GB2105393A - Offshore structures - Google Patents

Abstract

Marine oil field exploitation apparatus comprises a floating operations platform 114, an otherwise buoyant anchoring platform 103 which has been sunk to the sea bed 121, lattice-form legs 199 tethering the platform 114 to the platform 103, and an artificial haven comprising two vessels 200 arranged in a V- formation, a pontoon interconnecting the vessels 200 in such manner that they can move a significant extent relative to each other in substantially all senses except horizontally towards and away from each other, a turret around which the pontoon and the vessels 200 can rotate together, and lines 264 mooring the turret. <IMAGE>

Description

SPECIFICATION Improvements in or relating to marine apparatus According to a first aspect of the present invention, there is provided marine apparatus, comprising a floating operations platform, a buoyant anchoring platform sinkable to the sea bed, and tethering connections arranged to tether the floating operations platform to the sunk anchoring platform.

According to a second aspect of the present invention, there is provided a method of providing a tethered offshore operations platform, comprising providing in a coastal region a floating operations platform and a buoyant anchoring platform, interconnecting the two platforms by way of tethering connections, towing the two platforms out to sea, and sinking the buoyant anchoring platform to the sea bed while lengthening the tethering connections.

According to a third aspect of the present invention, there is provided a method of providing a tethered offshore operations platform, comprising providing in a coastal region a buoyant operations platform and a buoyant anchoring platform supporting said operations platform, towing the two platforms out to sea with the anchoring platform supporting the operations platform, and sinking the buoyant anchoring platform to the sea bed while lengthening tethering connections interconnecting the two platforms and while ieaving the operations platform floating in the sea.

According to a fourth aspect of the present invention, there is provided a frame member for subsea use, comprising a hollow body, and nonreturn inlet valve means whereby compressed gas can be supplied to the hollow interior of said body.

According to a fifth aspect of the present invention, there is provided an artificial haven, comprising two vessels arranged in a V formation, and means connecting the two vessels together in such a manner that they can move a significant extent relative to each other in substantially all senses except horizontally towards and away from each other.

According to a sixth aspect of the present invention, there is provided a slip ring for fluids, comprising first and second members arranged in a fluid-tight manner relative to each other and rotatable relative to each other about an axis of rotation, a plurality of separate flow paths each from the outside of the first member through the first and second members to the outside of the second member, each of the flow paths including in one of the members a substantially circular channel in the inside surface of that member and substantially concentric with said axis.

According to a seventh aspect of the present invention, there is provided, in combination, a windlass situated on a substantially vertical axis and a plurality of chain stoppers arranged around said axis for having respective chains therein tensioned in turn by said windlass, said windlass, on the one hand, and said chain stoppers, on the other hand, being turnable relative to each other about said axis.

According to an eighth aspect of the present invention, there is provided a method of tensioning chains in respective chain stoppers, comprising operating a windlass to tension a first chain in a first chain stopper aligned with the windlass, operating the windlass to relax the portion of the first chain between the windlass and the first chain stopper, the portion of the first chain beyond the first chain stopper remaining tensioned, turning the windlass, on the one hand, and the first chain stopper and a second chain stopper, on the other hand, relative to each other about a substantially vertical axis to align the windlass with said second chain stopper, the first chain being removed from the windlass, introducing a second chain in the second chain stopper into the windlass and operating the windlass to tension the second chain.

According to a ninth aspect of the present invention, there is provided a chain guiding apparatus, comprising a chain guiding pulley, a support, and means mounting rotatably said pulley on said support in such a manner as to allow a limited amount of movement of said pulley relative to said support in a sense transverse to the axis of rotation of said pulley, said means comprising a load measuring device by way of which said pulley is mounted on said support and which is turnable about said axis relative to said support and said pulley between first and second positions in both of which it performs the pulleymounting function.

According to a tenth aspect of the present invention, there is provided a well casing comprising a tubular wall formed with annular corrugations extending peripherally therearound.

