RECOVERY OF SUBMERGED STRUCTURES

Abstract: Equipment and procedures for recovering submerged structures (114), comprising a frame (100) having controllable buoyancy and which is first submerged to hang beside the submerged structure. The frame is attached to the submerged structure, along with controllably inflatable airbags (110) for lifting the structure towards the surface. The raised frame and structure are rotated to horizontal, and placed aboard a barge (120) or other marine vessel for transport elsewhere. The equipment and procedure are particularly suited to recovering redundant offshore platform jackets. The equipment is simple, economical, and does not require a special-purpose ship.

Description: "Recovery of Submerged Structures" This invention relates to recovery of submerged structures, and relates more particularly but not exclusively to methods and apparatus for the recovery of offshore platform jackets, i.e. seabed-mounted towers used for exploration and production of hydrocarbons from offshore locations. Offshore platform jackets are very numerous, and eventually become redundant, at which time it is necessary or desirable that the jackets be removed from their operational site for reasons such as avoidance of navigational hazards, minimisation of environmental pollution, and recovery of recyclable materials. Installation of an offshore platform jacket at a predetermined offshore location usually involves land- based prefabrication of the jacket, transport of the jacket on a barge from the fabrication site to the selected offshore location, and controlled sinking of the jacket to land upright at the location, followed by anchoring (e.g. by attaching the jacket base to the seabed using piles) . Recovery of the jacket when redundant cannot readily be undertaken simply by reversing the installation procedure. According to a first aspect of the present invention there is provided recovery apparatus for the recovery of a submerged structure, the recovery apparatus comprising the combination of a frame means, controllable buoyancy means, and attachment means for attachment of the frame means to the submerged structure. The frame means may be a generally planar construction dimensioned to conform to a generally planar facet of the submerged structure, e.g. one substantially vertical face of an offshore platform jacket. The controllable buoyancy means may comprise airbag means which are controllably inflatable/deflatable for controllable variation of their displacement volume and hence of their buoyancy when submerged. The controllable buoyancy means may either be permanently attached to the frame means, or selectively attachable to the submerged structure without being directly attached to the frame means. In the latter case, the frame means is preferably provided with its own controllable buoyancy means sufficient at least to provide for controllable flotation/submersion of the frame means when the frame means is not attached to the submerged structure. The attachment means for attachment of the frame means to the submerged structure may comprise a plurality of clamps, or may alternatively (or additionally) comprise a plurality of flexible straps capable of being secured around parts of the submerged structure such as the legs and/or struts of an offshore platform jacket. The recovery apparatus may additionally comprise a base means to which the frame means is slidably and rotatably coupled, the base means preferably comprising mounting means by which the base means can be detachably mounted on a marine vessel such as a barge or self-propelled ship by which the recovery apparatus can be transported to the location of the submerged structure, and by which the recovery apparatus together with a recovered structure can be transported elsewhere. The recovery apparatus of the present invention is distinguished from known forms of salvage equipment for sunken ships by the frame means being particularly suited by size and shape for attachment to one vertical face of a vertically elongate lattice-form tower whose upper end will normally be just above or just below the surface of the sea. (By contrast, a sunken ship will be horizontally elongate and of vertically limited extent, not of open tubular lattice form, and extending along the seabed rather than being upstanding therefrom) . According to second aspect of the present invention there is provided a method of recovering a submerged structure, the method comprising the steps of providing a recovery apparatus according to the first aspect of the invention, transporting the recovery apparatus to the location of the submerged structure, controllably submerging the frame means of the recovery apparatus to lie alongside the submerged structure, attaching the frame means to the submerged structure, attaching the controllable buoyancy means to the submerged structure if not already attached to the frame means, detaching the submerged structure from the seabed if not already detached therefrom, and controlling the buoyancy of the controllable buoyancy means to raise the submerged structure towards the surface of the sea. Where the submerged structure is vertically elongate and the recovery apparatus comprises a base means mounted on a marine vessel, the structure and attached frame means are preferably tilted to a substantially horizontal alignment after being raised towards the surface of the sea, and then translated on board the marine vessel for subsequent transport to another location. Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings wherein: Fig 1 is a schematic representation of various components of a first embodiment of recovery apparatus in accordance with the present invention; Figs 2-16 are schematic representations of the deployment and utilisation of the recovery apparatus of Fig 1; Fig 17 is a schematic representation of a first form of pivot utilisable in the recovery apparatus of Fig 1 ; Fig 18 is a schematic representation of a second form of pivot utilisable in the recovery apparatus of Fig 1; Figs 19 and 20 schematically represent a second embodiment of recovery apparatus in accordance with the present invention; Figs 21 and 22 schematically represent side and end elevations, respectively, of a third embodiment of recovery apparatus in accordance with the present invention; and Figs 23-26 schematically represent successive stages in the deployment of the third embodiment. Referring first to Fig.l, this figure is sub-divided as follows :- Figs 1A and IB are respectively side elevation and plan views of an elongate ladder frame 100 carried in a base frame 102. The ladder frame 100 is a substantially planer tubular lattice structure having a length and breadth similar to those of one vertical face of an offshore platform jacket (not shown in Fig.l) which is intended to be recovered by use of recovery equipment incorporating the ladder frame 100. The rectangular base frame 102 has a socket 104 (Fig IB) at each corner for mounting on four locating pins 106 (Fig.lC) secured on the deck of a barge (not shown in Fig.l) which serves for maritime transport of the recovery equipment to and from the location of the offshore platform jacket. Pivot means (not shown in Fig.l but detailed with reference to Figs 17-20) allow the ladder frame 100 to slide and pivot on the base frame 102. The ladder frame 100 and the base frame 102 are each hollow with access to their respective interiors controlled by respective valves (not shown) . With the frame interiors drained of water, the frames 100 and 102 float as depicted in Fig.lA wherein the sea surface is represented by the line 108. The recovery equipment also comprises a suitable plurality of airbags, of which one is depicted in Fig. ID in the form of a tubular closed-end envelope 110 with integral attachment straps 112. Fig. IE shows a side view of four airbags 110 attached by their respective straps 112 to a lattice-form submerged structure 114. Turning now to Fig.2, this schematically depicts a preliminary stage in installing the recovery equipment. A barge 120 which is controllably ballastable (e.g. by controlled internal flooding) to selectively submerge at least one deck area 122 has the locating pins 106 secured to the deck area 122. The barge 120 is ballasted to submerge the deck are 122 and the combination of the frames 100 and 102 is floated over the deck area 122 to align the sockets 104 with the pins 106. The barge 120 is then deballasted to lift the pins 106 into the socket 104 which are then preferably locked together. The barge 120 can then transport the recovery equipment to the location of jacket to be recovered. Referring now to Fig. 3, a redundant offshore platform jacket 114 is prepared for recovery by the attachment of airbags 110 of a capacity adequate to lift the entire jacket 114 to the sea surface 108 (Fig. 3A) , partially (but not yet totally) severing the jacket legs at or near the seabed (Fig.3B), and partially inflating the airbags 110 to an extent that the combination of the jacket 114 and the airbags 110 have neutral buoyancy (Fig.3C). Next (Fig.4) the barge 120 carrying the frames 100 and 102 is brought up to a point near the prepared jacket 114, the barge 120 is submerged by ballasting until the ladder frame 100 floats on the sea surface 108, and the ladder frame 100 is slid horizontally off the base frame 102 by a means of an on-barge deck winch 124. When the frame 100 has been moved nearly but not quite entirely off the base frame 102 (Fig.5) it is initially supported by self-flotation, and is then flooded at its outboard end (ie the end remote from the base frame 102; the right end as viewed in Fig.5) to sink the ladder frame 100 to a substantially vertical position (Fig.6) while still being pivotally coupled at its now upper end to the base frame 102. The barge 120 with the vertically suspended ladder frame 100 is moved alongside the jacket 114 (Fig.7) and the frame 100 is then attached to a vertical side of the jacket 114 (Fig.8) by suitable attachment means (not shown in Fig.8) . Next the barge 120 is deballasted to apply an upward force to the jacket 114 (Fig.9) which pre- tensions the jacket legs since they are not yet severed through and the jacket 114 thus remains anchored in the seabed. In the following stages of jacket recovery, the jacket legs are fully severed (Fig.10), for example by detonation of explosive cutting charges, while the barge 120 is manoeuvred by tugs such that local ocean current flows from the barge 120 towards the jacket 114. Then the airbags 120 on the side of the jacket 114 opposite to the side to which the ladder frame 100 is attached are inflated (Fig.11) so as to begin to rotate the jacket 114 by lifting the lower end of the jacket towards the surface 108 at which the upper end of the jacket is suspended. When the jacket 114 has partially rotated (to almost the extent possible by inflating the outer airbags, ie inflation of the airbags on the side opposite the frame 100) , further inflation of the outer airbags ceases, and greater rotation of the jacket 114 is brought about by inflating the inner airbags 110 (ie the airbags on the side of the jacket 114 to which the frame 100 is attached) as shown in Fig.12. As the jacket 114 rotates towards horizontal under the lifting influence of the inflating airbags 110 (Fig.13), the barge 120 is ballasted to lower the pivot point of the coupling of the ladder frame 100 to the base frame 102 until the side of the jacket 114 to which the frame 100 is attached becomes substantially horizontal (Fig.14) whereafter the ladder frame 100 (with the still -attached jacket 114) is winched back onto the base frame 102 (Fig.15) . Finally the barge 120 is fully deballasted (Fig.16), and the deck load, including the recovered jacket 114 is lashed down for transit of the barge and jacket, with the recovery equipment, to a remote site where the jacket can be recycled, dumped, or otherwise disposed of. Reference has previously been made to the combined sliding and pivoting connections between the ladder frame 100 and the base frame 102. Details of various explaining forms of such connection will now be given. A first form of sliding/pivoting connection 200 is shown in Fig.17 wherein Figs.l7A, 17B and 17C are respectively an elevation, a sectional side elevation, and a sectional plan view of the connection 200. The connection 