GB2351724A

GB2351724A - Offshore oil loading

Info

Publication number: GB2351724A
Application number: GB0012707A
Authority: GB
Inventors: Peter Eric Baylis Duncan; William R H Clifford
Original assignee: Kvaerner Oil and Gas Field Development
Current assignee: Kvaerner Oil and Gas Field Development
Priority date: 1999-05-24
Filing date: 2000-05-24
Publication date: 2001-01-10
Also published as: GB0012707D0; GB9912064D0

Abstract

A loading system for transferring oil from a seabed well or manifold to a tanker (11), and comprising a Floating Production Storage Offloading (FPSO) vessel (1) moored in deep water on a generally constant heading above a seabed wellhead or manifold, a buoyant Tanker Loading Unit (TLU) (7) spaced from the FPSO by a horizontal distance comparable to (e.g. half to twice) the depth of water beneath the FPSO, a combined flowline and mooring line (9) connecting the TLU to the FPSO, and a taut leg mooring (8) extending from the TLU to the seabed on the side of the TLU away from the FPSO, in which the TLU is secured in position by its own buoyancy, by tension in the combined flowline and mooring line which exerts a pull on the TLU in one direction (towards the FPSO) and by tension in the taut leg mooring which exerts a pull on the TLU in the opposite direction, whereby to position the TLU so that a tanker of opportunity (11) can draw oil off the FPSO via the TLU without fouling the FPSO or the combined flowline and mooring line. Preferably, the combined flow line and mooring line (9 ) has a plurality of spaced floats (10) and its tension is such as to avoid the line (9) floating on the water surface. A plurality of taut lines (8) can be provided. The FPSO can be moored by catenaries and/or taut leg moorings.

Description

2351724 LOADING SYSTEM - The invention relates to a loading system for

transferring oil from a seabed well or manifold to a tanker of opportunity.

In particular, the invention relates to a loading system for transferring oil from a seabed well in deep water (e.g. more than 2,000ft) to a tanker of opportunity. in this context, a 'tanker of opportunity' means a vessel without specific and sophisticated provisions for loading oil.

Heretofore it has been known to transfer oil from subsea oilfields to offtake vessels by several different techniques.

In one technique, a specially designed offtake vessel lifts a subsea buoy into a downwardly facing receptacle in the bottom of its hull near to its bow. The offtake vessel can then receive oil directly from a riser connected to the subsea buoy. This technique is illustrated in PCT Application No W093/11030. This requires at least one dedicated offtake vessel equipped to draw the subsea buoy up into its hull.

In another technique, a Floating Production Storage and Offloading (FPSO) vessel produces oil on a pivoting mooring (W085/03048). The FPSO vessel automatically takes up a position head to wind. A specifically designed offloading vessel (or shuttle tanker) can take up position astern of the FPSO vessel, In this position the shuttle tanker can be loaded by hose from the FPSO vessel. This requires a sophisticated pivoting mooring to draw oil from the seabed, and one or more shuttle tankers to transport oil away from the FPSO vessel.

In yet another technique, a Floating Production and Storage (FPS) vessel is arranged to transfer oil to a nearby loading buoy. The loading buoy is independently moored. The loading buoy has a pivotal loading arm so that tankers can moor up to the buoy, and take oil from the buoy. The Shell Brent Spar loading arrangement operated safely and effectively for many years in this mode in the northern North Sea. This technique is suitable for relatively shallow water, but mooring of the loading arrangement becomes more difficult in deep water.

The invention is concerned with a loading system for transferring oil from a seabed well or manifold in deep water to a tanker of opportunity.

The invention provides a loading system for transferring oil from a seabed well or manifold to a tanker, and comprising a Floating Production Storage Offloading (FPSO) vessel moored in deep water on a generally constant heading above a seabed wellhead or manifold, a buoyant Tanker Loading Unit (TLU) spaced from the FPSO by a horizontal distance comparable to (e.g. half to twice) the depth of water beneath the FPSO, a combined flowline and mooring line connecting the TLU to the FPSO, and a taut leg mooring extending from the TLU to the seabed on the side of the TLU away from the FPSO, in which the TLU is secured in position by its own buoyancy, by tension in the combined flowline and mooring line which exerts a pull on the TLU in one direction (towards the FPSO) and by tension in the taut leg mooring which exerts a pull on the TLU in the opposite direction, whereby to position the TLU so that a tanker of opportunity can draw oil off the FPSO via the TLU without fouling the FPSO or the combined flowline and mooring line.

Preferably the FPSO is ship shaped.

It is preferred that the combined flowline and mooring line incorporates sufficient buoyancy to relieve the tension in the combined line without causing the combined line to float on the sea surface.

It is further preferred that the buoyancy is spread along the combined line.

It is still further preferred that the buoyancy is spread along the combined line in discrete units.

The FPSO may be moored on an array of taut mooring lines, or an array of catenary moorings, or on a combination of taut and catenary moorings.

It is preferred that the taut leg mooring extending from the TLU to the seabed comprises an array of individual mooring lines.

It is also preferred that oil is led from the seabed to the FPSO up a rigid riser from the seabed to a subsea buoy, and along a flexible riser from the subsea buoy to the FPSO.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing, which is a sectional view through seawater above a subsea oilfield.

The specific description includes a discussion of the problems to be faced and considerations to be evaluated, and a brief description of some of the analytical techniques to be used in evaluation of the alternatives.

