GB2285017A - Connection of risers to floating platforms - Google Patents

Abstract

A floating platform comprises a vessel 50 having a deck or topsides portion mounted above the highest expected water level at the site where the platform is to be located, and a two part tension moored wellhead/manifold having a lower part 61 secured to the seabed against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed and a liftable connection assembly 63, adapted to rest upon the lower part; the arrangement being such that the vessel can be positioned in engaging relationship over the liftable connection assembly and can draw oil and/or gas from risers 64 and/or conductors terminating in that assembly for processing and export via the deck or topsides aforesaid. The liftable assembly may be connected via mooring lines to midwater structures 52. In this arrangement the platform itself may be held in partition by the lines 59 A, B via the assembly 63. The platform may be installed by positioning it over the assembly 63 and ballasting into engagement therewith. The arrangement facilitates temporary removal of the platform eg to avoid iceberg danger. <IMAGE>

Description

FLOATING PLATFORM The invention relates to a floating platform for the production of oil and/or gas at an offshore site, and to methods of installing, and removing and reinstalling, such a platform.

It has been proposed to use floating platforms for the production of oil and/or gas from deepwater sites, where fixed platforms would be impractical; and from sites in shallower water, where small reservoirs of oil and/or gas would make fixed platforms uneconomical. Floating platforms have been formed of a hull'or vessel with a deck or topsides portion mounted above the level of the highest expected wave. Well strings have been connected between subsea wells on the seabed and production facilities on the deck of the hull or vessel.

Floating platforms have been suggested for deployment in Arctic waters where icebergs or ice flows may threaten sites for offshore developments.

Several concepts have been proposed to resist the effects of ice impact on floating platforms. Some of these concepts have been described in OTC Paper 6596 (presented at the 23rd OTC at Houston Texas in 1991). That paper featured as a preferred concept a deep draft floating platform with four corner columns and one central riser protection column. The corner columns were protected by cable type fendering units. However, at sites where large icebergs or extensive ice flows are anticipated, it may not be possible to maintain a floating platform on site by the use of protective fendering, as this may be inadequate to prevent damage from floating ice, or indeed to prevent the platform from being pushed sideways off its site.

In circumstances where it may be desirable to move the floating platform temporarily - for example to avoid an iceberg or ice flow drifting towards the platform - the hull or vessel will have to disconnect from and lose it's well strings.

One proposal for moving a hull or vessel (in this case a TLP) to avoid an iceberg is shown in U.S. Patent No. 4,423,985 (Aagaard). In this proposal the tendons are untensioned, and flexible lines between the structure and the tendons are then gradually veered (payed out) to give the hull of the TLP freedom for limited lateral movement. This would involve disconnection from oil/gas risers extending up from the seabed. Moreover, the ability of the TLP to avoid ice flows would be limited by the lengths and weights of the flexible lines which could in practice be carried by the TLP.

There is a requirement for floating platforms to be able to move away from subsea oil/gas wells (e.g. when threatened by drifting icebergs), while leaving those wells readily available for reconnection and resumption of production when the platform returns.

The invention provides a floating platform comprising a vessel having a deck or topsides portion mounted above the highest expected water level at the site where the platform is to be located, and a two part tension moored wellhead/manifold having a lower part secured to the seabed against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed and a liftable connection assembly adapted to rest upon the lower part; the arrangement being such that the vessel can be positioned in engaging relationship over the liftable connection assembly and can draw oil and/or gas from risers and/or conductors terminating in that assembly for processing and export via the deck or topsides aforesaid.

It is preferred that there is means to disengage and reengage the vessel from and with the tension moored wellhead/manifold, and to disengage and reengage the risers from and to the deck or topsides aforesaid.

It is also preferred that the vessel has plan bracing at or near its point of greatest draught, and that plan bracing incorporates means to engage the liftable connection assembly.

In a particularly advantageous form of the invention the vessel is positioned with a midwater buoy mooring system, and the mooring lines are connected to the liftable connection assembly that can either, in an upper position, engage the vessel or, in a lower position, rest on the lower part of the subsea wellhead/manifold, so to release the vessel.

The invention also provides a method of installing a floating platform as described above comprising the steps of installing the tension moored wellhead/manifold, positioning the vessel over the wellhead/ manifold, and ballasting the vessel into engagement with the wellhead/manifold.

It is preferred that oil and/or gas wells have been predrilled through the tension moored wellhead/manifold, and after the vessel has been ballasted on to the wellhead/manifold, the predrilled wells are tied back to the platform.

The invention further provides a method of disengaging a floating platform installed in accordance with the method of installation described above comprising the steps of ballasting the vessel down, disconnecting the risers of the wellhead manifold, deballasting the vessel clear of the wellhead/manifold, and then floating the vessel laterally away from the site.

Two specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a floating platform; and Figure 2 is a view similar to Figure 1 but showing a variant of the floating platform's mooring system.

As shown in Figure 1, a floating platform 50 has corner columns 51 spaced apart by peripheral pontoons 54 and a deck 57. The columns 51 and pontoons 54 comprise a buoyant assembly which may be referred to as the "hull" (or vessel) of the floating platform 50. (In one form the floating platform may have a catamaran type hull.) The hull and deck of the floating platform are held in place by corner mooring lines 59, which are secured to anchors 60 on the seabed in a manner known per se. For simplicity, only one mooring line is shown at each corner. In practice at least two independent mooring lines would hang down from each corner of the hull to anchors on the seabed.