In order that the invention may be clearly understood and readily carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1 to 8 are diagrammatic illustrations of respective stages in the construction of marine apparatus including a buoyant operations platform and a buoyant anchoring platform, the towing of the apparatus out to sea and the lowering of the anchoring platform onto the sea bed, Figure 9 shows this apparatus half in fragmentary side elevation and half in fragmentary section on the line X-X of Figure 10, with the anchoring platform at the sea-bed and the operations platform in a restrained buoyant condition at sea level, Figure 10 shows a sectional, fragmentary plan view of this apparatus in the condition shown in Figure 9, plans being taken on the lines V-V, Y-Y and Z-Z of Figure 9, Figure 11 shows a sectional plan view of a sprag stopper arrangement of this apparatus, but with parts thereof disengaged, Figure 12 shows a section taken on the line E-E of Figure 11, but with those parts engated, Figure 1 3 shows a top plan view of the anchoring platform, Figure 14 shows a detail of Figure 13, Figure 1 5 shows diagrammatically a section taken on the line A-B of Figures 13 and 14, Figure 1 6 shows a section taken on the line D-D of Figure 15, Figure 1 7 shows a section taken on the line A-C of Figure 13, but also illustrating latticed pipe legs of this apparatus, Figure 1 8 shows a plan view of a universal coupling between a latticed pipe leg and the anchoring platform, Figure 1 9 shows a side elevation of that coupling, Figure 20 shows a fragmentary, top plan half view of a Judas sheep guideframe of this version, Figure 21 shows a section taken on the line M-M of Figure 20, Figure 22 shows a plan view of an ocean oil haven usable with the tethered buoyant operations platform, Figure 23 shows a plan view of a universal joint between a pontoon and a vessel of the haven, Figure 24 shows a side elevation of the universal joint, Figure 25 shows a plan view of the pontoon, Figure 26 shows a vertical section taken on the line N-N of Figure 25, Figure 27 shows two half-views, the left-hand one of which is a detail of Figure 26 and a section taken on the line 0--0 of Figure 28, and the righthand half of which is an elevation taken in the direction of the arrow P in Figure 28, but with a circular bulkhead cut away for the sake of clarity, and with a chain stopper in a stoppered condition, Figure 28 is two half-views, the upper half being a sectional plan taken on the lines Q-Q of Figure 27, and the lower half being a sectional plan view taken on the lines R-R of Figure 27, Figure 29 shows a section taken on the line S-S of Figure 27, Figure 30 shows a detail of Figure 27, but with the chain stopper in a free condition, Figure 31 is two quarter-views of an hydraulic slip ring of the pontoon of a modified version of the haven, the left-hand quarter being a top plan, and the right-hand quarter being an underneath plan, Figure 32 is two half-views of the slip ring, the left-hand half being a radial section, and the righthand half being an elevation, Figure 33 is a fragmentary side elevation of part of the apparatus, and Figure 34 is half a detail of Figure 33, and half a fragmentary axial section through items of that part.

Various stages of construction, towing, anchoring and use of a tethered buoyant operations platform for marine exploitation will initially be described.

Referring to Figure 1, an artificial lake is created by building a carrier 101 across a neck of a tidal inlet to separate the inlet from the sea. The lake is pumped out, and a construction site is prepared by levelling the ground and laying a concrete surface on a hard core base. A buoyant anchoring platform 103 to be described in more detail hereinafter is constructed to about 1 5 metres high using conventional construction plant. The barrier 101 is removed and the sea thus allowed to flow into the inlet and float the platform 103. The platform is then towed out from the inlet. It is taken to a sheltered anchorage with about 1 6 fathoms of water. It is there moored securely, by means of sliding claw floats 102, to anchored buoys 105 seen in Figure 2.The construction of the floating platform 103 is completed at this sheltered anchorage. The platform 103 is equipped with flooding and blowing pipe work and valving 106 seen in Figure 13, a drilling template 108, and a relay flood control unit 110 comprising a relay manifold for remotely controlling the valving 106, and flooding water accumulators for storing power for the relay manifold. The unit 110 is actuated by a remote flood control unit. The platform 103 is now towed away to a new sheltered position with 28 fathoms of water. The platform 103 is cellular, with eighteen outer hollow cells 111, and eighteen inner hollow cells 112 of which six cells 11 2a are coupled to latticed tube legs 199 seen in Figure 1 7. The cells are of reinforced concrete. The platform 103 is submerged by controlled flooding of the outer cells 111.