In developing a concept for a remote loading facility, important variables include:- station keeping of the FPSO and the Tanker Loading Unit (TLU); compliance required in the flowline; and routing of the flowline. Considerations relating to these variables are outlined below.

Station keeping of the FPSO depends in part on the configuration of its mooring spread - catenary or taut - and on the materials to be used chain, wire, or synthetic fibre ropes (nylon, polyamide, polyester, polyethylene, etc).

For the TLU, mooring possibilities include a spread mooring (with the variations noted above), and/or a vertical tether.

Flexibility of the flowline (and its support) is required in order to avoid unsustainable internal loads being generated by environmental loading on the flowline itself, and by relative motions of the FPSO and TLU. Resistance to fatigue damage is also of great importance.

There are four possibilities for routing the flowline. These are:- down to the seabed and back up again; submerged in mid water; on the surface; and above the surface. Each of these has its advantages and drawbacks. For instance, the first possibility removes the problem of relative motion between the FPSO and the TLU. However, it would be difficult (and therefore costly) to install, inspect and maintain. The simplest possibility - having the flowline at the water surface - gives the shortest route, but has possible safety problems. Routing the flowline above the surface involves further safety problems.

Accordingly, as shown in the accompanying drawing, the specific embodiment of the invention features a submerged midwater flowline route.

The system of the specific embodiment comprises an FPSO (1) with a spread mooring (2). This mooring may be either taut leg or catenary. Rigid risers (3) run up from the seabed (4) to a sub-surface buoy (5). Flexible risers (6) then run up to the FPSO (1). A TLU (7) floats some distance from the FIDS0 (1). The TLU is held in position by taut mooring lines (8). These mooring lines are deployed to counter the horizontal load from a flowline (9), and to maintain the position of the TLU in a direction perpendicular to the flowline (9).

The term 'flowline' is used here to describe the entire component (9) which connects the FPSO (1) to the TLU (7). This component includes a tension member and buoyancy units, in addition to piping for oil.

More specifically, the flowline (9) connecting the TLU to the FPSO includes a tension member (either integral with the pipe or as g separate item), together with distributed buoyancy units (10). The purpose of the buoyancy units is to reduce the effective weight of the flow)ine, and thus the tensile load.

The technical issues include basic sizing of the components (using a relatively straightforward analysis of the behaviour of the system under quasi-static loads); and then a more complex analysis of the system dynamics.

Potential effects influencing the dynamic response include excitation of the mooring line (218) andlor flowline (9) dynamics due to excitation at wave frequencies. There will be a response of the FPSO and the TLU to slowly varying wave drift loading, and it is possible this will give rise to large changes in flowline tension. Vortex-induced vibration (VIV) may be experienced by the mooring lines (8) or the flowline (9).

In use, oil flows up risers (3) and (6) to the FPSO (1). Primary production takes place on the FPSO to produce exportable crude oil. This oil is stored in the FPSO. From time to time, a tanker of opportunity (11) takes oil from the FPSO. The oil passes from storage on the FPSO, through flowfine (9) and TLU (7), and thence to the tanker of opportunity through conventional pipework. The tanker of opportunity (11) can then proceed on its way with a full or part load of crude oil.

Claims

Claims

1/ A loading system for transferring oil from a seabed well or manifold to a tanker, and comprising 5 a Floating Production Storage Offloading (FPSO) vessel moored in deep water on a generally constant heading above a seabed wellhead or manifold, a buoyant Tanker Loading Unit (TLU) spaced from the FPSO by a horizontal distance comparable to the depth of water beneath the FPSO, a combined flowline and mooring line connecting the TLU to the FPSO, and a taut leg mooring extending from the TLU to the seabed on the side of the TLU away from the FPSO, in which the TLU is secured in position by its own buoyancy, by tension in the combined flowline and mooring line which exerts a pull on the TLU in one direction (towards the FPSO) and by tension in the taut leg mooring which exerts a pull on the TLU in the opposite direction, whereby to position the TLU so that a tanker of opportunity can draw oil off the FPSO via the TLU without fouling the FPSO or the combined flowline and mooring line.

2/ A system as claimed in claim 1, in which the FPSO is ship shaped.

3/ A system as claimed in claim 1 or claim 2, in which the combined flowline and mooring line incorporates sufficient buoyancy to relieve the tension in the combined line without causing the combined line to float on the sea surface.

4/ A system as claimed in claim 3, in which the buoyancy is spread along the combined line.

5/ A system as claimed in claim 4, in which the buoyancy is spread along the combined line in discrete units.

6/ A system as claimed in any one of the preceding claims, in which the FPSO is moored on an array of taut mooring lines.

7/ A system as claimed in any one of claims 1 to 5, in which the FPSO is moored on an array of catenary moorings.

8/ A system as claimed in any one of claims 1 to 5, in which the /FPSO is moored on a 5 combination of taut and catenary moorings.

9/ A system as claimed in any one of the preceding claims, in which the taut leg mooring extending from the TLU to the seabed comprises an array of individual mooring lines.

10/ A system as claimed in any one of the preceding claims, in which oil is led from the seabed to the FPSO up a rigid riser from the seabed to a subsea buoy, and along a flexible riser from the subsea buoy to the FPSO.

11/ A system substantially as hereinbefore described by way of example with reference to 15 and as shown in the accompanying drawing.