the industry has been moving towards extremely large offshore platform decommissioning vessels which are capable of performing all the necessary operations for the installation and decommissioning of oil and gas platforms.

Recently, there has been the development of specialised ‘buoyancy lift’ vessels which are capable of lifting the topsides of offshore platforms in their entirety, i.e. without the-requirement to split a topside into smaller sections.

This operation (which is sometimes termed ‘single-lift’) is performed in principle by manipulation of the ballasting systems of the buoyancy lift vessels. A vessel which has been ballasted down is positioned below the topside of an offshore oil and/or gas platform. The vessel is then de-ballasted and rises in the water due to buoyancy, lifting the platform topside from its support structure.

This lift operation requires significantly less time than removing the topsides of offshore platforms using traditional crane vessels (as discussed above), and can take as little as 3 to 4 days to complete. However, the ballasting systems required to perform these steps are typically very large and the need for the vessel to encompass the jacket support structure whilst also positioning itself below the topsides to be lifted. As a consequence, the only existing buoyancy lift vessel designed to remove the topsides of offshore platforms (the ‘Pioneering Spirit) is extremely large. It is also true that the extremely large size of this vessel is dictated by its multifunctional capabilities.

The ‘Pioneering Spirit (formerly named ‘Pieter Shelte’) operated by Allseas Engineering (RTM), which is the largest lift vessel in the world, and entered service in mid-2016. Its topsides lift capacity is rated at 48,000 tonnes (48,000,000 kg) and with a jacket lift capacity of 25,000 tonnes (25,000,000 kg). In addition to providing buoyancy lift for the topsides of offshore platforms, the ‘Pioneering Spirit is presently fitted with pipelay equipment, diving support equipment and accommodation for over 500 crew. The vessel includes an additional area at its stem where a lifting system for steel jackets may be fitted in the future (discussed in more detail below). This multifunctional design means that the ‘Pioneering Spirit is extremely large, having a length of 382m and a beam (width) of 124m. Its construction and sea trials cost is quoted in the press as being in the order of US $3 billion.

Furthermore, there is a second ‘Pioneering Spirit’ vessel being built by Aliseas Engineering (RTM) at the moment which is designed to lift up to 72,000 tonnes (72,000,000 kg). This sister vessel is scheduled to come into operation sometime in 2020/21. Again the design has all the multifunctional features of the initial ‘Pioneering Spirit’ and its size will be significantly larger than that of the current ‘Pioneering Spirit’.

A further vessel, the ‘Twin Marine Lifted design concept developed by Shangdong Twin Marine (RTM), is currently being constructed and is planned to be in service in 2021. The topside lift capacity of the ‘Twin Marine Lifter’ is rated at 34,000 tonnes (34,000,000 kg). This vessel is significantly different to the ‘Pioneering Spirit’ in as much as it consists of two independent vessels with lifting arms which straddle the underneath of the topsides to be lifted working in unison. The lifted topside is then transferred to a third vessel for transport to topsides to shore. It is not known at present how the topsides will be then transferred to the quayside. Additionally, this vessel does not have the capacity to lift jacket structures. Although the cost of the three vessels is not publicly known it is envisaged that it could be in the order of 3 to US $4 billion.

Despite the significant reduction in time required to remove the topsides of offshore platforms, operating and leasing these colossal buoyancy lift vessels for the decommissioning of the topsides of offshore platforms remain highly expensive.

Once a buoyancy lift vessel has lifted the topside of an offshore platform the topside must be transferred to shore. This can be done in two distinct ways. The first requires the transfer of the topside to a transportation barge at sea or nearshore so that the topsides will be transferred onto a quayside for demolition. This requires the leasing of the additional transportation barge and tugs to manoeuvre it. The ‘Pioneering Spirit’ uses a 200m long 57m wide cargo purpose-built barge called the Iron Lady for transferring the topsides in sheltered water close to the demolition yard.

The issues identified above will equally apply to vessels designed to lift the support structures of offshore platforms.

However, there is currently no vessel commercially available which is configured in practice to lift both the topside and the support portion of larger offshore platforms without breaking either apart into smaller sections. As discussed above, the ‘Pioneering Spirit incorporates an additional area at its stem where a lifting system for steel jackets may be fitted in the future. However, at this time no jacket decommissioning has been completed by the ‘Pioneering Spirit.

The reported methodology by which the ‘Pioneering Spirit’ might remove a jacket relies on a crude “skidding” operation to move the jacket on to a transportation barge. In this proposed skidding operation the jacket is slid on rollers from the ‘Pioneering Spirit’ to a barge. This process would further require significant ballasting of the buoyancy lift vessel and significant de-ballasting of the transportation barge as the jacket is transferred to ensure a horizontal skid operation, i.e. that the ‘Pioneering Spirit and the barge remain level with one another. Furthermore, significant motive force would be required to be applied to the jacket and skid by the transportation barge in order to move the jacket. This requires a barge with complex, large and expensive equipment (e.g. winches) to apply these forces and similarly complex anchoring systems. In addition, this proposed skidding operation has significant safety concerns and an inherent risk of damage to the jacket, the transport barge and the ‘Pioneering Spirit.

The space provided for this steel jacket lifting system by its nature increases the size of the ‘Pioneering Spirit design over and above a vessel designed to lift topsides only. This largeness in size exacerbates the issues discussed above with reference to buoyancy lift vessels in general and is exacerbated further with the development of a second ‘Pioneering Spirit’ due in service in in 2020/21 which is approximately 50% larger than the current lift vessel in service.

In summary, there is currently no vessel available which is able to lift both the topside and the support portion of the largest offshore platforms without breaking either apart into smaller sections. This results in long and expensive decommissioning process. Furthermore, the large size and multifunctional aspect of the operations of the vessels in service or presently being built means that they require huge amounts of resources (e.g. materials, fuel and personnel) to build and operate.

In view of the issues identified above there is a clear need to improve known methods of decommissioning offshore platforms.

STATEMENT OF INVENTION

According to an aspect of the present disclosure, there is provided a system for decommissioning an offshore platform. The offshore platform comprises a topside and a support structure, wherein the system comprises: a topside lifting unit adapted to lift the topside from the support structure; a support structure lifting unit adapted to lift the support structure from the water such that the support structure is supported by the vessel; and a vessel configured to interchangeably receive the topside lifting unit and the support structure lifting unit wherein the vessel comprises a hull which is adapted to accommodate an installed support structure and a ballasting system for lifting the topside.

By interchangeably receive it is understood that the vessel may be fitted with either the topside lifting unit or the support structure lifting unit. As such, the lifting units may be switched, replaced or exchanged as necessary.

Thus, the vessel is capable of lifting offshore platform topsides using buoyancy lift techniques when the topside lifting unit is installed on the vessel and also lifting offshore platform support structures (e.g. a jacket, caisson, tension-leg structure, gravity based support structure, jack-up legs) when the support structure lifting unit is on-board the vessel.

Hence, the system and vessel discussed above are able to quickly, easily and economically lift and remove the topsides and support structures of offshore platforms. This is unlike existing heavy lift crane vessels (which must break the topsides and supports structures of larger offshore platforms down into smaller sections) or the ‘Twin Marine Lifted (which is only able to lift the topsides of offshore platforms) or the ‘Pioneering Spirit which can at present lift topsides and may in future be able to lift support structures but not as efficiently or safely as the vessel discussed above.

In addition, the ability to switch or exchange the topside lifting unit for the support lifting allows for a significant reduction in the vessel’s size in comparison to existing vessel designs. In particular, the inventors believe that the vessels in accordance with the present disclosure are able to lift both topsides and jacket structures, to efficiently interchange topsides and jacket lifting equipment as required, have the ability to offload both the topsides and jacket structures to a transportation barge using safe and efficient a soft landing lift technique whilst being approximately 35% of the size and cost of both the ‘Pioneering Spirit’ and the ‘Twin Marine Lifter’

This reduction in vessel size results in significant benefits over existing vessels with similar capabilities. These smaller vessels require significantly fewer resources to build and operate and are therefore significantly more economic than existing vessels. Additionally, the relatively small vessels in accordance with the present disclosure may enter a larger proportion of ports, reducing the time wasted as the vessels travel from to and from the offshore platform, increasing the vessel’s utilisation and reducing the time and cost required to decommission offshore platforms.

In addition, it is envisaged that the system may be used to perform installation operations for offshore platforms as the vessel and lifting units are able to cooperate to lift and manoeuvre the topsides and support structures of offshore platforms.

Herein, the term ‘vessel’ is understood to refer to a ship or large watercraft including ships and boats with traditional hull forms (e.g. monohull, catamaran, trimaran, SWATH), barges, and semi-submersibles. Furthermore, the term Offshore platform’ is understood to refer to any large permanent or semipermanent offshore man-made structure. These may include: platforms for the extraction of oil and/or gas or other resources located on or below the seabed; accommodation platforms; and platforms for the generation of renewable energy (e.g. wind turbines, tidal energy farms).

In certain embodiments, the vessel may comprise a ballasting system further configured for lifting the support structure. In such embodiments the vessel is capable of lifting the support structure through a buoyancy lift operation or through a combination of buoyancy lift and conventional lifting methods (e.g. using winch systems, cranes or other means).

Preferably, the topside lifting unit or the support structure lifting unit, or both, comprise a propulsion system configured to move the respective lifting unit.

A lifting unit with such a propulsion system may be self-propelled and able to locomote. In certain embodiments the propulsion system(s) may comprise any of wheels, tracks or rollers. These components may be located on one or more bogies.

Alternatively, or additionally the propulsion system(s) may comprise means of electromagnetically levitating (e.g. lifting or raising) either the topside lifting unit or the support structure lifting unit. This electromagnetic levitation (maglev) system may be adapted to move one or more of the lifting units or alternatively may reduce the forces and power which must be applied to the lifting unit(s) by the propulsion system (i.e. by components such as wheels, tracks, rollers or winches) or by handling equipment such as cranes or trucks.

In preferred embodiments the topside lifting unit and the support structure lifting unit each comprise a propulsion system.

Advantageously, such propulsion systems allow the lifting units to be quickly and easily transferred onto and off of the vessel (e.g. to exchange the lifting units or to remove a lifting unit from the vessel for maintenance) without any additional equipment such as cranes or towing machinery. Therefore, the lifting unit(s) may be easily transferred or exchanged whilst the vessel is at sea, nearshore or whilst the vessel is in a wide variety of ports.

Consequently the utilisation of the system may be improved because the vessel may spend more time ‘on-station’ performing lifting operations, rather than transiting to a specific port to switch or exchange its lifting units. Furthermore, the time required to decommission a single offshore platform is reduced as the time used to interchange the lifting units is significantly reduced.

Additionally, the position and arrangement of the lifting unit(s) on the vessel may be adjusted as necessary to accommodate the specific offshore platform to be decommissioned. As such, the system discussed above is able to decommission a wider variety of offshore platforms than existing vessels.

Alternatively, if either of the lifting units is not provided with a propulsion system it may be manoeuvred and transferred using cranes or other lifting equipment. These cranes or other lifting equipment may be located on the vessel, on an additional lift vessel or onshore (if the lifting units are to be transferred to the quayside). In further alternative embodiments the topside lifting unit or the support structure lifting unit may be moved using towing or handling equipment. This option is particularly suitable if the lifting units comprise unpowered bogies, wheels, tracks or rollers or if the lifting units are lifted,supported or moved by an electromagnetic levitation system.

In preferred embodiments the topside lifting unit comprises a propulsion system, wherein the propulsion system is configured to move the topside lifting unit and a topside supported by the topside lifting unit.

Consequently, the topside lifting unit is able to transfer itself and a topside it has lifted to and from the vessel. Thus the system is able to offload the topside to a transport barge or the quayside without any further specialised equipment and without breaking the topside down into smaller sections. This reduces the time and cost required to decommission an offshore platform and increases vessel utilisation.

Alternatively, topsides may be transferred to and from the vessel using cranes or other lifting equipment. These cranes or other lifting equipment may be located on the vessel, on an additional lift vessel or onshore (if the lifting units are to be transferred to the quayside). In certain embodiments, the topside may be broken down into smaller sections before it is removed from the vessel.

In further preferred embodiments the support structure lifting unit comprises a propulsion system, wherein the propulsion system is configured to move the support structure lifting unit and a support structure supported by the support structure lifting unit.

Consequently, the support structure lifting unit is able to transfer itself and a support structure that it has lifted to and from the vessel. Thus the system is able to offload the support structure to a transport barge or the quayside without any further specialised equipment and without breaking the support structure down into smaller sections. This reduces the time and cost required to decommission an offshore platform and increases vessel utilisation.

Alternatively, support structures may be manoeuvred and transferred to and from the vessel using cranes or other lifting equipment. These cranes or other lifting equipment may be located on the vessel, on an additional lift vessel or onshore (if the lifting units are to be transferred to the quayside). In certain embodiments, the support structure may be broken down into smaller sections before it is removed from the vessel.

In preferred embodiments, the hull comprises a slot arranged to accommodate an installed support structure.

Advantageously, the vessel may be positioned so that the slot interfaces (i.e. receives, cooperates or mates) with the installed support structure. In this position the installed support structure extends through the slot and the hull of the vessel is adjacent to and surrounds the support structure.

Consequently, the topside may be positioned directly over the slot when it is lifted by the topside lifting unit. As a result, the centre of mass of the topside is inboard of the vessel (i.e. over the hull of the vessel or within a rectangle formed by the maximum width and maximum length of the vessel) when it is lifted. This reduces the stress applied to the structure of the vessel and allows a reduction in the size of the vessel as the mass of the topside may be more easily counterbalanced.

