GB2390351A

GB2390351A - Fluid transfer apparatus

Info

Publication number: GB2390351A
Application number: GB0215283A
Authority: GB
Inventors: Bann Jacob De
Original assignee: Bluewater Terminal Systems NV; Bluewater Energy Services BV
Current assignee: Bluewater Energy Services BV
Priority date: 2002-07-02
Filing date: 2002-07-02
Publication date: 2004-01-07
Anticipated expiration: 2022-07-02
Also published as: GB0215283D0; GB2390351B

Abstract

Apparatus is described for transferring fluid between a first floating structure such as a floating production, storage and off-loading facility (FPSO)(1) and a second floating structure such as a CALM buoy (4) to which a tanker (7) may be moored. The apparatus includes at least one rigid underwater flow line (6) connected between the FPSO (1) and a submerged buoyant body (8) of adjustable buoyancy. One or more flexible flowlines (11) are connected between the submerged buoyant body (8) and the CALM buoy (4). The submerged buoyant body (8) is tethered to the seabed (3) by at least one anchor leg (12) which has a length greater than the depth of water in which the apparatus is located. The invention allows the submerged buoyant body (8) to be brought to the surface for inspection and maintenance without being untethered from the seabed. In addition, it allows larger diameter flowlines (6) to be used, increasing the flow rate of fluid and the pumping power required.

Description

239035 1

- 1 - Fluid Transfer Apparatus The present invention relates to apparatus for transferring fluid between two structures at sea, for 5 example between a floating production platform and a floating transport vessel.

In offshore oil production in deepwater, use is very often made of Floating Production, Storage and 10 Offloading facilities (FPSOs) spread moored in the vicinity of subsea oil wells. Especially in large oil fields, the FPSO facility itself is complex and costly

and it is therefore considered a risk to moor a transport tanker side-byside, or in tandem with, an 15 FPSO facility to offload the oil produced for further transport to refineries.

Therefore, an offloading buoy is often installed, such as a Catenary Anchored Leg Mooring (CALM) buoy as 20 is well known in the industry, at some distance away from the FPSO facility. The flow connection between the FPSO facility and the CALM buoy is made by steel flowlines which are suspended from the FPSO facility and the CALM buoy and which have a certain amount of 25 buoyancy at the midspan region.

A drawback with such a system is that the flowline diameter is limited to approximately 405mm to 460mm (16" to 18") diameter, because a CALM buoy will 30 respond to wave activity such that a very significant amount of fatigue damage will develop in these steel flowlines. At larger diameters this phenomena increases and makes the system impractical if a service life of over 10 years is required. In 35 addition, there is a large risk that tankers running up to the CALM buoy can seriously damage the steel flowlines.

2 - Larger diameter flowlines, for example up to 560mm (22") in diameter, permit greater flowrates of crude and consequently shorter tanker loading times and develop less back pressure and hence require less 5 pump horsepower.

Various systems have been developed in an attempt to facilitate the use of larger diameter pipelines.

One such system uses a tethered submerged body to 10 which the steel flowlines are connected and flexible hoses to connect the submerged body to the CALM buoy.

Due to the location of the submerged body below the highest wave energy zone, the fatigue loading on the pipelines is much lower. Additionally the steel 15 flowlines are positioned out of the way from any tanker. However, a drawback of systems using tethered submerged bodies is that divers are needed in order to 20 carry out any necessary inspections or maintenance to the submerged body itself or to the flow lines connecting to it.

Accordingly, the present invention provides 25 apparatus for transferring fluid between first and second floating and moored structures comprising one or more rigid underwater flowlines, each with a first end connected to the first structure and a second end connected to a submerged buoyant body of adjustable 30 buoyancy and one or more flexible flowlines each with a first end connected to the submerged buoyant body and a second end connected to the second structure, wherein the body is tethered to the seabed by at least one anchor leg which has a length greater than the 35 depth of the water.