200 comprises a hollow housing 202 having a slot 204. A bearing member 206 comprises a bearing shoe 208 having a flat face 210 which slidingly bears against a vertical barge wall 212 which extends fore and aft on either side of the stowed location of the ladder frame 100 (not shown in Fig.17) . The bearing shoe face 210 and the barge wall contacted by the face 210 are each faced with replaceable polymer linings 214 and 216 to protect the steelwork. A spigot 218 extends inward of the bearing shoe 208, and projects through the slot 204 for attachment to the ladder frame 100. The vertical slot 204 allows limited vertical movement of the ladder frame 100. In a second version 250 of the pivoting connection shown in Fig.18, the connection 250 is the same, apart from a feature to be detailed, as the connection 200 and therefore has the same reference numerals applied. The difference in the connection 250 with respect to the connection 200 lies in the slot 204 having a downwardly open end 252 to allow the bearing member 206 to be withdrawn from the housing 202 at choice. When the bearing member 206 is to be retained within the housing 202, the slot end 252 is closed against passage of the bearing member 206 by means of a removable transverse locking pin 254. A third form of sliding/pivoting connection, and its method of use, is depicted in Figs. 19 and 20. The base frame 202 is provided with an elongated horizontal slot or slide 300 along either side of the stowed location of the ladder frame 100 (refer to Fig. IB) . The ladder frame 100 is provided with a pair of spaced- apart spigots 302 on either side which run in the slots 300 and thereby support the ladder frame 100 on the base frame 102. During storage and transport, longitudinal sliding of the ladder frame 100 with respect to the base frame 102 is prevented by horizontal locking of the spigots 302 at one end (the right end as viewed in Fig 19A) of the ladder frame 100 by means of a pair of vertical locking pins 304 crossing the slot 300 on either side of the spigot 302 at that end. To allow the ladder frame 100 to commence its horizontal deplongment as shown in Fig. 4, both of the locking pins 304 are withdrawn (Fig. 19B) to allow the previously retained spigot 302 to pass outboard of the slot 300 whereafter the outboard one of the pair of locking pins (but not the inboard locking pin) is re- inserted (Fig. 19C) . When the spigot 302 at the other end of the ladder frame 100 reaches the locking pins, the inboard locking pin 304 is re-inserted (Fig. 19D) to anchor what will become the upper end of the ladder frame 100 while allowing pivoting of the ladder frame 100 with respect to the base frame as shown in Fig. 20A (which corresponds to Fig. 6) . The portion of the slide 300 adjacent the locking pins 304 is shown to an enlarged scale in Fig. 20B. The slide 300 is vertically extended between the locking pins 304, which allows for heave of the ladder frame 100 due to oceanic turbulence during jacket recovery operations. A further embodiment of recovery equipment in accordance with the present invention is schemetially depicted in Figs. 21 and 22 which are, respectively, an elevation and a side view of the third embodiment. The third embodiment 400 comprises a ladder frame 402 in five sections 404 mutually linked by inter-section hinges 406. Each section 404 is a cross-braced planar lattice having several transverse members 408 supporting controllably inflatable airbags 410. Each section 404 also comprises a pair of clips 412 by which the ladder frame 402 can be secured to the transport barge. Each section 404 can be secured to a jacket 114 by a pair of heavy-duty straps 414. As shown in Fig. 22, a similar frame with integral airbags can be attached to the opposite side of the jacket 114 to provide flotation functions in a manner analogous to that described with reference to the first embodiment. The hinged multi-section ladder frame 402 allows its submersion and attachment to a jacket in several stages which will probably be easier than the handling of the single rigid ladder frame of the first embodiment . By attaching initially only the top of the ladder frame 402 to the top of the jacket 114 (Fig. 23) and then progressively submerging the section 404, (Figs. 23, 24 and 25) until the entire frame 402 rests against the side of the jacket 114 (Fig. 26) , greater control of deployment is possible than with a rigid frame of the same overall size. The third embodiment 400 has advantages with respect to the first embodiment by comprising integrally mounted airbags which do not require separate deployment and attachment, an improved jacket-strengthening function and an improved distribution of dynamic stresses, and a skidding ability, i.e. the ability to act as a load- carrying sledge when being dragged across a deck or another solid surface (e.g land). All embodiments of the recovery equipment have the advantages, compared to alternative jacket recovery equipment, of being economical and allowing recovery of jackets into shallow water. While certain modifications and variations of the invention have been described, the invention is not restricted thereto. For example, the ladder frame has been described as lying fore and aft along the barge, but could alternatively lie athwart the barge, overhanging at each site. Instead of being installed along or across a single barge, the recovery equipment could be deployed between two barges or support vessels. While references have been made to the "sea", the applicability of the invention is to recovery of structures partially or wholly submerged in any body of water, whether in mid-ocean, coastal waters or inland water and waterways; references to "sea" or "water" are to be generalised accordingly. While the invention has particular applicability to the recovery of offshore platform jackets, the equipment and procedures of the invention can be applied to the recovery of other forms of submerged structure, whether or not such structures relate to exploration for and/or production of hydrocarbons. Other modifications and variations can be adopted without departing from the scope of the invention as defined in the appended claims.

