GB2299975A - Removable Buoyancy - Google Patents

Abstract

Removable additional buoyancy to fit around an elongate member 11 (e.g. a tubular forming part of an offshore structure), and comprising two buoyant half shells 21 and 22 capable of being arranged to surround substantially the whole of the periphery of the member, in which the two half shells are hingedly connected together along a hinge axis 23 on their inner surfaces parallel to the axis of the member, in combination with releasable connection means 28 to hold the half shells together or to allow them to move rotationally apart about the hinge axis.

Description

REMOVABLE BUOYANCY The invention relates to removable buoyancy for offshore structures, and in particular relates to removable buoyancy for use in the installation of support structures for offshore platforms.

Support structures for offshore platforms have conventionally been formed of steel lattice frameworks made from tubular steel members, and such structures are known in the offshore industry as "jackets".

In the past a typical jacket has been built in a fabrication yard, moved onto a barge alongside the yard, transported on that barge to the site of a proposed offshore oil/gas field, and then lifted or launched from the barge into the sea. After lift or launch the jacket must be manoeuvred into a suitable position/attitude to be fixed to the seabed (e.g. by piling). The basic jacket, which is designed for long term structural integrity, may not be sufficiently buoyant for the manoeuvring operation. In consequence the jacket has either been designed so that its tubular steel members are over sized to provide sufficient buoyancy1 or it has been fitted with temporary steel buoyancy tanks.

Experience on one project showed that an installation contractor with a particular offshore spread would have difficulties installing a jacket weighing more than 1400 tonnes. The contractor had intended to lift the jacket from a barge in its horizontal orientation and then place it in the water using a 2000 tonne main hoist with slings positioned part way along the jacket legs. The contractor had anticipated that the jacket would float, thereby allowing an operative to de-rig the main hoist and re-rig it to slings positioned on the tops of the jacket legs. This would have allowed the contractor to rotate the jacket into an upright orientation while it had positive buoyancy, and then to install it vertically.

The major problem with this scheme was that there was insufficient inherent buoyancy within the jacket to allow it to float unsupported with the required factors of safety. Consequently a substantial additional cost would have been incurred in making temporary steel buoyancy tanks, together with further costs for valves, flooding control systems and tank removal systems.

In our UK patent specification 2,1 99,290A we have described several schemes for using temporary additional buoyancy in the form of inflatable buoyancy bags. Figures 1, 2 and 3 of that specification show the use of such bags in relation to marine operations with jackets.

However, temporary additional buoyancy in the form of inflatable buoyancy bags is operationally awkward (and also expensive). As an alternative, the temporary additional buoyancy can be provided by the use of modular foam buoyancy around upper legs of the jacket. The initial cost of the foam buoyancy would be close to that for steel buoyancy tanks. The key to reducing costs lies in arranging for the modular foam buoyancy to be easily reused. If the cost of the buoyancy is recovered against a number of projects, modular foam manufacturers will be able to offer competitive prices, thereby enabling installation costs to be reduced.

The invention provides removable additional buoyancy to fit around an elongate member (e.g. a tubular forming part of an offshore structure), and comprising two buoyant half shells capable of being arranged to surround substantially the whole of the periphery of the member, in which the two half shells are hingedly connected together along a hinge axis on their inner surfaces parallel to the axis of the member, in combination with releasable connection means to hold the half shells together or to allow them to move rotationally apart about the hinge axis.

It is preferred that each half shell has inner and outer cylindrical surfaces parallel to the axis of the member, radial surfaces extending lengthwise parallel to that axis and end surfaces perpendicular to that axis, and in which the inner surface conforms generally to the outer surface of the member, and the radius of the outer surface is determined by the amount of the additional buoyancy required; In one form it is further preferred that the half shells are made of lightweight foam material.

In another form it is further preferred that the half shells are fabricated from plate material to form hollow buoyancy tanks.

It is also preferred that there are spacers to keep the half shells spaced apart longitudinally along the tubular member, and these spacers are also used as packers within the inner cylindrical surface to adapt the half shells to fit onto a tubular member of smaller external diameter.

In one preferred form the releasable connection comprises a locking rod parallel to the axis of the member and passing through lugs on the intersection between a radial surface and an outer surface of each of the half shells remote from the hinge, which rod can be withdrawn axially of the member.

In another preferred form the releasable connection comprises a locking pin extending radially with respect to the axis of the member and passing through lugs on opposed radial surfaces of the half shells, so that outward radial movement of the pin will release the half shells.

It is still further preferred that there are lever members to pull the half shells apart, and the lever members are linked to means to withdraw the pin radially.

It is still further preferred that a single release wire first withdraws the pin and then pulls the lever members to move the half shells apart.