While the platform 103 has been built, a buoyant operations platform 114 has been built by a shipyard. The platform 114 has been brought to rendezvous with the platform 103 and is now floated into position directly over the platform 103, as seen in Figures 3A and 38. This can be carried out in the ocean haven 200, 201 shown in Figure 22 if a suitable land-sheltered haven cannot be found. Wells 1 78 (see Figures 9 and 10) have been provided through the platform 114 for lowering of the latticed pipe legs 199.Locating pipes (not shown) guided vertically in sleeves (not shown) on the outside of the platform 114 are lowered down into lateral contact with the sides of the platform 103 to ensure accurate alignment for subsequent coupling of the legs 199 to the cells 11 2a. The platform 103 is raised by blowing out the cells 111. This raising of the platform 103 lifts the platform 114 out of the water, as shown in Figures 4A and 48.The universal couplings 1 98 are now connected to concreted-in leg extensions 1 97 of the cells 11 2a. Now the latticed pipe legs 1 99, which are in sections each of which is weightless in seawater, have their bottom sections connected to the respective universal couplings 1 98. The couplings 1 98 are connected to the extensions 1 97 and to the bottom sections of the legs 1 99 and the sections of the legs 1 99 are connected to each other by means of squnch joints 196 and 1968 (see Figures 14, 18 and 19).

The legs 1 99 are extended upwardly section by section until there are sections protruding from the upper part 130 of the platform 114. A drilling rig 131, a helicopter landing deck 195, pipe storing racks, mud hoppers etc. are installed.

Guide lines 1 94 (see Figure 17) are attached to the drilling template 108 on the anchoring platform 103, a multi-tube hose is connected to the unit 110 and the remote control cell-flooding system for the platform 103 is tested. Then the platform 103. carrying the platform 11 4, is towed to an oil field to be exploited. At the oil-field site, the outer cells 111 are flooded to obtain negative buoyancy, and the platform 103 is lowered, by motor-driven gear trains 191 engaging racks 192, to the position shown in dot-dash lines in Figure 5 at about 1 30 metres from the sea-bed 1 21 by allowing the legs to lower while adding further sections to the legs.Then upper universal joints 1 86 (see Figure 6) are connected onto the legs 1 99. Thereupon, the platform 103 is further lowered, by lowering the legs 1 99 and adding further sections thereto, to the position shown in chain lines in Figure 5 at about 38 metres from the sea-bed 121. Then the lower part 128 of the platform 11 4 is flooded to lower the platform 11 4 to its operating trim and to lower the anchoring platform 103 to the sea-bed 1 21. Now the anchoring platform 103 is flooded completely with the platform 114 in neutral buoyance. The apparatus is now in the condition shown in Figure 6.Then thousands of tons of water are pumped out of the platform 114 to create an equivalent tension in the legs 1 99. The trim is novs given its final adjustment. Sprags 1 93 (see Figured 9, 11 and 12) are engaged in the racks 192 extending along the legs 199. The gear trains 191 which have been meshing with the racks 1 92 in the controlled lowering of the sections of the legs 199, are now slackened off, so that load cells (of which one is seen and referenced 1 90 in Figure 12) can monitor the tensions in the legs 199, which tensions can be controlled by adjusting the quantity of water in the platform 144. The drilling programme is now commenced. A thirty-six inch bit and thirty inch conductor are guided to the template 108.The thirty inch conductor is cemented up to the template and the annuli of all of the other casings are cemented to the same level. Oil/gas and oil/water separators are installed. Throughout the drilling and production programmes, the tensions in the legs 199 are controlled by adjusting the quantity of water in the lower part 128 of the platform 114. Moreover, reversible thrusters 1 83 installed in the corners of the lower part 128, which is hexagonal in plan, can be utilised to maintain position in severe weather conditions.