The slot may be formed by a recess in the hull of a monohull vessel or by the gap between the demihulls in a multihull vessel (e.g. a catamaran, trimaran, etc). Alternatively, the vessel may be formed without a slot. In such a configuration the topside must be lifted outboard of the vessel.

In certain embodiments the slot is at least 30m wide, preferably at least 45m wide and more preferably still at least 60m wide. In further embodiments the slot is at least 15m long, and more preferably at least 60m long, and more preferably still at least 100m long.

Vessels may have substantially any size of slot so long as it is configured to accept an installed offshore platform support structure. However, advantageously, as the size of the slot increases, the number of installed offshore platform support structures which may be accommodated in the slot also increases. Hence, as the slot gets larger the number of offshore platforms which may be decommissioned by the system increases.

In certain embodiments, the vessel comprises a duct extending through the vessel from the base of the slot and a turbine located within the duct, wherein the turbine is adapted to generate energy as water passes through the duct. In particularly preferred embodiments the vessel comprises a plurality (e.g. at least four) ducts and a corresponding plurality (e.g. at least four) turbines located within said plurality of ducts.

Water may pass through the duct due to waves or currents passing the vessel or as the vessel moves or passes through the water. Such a system allows energy and power to be generated as waves or current pass the vessel whilst it is stationary (e.g. while conducting lifting operations) and for the vessel to recover energy spent propelling the vessel when it decelerates or slows down. Consequently, the load applied to the propulsion and electrical power systems of the vessel is reduced, prolonging the operational lifespan of the expensive equipment within these systems. As a result, the energy generation system discussed above reduces the costs to operate and maintain the vessel.

Furthermore, the ducts (which allow water to pass through them as the vessel moves through the water) reduce the drag of the vessel, reducing the resistance of the vessel as it moves (e.g. accelerates, decelerates, turns or maintains a steady course and speed). In turn this reduces the resources required to operate the vessel and may allow for a reduction in the size and expense of the propulsion system and propulsors (e.g. propellers) fitted to the vessel.

In preferred embodiments, the topside lifting unit comprises a cantilever beam assembly and wherein the cantilever beam assembly comprises a cantilever beam which is adapted to project outboard from the vessel and support the topside when the topside is lifted by the system.

In preferred embodiments of the system, the cantilever beam of the topside lifting unit is configured to project outboard from the vessel and to extend over the slot in the vessel (if present). Therefore, the cantilever beam may be adapted to support the topside from below (i.e. so that the cantilever beam contacts the underneath or bottom of the topside) when the topside is lifted.

In preferred embodiments the topside lifting unit comprises a plurality of the cantilever beam assemblies described above. In particular, the topside lifting unit preferably comprises six or eight or ten cantilever beam assemblies which may be arranged in pairs around the slot of a vessel (where present). This allows the forces, stresses and loads applied to the topside lifting unit to be evenly distributed over the entirety of the hull structure.

Alternatively, the topside lifting unit may not project outboard of the vessel and instead the topside may be lifted or supported in contact with or above the hull of the vessel.

Preferably the cantilever beam assembly comprises a propulsion system configured to move the cantilever beam assembly.

Consequently, the cantilever beam assembly is self-propelled and able to locomote. Therefore, the cantilever beam assembly may be transferred to and from the vessel without additional equipment (such as cranes). This increases the locations the cantilever beam assembly may be offloaded to and reduces the time and cost required to decommission an offshore platform.

Additionally, the cantilever beam may be repositioned or arranged on the vessel using the propulsion system. This provides increased flexibility and increases the number of offshore platform topsides which may be lifted by the system.

Preferably the propulsion system provided to the cantilever beam assembly is configured to move a topside which is supported by the cantilever beam assembly. Advantageously, this avoids the need for further equipment or machinery to offload or onload the topside and increases the number of ports and ancillary vessels (transport barges, crane vessels) that the topsides maybe offloaded to, as discussed above. This reduces the time and cost associated with decommissioning an offshore platform and can increase the utilisation of the vessel. Alternatively, the cantilever beam assembly or a topside supported by the cantilever beam assembly, or both, may be transported or repositioned using a crane or other handling equipment. Such cranes or handling equipment may be separate from the vessel or provided to the vessel.

In certain embodiments the cantilever beam assembly comprises a plurality of bogies configured to move the cantilever beam assembly, wherein preferably the plurality of bogies is further configured to raise and lower the cantilever beam assembly. A bogie is a framework or structure comprising a plurality of axles and wheels. Bogies distribute the forces applied to them through all of their axles and wheels, decreasing the stress applied to each.

Preferably at least one bogie is powered (e.g. by a motor) such that the cantilever beam assembly may be moved in a lateral or transverse direction. However, this is not essential if a towing vehicle or other equipment which is configured to move the cantilever beam assembly is provided. Additionally, in certain embodiments the bogies may be configured to raise and lower the cantilever beam assembly relative to the bogies themselves. This may be performed, for instance, by operating a suspension system provided to each bogie.

Alternatively, the propulsion system of the cantilever beam assembly may comprise tracks or rollers, or wheels which are not arranged on bogies.

In certain embodiments, the vessel comprises a crane configured to move the cantilever beam assembly. Advantageously the crane may be able to transfer the cantilever beam assembly from the vessel without any additional equipment. This may be provided in addition to, or as an alternative to, a cantilever beam assembly propulsion system. In the latter case, the crane will provide increased operational redundancy should the propulsion system of the cantilever beam assembly be broken or malfunction.

Alternatively, or additionally the propulsion system of the cantilever beam assembly may comprise means of electromagnetically levitating (e.g. lifting or raising) the cantilever beam assembly. Such an electromagnetic levitation (maglev) system may be adapted to move the cantilever beam assembly. Alternatively, the electromagnetic (maglev) system may be adapted to lift or raise the cantilever beam assembly so as to reduce the forces or power which must be applied to the cantilever beam assembly by the cantilever beam assembly propulsion system (i.e. by wheels, tracks or rollers) or by a crane or other handling equipment located on the vessel.

In preferred embodiments, the cantilever beam assembly further comprises an assembly base and a means of moving the cantilever beam relative to the assembly base.

Preferably, the cantilever beam assembly comprises a means of translating the cantilever beam relative to the propulsion system such that the cantilever beam is extended outboard from the vessel or retracted inboard toward the vessel.

In equally preferred embodiments, the cantilever beam assembly comprises a means of rotating the cantilever beam relative to the propulsion system such that the cantilever beam is extended outboard from the vessel or retracted inboard towards the vessel.

These means of moving the cantilever beam allow the beam to be positioned in a suitable position to safely contact and lift a topside. Where the vessel does comprise a slot, these means of moving, translating or rotating the cantilever beam allow the cantilever beam to be positioned over the slot. This feature increases the number and varieties of topsides that may be lifted safely by the system.

Additionally, the cantilever beam may be positioned so that the vessel may safely approach an installed offshore platform before the topside is lifted such that the offshore platform is adjacent to the vessel or accommodated in a slot in the vessel.

The means of moving, translating or rotating the cantilever beam relative to the assembly base may be provided by the cantilever beam assembly propulsion system discussed above or by another system. Any suitable means may be used to move, translate or rotate the cantilever beam, including: powered wheels, tracks, rollers or bogies; pneumatic pistons; hydraulic pistons; and a rack and pinion or screw actuator. Alternatively, the cantilever beams may be moved or re-positioned by cranes or other handling equipment.

Alternatively, the positions of the position of the cantilever beam relative to the assembly base may be fixed or constant. In this case the position of the cantilever beams relative to the sides of the vessel or a slot in the vessel may still be adjusted by a propulsion system provided to the cantilever beam assembly or by cranes or other handling equipment.

In preferred embodiments the cantilever beam assembly comprises a lifting pad positioned in contact with or above the cantilever beam, wherein the lifting pad is adapted to contact the topside when the topside is lifted by the system.

Advantageously, the lifting pad may be formed of a flexible or pliable material to protect the cantilever beam from damage when it contacts the topside. Alternatively, the cantilever beam assembly may not comprise a lifting pad and the cantilever beam may be configured to directly contact the topside when the topside is lifted by the system.

Preferably the system further comprises a motion compensation system configured to minimise motion of the lifting pad when the topside is lifted by the system. This motion compensation system may be positioned between the lifting pad and the cantilever beam, or between the vessel and the cantilever beam.

These motion compensation systems prevent motions of the vessel caused by waves or currents being transferred to the topside which may cause damage to either the topside, the vessel or the cantilever beam assembly.

In certain embodiments, the motion compensation system may be a heave compensation system configured to minimise vertical motions of the lifting pad when the topside is lifted. Alternatively, or additionally, the vessel may comprise a variety of other systems to minimise the motion of the vessel or the cantilever beams including anti-roll tanks, active fins and bilge keels.

Alternatively, where the cantilever beam assembly does not comprise a lifting pad, a motion compensation system may be provided to the cantilever beam such that the motion compensation system contacts the topside when the topside is lifted or between the cantilever beam and the vessel.

In preferred embodiments the system further comprises a means of moving the lifting pad, such that the lifting pad may be moved relative to the cantilever beam.

Consequently the lifting pad is movable and may be positioned independently of the position of the cantilever beam. This provides increased flexibility in the positioning of the lifting pad and advantageously increases the variety of topsides which may be lifted by the system. This is particularly, important where two or more cantilever beams are coupled to form a continuous beam and therefore cannot be independently positioned beneath the offshore platform during a lifting operation.

Preferably the lifting pad may be moved to substantially any position above or in contact with the cantilever beam. In particular, the movable lifting pad may be moved along the length of the cantilever beam, e.g. in a direction extending towards or away from the assembly base of the cantilever system, or further outboard or inboard of the vessel when the cantilever beam assembly is received on the vessel.

The movement of the lifting pad may be achieved using any suitable means including a hydraulic piston, a pneumatic piston, a linear screw actuator, a rack and pinion actuator or a motor. These means may be provided on the lifting pad or to the cantilever beam. Preferably the position of the lifting pad may be fixed, locked or secured by a locking apparatus during transit and lifting operations. Where the system comprises a motion compensation system directly coupled to the lifting pad or positioned between the lifting pad and the cantilever beam the means of moving the lifting pad may also be adapted to move the motion compensation system.

In particularly preferred embodiments, the topside lifting unit comprises two cantilever beam assemblies, each of said cantilever beam assemblies comprising a cantilever beam, wherein the cantilever beams are configured to detachably couple so as to form a continuous beam across the slot.

Advantageously, the continuous beam formed by the coupling of two of the cantilever beams ensures that the mass of a topside which is lifted by the vessel is evenly distributed across the slot by the two cantilever beams and that the forces applied to the structure of the vessel are balanced and well distributed. This beam configuration potentially reduces the size and weight of the lifting beams thereby reducing overall cost of the beam construction without loss of structural integrity. The cantilever beams are configured to detachably couple so that the vessel may approach an offshore platform to be decommissioned and accommodate the installed support structure within the slot. Once the vessel is in position the cantilever beams may be extended over the slot and coupled to one another to form a continuous beam.

Preferably, all of the cantilever beams are configured to be detachably coupled in this manner.

Furthermore, in preferred embodiments the topside lifting unit is further configured to lock the cantilever beams together when they are detachably coupled to form a continuous beam so as to prevent the cantilever beams from being separated. This locking may be performed by propulsion systems of the cantilever beam assemblies or the means of moving the cantilever beams discussed above or by a separate locking apparatus provided to the cantilever beams. Alternatively, any of these systems could be used in combination to lock the cantilever beams. The step of locking the coupled cantilever beams together prevents the beams from being unintentionally or accidentally separated. Thus the safety of the system is improved and damage to the vessel, the topside lifting unit and a topside lifted by the lifting unit is avoided.

In preferred embodiments the system further comprises a locking mechanism adapted to detachably secure the topside lifting unit to the vessel.

By securing the topside lifting unit to the vessel it is understood that the locking mechanism prevents the topside lifting unit from moving relative to the vessel. Advantageously, this increases the safety of the system by preventing the topside lifting unit which may be very heavy from moving once it has been secured to the vessel.

A wide variety of locking mechanisms may be used to secure the topside lifting unit. For instance, the locking mechanism may comprise locking pins configured to couple the topside lifting unit to the structure of the vessel (e.g. the upper deck of the vessel), mechanical clamps or locking projections provided to the vessel configured to interlock with the topside lifting unit, or magnetic locks. Alternatively, the topside lifting unit may be prevented from moving relative to the vessel by its self-weight or by its propulsion system (e.g. by brakes applied to its bogies, wheels, tracks or rollers, and/or by not applying any electromagnetic levitation or motive forces).

Preferably, the locking mechanism and the ballasting system are interlocked such that the topside may only be lifted if the locking mechanism is engaged. This interlock ensures the safety of the vessel and its crew and prevents damage to the system or the topside which might occur if the lifting unit or a portion of the lifting unit were to move relative to the vessel during a lifting operation.

Where the topside lifting unit comprises a cantilever beam assembly and a cantilever beam the locking mechanism is preferably adapted to secure the cantilever beam assembly or the cantilever beam to the vessel, such that the motions of the cantilever beam or the cantilever beam relative to the vessel are prevented.

