WO2020161340A1

WO2020161340A1 - Structure and method for decommissioning an offshore platform

Info

Publication number: WO2020161340A1
Application number: PCT/EP2020/053210
Authority: WO
Inventors: Jean-Christophe Le Gal
Original assignee: Petrodec B.V.
Priority date: 2019-02-08
Filing date: 2020-02-07
Publication date: 2020-08-13
Also published as: EP3693513B1; DK3693513T3; EP3693513A1

Abstract

Structure and method for decommissioning offshore platforms presenting topsides (12) mounted on a jacket having a plurality of jacket legs. The offshore decommissioning structure comprises a jack-up-vessel (1) comprising a hull, a main deck (5), an elevating jacking device and a plurality of jack-up legs, wherein said jack-up vessel (1) is elevated on its legs above the sea level by the elevating jacking device. Furthermore, the structure comprises a carrying and skidding system for shifting the topsides (12)from the platform jacket onto the main deck (5) of the jack-up vessel (1).Besides, the method comprises the following steps: position the jack-up vessel (1) close to the platform (12); position skid beams (18) on jack-up main deck (5) relative to the platform legs; push jacket skid beams toward the jacket legs;cut the platform jacket legs below the topside (12); and shift the topsides (12) on the skid beams (18) to the jack-up main deck (5).

Description

STRUCTURE AND METHOD FOR DECOMMISSIONING AN

OFFSHORE PLATFORM

The current invention relates to a structure and a method for decommissioning an offshore platform.

The invention can be implemented in the oil and gas extraction activities, in particular for disassembling end-of-life platforms located in offshore oil and gas fields such as in the North Sea. However, it can also be used for dismantling any offshore structure.

Decommissioning an oil or gas extraction platform is a complex and costly activity due to the size of the platform and its offshore location. The dismantling of platform comprises different stages. The first one consists in the well plugging and abandonment. Then the platform topsides are removed after having been separated from the platform jacket. Eventually, the jacket is removed. Currently, the topsides removal usually requires the mobilization of a heavy lift vessel and transportation barges. However, this operation requires favorable weather conditions and in oil fields such as those in the North Sea favorable weather windows are limited. Therefore, in order to save time, the topsides are removed in one piece which requires the involvement of heavy lift vessel with sufficient lifting capacities. There are not so many vessels with sufficiently large crane for removing in one piece the topsides of an oil platform and they are very expensive.

WO 2012/144952 A1 discloses a multi-functional jack up system for decommissioning an offshore platform wherein modules of the topsides of the offshore platform to be disassembled are lifted away by a crane. The crane is mounted on a skidding base cooperating with two parallel crane skidding beams located on the top deck of a cantilever platform for enabling the crane to travel back and forth on the skidding base top deck. Additionally, the cantilever platform supporting the skidding base and the crane can be also shifted back and forth on the jack up main deck to bring the crane close to the offshore platform to be disassembled. The object of the present invention is to propose a solution to the problems of decommissioning offshore platforms which is safer, less sensitive to weather conditions and cheaper than the currently used methodology and equipment.

To this aim, the invention relates to an offshore decommissioning structure for disassembling an offshore platform presenting topsides mounted on a jacket having a plurality of jacket legs, wherein said decommissioning structure comprises on the one hand a jack-up-vessel which comprises a hull, a main deck, an elevating jacking device and a plurality of jack-up legs, wherein said jack-up vessel can be elevated on its legs above the sea level by the elevating jacking device; and, on the other hand a carrying and skidding system for shifting the topsides of the platform to be disassembled from the platform jacket onto the main deck of the jack-up vessel. The carrying and skidding system comprises a skid beams set for bridging the platform jacket and the jack up vessel.

The invention concept lies in the utilization of a jack-up which is converted from its usual drilling activities to platform decommissioning. A jack-up vessel can be lifted on its legs sufficiently high above the sea level for enabling most of the preparation works to be performed in unrestricted weather conditions. While said preparation works can be done with the jack-up main deck at the same level as the platform main deck, namely at survival level, only the installation of the skidding system and the shifting of the platform topsides requires favorable weather conditions even though the resulting restriction are not so stringent as when floating vessel are used. This means that a less expensive equipment is mobilized over a shorter period of time as less interruption caused by weather deterioration will occur.

Preferably, the skid beams set comprises: a plurality of jack-up skid beams which are fastened on the jack-up main deck; and a plurality of jacket skid beams which can be transferred from a first position where they rest on the jack up main deck to a second position outside of the jack-up main deck where they can be attached to the jacket legs of the platform. This arrangement generates a skidding plan over which the topsides can be skidded for being transferred from the jacket to the jack-up main deck. Additionally, the skid beams set may comprise a plurality of link beams, each of said link beams being attached by a hinged end to an aft end of a jack-up skid beam, wherein said link beams can be lowered for connecting said jack-up skid beam to a corresponding jacket skid beam, when it is in the second position, and raised for disconnecting them. The hinged link beams make it possible to connect or disconnect as appropriate the jack-up vessel to the platform, enabling disconnection of the two structures in case of weather deterioration.

Preferably, the skid beams set comprises at least two parallel skid beams subset each comprising a pair of jack-up skid beams aligned with a corresponding pair of jacket skid beams, each one being adapted to be positioned on opposite sides of at least two jacket legs when they are in said second position. This skid beams arrangement allows a balanced load transfer on the platform legs.

Advantageously, the offshore decommissioning structure comprises further skid beam support assemblies which can be attached on the jacket legs for supporting the jacket skid beams when in said second position.

Preferably, each skid beam support assembly comprises: a support ring which can be welded on a jacket leg, and a skid beam support clamp which can be mounted on the jacket leg above the support ring and which presents lateral skid beam brackets for supporting the jacket skid beams on both side of the jacket leg. This arrangement optimizes the load transfer from the jacket skid beams to the jacket legs via the welded support rings.

