GB2398762A

GB2398762A - Buoyancy module for riser pipes

Info

Publication number: GB2398762A
Application number: GB0304349A
Authority: GB
Inventors: Stephen Anthony Hatton
Original assignee: 2H Offshore Engineering Ltd
Current assignee: 2H Offshore Engineering Ltd
Priority date: 2003-02-26
Filing date: 2003-02-26
Publication date: 2004-09-01
Anticipated expiration: 2023-02-26
Also published as: GB0304349D0; GB2398762B

Abstract

A buoyancy module 10 for imparting buoyancy to a riser pipe (30, Fig 2) comprises a plurality of hollow tubes 14 secured together in a parallel array with each tube having an opening at its bottom end (24, Fig 3a) and being closed at its top end. Each tube communicates with an adjacent tube through a passage (38, Fig 4) at a point close to but above its bottom end such that all tubes are in communication with each other. Means (36, Fig 4) for injecting air into the top of at least one of the tubes are provided to displace water sequentially from as many of the tubes as is required to provide the required degree of buoyancy.

Description

Buoyancy Module This invention relates to a composite buoyancy module used

for providing buoyancy to riser systems. Such risers are used in the search and extraction of offshore hydrocarbons and are the link between the seabed and a floating vessel.

Risers are long tubular structures up to 3000m long assembled from short lengths of steel pipe typically 12m long. In service they are subjected to high loads resulting from self- weight and environmental loading such as waves and current. For structural stability they must be supported by applying tension either directly from the vessel or by use of submerged buoyancy modules attached directly to the riser.

A range of alternate buoyancy system designs and .materials have been proposed and used. The most common '.. 20 is the use of large diameter steel tanks, which are 1 À filled with air. The air is often pressurized to resist À.

À collapse of the aircan as it is lowered into the water, sometimes to a considerable depth where the hydrostatic pressure is high. Alternatively the aircan is allowed to free flood as it is lowered and then filled with air À . when it reached its service depth.

These steel aircans are relatively inefficient with respect to their ability to provide buoyancy due to the high weight of steel and corrosion prevention - 2 requirements that need a steel plate thickness corrosion allowance and the attachment of anodes. The need to resist high collapse pressures requires the utilization of rolled plate with internal stiffeners and bulkheads. Such large steel structures are complex to design and fabricate and this results in a high cost. The relatively high density of the structure results in a non-optimised riser response due to the large mass and drag diameter of the steel air can.

Other buoyancy solutions use hollow glass beads in an epoxy matrix to form a strong and light material that can be moulded to a range of shapes to fit around the riser pipe. However these materials are expensive and the buoyancy they provide is fixed and cannot be adjusted in service.

According to the present invention, there is provided a buoyancy module for attachment to a riser pipe, the module comprising a plurality of hollow tubes secured together in a parallel array, each tube having an Àopening at its bottom end and being closed at its top end, each tube communicating with an adjacent tube at a point close to but above its bottom end such that each À . 25 tube is in communication with another tube, and means À for injecting air into the top of at least one of the tubes.

In the present invention the buoyancy module is configured by bundling together relatively small - 3 diameter air filled glass fibre reinforced tubes to produce a lightweight but stiff structure that does not suffer from corrosion. The tubes are hexagonal in cross section to achieve an efficient packing density with minimum gaps between tubes.

Each tube is configured vertically with a closed top and open bottom. Air is injected into the tube to displace the water. The wall thickness of the tube is sized to accommodate the differential pressure across the wall resulting from the external hydrostatic pressure.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a buoyancy module in À . accordance with the invention; Figure 2 shows the buoyancy module of Figure 1, partly 1.

cut away; Figure 3a shows a single tube in vertical cross section; À À.

Figure 3b shows a section on the line A-A from Figure 3a; Figure 4 is a schematic illustration showing how the module is operated; Figure 5 shows the method of attachment of the module to a riser; and Figure 6 shows an alternative embodiment of the module according to the invention.

The module 10 shown in Figure 1 has a matrix '2 of parallel hollow tubes 14 held together in a bundle and with a circumferential belt 16 and top 18 and bottom 20 discs. The top and bottom discs are held to the bundle by elongate bolts 22. The discs, in the form of GRP gratings, are lightweight but stiff discs approx. 100 mm thick and of the same diameter as the buoyancy module. They are used above and below to sandwich the tubes 14 and to provide the stiffness to transmit the À .buoyancy force into the riser pipe 30. The outside In.20 diameter of the GRP discs 18, 20 has a rolled edge.

This is a section of hard rubber to act as a 'bumper' Àduring handling.

Figure 2 shows the arrangement of the individual GRP À Visa tubes 14. The tubes are manufactured using a continuous a pultrusion process and are then cut up into lOm long sections. The tube is manufactured with a hexagonal cross section (see Figure 3b) so that they can be closely packed with minimum voids. A combination of triangles, octagons and squares or other tessellated shaped tubes can be used to maximise packing density and obtain the required shape. Each tube is open at the lower end 24 and closed off at the upper end using a GRP plug 26 that is bonded into the tube using adhesive.

The tubes 14 are assembled to form a bundle by bolting each tube to its adjacent tube at both ends. To minimise weight the bolting materials are non metallic such as nylon. At the top end the tube is bolted through holes 28 in the plug 26. At the bottom end the tubes are bolted directly through the wall of the tube, at a location close to the bottom end (see Figure 4).

The bolting system used at the lower end has a hollow core to allow air to pass from one tube to another during the air-up process.