In preferred embodiments, the support structure lifting unit comprises:

a base adapted to be received on the vessel;

a winch system configured to detachably couple to the support structure and raise or lower the support structure;

a bearing housing positioned on the base;

a frame rotatably coupled to the base by the bearing housing and configured to support the support structure when the support structure is lifted by the system; and, wherein the support structure lifting unit is adapted to rotate the frame and a support structure about the bearing housing such that the support structure is transferred on to and off of the vessel.

Advantageously, this system is able to lift an offshore platform support structure out of the water and onto the vessel without cutting or dividing the support structure into small sections.

It should be noted that in this embodiment the winch system is configured to lift a support structure from the water, whilst the frame is configured to position the support structure onto the vessel once it has been lifted from the water. In further alternative embodiments, the support structure may not be positioned upon the vessel after it has been lifted from the water, as such the frame may not be required to be rotatably coupled to the base.

In certain embodiments, the base may comprise a plurality of base members.

In certain embodiments the frame may be rotated relative to the base by the winch system. Alternatively or additionally, the frame may be rotated by any of:

a hydraulic piston; a pneumatic piston; a rack and pinion actuator; a linear screw actuator.

Preferably, the frame is adapted to rotate between a position where it is parallel to the base member and to a position where it forms an angle of at least 90 degrees with the base member, more preferably at least 135 degrees. Advantageously, angles of greater than 90 degrees allow the frame and cables to be extended outboard of the vessel so that the support structure does not contact the vessel and potentially cause damage to the vessel during lifting operations.

Preferably, the winch system comprises:

a winch;

a cable configured to detachably couple to the winch and the support structure;

a lifting gear apparatus positioned on the frame; and, wherein a portion of cable between the winch and the support structure passes around the cable roller.

In certain embodiments, the cable may be detachably coupled to the support structure using a hook, sling, clamp or electromagnet.

In preferred embodiments either the winch may be translated laterally relative to the frame, or the lifting gear apparatus may be translated laterally relative to the frame, or both. Advantageously, this movement of the winch and/or the lifting gear allows the supports structure lifting unit to couple to a wide variety of offshore platform support structures.

Preferably the lifting gear apparatus comprises either a tensioner or a roller, or both. Furthermore, preferably the lifting gear apparatus is configured to damp the motions of the cable and the forces applied to the cable by a support structure or the support structure lifting unit.

In preferred embodiments the system comprises a motion compensation system configured to minimise motion of the winch when the support structure is lifted by the system. This motion compensation system may be positioned either between the winch and the support structure lifting unit or between the vessel and the support structure lifting unit. In particularly preferred embodiments the motion compensation system is a heave compensation system configured to minimise the absolute vertical motions (i.e. heave) of the winch when the support structure is lifted by the system.

Advantageously, such motion compensation systems prevent motions of the vessel caused by waves or currents being transferred to the winches, cables and ultimately the support structure lifting unit during lifting operations. This prevents damage to either the support structure, the vessel or any component of the support structure lifting unit. Alternatively, or additionally, the vessel may comprise a variety of other active or passive systems to minimise the motion of the vessel or the cantilever beams including anti-roll tanks, active fins and bilge keels.

In preferred embodiments the support structure lifting unit further comprises a counterweight rigidly coupled to the frame such that the counterweight is positioned on an opposing side of the bearing housing to the cable roller. Advantageously, this counterweight minimises the force required to rotate the frame. In alternative embodiments of the support structure lifting unit no counterweight is provided.

Preferably, the vessel comprises a hull recess to accommodate the counterweight when the frame is lifted. Advantageously, the recess within the hull allows the centre of mass of the frame and the centre of mass of the support structure to be further inboard when the support structure is lifted. This reduces the stresses applied to the structure of the vessel.

In preferred embodiments, the system comprises a locking mechanism adapted to detachably secure the support structure lifting unit to the vessel.

By securing the support structure lifting unit to the vessel it is understood that the locking mechanism prevents the support structure lifting unit from moving relative to the vessel. Advantageously, this increases the safety of the system by preventing the support structure lifting unit which may be very heavy from moving once it has been secured to the vessel.

A wide variety of locking mechanisms may be used to secure the support structure lifting unit. For instance, the locking mechanism may comprise locking pins configured to couple the support structure lifting unit to the structure of the vessel (e.g. the upper deck of the vessel), mechanical clamps or locking projections provided to the vessel configured to interlock with the support structure lifting unit, or magnetic locks. Alternatively, the support structure lifting unit may be prevented from moving relative to the vessel by its self-weight or by its propulsion system (e.g. by brakes applied to its bogies, wheels, tracks or rollers and/or electromagnetic levitation or motive forces).

Preferably, the locking mechanism and the support structure lifting unit are interlocked such that the supports structure lifting unit is prevented from being operated to lift the support structure if the locking mechanism is engaged. Consequently, in certain embodiments the winch system may be prevent from operating. This interlock ensures the safety of the vessel and its crew and prevents damage to the system or the support structure which might occur if the lifting unit or a portion of the lifting unit were to move relative to the vessel during a lifting operation.

In certain embodiments the support structure lifting unit comprises a plurality of bogies configured to move the support structure lifting unit, wherein preferably the plurality of bogies is further configured to raise and lower the support structure lifting unit.

Preferably at least one bogie is powered (e.g. by a motor) such that the support structure lifting unit may be moved in a lateral or transverse direction. However, this is not essential if a towing vehicle or other equipment which is configured to move the cantilever beam assembly is provided. Additionally, in certain embodiments the bogies may be configured to raise and lower the support structure lifting unit relative to the bogies themselves. This may be performed, for instance, by operating a suspension system provided to each bogie.

According to a further aspect of the present disclosure, there is provided a vessel for decommissioning an offshore platform, the offshore platform comprising a topside and a support structure, wherein the vessel is configured to interchangeably receive a topside lifting unit adapted to lift the topside from the support structure and a support structure lifting unit adapted to lift the support structure from the water such that the support structure is supported by the vessel, and wherein the vessel comprises a first hull portion adapted to accommodate an installed support structure and a ballasting system for lifting the topside.

Further embodiments of the vessel may comprise any of the features of the vessels in the system discussed above.

In accordance with a further aspect of the present disclosure, there is provided a method of decommissioning an offshore platform comprising a topside and a support structure, the method comprising the steps of:

receiving a topside lifting unit onto a vessel configured to interchangeably receive the topside lifting unit and a support structure lifting unit, wherein the vessel comprises a first hull portion adapted to accommodate an installed support structure and a ballasting system for lifting the topside;

lifting the topside from the support structure using the topside lifting unit; transferring the topside from the vessel;

transferring the topside lifting unit from the vessel; receiving the support structure lifting unit onto the vessel; lifting the support structure using the support structure lifting unit; and transferring the support structure from the vessel.

Thus, the vessel is capable of lifting offshore platform topsides using the topside lifting unit and lifting offshore platform support structures when the support structure lifting unit is on-board the vessel. Advantageously, there is no need to separate or split the topside or the support structure into smaller sections.

In preferred embodiments, the method comprises the further steps of:

ballasting the vessel such that the vessel sinks in the water topside lifting unit is lower than bottom surface of the topside;

positioning the vessel such that topside lifting unit is below the topside; and, de-ballasting the vessel such that the vessel rises in the water and the topside is lifted from the support structure by the topside lifting unit.

Preferably lifting the support structure using the support structure lifting unit comprises the further steps of:

raising the support structure using a winch system adapted to detachably couple to the support structure and raise or lower the support structure in a vertical direction; and, rotating the support structure such that the support structure is transferred on to the vessel using a frame adapted to rotate about a bearing housing.

In preferred embodiments the topside and/or the topside lifting unit are transferred from the vessel to an ancillary vessel whilst the vessel is at sea. Advantageously the ancillary vessel (which may be a barge or heavy lift ship) may transfer the topside or the topside lifting unit to shore. This avoids the need for the vessel to transit to port and allows the vessel to remain on station for a greater proportion of time and increases the utilisation of the vessel. Thus the method is quicker, more efficient and more economic than existing methods.

Alternatively, the topside and/or the topside lifting unit may be transferred to the quayside or to an ancillary vessel whilst the vessel is in port.

In particularly preferred embodiments the steps of transferring the topside from the vessel and transferring the topside lifting unit from the vessel are performed simultaneously using a propulsion system provided to the topside lifting unit, the propulsion system configured to move the respective lifting unit.

Consequently, the topside lifting unit is able to transfer itself and a topside it has lifted to and from the vessel. Thus the vessel is able to offload the topside to a transport barge or the quayside without any further specialised equipment and without breaking the topside down into smaller sections. This reduces the time and cost required to decommission an offshore platform and increases vessel utilisation.

Alternatively, topsides and the topside lifting unit may be transferred to and from the vessel using cranes or other lifting equipment.

In preferred embodiments, the support structure and/or the support structure lifting unit are transferred from the vessel to an ancillary vessel whilst the vessel is at sea.

Alternatively, the support structure and/or the support structure lifting unit may be transferred from the vessel to the quayside or to an ancillary vessel whilst the vessel is in port.

In particularly preferred embodiments wherein the steps of transferring the support structure from the vessel and transferring the support structure lifting unit from the vessel are performed simultaneously using a propulsion system provided to the support structure lifting unit, the propulsion system configured to move the support structure lifting unit.

Consequently, the support structure lifting unit is able to transfer itself and a support structure it has lifted to and from the vessel. Thus the vessel is able to offload the support structure to a transport barge or the quayside without any further specialised equipment and without breaking the support structure down into smaller sections. This reduces the time and cost required to decommission an offshore platform and increases vessel utilisation.

Alternatively, support structures and the support structure lifting unit may be transferred to and from the vessel using cranes or other lifting equipment.

BRIEF DESCRIPTION OF DRAWINGS

Examples of vessels and lifting units for the disposal and decommissioning of offshore oil and gas hydrocarbon platforms will now be described with reference to the accompanying drawings, in which:Figure 1 depicts a simplified side view of a conventional fixed offshore platform.

Figure 2(a) invention.

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Figure 2(b) invention.

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Figure 3(a) invention.

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Figures 3(d) and 3(e) show simplified plan view of a vessel with ducts in accordance with the present location.

Figure 4(a) shows a side view of a vessel in accordance with the present invention.

Figure 4(b) shows a plan view of a vessel in accordance with the present invention.

Figure 4(c) shows a front view of a vessel in accordance with the present invention.

Figure 5 shows a plan view of a topside lifting unit which is received on a vessel in accordance with the present invention.

Figure 6 shows cross sections of a topside lifting unit in accordance with the present invention.

Figure 7 shows cross sections of a further topside lifting unit in accordance with the present invention.

Figures 8(a), (b) and (c) show cross sections of topside lifting units in accordance with the present invention.

Figures 9(a) and 9(b) respectively show side and end views of a system suitable for detachably connecting two cantilever beams in accordance with the present invention.

Figures 10(a) to 10(e) illustrate the steps involved in lifting an offshore platform topside using a vessel and a topside lifting unit in accordance with the present invention.

Figures 11(a) and 11(b) show side and front views of a vessel 400 which has received a support structure lifting unit in accordance with the present invention.

Figures 12(a) to 12(g) illustrate the steps involved in lifting an offshore platform support structure using a vessel and a support structure lifting unit in accordance with the present invention.

Figures 13(a) and 13(b) show embodiments of support structure lifting units and vessels carrying said lifting units in accordance with the present invention.

Figure 14 shows a cross section of an embodiment of a support structure lifting units and a vessels carrying said lifting unit in accordance with the present invention.

Figures 15(a) and 15(b) show embodiments of support structure lifting units and vessels carrying said lifting units in accordance with the present invention.

Figures 16(a) to 16(d) illustrate the steps involved in replacing a topside lifting unit mounted on a vessel with a support structure lifting unit whilst at port.

DETAILED DESCRIPTION

The present disclosure offers improved offshore vessels, systems and methods for the decommissioning of offshore platforms. Additionally, it is envisioned that these vessels may equally be used for the installation of offshore platforms and the installation and decommission of other permanent offshore structures.

The vessels described below are able to lift the topside and the support structure of offshore platforms without the need to separate either structure into smaller sections. Once the topside or support structure has been lifted by the vessel it may be transferred to shore for disposal or deconstruction, or installed at a new location.

The lift of the topside and the support structure of an offshore platform are achieved using a topside lifting unit and a support structure lifting unit, respectively. These lifting units are interchangeable - such that they may be alternatively loaded on a vessel depending on the portion of an offshore platform to be lifted. Consequently, the vessel size may be minimised without diminishing or impairing the capability of the vessel. This offers significant reduction in the time and resources required to decommission offshore platforms and a corresponding increase in the utilisation of vessel in accordance with the present disclosure. Each of the embodiments below are optional examples of how the above principle may be implemented in practice.

Before describing suitable vessels, topside lifting units, support structure lifting units and methods of decommissioning offshore platforms in turn, the typical features of conventional offshore platforms will be briefly discussed.

Figure 1 shows a simplified side view of a fixed offshore platform 10 for exploiting a subsea oil and/or gas field. The platform 10 comprises two major sections, a topside 12 and a support structure 14, in the form of a steel jacket. The jacket 14, which is formed of tubular steelwork, typically extends from the sea bed 22 above the surface 24 of the sea 20 and supports the topside 12 above the sea surface 24.

In addition to the typical offshore platform 10 shown in Figure 1 (which is supported by a steel jacket 14) the vessels, systems and methods discussed below may equally be used to decommission or install a wide variety of alternative offshore platforms 10. Such offshore platforms 10 may have a variety of support structures which may include a jacket, a tension-leg structure, a gravity based support structure such as a caisson or jack-up legs.