Preferably the length of the anchor leg is

between 1 01 f the water d b M re Preferably th 5 en 105t and 1l59:i en9th of the a i preferabl y, the i ne9 9eneral l y 9 C mprlSes tw 10 In a further a1t three anchor 1 the anChor leg 120 de r 1nes g eeS to one a SPaced at ap Polyest a1 1y' the or each 15 mbination thereOf or anCh preferably, t 460 ha rlDid Und {1B to 22'. ines are a ln diameter t 20 Preferably' the b ntro1 means perab1 g d body Comprises Yancy of the tank reaSe and decre 25 In particular th and a cOmp nt rO1 means se buoyanc b Y and 1s 0 er Y Y SUPplying as. P able to rease buoyancy by g lntO the tan gaS from the tank 30inventlo r art and an emb di Y' With reference t Y way of FiDure 1 sho 5chains and w. ws an FpSo faci1 reS to the seabe ity anChored y ls knOwn ln the FigUre 2 shO s a d s Ubmerged bOdy

4 - as is known in the art; and Figure 3 shows a fluid transfer system in accordance with one embodiment of the present 5 invention.

Figure 1 shows a known system in which an FPSO facility 1 is shown anchored to the seabed 3, by chains and/or wires 2, using any conventional and 10 appropriate means. Some distance from the FPSO facility 1 a CALM buoy 4 is anchored to the seabed 3, by chains and/or wires 5, using any conventional and appropriate means. One or more catenary flowlines 6 are suspended between the FPSO facility 1 and the CALM 15 buoy 4. The catenary flowlines 6 are equipped with buoyancy aids in their mid region. These buoyancy aids resist the catenary flowlines 6 tendency to form a single curve and try to retain the flowlines in as straight a line as is practicable. A tanker 7 is 20 moored to the CALM buoy 4 for receiving oil from the FPSO facility 1.

Figure 2 shows another known system which is similar to Figure 1 but which includes a submerged 25 body 8 connected to the end of the catenary flowlines 6 and tethered to the seabed 3 by an anchor leg 9 attached to an anchor point 10. Flexible hoses 11 connect the catenary flowlines 6, via the submerged body 8, to a CALM buoy 4. In this system the buoyancy 30 aids attached to the catenary flow lines 6 additionally help to reduce the loads imposed on the submerged body 8.

Figure 3 shows an embodiment of the present 35 invention. An FPSO facility 1 and a CALM buoy 4 are shown anchored to the seabed 3 by chains and/or wires, 2 and 5 respectively, using any conventional and

5 - appropriate means. A tanker 7 for receiving oil from the FPSO facility is shown moored to CALM buoy 4. A submerged body 8 is connected by one set of catenary flowlines 6 to an FPSO facility 1 and by a second set 5 of flexible supply lines 11 to a CALM buoy 4. In this case, the submerged body 8 is tethered to the seabed 3 by an anchor leg 12 having a length greater than the water depth (it should be noted that although Figure 3 shows the anchor leg as being taut, in practice it 10 would assume a catenary curve) It is envisaged that in the apparatus of the present invention the length of the anchor leg is preferably in the range of 101% to 150% of the depth 15 of the water. It is further envisaged that the length of the anchor leg is more preferably in the range of 105% to 115% of the depth of the water.

A specific advantage of the present invention 20 with regard to the prior art is that the submerged

body 8 can be brought to the surface at will without having to be untethered from the seabed 3. This is a useful feature as it allows surface access to the submerged body 8 should inspection or repairs be 25 necessary. Consequently it disposes with the need for deep divers, as was the case with the prior art

systems, both saving money and increasing operational safety. 30 Another advantage of the present invention is that it allows larger catenary flowlines to be used, e.g. 460mm to 560 mm (18" to 22") or more in diameter.

These larger diameter flowlines facilitate reductions in tanker loading times, by permitting greater 35 flowrates of oil, and reduce the necessary pump horsepower by developing less back pressure within the flowlines.

6 - An important feature of the fluid transfer system described by the present invention is that the submerged body 8 supports the catenary flow lines 6 at a depth such that they are both out of the zone of 5 action of the waves and out of the way of passing tankers. It is therefore crucial that the submerged body 8 is always located at the correct depth. In the system known in the prior art the range of depths of

the submerged body 8 is constrained only by the length 10 of the anchor leg 9, the position of the anchoring point 10 on the seabed 3 and the length and positioning of the catenary flowlines 6 and flexible supply lines 11. In the present invention there is no depth constraint placed upon the submerged body 8 by 15 the anchor leg 12. Consequently the system of the present invention relies on changing the buoyancy of the submerged body 8 in order to control its depth.

