GB2303337A - Offshore operations vessel - Google Patents

Description

OFFSHORE OPERATIONS VESSEL The invention relates to an offshore operations vessel, and to a method of using that vessel.

In general, offshore operations vessels are designed to provide a stable working plafform in moderate seas, in combination with sufficient buoyancy to carry all the equipment and stores needed for the offshore operations.

One generic offshore operations vessel is a Mobile Offshore Drilling Vessel or MODU.

This has pontoons at or near its deepest draft, relatively slender columns which during operation extend through the wave effected zone, and a deck or payload area for equipment and stores. Existing designs of MODU's, such as the SEDCO FOREX 700 series semi semisubmersibles currently in service, have worked well for their intended purpose. However, existing designs of MODU's do not have sufficient payload capacity for certain tasks, such as assembling transporting, installing and/or removing major offshore structures.

Proposals have been made - e.g. in U.K. Patent Specification 2223458 - to use a pair of tanker hulls to transport offshore plafforms. Proposals have also been made - e.g. in U.K.

Patent Specification 2165188 - for specifically constructed twin hull installation vessels. These proposals, while suited to transportation and installation of certain types and sizes of offshore plafforms, may lack flexibility in respect of jacket sizes, and may not be appiicable for assembly and decommissioning tasks.

The invention provides a vessel capable of assembling, transporting, installing and/or removing an offshore structure designed to stand upright on a seabed, and comprising an arrangement of buoyant elongate pontoons and at least three stabilising columns upstanding from those pontoons, in which there are connectors attached to at least three of the columns, the connectors being capable of directly or indirectly engaging primary members of the offshore structure and being arranged to lift the offshore structure or parts thereof in a stable vertical orientation between the columns (for marine operations).

In one form the pontoons are connected together to form a U shape.

In another form there are two pontoons which are parallel to and spaced apart from each other, and each pontoon has one stabilising column upstanding at or near each of its ends.

In this form it is preferred that the pontoons are spaced apart from each other by removable elongate support braces, and the length of the removable support braces is adjustable.

Preferrably lower connectors attached to the pontoons are designed to carry horizontal and vertical loads, and upper connectors attached to the columns are designed to cany horizontal loads only.

The vessel may have winching and/or ballasting systems to adjust the height of the structure during operations.

The invention also provides a method of assembling, transporting, installing and/or removing an offshore structure comprising the steps of positioning vessel halves on either side of the structure or a part thereof, and then engaging the structure or part thereof with connectors, so that the structure or part thereof can be raised or lowered on winches on the vessel halves, and/or by ballasting the vessel halves.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of an offshore operations vessel; Figure 2 is a plan view of that vessel; Figure 3 is an end view of the vessel seen from the direction of arrow Ill in Figure 2; Figure 4 is a perspective view of a multi portion jacket, the assembly of which, using the vessel of Figures 1 to 3, is illustrated in the remaining figures; Figure 5 shows base portions of the multi portion jacket being assembled; Figure 6 shows the multi portion jacket near completion; Figure 7 shows that jacket being transported; and Figure 8 shows the jacket being installed.

The vessel 10, shown in Figures 1 to 3, has been developed to assist the assembly, transportation, installation and removal of a multi portion jacket forming a substructure for an offshore platform. However, because the vessel can be reused and can accommodate different jacket geometries, it can also assist in the transportation, installation and removal of other offshore plafforms.

The major components of the vessel 10 are two identical (handed) buoyant halves. Each half is constructed of a pontoon 11 to provide the main buoyancy, and two upstanding columns 12 near the ends of the pontoon to ensure stability of the jacket/vessel system during marine operations. The vessel 10 includes the following equipment; connectors to keep the jacket in position, a jacking system to assist jacket assembly, and a ballast system to control the draft of the vessel and hence facilitate the installation and removal of jackets. The two halves, each comprising one pontoon 11 and two columns 12, can be kept in position by adjustable braces 14 while the vessel is being attached to a jacket. However, the main transverse stiffness during operation is provided by the jacket itself, and not by the braces.

The vessel can be designed to suit all types of jacket. However, preliminary assessment indicates that the vessel is most suited for small and medium sized jackets.

The vessel 10 will now be described in more detail.

The vessel 10 is configured like a lower part of a semisubmersible MODU, and comprises the two parallel horizontal pontoons 11 and four vertical columns 12. The two halves are connected by the adjustable braces 14, though the main stiffness between the two halves is provided by the jacket through eight connecting hydraulic clamps.

The two halves are stable on their own at a certain draft, hence the adjustable braces 14 can be removed before the halves are moved into position for connection to a jacket. When the halves are attached to a jacket, the adjustable braces are not required for strength or stability.

Details of the system to connect the jacket to the buoyant halves of the vessel are not important and several altematives can be used. Hydraulic Clamps are described to exemplify a particular form of connectors.