Figures 2, 3 and 4 illustrate three separate exemplary offshore vessels 100, 200, 300 in accordance with the present invention. Each of these vessels 100, 200, 300 are adapted to interchangeably receive a topside lifting unit or a support structure lifting unit and to cooperate with these lifting units to respectively lift the topside and support structure of an offshore platform. The vessels are a newly built lifting vessel 100, an adapted vessel 200, and a vessel 300 formed by combining two existing ships. It will be understood that these are merely examples of suitable vessels.

Figures 2(a), 2(b) and 2(c) show a newly built offshore vessel 100. The vessel 100 is approximately 120m to 150m in length with a total width of approximately 120m.

The hull 110 of the vessel 100 comprises a slot 112 or recess which begins at the bow (front) of the vessel at the far right as drawn and extends towards the stem (rear) of the vessel at the far left as drawn. Consequently, the vessel 100 is ‘U’ shaped, much like a “tuning fork”, when viewed from above, as seen in Figure 2(b).

This slot 112 is capable of accommodating or receiving the support structure 14 of an installed offshore platform 10. This allows the vessel 100 to be positioned so that the support structure 14 is located within the slot 112, extending vertically through the slot 112. Consequently, slot 112 is preferably wider and longer than the dimensions of the majority of offshore platform support structures 14 at the sea surface 24. In certain embodiments, slot 112 has a width Wi of approximately 60m (measured in a direction perpendicular to the length of the vessel 100) and a length Li of approximately 90m to 100m (measured along the length of the vessel 100). This size enables the platform’s topside 12 to be lifted in a single lift.

The portion 120 of the upper deck 114 of the vessel 100 (the highest deck of the hull extending along the length of the vessel 100) surrounding the slot 112 is substantially flat and free of equipment and vessel systems with the exception of a retractable foremast 116 (a legislative requirement). This open portion 120 of the weather deck 114 is able to interchangeably receive a topside lifting unit (which is adapted to lift a topside 12 from a support structure 14) and a support structure lifting unit (which is adapted to lift a support structure 14 from the water 20). In other words, the vessel 100 can hold either the topside lifting unit or the support structure lifting unit on the open portion 120 of its weather deck 114 at a given time. The lifting units are discussed in detail below. In certain embodiments the vessel’s structure is reinforced or strengthened below this open portion 120 may so that it is able to support the lifting units.

The vessel 100 further comprises an energy recovery system comprising four ducts 118a extending from the rear of the slot 112 through the vessel 100. As shown these ducts 118 have a circular cross-section, however, any suitable cross section may be used. Furthermore, the cross section of the ducts 118 may vary along the length of the ducts 118.

Within each duct 118a is provided at least one turbine 118b which can generate energy as water passes through the duct 118. For instance, the energy used to power the vessel 100 through the water may be recovered whilst sailing or as the vessel 100 slows down or comes to a stop, whilst energy may be generated directly as currents or waves pass the vessel 100 whilst it is stationary (e.g. when performing lift operations or at anchor). Additionally, the ducts 118a reduce the water resistance of the vessel 100 as it moves. This increases the efficiency of the vessel 100 and reduces the resources required to operate it.

Within the vessel 100, ducts 118a extend along an axis substantially parallel to the length of the vessel 100, however, this is not essential. In some embodiments, the duct 118a will have a bell shaped entry and discharge and be formed as a venture nozzle to increase the water velocity through the duct 118a and increase the power generated by the turbines 118b. It will be appreciated that any suitable number of ducts 118a and turbines 118b may be provided at the rear of the slot 112.

Whilst Figures 2(a), 2(b) and 2(c) show a purpose built vessel design suitable for decommissioning offshore platforms, similar capability and functionality may also be achieved by converting or retrofitting existing vessels. Two embodiments formed of pre-existing vessels which have been adapted to decommission offshore platforms are shown in Figures 3 and 4.

Figures 3(a), 3(b) and 3(c) show a further offshore vessel 200 design. The vessel 200 is again approximately 120m to 150m in length with a maximum width at its bow of approximately 120m.

Unlike a purpose-built vessel 100 (such as the one shown in Figure 1 and discussed above), the offshore vessel 200 comprises a rear section 210a which is formed from the rear section of an existing ship or vessel (e.g. an oil tanker, bulk carrier, containership, etc.) to which a new front section 210b has been attached. The front section 210b comprises a slot 212 and an open portion 220 of the upper deck 214 substantially clear of equipment and systems which is suitable to interchangeably accommodate a topside lift unit and a support structure lift unit. Thus the vessel 200 has a ‘U’ or ‘tuning fork’ shape when viewed from above. Beneath the open portion 220 of the upper deck 214, the vessel 200 may be reinforced to support the lifting units. The vessel 200 comprises a retractable foremast 216.

The slot 212 is again preferably wider and longer than the dimensions of the majority of offshore platform support structures 14 at the sea surface 24, preferably having width W₂ of approximately 60m (measured in a direction perpendicular to the length of the vessel 100) and a length L₂ of at least 30m and more preferably 90m to 100m (measured along the length of the vessel 100). This size of slot 212, which is able to receive the support structure 14 of an offshore platform 10 as installed, allows the platform’s topside 12 to be lifted in a single lift.

By reusing the rear section 210a of an existing vessel 200 the marine systems, equipment, accommodation and superstructure typically found in at the rear of merchant vessels may be reused. Thus the time and cost associated with procuring and building these valuable systems and features are significantly reduced. Nonetheless, the interface or transition between the front and rear sections 210a, 210b must be carefully designed to ensure structural integrity and continuity between the sections 210a, 210b.

In certain embodiments the vessel 200 shown in Figures 3(a), 3(b) and 3(c) may be provided with an energy recovery system similar to that discussed with reference to the vessel 100 of Figures 2(a), 2(b) and 2(c). This may allow the generation of energy from water passing through one or more ducts 218 and reduce the resistance or drag on the vessel 200 as it passes through the water.

Two alternative duct 218 arrangements for this system are shown in Figures 3(d) and 3(e) which show simplified plan views of the vessel 200 of Figures 3(a), 3(b) and 3(c). In each case, the vessel 200 comprises two ducts 218 extending through the vessel 200 from the slot 212 (although substantially any number of ducts 218 may be used).

In Figure 3(d) the ducts 218 extend from the base of the slot to the stem (rear) of the vessel 200 in a direction which is parallel to the length of the vessel 200 and the length of the slot 214. Advantageously, this arrangement - where the ducts 218 are aligned with the vessel 200 and its direction of travel - maximises the amount of energy which may be recovered by turbines (not shown) within the ducts 218 and maximises the reduction in the drag of the vessel 200. However, it may be difficult to adapt or convert the rear section 210a of an existing vessel to have such ducts 218 passing through it as the positioning of the ducts may interfere with machinery or systems within the existing vessel. Therefore, instead the ducts 218 may be positioned externally on the keel (i.e. the underside or bottom) of the vessel 200, or adjacent or in contact with to the keel inside the vessel 200.

Alternatively, the ducts 218 may only pass through the front section 210a of the vessel 200, as shown in Figure 3(e). This allows the vessel to be designed to accommodate such ducts 218 rather than adapting the ducts to avoid the existing systems of the rear section 210a of a conventional vessel. Specifically, the ducts 218 extend from opposing sides of the slot through the vessel 200 to the corresponding opposing sides 210c of the vessel 200, as shown in the figure. These ducts 218 are inclined at an angle to both an axis extending along the length of the vessel and the length of the slot and a transverse extending across the width of the vessel 200. Whilst this arrangement may result in a loss in energy recovered or reduced drag reduction it is significantly easier to integrate into the vessel design.

Figures 4(a) 4(b) and 4(c) show a further embodiment of an offshore vessel 300 which has been constructed from two identical existing ships 310a, 310b (e.g. an oil tanker, bulk carrier, containership, etc.). The hulls of these ships 310a, 310b have been coupled by a central section 310c at the stem (rear) of the vessel 300. This central section 310c does not extend below the waterline of the vessel

300. Thus, the vessel 300 is a catamaran having two parallel demi-hulls 310a, 320b of equal size.

Once again, from above the hull 310 of the vessel appears ‘LT or ‘Tuning Fork’ shaped with a slot 312 between the two demi-hulls 310a, 310b (as seen in Figure 3(b)). However, unlike the vessels 100, 200 discussed above, the demihulls 310a, 310b are not connected at the waterline 24. Consequently, the slot 312 is continuous and passes completely through the vessel 300 at the waterline. Above the waterline the slot 312 extends from the bow of the vessel 300 to the front of the central section 310c (as shown).

Either the topside lift unit when connected in a beam configuration, and the support structure lift unit would be in place during transit which would provide additional lateral support to the front reinforced hull sections (310).

The slot 312 is sized to accommodate or receive the majority of offshore platform support structures 14 when the platforms 10 are installed on the sea bed 22. Thus the vessel 200 may be positioned around the support structure 14, so that the support structure 14 extends vertically through the slot 312. In preferred embodiments the slot has a minimum width W₃ of approximately 60m (from the bow of the vessel 300 to central section 310c) and a length L₃ of approximately 90m to 100m. The vessel 200 comprises a retractable foremast 316.

The forward portion of the upper deck 314 of each demi-hull 310a, 310b on either side of the slot 312 has been cleared of existing equipment and systems. Thus, the vessel comprises an open portion 320 which is adapted to interchangeably accommodate a topside lift unit and a support structure lift unit (discussed in detail below).

Advantageously, by reusing two existing ships significant savings in the time and cost required to build the hull of vessel 300 may be achieved. However, many of the systems onboard the combined vessel 300 will be duplicated across the two hulls 310a, 310b. Positively, this duplication provides redundancy for systems within the vessel, thus reducing the impact of any mechanical failures and increasing the utilisation of the vessel 300. However, the systems with redundant equipment may be difficult to integrate and control since the equipment is spread across both hulls 310a, 310b. Additionally, the duplicated equipment and may require more resource requirements to run, maintain and crew. Consequently, in certain embodiments of the vessel 300 some or all of the duplicated equipment may be combined or removed to alleviate or avoid these issues.

All three of the vessels 100, 200, 300 discussed above comprise an open portion 120, 130, 140 of the upper deck 114, 214, 314 configured to receive lifting units interchangeably (i.e. where one lifting unit may be switched out and replaced with another unit) and a slot 112, 212, 312 which can accommodate the portion of an offshore platform support structure 14 which is at the surface 24 of the sea 20 when the offshore platform 10 is installed.

Additionally, each vessel 100, 200, 300 comprises a ballasting system (not shown) capable of lifting an offshore platform topside 12 in a single lift. Such a ballasting system must comprise water intakes and outtakes, ballast pumps and ballast tanks which are capable of receiving water with a mass equal to at least the mass of the topside 14 to be lifted. This is because in order to lift the topside 14 using buoyancy lift the mass of the topside 14 must be offset by an equivalent mass of water expelled from the lifting vessel 100, 200, 300. In certain embodiments, each vessel 100, 200, 300 must be capable of receiving at least 25,000 tonnes (25,000,000 kg) of water, whilst more preferably the vessels 100, 200, 300 are capable of receiving at least 30,000 tonnes (30,000,000 kg) of water. Such ballast systems may be designed using conventional methods.

Furthermore, each vessel may comprise: propulsion machinery (e.g. diesel engines, steam turbines, electrical generators); propulsers (e.g. thrusters, propellers, anchors); fresh water machinery; accommodation; fire suppression and safety systems; heating, ventilation and air conditioning (HVAC); dynamic positioning control; steering, communication and navigation systems; lifeboats; a helideck; at least one crane. These systems may be designed using methods known in the art.

As discussed above, the vessels 200 and 300 shown in Figures 3 and 4 are built by converting existing (second-hand) ships. Alternatively, vessels with similar hull forms to vessels 200 and 300 may be newly built. This avoids the need to procure existing ships or vessels for conversion and may avoid a long, expensive and complicated retrofitting process.

In addition, although the discussion above has focused on the decommissioning of offshore platforms, it will be appreciated that the vessels shown in Figures 2, 3 and 4 may equally be used to lift, position and install topsides and support structures during the installation of offshore platforms.

Exemplary and optional topside lifting units which are adapted to be received on vessels 100, 200, 300 for the decommissioning of offshore platforms (such as the vessels shown in Figures 2, 3 and 4) will now be discussed with reference to Figures 5 to 9. Once a topside lifting unit is loaded on any of the vessels 100, 200, 300 discussed above, the vessel and the topside lifting unit may cooperate to lift the topside 12 of an offshore platform from the support structure it is installed upon through buoyancy lift. As will be seen, this process does not require the topside 12 to be split or separated into smaller sections and as such the removal of the topside is a ‘single-lift’ procedure.

Figure 5 shows a plan view of a vessel 400 with a slot 412 in its hull 410, the slot 412 being sized to accommodate the support structure 14 of an offshore platform 10 as installed. On the upper deck 414 of the vessel 400 is a topside lifting unit 500 arranged so as to lift an offshore platform topside 12.

The topside lifting unit 500 comprises eight cantilever beam assemblies 500a to 500h arranged in four pairs on opposing side of the slot 412. Each cantilever beam assembly comprises a cantilever beam 510 which extends in a lateral direction from the vessel hull 410 over the slot 412. This lateral direction is perpendicular to the length of the slot 412 as shown.