This buoyancy must be sufficent to support the catenary flowlines 6, the flexible supply lines 11 and 20 the weight of the one or more anchor lines.

In a preferred embodiment of the present invention the buoyancy of the submerged body 8 is altered by increasing or decreasing the amount of air 25 retained within the submerged body 8. For this reason, it is preferred that the submerged body 8 of the present invention is in the form of a buoyant tank, in which the buoyancy is controlled, for example, by the ingress or expulsion of water as a result of air being 30 supplied or vented. Air is vented from the buoyant tank to the surrounding water and is replaced, when required, from compressed air cylinders integral with the buoyant tank. The supply and venting of air is achieved by using remotely operated valves preferably 35 operated by an active buoyancy control system. It is envisaged that this buoyancy control system is able to detect and react to changes in the loads applied to

- 7 - the buoyant tank such that the depth of the buoyant tank always remains within the desired range. The most significant loads changes will occur as a result of changes to the type of oil (e.g. light or heavy crude) 5 in each of the catenary flowlines 6 and each of the flexible supply lines 11 during pumping operations.

When inspection or maintenance of the tank is required, the buoyancy of the tank may be increased by 10 expelling water from within the tank, using air from the integral compressed air supply, causing the tank to rise to the surface. Retrieval of the buoyant tank may be facilitated by one or more lines attached to the tank such that the tank may be manoeuvred into any 15 desired location.

In very deep water, it is generally found that a straight steel or synthetic wire anchor leg between the submerged body and the seabed is sufficiently long 20 to absorb dynamic motions of the submerged body.

However, in shallower waters there is insufficient length in the anchor leg to produce such an effect. In such cases the lower part of the anchor leg can be replaced with a chain catenary which is intended to 25 lie along the seabed. When it is desired to raise the submerged body the length of chain running along the seabed is sufficient to allow the submerged body to resurface. 30 In the embodiment of the present invention shown in Figure 3 there is a single anchor line forming the anchor leg 12 tethering the submerged body 8 to the seabed 3. Alternative means of tethering the submerged body 8 are envisaged. For example, the single anchor 35 line could be supplemented with one or more additional anchor lines generally parallel to the first anchor line in order to provide redundant members in the

8 - anchor leg 12 Another optio nClUde three anchO 1 spaced at approxi degrees f rom each order to prOvi ys restraint to the body so that t n of the buoyant ta k ntr lled more c10sely The Skilled Will appreciate th 10 present invent) es improved apparat transfer of flui tWo fl oa t ing S t ruct ll be appreciat mber of modi fi cat i variations to t ic ebodiments desc ib also pOssible, wi eparting from the sc 15 claims.

Claims

- 9 Claims

1. Apparatus for transferring fluid between first and second floating and moored structures comprising one 5 or more rigid underwater flowlines, each with a first end connected to the first structure and a second end connected to a submerged buoyant body of adjustable buoyancy, and one or more flexible flowlines each with a first end connected to the submerged buoyant body 10 and a second end connected to the second structure, wherein the body is tethered to the seabed by at least one anchor leg which has a length greater than the depth of the water.

15

2. Apparatus as claimed in claim 1, wherein the length of the anchor leg is between 101% and 150% of the water depth.

3. Apparatus as claimed in claim 2, wherein the length 20 of the anchor leg is between 105% and 115% of the water depth

4. Apparatus as claimed in any preceding claim, wherein the anchor leg comprises two or three 25 identical lines generally parallel to each other.

5. Apparatus as claimed in any preceding claim, wherein the anchor leg comprises three anchor lines spaced at approximately 120 degrees to one another.

6. Apparatus as claimed in any preceding claim, wherein the or each anchor line consists of polyester, or steel wire, or anchor chain or a combination thereof.

7. Apparatus as claimed in any preceding claim, wherein the rigid underwater flowlines are at least

- 10 460mm to 560 mm (18" to 22") in diameter.

8. Apparatus as claimed in any preceding claim, wherein the submerged body comprises a tank and 5 control means operable to increase and decrease the buoyancy of the tank.

9. Apparatus as claimed in claim 8, wherein the control means comprises valve means and a compressed 10 gas supply and is operable to increase buoyancy by supplying gas into the tank and to decrease buoyancy by venting gas from the tank.

10. Apparatus substantially as hereinbefore described 15 with reference to the accompanying drawings.