GB2168293A - Vessel for the installation of production platforms on fixed jackets - Google Patents

Abstract

For installation of an offshore platform 30 on a fixed leg jacket 29 of an offshore oil production well, a semi- submersible installation vessel is provided which has twin hulls or a two-limbed hull 13 with superstructure comprising towers 15 upstanding on the hulls or hull limbs and bridging structure connecting the tops of the towers. The platform 30 is suspended on winches 28 in the region 26 between the hulls or hull limbs at a height greater than its final height, the installation vessel is manoeuvred into position with the platform 30 above the jacket 29 and then a multiple winch system 28 on the vessel is employed to haul down the vessel over the jacket until the hull or hulls are submerged. Dynamic positioning and self-tensioning auxiliary winches can be employed to reduce the relative motions of the vessel and the jacket during mating and fixing of the platform to the jacket. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the installation of production plafforms on fixed jackets This invention relates to offshore oil-field production platform.

A commonly used method of production in offshore oii-fields is the installation of a fixed leg jacket over the well and on top of that a production platform. The installation procedure for the production platform has, generally speaking, been in the form of assembling sub units of the platform into the completed platform in situ on top of the jacket legs. This procedure is time consuming and costly but, above ali, means that after the completion of the jacket there is a considerable period before the platform can commence production.

It has been found in some installations that if the complete working platform can be installed as one unit on top of the jacket or similar support structure, then production at the field can be commenced very much sooner than would be the case if the working platform were assembled from sub units in situ.

It is thus desirable that a method be developed to install a complete working platform in place over a jacket bearing in mind that for major units this may mean the installation of a weight of the order of 30,000 tons in one piece and in one installation operation.

While obviously suitable weather would be chosen for such an operation it is clear that the production platform must be at the oil field site and that weather predictions are not sufficiently reliable to guarantee completely calm conditions over a sufficient period. A combination of these two factors means that the installation procedure must take into account the relative motions of the fixed jacket and the floating installation unit in practical open sea conditions. It is not proposed that any such installation would take place during a storm but the procedure should be capable of operation during average to low weather conditions.

The invention seeks to carry this out substantially avoiding the relative motions of the unit and the fixed jacket and ensuring that the mating of the working platform on the jacket is carried out without the possibility of significant movement or impact damage during the mating.

According to the present invention there is provided a method of installing an offshore platform on a fixed leg jacket of an offshore oil production well or the like, wherein the platform is suspended, at a height above sea level greater than that it will have when installed, on superstructure rising above the hull or hulls of a semi-sumbersible installation vessel which vessel is so configured that there is a region of clear water below the platform when so suspended, the installation vessel is manoeuvred at the site of the jacket to bring the jacket into said region of clear water and thereby move the platform into position over the jacket, and a system of winches on the vessel is used to haul down the vessel over the jacket to submerge the hull or hulls and minimise relative motion of the installation vessel and the jacket during fixing of the platform to the jacket.

Although this haul-down method can be performed using a single-hull barge as the installation vessel, this entails large forces due to the large waterplane area required for the stability of a monohull. It is therefore much preferred to employ a twin-hulled or catamaran type vessel, or a vessel having a hull of U-configuration in plan with two forward-extending side limbs.

The invention also provides apparatus for performing the method including an installation vessel which comprises twin hulls or a hull of Uconfiguration in plan with longitudinally-extending side limbs, columns or towers upstanding on the hulls or hull side limbs, decking or bridging structure bridging the tops of the columns or towers, means for suspending an offshore platform, which is to be installed on a fixed leg jacket, over the region of clear water between the hulls or hull side limbs, and a system of multiple winches whereby the vessel can be hauled down to submerge the hull or hulls upon attachment of the winch wires to the jacket.

Means for carrying the invention into practice will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic elevation of a first installation vessel for use in the method according to the invention, Figure 2 is a diagrammatic half-plan of the vessel of Figure 1, Figure 3 is a diagrammatic cross-section of the vessel, Figures 4and 5 are views similarto Figures 2 and 3 of a second installation vessel, and Figures 6 and 7 are views similar to Figures 2 and 3 of a third installation vessel.