Hexagonal tubes can be packed to form a wide range of .buoyancy module cross sections. The simplest *.20 arrangement is cylindrical with a central aperture for

I

the riser pipe 30 to be located.

After the hexagonal tubes have been assembled, the top and bottom gratings 18, 20 are attached to both ends. À

Top and bottom plates 33 (only the top plate is visible) of steel are attached to the top and bottom gratings. These steel plates 33 incorporate a short section 32 of steel pipe that prevents the stee: riser pipe 30 from contacting the GRP tubes 14 and possibly causing wear. The steel top and bottom plates are of much smaller diameter than the grating discs 18, 20 and are connected to the central steel pipe 32. The function of the steel plates is to transmit the buoyancy load into the riser pipe 30. As this can be a large force which must be transmitted through a relatively small contact area, these plates must be of steel rather than GRP. The top and bottom gratings and their steel plates are retained by the tensioned GRP rods that connect between the upper and lower steel plates. These are inserted through evenly distributed hexagonal tubes and tensioned using a 'threaded section and nut assembly.

The buoyancy module is fitted over the riser pipe and reacts against a load shoulder 34 on the riser pipe.

The tubes are initially full of air. As the buoyancy module is lowered into the water the trapped air is compressed due to the increasing water pressure with depth ensuring that the tubes remain pressure balanced.

Due to the relatively large opening at the base 24 of each tube the buoyancy module can be lowered quickly into the water without concern for collapse.

After the riser is latched at the seabed the -mmersion depth of the module will be fixed. The module -s then aired-up using a flexible air line 36 (F cure 4) connected co the buoyancy module by an ROV. :^e air line allows air to be injected into one of:ee hex - 7 tubes 14a displacing water through the open bottom end 24. When the air/water interface approaches the bottom of the tube, the air begins to pass into the adjacent tube 14b via the hollow bolts 38 at the base of each tube which connect the bottom ends of the tubes to each other. Once the air/water interface in that tube approaches the bottom, air starts to enter the tube 14c and the air displaced the water as shown in Figure 4.

The remaining tubes fill in this cascade manner. When the module is completely full of air, air will be seen to escape from the bottom of the module. Clearly, the air-up process can be stopped at any stage when sufficient buoyancy has been imparted to the module by the air-up process.

The module can be configured with a number of separate air lines if required for redundancy or to air-up non communicating groups of tubes. It is noted that in the event of a single tube being damaged or leaking this does not effect the buoyancy contribution of the other

R

tubes and hence there is only a small reduction in the ..

Àoverall buoyancy force.

If it is necessary to reduce the buoyancy force the air can be removed from the tubes via small diameter pipes e inserted into the base of each tube and extending to a location near the top of each pipe where it is fitted with a float valve. - 8

The air can then be pumped out of the tubes using an ROV which connects to a manifold to which all the small diameter air pipes are connected. As the ROV 'sucks' the air out of each tube the water enters the bottom filling the tubes.

An alternative configuration to the buoyancy module described above is shown in Figure 6. This shows a module 40 that allows a radial group 42 of tubes to be removed in order to allow the module to be fitted on to a riser pipe 30. The radial group 42 of tubes is then replaced after fitting to the riser to regain the structural stiffness. À 1 À À.e.e À À À À À - 9

Claims

Claims 1. A buoyancy module for attachment to a riser pipe, the module

comprising a plurality of hollow tubes secured together in a parallel array, each tube having an opening at its bottom end and being closed at its top end, each tube communicating with an adjacent tube at a point close to but above its bottom end such that each tube is in communication with another tube, and lo means for injecting air into the top of at least one of the tubes.
2. A buoyancy module as claimed in Claim 1, wherein each tube has a plug to close its upper end, and the plugs are connected to one another to secure the tubes together.
3. A buoyancy module as claimed in Claim 1 or Claim 2, wherein the bottom ends of the tubes are secured to one another by bolts passing through the tube walls at If a point close to but above their bottom ends, the bolts being hollow to provide a fluid flow passage from one À.

tube to adjacent tubes. À . . al e
4. A buoyancy module as claimed in any preceding claim, wherein the tubes are polygonal in cross-section À e. À

and are arranged in a close-packed, tessellated array. À À À Àe
5. A buoyancy module as claimed in Claim 4, wherein the tubes have hexagonal cross-sections.
6. A buoyancy module as claimed in any preceding claim, wherein the tubes are made from a glass- reinforced resin.
7. A buoyancy module as claimed in any preceding claim, wherein the tubes are made by a pultrusion process.
8. A buoyancy module as claimed in any preceding claim, wherein means are provided for inject ng air into the tops of several of the tubes.
9. A buoyancy module as claimed in any preceding claim, wherein cover plates extending over the tops and bottoms of all the tubes are provided at the top and bottom of the module, and elongate bolts pass through the module, from top to bottom, to secure the cover plates to one another.
10. A buoyancy module as claimed in Claim 9, wherein :.c.. the top cover plate has means for securing the module . . on a riser pipe. A..

I e e.
11. A buoyancy module as claimed in any preceding claim, wherein the array of tubes is centred around a passage through which a riser pipe will be located, and ....

a group of tubes extending from the outer periphery of . the module -o he passage can be removed from the array to allow tee module to be mounted on a riser Ripe by guiding the Module laterally into the passage through the space left by the removed group, before the removed group is replaced.
12. A buoyancy module as claimed in Claim 11, wherein the removable group of tubes has its own air injection means.
13. A buoyancy module A buoyancy module for attachment to a riser pipe substantially as herein described with reference to any one embodiment shown in the accompanying drawings. e

- À.. À.e À :e e. . e. À