Additionally, the offshore decommissioning structure comprises skid assemblies for shifting the platform topsides from the jacket to the jack-up main deck, said skid assemblies being able to move on the skid beams top surface when the jacket skid beams are in said second position. The skid assemblies comprise skid shoes positioned on the top surface of the skid beams; vertical jacks mounted on the skid shoes for lifting the platform topsides; and horizontal jack assemblies for shifting the platform topsides to and off the jack-up main deck.

Preferably, the horizontal jack assemblies are push-pull units which can move back and forth the platform topsides on skid tracks formed on the top surface of the skid beams, and the jacket skid beams on the jack-up skid beams for moving between said first and second positions. Push-pull units can be found easily by different vendors and are completely autonomous.

Advantageously, the vertical jacks and the horizontal jack assemblies are hydraulic jacks and are actuated by a hydraulic pump controlled by a control unit.

Additionally, the offshore decommissioning structure comprises further: at least a supporting crane for lifting loads between the jack-up vessel and the platform topsides, and/or at least a mobile working platform for reaching working areas in particular below the platform topsides. This equipment enhances the safety and facilitates the preparation works on the platform.

Advantageously, the jack-up vessel further comprises: at least a gangway/bridge for enabling access from the jack-up vessel to the platform; and/or walkways attached on at least one side of each jacket skid beam for reaching all areas along the jacket skid beams.

According to a second aspect of the invention, it is proposed a method for disassembling an offshore platform using the offshore decommissioning structure as defined above and comprising the following steps: position the jack up vessel close to the platform with the stern oriented toward the platform, lift the jack-up vessel at working height; position skid beams on jack-up main deck relative to the platform legs; push the jacket skid beams toward the platform jacket legs in said second position; cut the platform jacket legs below the bottom part of the platform topsides; and shift platform topsides on the skid beams to the jack-up main deck.

Additionally, a preparation stage may be performed on the platform before the step of cutting the platform jacket legs, the preparation stage comprising the following steps: perform the plug and abandonment campaign if not already done before; remove a communication tower of the platform if not already done during the plug and abandonment campaign; remove a vent boom of the platform; remove at least a part of a helideck; remove all elements of the platform bottom part which clash with the skid beams; and cut all secondary parts between platform jacket and topsides for providing a passage for the jacket skid beams and the topsides shifting.

Preferably, skid beams support assemblies are installed before the step of pushing jacket skid beams toward the jacket platform legs, the installation of the skid beam support assemblies comprising the following steps: weld the support rings on the platform jacket legs; bolt together half tubular parts of the skid beam support clamps around the platform jacket legs above the support rings; and put grout between the skid beam support clamps and the platform jacket legs for compensating the diameters difference if necessary.

Advantageously, the following steps are performed before the step of pushing the jacket skid beams toward the platform: lower the jack-up vessel to a level below a platform shifting elevation; verify the skid beams position relative to the platform legs and reposition as appropriate; and install support and stiffening structures on the lower of the platform topsides as required for lifting the topsides.

Preferably, the following steps are performed before the step of cutting the platform jacket legs: position the link beams to connect the jack-up skid beams to their respective jacket skid beams; move the skid assemblies under the platform topsides supporting points; connect to a pump and test hydraulic jacks for lifting and shifting the topsides; and extend the vertical jacks for preloading the topsides fastening to the jacket legs.

Additionally, the following steps are performed before the step of pushing the platform topsides to the jack-up vessel: lift the platform topsides with the vertical jacks for checking the weight of the topside with an integrated weighting system; if topsides weight is within an acceptable threshold, then continue to lift the topsides until the defined clearance with the jacket is reached; if topsides weight is above the acceptable threshold check, then the platform topsides are lowered back and secured.

Preferably, the following steps are performed when the platform topsides have reached their final position on the jack-up main deck: lower topsides on transport support points on the jack-up main deck; open skid beam connections at link beams; fasten topsides on jack-up vessel main deck; pull back the jacket skid beams on the jack-up main deck; lower the jack-up vessel on the sea; and tow the jack-up vessel with the topsides on jack-up main deck to the offloading location.

Additionally, the following steps may be performed before critical stages of the method: check weather forecast; if weather is deteriorating disconnect the jack-up vessel from the platform and lift up the jack-up vessel to safe elevation, or if weather forecast is favorable, then perform the planned critical stage.

Further features and advantages of the invention will be revealed in the below description of non-limiting examples of achieving the different aspects of the invention. The description will be supported by figures also provided as non-limiting examples.

- Figure 1 a and 1 b represent a top and side view of a jack-up vessel of an offshore decommissionary structure according to one embodiment of the invention;

- Figure 2a and 2b represent a top and a side view of a platform PKA;

- Figures 3a represents a top view of the jack-up vessel positioned with respect to the platform;

- Figures 3b represents a top view of the jack-up vessel with the platform topsides shifted on the jack-up main deck;

- Figure 4 illustrates a skid beam support assembly mounted on a platform jacket leg;

- Figure 5a illustrates a pair of link beams with the connection in open situation;

- Figure 5b illustrates said pair of link beams of figure 5a in connected position;

- Figure 6 represents a side view of jacket skid beams with skid assemblies;

- Figure 7 represents a side view of a skid assembly;

- Figures 8a to 8c represent different stages of the jacket skid beams positioning; and

- Figure 9 represents in perspective the skid beams arrangement in skidding position. In the following, the description and figures are based on the removal of the platforms Pickerill A (PKA) or Pickerill B (PKB) topsides. All the indicated values are based on these two examples and are not limitative for the definition of the invention and it must be understood that they may vary according to the kind of offshore structure to be dismantled by the structure of the invention. The weight of the PKA topsides is estimated at 1.536 tones and the weight of PKB topsides is estimated at 1.398 tones. However, in order to provide sufficient safety margin and to allow the reutilization of the invention equipment the decommissioning structure should be able to remove topsides weighting 1.800 tones. This allows the removal of the topsides for this kind of platform in one piece and one operation.