Referring particularly to Figures 9 to 12, each section of each leg 1 99 comprises four vertical tubes 1 85 interconnected by lattice work 1 84 and terminating in squnch joints 1 96. Prior to lowering into the sea, each tube 1 85 is filled with compressed air through a non-return inlet valve (not shown). The provision of this compressed air helps to prevent implosion of the tubes under hydrostatic pressure. The valves also allow the inflow of sea water at relatively great depths in the sea and thus prevent implosion of the tubes at those depths.The sections have fixed to the outermost surface portions of their tubes the respective racks 192, so that the sections can be lowered by means of the gear trains 1 91 provided in the upper part 130. The sprags 193, which are arranged in the wells 178 in the upper part 130 can be moved into and out of engagement with the racks 1 92 by rotation of screws 1 82 which carry the sprags 1 93. The female parts of the squnch joints 1 96a (see Figure 19) connecting the universal couplings 1 98 to the extensions 1 97 are provided with hydraulic release pots arranged to act on the resilient split rings of the wall parts so that, by means of a remote control leg-release unit provided at the platform 114 operating a relay leg release unit 180, the hydraulic release pots can be actuated to disconnect the universal couplings 1 98 from the platform 103, so that either individual legs can be released and then lifted for servicing or all of the legs can be released and the whole apparatus, other than the platform 103 can be removed for re-allocation to another area.

Following this, the platform 103 can be purged of the sea water from the surface and thus recovered. As can be seen from Figures 13 to 16, each cell 111 or 112 is a circular cylindrical tube with internal and external reinforcing ribs 179, of which ribs 1 79A are common to adjacent cells.

The Judas sheep guideframe 1 76 is coextensive in plan with the template 108 and is lowered concurrently with a drilling bit or an outermost casing and is then raised again, it being itself guided by the guide lines 1 94 aurind lowering and raising.

Referring to Figures 22 to 32, the ocean oil haven comprises two vessels 200 connected to a pontoon 201. Each vessel 200 can be a 300,000 ton tanker and can contain oil treatment apparatus. The vessels 200 are interconnected at the bows by a universal joint 202 and, aft of the bows, by way of respective universal joints 202 to the bows of the pontoon 201 and by respective universal joints 202 to the stern of the pontoon 201. The joints 202 connecting the pontoon 201 to the vessels 200 are attached to the vessels at locations where isolated and strengthened void spaces 203 have been provided. Chains 204 extending ahead and astern from the pontoon 201 and supported by intermediate nylon spring purchases 205 also connect the pontoon 201 to the vessels 200. By means of respective cableand-pulley arrangements, the forward vessel-side mooring attachments of the forward chains 204, which attachments run in vertical guides 290, are kept at the same level as the after pontoon mooring attachments thereof. Respective similar arrangements are used for the after chains 204, except that these are supported by twin floats 291. Yokohama fenders 206 are interposed between the vessels 200 and the pontoon 201 and provided at the stern of the pontoon 201.

The vessels 200 are thereby so interconnected as to subtend an angle of about 200, in fact 22.50 so that each vessel can be 1 point off wind. The haven would have at least one tug or supply vessel 207. The pontoon 201 carries a turret 208 which is moored to the ocean bed and around which the pontoon 201 and thus the vessels 200 may turn to weathervane into the wind. Figure 22 shows a service tanker 209 moored by lines 210 in the haven so provided.

Referring to Figures 23 and 24, which show one of the joints 202, the joint comprises a vertical guide 211 extending the full height of the side of the vessel 200 and fixed to the hull of the vessel. It provides a partially enclosed track in which runs a carriage 212. The design of the guide 211 and the carriage 212 is such that the only movement which the carriage can perform relative to the guide is movement to and fro along the guide. Connected by means of a horizontal swivel pin 213 to the carriage 212 is a yoke 214 between the free ends of the limbs of which a block 21 5 is so pivotally mounted as to be turnable about an axis perpendicular to that of the swivel pin 213.An H-piece 216 is, at one end, so pivotally connected to the block 21 5 as to be turnable relative thereto about an axis perpendicular to the axis of turning of the block 21 5 relative to the yoke 21 4. The other end of the H-piece 21 6 is articulated to a yoke 21 7 by way of a block 218 in a manner identical to its articulation to the yoke 214. The yoke 21 7 is fixed to the pontoon 201. The effect of the universal joints 202, with the aid of the fenders 206, is that vessels 200 can move a significant extent relative to each other in substantially all senses except horizontally towards and away from each other.

