A method and equipment arrangement for relocation of an object from one location to another on the seabed

FIELD OF THE INVENTION The present invention relates to a method of moving objects on seabed, particularly related to the moving of objects that are part of the infrastructure in oil and gas fields.

Conventional offshore removal methods are normally based on lifting an object or structure from the seabed offshore with a crane construction vessel and transporting the structure to an inshore destination on the deck of the same vessel or a transportation barge. For an offshore installation operation onto the seabed the structure is transported from an inshore site to the field on the deck of an offshore crane construction vessel or a barge with a subsequent offshore lift from vessel deck or barge deck onto the seabed. Such operations set high demands to crane capacity and deck space and can be very weather sensitive operations and can consequently be tying up very costly crane construction vessels for long periods of time.

From GB A-2330854 a method is known for transporting and installing an offshore structure consisting of a tower extending from a concrete base. The base with the tower is constructed in a dry dock, whereupon the dock is flooded and a barge is floated on top of the base and attached thereto by means of strand jacks. This assembly is floated to the installation site, where the base is lowered to the water bed by means of the strand jacks. This method is only suitable for shallow waters and calm weather conditions due to the large waterline area of the barge making it very responsive to wave and swell movements.

GB 1419266 suggests a method for transporting and installation of a submerged structure suspended in cables underneath a schematically drawn "semi-submersible" provided with power winches for the cables. Such cable winches are relatively heavy and expensive for a given pulling capacity, and the semi-submersible must be a manned vessel since an excavating vehicle is to be serviceable therefrom.

This has led to the present invention, which involves the principles of "moving an object on seabed by lifting it from the seabed, submerged towing and lowering onto seabed from a buoy". The invention is defined in the independent claims.

For the means of lifting, the buoy can be fitted with one or more anchor chain jacks or strand jacks, the latter employing sets of cable strand bundles to be attached to an object

situated on the bottom of the sea underneath the buoy. In order to have a minimum of vertical movement and a more or less constant force acting on the attachment points of the object, the means of lifting may be fitted on top of a series of vertical resilient mountings made of elastic flexible materials suitable for the task. Such materials may be rubber, elastomer, polyurethane or equivalent, acting like shock absorbers and load compensators as the buoy is moving up and down in the waves. By specifying a proper load-deflection curve for the resilient mountings, the vertical movement and the tension in the cable strand bundles can be kept under control during the lifting , towing and lowering of the object.

In order to save the strand bundles from wear and tear, especially during towing operation, a heavy chain can be introduced in the lower part of the strand bundle.

In order to be able to use the resilient mountings for different objects with different weight and in different water depth and environmental conditions, fibre rope made typically of polyester can be connected to the lower part of the cable strand bundles or chains with a size and length suitable to compensate for any short-comings in the load- deflection of the resilient-mountings. Polyester has a typical 14 % extension at break, while steel in comparison has less than 1 %. Loading polyester to 30 % to MBL gives about 4,5 % extension of the rope, which means that a 50 m long polyester rope will extend about 2 m. This property can therefore be used to enhance or adjust the load deflection characteristic of the lift system as required. With a stroke of 1 - 2 m on the resilient mounting, the total stroke of the combined lift system including the polyester part can then be 3 - 4 m, allowing lifting or lowering a heavy structure in relative rough sea states without overstress.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention shall be described in the following with a reference to the attached drawings which illustrate preferred embodiments, wherein:

Fig. l is a transverse section of a load compensated lifting assembly consisting of a resilient mounting acting as compensator and situated on a grillage on top of a buoy, with a strand jack mounted on top of the resilient mounting and with the load-carrying cable strand bundle from the strand jack led through the centre of the resilient mounting for connection to the object being lifted or lowered. The off-loaded cable strand bundle is led off from the top of the strand jack to means of storage. The storage can be a locker- space inside the buoy or a reel, e.g. as disclosed in EP-A-1275610.