Claims: CLAIMS 1. Recovery apparatus for the recovery of a submerged structure, the recovery apparatus comprising the combination of a frame means, controllable buoyancy means, and attachment means for attachment of the frame means to the submerged structure . 2. Recovery apparatus as claimed in Claim 1 wherein the frame means is a generally planar construction dimensioned to conform to a generally planar facet of the submerged structure . 3. Recovery apparatus as claimed in Claim 2 wherein the submerged structure is an offshore platform jackets and the generally planar facet thereof is one substantially vertical side of the jacket. 4 » Recovery apparatus as claimed in any preceding claim wherein the controllable buoyancy means comprises airbags which are controllably inflatable/deflatable for controllable variation of their displacement volume and hence of their buoyancy when submerged. 5. Recovery apparatus as claimed in any preceding claim wherein the controllable buoyancy means is permanently attached to the frame means. 6. Recovery apparatus as claimed in any of claims 1-4 wherein the controllable buoyancy means is selectively attachable to the submerged structure without being directly attached to the frame means . Recovery apparatus as claimed in claim 6 wherein the frame means is provided with its own controllable buoyancy means sufficient at least to provide for controllable flotation/submersion of the frame means when the frame means is not attached to the submerged structure. Recovery apparatus as claimed in any preceding claim wherein the attachment means for attachment of the frame means to the submerged structure comprises a plurality of clamps and/or comprises a plurality of flexible straps capable of being secured around parts of the submerged structure such as the legs and/or struts of an offshore platform jacket. Recovery apparatus as claimed in any preceding claim wherein the recovery apparatus additionally comprises a base means to which the frame means is slidably and rotatably coupled. Recovery apparatus as claimed in claim 9 wherein the base means comprises mounting means by which the base means can be detachably mounted on a marine vessel such as a barge or self-propelled ship by which the recovery apparatus can be transported to the location of the submerged structure, and by which the recovery apparatus together with a recovered structure can be transported elsewhere. Recovery apparatus as claimed in any preceding claim wherein the frame means comprises a plurality of frame sections which are mutually hinged together to form an articulated frame. Recovery apparatus as claimed in claim 11 wherein the controllable buoyancy means is distributed between the frame sections such that at least a majority of the frame sections can individually apply flotation forces to the submerged structure when the frame means is attached thereto. A method of recovering a submerged structure, the method comprising the steps of providing recovery apparatus as claimed in any preceding claim, transporting the recovery apparatus to the location of the submerged structure, controllably submerging the frame means of the recovery apparatus to lie alongside the submerged structure, attaching the frame means to the submerged structure, attaching the controllable buoyancy means to the submerged structure if not already attached to the frame means, detaching the submerged structure from the seabed if not already detached therefrom, and controlling the buoyancy of the controllable buoyancy means to raise the submerged structure towards the surface of the sea. A method as claimed in claim 13 wherein the submerged structure is vertically elongate and the recovery apparatus comprises a base means mounted on a marine vessel, the structure and attached frame means are tilted to a substantially horizontal alignment after being raised towards the surface of the sea, and then transported on board the marine vessel for subsequent transport to another location.

Inventors: DIXON-ROCHE KEITH, DIXON-ROCHE, KEITH
Applicants: DIXON ROCHE KEITH, DIXON-ROCHE, KEITH
Application year: 1999
Main classification: E21B19
IPC codes: E02B17, E21B19