GB2274476A

GB2274476A - Floating platform for offshore hydrocarbon production

Info

Publication number: GB2274476A
Application number: GB9401142A
Authority: GB
Inventors: John William Waddell
Original assignee: Kvaerner Earl and Wright
Current assignee: Kvaerner Earl and Wright
Priority date: 1993-01-22
Filing date: 1994-01-21
Publication date: 1994-07-27
Anticipated expiration: 2014-01-21
Also published as: GB2274476B; GB9401142D0

Abstract

A floating platform (10) comprises a vessel (11, 12) having a deck or topsides portion (17) mounted above the highest expected water level at the site where the platform is to be located, and a tension moored wellhead/manifold assembly (21) secured to the seabed by at least two tethers (22) against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed, the arrangement being such that the vessel can be positioned directly over the tension moored wellhead/manifold assembly (21) and can draw oil and/or gas from risers and/or conductors (24) terminating in that assembly for processing and export via the deck or topsides. The vessel may be fixed in position by catenary anchors 19 or tension legs (not shown). <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. Such platforms have been restrained at sites for offshore developments in two principal modes.

In one mode (known in the offshore industry as a Catenary Anchored Floater or CAF), mooring lines are fixed to the vessel and are laid out to hang in catenaries around the location of the platform, and the position of the platform is controlled by heaving in or paying out the mooring lines.

In a second mode (known in the offshore industry as a Tension Leg Platform or TLP), vertical tethers are disposed below the hull of the platform to constrain the platform by holding it down against the action of its own buoyancy.

Floating platforms restrained in the modes described above 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). Thai paper featured, as a preferred concept, a deep draft TLP 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.

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 - neither of the above described modes of restraint may be applicable. The catenary anchored floater can be moved, but will have to disconnect from and lose it's well strings; and the tension leg platform may not be able to move far enough laterally to avoid the iceberg.

One proposal for moving a TLP to avoid an iceberg is shown in U.S.

Patent No. 4,423,985 (Aagaard). In this case 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 tension moored wellhead/manifold assembly secured to the seabed by at least two tethers against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed, the arrangement being such that the vessel can be positioned directly over the tension moored wellhead/manifold 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 well head/manifold assembly, 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 locate the tension moored well head/manifold assembly.

When the floating platform is in the form of a TLP, there is locating means to hold the tethers of the TLP in spaced apart relationship with each other and with the means to secure the tension moored wellhead/manifold assembly to the seabed.

When the floating platform is in form of a CAF, the subsea wellhead/manifold assembly is in separable upper and lower parts, such that the upper part can be raised into engagement with the underside of the deck.

The invention provides a method of installing a floating platform as defined above, and comprising the steps of installing the tension moored welihead/manifold assembly, positioning the vessel over the wellhead/manifold assembly, and ballasting the vessel into engagement with the wellhead/manifold assembly, or lifting part of that assembly into engagement with the under side of the deck.

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

The invention further includes a method of disengaging a floating platform installed in accordance with the method defined above, comprising the steps of ballasting the vessel down, disconnecting the risers at the wellhead manifold assembly, deballasting the vessel clear of the wellhead/manifold assembly, 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 diagrammatic side view of a catenary anchored floating platform; Figure 2 is a cross section on the line A-A in Figure 1; Figure 3 is a diagrammatic side view of a tension leg platform; Figure 4 is a cross section on the line A-A in Figure 3; As shown in Figure 1, a buoyant platform comprises a Catenary Anchored Floater (CAF) 10 having four corner columns 11 spaced apart at their lower ends by pontoons 12. The columns 11 and pontoons 12 comprise a buoyant assembly which may be referred to as the "hull" (or vessel) of the CAF 10.

A deck 17 (supporting topsides) is mounted on the columns 11, and the deck may carry a drilling derrick 18.

The CAF hull and deck are held in place by corner mooring lines 19 which hang in the water as catenaries, and which are secured to anchors 20 on the seabed in a manner known per se. For simplicity, only one mooring line 19 is shown at each corner. In practice at least two and probably three independent mooring lines would hang down from each corner of the hull to anchors on the seabed.

The foregoing description of the CAF 10 deals with a generally conventional arrangement.

If the CAF 10 is to be used at a location where there are drifting icebergs, it will be necessary to move the hull and deck out of the expected path of an iceberg, 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 wellhead/manifold assembly 21, held below the water surface at a predetermined height above the seabed against its own buoyancy by well head/manifold tethers 22, so forming a sub-surface wellhead/manifold TLP. Conductors 24 from a seabed drilling/production template (not shown) lead up to the underside of the assembly 21.

The subsea wellhead/manifold assembly 21 is split into two parts. A lower part 25 is connected to the tethers 22. Thus the lower part 25 can remain at a fixed height above the seabed (held down by the tethers 22) while the CAF 10 is free to make limited vertical (heave) movements in response to waves and tides. An upper part 27 is adapted to rest upon the lower part 25 (as shown in dotted lines), and is sized so that it can be lifted off that lower part, and drawn up towards the underside of the deck 17 of the CAF (as shown in full lines). Flexible riser lengths 28 extend from the lower part to the upper part.

When the CAF has located over the subsea wellhead/manifold TLP assembly 21, the upper part 27 can be hoisted off the lower part 25. The risers on the upper part 27 can then be coupled up to the processing facilities on the deck 17 of the CAF. The CAF 10 would be able to carry out limited heave motions, while the lower part 25 of the subsea well head/manifold TLP was restrained by the tethers 24.

In the event of a prediction that an iceberg will approach the moored platform on a collision course, the columns 11 are ballasted down and the upper part 27 of the wellhead/manifold assembly 21 is disconnected from the deck. The upper part 27 is then lowered on to the lower part 25. The mooring lines 19 are veered (payed out) or released from the columns 11, and the hull of the CAF 10 is then deballasted and floated away from the path of the iceberg.