Fig. 2 is a transverse section of a top of a buoy showing a plurality of load compensated lifting assemblies of the strand jack type with a number of resilient mountings, typical two or four off, each with a strand jack mounted on top of the resilient mounting assembly. Their load-carrying set of strand bundles are led through the centre of the assembly for connection to the object, and the off-loaded bundles are led off from the top of the jacks onto means of storage.

Fig. 3 shows a preferred embodiment of a load compensated lifting assembly in the form of resilient mountings with strand jacks. These are located on a buoy fitted with ballast tanks and ballast system, enabling the buoy to be upended and loaded and unloaded through de-ballasting and ballasting operations.

Fig. 4 shows a preferred embodiment of a plurality of load compensated lifting assemblies consisting of resilient mountings with associated strand jacks located on a plurality of coupled buoys of the type shown in figure 3. They are fitted with ballast tanks and ballast systems, enabling them to be upended and loaded and unloaded through de- ballasting and ballasting operations.

Fig. 5 is a plan view illustrating how the buoys of Fig. 4 are kept on station during lift- ing or lowering.

Fig. 6 a and b are a transverse section and plan, respectively, of the top of a floating buoyancy body assembly consisting of two buoys showing two load-compensated lifting assemblies each with a chain jack winch mounted on a common table resting on a number of resilient mountings, typically four off, for the purpose of minimizing the landing speed of the lift-object onto seabed.

The buoyancy body assembly with two buoys is fitted with a number of ballast tanks and ballast system enabling the assembly to be rotated from a vertical to a horizontal position, and visa versa.

The load-carrying chain is shown led along the side of the buoys to a guide at the base of the buoy, and the off-loaded chain is led off from the top of the chain jack winch to means of storage in the buoy.

The top of the buoy has a small cross-section area minimizing the heave action when the waterline is at this section during offshore installation operation, thus improving the landing condition for the lift-object onto the seabed.

Fig. 7 shows an up-ended buoyancy body assembly ready loaded and prepared for the offshore towing on towing draft line 42 as well as prepared for offshore installation by ballasting to deeper draft line 43. s

Fig. 8 shows a special scenario of the preferred embodiment with one load compensated lifting assembly for smaller loads in the form of resilient mountings with one chain jack winch on top of the buoyancy body assembly, the assembly comprising two buoys with ballast tanks and ballast system enabling it to be rotated from a vertical to a horizontal io position and visa versa.

The buoyancy assembly is shown in a "near horizontal position" for improved operation in shallow water during pick-up of smaller loads with subsequent inshore towing. The load-carrying chain is shown located off centre of the assembly but can also as an alter- I ₅ native be led through the centre section between the buoys of the buoyancy body assembly, depending on the size of the load to be lifted and the need for stability during this special operation.

Fig. 9 is a transverse section of a floating buoyancy body assembly consisting of two 20 buoys where the two load-compensated lifting assemblies with chain jacks shown in fig. 6a and b are replaced with four load-compensated lifting assemblies, each with a strand jack winch mounted on a resilient mounting, typically, four off totally, for the purpose of minimizing the landing speed of the lift-object onto seabed.

2 ₅ DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 is showing a load compensated lifting assembly 17 made up by a strand jack 12 and a resilient mounting assembly 1 consisting of a piston-cylinder assembly 18 stroking when external load is applied and acting on a stack of elastic flexible material 2, thus working as a load compensator or spring designed for a load deflection curve re-

30 quired by the dynamic loads imposed by the buoy-movement onto the lifted object and caused by the environmental conditions during lifting, towing and lowering.

The figure showing the lifting assembly is also indicating the outlines of the strand jack 12 resting on top of the resilient mounting assembly 1 and comprising (partly shown) an 3 ₅ internal central hydraulic piston and cylinder assembly 3 with an upper and lower anchor heads 4, gripping and releasing the individual cable strands by a hydraulic wedge mechanism following the movement of the central piston as it is extending or retracting, just as the method applied when moving the hand over hand when heaving a load with a

rope manually. The cable strand bundle 5 is built up by a number of single strand wires, each consisting of a number of twisted steel strings. The bundle can be stored on reels, in locker-space in the buoy or on deck of the buoy.

