GB2372965A

GB2372965A - Water intake for a floating offshore platform

Info

Publication number: GB2372965A
Application number: GB0129731A
Authority: GB
Inventors: Donald Ferns
Original assignee: JOHN BROWN HYDROCARBONS Ltd
Current assignee: JOHN BROWN HYDROCARBONS Ltd
Priority date: 2000-12-12
Filing date: 2001-12-12
Publication date: 2002-09-11
Also published as: GB0129731D0; GB0030256D0

Abstract

A water intake arrangement (15, Fig 1) for a floating offshore platform (10, Fig 1) comprises a conduit 23 adapted to occupy a first position in which it does not extend significantly below the maximum draft of the platform Fig 2 and a second position in which an orifice near a free end of the conduit is capable of drawing water from a mid water position generally beneath the platform Fig 3. Means to move the free end of the conduit from the first position to the second position are also provided. The conduit may be housed vertically within the platform and may comprise a single length of pipe, a plurality of pipe lengths arranged telescopically within one another or a plurality of pipe lengths hingedly attached to one another. Alternatively the conduit may be extensible in the manner of a concertina or may be formed from a single length of flexible non-collapsible pipe.

Description

WATER INTAKE The invention relates to a water intake arrangement to allow the uptake of clean seawater for a floating platform at an offshore location.

Offshore oil and gas fields in shallow water depths (e. g. up to 200mw. d. in the North Sea) have been developed using fixed platforms standing on the seabed. Seawater to run services on these fixed platforms has been taken from caissons rigidly connected to the platforms. The caissons have extended down platform legs or down through conductor guide framing to draw water from significant depths below the sea surface. Seawater drawn from these depths is generally uncontaminated by process water or sewage from the platform.

In deeper water depths, many offshore oil and gas fields are currently being exploited using floating platforms. The supporting vessels for these platforms may be monohull, conventional twin hulled semi-submersibles, Tension Leg Platforms (TLPs), or Spars.

Spars extend down for a substantial distance into the water and so can draw water from depths uncontaminated by discharges from the floating platform.

The other types of floating platforms (e. g. monohull, semis and TLP's) have relatively shallow drafts as compared with the water depths in which they may be anchored. Seawater to run the services on these other types of floating platforms is typically drawn from intakes in the sides of the hulls or columns, i. e. from a depth no deeper than the hull draft. At hull draft depths, seawater can be contaminated by activities on the floating platform. Hence it may be desirable to draw water from a mid water position below the platform.

However, if the draft of the vessel's hull were to be increased in order to position a water intake in the side or bottom of the hull at such a mid water position, the vessel would require a very deep draft. This deep draft would preclude movement of the vessel into shallow water at near shore locations, e. g. for maintenance, repair or conversion.

The invention provides a water intake arrangement for a floating offshore platform, comprising a conduit adapted to occupy a first position, in which it does not extend significantly below the maximum draft of the platform, and a second position, in which a free end of the conduit is capable of drawing water from a mid water position generally beneath the platform, and means to move the free end of the conduit from the first position to the second position.

Preferably there is means to retract the conduit from the second position to the first position.

It is preferred that there is means to stabilise the conduit relative to the platform in its second position against the action of environmentally induced motions of the platform.

in one preferred form the conduit consists of a single length of straight pipe forming a caisson which is housed generally vertically within the platform, and there is means to lower the caisson so that the lower end of the caisson is at the required mid water position.

In another form the conduit consists of a plurality of lengths of straight pipe arranged telescopically one within another and housed generally vertically within the platform, and there is means to extend the lengths of pipe downwardly so that the lower end of the lowest pipe is at the required mid water position.

In yet another form the conduit consists of a plurality of straight lengths of pipehingedly connected together to permit water to be drawn between the lengths of pipe from a lower end of the lowest pipe to the platform when the conduit is in its second position, and there is means to let down the lengths of pipe so that the lower end of the lowest pipe is at the required mid water position.

In still yet another form the conduit is extensible in the manner of a concertina, and, in its extended configuration, is of sufficient length to extend down from the platform to the required mid water position, and there is means to extend the conduit to that position.

