BACKGROUND

Field of the invention

[0001] This invention regards a barge system for offshore operations. Prior and related art

[0002] Offshore installations include wind turbines and various structures and modules used in the oil and gas industry. Depending on water depth and other factors, an offshore installation may comprise structures above the sea surface or fully submerged subsea modules. For example, in water depths up to 300m, a jacket resting on the seabed may be sufficient to keep the topside above extreme 100 years return waves that may occur on the field. In more shallow waters, a jackup rig may provide a sufficient distance from the sea surface to the topside. In deeper waters, a floating rig, a semisubmersible platform, tension leg platform or a subsea module may be the most likely alternative.

[0003] In the following description and claims, a "subsea module" is a structure designed for operation on the seabed, e.g. a unit comprising a wellhead, valves and couplings for use in a satellite field. The subsea module is typically protected by a frame, e.g. to avoid damage if a trawl is inadvertently towed over the structure. In this document, the term "subsea module" dos not include a jacket or tower resting on a seabed, nor the legs of a floating rig.

[0004] In this document, the term "dynamic positioning" (DP) includes six degrees of freedom. Three are related to load shifts, i.e. surge for shifts along a vessel, sway for sideway shifts and heave for vertical motion. The other three are pitch, roll and yaw, and describe vessel rotation about three mutually perpendicular rotation axes. Heave compensation and related systems handle a subset of the six degrees of freedom. DP-systems are classified as DPI, DP2 and DP3 according to their redundancy. For example, DP3 allows operation even if an entire subsystem such as a one engine room failure.

[0005] Traditional offshore construction vessels (OCVs) are designed for specific purposes, e.g. subsea installations or heavy lifts. In order to be competitive, OCVs are still generally outfitted with equipment that is not used in all operations. For example, a crane vessel with DP3 and a large main crane may lift structures and modules to or from installations that require DP3, DP2 or DPI, and any load determined by the lifting capacity, lifting height and outlay of its crane(s). This versatility adds to the day rates for leasing such versatile OCVs, and less expensive, less versatile vessels are generally preferred for specialized and less demanding operations. For example, a transport barge may be used to lower a topside onto a jacket in a floatover operation.

[0006] Historically, ballasting were used to mate topsides with support structures, e.g. to produce Condeep-platforms in Norwegian fjords during the 1990's. Another example is provided in WO9906270A1, which discloses a transporter using ballasting to lift a topside from a barge and lower the topside on the jacket during installation. Ballasting is too slow to compensate for wave motion, and hence mating with such methods requires sheltered waters.

[0007] In less sheltered waters, significant vertical motion may be caused directly by wind- generated waves or indirectly by resonance from low amplitude swell. Current transport barges typically have hydraulic jacks that lower the topside onto the jacket in less than a minute to minimize the effect from waves.

[0008] During deployment of a subsea module, underwater currents may affect the position of the subsea module significantly. Thus, a work ROV (WROV) may be required to push or pull the suspended module to its intended position on the seabed. An OCV with a WROV and trained personnel is typically required for deployment on a seabed, down to several thousand meters below the sea surface. The subsea module may be carried to the field on a semi- submersible transport barge, and buoyancy elements on the subsea module may reduce the required lifting capacity of the OCV.

[0009] Before ISO standard cargo containers and associated container terminals became commonplace, standardized barges known as "lighters" were used to carry cargo from shallow ports, rivers etc. to a barge carrier suited for ocean transport. In the "lighter aboard ship" (LASH) system, the barge carrier had two parallel hull elements extending rearward to provide a stern recess, and a gantry crane running along the length of the ship. The gantry crane lifted the lighters between the stern recess and a location in the hull or on the deck of the barge carrier. A related system, Sea Bee, used larger lighters and comprised a submersible platform in the stern recess. The lighters were floated over the submerged platform, lifted, and moved on rails to their intended position on the barge carrier. The Sea Bee vessels had three decks, and could carry 12 + 12 + 14 = 38 lighters. The LASH, Sea Bee and similar systems have significantly higher operational costs than current container systems. For example, a lighter is a vessel that may require anchoring and other systems to comply with vessel regulations. The added cost for such systems increases with the number of lighters. In addition, ice may inhibit operation of lighters during the winter in some regions. [0010] WO 2011108932 Al discloses a barge for installation of an elongated offshore structure, e.g. a jacket or tower for a wind turbine. The barge may be towed and/or self- propelled, and has two parallel hull elements extending rearward forming a stern recess. A hinge at the fore end of the stern recess connects a support frame with the barge. During transport, the support frame holds the elongated structure in a horizontal position. During installation, hydraulic pistons and/or winches pivot the frame about the hinge from 0° to 100° relative to the barge deck. When the frame is in the raised position, winches suspend and lower the elongated offshore structure through the stern recess to the seabed. While the support frame is suitable for small and medium sized jackets and towers, it is less practical for floatover operations, and may need to be removed for transport of large cargo such as a topside or a subsea module.

[0011] The general objective of the present invention is to solve or alleviate at least one of the problems above while retaining the benefits of prior art.

[0012] A more specific objective of the present invention is to provide a versatile barge system for installing, maintaining and removing offshore installations of various types, including jackets, towers and other support structures as well as topsides and subsea modules. In particular, the barge system should be able to transport and install a jacket or module offshore, yet allow loading and unloading at an onshore facility with limited water depth and quay facilities. Further, the barge system should allow transport and installation in normal swell and waves in an efficient manner. This involves reducing the dynamic loads imposed on the barge system from the cargo. Yet another objective is to provide a barge system that is readily adapted to one or more special applications, e.g. for floatover operations, for deploying and retrieving subsea modules, for use as an assisting crane vessel etc.