Referring to Figures 25 and 26, the turret 208 comprises framework 219, and chain stoppers 220, chain fairleads 221, chain guide tubes 222 between the chain stoppers 220 and the fairleads 221, chain storage tubes 223, crude oil supply lines 224, and a spider 225, all supported by the framework 219. The pontoon 201 includes framework 226 supporting flexible import hoses 227 connected to the tanks of the tankers 200, rollers 228 guiding the pontoon 201 on the turret 208, and other rollers 229 supporting the overhung central portion of the framework 226 on the turret 208. The pontoon 201 is provided with a breakwater 230.

Referring to Figures 27 to 30, the framework 226 includes a turntable 231 which, by means of the rollers 229, can rotate on tracks 232 fixed to the framework 219. The supply lines 224 are connected at their upper ends, through respective va!ves 233, to the spider 225, of which a vertical central pipe 234 is connected via a swivel joint 235 to a vertical central pipe 236 fixed to the framework 226 and leading to the hoses 227. An electrical supply cable 237 mounted on the framework 226 is connected to a moving part 238 of an electrical slip ring 238, 239, encircling the pipe 236. The moored part 239 of the slip ring is fixed, by means of a cage 240, to the pipe 234.

Electrical leads 241 extend from the slip ring part 239, via a gap between the pipe 234 and the turntable 231, to transducers 242 and thence to load cells 243. A water-tight hatch 244 gives access to the interior of the turret, whilst an access to a moonpool spider deck is indicated at 245.

Each chain stopper 220 comprises a frame 246 in which is mounted for rotation about a horizontal axis a chain-guiding pulley 247 provided with a circular ratchet 248 with which co-operates a pawl 249 turntable about a parallel horizontal axis by means of an hydraulic ram 250. The piston rod of the ram 250 is articulated to a radial arm 251 fixed to one end of a horizontal shaft 252 to which the pawl 249 is fixed. The pulley 247 is itself rotatably mounted on an axle 253 which has fixed to one end thereof a radial arm 254 articulated to the piston rod of an hydraulic ram 255. At that same end the axle 253 is mounted in the frame 246 by way of a spherical bearing 256. At its other end, it is received with play in a housing 257. Two U-bolts 258 embrace the axle 253 in the housing 257 to support the same, and are themselves supported on the load cell 243 by way of a plate 259.The axle 253 is provided with an end plate 260, whilst the housing 257 is provided with an end cover 261 , but both of these have been omitted from Figure 27 for the sake of clarity. The load cell 243 is supported on the circular, cylindrical external surface of the housing 257 by way of a complimentarily shaped plate 262. Moreover, the shaft 253 is connected to the U-bolts 258 by keying 263. Thereby, through operation of the ram 255, the items 243, 253, 258, 259, 260, 262 and 263 can be turned between the position shown in Figures 27 to 29, which they occupy when the chain stopper is in the stoppered condition, and the position shown in Figure 30, which they occupy when the chain stopper is in the free condition.The dimensioning of the chain stopper, including the size of the angle through which the load cell 242 is turned between the positions shown in Figures 27 and 30, as well as the angle enclosed by the chain 264, are so selected that the resultant load on the axle 253, and thus the output signal of the load cell 243, is the same in the stoppered condition shown in Figure 27 as it is in the free condition shown in Figure 30, even though the chain tension at full scale of a stoppered chain tension dial of a gauge connected to the load cell is considerably greater (at 1000 tons) than the chain tension (at 400 tons) at full scale of a free chain tension dial of the gauge.