The pontoon/column configuration was selected considering the wave loading as benign in the submerged condition, hence installation and removal operations can be carried out within acceptable seastate limitations. Further, the reduced waterplane area in the final installation phase/early removal phase results in minimum ballast requirements to ensure a fast installation/removal operation. The pontoons 11 provide both the required buoyancy to lift the entire jacket and the hydrostatic stiffness in the assembly mode to ensure a safe working area; whereas the four columns 12 give the required hydrostatic stability during transport and installation.

Each column 12 has a cone shaped section midway up its height to reduce hydrodynamic loading. To allow selective ballasting, the pontoons 11 are divided into several compartments, which also ensures stability in a "one compartment damage" condition. The braces 14 connecting the two halves of the vessel are adjustable in length to ensure flexibility with respect to jacket dimensions. Two of the braces can be partly removed to allow separation of the vessel and jacket.

The size of the vessel is determined by the weight and CG position of the largest jacket to be installed/removed. The vessel illustrated by way of example in Figures 1 to 3 (with plan dimensions of 56m x 56m and an overall height of 26m) can handle a jacket with a total weight of approximately 7000 tonnes. If the CG position of the jacket is substantially offset from the centre line of the vessel, the load carrying capacity of the vessel will be reduced.

The vessel is fitted with the following items of equipment and systems to enable it to carry out marine operations: Four (lower) hydraulic clamps 15 for transfer of lateral and vertical loads.

Four (upper) hydraulic damps 16 for transfer of lateral loads only.

A winch system for lowering/hoisting a jacket in assembly/decommissioning mode.

A ballast system.

The eight hydraulic clamps 15 and 16 (or equivalent connectors) are a key element in the vessel system. The clamps connect the pontoons 11 and columns 12 to the jacket and ensure that the vessel and the jacket act as one rigid body during transportation, installation and removal operations.

A hydraulic release system has been adopted to ensure fast diverless connection/deconnection operations between the jacket and the vessel. The support arrangements between the pontoons and columns and the lower and upper clamps respectively are adjustable to accommodate different jacket configurations. The four lower clamps 15 are attached to the pontoon/column intersections and take both lateral and vertical loads; whereas the four upper clamps 16 are attached to the columns, and transfer only lateral loads to reduce the deflections and racking of the vessel.

As an altemative to hydraulic clamps, pinned connectors may be used.

The winch system comprises four winches for lowering/hoisting of the jacket in assembly and decommissioning modes. The lowering/hoisting is performed using four wires attached to the lower part of the jacket. A central control system could be used for controlling the winch system. However, the winch system is off-the-shelf technology and details of the system are not important for this description.

The ballast system is designed to adjust the draft of the vessel, so that the vessel can carry out installation and removal of jackets and complete plafforms. A pump room is located in each pontoon. The pumps are connected to all the compartments to allow differential ballasting operations to be performed.

Power requirements can either be supplied through an umbilical from a stand-by vessel, or generators can be carried on the vessel. To control ballast operations and the operation of the hydraulic clamps, a control station 17 is located on top of one of the columns. Further miscellaneous equipment and systems, e.g. tank vent systems, towing equipment and temporary walkways and access plafforms, are also incorporated.

During the life of a development incorporating a multi portion jacket like that shown in Figure 4, the vessel can be used in the following four phases - viz assembly, transportation and installation; and then finally removal.

The vessel is shown in its assembly mode in Figures 5 and 6. The assembly sequence for the multi portion jacket (shown in Figure 4) is as follows: Moor vessel at a sheltered deep water site.

Lift base portion 21 with a sheerleg crane (not shown) and lower into water.

Attach vessel and hook up winch system.

Lift portion number two (22) using a sheerleg and stab into base portion 21.

Connect the two portions by welding.

Lower structure using the winch system until in position for stab in of next portion (23) Perform the above steps for remaining jacket portions 23-25 and topsides 26.

Complete near shore hook-up and commissioning.

During assembly, the vessel is ballasted to a minimum draft to ensure sufficient stability to give a safe and stable configuration. Temporary access walkways and platforms allow access to the legs of the jacket for welding the jacket portions (21 to 25) together.

The configuration of the vessel and a jacket in the transportation mode is shown in Figure 7. The transportation sequence is as follows: Winch jacket to transportation position.

Attach hydraulic clamps 15, 16 to jacket legs.

Disconnect winch system and other temporary equipment.

Attach tug or tugs (not shown) to vessel.

Perform tow to installation site.

The vessel and jacket in touch-down condition are shown in Figure 8. The installation sequence is as follows: Position jacket over installation site using tugs or prelaid mooring system.

Carry out controlled ballasting of the vessel until jacket is set down onto seabed 27.

Perform further ballasting until vessel is neutrally buoyant.

Release hydraulic clamps 15, 16 Remove vessel from installation site.

Secure jacket to seabed (by bucket foundations or piling).

During the controlled ballasting of the vessel, the wave loading and hence the wave induced motion will be reduced to a minimum, because the waterplane area reduces and the draft of the pontoons increases.