Fig. 1 is also showing the complete load compensated lifting assembly 17 resting on a structural grillage 13 which can be designed to distribute the load onto the buoy.

Fig. 2 is showing a transverse section of a plurality of load compensated lifting assemblies 17 located on a grillage 13 on top of a buoy 8, typically, two or four off, with a number of strand jacks 12 mounted on a common table 19 as required and resting on top of the piston-cylinder assemblies 18 as part of the resilient mounting assemblies 1 acting as load compensators, with their load-carrying set of strand bundles 5 led through the centre or in between the resilient mounting assemblies and further through the buoy 8, for connection to the lift-object 11, and the off-loaded bundles led off from the strand jacks to means of storage on reels or in the locker-space 14 in the buoy.

Fig. 3 is showing a load compensated lifting assembly 17 with strand jacks 12 and a load compensator of the resilient mounting type 1 located on top of a buoy 8. The buoy is fitted with a number of ballast-tanks 20 and associated ballast-system with distributing manifolding and control valve panels 22 located in the top of the buoy, enabling the buoy to be upended and to allow it to be loaded and unloaded through de-ballasting and ballasting operations during a lifting or lowering operation of an object 11. The unloaded strand bundles 5 are strapped every one meter prior to being stored coiled "bottom up" in a circular bundle-locker 14 inside the buoy. The loaded part of strand bundle is penetrating the hull of the buoy through a guiding 26 at the base of the buoy for connection to the object 11 to be lifted. The part of the bundle penetrating the hull during a towing operation can be replaced with a heavy chain to avoid undue wear and tear on bundle. Lifting an object from seabed can be done through a combined operation of hauling by the strand jack and de-ballasting the buoy. De-ballasting is accomplished by distributing compressed air to the tanks 20 through the distributing manifold in 22. Installing an object onto seabed can be done by a combined operation of lowering by the strand jack and ballasting the buoy. Ballasting is accomplished by opening ballast control valves to sea from the control panels in 22 or from the operation control vessel 21. During lifting and lowering operation the buoy is kept on station by one tug and one anchor handling vessel, the latter acting as operation control vessel. The latter will also supply the necessary compressed air, hydraulic power and control to the buoy through an umbilical connection. When the object has been lifted and suspended from the buoy,

the anchor handling vessel will connect towing lines to the object and the buoy for towing to a new location.

Fig. 4 is showing four buoys 8 similar to the one shown in figure 3, coupled together to a buoy assembly 23 with four load compensated lifting assemblies 17 enabling it to lift, transport and lower an extra large object 11. The figure is showing the buoy assembly 23 and the object 11 being towed by an anchor handling vessel 21 through a combination of one or two towing lines 24, 25, with the object 11 suspended from the strand jacks 12 via strand bundles 5.

Fig. 5 is showing a buoy assembly 23 kept on station during the lifting or lowering operation of an object by one tug 27 and one anchor handling vessel 21 with DP facilities. These are connected by wire lines 29 to the respective sides of the buoys. The latter vessel is supplying the necessary compressed air, hydraulic power and controls through the umbilical 28 for the operation.

Fig. 6a and b is transverse section and plan respectively showing two load compensated lifting assemblies 17 located on cantilevered grillage structures 13 attached to a control section 22 on top of a separate ballast tank 38 having a small cross-section area and situated on top of the floating buoyancy body assembly 8 consisting of two buoys, each lifting assembly 17 being fitted with a chain jack winch 30 mounted on a common table 19 resting on top of the piston-cylinder assembly 18 as part of the resilient mounting assembly 1 acting as load compensators or dampers, with the load-carrying part of the chain 31 in a chain pipe 36 along the side of the buoyancy body assembly 8, for connec- tion to the lift-object 11, and the off-loaded part of chain 31 led off from the chain jack winch 30 through chain pipe 35 to means of storage in the locker space 14 in the base of the buoyancy body assembly 8.

The figures showing the lifting assembly are also indicating the outlines of the chain jack winch 30 mounted on the common table 19 and consisting of a hydraulic driven pawl-mechanism 32 driven by hydraulic cylinder 33 and acting against the chain 31 on the chain wheel 34.