As shown in figures 1 a and 1 b, the invention concept is based on the utilization of a converted drilling jack-up (JU). The example chosen for describing the invention is the JU Energy Endeavour which is a drilling platform comprising a JU vessel 1. The jack-up vessel 1 has a hull 2 which can float on the sea so that the JU vessel 1 can be towed to the drilling area on an oil or gas filed located. It further comprises three legs 3 which can be jacked down to the sea bed for lifting the hull 2 above sea level 8. Similarly, the legs 3 can be jacked up for lowering the hull 2 as appropriate. In general, JU Energy Endeavour operates on the North Sea. Once the jack-up vessel 1 has reached the drilling area, the three legs 3 are lowered to the sea bed 4 (see figure 2b) by an elevating jacking device for lifting the JU vessel 1 above the sea level 8 at the desired working level or at survival level in case of storm when the work to be performed is below the height of the waves plus a safety margin. The JU vessel 1 comprises further spuds at the lower end of the legs 3 and ballast tanks which are filled with a determined amount of water in order to preload the JU vessel 1 on the sea bed 4 and stabilize the jack-up vessel on the sea bed or in floating condition.

This JU vessel 1 has been converted for offshore structure removal, in particular, but not exclusively, for topsides removal operations in the North Sea. However, it must be understood that other type of JU vessel can be used and that the decommissioning operations according to the invention can be performed in offshore oil or gas fields in all seas and oceans. The intended main tasks of the converted JU vessel 1 are the plugging and abandonment, the preparation works on platforms and in particular the removal and transportation of the topsides 12. For this purpose, the drilling equipment such as drilling tower and support cantilevers are removed and a grid of load distribution beams 35 are installed on the JU main deck 5 for receiving the platform topsides 12 once removed from the platform jacket 10. Furthermore, the JU vessel 1 is equipped with a new crane 6 located at the JU stern 7 for assistance works. The JU vessel

I is for example 68 m long and 70 m wide. It weights 11.603 tones.

The supporting crane 6 at the stern 7 of the JU 1 is mainly used for preparing the topsides before their removal. In the following the crane 6 used as example for describing the invention is a 300mt PMC-6200-300 from Fluisman. The crane 6 can lift a maximum load of 300 tones with a 12,6 m outreach. The maximum working radius for the hoist is 48 m with a maximum load of 50 tones. The wind speed that the jack-up vessel can stand is an important design parameter, for example a 20 m/s wind speed during operation at JU main deck level, 36,7 m/s in survival conditions and 30 m/s during transport.

Besides the stern 7 of the JU vessel 1 , a platform 9 to be decommissioned, for example PKA, is shown in figure 3a. The PKA platform operates in the Southern North Sea. As shown in figures 2a and 2b, the platform 9 comprises a jacket 10 with four legs 11 anchored in the sea bed 4. The jacket

10 supports the topsides 12 which are fastened at the top end of the jacket legs

I I at sufficient elevation above the sea level 8. The upper part of the topsides 12 consists in a main deck 13 located at 30,5 m above the sea bed and the lower part consists mainly in a cellar deck 14. Above the main deck 13 of the topsides 12, there is a helideck 15 where a helicopter can land, a communication tower 16 and a vent boom 17. The water depth at PKA location is between 20,3 m (lowest astronomic tide) and 25,54 m (highest astronomic tide) with storm surge it can go from 22,13 m (LAT) to 27,37 m (HAT).

As shown in figure 3a, the jack-up vessel 1 is positioned beside the platform 9 with the stern 7 pointing to the platform. The jack-up vessel 1 is used for preparing the platform topsides 12 for its removal from the jacket 10 and the topsides towing to its offloading place on the shore or on a barge where it will be dismantled. The distance between the jack-up vessel hull 2 and the platform topsides 12 should be comprised for example between 0,75 m and 2,25 m, e.g. 2,15 m. The removal sequence is mainly the same for removing any kind of platforms besides PKA such as Pickerill B (PKB) for example. The main difference between the removal of different platforms lays mostly in the preparation works which need to be adapted to the features of the platform to be decommissioned.

As shown in figures 8a a set of skid beams 18 and 19 are installed on to the JU main deck 5. The skid beams set consists of two parallelly positioned subsets of skid beams 18 and 19. Each subset comprises two parallel adjusted skid beams 18 and 19. The skid beams are split into JU skid beams 18, which are located on the JU main deck 5 via the load distribution beams, and jacket skid beams 19 which are moved from a first position on the JU skid beams 18 on the JU main deck 5, as shown in figures 3a and 8a, to a second position outside of the JU main deck 5 onto the existing jacket legs 11 , as shown in figures 8c and 9. When the jacket skid beams 19 are in the second position, the topsides 12 supported by the skid beams 18 and 19 can be skid from the jacket 10, as shown in figure 3a, to the JU main deck 5, as shown in figure 3b. As shown in figures 5a and 5b, each JU skid beam 18 is connected to its respective jacket skid beam 19 by a link beam 21 which is connected by a hinged end 22 with pin connection to the JU skid beam 18 at the JU aft edge 23.

At the beginning of the removing sequence, the jacket skid beams 19 are in the first position on the top of the JU skid beams 18 on the JU main deck 5, as shown in figure 8a. Then the jacket skid beams 19 are pushed toward their second position on the jacket 10 by skidding on the JU skid beams 18 as shown in figure 8b.

As shown in figure 8c, when they reach their second position, the jacket skid beams 19 are supported and connected to the jacket legs 11 by support assemblies 20. The support assemblies 20 are connected to each one of the four jacket legs 11 below the topsides cellar deck 14. As shown in figure 4, each of the support assemblies 20 comprise a support ring 24 welded on the jacket leg 11 and a support clamp 25 consisting of two half tubular parts 26 bolted together around the leg 11. Grout can be put between the support clamp 25 and the jacket leg 11 if the jacket legs outer diameter is smaller than the inner diameter of the support clamp 25. The half tubular parts 26 present sidewise support brackets 27 on which the jacket skid beams 19 of a sub set of skid beams will rest on each side of the jacket leg 11.