As shown, the length of the cantilever beams 510 (measured in the direction the cantilever beam 510 extends over the slot) is significantly larger than the width of the cantilever beams 510 (perpendicular to the length of the beams i.e. direction along the length of the slot in Figure 5). This allows multiple cantilever beam assemblies 500a to 500h to be positioned on the vessel 400 on either side of the slot 412, but is not essential. Indeed, the cantilever beams 510 may have substantially any width so long as they are capable of supporting a topside 12.

In preferred embodiments a cantilever beam 510 has a length of at least 45m, more preferably 60m such that the cantilever beam 510 may extend from the vessel 400 to at least the centre of the slot 412. Preferably the length of each cantilever beam is approximately equal to the combined length of half the width of the slot 412 when added to the width of the vessel adjacent to the slot (i.e. the width of each of the arms of the ‘tuning fork’ or the width of each of the hulls of a catamaran). For instance, if the slot is 60 m and the vessel width on either side of the slot is 30 m and then each beam may be approximately 60m as we would try to lift always in a beam configuration.

Each cantilever beam 510 is provided on an assembly base 520 which is located on the upper deck 414 of the vessel 400. Each assembly base 520 comprises two connection portions 530 configured to couple the assembly base 520 to its respective cantilever beam 510. These connection portions 530 are discussed in more detail with reference to Figure 8 below.

In further embodiments the assembly base 520 comprises at least one integrated connection portion 530. The number and distribution of the connection portions 530 may be determined based on the maximum size and weight of the topside lifting unit to be lifted by the topside lifting unit 500.

Two of the pairs of cantilever beam assemblies 500a &500e and 500d & 500h, are arranged such that their cantilever beams 510 are detachably coupled to form a continuous beam which spans (i.e. extends across) the slot 412.

In preferred embodiments all the cantilever beams may be detachably coupled to one another such that any combination of cantilever beams may be coupled to form continuous beams. This allows the layout of the cantilever beams to be adjusted to accommodate a wide variety of topsides.

The even arrangement of cantilever beams 510 shown in Figure 5 (in pairs, with two pairs of beams coupled at the centre of the slot 412) allows a balanced distribution and transmission of forces from the offshore platform topside 14 to the vessel hull 410. Nonetheless, it will be appreciated that there are a wide variety of distributions of cantilever beams 510 that are suitable to lift a topside 14, so long as at least one cantilever beam 510 extends over the slot 412 which is adapted to accommodate the support structure. Indeed, the specific topside 14 to be lifted may require variations in the number and positioning of the cantilever beam assemblies 500 and the angle of the cantilever beam 510 relative to the slot 412 and/or the length of the vessel 400.

The cantilever beam assemblies 500a to 500h shown in Figure 5 will now be discussed in more detail with reference to Figures 6 to 8.

Figures 6 shows a cross section A-A of a cantilever beam assembly 500a to 500h, as indicated on the cantilever beam assembly labelled 500b on Figure 5.

The assembly base 520 shown in Figure 6 is provided with five rows of bogies 522 positioned between the assembly base 510 and the upper deck 414 of the ship 400 adapted to move the assembly 500. However, it will be appreciated that substantially any number of bogies 522 or rows of bogies 522 may be used. The bogies 522 comprise a plurality of wheels located on a plurality of axles and allow the cantilever beam assembly 500a to 500h to move. Projections 416 extend above the surface of the upper deck 414 at the edges of the upper deck 414 for personnel safety, and such that the wheels in the bogies 522 do not fall off the vessel 400. Preferably, the vessel or the topside lifting unit will further comprise safety systems (e.g. alarms, cameras, locking brakes) to ensure that the bogies cannot steer over the edge.

In preferred embodiments the bogies 522 are powered or motorised such that the cantilever beam assemblies 500a to 500h are self-propelled and able to locomote. In further preferred embodiments one or more bogies 522 (and the axles in said bogies 522) may rotate and by positioned relative to the assembly base 520 so as to turn the cantilever beam assemblies 500a to 500h.

Alternatively, the system may be provided with any other suitable means of moving the cantilever beam assemblies 500a to 500h. For instance, the cantilever beam assemblies 500a to 500h or the vessel 400 may be provided with wheels, tracks or rollers, hydraulic or pneumatic pistons, rack and pinion actuators, screw actuators, electromagnetic levitation (maglev) or a winch system configured to move the cantilever beam assemblies 500a to 500h. Alternatively, the vessel 400 may be provided with a crane configured to move the cantilever beam assemblies 500a to 500h.

The assembly base 520 further comprises projections 524 which from a bottom surface of the assembly base 520. The length of the projections 524 is such that the free end of the projections 524 is located approximately coincidently with the lowest point of the wheel provided to each bogie 522. Consequently, the projections 524 are adapted to support the cantilever beam assembly 500 on the upper deck 414 of the vessel 400.

Alternatively, a projection which is configured to support the cantilever beam assembly 500 may be provided to the upper deck 414 instead of, or in addition to, the projections 524 extending from the assembly base 520. For instance, in Figure 7 (which shows a portion of an alternative cross section A-A of a cantilever beam assembly 500a to 500h, as indicated on the cantilever beam assembly labelled 500b on Figure 5) there are projections 414a provided on the upper deck 414 of the vessel. These projections 414a extend away from the vessel 400 and are configured to support the cantilever beam assembly 500 when installed on the vessel 400.

Returning to Figure 6, each cantilever beam assembly 500a to 500h comprises at least one lifting pad 550 located on the respective cantilever beam 510 and configured to contact the topside 14 during lifting operations. Between each lifting pad 550 and the cantilever beam 510 there is provided a heave compensation system 560 which reduces the absolute vertical motion of the lifting pad 550. In effect, the heave compensation system decouples the vertical motion of the lifting pad 550 from that of the vessel 400 such that the vertical position of each lifting pad 550 relative to the topside 14 may be held constant during a topside lifting operation.

In particularly preferred embodiments a cantilever beam assembly 500 comprises one or more moveable lifting pads 550, such that the movable lifting pad 550 may be moved in a transverse direction along the length of the cantilever beam 510 (i.e. along the cantilever beam 510 in a direction extending towards or away from the assembly base 520). This may be achieved using any suitable means (not shown) including a hydraulic piston, a pneumatic piston, a linear screw actuator, a rack and pinion actuator or a motor. Alternatively, the lifting pad 550 may be moved in substantially any direction along the top face of said cantilever beam 510. This allows the position of lifting pads 510 to be adjusted based on the specific topside 12 being lifted by the topside lifting unit, thus increasing the range of offshore platform topsides 12 that may be lifted by the topside lifting unit. This is especially important where two or more cantilever beams 510 are coupled to form a continuous beam and therefore cannot be independently positioned beneath the offshore platform 12 during a lifting operation.

The topside lifting unit 500 further comprises a locking assembly 570 provided to each cantilever beam assembly 500a to 500h which is adapted to secure the cantilever beam assembly 500a to 500h to the upper deck 514 of the vessel 400. When engaged the locking assembly 570 acts to prevent the cantilever beam assembly 500a to 500h moving relative to the vessel 400.

As shown in Figure 6, the locking assembly 570 comprises a plurality of locating pins 572 which pass through holes 574 in the upper deck 414 of the vessel 400 and corresponding holes 576 in the assembly base 520 and the projections 524 of the assembly base 520. Thus each cantilever beam assembly 500a to 500h may be secured to the vessel at any location where the necessary holes 574 in the upper deck 414 of the vessel 400 are provided.

The locking system in Figure 7 works similarly, comprising a plurality of locating pins 572 which passes through corresponding holes 574 in the upper deck 414 and the projections 414a of the upper deck 414 and through corresponding holes 576 in the assembly base 520.

Figures 8(a), 8(b) and 8(c) show alternative embodiments of the connection portion 530 of the assembly base 520 of a topside lifting unit. The figures provide cross sections through a cantilever assembly 500a to 500h, as indicated by line B-B in Figure 5. As shown the connection portion 530 and the assembly base 520 are formed as a single component (e.g. as a single continuous casting). However, this is not essential and the connection portion 530 and assembly base 520 may be formed separately and welded or otherwise secured together. In certain embodiments the connection portion 530 and the assembly base may be detachably coupled so that the topside lifting unit may be adapted based on a topside 12 to be lifted.

Each connection portion 530 prevents the cantilever beam 510 of the cantilever beam assembly 500a to 500h from moving in at least two orthogonal directions. This increases safety and prevents damage to the vessel 400 or the offshore platform 10 during lifting operations.

All of the embodiments of the connection portion 530 shown in the figures comprise an annular cross section which is configured to receive the cantilever beam 510 within the annular ring. In alternative embodiments, the connection portion 510 may not be formed as a closed annulus and may instead be formed as, for instance, a ‘LT shaped portion adapted to receive the cantilever beam 510

In Figures 8(a) and 8(b) the connection portion 530 further comprises two slots 532 in an inner wall 534 of the annulus. The slots 532 widen as the distance from the inner wall 534 of the connection portion increases. In Figure 8(a) the slots 532 widen continuously and are tapered, whilst in Figure 8(b) the slots 532 widen discontinuously and have a T-shape.

Furthermore, in Figures 8(a) and 8(b) the cantilever beam 510 comprises two projections 514 on an outer surface 516 of the cantilever beam 510. These projections are configured to be received in the slots 532 of the connection member 530. The shape of these projections 514 corresponds to those of the slots 532. In both embodiments the projections 514 widen as distance from the outer surface 516 of the cantilever beam 510 increases. In Figure 8(a) the projection is tapered, whilst in Figure 8(b) the projection is T-shaped.

In the embodiment shown in Figure 8(c) the connection portion 530 comprises two projections 536 extending from an inner wall 534 of the connection portion 530. These projections 534 do not widen or narrow with distance from the inner wall 534 of the connection portion 530. The cantilever beam 510 comprises a projection 514 adapted to be received between the two connection portion projections 536. This cantilever beam projection 514 does not widen or narrow with distance from the cantilever beam 510. There is provided a locating pin 518 which extends through the cantilever beam projection 514 and both connection portion projections 536, restricting the movement of the cantilever beam relative to the connection portion 530 and the assembly base 520.

In particular, it will be seen that the projections 514 and slots 532 shown in Figures 8(a) and 8(b) restrict the motion of the cantilever beam 510 in two directions - vertically and laterally as shown by axes x and z. In comparison the locating pin 518 in Figure 8(c) is adapted to restrict or prevent any motion of the cantilever beam 510 relative to the assembly base (i.e. the cantilever beam 510 may not move in either x, y or z as shown).

Each of the cross sections shown in Figures 8(a), 8(b) and 8(c) show a screw actuator 536 adapted to move the cantilever beam 510 relative to the assembly base 520 such that the cantilever beam 510 is extended over the slot 412. As indicated, this movement is in the y direction (i.e. into or out of the page). Alternatively, any other suitable means of including wheels, rollers, tracks, linear actuators, ratchets, rack and pinion devices, winches, hydraulic or pneumatic pistons, magnetic levitation or motors.

Figures 9(a) and 9(b) respectively show side and end views of a system suitable for detachably connecting two cantilever beams 510a and 51 Oe together in a offshore platform decommissioning vessel 400.

The first cantilever beam 510a comprises three pins 582. Each pin 582 extends from the free end 512a of the first cantilever beam 510a in a direction which is parallel (or substantially parallel) to the length of the cantilever beam 510a. These pins 582 are adapted to be received in a set of three corresponding holes 584 formed within the free end 512e of the second cantilever beam 51 Oe.

The pins 582 and holes 584 may be meshed or interlocked by extending either cantilever beam 510a or cantilever beam 51 Oe (or both beams simultaneously) over the slot 412 such that the pins 582 are received in the holes 584. Thus the two cantilever beams 501a and 51 Oe are coupled to form a single beam which spans the width of the slot 412. This allows the forces to be transmitted evenly from the topside 12 to the vessel 400 through both cantilever beams 510a, 51 Oe. This evenly distributed load will allow an increase in the lifting capacity of the vessel 400.

To decouple the two cantilever beams 510a, 51 Oe one, or both, cantilever beams 510, 51 Oe is retracted towards the vessel 400 from over the slot 412, such that the pins 582 are removed from the holes 584.

As discussed above, the holes 584 must be adapted to receive or mesh with the pins 582 such that forces may be transmitted between the cantilever beams 510a, 51 Oe on which the pins 582 and holes 584 are provided. Although three pins 582 and three corresponding holes 584 are shown in Figures 9(a) and 9(b), substantially any number of pins and holes may be used.

The end portion 586 of each pin 582 narrows or tapers as distance increases from the cantilever beam 510a on which it is mounted. Specifically, the end portion of each pin 582 is bell-shaped. Similarly, the entrance 588 of each hole 584 narrows or tapers as the distance from the free end 512e of the cantilever beam 51 Oe increases (i.e. each hole narrows as its depth increases). Specifically, the entrance 588 of each hole 584 is bell-shaped.

This shaping means that a pin 582 and a hole 584 will self-centre, i.e. align to each other, as the two cantilever beams 510a, 51 Oe are brought into contact. If the pins 582 and holes 584 are brought into contact along a first direction whilst there is a small misalignment between the pins 582 and holes 584 the entrance 588 of each hole 584 will contact the end portion 586 of its corresponding pin 582. This contact will force the pin 582 and hole 584 (and hence the cantilever beams 510a, 51 Oe) to move laterally relative to the first direction and i.e. laterally relative to one another.