Fig. 7 is showing the buoyancy body assembly 8 in a vertical up-ended position with offshore towing draft on line 42. When arrived at the offshore site, the buoyancy body assembly is ballasted to installation draft indicated on line 43. This is done from the towing and control vessel 21 connecting up the control umbilical 28 to the control section 22 and operates the tank-ballast system 20 of the buoyancy body assembly 8. BaI-

lasting is accomplished by relieving air from the tanks 20, while de-ballasting is accomplished by supplying compressed air to the tanks.

The loaded part of the chain 31 is penetrating through the base of the buoyancy body assembly 8 at a support guide 26 giving the lateral support during the operations. The unloaded portion of chain 31 is led to the chain locker 14 through the chain pipe 35.

Fig. 8 is showing a transverse section of the buoyancy body assembly 8 consisting of two buoys with one chain jack winch 30 in a "near to horizontal" position achieved by operating the ballast-system for the tanks 20. This position of the buoy will ease the operation during load pick-up or towing in shallow water but for small lift loads 11 only.

The angle of 10 — 20 degrees can be adjusted by the amount of ballast water in the dif- ferent ballast tanks 20 prior to the start of picking up load by the winch in order to overcome the static friction of the chain in the chain pipe 35 to locker 14. Pick up of a load inshore can also alternatively be accomplished by operating the ballast system for the tanks 20 only, with no winch operation.

The loaded part of the chain 31 is shown directed through a chain pipe 36 with support guiding 26 before being directed to the submerged lift object 11.

Inshore towing of an object 11 can be carried out with the object suspended underneath the buoyancy body assembly 8 in this position and towing power applied by an anchor handling and control vessel 21.

When the buoyancy body assembly 8 is towed inshore in a "near to horizontal" position with a lift-object 11 suspended below, the assembly is to be up-ended prior to entering the offshore transport leg. This is done from the towing and control vessel 21 by con- necting up the control umbilical 28 to the control section 22 and operating the ballast system 20 of the buoyancy body assembly 8 (see Fig. 5)

Fig. 9 is a transverse section of a floating buoyancy body 8 assembly consisting of two buoys showing four load-compensated lifting assemblies 17, each with a strand jack winch 12 mounted on a resilient mounting 18, typical four off totally, for the purpose of minimizing the landing speed of the lift object 11 onto seabed.

The buoyancy body assembly 8 with the two buoys are fitted with ballast tank system 20, thus enabling it to be rotated from a vertical to a horizontal position and visa versa by control of compressed air supply and relieve to and from tanks via an umbilical 28 connected to an anchor handling control vessel 21.

The load-carrying part of wire bundles 5 are shown led along the side of the buoyancy body assembly 8 to a support guide 26 at the base of the buoyancy body assembly, and the off-loaded part of wire bundle 5 are led off from the top of the strand jack winches 12 onto means of storage on the buoyancy body assembly shown as four reels 37, but can also as an alternative be stored in bundle lockers in the buoys 8. To avoid wear and tear on the bundles 5 in the support guide 26 during towing, the lower end of the bundles is fitted with a chain 39.

The buoyancy body assembly 8 is shown in a towing draft situation indicated on line 42. The top of the buoyancy body assembly 8 with ballast tank 38 has a small cross- section area, thereby minimizing the heave action when the waterline indicated by line 43 is at this section during offshore installation operation, thus improving the landing condition for the lift object 11 onto seabed.

The four chains 39 are connected to a spreader bar 40 which are suspending the lift-load 11 by four slings 41.

The present invention is especially attractive because it can reuse, with only minor alterations, the elongate buoyancy bodies employed in the transportation method de- scribed in GB 2402422. Also the jack winches are readily available, e.g. as anchor chain winches used for semi-submersible offshore platforms and exemplified in US 6655661. Both these publications are incorporated by reference.

The invention is not limited to the examplifying embodiments described above, but may by the skilled person be varied and modified within the scope of the following claims.