APPARATUS FOR MAINTENANCE OF OIL RIG LEGS

Abstract: In the area just above and below the surface of the water the swell is so bad that no divers or boats can get to the area to clean the legs. According to the invention there is provided apparatus (1) for maintenance of a length of an oil rig leg (2) in situ comprising a sleeve-like vessel (3) for surrounding a given length of an oil rig leg, and including a releaseable leg clamp for detachably clamping the vessel to the oil rig leg; the sleeve-like vessel consisting of at least two pieces (5, 6) which are clamped together by clamps which are capable of being released to allow separation of the two pieces to allow removal of the vessel from the oil rig leg (2), the vessel being arranged to define a space between the leg and the inner wall of the vessel, which extends around the periphery of the leg and is large enough to allow an operator to access the whole surface of the given length of the leg to maintain it, and the vessel also including a base at its lower end which spans the space between the inner wall of the vessel and the surface of the leg; the apparatus also including an entrance (11) to allow entry of an operator (10) and a pump for pumping water out of an output opening in the vessel.

Description: APPARATUS FOR MAINTENANCE OF OIL RIG LEGS Field of Invention This invention relates to apparatus for maintenance of oil rigs and in particular to maintenance of offshore oil rig legs . In the following specification the term "oil rig" will be used to encompass all drilling rigs and includes both rigs for drilling for oil and for gas . Offshore oil rigs are subject to the effects , both of extremes in weather conditions , and of the sea on the oil rig legs . The major length of the oil rig leg lying beneath the water is inspected and maintained by divers . The legs have to be inspected so that they can be cleaned and repaired if they start rusting . A major problem occurs in the area just above and below the surface of the water . This area is about 12 feet deep and is known as the "splash zone" . At this zone the swell is so bad that no divers or boats can get to the area to clean the legs . Unfortunately this area of the leg is the very one which takes the worst weathering and deteriorates very quickly . Also in the splash zone there is no possibility of any of the marine growth occuring on the legs which can grow on other regions of the legs . This is unfortunate because such marine growth serves to protect the surface of the legs from the effects of the water . However , because of the heavy swell the marine growth cannot manage to grow in the swell zone. The result is that the swell zone of the oil rig legs gets the worst weathering , has no protection and cannot easily be Inspected and maintained . In many cases, the area just never gets cleaned and maintained . In some cases this has resulted in the oil rig collapsing because the legs at this point have been allowed to rot away. Summary of the Invention According to a first aspect of the invention there is provided apparatus for maintenance of a length of an oil rig leg in situ comprising a sleeve like vessel for surrounding a given length of an oil rig leg, and including a releasable leg clamp for detachably clamping the vessel to the oil rig leg; the sleeve-like vessel consisting of at least two pieces which are clamped together by clamps which are capable of being released to allow separation of the two pieces to allow removal of the vessel from the oil rig leg, the vessel being arranged to define a space between the leg and the inner wall of the vessel , which extends around the periphery of the leg and is large enough to allow an operator to access the whole surface of the given length of the leg to maintain it, and the vessel also including a base at its lower end which spans the space between the inner wall of the vessel and the surface of the leg; the apparatus also including an entrance to allow entry of an operator into the vessel , and a pump f r pumping water out of an opening in the vessel . In use the two pieces of the vessel may be lowered into the water and placed around an oil rig leg at a point where the leg Is to be maintained . The two pieces are clamped together to form a rigid body and the whole arrangement is clamped to the oil rig leg. The pump is then operated to pump water out of the vessel and the operator can then enter into the vessel to inspect and maintain the oil rig leg. This provides the only available method for inspecting and maintaining oil rig legs in the splash zone since the operators will be protected from the swell whilst being able to clean the legs . This will considerably extend the lifetime of oil rigs. The top of the vessel may be open so that an operator may simply climb into the vessel over the top edge . However , in cases where the swell is very bad this could be very dangerous since it could allow the entry of waves into the vessel . It is therefore preferred in this case that a pressurised container serving as a lid is attachable to the top of the vessel to seal it completely from the outer elements . This would mean that an operator could climb into the vessel , close the lid and then be completely protected from the outside elements whilst inspecting and cleaning the legs . Preferably , however , the vessel also includes an integral top which spans the space between its inner wall and the leg. In this case it is preferred that the operator