It is noted that a similar self-centring may be achieved with if only one of the pins 582 or the holes 584 narrow or taper. Further, a variety of pin 582 and hole 584 geometries are possible. For instance, the pins 582 or holes 584 may narrow continuously or discontinuously. In certain embodiments either the pins 582 or the holes 584 or both may be formed as truncated cones.

As shown, the pins 582 and the holes 584 have a circular cross section. However, this is not essential. For instance, the pins 582 and/or holes 584 may be square, slotted, or slotted-square keys. These square, slotted or squareslotted pins 582 and holes 584 may be adapted to self-centre as they are brought together to correct any misalignment or relative rotation. This may include a centring configuration located at the free end of each pin 582.

In addition, Figures 9(a) and 9(b) show a locking apparatus configured to secure the cantilever beams 510a, 51 Oe together such that movement of the cantilever beams 510a, 51 Oe relative to one another is restricted. The locking apparatus comprises two piston 592 positioned on opposing sides of a first cantilever beam 510a. Each piston 592 is configured to be extended in a direction along the length of the first cantilever beam 510a so that it is received a corresponding piston receiving portion 594 positioned on a second cantilever beam 51 Oe when the first and second cantilever beams 510a, 51 Oe are detachably coupled to form a continuous beam. Thus the locking apparatus further comprises two piston receiving portions 594 positioned on opposing sides of the second cantilever beam. By operating the pistons 592 to be received in the piston receiving portions 594 the cantilever beams 510 may be selectably locked or secured together.

It will be appreciated that in alternative embodiments substantially any number and arrangement of pistons 592 and corresponding piston receiving portions 594 may be provided on the cantilever beams 510 so long as the movement of the cantilever beams 510 relative to one another is restricted. In addition or alternatively the movement of the cantilever beam may be further restricted by the action of components, features or means adapted to move the cantilever beams 510 relative to their respective assembly bases 510 or by the propulsion system adapted to move the cantilever beam assemblies 500.

Having discussed the features of suitable vessels 100, 200, 300 and topside lifting units 500 above an exemplary method of operating these systems to lift the topside 12 of an offshore platform 10 from its support structure 14 will now be discussed. As will be seen, this process utilises buoyancy lift principles and does not require the topside to be subdivided before it is lifted.

Figures 10(a) to 10(e) illustrate the steps involved in lifting an offshore platform topside 12 using a vessel 400 and a topside lifting unit 500 so that the platform may be decommissioned.

As will be seen from the figures, the topside 12 is initially supported by support structure 14. The vessel 400 comprises a slot 414 (not shown) configured to accommodate or receive the support structure 14 as installed, so that the vessel 400 may be positioned around the support structure 14. Additionally, the vessel 400 comprises a ballasting system (not shown) configured to lift the topside 14.

The topside lifting unit 500 comprises four cantilever beam assemblies 500a, 500b, 500c and 500d which are arranged in two pairs 500a & 500b and 500c &

500d. Initially, each cantilever beam assembly 500a, 500b, 500c, 500d is positioned such that it is retracted, so as to not extend outboard from the vessel 400 over the slot 414. The vessel 400 may accordingly approach the offshore platform and receive the support structure 14 within the slot 414.

Between Figures 10(a) and 10(b) the vessel 400 operates its ballasting system to lower itself in the water. This may be achieved by taking water into the ballast tanks of the ballasting system. Specifically, the vessel 400 sinks or is lowered in the water such that the cantilever beam assemblies 500a, 500b, 500c, 500d are positioned below the topside 12.

The vessel 400 is then manoeuvred such that the support structure 14 mates with the vessel 400 so that the support structure 14 extends through the slot 414 (see Figure 10(c)). Once the vessel 400 is positioned around the support structure 14 the cantilever beam assemblies 500a, 500b, 500c, 500d are extended outboard from the vessel 400 so that they are positioned over the slot 414 and beneath the topside 12 of the offshore platform. More specifically, the cantilever beam assemblies 500a, 500b, 500c, 500d may be positioned such that the lifting pads 550 discussed above with reference to Figures 5 and 6 are located directly under points on the topside 12 which are capable of supporting the mass of the topside 12. In some embodiments at least one pair of cantilever beams 500a & 500b and 500c & 500d may be detachably coupled to form a continuous beam which spans (i.e. extends across) the slot 412. In preferred embodiments all pairs of cantilever beams 500a & 500b and 500c & 500d are coupled in this manner.

With the cantilever beam assemblies 500a, 500b, 500c, 500d in position the vessel 400 is de-ballasted (e.g. by expelling water from its ballast tanks) and rises in the water such that the cantilever beam assemblies 500a, 500b, 500c, 500d of the topside lifting unit 500 contact the topside 12 and lift the topside 12 from the support structure 14. This arrangement is shown in Figure 10(d).

Once the topside 12 has been lifted from the support structure 14 by the vessel 400 and the topside lifting unit 500 the vessel 400 may be manoeuvred away from the support structure 14 which remains in situ (see Figure 10(e)). The vessel 400 may then offload the topside 12, either to a quayside (i.e. in port or at the shore) or to an ancillary vessel such as a transportation barge or heavy lift vessel whilst still at sea or at nearshore.

Transferring the topside 12 to an ancillary vessel may be achieved by substantially reversing the process described above - i.e. the vessel 400 is ballasted such that the vessel 400 sinks in the water and the topside 12 is deposited or lowered onto the transport barge. This process requires the prior step of positioning the ancillary vessel (e.g. a transportation barge) underneath the topside 12. In more detail the ancillary vessel may be positioned within the slot 414 of the vessel such that it is located below the topside 12. This may be achieved using tugs or winches located on ether the ancillary vessel or the lift vessel 400.

The topside lifting units 500 discussed above may be replaced on a vessel 100, 200, 300, 400 with a support structure lifting unit. When the support structure lifting unit is installed it may cooperate with the vessel 100, 200, 300, 400 to lift the support structure of an offshore platform from the water and onto the vessel 100, 200, 300, 400 or onto an awaiting transportation barge or other ancillary vessel.

In addition to performing the methods of removing a topside 12 from offshore platforms 10 during a decommissioning process, it will be understood that the topside lifting units 500 vessels 100, 200, 300, 400 may also be used to install topsides onto support structures 14. This may be achieved by, for instance, reversing the steps of the methods discussed above.

Exemplary and optional embodiments of support structure lifting units will not be discussed with reference to Figures 11(a) and 11(b). These figures show a vessel 400 which has received a support structure lifting unit 600 in accordance with the present invention. The vessel 400 has a slot 412 within its hull 410, the slot 412 being adapted to interface with or accommodate an installed offshore platform support structure 14.

The support structure lifting unit 600 is configured to be interchangeable with the topside lifting portion 500 discussed above. Consequently, the support structure lifting unit 600 is positioned on top of (i.e. in contact with) the clear area of the upper deck 414 of the vessel 400 which is suitable to receive a plurality of cantilever beam assemblies 500a to 500h, and surrounds the slot 412 in the vessel 400.

The support structure lifting unit comprises a base formed of two base portions 610 which are located on the upper deck 414 of the vessel 400 on either side of the slot 412. A frame 620 is rotatably coupled to each base portion 610 by a bearing housing 630. Each bearing housing 630 comprises a bearing 632 which is configured to rotate and is located over the bow (front) of the vessel 400 when the support structure lifting unit 600 is installed on the vessel 400.

Figures 11(a) and 11(b) show the vessel 400 and support structure lifting unit 600 whilst the frame 620 is raised at 90° to the base. The frame 620 may also be stored in a laydown position where it is at an angle of approximately 0° relative to the base. Whilst the frame 620 is raised for lifting operations, it is stored in the laydown position during transit to protect it from damage. In preferred embodiments the frame 620 is configured to be raised to an angle of 135 degrees with the base.

The structure of the frame 620 will be discussed further below with reference to Figures 12(a) and 12(b).

On each base portion 610 there is provided a winch 642 which is coupled to a first end of a cable 644. Each cable 644 extends from the winch 642 over a cable roller 646 mounted on the frame 620. At a second end of the cable 644 there is provided a connector 648 which is configured to couple to an offshore platform support structure 14. The connector 648 may be a hook, clamp, sling or electromagnet. The winch 642 is configured to raise and lower the support structure 14 of an offshore platform and to rotate the frame 620 relative to the base portions 610. Specifically, the winch 642 is able to rotate the frame 620 such that the angle between the frame 620 and the base portions 610 is reduced.

Additionally, on each base portion 610 there is provided a hydraulic piston 612. The piston 612 is coupled to the frame 620 by rod 614. The hydraulic piston 612 may also be operated such that the frame 620 is rotated relative to the base portions 610 such that the angle between the frame 620 and the base portions 610 is increased. Additionally, the hydraulic piston 612 may be operated to damp, reduce or oppose the motions of the frame 620, e.g. during lifting operations whilst a support structure 14 is being lifted by the support structure lifting unit 600. Alternatively, the hydraulic piston 612 may be replaced by any suitable actuator such as a pneumatic piston, a linear screw actuator, a rack and pinion actuator or a motor.

The support structure lifting unit 600 further comprises a counterweight 650 which is fixedly coupled to the frame 620. Thus the frame 620 and counterweight 650 both rotate relative to the base around the bearings 632. The mass of the counterweight 650 is positioned such that it opposes (i.e. counteracts or counterbalances) the mass of the frame 620. In other words, the centre of gravity of the counterweight 650 is located on the opposite side of the bearings 632 than the centre of gravity of the frame 620. This reduces the work required to rotate the frame 620 (and any attached offshore platform support structure 14) relative to the base and base portions 610 by the hydraulic ram 612 and the winches 642.

In certain embodiments, the mass of the counterweight may bias the position of the frame 620 such that, without the action of the winch system 642, the frame 610 will remain in an upright position. In such embodiments a hydraulic piston 612 may not be required. In certain embodiments, the moment of the counterweight 650 relative to the bearing housing 630 may be varied dependent upon the platform support structure to be lifted. This may be achieved by fixing additional weights or masses to the counterweight 650 or by moving the counterweight 650 (or one or more weights or masses which comprise counterweight 650) relative to the bearing housing.

In particularly preferred embodiments, the support structure lifting unit 600 or the vessel 400 is provided with a motion compensation system (not shown) adapted to reduce the motions of at least one winch 642 during lifting operations. For instance, a heave compensation system may be employed to reduce the heave motions (i.e. the absolute vertical motions) of the winches 642. Preferably such compensation systems are able to reduce the motions of all of the winches 642 of the support structure lifting unit 600.

The vessel 400 further comprises a hull recess 460 on either side of the slot 412 at the bow (front) of the vessel 400. These hull recesses are sized to accommodate the counterweight 650 whilst the frame 620 is raised from the base portions 610. This increases the range of angles that the frame 620 may be rotated through without the counterweight 650 contacting the vessel 400.

Additionally, it will be seen that each counterweight 650 is formed as a beam which extends from the bearing housing 630. As shown, the beam of the counterweight 650 is arranged to form an obtuse angle of approximately 145° relative to the frame 620. This further increases the angle through which the frame 620 may be rotated relative to the base portions 610 without the counterweight 650 (to which the frame is rigidly coupled) contacting the vessel 400. In certain embodiments the counterweight 650 is formed as a beam at an angle of between 135° and 180° relative to the frame.

Each base portion is provided with bogies 616 which are adapted to allow the support structure lifting unit to move 600 and support the lifting unit 600 on the upper deck 414 of the vessel 400. Each bogie 616 comprises a framework having a plurality of wheels on three axles (although substantially any suitable number of wheels and/or axles may be used). Each bogie 616 is able to rotate about a substantially vertical axis such that the support structure lifting unit 600 may be turned.

Alternatively, the vessel 400 or the support structure lifting unit 600 may be provided with another means of moving the support structure lifting unit 600.

Such equipment may include wheels, tracks or rollers, hydraulic or pneumatic pistons, rack and pinion actuators, screw actuators or a winch system configured to move the support structure lifting unit. These components may be provided to the vessel 400 or the support structure lifting unit 600, or both. Alternatively, the vessel 400 may be provided with a crane configured to move the support structure lifting unit 600.

Having discussed the features of preferred embodiments of support structure lifting units 600 and a vessel 400, exemplary methods of operating the lifting unit 600 and the vessel 400 to lift the support structure 14 of an offshore platform 10 will now be discussed. These methods do not require the support structure 14 to be divided into smaller sections and are significantly quicker than the previously available methods.

Figures 12(a) to 12(g) outline the steps required to lift a support structure 14 using a vessel 400 carrying the support structure lifting unit 600 discussed above and transferring it from the vessel 400 using an ancillary barge 700.

Before the start of lifting operations, the ancillary transportation barge 700 is positioned within a slot 412 in the hull 410 of vessel 400, as seen Figure 12(a). Advantageously, this positioning ensures the correct location of the barge 700 relative to the vessel 400 and shields the barge 700 from waves and currents.

In Figure 12(b) the offshore lifting vessel 400 and the barge 700 (not shown) are then positioned adjacent to an offshore platform support structure to be decommissioned (i.e. removed) and the frame 620 is raised such that the cable rollers 646 and connectors 648 are positioned above the support portion 14 outboard of the vessel 400 by the action of the counterweight 650 and/or using the hydraulic pistons 612 and/or winches 642. Preferably, at this stage the frame 620 is located at an angle of approximately 135° relative to the base of the support portion lifting unit 600 to prevent the vessel 400 accidentally contacting the support portion 14 before or during the lifting operation. In this arrangement the counterweights 650 enter or are received within recesses 460 in the hull 410 of the vessel 400. Alarms and interlocks (not shown) are provided to control the max rotation of the frame 620 to ensure compliant operation. Accordingly the frame 620 may be prevented from rotating past a pre-determined angle.