The JU skid beams 18, the link beams 21 and the jacket skid beams are welded box profile. For example, the dimensions of the jacket skid beams in mm can be: height 1500, width 500 x vertical walls thickness 40 and horizontal walls thickness 70, and the dimensions of the jack-up skid beams and link beams profile can be: height 900, width 500, vertical walls thickness 40 and horizontal walls thickness 70. The skid beams 18 and 19 are planned to be fabricated as welded boxed girder made from material with yield strength of 355 MPa. Local parts of the skid beams are designed with yield strength of 460 MPa.

For supporting the topsides 12, the jacket skid beams 19 are planned to be arranged sidewise of the legs 11 , as shown in figure 9, with an eccentricity of 1000 mm to the leg axis. Therefore, as shown in figure 4, it is planned to weld the support ring 24 onto each jacket leg 11 and the support clamp 25 with sidewise support brackets 27 will be mounted onto the legs. The bracket clamp 27 transfers the vertical loads via the support ring 24 underneath into the legs 11.

As shown in figure 5b, each link beam 21 connects together a JU skid beam 18 with its respective jacket skid beam 19 in order to form a continuous skidding track extending from the extremity of the platform jacket 10 to the end of the topsides storage area on the JU main deck 5, as shown in figure 9. As it can be seen in figure 5b, the link beam 21 is lowered to a connected position for connecting the JU skid beam 18 to its respective jacket skid beam 19, and it can be raised to a disconnected position shown in figure 5a where the skid beams 18 and 19 are disconnected. The jacket skid beams 19 and the JU skid beams 18 are only connected temporarily by the link beams 21 , mainly when the topsides 12 are skidded from the platform jacket 10 to the topsides storage area on the JU main deck 5.

After topsides preparation is completed, the four legs 11 of the platform 9 are cut below the cellar deck 14, and the topsides 12 is lifted by means of sixteen skid beams assemblies 28 positioned on the jacket skid beams 19 (see figure 7). Further on, the topsides 12 are moved on the skidding track formed by the top surface of the jacket skid beams 19, over the link beam 21 and then on the JU skid beams 18 toward the JU main deck 5. When the topsides 12 have reached their final position, they are lowered on dedicated transport supports on the storage area on the JU main deck 5 and are fastened for being towed. Eventually, the JU jacking system lower the JU vessel 1 at sea level with the topsides 12 fastened on the JU main deck 5 and it can be, for example, towed to the shore at the offloading location where the topsides will be dismantled.

This skidding sequence is performed by skid assemblies 28 shown in figures 6 and 7. The skid assemblies 28 are interposed between the bottom parts of the topsides 12 and the top surface of the jacket skid beams 19. The skid beam assemblies 28 ensure the lifting and lowering of the topsides 12 as well as the pushing and pulling movement of the topsides 12 on the skid beams 18 and 19. The skid assemblies 28 are suitable to move the topsides 12 from the jacket 10 onto the jack-up main deck 5, and then to offload the topsides 12 to a quay side or a barge. The total topsides weight to be skidded is 1.600 tones including the weight contingency. The skid assemblies 28 are able to deal with at least 1.800 tones topsides weight in order to provide a 200 tones safety margin. The skid assemblies 28 comprise skid shoes 29 which glide on the skidding tracks constituted by the skid beams top surface. A sliding interface may be provided on the skid beams top surface for lowering the friction during the skidding stages. During the topsides skidding, the loads in the skid shoes should be actively controlled by vertical jacks 30 with a lifting stroke of at least 150 mm. These jacks 30 can be hydraulic or of other type, e.g. electrical jack. The vertical jacks lift the topsides above the platform jacket 12 in order to provide sufficient clearance for skidding the topsides 12 away from the jacket legs 11 and lower the topsides on the JU main deck 5 once the topsides skidding is completed. Push-pull units 31 comprising horizontal hydraulic jacks control the topsides movements on the skid tracks provided on the top of the skid beams 18 and 19. The horizontal jack of the push-pull units 31 are hydraulic in this example but other type of jacks can be used such as electrical jacks. A control system is required to operate and control the vertical hydraulic jacks 30 and the horizontal hydraulic jacks of the push-pull units 31 remotely from a central control console.

The topsides skidding distance from the platform jacket 12 to the final position on the storage area of the JU main deck 5 is about 30 to 40 m. The topsides traveling speed of the skid assemblies is at least 20m/h. This results in a required time window of less than 2 hours for the topsides skidding operation itself.

The skid beams arrangement creates two skid tracks on each side of the jacket leg 11 to avoid eccentric loading of the legs 11 during the topsides skidding. A total of sixteen skid assemblies 28 are arranged underneath the topsides 12. The vertical jacks 30 are grouped in order to apply the same hydraulic pressure to the jacks within the same group. The vertical jacks grouping is defined for optimizing the required jacking forces considering the center of gravity location.

The skid assemblies 28 should comply with different technical requirements for obvious safety reasons. For example, the skidding should be possible in the two directions on the one hand for pushing the jacket skid beams 19 to the platform jacket legs 11 and to pull them back on the jack-up main deck once the topsides skidding is completed, and for skidding the topsides 12 from the jacket 10 to the JU main deck 5 and for offloading the topsides 12 from the jack-up main desk 5. In particular, the push-pull units 31 must generate a sufficient pulling and pushing force. The vertical hydraulic jacks 30 must provide sufficient lifting force for lifting the topsides 12 and weighting of the topsides should be possible during lifting. Given that weather can deteriorate very quickly, the skid assemblies 28 should make it possible to stop and resume the topsides skidding at every stage of the skidding sequence and at any position. A robust and adaptive design of the skid assemblies 28 is of high importance as the structure of the invention is supposed to be reusable for decommissioning other platforms than PKA and PKB. Robustness is particularly important because the structure will operate offshore. For this reason, the skid assemblies 28 must be easy to handle in an offshore context and provide sufficient system redundancy in case of failure of components (such as pumps, valves, hoses, hydraulic cylinders, control unit). Moreover, the equipment must be exchangeable in offshore conditions.