In a simple form, the conduit may consist of a single length of flexible, non-collapsible pipe which is of sufficient length to extend down from the platform to the required mid water position, and there is means to veer the lower end of that non-collapsible pipe to that mid water position.

The invention includes an offshore platform having a water intake arrangement as described above.

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 a Floating Production Storage & Offloading (FPSO) vessel ; Figure 2 is a diagrammatic transverse cross section through such a vessel with a water intake arrangement housed within the vessel ; and Figure 3 is a corresponding cross section with the intake arrangement extended.

As shown in Figure 1, a monohull Floating Production Storage & Offloading vessel (FPSO) 10 is secured at a deepwater offshore location by a mooring turret 11 and cables 12 leading to seabed anchors (not shown). The vessel and its production facilities constitute a'floating platform'. No claim is made to the particular configuration of the FPSO.

Risers 14 lead up from the seabed into the mooring turret 11. The risers carry oil up from one or more subsea wells (not shown) to the FPSO. The oil is subjected to primary processing, is stored, and then is intermittently removed from the FPSO in a shuttle tanker (not shown).

Processing facilities on the FPSO require clean seawater for services e. g. for cooling purposes.

Following the invention, clean seawater is drawn from a mid water position below the FPSO by a water intake arrangement 15.

Figures 2 and 3 show how one particular form of water intake arrangement is deployed. A hull 20 is pierced by a watertight vertical passage 21. At the foot of the vertical passage 21 there is a caisson retaining ring 22 extending horizontally around the internal surface of the passage. The ring 22 is close to and just above the bottom of the hull 20.

As shown in Figure 2, there is a caisson 23 within the passage 21, and the caisson 23 has a rim 24 at its upper end. The caisson 23 is moveable vertically at will within the passage. In its retracted position, the lower end of the caisson 23 may project slightly below the bottom of the hull 20 locally, but it does not project significantly below the maximum draft of the hull. Thus in its retracted position, the caisson does not significantly increase the draft of the FPSO, so that the FPSO can be moved into shallow water for maintenance, repair or conversion.

When on station at the intended deepwater offshore location, the caisson 23 is deployed to draw seawater from a mid water position below the hull 20.

As shown in Figure 3, the caisson 23 is lowered e. g. by a cable or rack mechanism (not shown), so that its rim 24 rests on the ring 22. The lower end of the caisson 23 is now at a mid water position that is relatively unpolluted by discharges from the FPSO. A submersible pump 25 draws seawater from the upper end of the caisson 23 for use on the FPSO.

The caisson 23 may be stabilised by members 26 extending below the bottom of the hull 20. The members in Figure 3 are disposed beamwise of the hull. Other stabilising members (such as those shown in dotted lines in Figure 1) may be disposed lengthwise of the hull. The stabilising members would be purpose designed for each installation depending upon caisson length, anticipated vessel motions, and where on the hull they could be secured.

As alternatives to the simple lowerable caisson 23 described above, the conduit for clean seawater might comprise two or more telescopic pipes; two or more articulated pipes; a concertina conduit; or merely a flexible pipe which can be veered from the hull. The stabilising members 26 described above might be unnecessary in the latter case. Otherwise, the pipes or conduits could be stabilised by extendable struts, or ties, or a mixture of the two. As with the stabilising members 26, the struts and/or ties would be purpose designed for each installation depending upon caisson length, anticipated vessel motions, and where on the hull they could be secured.

The FPSO 10 would normally remain on station for many months, or even years. In these circumstances the conduit might have no means for retraction, and might be dumped on the seabed when the FPSO was withdrawn.

Claims

n l A s CLAIMS 1/A water intake arrangement for a floating offshore platform, comprising a conduit adapted to occupy a first position, in which it does not extend significantly below the maximum draft of the platform, and a second position, in which an orifice near a free end of the conduit is capable of drawing water from a mid water position generally beneath the platform, and means to move the free end of the conduit from the first position to the second position.