The conductors 24 remain connected to the well head/manifold assembly 21 below the water surface.

When the danger has passed, the CAF 10 can return to the site. It can then deploy its mooring lines 19 and relocate over the well head/manifold assembly 21 by ballasting down onto that assembly.

The foregoing specific embodiment of the invention has been described with reference to a Catenary Anchored Floater (CAF) but the invention may be applied to any floating vessel having provision for substantial ballasting/deballasting, and capable of engaging risers on a subsea TLP.

Another embodiment of the invention will now be described in respect of a primary tension leg platform or TLP.

As shown in Figure 3, a tethered buoyant platform comprises a primary Tension Leg Platform (TLP) 30 having four corner columns 31 spaced apart by upper and lower horizontal members 32 and 34 respectively. The members 32 and 34 are braced by diagonal members 35 (in vertical planes) and by plan bracing 36 (in a horizontal plane). The columns 31, horizontal members 32 and 34, diagonal members 35 and plan bracing 36 comprise a buoyant assembly which may be referred to as the "hull" (or vessel) of the primary TLP 30.

A deck 37 (supporting topsides) is mounted on the columns 31, and the deck may carry a drilling derrick 38.

The TLP hull and deck are held in place by four vertical corner tethers 39 secured to foundation units 40 on the seabed in a manner known per se.

The tethers 39 are releaseably secured at their top ends to housings in the columns 31, and hold the hull down against its own buoyancy. For simplicity, only one tether 39 is shown at each corner. In practice probably three or four independent tethers would extend vertically downward from each corner of the hull to foundation units on the sea bed.

The foregoing description of the primary TLP 30 deals with a generally conventional arrangement.

If the primary TLP 30 is to be used at a location where there are drifting icebergs, it will be necessary to move the hull and deck out of the expected path of an iceberg, without disturbing subsea wells and risers used for the production of oil and/or gas.

Following the invention, there is a tension moored wellhead/manifold assembly 41, held below the water surface at a predetermined height above the seabed against its own buoyancy by well head/manifold tethers 42, so forming a sub-surface wellhead/manifold TLP.

The assembly 41 is configured to engage framing 43 arranged within the plan bracing 36 at or near the point of greatest draught of the hull of the primary TLP. Conductors 44, from a seabed drilling/production template (not shown), are connected in the assembly 41 to risers 45, which extend up to production facilities (also not shown) in the topsides.

In the event of a prediction that an iceberg will approach the tethered platform on a collision course, the columns 31 are ballasted down so that the primary tethers 39 are unloaded, the risers 45 are disconnected from the wellhead/manifold assembly 41, the tethers 39 are disengaged from the columns 31, and the hull of the primary TLP is deballasted and floated away from the path of the iceberg.

The conductors 44 remain connected to the wellhead/manifold assembly 41 below the water surface.

When the danger has passed, the primary TLP can return to the site. It can then pick up its own tethers 39 and also the wellhead/manifold assembly 41 by ballasting down onto them.

During the time that the primary TLP 30 is away from the site, the upper ends of the tethers 39 can be supported by a lightweight buoyant locating frame (not shown). This frame will maintain the tethers generally vertical and in a spaced apart relationship with each other and with the wellhead/manifold assembly 41 so that re-engagement will be a simple operation when the primary TLP 30 returns to site.

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

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 tension moored wellhead/manifold assembly secured to the seabed by at least two tethers against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed, the arrangement being such that the vessel can be positioned directly over the tension moored wellhead/manifold 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 assembly, 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 locate the tension moored well head/manifold assembly.

4. A floating platform as claimed in any one of the preceding claims and in the form of a TLP, in which there is locating means to hold the tethers of the TLP in spaced apart relationship with each other and with the means to secure the tension moored wellhead/manifold assembly to the seabed.

5. A floating platform as claimed in any one of the preceding claims 1 to 4 and in the form of a CAF, and in which the subsea wellhead/manifold assembly is in separable upper and lower parts, such that the upper part can be raised into engagement with the underside of the deck.

6. 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 assembly, positioning the vessel over the wellhead/ manifold assembly, and ballasting the vessel into engagement with the well head/manifold assembly, or lifting part of that assembly into engagement with the under side of the deck.

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

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

9. A method of installing and disengaging a floating platform substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

9. A floating platform substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

10. A method of installing and disengaging a floating platform substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

Amendments to the claims have been filed as follows 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 plan bracing at or near its point of greatest draught, and a tension moored wellhead/manifold assembly secured to the seabed by at least two tethers against the action of its own buoyancy and floating below the water surface at a predetermined height above the seabed, in which the plan bracing incorporates means to locate the tension moored wellhead/manifold assembly, the arrangement being such that the vessel can be positioned directly over the tension moored wellhead/manifold 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.

3. A floating platform as claimed in Claim 1 or Claim 2 and in the form of a TLP, in which there is locating means to hold the tethers of the TLP in spaced apart relationship with each other and with the means to secure the tension moored wellhead/manifold assembly to the seabed.

4. A floating platform as claimed in Claim 1 or Claim 2 and in the form of a CAF, and in which the subsea wellhead/manifold assembly is in separable upper and lower parts, such that the upper part can be raised into engagement with the underside of the deck.

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 assembly, positioning the vessel over the wellhead/ manifold assembly, and ballasting the vessel into engagement with the wellhead/manifold assembly, or lifting part of that assembly into engagement with the under side of the deck.

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 assembly, and after the vessel has been ballasted on to the wellhead/manifold assembly, 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 at the well head manifold assembly, deballasting the vessel clear of the wellhead/manifold assembly, 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 Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.