The maximum jacking force per skid shoe is 138 tones considering a topsides weight of 1 ,800 tones. Vertical hydraulic jacks with at least 150 tones capacity and 150 mm stroke are considered for jacking the topsides 12 after that the jacket legs 11 are cut. During skidding, the topsides 12 is supported by activated vertical jacks 30 which are pressure/force controlled. Thereby, the load in the vertical jacks 30 is monitored and controlled during skidding. This avoids overloading of skid assemblies 28, jack-up vessel 1 and topsides 12. During the topsides skidding stage, deflections and installation tolerances are compensated by the stroke of the vertical jacks 30. Furthermore, the hydraulic vertical jacks 30 are capable to withstand 5% of their vertical capacity as horizontal load.

The vertical jacking system is able to determine the topsides weight and center of gravity with an accuracy of 5% for enabling the structure to be usable for other platforms than PKA and PKB.

Push-pull units 31 are used for skidding the topsides 12 because the push-pull-units are able to move the topsides 12 back and forth without additional preparation. This is beneficial for offloading operation and can also be considered as a safe return option during topsides removal offshore. The push-pull units 31 are combined with horizontal guiding system. However, alternatives to push-pull units such as horizontal hoist could be considered for moving the jacket skid beams and/or the topsides 12.

The push-pull unit capacity is about 300 kN, which is sufficient to overcome the friction loads of maximum vertical capacity, a slope of 1/100 and wind loads. The topsides traveling speed during the skidding operation is about 24 m/h and the topsides skidding distance is about 40 m which results in the topsides skidding being performed within 2 hours. Another advantage of using push-pull-units 31 for skidding the topsides 12 is that they can also be used for installing the jacket skid beam 19 on the jacket legs 11 and for bringing the skid assemblies in position underneath the topsides. In addition to skidding longitudinally the topsides from the jacket 10 to the jack-up main desk 5, it may also be useful to move the topsides 12 transversally after that it has been skidded on the JU main deck 5, e.g. for balancing the load on the jack-up vessel.

Different equipment may be needed for completing the offshore decommissioning structure, such as lifting devices, mobile working platforms and walkways for provide a safe working area and supporting the preparation work on the topsides, the removal and the tow preparation. For example, a bridge or a gangway is required to provide access form the jack-up vessel 1 to the platform 12 and also to guide service lines (e.g. electrical power). As the main preparation work is performed below the topsides which is not easily reachable by the JU crane 6, a mobile working platform is foreseen which is connected to the stern of the jack-up vessel and which can reach the different working areas. With this mobile platform, rope access works can be avoided, and scaffolding reduced which also enhances safety. Similarly, a walkway will be attached to the side of each jacket skid beam 19 opposite to the platform leg 11. From these walkways all areas along the jacket skid beams 19 and below the topsides 12 can be reached safely and conveniently.

According to the invention, a method for disassembling an offshore platform using the offshore decommissioning structure described above is explained in the following, in particular the removal of the platform topsides 12 from the platform jacket 10 to the main deck 5 of the jack-up vessel 1. This topsides removal sequence can be split in several stages: the jack-up vessel positioning, the platform preparation, the installation of support assemblies for the jacket skid beams, the installation of the jacket skid beams, the installation of topsides supports, the installation of the skid assemblies, topsides lifting and skidding on the jack-up main deck 5, and jack-up vessel lowering and towing.

In the following the jack-up positioning stage, as all the other stages, is described based on the example of decommissioning the PKA platform. As shown in figures 3a, the jack-up vessel 1 is positioned north of the platform 9 using, for example, the same position as planned during the plugging and abandonment activities. Distance range between the stern of the jack-up vessel and platform has been given above. The jacket skid beams 19, the skid assemblies 28 and all other elements of the decommissioning structure, such as the support assemblies 20, are located on the jack-up main deck 5.

After positioning of the jack-up vessel 1 , the jack-up legs 3 are lowered to the sea bed 4 and the jack-up hull 2 is lifted above the sea level 8. Preloading is performed considering maximum loads resulting from survival conditions including the removed topsides'! 2 on jack-up main deck 5. After preloading, the jack-up vessel 1 is lifted to a higher elevation used for the preparation activities. If the jack-up vessel has been previously located beside the platform, spud cans 32 shall be positioned into the existing depressions formed during the first jack- up vessel positioning. The ballast tanks are filled for preloading the jack-up legs 3 and then the jack-up hull 2 is lifted at the required elevation for performing the preparation works of the platform and the plugging and abandonment activities if still required.

Once the jack-up vessel 1 is in position and before removing the topsides 12, several activities need to be performed. This includes removing a part of the helideck 15 and the vent boom 17, which both would clash with parts of the jack-up vessel during the skidding. These elements can be cut and lifted off with the jack-up crane 6 to the JU main deck 5 and afterwards cut in smaller pieces. The communication mast 16 is removed if not already done during plugging and abandonment campaign. Furthermore, secondary platform elements located between the jacket 10 and the topsides 12 like risers, ladders are also removed, if not already done, because they can clash with the skid beams 18 and 19.

Before that the skid beams can be installed, four skid beam support assemblies 20 are fastened to the jacket legs 11. For each support assembly 20, the support ring 24 is welded to the jacket legs 11 to provide a vertical support. Then, a support clamp is mounted around the legs 11 by bolts. The support assembly parts are lowered from the cellar deck 14 and temporary secured by lifting devices before being finally welded and bolted to the legs 11. Access to the area for performing these activities is provided by the mobile work platform. The support clamp inner diameter is 1.25 m in order to fit jacket leg of other platforms as well. The gap between the clamp and the PKA jacket legs (outer diameter 0.9 m) will be filled with grout or similar material.