2/An arrangement as claimed in claim 1, in which there is means to retract the free end of the conduit from the second position to the first position 3/An arrangement a claimed in claim 1 or claim 2, in which there is means tostabilise the conduit relative to the platform in its second position against the action of environmentally induced motions of the platform.

4/An arrangement as claimed in any one of the preceding claims, in which the conduit consists of a single length of pipe forming a caisson which is housed generally vertically within the platform, and there is means to lower the caisson so that a lower end of the caisson is at the required mid water position.

5/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a plurality of lengths of straight pipe arranged telescopically one within another and housed generally vertically within the platform, and there is means to extend the lengths of pipe downwardly so that a lower end of the lowest pipe is at the required mid water position.

6/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a plurality of lengths of pipe hingedly connected together to permit water to be drawn between the lengths of pipe from a lower end of the lowest pipe to the platform when the conduit is in Its second position, and there is means to let down the lengths of pipe so that the lower end of the lowest pipe is at the required mid water position.

7/An arrangement as claimed in any one of claims 1 to 3, in which the conduit is extensible in the manner of a concertina, and, in its extended configuration, is of sufficient length to extend down from the platform to the required mid water position, and there is means to extend the conduit to that position.

8/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a single length of flexible, non-collapsible pipe which is of sufficient length to extend down from the platform to the required mid water position, and there is means to veer the lower end of that non-collapsible pipe to that mid water position.

9/An arrangement substantially as hereinbefore described by way of example with reference to and as shown in Figures 2 and 3 of the accompanying drawings.

10/An offshore platform having a water intake arrangement according to any one of the preceding claims.

11/An offshore platform substantially as hereinbefore described by way of example with reference to and as shown in the accompanying drawings, but excluding the particular configuration of the vessel shown in Figure 1.

Amendments to the claims have been riled as follows 1/A water intake arrangement for a permanently positioned floating offshore platform, comprising a conduit adapted to occupy a first position, in which it does not extend significantly below the maximum draft of the platform, and a second position, in which an orifice near a free end of the conduit is capable of drawing water from a mid water location generally beneath the platform, and means to move the free end of the conduit from the first position to the second position; in which there is means comprising ties and/or extendable struts which may be disposed below the maximum draft of the platform tostabilise the conduit lengthwise relative to the platform against the action of environmentally induced motions of the platform when the conduit is in its second position.

2/An arrangement as claimed in claim 1, in which the means comprising ties and/or extendable struts additionally stabilize the conduit beamwise relative to the platform.

3/An arrangement as claimed in claim 1 or claim 2, in which there is means to retract the free end of the conduit from the second position to the first position.

4/An arrangement as claimed in any one of the preceding claims, in which the conduit consists of a single length of pipe forming a caisson which is housed generally vertically within the platform, and there is means to lower the caisson so that a lower end of the caisson is at the required mid water location.

5/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a plurality of lengths of straight pipe arranged telescopically one within another and housed generally vertically within the platform, and there is means to extend the lengths of pipe downwardly so that a lower end of the lowest pipe is at the required mid water location.

6/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a plurality of lengths of pipe hingedly connected together to permit water to be drawn between the lengths of pipe from a lower end of the lowest pipe to the platform when the conduit is in its second position, and there is means to let down the lengths of pipe so that the lower end of the lowest pipe is at the required mid water location

7/An arrangement as claimed in any one of claims 1 to 3, in which the conduit is extensible in the manner of a concertina, and, in its extended configuration, is of sufficient length to extend down from the platform to the required mid water location, and there is means to extend the conduit to that location.

8/An arrangement as claimed in any one of claims 1 to 3, in which the conduit consists of a single length of flexible, non-collapsible pipe which is of sufficient length to extend down from the platform to the required mid water location, and there is means to veer the lower end of that non-collapsible pipe to that mid water location.

9/An arrangement substantially as hereinbefore described by way of example with reference to and as shown in Figures 2 and 3 of the accompanying drawings.

10/A permanently positioned offshore platform having a water intake arrangement according to any one of the preceding claims.

11/A permanently positioned offshore platform substantially as hereinbefore described by way of example with reference to and as shown in the accompanying drawings, but excluding the particular configuration of the vessel shown in Figure 1.