The arrangement of the chain stoppers 220 in a ring around a central turret round which a pontoon is turnable also has the advantage of facilitating the establishment of the moorings. For this purpose, a single windlass, indicated in broken lines at 265 in Figure 27 can be mounted on the turntable 231 prior to the mounting on the pipe 234 of the items 236 to 241, and with the windlass aligned with the chain stopper, the chain 264 is passed through the relevant chain stopper 220 to the windlass 265, as indicated by the broken lines 264' between the chain stopper and the windlass and in the pipe 234. The chain 264 is then drawn in until it is under a desired tension, as measured by the load cell 243 in the condition shown in Figure 30. Thereupon, the pawl 249 is applied and that chain is then removed from the windlass and fed into the tube 223.Then the pontoon is turned about the turret 208 until the windlass is in alignment with another chain stopper 220, and the individual chain 264 thereof is again tensioned in the same manner and removed from the windlass in the same manner.

This process continues until all of the mooring lines 264 have been established. Then the windlass is removed from the turret.

In the oil supply system shown in Figures 22 to 28, mixing of the oil from the respective lines 224 in the common pipe 234 has been assumed to be permissible. However, if the crude oil supply is from a number of platforms or fields, or direct from individual sub-sea wellheads, it may be necessary to import it by separate lines from the turret 208, in a case where differential processing, metering, or sampling, or segregated storage, may be required. For this purpose, an hydraulic slip ring as shown in Figures 31 and 32 is provided in place of the spider 225. This slip ring comprises a horizontal upper disc 270 fixed relative to the framework 226, and a horizontal lower disc 271 fixed relative to the framework 21 9. The upper disc 270 is guidedly supported on the lower disc 271 by means of an inner ring and an outer ring of rollers 229'.Formed in the underneath surface of the disc 270 concentrically with the common axis of the discs 270 and 271 is a series of annular recesses 272, whilst formed in the top surface of the disc 271 and in registry with the respective recesses 272 is a series of annular recesses 273.

Interposed between each pair of registering recesses 272 and 273 and each adjacent such pair, as well as radially inwardly of the innermost pair and radially outwardly of the outermost pair, are annular packing devices 274 to prevent leakage of oil from the individual recess pairs.

Spaced around the disc 271 and communicating with the respective recesses 273 are connection holes 275 for oil inlet lines (not shown), whilst arranged at intervals around the disc 270 and communicating with the respective recesses 272 are connection holes 276 for respective oil outlet lines (also not shown). The framework 219 includes a spoked support ring 277. However, the rim of this ring is liable to obstruct the path of at least the outermost of the oil inlet lines, and in order to avoid this, a segment 278 of this ring can effectively be shifted radially inwardly to make room for the outermost inlet line. If the next inlet line is also liable to be obstructed, then another segment 279 of the support ring 277 can be formed with a recess 280. Tell-tale cocks 282 can be provided beneath covers 281 in the disc 270, for use in inspecting for any leakage past the packings 274.

Referring to Figures 33 and 34 the rig 1 31 is traversable along guide rails 1 32 themselves traversable along guide rails 133 perpendicular to the guide rails 132 and fixed to a top deck 134 itself fixed to a well head deck 135. Well heads 1 36a and 1 36b are fixedly mounted on the well head deck 135 and casing strings 137 extend therefrom through the lower part 1 28 (not shown) of the platform 114 to the platform 103 and thence into the sea bed 121. Thus the rig 131 is fixed in a vertical sense relative to the casing strings 137.Figure 33 shows a drilling well head 1 36a with a blow-out preventing arrangement 136c, and production well heads 1 36b. The deck 1 35 has fixed thereto constant-tension winches 1 43 which supply a constant tension to the guide lines 1 94 in the form of thin cables fixed at their lower ends to the template 108 of the platform 1 03. These guidelines serve for running the bits and casing shoes prior to casing conductors being established. They also indicate any distance variation between the platform 103 and the deck 135. Other winches 292 drive cables 293 serving to lift and lower the guideframe 1 76.In practice, after cementation, the casing strings 137 are pulled, if necessary, to ensure that they are hungoff in tension, that is to say are under a tension of for example 5% greater than their platform-totemplate hanging weight. Each casing string 137 includes, just above the platform 103, a corrugated section 145 whereof the corrugations extend peripherally around the casing. These sections 1 45 terminate at their lower ends in cement returns 1 46. The sections 145 are about 50' in length and must not be cemented, as their purpose is to provide compliancy with any fluctuation in the residual tensions of the respective casings, as well as with any lateral movement that might occur.

