GB2269619A

GB2269619A - Midwater support structures

Info

Publication number: GB2269619A
Application number: GB9316217A
Authority: GB
Inventors: John William Waddell; Poul Erik Christiansen
Original assignee: Kvaerner Earl and Wright
Current assignee: Kvaerner Earl and Wright
Priority date: 1992-08-05
Filing date: 1993-08-05
Publication date: 1994-02-16
Also published as: GB9316217D0; GB9216609D0

Abstract

A method of installing a buoyant mid-water support or the like comprising an elongate base (32) and an elongate buoyant tank (31) joined at or near their corresponding ends by tension members, includes the steps of placing the tank on top of the base in aligned relationship, transporting the tank and base, eg on a semisubmersible vessel, to an offshore site at which the subsea support is required, lowering the base, and drawing the tank down with the base so that the tank is held by the tension members in a midwater position above the base. <IMAGE>

Description

METHOD FOR SUBSEA INSTALLATION The invention relates to a method of installing a midwater support structure, and in particular relates to a method of installing a subsea support arch.

The progressive development of offshore oil/gas fields in deeper and deeper water has led to the development of Floating Production Facilities (FPF's). In cases where a fixed platform would be uneconomic, an FPF can be moored over the subsea oil/gas deposit, and can process crude oil or raw gas produced from that deposit. The crude oil/raw gas passes up from a subsea well, through a wellhead manifold on the seabed, and through a riser leading from the manifold up to the FPF. Support of the riser as it is suspended in the water between the seabed manifold and the FPF is an important requirement for efficient working of the field.

For large accumulations of oil or gas, it has been found necessary to have multiple risers suspended between the seabed manifold and the FPF.

Heretofore, it has been proposed to have fixed riser support structures upstanding from the seabed (as shown in Figure 1), or to have midwater riser support floats which are moored to the seabed. One such midwater support float arrangement is shown in Figure 3 of our UK Patent Specification No.

2250253.

Difficulties may occur in setting a fixed riser support structure on the seabed, and the carrying capacity of a single midwater float may not be sufficient to support multiple risers.

The invention provides a method of installing a midwater support structure comprising a base and a buoyant tank joined at or near corresponding spaced points by at least two tension members, such method including the steps of placing the tank on top ofthe base in aligned relationship, transporting the tank and base to an offshore site at which the subsea support is required, lowering the base, and drawing the tank down with the base so that the tank is held by the tension members in a midwater position above the base.

More specifically the invention provides a method of installing a subsea support arch comprising an elongate base and an elongate buoyant tank joined at or near their corresponding ends by tension members, such method including the steps of placing the tank on top of the base in aligned relationship, transporting the tank and base to an offshore site at which the subsea support is required, lowering the base, and drawing the tank down with the base so that the tank is held by the tension members in a midwater position above the base.

It is preferred that the tank is drawn down towards the base by the tension members that will subsequently hold the tank in the midwater position.

It is also preferred that the tank and base are floated to the offshore site on a vessel.

In this form it is further preferred that the vessel can be submerged from beneath the tank to leave the tank either floating, or partially floating and partially suspended from the vessel.

It is still further preferred that the tank and base are lifted off the vessel using a spreader beam arrangement having sheeves reeved to carry lowering cables or the like.

In this last mentioned form it is preferred that the spreader beam arrangement is adapted to provide guidance for and/or constraint to the tension members such that they can be lowered towards the seabed in alignment with the tank and base.

Preferrably the base is floodable for lowering to the seabed.

The invention includes a subsea riser support arch when installed in accordance with the method described above.

More generally the invention includes a midwater support structure when installed in accordance with the method first described.

A specific embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of a Floating Production Facility located over a subsea well head manifold, showing a fixed subsea riser support structure (prior art); Figure 2 is a side elevation of a subsea riser support arch accordance with the invention; Figure 3 is an cross-section of the riser support arch shown in Fig. 2; Figure 4 is a detail of the top of Figure 3; Figure 5 is a view on arrows V-V in Figure 4; Figure 6 is a detail of the base of Figure 3; Figures 7 to 9 are a side elevation, cross-section and plan respectively, showing the transportation of a riser support arch on a submersible heavy lift vessel; and, Figure 10 shows nine successive stages in the installation of the riser support arch.

As shown in Figure 1 (Prior Art) an FPF 20 (in this case a column stabilized semisubmersible vessel) is moored by cables 21 over a subsea wellhead manifold 22. Risers 23 connect the manifold 22 to the FPF 20. A fixed subsea riser support structure 24 carries an intermediate point on the risers 23.

The present invention is concerned with the installation of a riser support arch to replace the fixed subsea riser support structure 24.

The riser support arch is formed of an elongate tank 31 tethered to a gravity base foundation 32, and is shown in outline in Figures 2 to 6.

The riser support arch comprises an elongate tank 31 of square cross section which is tethered at both ends to a concrete gravity base foundation 32 (Figure 2). The tank is 5.2m x 5.2m in cross section and 30 metres long, and is tethered so that one diagonal across each end is disposed vertically.

The tank 31, which is open to the sea, is filled with syntactic foam which provides a net buoyancy of around 300 tonnes. The steel weight of the tank is around 120 tonnes and the weight of the syntactic foam within the tank is around 320 tonnes.

The tank 31 is fabricated from 12.7mm steel plate stiffened by external transverse frames at 2m centres and longitudinal bulb flat stiffeners at 1.3m centres. A total of four internal X-braces are provided for rigidity during construction. A rectangular tank shape is adopted for ease of fabrication, and to suit the installation of pre-formed rectangular blocks of syntactic foam.