entrance includes a sealable manway in the top of the vessel comprising a cylin¬ drical opening into the vessel having a lockable flap lid. Preferably the flap lid includes sealing means to ensure that no water can enter the vessel when an operator is working within. Since the splash zone is around 12 feet deep it is preferred ^ that the vessel is deep enough to cover the whole of the splash zone which may conveniently be up to 18 feet long . In this case it is preferred that a ladder extends up the inner wall of the vessel to allow the operator to climb up and down the vessel to access different areas of the oil rig leg. In this case it is preferred that the vessel includes intermediate platforms which extend around the inner wall of the vessel to allow the operator to stand at different levels to inspect and clean the oil rig legs . In the case where platforms are provided , it is preferred that the platforms do not extend up to the oil rig legs but leave a gap between the leg and the platform so that if the leg is being cleaned , for example by a blasting the debris may fall between the platform and the leg to the base of the vessel where it can be pumped out using the pump to keep the vessel clean inside. The intermediate platforms may be permanent platforms attached to the wall of the vessel . Preferably however the platforms are temporary and the vessel includes a plurality of support means at different heights within the vessel , on which the temporary platforms may be mounted . This allows greater flexibility of operation within the vessel ; The platforms may, for example , be made of timber or of scaffolding. Conveniently the vessel is cylindrical for the' major portion of its depth between its base and top . The vessel has to be made of a strong but lightweight material . Typically this material will be steel . The vessel will preferably include a plurality of stiffening rings , extending about the periphery of the vessel to give added strength to the vessel . The stiffening rings may extend from the outer periphery of the vessel outwards . However it is preferred that the stiffening rings extend inwardly of the periphery of the inside face of the wall of the vessel . In this case the stiffening rings may conveniently also constitute the support means for temporary intermediate platforms . The stiffening rings are conveniently made of the same material as the rest of the vessel . It is preferred that the vessel is made up of two substantially identical halves which helps reduce manufacturing costs . However in some cases the halves are identically shaped apart from the clamping means which they include. One piece may incorporate male -clamping means and the second piece include mating female clamping means . In some cases each -piece includes a leg clamp for clamping each piece to the oil rig leg . However it has been found to be convenient to include only one piece including the leg clamp to clamp the piece to the oil rig leg , with the other piece of the vessel including only means to clamp it to the one piece . In the most preferred arrangement each piece includes a semi- cylindrical wall each having flanges extending from their longitudinal peripheral edges which are used to clamp the two pieces together . The apparatus may also include buoyancy rings attached to the vessel . These keep the vessel buoyant , especially when the pieces are being moved around about the leg before fitting a clamp to the leg. In this case it is preferred that at least one of the buoyancy rings is of adjustable buoyancy. An air supply with cocks leading to the buoyancy ring may be used by which the buoyancy can be increased or decreased to raise or lower the vessel in the water . This allows for complete control of the halves of the vessel when they are being manoeuvred into position about the oil rig leg . It is noted , however , that provision of such buoyancy rings may be unnecessary if the vessel body is made of a lightweight material . Moreover in some cases it is preferred that no buoyancy rings are included since they may exert unnecessary stress on the walls of the vessel . Preferably around the top of the vessel there are provided lifting eyes which can be engaged by lifting hooks on a boat or on the oil rig to help control movement of the halves of the vessel , during installation of and removal of the vessel . Installation of the vessel onto an oil rig leg itself produces problems . This is because a piece of the vessel can only be lowered into the water and moved against the oil rig leg in a controlled manner over the outer edge of the oil rig. Thus there is no difficulty in positioning one piece (the piece lying over the surfaςe of the leg facing outwards of the rig) but there is great difficulty in positioning the second piece which covers the inwardly facing surface of the leg. According to a second aspect of the invention there is provided a method of installing apparatus in accordance with the first aspect of the invention on an oil rig leg comprising the steps of : positioning a collar around the oil rig leg at a position lying just beneath the required position of the apparatus ; lowering a first piece of the apparatus down the outer edge of the oil rig and oil rig leg until it bears against the collar ; loosely clamping the first piece to the oil rig leg to locate the piece onto the leg but allow rotation of the piece with respect to the leg ; rotating the first piece through 180° about the