The apparatus described with reference to the drawings has the following advantages: 1. It lends itself to speedy emplacement, leading to a reduction in "on site" building and installation costs.

2. Its top side drilling and production control permit large bore as-on-land operations.

3. Deep water recourse to sophisticated subsea well heads and gathering systems, with their inherent lack of reliability and servicing facility, is unnecessary.

4. It can operate in remote areas with water depths beyond pipe-laying feasibility.

5. It can include provision in the vessels 200 of intermediate storage of treated field production to avoid well shut-ins between load-out operations.

6. The rig, the drill string and its appropriate selection of chemicals can remain on the platform 114 after completion of drilling for down hole repairs.

7. In having six legs, it has leg redundancy to allow them to be raised individually for servicing so that they can possibly outlast the field life.

8. The tanker need not be directly attached to the platform 1 4.

9. It may be possible to effect ioad-out at a rate of ten thousand tons per hour. In that case the disparity between the loading rate and the field flow rate can mean that only small "weather windows" are sufficient to maintain continuous field production.

1 0. Recoverability of the whole apparatus permits re-allocation to other areas.

11. It does not require a shore-based terminal.

12. It can function in deep, remote and tempestuous waters.

Claims (27)

1. A marine apparatus, comprising a floating operations platform, a buoyant anchoring platform sinkable to the sea bed, and tethering connections arranged to tether the floating operations platform to the sunk anchoring platform.

2. A method of providing a tethered offshore operations platform, comprising providing in a coastal region a floating operations platform and a buoyant anchoring platform, interconnecting the two platforms by way of tethering connections, towing the two platforms out to sea, and sinking the buoyant anchoring platform to the sea bed while lengthening the tethering connections.

3. A method of providing a tethered offshore operations platform, comprising providing in a coastal region a buoyant operations platform and a buoyant anchoring platform supporting said operations platform, towing the two platforms out to sea with the anchoring platform supporting the operations platform, and sinking the buoyant anchoring platform to the sea bed while lengthening tethering connections interconnecting the two platforms and while leaving the operations platform floating in the sea.

4. Apparatus as claimed in claim 1 or a method as claimed in claim 2 or 3, wherein said tethering connections comprise lattice-form legs.

5. Apparatus or a method as claimed in claim 4, wherein said legs each comprise substantially vertical tubes and latticework interconnecting the tubes.

6. Apparatus or a method as claimed in claim 5, wherein each of said legs is substantially weightless in seawater.

7. Apparatus or a method as claimed in claim 5 or 6, wherein each tube is provided with nonreturn inlet valve means whereby compressed gas can be supplied to the hollow interior of the tube.

8. Apparatus or a method as claimed in claim 4, 5, 6, or 7, wherein each leg comprises at least one rack extending therealong, and wherein said operations platform carries, for each leg, at least one pinion for meshing with the rack(s) and for use in lowering the leg.

9. Apparatus or a method as claimed in any one of claims 4 to 8, wherein the legs are releasably connected to the anchoring platform by joints, and wherein joint control means is operable to release said joints even when said joints are under the surface of the sea.

10. Apparatus or a method as claimed in any preceding claim, wherein there is provided depending from said operations platform a well casing comprising a tubular wall formed with annular corrugations extending peripherally therearound.

11. Apparatus or a method as claimed in any preceding claim, wherein there is provided an artificial haven, comprising two vessels arranged in a V-formation, and connecting means connecting the two vessels together in such a manner that they can move a significant extent relative to each other in substantially all senses except horizontally towards and away from each other.