The following removal sequence is suitable for removal of both the jacket shown in Figure 4 and any other suitably sized jacket type platform: Transport vessel to removal site.

Ballast vessel to required draft.

Attach vessel to jacket legs using the hydraulic clamps.

Cut jacket foundations.

Deballast vessel to transport draft.

Transport jacket to decommissioning site.

Attach winch system to jacket.

Carry out decommissioning of the topside and modules.

Perform hoisting of jacket when required.

Demobilise vessel and refurbish for reuse.

The main advantages of using the vessel for installation of a newly built jacket are as follows: the principle fabrication advantages for the multi portion jacket are retained; minimum installation spread is required and a large heavy lift vessel is avoided; extensive hook up and commissioning may be performed near shore; offshore installation and HUC activities are of short duration; and the vessel and installation equipment can be reused.

The main advantages of using the vessel for removal of existing jacket type structures are as follows: minimum structural modification and removal equipment are required, e.g. no padeyes or other lifting equipment are required offshore; no large heavy lift vessel is required; the vessel is attached to the jacket legs, hence the load paths during removal are similar to the load paths in the in-place condition, and therefore only a minimum of survey and testing activities are required to confirm the structural integrity of the jacket and topside during the removal phase, and most of the decommissioning activities can be performed near shore.

The specific embodiment of the invention has been described in terms of dealing with the multi portion jacket illustrated in Figure 4. However, it will be understood that the vessel of Figures 1 to 3 can be used in connection with marine operations covering other types of jackets, or indeed complete plafforms comprising the combination of topsides on top of jackets.

Claims (9)

1. A vessel capable of assembling, transporting, installing and/or removing an offshore structure designed to stand upright on a seabed, and comprising an arrangement of buoyant elongate pontoons and at least three stabilising columns upstanding from those pontoons, in which there are connectors attached to at least three of the columns, the connectors being capable of directly or indirectly engaging primary members of the offshore structure and being arranged to lift the offshore structure or parts thereof in a stable vertical orientation between the columns (for marine operations).

2. A vessel as claimed in Claim 1 in which the pontoons are connected together to form å U shape.

3. A vessel as claimed in Claim 3 in which there are two pontoons which are parallel to and spaced apart from each other, and each pontoon has one stabilising column upstanding at or near each of its ends.

4. A vessel as claimed in Claim 3 in which the pontoons are spaced apart from each other by removable elongate support braces, and in which the length of the removable support braces is adjustable.

5. A vessel as claimed in any one of the preceding claims in which lower connectors attached to the pontoons are designed to carry horizontal and vertical loads, and in which upper connectors attached to the columns are designed to carry horizontal loads only.

6. A vessel as claimed in any one of the preceding claims, in which the vessel has winching and/or ballasting systems to adjust the height of the structure during operations.

7. A vessel substantially as hereinbefore described by way of example with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

8. A method of using a vessel as claimed in any one of Claims 1 to 7, and generally as described herein, with reference to Figures 5 to 9 of the accompanying drawings.

9. A method of assembling, transporting, installing and/or removing an offshore structure comprising the steps of positioning vessel halves on either side of the structure or a part thereof, and then engaging the structure or part thereof with connectors, so that the structure or part thereof can be raised or lowered on winches on the vessel halves, and/or by ballasting the vessel halves.

9. A method of assembling, transporting, installing andlor removing an offshore structure comprising the steps of positioning vessel halves on either side of the structure or a part thereof, and then engaging the structure or part thereof with connectors, so that the structure or part thereof can be raised or lowered on winches on the vessel halves, and/or by ballasting the vessel halves.

Amendments to the claims have been fulled as follows 1. A vessell capable of assembling, transporting, installing and/or removing an offshore structure designed to stand upright on a seabed, and comprising an arrangement of buoyant elongate pontoons and at least three stabilising columns upstanding from those pontoons, in which there are connectors attached to at least three of the columns, the connectors being capable of directly or indirectly engaging primary members of the offshore structure and being arranged to lift the offshore structure or parts thereof in a stable vertical orientation between the columns (for marine operations).

2. A vessell as claimed in Claim 1 in which the pontoons are connected together to form a U shape.

3. A vessell as claimed in Claim 2 in which there are two pontoons which are parallel to and spaced apart from each other, and each pontoon has one stabilising column upstanding at or near each of its ends.

4. A vessel as claimed in Claim 2 in which the pontoons are spaced apart from each other by removable elongate support braces, and in which the length of the removable support braces is adjustable.

5. A vessel as claimed in any one of the preceding claims in which lower connectors attached to the pontoons are designed to carry horizontal and vertical loads, and in which upper connectors attached to the columns are designed to carry horizontal loads only.

6. A vessel as claimed in any one of the preceding claims, in which the vessel has winching and/or ballasting systems to adjust the height of the structure during operations.

7. A vessel substantially as hereinbefore described by way of example with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

8. A method of using a vessel as claimed in any one of Claims 1 to 7, and generally as described herein, with reference to Figures 5 to 9 of the accompanying drawings.