The gravity base foundation 32 has an "egg box" configuration which is simple and economical to construct from concrete. A caisson base area of around 360m2 is proposed although this will depend upon foundation soil strength. The caisson is provided with downstanding peripheral steel skirts 30 (see Figure 6) to prevent scour.

As shown particularly in Figure 3, the elongate tank 31 is tethered to the gravity base foundation 32 with one primary (P) and two secondary (S) lines at each end. Each primary line (P) forms the main support cable for carrying direct buoyancy loads, whilst the secondary lines (S) provide stabilising support to resist rocking motions. This arrangement provides well defined load paths from the elongate tank 31 into the gravity base foundation 32. The secondary lines (eg of 75mm OD) are sized to resist uplift loads in the event of failure of a primary line (eg. of 90mm OD).

These lines or tethers are anchored at strengthened end points (33, 34, 35) located at corners of the ends of the elongate tank 31 as shqwn in Figure 4.

Point 34 is at or near the lowest point on the vertical diagonal, and points 33 and 35 are spaced apart at opposite ends of the horizontal diagonal on the square ends of the tank.

The tether anchoring system is engineered to allow removal and replacement of individual tethers in the event of line failure.

The riser support arch provides support to fourteen risers (eg. 36), each riser being located in a separate riser tray 37 fixed to the tank (Figure 5). The risers are arranged at a 2m spacing, and transverse stiffness are disposed within the riser support tray at the same spacing.

The riser support tray is configured to carry the risers 36 over the tank 31 in an easy curve.

The tank 31 could be floated to site using the caisson 32 as a transportation raft, although this might require an excessively wide base.

Alternatively, the tank/caisson can be transported to site on a semisubmersible heavy lift vessel such as the Super Servant (Figures 7, 8 and 9).

As shown in Figure 7, the semisubmersible heavy lift vessel has a buoyant water piercing forecastle 41, and two buoyant water piercing towers 42 and 43 at its aft extremity. The vessel has a submersible carrier deck 44.

The tank 31 and caisson 32 can be floated together onto the deck 44 when that deck is submerged, and the vessel can then be deballasted so that the deck 44 is raised above the water level. In this configuration the tank/caisson can be sea fastened to the deck for transportation to site.

A spreader beam 46 (for use during the installation) can be carried to site on top of the tank 31. In this case temporary supports 45 can be added to hold the spreader beam in place during transportation.

Nine successive stages in the installation of the riser support arch are shown in Figure 10.

At stage 1 the semisubmersible heavy lift vessel is deballasted for removal of the sea fastenings between the tank/caisson and the vessel. The vessel is then ballasted down to leave the--tank 31 floating on the caisson 32, shown-as Stage 2. In Stage 3 a crane (not shown) raises the spreader beam 46 to support the tank on rigging lines 47. The base of the caisson 32 is then partially flooded (Stage 4) until the tank 31 is buoyant, so that sea fastenings 48 between tank 31 and caisson 32 can be removed (Stage 5).

After severing the temporary connections between tank and caisson, the caisson base is then flooded and lowered to around 60 metres, corresponding with the length of the tether lines, (Stage 6). With the weight of the caisson 32 on the rigging lines 47, the spreader beam 46 is lowered by the crane. Further ballasting of around 800 tonnes -is required (Stage 7) to lower the riser support arch below the waterline and down towards the seabed. Crane hook loads (Static) should not exceed 500 tonnes. Lowering is achieved by flooding previously buoyant caisson cells to ensure that no part of the base structure is subjected to external hydrostatic pressure loads.

The caisson 32 is set on to the seabed (Stage 8). This method ensures that installation loads are carried on the robust concrete base structure rather than on the more slender steel tank. Following set-down on the seabed (Stage 9) the spreader beam and lowering slings can be removed by ROV disconnection or by using explosive bolts.

While the specific embodiment of this invention described above has featured an elongate riser support arch, the invention includes the installation of any midwater support structure which is tethered to the seabed by two separate lines or tethers.

Claims

1. A method of installing a midwater support structure comprising a base and a buoyant tank joined at or near corresponding spaced points by at least two tension members, such method including the steps of placing the tank on top of the base in aligned relationship, transporting the tank and base to an offshore site at which the subsea support is required, lowering the base, and drawing the tank down with the base so that the tank is held by the tension members in a midwater position above the base.

2. A method of installing a subsea support arch comprising an elongate base and an elongate buoyant tank joined at or near their corresponding ends by tension members, such method including the steps of placing the tank on top of the base in aligned relationship, transporting the tank and base to an offshore site at which the subsea support is required, lowering the base, and drawing the tank down with the base so that the tank is held by the tension members in a midwater position above the base.

3. A method as claimed in Claim 1 or Claim 2 in which the tank is drawn down towards the base by the tension members that will subsequently hold the tank in the midwater position.

4. A method as claimed in any one of the preceding claims in which the tank and base are floated to the offshore site on a vessel.

5. A method as claimed in Claim 4 in which the vessel can be submerged from beneath the tank to leave the tank either floating, or partially floating and partially suspended from the vessel.

6. A method as claimed in Claim 4 or Claim 5 in which the tank and base are lifted off the vessel using a spreader beam arrangement having sheeves reeved to carry lowering cables or the like.

7. A method as claimed in Claim 6 in which the spreader beam arrangement is adapted to provide guidance for and/or constraint to the tension members such that they can be lowered towards the seabed in alignment with the tank and base.

8. A method as claimed in any one of the preceding claims in which the base is floodable for lowering to the seabed.

9. A method substantially as hereinbefore described with reference to Figures 2 to 10 of the accompanying drawings.

10. A subsea riser support arch when installed in accordance with the method of any one of the preceding claims 2 to 9.

11. A midwater support structure when installed in accordance with the method as claimed in Claim 1.