The foregoing description of the floating platform 50 deals with a generally conventional arrangement.

If the floating platform 50 is to be used at a location where there are drifting icebergs or iceflows, it will be necessary to move the hull and deck out of the expected path of an iceberg or iceflow, without disturbing subsea wells and risers used for the production of oil and/or gas.

Following the invention, there is a two-part tension moored well head/manifold. This includes a lower part 61, which is held below the water surface at a predetermined height above the seabed against its own buoyancy by wellhead/manifold tethers 62, so forming part of a sub-surface well head/manifold TLP.

The lower part 61 of the wellhead/manifold can support a liftable connection assembly 63 configured to engage framing within the pontoons 54 at or near the point of greatest draught of the hull of the floating platform 50. Tapered guides may be used to ensure accurate alignment and then close engagement of the assembly 63 and the framing within the pontoons. (If the floating platform has a catamaran type hull, the engagement can be between the hulls at one end of the platform.) Conductors 64, from a seabed drilling/production template (not shown), are connected to the underside of the lower part of the wellhead/manifold.

Flexible risers 65 extend between the lower part 61 of the wellhead/manifold and the underside of the liftable connection assembly 63, and thence can be connected up by means outwith the scope of the present invention to production facilities (also not shown) in the topsides.

Figure 2 shows a particularly advantageous variation of the mooring arrangement of the embodiment shown in Figure 1, in which the hull and deck are held in place at an offshore site by mooring lines 59A/59B. These lines hang in the water as catenaries 59A between the floating platform 50 and midwater buoys 52, and as catenaries 59B between the midwater buoys 52 and anchors 60 on the seabed. For simplicity, only one mooring line 59A/59B is shown at each corner. In practice at least two independent mooring lines would hang down from each corner of the hull to anchors on the seabed.

With this variation, the mooring lines 59A which hold the floating platform 50 in place at the offshore site are connected to the corners of the liftable connection assembly 63. The floating platform 50 is thus held in position by the action of the mooring lines 59A on the liftable connection assembly 63, which is itself engaged within a 'moonpool' between the peripheral pontoons 51 of the platform.

In the event of a prediction that an iceberg will approach the floating platforms of Figures 1 and 2 on a collision course, the liftable connection assembly 63 is disconnected from the moonpool, and lowered onto the top of the lower part 61 of the subsea wellhead/manifold. The mooring lines 59 would be released (or the mooring lines 59A would be lowered) and the flexible risers 65 would adopt the positions shown in broken lines. The floating platform 50 would now be free to be deballasted and floated clear of the path of the iceberg.

The conductors 64 would remain connected to the lower part 61 of the well head/manifold below the water surface.

In accordance with the variation shown in Figure 2, the midwater buoys 52A remain connected to the liftable connection assembly 63 (as shown in broken lines) which rests on top of the lower part 61 of the wellhead/manifold. If advantageous the assembly 63 can be supported by the mid-water buoys 52, or by combinations of both the lower part of the well head/manifold and the midwater buoys 52.

When the danger has passed, the floating platform 50 can return to the site. Using marker buoys or transponders it can be positioned over the subsurface wellhead/manifold TLP. It can then pick up the liftable connection assembly 63, so that it is connected both to the mooring lines 59 (or, if applicable, to the mooring lines 59A) and to the flexible risers 65.

The specific embodiments of the invention have been described in relation to moving floating platforms out of the paths of icebergs. It will be appreciated that the invention may have utility in any circumstance in which floating platforms over subsea wells may be required to move off those wells, and then to return and reengage the wells later.

Claims (9)

1. A floating platform comprising a vessel having a deck or topsides portion mounted above the highest expected water level at the site where the platform is to be located, and a two part tension moored wellhead/manifold having a lower part secured to the seabed against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed and a liftable connection assembly adapted to rest upon the lower part; the arrangement being such that the vessel can be positioned in engaging relationship over the liftable connection assembly and can draw oil and/or gas from risers and/or conductors terminating in that assembly for processing and export via the deck or topsides aforesaid.

2. A floating platform as claimed in Claim 1 in which there is means to disengage and reengage the vessel from and with the tension moored well head/manifold, and to disengage and reengage the risers from and to the deck or topsides aforesaid.

3. A floating platform as claimed in claim 1 or claim 2 in which the vessel has plan bracing at or near its point of greatest draught. and that plan bracing incorporates means to engage the liftable connection assembly.

4. A floating platform as claimed in any one of the preceding claims in which the vessel is positioned with a midwater buoy mooring system, and in which the mooring lines are connected to the liftable connection assembly that can either, in an upper position, engage the vessel or, in a lower position, rest on the lower part of the subsea wellhead/manifold, so to release the vessel.

5. A method of installing a floating platform as claimed in any one of the preceding claims comprising the steps of installing the tension moored wellhead/manifold, positioning the vessel over the wellhead/ manifold, and ballasting the vessel into engagement with the wellhead/manifold.

6. A method of installing a floating platform as claimed in Claim 5, in which oil and/or gas wells have been predrilled through the tension moored wellhead/manifold, and after the vessel has been ballasted on to the well head/manifold, the predrilled wells are tied back to the platform.

7. A method of disengaging a floating platform installed in accordance with Claim 5 or Claim 6, comprising the steps of ballasting the vessel down, disconnecting the risers of the wellhead manifold, deballasting the vessel clear of the wellhead/manifold, and then floating the vessel laterally away from the site.

8. A floating platform substantially as hereinbefore described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings.

9. A method of installing and disengaging a floating platform substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.