oil rig leg so that it lies underneath the oil rig ; clamping the first piece firmly to the leg ; lowering the second piece of the apparatus down the outer edge of the oil rig and oil rig leg until it bears against the collar ; and , clamping . the second piece to the first piece of the vessel . The pump is then operated to pump all water out of the vessel to allow an operator to be able to enter the vessel to work on the oil rig leg . Brief Description of the Drawings Two examples of apparatus in accordance with the first aspect of the invention, together with a method of installation in accordance with , the second aspect of the invention will now be described , by way of example only , with reference to the accompanying drawings , in which: - Figure 1 is a partially cutaway schematic " perspective view of a first example of the apparatus ; Figure 2 is a partly sectioned elevation, of the apparatus in Figure 1 ; Figure 3 is a sectional elevation in direction I-I in Figure 2; Figure 4 is a sectional elevation in direction II-II of Figure 2 ; Figure 5 is a sectional elevation in direction III-III of Figure 2; Figure 6 is an enlarged section through line IV-IV of Figure 2; Figure 7 is an enlarged view of the ceiling arrangement shown in Figure 2 ; Figure 8 is a schematic view of an alternative deck structure * of Figure 2; Figure 9 is a sectional elevation of a second example of the apparatus; Figure 10 is a sectional elevation in direction A-A of .Figure 9; Figure 11 is a sectional elevation in direction B-B of Figure 9 ; Figure 12 is an enlarged section showing detail C of Figure 10; Figure 13 is an enlarged section showing detail D of Figure 11 ; Figure 14 is an enlarged section showing detail E of Figure 9 ; Figure 15 is an enlarged section showing detail F of Figure 11 ; Figure 16 is a section along G-G of Figure 15 ; Figure 17 is an enlargement showing detail H of Figure 11 ; Figure 18 is an exploded enlargement showing detail I of Figure 11 ; Figures 19 to 24 are schematic drawings showing 6 steps of the method of installing the apparatus . Description of Preferred Embodiment Both the first and second examples of apparatus are designed for maintaining a given length of an oil rig leg 2. The mainten¬ ance includes inspection, cleaning and repair . Clearly the appar¬ atus may equally be used to maintain a gas rig leg . The apparatus 1 comprises a sleevelike vessel 3 for surrounding the given length of oil rig leg 2 and releaseable clamps 4 - for detachably clamping the vessel 3 to the oil rig leg 2. The vessel 3 is made up of two pieces 5 and 6 which clamp together at 7 to form a rigid body, but which are readily separable to remove the vessel from the oil rig leg 2. The vessel 3 has a closed base 8 and is hollow to define a space 9 defined between the leg 2 • and the inner wall of the vessel 3 which extends around the periphery of the leg 2 and is large enough to allow an operator 10 access to the total surface of the given length of leg 2 so that he can main¬ tain it . The top end 11 of the vessel 3 is open to provide an entrance to allow entry of an operator 10 in to the vessel 3 . A pump (not shown) is situated at base of the vessel 3 and pumps water out of the vessel . The two halves 5 and 6 are fastened together using three lever toggle release clamps (not shown) . The halves 5 and 6 are sealed together by a flexible bead seal 34 which is a sponge rubber gasket . The halves 5 and 6 are sealed to the leg using collar 33 which is a polypropylene former of neutral buoyancy . Between collar 33 and halves 5 and 6 is flexible bead seal 34. The vessel 3 is made of mild steel which is light and easy to assemble. Also it has high corrosion resistance and is easy to repair. The two halves 5 and 6 are identical and comprise semi-cylinders with flanges 12 by which they can be sealed together . Extending the entire length of each half 5 or 6 is a ladder 13 which allows the operator 10 access to the entire length of oil rig leg 2 surrounded by the vessel 3. At the top 11 of the vessel 3 a collar 14 extends outwardly to form a flat" flange which provides a platform for the operator to use when he is entering or leaving the vessel 3. Handrails 15 are shown in dotted line as the top of the vessel 3. These can be included if required to extend the work zone or for safety reasons and bad weather conditions . If the weather is really bad a cover similar to the base sealed to the leg 2 may be included at the top of the vessel 3 ( shown in dotted line 16 in Figure 2 ) to provide a pressurised chamber in which the operator 10 Is completely sealed from bad weather or bad swell . Within the vessel 3 are two intermediate decks 17 and 18 and a lower deck 19 so that the operator 10 may stand safely at different levels in the vessel 3. These platforms 18 and 19 do not extend up to the leg but leave a space 20 to allow any debris produced when the leg 2 is being cleaned to fall down to the base 8 of the vessel 3. A large bore drain connection 21 is included in each half 5 , 6 for removal of entrapped water and debris using the pump (not shown) To strengthen the whole structures of the apparatus 1 stiffening rings 22 , 23 are provided which extend around the body of the vessel 3. These stiffening rings 22 , 23 also include buoyancy rings to act as buoyancy collars . The buoyancy collar 22 is neutral and of fixed buoyancy but the upper buoyancy collar 23 is of variable buoyancy . An air suppl y manifold 24 is connected to