The winches 642 are then actuated to slacken off cable 644 to lower the connectors 648. The connectors 648 are coupled to the support portion 14 and the winches 642 are operated to retract cable 644 and raise the support portion 14 from the sea bed 24. The support portion is thus suspended from frame 620 by cables 644, as shown in Figure 12(c).

Where the support portion 14 is supported by piles driven into the sea bed 24 or is sunk into the seabed 24 it may be necessary to cut the support portion 14 close to the seabed 24 before the support portion 14 may be raised. Similarly, it may be necessary to sever or decouple any connections between the support portion 14 and the seabed. Preferably this cutting step is performed after the cables 644 are connected to the support portion 14 via connectors 648 such that the support portion 14 is held in position and supported by the cables 644. These cutting or decoupling operations may be performed manually using divers or using remotely operated vehicles (ROVs) or unmanned underwater vehicles (UUVs).

Once the support portion 14 is suspended by the cables 644 near the frame 620, the frame 620 is rotated using the winches 642 and the hydraulic pistons 612 towards the base portion until the frame 620 is in upright position forming an angle of 90° relative to the base portion, as shown in Figure 12(d). This rotation causes the support portion 14 to be raised vertically and translated towards the vessel 400 such that the support portion 14 is raised out of the water completely and is supported against the frame 620. In certain embodiments, the lifting of the support portion 14 further comprises the steps of attaching buoyancy aids to the support portion 14 so as to reduce the force which must be applied by the support portion lifting unit 600.

Once the support portion 14 is raised from the water, the barge 700 exits the slot 412. Once the barge 700 is under the support portion 14 the winches 642 and cables 644 are slackened off (i.e. released or reeled out), lowering the support portion 14 onto the barge 700 (Figure 12(e)). The barge 700 continues to move away from the vessel 400 and the winches 642 and cables 644 are released or reeled out so that the support structure 14 is laid down flat on the barge 700 and then suitably sea fastened i.e. secured so that movement of the support structure is prevented (Figure 11(f)). At this point the support structure is fully supported by the barge 700 and no longer supported by the support structure lifting unit 600 and the vessel 400.

Finally, the connectors 648 are removed from the support structure 14 and the frame 620 is moved (rotated) into its laydown position against the base portions 610 as shown in Figure 12(g). The transportation barge 700 is then able to transmit the support portion to shore for disassembly or further decommissioning or to another location for reinstallation as required. The vessel 400 is then able to continue its operations. For instance, the vessel 400 may replace the support structure lifting unit 600 with a topside lifting unit 500 (either at sea or in port) or relocate to another support structure 14 which is scheduled for decommissioning.

Alternatively, once the support section 14 is lifted from the water and is supported by the vessel 400 and the support structure lifting unit 600 (as shown in Figure 12(d)) it may be transferred onto the vessel 400. Specifically, rather than lowering the support portion onto the transport barge 700 by releasing the cables 644, the winch system may be operated to rotate the frame 620 and the support structure 14 relative to the base portions 610 until the frame 620 is arranged in its laydown position against the base portions 610 (forming an angle of approximately 0° with the base portions). In this arrangement the support structure 14 will be stably supported directly above the vessel 400 and the base portions 610 of the support structure lifting unit 600. The support structure 14 may then be secured to the vessel 400 for transit before it may be offloaded.

Additionally, it will be readily appreciated that the support structure lifting units 600 and vessels 100, 200, 300, 400 may also be used to install offshore platform support structures 14 by reversing the processes discussed above, such that a support structure 14 is positioned on the sea bed.

Following the discussion of the methods of operating a support structure lifting unit 600 and a vessel 400 above further details of the support structure lifting unit will now be described with reference to Figures 13 to 15.

Details of the frame 620 and the winch system 640 of a support structure lifting unit 600 are illustrated in Figures 13(a) and 13(b). Each of these figures shows a vessel 400 with a support structure lifting unit 600 wherein the frame 620 of the lifting unit 600 has been raised to an upright position such that it forms an angle of 90° with the base portions 610 of the lifting unit 600.

In both embodiments, the frame 610 comprises two main lifting arms 622, and two main cross-members 624a, 624b located at either end of the main lifting arms. The main cross-members 624a, 624b are substantially perpendicular to the main lifting arms 622. Each of the main lifting arms 622 is rotatably coupled at a first end to a base portion 610 through bearing housings 630. A first main cross-member 624a connects these first ends of the main lifting arms 622 which are coupled to the bearing housing 630. A second main cross-member 625a is provided between the free second ends of each main lifting arm 622 which are not coupled to the bearing housing 620. Thus the first main cross-member 624a is lower than the second cross-member 624b when the frame 620 is raised or in an upright position forming an approximately 90° angle with the base members 630.

Between the main lifting arms 622 and the main cross-members 624a, 624b is provided a structural framework 626 adapted to support an offshore platform support structure 14.

In the embodiment shown in Figure 13(b) a structural framework extension 628 extends from the first main cross-member 624a which is nearest the bearing housing and between the first ends of the main lifting arms 622. The structural framework extension is substantially co-planar with the structural framework 626, such that when the frame 620 is positioned in the upright position shown in Figure 13(b) (where the frame 620 forms an angle of 90° with the base portions

610) the structural framework extension 628 projects into the slot 412 of the vessel 400.

This structural framework extension 628 provides two benefits. Firstly, the structural framework extension 628 provides additional support to the support structure 14 to be lifted by the vessel 400. Secondly, as the structural framework extension 628 extends from the lower its centre of gravity is located on the opposing side of the bearing housing 630 than the remaining portions of the frame 620. Thus, the mass of the structural framework extension 628 acts to counterbalance the mass of the frame 620. This reduces the force required to rotate the frame 620 and the tension that must be carried by the winch system.

In certain embodiments, one or more of the components of the frame 620 discussed above may be provided with protective material or padding adapted to absorb impact forces so as to prevent damage to the frame 620 from a support structure 14 which has been lifted by the support structure lifting unit 600. In preferred embodiments the protective material or padding is detachable so that it may be maintained or replaced if damaged. This protective material may include any suitable material such as absorbent or foamed materials, rubber tyres or timber sleepers.

The embodiments shown in Figures 12(a) and 12(b) have identical winch systems 640. In each case, four lifting gear apparatuses 646 are provided to the frame 620.

A lifting gear apparatus 646a is provided at the comers of the frame 620 where the first main cross-member 624a intersects with each of the main lifting arms 622 (i.e. at the comers of the side of the frame 620 furthest from the bearing housing 630). The position of these lifting gear apparatuses 646 is fixed.

The remaining two lifting gear apparatuses 646b are provided on the first main cross-member 624a between the main lifting arms 622. These lifting gear apparatuses 646b may move in direction Di along the main cross-member (i.e. parallel to the second main cross member 624b and perpendicular to the main lifting arms 622). Thus the lifting gear apparatus 646b provided between the main lifting arms 622 may be adjusted to accommodate the specific support structure 14 to be lifted and decommissioned by the vessel 400. Any suitable system or apparatus may be used to move these lifting gear apparatuses, including hydraulic, pneumatic, electric or mechanical actuators.

Two cables 644 pass through each lifting gear apparatus 646. Each cable 644 is provided with a connector 648 at its end suitable to couple to a support structure 14. Each cable 644 may be formed of metal (e.g. steel as in a “wire rope”), plastics or natural fibres. Suitable connectors include hooks, slings, clamps or electromagnets. The lifting gear apparatus 646 may comprise a tensioner, a roller, or both.

In alternative embodiments the support structure lifting system may comprise a plurality of cables 644 wherein the number and lifting capacity (i.e. rating) of each cable 644 is determined based on the support structure 14 to be lifted.

Figure 14 is a cross section which shows the connection between a frame 620 and a base portion 610 of the support structure lifting unit 600 in more detail. Additionally, Figure 14 shows bogies 616 which are adapted to enable the support structure lifting unit 600 to move and a locking system 670 which secures the lifting unit 600 to the vessel 400.

The main lifting arm 622 of a lifting arm is fixedly coupled to bearing housing 630. A bearing 631 passes through the bearing housing 630 and the base portion 610. Thus, the frame 622 is able to rotate relative to the base portion 610 (which is secured to the vessel 400) about the bearing 631.

The base portion 610 comprises five rows of bogies 616 as shown (although it will be appreciated that substantially any number of bogies may be used). The bogies are frameworks or trucks comprising a plurality of wheels mounted on axles. These bogies allow the support structure lifting portion 600 to move. Each bogie is free to rotate about a substantially vertical axis allowing the support structure lifting portion 600 to turn. In particularly preferred embodiments the support structure lifting portion 600 is self-propelled comprising a plurality of motors attached to the bogies 616, however, this is not essential.

As shown in Figure 6 the upper deck 414 of vessel 400 comprises projections 416 at its edges for personnel safety. These projections act to prevent the bogies 616 leaving the surface of the upper deck 414. In preferred embodiments the vessel or supports structure lifting unit comprises safety systems (e.g. alarms, cameras, locking brakes) to ensure that the bogies cannot steer over the edge

The locking system 670 secures the base portion 610 (and thus the rest of the support section lifting unit 600 to the upper deck 414 of the vessel 400). The system comprises a plurality of locking pins 672 which extend through a corresponding plurality of holes 673 through the base portion 610 and a corresponding plurality of holes 674 through the upper deck 414 of the vessel 400. The locking pins 672 may be inserted through the holes in the base portion 673 and the holes in the upper deck 674 manually or automatically (e.g. on instruction from a control system on-board the ship).

Figures 15(a) and 15(b) each show a simplified plan view of vessels 400 on which support structure lifting unit 600 with a frame 620 has been installed in their laydown position. The figures illustrate how the support structure lifting units 600 discussed above may be arranged on vessels 400 with and without hull recesses 460 adapted to accommodate counterweights 650. The base and winch system of the support structure lifting unit have been omitted from the drawings for clarity.

As shown in Figure 15(a) the support structure lifting unit 600 is positioned such that the bearing housings 630 are located inboard of the open end of the slot 412, being located directly above the upper deck 414 of the vessel 400. The vessel comprises a hull recess 460 on either side of slot 412, as discussed above with reference to Figures 10(a) and 10(b). This hull recess 460 accommodates the counterweight 650 when the frame 620 of the support structure lifting unit 600 is rotated and raised from the base of the lifting unit 600.

Advantageously, this inboard positioning means that the mass of the frame 620 and any offshore platform structure attached to it are transferred directly to the structure of vessel 400 and reduces the amount of mass outboard of the vessel 400.

Alternatively, Figure 15(b) shows how the bearing housings 630 are located outboard of the open end of the slot 414 and project out from the vessel 400. This change in the location of the bearing housings 630 means that the counterweight 650 will not interfere or contact the vessel 400 for over greater range of angles relative to the base of the support structure lifting unit 600. This arrangement allows the support structure lifting unit 600 to be easily retrofit onto any existing vessel 400 as there is no need to provide hull recesses 460.

Figures 16(a) to 16(d) illustrate an exemplary method of interchanging a topside lifting unit 500 on a vessel 400 for a support structure lifting unit 600 (and vice versa).

As shown, this transfer is performed in port where the lifting units 500, 600 are transferred to and from a quayside 800. However, the same steps may also be performed when transferring a lifting unit 500, 600 to or from a barge (or other ancillary vessel) whilst the vessel 400 carrying the lifting unit 500, 600 is at sea.

Figure 16(a) shows a topside lifting unit 500 comprising three cantilever beam assemblies 500a, 500b, 500c located on the vessel 400. Each cantilever beam assembly 500a, 500b, 500c is self-propelled, comprising powered bogies 522 adapted to move the cantilever beam assembly 500a, 500b, 500c.

The vessel 400 is in port adjacent to a quayside 800. The vessel has been ballasted such that the upper deck 414 is level or substantially level with the quayside 800 so that the cantilever beam assemblies 500a, 500b, 500c may be transferred. To achieve this the ballasting system (not shown) of the vessel 400 has received water into its ballast tanks such that the vessel 400 sinks in the water or has expelled water from its ballast tanks so that the vessel 400 rises in the water, as required.

A gangway 810 is positioned between the upper deck 414 of the vessel 400 and the quayside 800. Alternatively, if the gap between the vessel 400 and the quayside 800 is relatively small - such that the lifting units 500, 600 are able to traverse or cross the gap without issue - a gangway 810 may not be required.

To transfer the three cantilever beam assemblies 500a, 500b and 500c from the vessel 400 (i.e. to discharge the topside lifting unit) the cantilever beam assemblies 500a, 500b, 500c are then moved (i.e. driven) from the vessel 400 over the gangway 810 and onto the quayside 800 by their bogies 522. The vessel may be ballasted, taking water into its ballast tanks, so that the upper deck 414 of the vessel 400 remains substantially level or co-planar with the quayside 800. The resulting arrangement is shown in Figure 16(b).

Once the topside lifting unit 500 is removed it may be replaced by a support structure lifting unit 600. The transfer of a support structure lifting unit 600 onto the vessel 400 is shown in Figures 15(c) and 15(d).