The JU skid beams 18 are positioned on the JU main deck 5 for being aligned with the jacket legs and then fixed with brackets. Afterwards, the jacket skid beams 19 are positioned on the top of the JU skid beams 18 and fixed by lateral guiding. After the JU vessel 1 has been lowered, the jacket skid beams 19 are pushed towards the jacket 10 by hydraulic jacking units, such as push-pull units 31. The jacket skid beams 19 are secured against uplifting before the second support assembly 20 on the jacket 10 is reached. The support assemblies

20 on the jacket legs 11 will also have some lateral guides to prevent the jacket skid beams 19 from capsizing/falling of the supports.

When the jacket skid beams 19 are in their second position, they are linked together and secured to the legs 11.

After the jacket skid beams 19 are installed, the link beams 21 are installed at the aft edge 23 of the JU skid beams 18. The link beams 21 can be closed and opened depending on the planned operation and required airgap between the sea level 8 and JU hull 2.

For lifting the topsides, the vertical hydraulic jacks 30 which mounted on the skid assemblies 28, will be used after their positioning on the jacket skid beams 19 below the topsides 12. To this end, primary beams 33 on the topsides 12 are used as supports wherever possible (see figure 6). They can be reinforced with local stiffeners if necessary. In areas were only secondary beams are available on the topsides 12, there is a need to add a more robust section. Topsides supports 34 are also fastened to the topsides structure where the vertical jacks cylinders will exert their lifting force. All the reinforcements and added supports will be welded to the existing structure and can be installed from the walkway below, which is connected to the jacket skid beam as indicated above.

The skidding of the topsides 12 is done by the skid shoes 29 of the skid assemblies 28 which are moving on the skidding track formed by the skid beams top surface (see figures 6 and 7). The skid assemblies 28 include the vertically oriented hydraulic jacks 30 which can lift the topsides 12 and horizontal hydraulic jacks included in push-pull units 31 for moving horizontally the topsides 12. The skid shoes 29 which are part of the decommissioning structure are pre installed on the skid beams pre-positioned on the JU vessel 1. During the operation, they can move from the JU main deck 5 to the platform for being positioned underneath the topsides 12 by their own horizontal jacks. The skid assemblies 28 are accessible from the walkway (not illustrated) connected to the jacket skid beams 19. Afterwards, the skid assemblies are installed and tested and the vertical jacks cylinders are extracted until they come in contact with the topsides supports 34. Eventually, the weather is checked and if it is favorable, then the JU vessel 1 is lowered from the jacked skid beams installation level (see figure 8c) to the skidding elevation where the top surfaces of the jacket skid beams 19 is at the same level as the JU skid beams 18 (see figure 6 and 9). Then, the link beams 21 are installed on the JU skid beams 18, as shown in figure 5a, and lowered to connect the JU skid beams 18 with the jacket skid beams 19 as shown in figure 5b. In an event of bad weather, the connection can be opened by lifting the link beam 21 and the Jack-up vessel may be lifted to safe elevation. Once the skidding track on the top the skid beams and link beams, the skid assemblies 28 are moved under the topsides supporting points 34. Again, the weather forecast is checked and if weather is deteriorating, the link beams are lifted for disconnecting the skid beams 18 and 19 and the JU vessel 1 is lifted to safe elevation. If weather forecast is favorable, then the operation can proceed further with the topsides removal.

The removal of the topsides 12 will start with the cutting of the platform legs 11 below the cellar deck 14. Cutting may be done by flame cutting, water jet cutting or diamond wire. The vertical jacks 30 will be regulated to keep the vertical level of the topsides 12 constant during the cutting step and to exert a preload on the jacket legs 11. After that the four legs 11 have been cut, the weight of the topsides 12 can be estimated by reading the pressure in the hydraulic system of the vertical jacks 30.

If the topsides weight is considered too high for the removal, then the topsides 12 will be lowered back on the jacket legs 11. In this event, a temporary fixation is added to secure the topsides 12 until weight shedding has been performed. A survey with regards to topsides weights will be performed during the preparation and the plugging and abandonment phase to verify the weight report. However, as all skidding equipment and strength checks are based on a topsides weight estimation of 1 ,800 tones while the expected weights of the topsides is, for example, 1536 tones for PKA and 1398 tones for PKB including weight factors, an overweight occurrence for the topsides is unlikely. Therefore, it is not expected that weight shedding is required.

If, as very likely, the topsides weight is considered acceptable for the removal, the topsides 12 will be further lifted by 75 mm to provide sufficient clearance underneath the topsides 12. Subsequently, the topsides skidding is performed moving the skid assemblies along the skid beams 19, 21 and 18 until it reaches the final position on the JU main deck 5. Finally, the topsides 12 is lowered on transport supports on the JU main deck 5.

When, the topsides 12 is completely moved onto the JU main deck 5, as shown in figure 3b, the link beams 21 are lifted for disconnecting the skid beams 18 and 19 and the JU vessel 1 is lifted to a higher elevation.

When the topsides have been put down on the JU main deck 5, it will be finally secured for the tow by sea-fastening. In parallel, the jacket skid beams 19 which are still located on the jacket 10 are removed, for example by lifting them with the JU crane 6 to the JU main deck 5. After confirmation of adequate weather forecast, the JU vessel 1 will then be ballasted to equalize heel/trim and lowered to sea. Then a tow line is connected to tugs and jack-up legs 3 are lifted. Eventually, the Jack-up vessel 1 with topsides 12 on main deck 5 is towed to the offloading location.

After removal of the topsides 12 from the jacket 10 and transport on the jack-up main deck 5, eventually, the topsides 12 must be offloaded from the jack-up vessel 1. Different options are possible and four examples are listed below.