A specific embodiment of the invention (and a variant thereof) will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows diagrammatically a jacket on a barge; Figure 2 shows the jacket floating horizontally in the water; Figure 3 shows the jacket raised vertically on a hook; Figure 4 shows the jacket partially lowered towards the seabed; Figure 5 shows the jacket lowered further towards the seabed; Figure 6 is a cross section on the line VI VI in Figure 1, showing two half shells of additional buoyancy surrounding a tubular leg of the jacket; Figure 7 is a similar section showing the half shells released; Figure 8 is a view on the tubular leg showing a line of half shells, and their release mechanisms; Figure 9 is a cross section similar to Figure 6 and showing a variant of the invention;; Figure 10 is a similar section showing the half shells released; and Figure 11 is a side view on the area Xl in Figure 10.

As shown in Figure 1 a jacket 10 has four legs 11 spaced apart by face bracing 1 2.

The legs 11 and face bracing 12 are formed of steel tubulars, which are welded to together at nodes to make a steel lattice framework.

The jacket 10 is shown lying on a barge 14 with one pair of legs parallel with the deck 1 5 of the barge. In this configuration the jacket 10 is transported on the barge 14 from the yard where it was fabricated to its intended offshore site. To enable the jacket to be transferred safely from the barge to the seabed it has temporary additional buoyancy in the form of a series of foam floats arranged to surround its upper legs (as seen when the jacket is lying on the barge). Each foam float consists of two half shells which wrap around the leg of the jacket. The two halves are linked together by rods passing through interlocking rings on one side, and by a hinge mechanism on the other side. The half shells are shown in more detail in Figures 6 to 8 and 9 to 11.

The foam floats consisting of pairs of half shells are placed on the jacket during fabrication, taking advantage of the fabricator's cranes. It is possible to place the floats on the jacket once it has been loaded onto the barge, but crane access may be more difficult. The floats are installed individually. The rigging lines (described later) may be attached after all the floats are in place.

In Figure 2 the jacket has been launched (or lifted) into the sea, and is seen floating with its upper pair of legs surrounded by the buoyant half shells in the plane of the sea surface. A hook 1 7 has been reeved to the tops of the legs 11 so that the jacket can be upended.

In Figure 3 the jacket has been lifted and rotated on the hook 17, which is supported on the main hoist of a heavy lift vessel (not shown). A rod release line 1 8 and a hinge operating line 19 are connected to and operable from the HLV and a tug (also not shown) respectively.

In Figure 4 the jacket 10 is being lowered vertically towards the seabed. As this operation is carried out, the temporary additional buoyancy may be released by a pull on the release line 1 8 (from the HLV), and then removed by a tug pulling on the hinge operating line 19. The line 1 9 is attached to each of the floats by two short wires. The two short wires are attached to opposite arms of the float which are pulled together by the line 19, thereby opening the half shells of the float and allowing it to detach from the leg. The line 19 then draws the half shells away from the jacket.

The hinge operating line 1 9 is connected loosely to the next float where the process is repeated as the jacket is lowered until all floats are removed. The key to this operation is a scissors system on the hinge which springs the half shells of the float away from the leg of the jacket. This system is simple and has a low risk of failure. Because of the batter on the jacket leg, buoyancy in the floats forces them away from the jacket, and so assists the action of the line 1 9 in ensuring the floats detach from the jacket.

Figure 5 shows the lowering operation almost completed.

Once removed the floats are still linked by the line 1 9 and can either be lifted onto a barge (or onto the HLV itself) by the HLV, or towed directly to shore by the tug.

In Figures 3, 4 and 5 there are two primary lines 18 and 1 9. The first (18) releases the locking rods (28). The second line 19, attached to the tug (or to the HLV), opens the hinges and also pulls the floats away from the jacket.

The foregoing description relates to a four legged jacket as shown in Figures 1 to 5.

However, it will be understood that floats in accordance with the invention can be applied to the upper most legs of any jacket which is transported to its intended offshore site on a barge. The jacket may have six or eight or possibly more legs, and floats can be used on any or all the legs which are uppermost when the jacket has been lifted or launched into the water.

Figures 6, 7 and 8 show the release arrangements for the floats in more detail.

As shown in Figure 6, a jacket leg 11 is substantially surrounded by two buoyant half shells 21 and 22 which are hinged at 23. The half shells are formed of foam material, and are protected by a casing which is rebated next to the hinge. The casing has limit plates 24 and 25 which are welded to radial arms 26 and 27 respectively.

Initially the half shells are held together by a locking rod 28 on the opposite side of the half shells to the hinge 23. At the tops of the radial arms there are short wire links 29 and 31 connected to a steel ring 30. The ring 30 is attached to the line 19. Optionally, a short strop 1 9A is used to attach the line 1 9 to the ring 30.