The support structure lifting unit 600 is self-propelled, comprising powered bogies 616 configured to move or drive the support structure lifting unit 600. To transfer the support structure lifting unit 600 onto the vessel 400 the bogies 616 are operated to move the support structure lifting unit 600 over the gangway 810 and onto the upper deck of vessel 400. The vessel 400 may be deballasted (i.e. expel water from its ballast tanks) as it receives the mass of the support structure lifting unit 600 such that the upper deck 414 of the vessel 400 remains substantially level and co-planar with the quayside.

To interchange or replace a support structure lifting unit 600 on a vessel with a topside lifting unit 500 the steps above may be reversed. Firstly, the support structure lifting unit 600 may be driven from the vessel 400 onto the quayside 800 or an ancillary vessel. Subsequently, the topside lifting unit can be moved or driven onto the vessel 400 from the quayside 800 or from an ancillary vessel.

Advantageously, the steps discussed above required to interchange or transfer the lifting units 500, 600 are very simple and do not require any external systems or equipment such as cranes or towing machinery. Consequently, these methods may be performed at a wide variety of ports and whilst at sea.

Similar steps may be implemented to offload an offshore platform topside 12 or a support structure 14 which has been lifted by the vessel 400.

A topside 12 which has been lifted by a topside lifting unit 500 may be supported by a plurality of cantilever beam assemblies 500a to 500h, as discussed above with reference to Figure 6. The cantilever beam assemblies 500a to 500h may comprise a propulsion system configured to move the assemblies such as bogies 522 which are powered by motors. Thus the topside 12 may be transferred from the vessel 400 by the cantilever beam assemblies 500a to 500h. Specifically, the bogies 522 may be driven to move the cantilever beam assemblies 500a to 500h - and a topside 12 supported on these assemblies 500a to 500h - onto a quayside 800 or onto an ancillary vessel (e.g. a transportation barge or heavy lift ship).

Subsequently, the topside 12 may be deposited onto the quayside 800 or onto the ancillary vessel by the topside lifting unit 500. For instance, this may be performed using either cranes or a conventional jacking system adapted to lift the topside 12 from the cantilever beam assemblies 500a to 500h and further configured to subsequently lower the topside 12 onto supports or blocks located on the quayside 800 or ancillary vessel once the cantilever beam assemblies 500a to 500h have been removed. Alternatively, the topside 12 may be lowered onto the supports or blocks by the topside lifting unit 500 itself, for instance by operating the bogies 522 to lower the topside 12. Consequently, the bogies 522 must be adapted to move the cantilever beam assemblies 500a to 500h in a lateral direction and raise and lower the cantilever beam assemblies 500a to 500h relative to the bogies 522.

Once the topside 12 is supported by the quayside 800 or by the ancillary vessel, the topside lifting unit 500 may be returned to the vessel 400 using its propulsion system or interchanged with a support structure lifting unit 500.

An offshore platform support structure 14 which has been lifted and is supported by the support structure lifting unit 600 comprising a propulsion system can also be transferred from the vessel by the support structure lifting unit 600. The propulsion system which may include bogies, wheels, tracks or rollers may transfer the support structure lifting unit 600 and the support structure 14 to a quayside 800 (for an ancillary transportation vessel transfer see above). The support structure 14 may then be deposited onto the quayside 800 by the use of heavy lift cranes lifting the support structure 14 clear of the lifting unit 600 whilst the support structure lifting unit 600 is removed. The support structure lifting unit 600 may then be returned to the vessel using its propulsion system or interchanged with a topside lifting unit 500.

Again these steps to offload a topside 12 or a support structure 14 are very simple and do not require any complex external systems or equipment other than cranes for the support structure lift. Thus the system discussed above is able to offload topsides 12 and support structures 14 from the vessel 400 at a wide variety of ports and whilst the vessel 400 is at sea.

Alternatively, a topside 12 or a support structure 14 lifted by the vessel may be transferred directly from the vessel 400 using cranes or skids (i.e. by moving the topside across a series of rollers mounted on the ancillary vessel or on the quayside).

In summary, the present disclosure offers improved offshore vessels, systems and methods for the decommissioning and installation of offshore platforms. Specifically, providing a vessel adapted to interchangeably receive a topside lifting unit and a support structure lifting unit enables the size of the vessel to be reduced (resulting in corresponding reductions in the resources required to operate the vessel), increases in the utilisation of the vessel and reduces the amount of time required to remove or install an offshore platform. Consequently, the vessels, systems and methods discussed above are significantly quicker, more efficient and more economic than the existing alternatives available to the operators of offshore platforms.

Claims

1. A system for decommissioning an offshore platform, the offshore platform comprising a topside and a support structure, wherein the system comprises:

a topside lifting unit adapted to lift the topside from the support structure;

a support structure lifting unit adapted to lift the support structure from the water such that the support structure is supported by the vessel; and a vessel configured to interchangeably receive the topside lifting unit and the support structure lifting unit wherein the vessel comprises a hull which is adapted to accommodate an installed support structure and a ballasting system for lifting the topside.

2. The system according to claim 1, wherein the topside lifting unit comprises a propulsion system configured to move the topside lifting unit.

3. The system according to claim 2, wherein the propulsion system is configured to move the topside lifting unit and a topside supported by the topside lifting unit.

4. The system according any preceding claim, wherein the support structure lifting unit comprises a propulsion system configured to move the support structure lifting unit.

5. The system according to claim 4, wherein the propulsion system is configured to move the support structure lifting unit and a support structure supported by the support structure lifting unit.

6. The system according to any preceding claim, wherein the hull comprises a slot arranged to accommodate an installed support structure.

7. The system according to claim 6, wherein the slot is at least 30m wide, preferably at least 45m wide and more preferably still at least 60m wide.

8. The system according to either claim 6 or claim 7, wherein the slot is at least 15m long, and more preferably at least 60m long, and more preferably still at least 100m long.

9. The system according any preceding claim, wherein the vessel comprises a duct extending through the vessel from the base of the slot and a turbine located within the duct, wherein the turbine is adapted to generate energy as water passes through the duct.

10. The system according to any preceding claim, wherein the topside lifting unit comprises a cantilever beam assembly and wherein the cantilever beam assembly comprises a cantilever beam which is adapted to project outboard from the vessel and support the topside when the topside is lifted by the system.

11. The system according to claim 10, wherein the cantilever beam assembly comprises a propulsion system configured to move the cantilever beam assembly.

12. The system according to claim 10, wherein the propulsion system is configured to move the cantilever beam assembly and a topside which is at least partially supported by the cantilever beam assembly.

13. The system of claims 11 or 12, wherein the cantilever beam assembly comprises a plurality of bogies configured to move the cantilever beam assembly, wherein preferably the plurality of bogies is further configured to raise and lower the cantilever beam assembly.

14. The system according to any of claims 10 to 13, wherein the vessel comprises a crane configured to move the cantilever beam assembly.

15. The system according to any of claims 10 to 14, wherein the cantilever beam assembly comprises an assembly base and a means of moving the cantilever beam relative to the assembly base.

16. The system according to claim 15, wherein the cantilever beam assembly comprises a means of translating the cantilever beam relative to the propulsion system such that the cantilever beam is extended outboard from the vessel or retracted inboard toward the vessel.

17. The system according to either claim 15 or claim 16, wherein the cantilever beam assembly comprises a means of rotating the cantilever beam relative to the assembly base such that the cantilever beam is extended outboard from the vessel or retracted inboard towards the vessel.

18. The system according to any of claims 10 to 17, wherein the cantilever beam assembly comprises a lifting pad positioned in contact with or above the cantilever beam, wherein the lifting pad is adapted to contact the topside when the topside is lifted by the system.

19. The system according to claims 18, further comprising a motion compensation system configured to minimise motion of the lifting pad when the topside is lifted by the system.

20. The system according to claim 19, wherein the motion compensation system is positioned between the lifting pad and the cantilever beam.

21. The system according to claim 19, wherein the motion compensation system is positioned between the vessel and the cantilever beam.

22. The system according to any of claims 19 to 21, wherein the motion compensation system is a heave compensation system configured to minimise the vertical motions of the lifting pad when the topside is lifted by the system.

23. The system according to any of claims 18 to 22, further comprising a means of moving the lifting pad, such that the lifting pad may be moved relative to the cantilever beam.

24. The system according to any of claims 10 to 23 when dependent on claim 2, wherein the topside lifting unit comprises two cantilever beam assemblies, each of said cantilever beam assemblies comprising a cantilever beam, wherein the cantilever beams are configured to detachably couple so as to form a continuous beam across the slot.

25. The system according to any preceding claim, further comprising a locking mechanism adapted to detachably secure the topside lifting unit to the vessel.

26. The system according to claim 25, wherein the locking mechanism and the ballasting system are interlocked such that the ballasting system is prevented from being operated to lift the topside if the locking mechanism is engaged.

27. The system according to either of claims 25 or 26 when dependent on any of claims 10 to 22, wherein the locking mechanism is adapted to secure each cantilever beam assembly to the vessel, such that the motions of the cantilever beam relative to the vessel are prevented.

28. The system according to claims 25 to 27, when dependent on any of claims 10 to 22, wherein the locking assembly is adapted to secure the cantilever beam to the vessel, such that the motions of the cantilever beam relative to the vessel are prevented.

29. The system according to any of claims 1 to 28, wherein the support structure lifting unit comprises:

a base adapted to be received on the vessel;

a bearing housing positioned on the base;

a frame rotatably coupled to the base member by the bearing housing and configured to support the support structure when the support structure is lifted by the system; and, wherein the support structure lifting unit is adapted to rotate the frame and a support structure about the bearing housing such that the support structure is transferred on to and off of the vessel.

30. The system according to claim 28, wherein the frame is adapted to rotate between a position where it is parallel to the base member and to a position where it forms an angle of at least 90 degrees with the base member, more preferably at least 135 degrees.

31. The system according to either of claims 29 or 30, wherein the winch system comprises:

a winch;

a cable configured to detachably couple to the winch and the support structure;

32. The system according to claim 31, wherein either the winch may be translated laterally relative to the frame or the lifting gear apparatus may be translated laterally relative to the frame, or both.

33. The system according to either of claims 31 or 32, wherein the lifting gear apparatus comprises either a tensioner or a roller, or both.

34. The system according to any of claims 31 to 33, further comprising a motion compensation system configured to minimise motion of the winch when the support structure is lifted by the system.

35. The system according to claim 34, wherein the motion compensation system is positioned between the winch and the support structure lifting unit.

36. The system according to claim 35, wherein the motion compensation system is positioned between the vessel and the support structure lifting unit.

37. The system according to any of claims 34 to 36, wherein the motion compensation system is a heave compensation system configured to minimise the vertical motions of the winch when the support structure is lifted by the system.

38. The system according to any of claims 29 to 37 wherein the support structure lifting unit further comprises a counterweight rigidly coupled to the frame such that the counterweight is positioned on an opposing side of the bearing housing to the cable roller.

39. The system according to claim 38, wherein the vessel comprises a hull recess to accommodate the counterweight when the frame is lifted.

40. The system according to claims 29 to 39, comprising a locking mechanism adapted to detachably secure the support structure lifting unit to the vessel.

41. The system according to claim 40, wherein the locking mechanism and the support structure lifting unit are interlocked such that the supports structure lifting unit is prevented from being operated to lift the support if the locking mechanism is engaged.

42. A vessel for decommissioning an offshore platform, the offshore platform comprising a topside and a support structure, wherein the vessel is configured to interchangeably receive a topside lifting unit adapted to lift the topside from the support structure and a support structure lifting unit adapted to lift the support structure from the water such that the support structure is supported by the vessel, and wherein the vessel comprises a first hull portion adapted to accommodate an installed support structure and a ballasting system for lifting the topside.

43. A vessel according to claim 42, adapted for use in the system of any of claims 1 to 42.

44. A method of decommissioning an offshore platform comprising a topside and a support structure, the method comprising the steps of:

transferring the topside lifting unit from the vessel;

receiving the support structure lifting unit onto the vessel;

lifting the support structure using the support structure lifting unit; and transferring the support structure from the vessel.

45. The method of claim 44, wherein lifting the topside from the support structure using the topside lifting unit comprises the further steps of:

46. The method of either claim 44 or claim 45, wherein lifting the support structure using the support structure lifting unit comprises the further steps of:

47. The method of any of claims 44 to 46, wherein the topside or the topside lifting unit, or both are transferred from the vessel to an ancillary vessel whilst the vessel is at sea.

48. The method of any of claims 44 to 47, wherein the topside or the topside lifting unit, or both are transferred from the vessel to the quayside or to an ancillary vessel whilst the vessel is in port

49. The method of any of claims claim 44 to 48, wherein the steps of transferring the topside from the vessel and transferring the topside lifting unit from the vessel are performed simultaneously using a propulsion system provided to the topside lifting unit, the propulsion system configured to move the topside lifting unit.

50. The method of any of claims 44 to 45, wherein the support structure, or the support structure lifting unit, or both are transferred from the vessel to an ancillary vessel whilst the vessel is at sea.

51. The method of any of claims 44 to 46, wherein the support structure or the support structure lifting unit, or both are transferred from the vessel to the quayside or to an ancillary vessel whilst the vessel is in port.

52. The method of any of claims 44 to 47, wherein the steps of transferring the support structure from the vessel and transferring the support structure lifting unit from the vessel are performed simultaneously using a propulsion system provided to the support structure lifting unit, the propulsion system configured to move the support structure lifting unit.