adjustable buoyancy tanks 25 within collar 23 via three way cocks 26. These adjustable buoyancy collars are used in helping to manoeuvre the halves 5 and 6 into position on the oil rig leg 2. Additional ballast 27 is fixed beneath the lower deck 19 to help add to the buoyancy and stability of the structure . Bracing steadies 28 which are adjustable extend between upper deck 17 and the oil rig leg 2 to brace the vessel 3 against the leg 2 . Deck supports 29 lie beneath the decks to support them in position. Lifting eyes 30 are positioned around the top of the vessel 3 to aid in lifting and manoeuvring the halves 5 , 6 into position. A sealing arrangement of the base 8 to the oil rig leg 2 i s shown in detail in Figure 7. An alternative top- deck 31 is shown in Figure 8 which provides for a cover to be placed over the vessel 3 . The apparatus 1 is placed into position by the following steps . The area just below 32 , the area to be cleaned of the oil rig leg 2 , is carefully cleaned by divers to allow a good gripping surface for the sealing collar 33 of the vessel 3 . The first half 5 is lowered into position using the lifting eyes 30 and using the adjustable buoyancy tanks 25 to help lower the half into position . The half 5 is temporarily clamped to the oil rig leg and half 6 is then lowered into position and is aligned with half 5 and then sealed both to the leg 2 and to the other half 5 by seal ing arrangement 7 . The pump is then operated to evacuate all the water from inside the vessel 3 and an operator may stand on the platform 14 and then climb down the side ladder 13 to the required platform 18 to start work on the leg . 5 This arrangement is particularly useful for cleaning the area known as the splash zone of the oil rig leg . A second example of apparatus in accordance with the first aspect of the invention is shown in Figures 9 to 24. The apparatus 40 comprises a sleeve-like vessel 41 and release-. 10 able leg clamps 42 for clamping the sleeve-like vessel 41 to an oil rig leg 43. The sleeve-like vessel 41 is generally cylindrical and , in use , defines a space 44 between its inner surface 45 and the oil rig leg 43 . 5 The vessel includes a base 46 which spans the space 44 between the oil rig leg and the inner wall 45 of the vessel and also " includes an integral top 47 which also spans the gap 44 between the inner wall 45 and the oil rig leg 4"3. The vessel includes a sump 48 which provides an output opening through which 20. water may be pumped out of the vessel 41 by a pump forming part of the apparatus (not being shown) . The top of the vessel 47 includes an entrance 49 comprising a cylindrical bore of a diameter such that an operator may enter through the bore into the vessel 41 . The entrance 49 5 has a flap lid 50 which can be opened to allow movement of an operator but which can be locked closed to form a sealed unit within the vessel 41 . A seal 51 extends around the edge of the flap lid 50 to ensure that the opening is watertight . This means that if an operator is working within the vessel 0 41 water cannot enter the vessel through , the entrance 49 . The vessel 41 consists of two pieces 52 and 53 , each piece 52 and 53 is generally semi-cylindrical but includes a pair of peripheral flanges 54 , 55 , 56 and 57 respectively . The two pieces 52 and 53 may be clamped together by passing bolts 58 through the pairs of flanges 54 , 56 and 55 , 57. However , the bolts may be released to allow the pieces 52 and 53 to be separated to allow the vessel 41 to be removed from an oil rig leg . The vessel 41 has an overall depth of six meters so that the vessel may enclose the "splash zone" even in regions where there is a heavy swell . The internal diameter of the vessel 41 is 3.6 meters which gives an operator room to work within vessel 41. The internal diameter of the base 46 and top 47 of the vessel are 1.092 meters . The vessel 41 and the major components of the apparatus are all made of mild steel of grade 50D to British Standards BS4360. In the lower region of the vessel 41 the outer wall 59 is of 12 millimeter steel plate but in the upper region where the pressures on the shell will not be- as great the thickness of the shell is 10 millimeters . Details of the seal between the two pieces 52 and 53 are shown in detail in Figures 12 and 13. Between flange 54 and 56 are placed a pair of seals . The inner seal 60 is a hard neoprene seal and an outer seal 61 is made of soft neoprene . The bolt 58 is passed through bores 62 in the flange 54 and 56 and a nut 63 is used to tighten the bolt in position. At the region where the bolt passes through the flanges a 15 x 12 millimeter compression stop 64 is placed between the two pieces at the inner edge of the flange 54. In order to stabilise the flanges a plurality of brackets 65 support the flange at right angles to the shell wall of the vessel 41 . The shape of flanges 65 are shown in more detail in Figure 13. Extending around the inner periphery of the inner wall 45 of the vessel 41 are five stiffening rings 66. The lower stiffen¬ ing rings 66 are made of 12 millimeter thick stee.l with the upper ones being made of ' 10 millimeter thick steel . The shape of the stiffening rings are annular * and their cross section is shown in detail in Figure 14. Temporary platform (not shown in the diagram) can be supported by the stiffening rings 66 to provide a temporary landing on which an operator may work. The two pieces 52 and 53 are not identical and their differences