The first option consists in offloading the topsides 12 onto a quay with the jack-up vessel elevated at quay level skidding the topsides directly on the quay. The second option is to offload the topsides on a barge with the jack-up floating. The third option consists in lifting the topsides 12 from the jack-up vessel 1 with a floating crane and then to lower it on a quay. The fourth option consists in cutting the topsides in small piece while being on the jack-up main deck 5 and then removing the small pieces with the jack-up crane 6 or any other mobile crane directly on a quay or on a barge.

The first offloading option is the preferred one, as it does not require an additional barge or vessel and the dismantling of the topsides on land is the easiest way. In this scenario, the jack-up is elevated beside a quay at quay level. The jacket skid beams are installed on the quay and are connected to the jack- up skid beams by the link beams. Then, the topsides is skidded on the quay as a reverse operation to the offshore removal from the platform jacket. Afterwards topsides can be dismounted on the quay. Jacket skid beams can be moved back on the jack-up main deck as soon as the topsides is lowered to support points on the quay. The advantages of this option are the usage of the same skidding equipment as for the offshore skidding, and a relatively short duration until the topsides is on land and the jack-up vessel can be used for the next job. However, it requires that an adequate quay side can be found with respect to the loading capacity and height, and additional structure on the offloading quay. Moreover, this may require the jack-up vessel ballasting when the topsides is skidded to land to avoid floating up the JU.

Offloading the topsides on a barge requires that the jack-up vessel is floating in a sheltered area and a barge is moored to the stern of the jack-up vessel. Then, the topsides is skidded on the barge exactly as on the quay for the first option. Eventually, the topsides can be dismounted on the barge and the parts lifted to quay by mobile cranes. This option requires a barge with sufficient strength and adequate height with ballasting capacities. It also requires sufficient ballasting capacity of the jack-up vessel in order to overcome change of the center of gravity. The advantages of the second option are that the same skidding equipment as for the offshore topsides removal can be used, and the skidding to the barge is independent from the water depth and tide level. The drawbacks are the hiring costs for the barge until final removal of topsides, and more complex operation compared to skidding to quay due to the hydrostatic stability of the barge and jack-up vessel during the topsides skidding.

For the third option, the jack-up vessel is floating in a sheltered area (e.g. a harbor). A floating heavy lift crane approaches the jack-up vessel from the stern and lifts the topsides. Afterwards, the floating crane is moved to the quay and lowers the topsides on the quay for final dismounting. The constraints of this option lie in finding a large floating crane with sufficient lifting capacity, outreach and hook height. The advantages of the third option are a relatively short duration until topsides is on land which means that the JU can be used quickly for next job. However, the disadvantages are in the hiring costs for the crane vessel and finding appropriate quay side.

The fourth option consists in removing the topsides in small pieces with the jack-up crane. This means that the jack-up vessel must be elevated beside a quay, the topsides is dismantled on the JU deck in small parts and the parts are then lifted by the jack-up crane to the quay. The advantages of this solution are that no additional equipment is required, and it does not involve any critical operation. The main drawback is that this option is very much time consuming and the jack-up vessel cannot be used for the next job until dismantling of the topsides.

Despite that, in the above description, specific aspects of the invention have been presented in the context of the decommissioning of PKA and PKB, it can be implemented in different contexts requiring technical adaptation which would not escape from the scope of the invention. All the values indicated in the above description are given as example based on the removal of PKA platform. It must be understood that they may vary for decommissioning other platforms or offshore structures.

Claims

1. Offshore decommissioning structure for disassembling an offshore platform (9) presenting topsides (12) mounted on a jacket (10) having a plurality of jacket legs (11 ), said decommissioning structure is characterized in that it comprises:

• a jack-up-vessel (1 ) comprising a hull (2), a main deck (5), an elevating jacking device and a plurality of jack-up legs (3), wherein said jack-up vessel (1 ) can be elevated on its legs (11 ) above the sea level (8) by the elevating jacking device; and

• a carrying and skidding system for shifting the topsides (12) of the platform to be disassembled from the platform jacket (10) onto the main deck (5) of the jack-up vessel (1 ), wherein said carrying and skidding system comprises a skid beams set for bridging the platform jacket (10) and the jack up vessel (1 ).

2. Offshore decommissioning structure according to claim 1 wherein said skid beams set comprises:

• a plurality of jack-up skid beams (18) which are fastened on the jack-up main deck (5); and

· a plurality of jacket skid beams (19) which can be transferred from a first position where they rest on the jack-up main deck (5) to a second position outside of the jack-up main deck where they can be attached to the jacket legs (11 ) of the platform (9).

3. Offshore decommissioning structure according to claim 2 wherein said skid beams set further comprises a plurality of link beams (21 ), each of said link beams (21 ) being attached by a hinged end (22) to an aft end (23) of a jack up skid beam (18), wherein said link beams (21 ) can be lowered for connecting said jack-up skid beam (18) to a corresponding jacket skid beam (19), when it is in the second position, and raised for disconnecting them.

4. Offshore decommissioning structure according to claim 2 or 3 wherein said skid beams set comprises at least two parallel skid beams subset each comprising a pair of jack-up skid beams (18) aligned with a corresponding pair of jacket skid beams (19), each one being adapted to be positioned on opposite sides of at least two jacket legs (11 ) when they are in said second position.

5. Offshore decommissioning structure according to anyone of claims 2 to 4 comprising further skid beam support assemblies (20) which can be attached on the jacket legs (11 ) for supporting the jacket skid beams (19) when in said second position.

6. Offshore decommissioning structure according to claim 5 wherein each skid beam support assembly (20) comprises:

• a support ring (24) which can be welded on a jacket leg (11 ), and

• a skid beam support clamp (25) which can be mounted on the jacket leg (11 ) above the support ring (24) and which presents lateral skid beam supports brackets (27) for supporting the jacket skid beams (19) on both side of the jacket leg (11 ).