In the first arrangement, shown in Figures 1 to 3, the installation vessel has a semi-submersible pontoon hull 11 that in plan has substantially the form of a U with a full width aft end portion 12 and two spaced parallel side limbs 13 extending forward from it, the hull structure being substantially symmetrical about the waterplane 14 at minimum draft. Each side limb 13 of the hull has upstanding from it several vertical columns or towers 15, in this example two on each hull limb, which supports at their tops a deck 16 which is also U-shaped in plan, the full width aft end portion of this deck being cantilevered and propped by braces 17 extending from the after pair of towers 15. Preferably, the centre of gravity of the deck is directly over the centre of flotation of the pontool hull to avoid trimming problems.

In the second arrangement shown in Figures 4 and 5, the installation vessel basically consists of two buoyant hull bodies 18, either barges or semisumbersible pontoon rig units, which are bridged at their after portions by a deck structure 19 attached to tower erections or columns 20 onthehullsto form a catamaran.

In the third arrangement, shown in Figures 6 and 7, a SWATH vessel is constructed from two cut down tanker hulls 21, on the inboard sides of which are erected the towers 22 and the transverse bridging structure 23. The towers are preferably constructed with longitudinal bulkheads, the inboard bulkhead being in line with the inboard bulkhead of the tanker hull and the outboard tower bulkhead being positioned at an appropriate location towards the centreline of the hull. Each tower 22 is continuous over a suitable length, generally of the order of half the length of the tanker hull, and it is watertight and used not only for supporting the bridging structure 23, and hence the platform, but also for supplying waterplane area when the tanker hulls, suitable stiffened and reinforced, are flooded down.By this means a restricted waterplane area can be provided, the whole structure being converted into a very large semi-submersible unit.

In order to limit stresses, a transverse structure may extend across between the two hulls under water at one end, leaving entry open at the other end to enable the vessel to be positioned over the jacket.

The bottom of the bridging structure is at a sufficient height above the top of the jacket 29, and the inside walls of the twin hulls or hull side limbs are at a sufficient distance from the jacket sides, to ensure adequate clearance to allowforthe relative motions. The bridging structure is arranged to define an open space 26 between side members, and possibly between fore and aft members (a removable forward member can be fitted, if desired), this open space ailowing the suspension of the working platform 30 from the bridging structure and/orthe side towers so that the platform is suspended in this area similarly in principle to the use of a moonpool on a drilling vessel.

For loading the platform 30 on to the installation vessel, the platform can be lifted into the 'moonpool' space, thereafter to be suspended from the bridging structure fore and aft and/orthe structures port and starboard, or it may be lowered into position if suitable lowering gear is provided. A system of Synchrolift dock type winches 28 mounted on the bridging structure and/or the side structures can be employed for lifting the platform into the working space. Generally, the procedure will be to ballast down the installation vessel, to a waterplane level such as that indicated at 24 in Figure 1,to facilitate positioning of the platform 30 in the installation position in the 'moonpool'.

The subsequent installation procedure is to guide the vessel into position over the jacket so that the platform is above the jacket 29 and ready to be lowered on to it. In a preferred form, the two semisubmersible hulls or hull limbs are dynamically positionable so that the vessel can maintain both transverse and longitudinal station relatively to the jacket. Transponders or tight wires actually attached to the jacket 29 is a possible way, if so desired, of providing location referenceforthe installation vessel.

The side structures of the installation vessel, either the side portions of the deck or bridging structure or the towers, have further synchronised winches 28, similar two those used in the Synchrolift dock system, which can be attached by their wires 31 to the legs of the jacket 29 of the platform, or alternatively the wires can be taken through sheaves on the jacket and attached to the underside af the deck orto the side structures.These winches are used to haul the entire installation vessel down by an amount such that the hull or hulls are submerged to a depth exceeding the amplitude of the maximum wave to be met, for example to the waterplane level indicated at 25 in Figure 1.The load in so doing will be the product of the tons per inch, or tones per centimetre, immersion ofthe installation vessel and the amount by which the unit is hauled down, this representing excess buoyancy.

By suitable programming of fore and aft winches, including winches on the bridging structure as well as the side structure or towers, it is possible to apply a constraint in the transverse and longitudinal planes as well as the vertical. By this means all relative motion of the installation vessel and the fixed facket 29 can be substantially prevented in heave, pitch and roll provided alwaysthatthe amount of haul-down exceeds the linear motion that would take place without restraint.