1 2. Apparatus or a method as claimed in claim 11 , wherein said connecting means includes a pontoon.

13. Apparatus or a method as claimed in claim 12, wherein said haven further comprises a movable turret encircled by said pontoon and around which said pontoon and said vessels can rotate together.

14. Apparatus or a method as claimed in claim 13, wherein said haven further comprises a slip ring for fluids, comprising first and second members arranged in a fluid-tight manner relative to each other and rotatable relative to each other about the axis of said turret, the first member being fixed to said turret and the second member being fixed to said pontoon, a plurality of separate flow paths each from the outside of the first member through the first and second members to the outside of the second member, each of the flow paths including in one of the members a substantially circular channel in the inside surface of that member and substantially concentric with said axis.

1 5. Apparatus or a method as claimed in claim 14, wherein each of said flow paths also includes in the other of the members a substantially circular channel in the inside surface of that member and substantially registering with the associated channel in said one of the members.

1 6. Apparatus or a method as claimed in claim 13, 14, or 15, wherein said haven further comprises a windlass situated on the axis of said turret, and a plurality of chain stoppers arranged around said axis for having respective chains therein tensioned in turn by said windlass, said chain stoppers being attached to said turret and said windlass being turnable relative to said chain stoppers about said axis.

1 7. Apparatus or a method as claimed in claim 16, wherein each chain stopper comprises a chain guiding pulley, and mounting means mounting rotatably said pulley on said turret in such a manner as to allow a limited amount of movement of said pulley relative to said turret in a sense transverse to the axis of rotation of said pulley, this means comprising a load measuring device by way of which the pulley is mounted on said turret and which is turnable, relative to said turret and the pulley, about said axis of rotation of said pulley between first and second positions in both of which it performs the pulley-mounting function.

1 8. Apparatus or a method as claimed in any one of claims 11 to 1 7 wherein the enclosed angle of the V-formation is about 200.

1 9. A frame member for subsea use, comprising a hollow body, and non-return inlet valve means whereby compressed gas can be supplied to the hollow interior of said body.

20. An artificial haven, comprising two vessels arranged in a V formation, and means connecting the two vessels together in such a manner that they can move a significant extent relative to each other in substantially all senses except horizontally towards and away from each other.

21. A slip ring for fluids, comprising first and second members arranged in a fluid-tight manner relative to each other and rotatable relative to each other and rotatable relative to each other about an axis of rotation, a plurality of separate flow paths each from the outside of the first member through the first and second members to the outside of the second member, each of the flow paths including in one of the members a substantially circular channel in the inside surface of that member and substantially concentric with said axis.

22. A windlass situated on a substantially vertical axis and a plurality of chain stoppers arranged around said axis for having respective chains therein tensioned in turn by said windlass, said windlass, on the one hand, and said chain stoppers, on the other hand, being turnable relative to each other about said axis.

23. A method of tensioning chains in respective chain stoppers, comprising operating a windlass to tension a first chain stopper aligned with the windlass, operating the windlass to relax the portion of the first chain between the windlass and the first chain stopper, the portion of the first chain beyond the first chain stopper remaining tensioned, turning the windlass, on the one hand, and the first chain stopper and a second chain stopper, on the other hand, relative to each other about a substantially vertical axis to align the windlass with said second chain stopper, the first chain being removed from the windlass, introducing a second chain in the second chain stopper into the windlass and operating the windlass to tension the second chain.

24. A chain guiding apparatus, comprising a chain guiding pulley, a support, and means mounting rotatably.said pulley on said support in such a manner as to allow a limited amount of movement of said pulley relative to said support in a sense transverse to the axis of rotation of said pulley, said means comprising a load measuring device by way of which said pulley is mounted on said support and which is turnable about said axis relative to said support and said pulley between first and second positions in both of which it performs the pulley-mounting function.

25. A well casing comprising a tubular wall formed with annular corrugations extending peripherally therearound.

26. Marine apparatus, substantially as hereinbefore described with reference to the accompanying drawings.

27. Marine method, substantially as hereinbefore described with reference to the accompanying drawings.