may most clearly be seen by referring to Figure 11 . Piece 53 has rigidly mounted to it a half of leg clamp 42 whereas piece 52 is only temporarily attached to the second half of the leg clamp 42. The leg clamp consists of two semi -cylinders 67 , 68 of depth 30 centimeters .each having - a pair of flanges 69 , 70 , 71 , 72 respectively by which they can be clamped together . The first semi-cylinder 67 is attached to the piece 53 but the other semi-cylinder 68 is not attached to the vessel 41. The diameter of the semi-cylinders 67 and 68 is 1.016 meters and a layer of 12 millimeter thick neoprene 73 lies on the inner surface of each semi-cylinder 67 , 68 to seal it to the oil rig leg. The semi-cylinder 67 is joined to the piece 53 by connecting arm 74 shown in more detail in Figures 15 and 16. The connecting arm 74 comprises a 12 millimeter thick plate 75 having depending downwards from it two plates 76 to strengthen the arm 74. The width of the arm 74 is 25 centimeters . The arm 74 is welded both to the ring stiffener 66 and to the leg clamp 67. In use piece 53 of the vessel is presented first to the oil rig leg with the leg clamp half 67 bearing against the oil rig leg. The second semi-cylinder 68 of the leg clamp is then clamped to the mating semi-cylinder 67 by passing bolts 77 through the flanges 69 and 71 to clamp the two semi- cylinders together to clamp piece 53 to the oil rig leg . Once the piece 53 has been clamped in place to the oil rig leg the piece 52 may then be moved to the piece 53 so that the flanges 54 and 56 and 55 and 57 are aligned and then bolts ' 58 are then used to fasten the flanges together . So a temporary connecting arm 78 is used to attach the piece 52 to the semi- cylindrical p -e 68 of the leg clamp 42. This is shown in more detail in Figure 18. The leg clamp 68 has mounted upon it a lug 79 including a 32 millimeter diameter hole 80. The " ring stiffener 66 of the ' piece 52 includes an aligned lug 81 also including a 32 millimeter diameter bore 82. Onto these is placed temporary connecting arm 78 which is a plate of a length so that it can extend across the space between the oil rig leg and the stiffening ring and includes a pair of slots 83 through which a bolt may be passed to fasten the connecting arm slideably to the lugs 79 and 81 respectively . This provides further support for the piece 52 and braces it from the inner leg clamp 68. The overall weight of the apparatus is ten tonnes . In this case , there is no requirement for any additional buoyancy and the vessel may be easily installed and removed as shown in the Figures 19 to 24 which show the method of installation in detail . The relevant parts . of an oil rig will first be identified . The oil rig 80 includes a cellar deck 81 and a spider deck 82 beneath the cellar deck 81 which is the deck closest to sea level 83 . The oil rig 80 has a platform crane mounted on it (not shown) which has extending from it chain 84 with a hook 85 which can engage the vessel to move it . The arrow X indicates outboard direction and arrow Y denotes inboards direction. In order to install the apparatus the following steps are taken. A winch 86 is installed on the spider deck and hoist attachment 87 are attached underside of the cellar deck. A collar 88 is then attached to the jacket leg of the oil rig below sea level 83 at a level just beneath the required position of the apparatus . Piece 53 of the vessel is coupled to hook 85 and is lowered close to sea level 83 using the platform crane . In the next stage , as shown in Figure 20 , the section 53 is transferred onto the winch line 89 and is lowered into position at the outboard face of the jacket leg 90 until it sits against collar 88. At the third stage as shown in Figure 21 the second semi- cylinders 68 of the leg clamps are brought into alignment with the semi-cylinders 67 of the leg clamps and are bolted together to ensure parallel alignment of the piece 53 with the jacket leg 90. The bolts are sufficiently slack to allow rotation of the piece 53 about the jacket leg 90. The fourth stage of operation, as shown in Figure 22 , the winch line 89 is released and tirfors (not shown) are used to roate piece 53 about 180° about the jacket leg 90 until it is in an inboard position on the leg 90. The bolts of the inside clamps are tightened to lock the piece 53 onto the jacket leg 90 and the tirfors can then be released . At the fifth stage , as shown in Figure 23 of the drawings , the platform crane is used together with winchline 89 to lower piece 52 of the vessel onto the collar 88 on the jacket leg 90. The seam bolt between the flanges of pieces 52 and 53 are tightened to a pre-determined torque using a pneumatic torque wrench . The piece 52 is also attached to the leg clamp 68. Winch line 89 is then removed and operational tirfors with induced tension are installed (not shown) , the tension being sufficient to resist any further downward movement of the vessel . Stuffing glands (not shown) are inserted at the base of the vessel between the vessel and the leg to tighten the seal between the two. Figure 24 shows the operational tirfors 91 in position and also shows the position 92 of the lower stuffing gland and 93 of upper suffing glands which may be inserted at the top of the vessel if necessary . The excess water is then pumped out of the inside of the vessel through the sump . The lower seam bolts and stuffing gland bolts are tightened to minimise or stop seepage of water.