7. Offshore decommissioning structure according to anyone of claims 2 to 6 further comprising skid assemblies (28) for shifting the platform topsides (12) from the jacket (10) to the jack-up main deck (5), said skid assemblies (28) being able to move on the skid beams top surface when the jacket skid beams (19) are in said second position; said skid assemblies (28) comprising:

• skid shoes (29) positioned on the top surface of the skid beams;

• vertical jacks (30) mounted on the skid shoes (29) for lifting the platform topsides (12); and

• horizontal jack assemblies (31 ) for shifting the platform topsides (12) to and off the jack-up main deck (5).

8. Offshore decommissioning structure according to claim 7 wherein the horizontal jack assemblies are push-pull units (31 ) which can move back and forth the platform topsides (12) on skid tracks formed on the top surface of the skid beams, and the jacket skid beams (19) on the jack-up skid beams (18) for moving between said first and second positions.

9. Offshore decommissioning structure according to claim 7 or 8 wherein the vertical jacks (30) and/or the horizontal jack assemblies (31 ) are hydraulic jacks and are actuated by a hydraulic pump controlled by a control unit.

10. Offshore decommissioning structure according to anyone of the preceding claims, wherein the jack-up vessel (1 ) further comprises:

• at least a supporting crane (6) for lifting loads between the jack-up vessel

(I ) and the platform topsides (12), and/or

· at least a mobile working platform for reaching working areas in particular below the platform topsides (12).

11. Offshore decommissioning structure according to anyone of the preceding claims, wherein the jack-up vessel (1 ) further comprises:

· at least a gangway/bridge for enabling access from the jack-up vessel (1 ) to the platform (12); and/or

• walkways attached on at least one side of each jacket skid beam (19) for reaching all areas along the jacket skid beams (19).

12. Method for disassembling an offshore platform using the offshore decommissioning structure for disassembling an offshore platform (9) presenting topsides (12) mounted on a jacket (10) having a plurality of jacket leg

(I I ), according to anyone of claims 2 to 9, said method comprising the following steps:

· position the jack-up vessel (1 ) close to the platform (12) with the stern (7) oriented toward the platform (9);

• lift the jack-up vessel (1 ) at working height; • position skid beams (18, 19) on jack-up main deck (5) relative to the platform legs (1 1 );

• push jacket skid beams (19) toward the platform jacket legs (1 1 ) in said second position;

· cut the platform jacket legs (1 1 ) below a bottom part of the platform topsides (12); and

• shift platform topsides (12) on the skid beams (18, 19) to the jack-up main deck (5).

13. Method for disassembling an offshore platform according to claim 12 wherein a preparation stage is performed on the platform before the step of cutting the platform jacket legs (1 1 ), the preparation stage comprising the following steps:

• perform the plug and abandonment campaign if not already done before;

• remove a communication tower (16) of the platform (9) if not already done during the plug and abandonment campaign;

• remove a vent boom (17) of the platform (9);

• remove at least a part of a helideck (15);

• remove all elements of the platform bottom part which clash with the skid beams (18, 19); and

• cut all secondary parts between platform jacket (10) and topsides (12) for providing a passage for the jacket skid beams (19) and the topsides shifting.

14. Method for disassembling an offshore platform according to claim 12 or 13 wherein skid beams support assemblies (20) are installed before the step of pushing jacket skid beams (19) toward the platform jacket legs (1 1 ), the installation of the skid beam support assemblies (20) comprising the following steps:

• weld support rings (24) on the platform jacket legs (1 1 );

• bolt together half tubular parts (26) of skid beam support clamps (25) around the platform jacket legs (1 1 ) above the support rings (24); and • put grout between the skid beam support clamps (25) and the platform jacket legs (11 ) for compensating the diameters difference if necessary.

15. Method for disassembling an offshore platform (9) according to anyone of claims 12 to 14 wherein the following steps are performed before the step of pushing the jacket skid beams (19) toward the platform (9):

• lower the jack-up vessel (1 ) to a level below a platform shifting elevation;

• verify the skid beams position relative to the platform legs (11 ) and reposition as appropriate; and

· install support and stiffening structures (34) on the lower of the platform topsides (12) as required for lifting the topsides (12).

16. Method for disassembling an offshore platform (9) according to anyone of claims 12 to 15 wherein the following steps are performed before the step of cutting the platform jacket legs (11 ):

• position link beams (21 ) to connect the jack-up skid beams (18) to their respective jacket skid beams (19);

• move skid assemblies (28) under the platform topsides (12) supporting points;

· connect to a pump and test hydraulic jacks for lifting and shifting the topsides (12); and

• extend the vertical jacks (30) for preloading the topsides fastening to the jacket legs (11 ).

17. Method for disassembling an offshore platform according to anyone of claims 12 to 16 wherein the following steps are performed before the step of pushing the platform topsides (12) to the jack-up vessel (1 ):

• lift the platform topsides (12) with the vertical jacks (30) for checking the weight of the topside (12) with an integrated weighting system;

· if topsides weight is within an acceptable threshold, then continue to lift the topsides (12) until the defined clearance with the jacket (19) is reached; • if topsides weight is above the acceptable threshold, then the platform topsides (12) are lowered back and secured.

18. Method for disassembling an offshore platform according to anyone of claims 12 to 17 wherein the following steps are performed when the platform topsides have reached their final position on the jack-up main deck (5):

• lower topsides (12) on transport support points on the jack-up main deck

(5);

• open skid beam connections at link beams (21 );

· fasten topsides (12) on jack-up vessel main deck (5);

• pull back the jacket skid beams (19) on the jack-up main deck (5);

• lower the jack-up vessel (1 ) on the sea; and

• tow the jack-up vessel (1 ) with the topsides (12) on jack-up main deck (5) to the offloading location.