Each of the half shells 21 and 22 is encased in a rigid casing. The half shells are positioned around the leg of the jacket when the jacket is built in the fabrication yard.

The floats have the hinge 23 on one side, and the half shells forming the floats are locked into position by the locking rod 28 passing through interlocking rings on the opposite side to the hinge. These are set up before the jacket 10 and barge 14 leave the fabrication yard.

Removal of the half shells 21 and 22 is achieved by withdrawing the locking rod 28 and then opening the hinge 23. These operations may be performed by pulling on the lines 1 8 and 1 9 linked to the HLV and tug respectively. Alternatively the hinges may be spring loaded.

To allow the floats to be attached to a variety of jacket members, they are designed to fit a 2300mm diameter leg, but can be applied to smaller legs by the addition of packers.

The intended location of the floats on the legs needs to be accounted for when attaching sacrificial anodes to the legs.

Figure 7 shows how the buoyant half shells 21 and 22 are released from the jacket leg 11. The locking rod 28 is withdrawn, using the release line 18, and the ring 30 is pulled away from the jacket leg using the hinge operating line 1 9. The effect is to swing the buoyant half shells outwardly away from the leg, so that the plates 24 and 25 abut in face to face relationship. The buoyancy can then be recovered from the jacket.

Figure 8 illustrates a series of buoyant half shells disposed along a jacket leg. This shows how a single line 1 9 can be arranged to pull all the pairs of half shells loose in sequence. Each pair of half shells is connected to the line 1 9 (optionally by the short strop 1 9A). As the jacket is lowered into the water, buoyancy in the half shells forces them away from the battered jacket leg. The buoyant floats (formed of pairs of half shells) are attached together to facilitate towing to shore or towing to a barge for later recovery.

There should be enough slack in line 1 9 between adjacent floats to prevent them interfering with each other when floating freely after having been detached from the jacket leg. In the event of a float jamming, and not floating free of the leg, the ring 30, the wires 29 and 31, or the short strop 1 9A connecting it to the hinge operating line 1 9 will break, allowing the rest of the floats to be removed. (The wires 29 and 31 or the short strop 1 9A will have a lower breaking load than the main line 19.) No diver intervention is anticipated. Once removed from the jacket, the floats will be towed to the HLV for lifting on to a barge (or onto the HLV itself) or taken directly to shore by the tug.At the shore they may then be lifted individually or in groups by a crane for reuse on another jacket.

The hinges of the half shells are placed on the top of the leg 11 because this is the easiest way to install the floats. They will be lifted in the open position using the existing lifting lugs on the ends of limit plates 24 and 25, lowered onto the leg 11 and, as the crane is lowered further, the weight of the half shells will cause them to close around the leg. The final positioning and locking can be achieved by jacking the plates 24 and 25 apart to allow insertion of the locking rod 28.

In Figure 8 it can be seen that the floats are located at specific positions along the leg by spacers 32. These spacers can also form part of, or act to separate, packers to allow standard sized floats to be used on jackets with smaller than standard diameter legs.

Figures 9 to 11 show a variant of the release arrangement for the buoyant floats.

In this variant the jacket leg 11 is substantially surrounded by two buoyant foam half shells 21 and 22 which are hinged at 23 (as in Figure 6). However, the locking rod release function and the hinge operating function are effected by a single line 40 (i.e.

combining the effects of lines 1 8 and 1 9 in the previous embodiment). In Figure 9 the locking pin (44) is withdrawn from all the sleeves by the action of pulling on line 40.

There will be some fine tuning of the lengths of the various strops and wires to ensure this happens. Once the pin is free of its sleeves it will fall away and clear the mechanism.

Upper and lower locking sleeves 41 and 42 project along arcs from the limit plate 24. There is another locking sleeve 43 projecting in an arc from the limit plate 25. The sleeves 41, 42 and 43 are arranged so that when the half shells 21 and 22 are in position surrounding the leg 11, a locking pin 44 can pass through all three sleeves to hold the half shells securely in position. Thus the half shells are secured from the top of the leg 11 as it is built in the fabrication yard, and rod 28A is merely an additional safety device which can be removed prior to instalTation.

In use a strop 40A may optionally be used on each float sequentially to remove the pin 44 from the sleeves, and then to draw the half shells apart by pulling on the wire links 29 and 31 as before. As shown in Figure 11, the sleeves will be accommodated within the half shells when the half shells 21 and 22 are free of the leg 11. In this way a single pull can act to release the half shells and to draw them clear of the leg. The lengths of the locking pin 44 and the links 29 and 31 will need to be set so that the pin 44 is clear of the sleeve 41 before the links 29 and 31 begin to draw the half shells together.