Claims: CLAIMS: 1. Apparatus for maintenance of a length of an oil rig leg in situ comprising a sleeve like vessel for surrounding a given length of an oil rig leg, and including a releaseable leg clamp for detachably clamping the vessel to the oil rig leg; the sleeve-like vessel consisting of at least two pieces which are clamped together by clamps which are capable of being released to allow separation of the two pieces to allow removal of the vessel from the oil rig leg, the vessel being arranged to define a space between the leg and the inner wall of the vessel , which extends around the periphery of the leg and is large enough to allow an operator to access the whole surface " of the given length of the leg to maintain it , and the vessel also including a base at its lower end which spans the space between the inner wall of the vessel and the surface of the leg; the apparatus also including an entrance to allow entry of an operator into the vessel , and a pump for pumping water out of an output opening in the vessel . 2. Apparatus according to Claim 1 , in which the top of the vessel is open, and the apparatus includes a lid attachable to the top of the vessel to form a sealed vessel . 3. Apparatus according to Claim 1 , in which the vessel includes an integral top spanning the space between the inner wall of the vessel and the oil rig leg, the top including a sealable opening openable to allow entrance of the operator into the vessel . 4. Apparatus according to any one of the preceding Claims in which the vessel is deep enough to span the whole of the splash zone of the oil rig leg. 5. Apparatus according to Claim 4 , in which the vessel includes at least one intermediate platform extending around the inner wall of the vessel to allow the operator to stand at different levels within the vessel . 6. Apparatus according to Claim 5 , in which the intermediate platform does not extend up to the oil rig legs but leaves a gap between the leg and the platform. 7. Apparatus according to any one of the preceding Claims , in which the major portion of the depth of the vessel between its base and top is cylindrical . 8. Apparatus according to Claim 7 , including a plurality . of stiffening rings extending about the periphery of the vessel . 9. Apparatus according to Claim 1 , in which the vessel includes at least one buoyancy ring consisting of a buoyant material . 10. A method of installing apparatus in accordance with Claim 1 , on an oil rig leg comprising the steps of : positioning a collar around the oil rig leg at a position lying just beneath the required position; lowering a first piece of the apparatus down the outer edge of the oil rig and oil rig leg until it bears against the collar ; loosely clamping the first piece to the oil rig leg to locate the piece onto the leg but allow rotation of the piece with respect to the leg; rotating the first piece through 180° about the oil rig leg so that it lies underneath the oil rig; clamping the first piece firmly to the leg; lowering the second piece of the apparatus down the outer edge of the oil rig and oil rig leg until it bears against the — . -20- "■' collar; and, clamping the second piece to the first piece of the vessel.

Inventors: ALGER DAVID, HARWOOD ANTHONY JACK, HARWOOD SUSAN BRENDA, ALGER, DAVID, HARWOOD, ANTHONY, JACK, HARWOOD, SUSAN, BRENDA
Applicants: HARWOOD SUSAN BRENDA, MAPLE ENG PROJECTS LTD, HARWOOD, SUSAN, BRENDA, MAPLE ENGINEERING PROJECTS LIMITED
Application year: 1986
Main classification: E02B17
IPC codes: B63C11, E02B17