A preferred form of the installation vessel is one in which the waterplane area is minimised, this giving the lowest haul-down forces in all three modes. The preferred embodiment employs a U-hull or two semi-submersible rigs of adequate loading capacity, which can be ballasted down either for the transit mode or at the jacket site so that the waterplane is through the columns ortowers and not the two hulls at the commencement of haul-down.It is normal with semi-submersible rigs to experience a minimum condition for stability just after flooding down beyond the pontoon hulls, and for the purpose of the installation procedure it is desirable to attach auxiliary waterplane area, which may be in the form of removable pontoons, to the tops of the hulls, thereby increasing the waterplane area over the first few metres of submersion of the unit be!ow the tops of the hulls.

Transverse and longitudinal location ofthe installation vessel relative to the platform is basically by directional positioning but auxiliary self-tensioning winches fitted on each of the side towers and on the transverse bridging structure, for trimming purposes, can reduce the residual motion of the vessel from the order of one or half a metre to the 5 cm or so necessary for the actual mating procedure. This procedure, using the dynamic positioning capability of the installation vessel, reduces the transverse and longitudinal forces on the jacket to an acceptable level.

It is desirable that the platform on the installation vessel should have its LCG not too fardistantfrom the LCB, LCF and LCG of the hauled down installation vessel in order to avoid excessive trimming moments when the platform is unloaded on to the jacket. However, trimming moments can be accommodated by ballasting providing the vessel has enough excess displacement available.

Claims (12)

1. A method of installing an offshore platform on a fixed leg jacket of an offshore oil production well or the like, wherein the platform is suspended, at a height above sea level greater than that it will have when installed, on superstructure rising above the hull or hulls of a semi-submersible installation vessel which vessel is so configured that there is a region of clearwater below the platform when so suspended, the installation vessel is manoeuvred at the site of the jacket to bring the jacket into said region of clear water and thereby move the platform into position over the jacket, and a system of winches on the vessel is used to haul down the vessel over the jacket to submerge the hull or hulls and minimise relative motion of the installation vessel and the jacket during fixing of the platform to the jacket.

2. A method according to Claim 1, wherein the installation vessel employed is twin-hulled or has a hull of U-configuration in plan with two side limbs.

3. A semi-submersible installation vessel for performing the method of Claim 1, comprising twin hulls or a hull of U-configuration in plan with longitudinally-extending side limbs, columns or towers upstanding on the hulls or hull side limbs, decking or bridging structure bridging the tops of the columns or towers, means for suspending an offshore platform, which is to be installed on a fixed leg jacket, over the region of clear water between the hulls or hull side limbs, and a system of multiple winches whereby the vessel can be hauled down to submerge the hull or hulls upon attachment of the winch wires to the jacket.

4. A vessel according to Claim 3, comprising twin hulls of SWATH type constructed from two cut down tanker hulls.

5. A vessel according to Claim 3 or Claim 4, wherein the columns or towers are watertight for at least the lower portions of their height to provide waterplane area.

6. A vessel according to Claim 3 or Claim 4 or Claim 5, wherein the means for suspending the platform comprises synchronously operating winches on the decking or bridging structure or the tops of the towers.

7. A vessel according to any one of Claims 3 to 6, including means for dynamically positioning the vessel laterally and longitudinally in relation to the jacket during installation.

8. A vessel according to any one of Claims 3 to 7, including auxiliarly self-tensioning winches for trimming purposes during the mating of the platform with the jacket.

9. A vessel according to any one of Claims 3 to 8, wherein the decking or bridging structure bridges the vessel forward and aft, the forward bridging structure being removable.

10. A vessel according to any one of Claims 3 to 9, comprising twin hulls with a transverse structure connecting the hulls under water at one end only.

11. A vessel according to any one of Claims 3 to 10, comprising means for adding removable waterplane area at the tops of the hull or hull limbs.

12. A semi-submersible installation vessel for performing the method of Claim 1, substantially as described with reference to Figures 1 to 3, or Figures 4 and 5, or Figures 6 and 7, of the accompanying drawings.