It is a feature of the invention that use of the locking pin 44 on a generally horizontal axial resists withdrawal when the line 40 is not along the axis of the pin, and so resists release when the float is above water level.

Claims (11)

1. Removable additional buoyancy to fit around an elongate member (e.g. a tubular forming part of an offshore structure), and comprising two buoyant half shells capable of being arranged to surround substantially the whole of the periphery of the member, in which the two half shells are hingedly connected together along a hinge axis on their inner surfaces parallel to the axis of the member, in combination with releasable connection means to hold the half shells together or to allow them to move rotationally apart about the hinge axis.

2. Removable buoyancy as claimed in Claim 1 in which each half shell has inner and outer cylindrical surfaces parallel to the axis of the member, radial surfaces extending lengthwise parallel to that axis and end surfaces perpendicular to that axis, and in which the inner surface conforms generally to the outer surface of the member, and the radius of the outer surface is determined by the amount of the additional buoyancy required;

3. Removable buoyancy as claimed in Claim 2 wherein the half shells are made of lightweight foam material.

4. Removable buoyancy as claimed in Claim 2 wherein the half shells are fabricated from plate material to form hollow buoyancy tanks.

5. Removable buoyancy as claimed in any one of Claims 2 to 4, in which there are spacers to keep the half shells spaced apart longitudinally along the tubular member, and these spacers are also used as packers within the inner cylindrical surface to adapt the half shells to fit onto a tubular member of smaller external diameter.

6. Removable buoyancy as claimed in any one of the preceding claims in which the releasable connection comprises a locking rod parallel to the axis of the member and passing through lugs on the intersection between a radial surface and an outer surface of each of the half shells remote from the hinge, which rod can be withdrawn axially of the member.

7. Removable buoyancy as claimed in any one of Claims 1 to 5 in which the releasable connection comprises a locking pin extending radially with respect to the axis of the member and passing through lugs on opposed radial surfaces of the half shells, so that outward radial movement of the pin will release the half shells.

8. Removable buoyancy as claimed in Claim 7 in which there are lever members to pull the half shells apart, and the lever members are linked to means to withdraw the pin radially.

9. Removable buoyancy as claimed in Claim 8 in which a single release wire first withdraws the pin and then pulls the lever members to move the half shells apart.

10. Removable buoyancy substantially as hereinbefore described with reference to and as shown in Figures 1 to 5, and Figures 6 to 8 or Figures 9 to

11.

10. Removable buoyancy substantially as hereinbefore described with reference to and as shown in Figures 1 to 5, and Figures 6 to 8 or Figures 9 to 11.

Amendments to the claims have been filed as follows 1. Removable additional buoyancy to fit around an elongate member (e.g. a tubular forming part of an offshore structure), and comprising two buoyant half shells capable of being arranged to surround substantially the whole of the periphery of the member, in which the two half shells are hingedly connected together along a hinge axis on their inner surfaces parallel to the axis of the member, and there is a pair of radial arms forming levers which extend away from the hinge axis on respective half shells such that movement of the levers towards each other will rotate the half shells outwardly away from each other; in combination with releasable connection means to hold the half shells together or to allow them to move rotationally apart about the hinge axis.

2. Removable buoyancy as claimed in Claim 1 in which each half shell has inner and outer cylindrical surfaces parallel to the axis of the member, radial surfaces extending lengthwise parallel to that axis and end surfaces perpendicular to that axis, and in which the inner surface conforms generally to the outer surface of the member, and the radius of the outer surface is determined by the amount of the additional buoyancy required; 3. Removable buoyancy as claimed in Claim 2 wherein the half shells are made of lightweight foam material.

4. Removable buoyancy as claimed in Claim 2 wherein the half shells are fabricated from plate material to form hollow buoyancy tanks.

5. Removable buoyancy as claimed in any one of Claims 2 to 4, in which there are spacers to keep the half shells spaced apart longitudinally along the tubular member, and these spacers are also used as packers within the inner cylindrical surface to adapt the half shells to fit onto a tubular member of smaller extemal diameter.

6. Removable buoyancy as claimed in any one of the preceding claims in which the releasable connection comprises a locking rod parallel to the axis of the member and passing through lugs on the intersection between a radial surface and an outer surface of each of the half shells remote from the hinge, which rod can be withdrawn axially of the member.

7. Removable buoyancy as claimed in any one of Claims 1 to 5 in which the releasable connection comprises a locking pin extending radially with respect to the axis of the member and passing through lugs on opposed radial surfaces of the half shells, so that outward radial movement of the pin will release the half shells.

8. Removable buoyancy as claimed in Claim 7 in which the levers are linked to means to withdraw the pin radially.

9. Removable buoyancy as claimed in Claim 8 in which a single release wire first withdraws the pin and then moves the levers to rotate the half shells apart.