SUMMARY OF THE INVENTION

[0013] The objectives above are achieved by a barge system according to claim 1. Further features and benefits appear from the dependent claims.

[0014] More particularly, the present invention concerns a barge system for offshore operations comprising a self-contained mother barge with a deck and two parallel hull elements extending rearwards to form a stern recess in the mother barge. The barge system is distinguished by a releasable connection suitable for attaching a feeder barge in the stern recess during transport to an offshore field.

[0015] The self-contained mother barge comprises facilities for operating independently of other vessels. The facilities typically include thrusters for propulsion, steering and dynamic positioning; a ballasting system for load balancing and counter ballasting; an electric power generator for supplying electric power to the thrusters and other sub-systems; a hydraulic power unit as required for linear and/or rotary hydraulic motors; communication and navigation systems and accommodation for a crew.

[0016] The releasable connection may, for example, comprise a hinge with hydraulic operable bolts or sheaths and/or retractable bolts capable of moving laterally into and out of corresponding grooves. The releasable connection also include associated power and communication systems required to operate the connection. The connection may allow a vertical and/or rotary motion of the feeder barge relative to the mother barge, and the associated communication system may operate a ballasting system in the feeder barge.

[0017] The width of the feeder barge must be less than the width of the stern recess. The length of the feeder barge may exceed the length of the stern recess, i.e. the length of the parallel hull elements, and be adapted to an intended load.

[0018] The draft of the feeder barge is preferably less than the draft of the mother barge. Thus, the feeder barge may access shallow ports that are inaccessible to the mother barge.

[0019] In some embodiments, the releasable connection comprises a hinge mechanism with an axis of rotation that extends across the stern recess parallel to the deck. The hinge mechanism allows a pivoting member, e.g. the feeder barge, a support arm or a rocker, to pivot about the axis of rotation while being attached to the mother barge.

[0020] Some embodiments comprise a releasable support arm configured to pivot about the hinge mechanism. The purpose of the support arm is to distribute the load over a length of an elongate load, e.g. a jacket or a tower, during transport. The hinge mechanism or pivot may be located anywhere in the stern recess and anywhere along the support arm. The support arm is releasable, and may be interchangeable with a feeder barge having complementary

components for the hinge mechanism.

[0021] In embodiments with a support arm, a partially submerged buoyancy element may exert a buoyancy force on the aft end of the support arm. The buoyancy increases when the aft end of the support arm moves downward and the buoyancy element displaces more water, and decreases when the aft end of the support arm moves upward such that the buoyance element displaces less water. The same effect is obtained by air-filled ballasting tanks in a feeder barge configured to pivot about the hinge mechanism. Either way, the variable buoyancy opposes the dynamic load imposed on the mother barge. If dynamic load compensation is required or desired, the variable buoyancy particularly simplifies the task of compensating for heave and surge. The buoyancy elements may include any permanent or temporary elements known in the art, e.g. permanent tanks within the support arm, buoys temporarily attached to the end of a load for a longest possible moment arm, air filled legs of a jacket or tower, etc.

[0022] In addition to or instead of a hinge element, the releasable connection may comprise retractable pins configured to enter corresponding grooves. The pins may be arranged in the stern walls and the grooves in the feeder barge or vice versa. The support arm may comprise similar guide pins or grooves.

[0023] In some embodiments, the grooves extend perpendicular to the deck. In addition, they may extend parallel to the deck. That is, a vertical motion of the feeder barge relative to the mother barge guided by pins sliding in the grooves does not exclude a horizontal translation guided by inclined grooves and/or a rotation relative to the mother barge.

[0024] In some embodiments, the grooves follow arcs of circles that are concentric with the hinge mechanism. Thus, the pins and grooves may take up forces when the support arm or feeder barge pivots about the hinge mechanism, i.e. prevent excessive loads on the hinge mechanism. The feeder barge may be displaced downward if its fore end is disconnected from the hinge mechanism. However, concentric guide grooves imply a rotation, so the deck of the feeder barge will not remain parallel with the deck of the mother barge in these embodiments.

[0025] The barge system preferably further comprises a conveyor system for moving cargo on a deck without lifting the cargo. The main purpose is to avoid unnecessary energy consumption for lifting and lowering. The conveyor system may comprise actuators such as winches with associated tackle and/or hydraulic pistons etc., as well as rails, skid beams, rollers etc. on the decks of the mother barge and feeder barge to reduce friction and guide the cargo. The conveyor system should enable transfer of cargo between the feeder barge and the mother barge, and should preferably facilitate transfer of cargo from the feeder barge to or from an onshore facility. For example, equipment may be transferred from the onshore facility to the feeder barge using rails on the deck of the feeder barge. Once the feeder barge has entered the stern recess, the equipment may be transferred to the deck of the mother barge by means of the conveyor system.

[0026] The barge system preferably further comprises a lifting system at the stern of the mother barge. The location aft on the mother barge provides a long moment arm to fore ballasting tanks in the mother barge, and hence enable a relatively quick counter ballasting. In addition, the location near the stern recess facilitates loading and unloading a feeder barge.

[0027] The lifting system may comprise a main crane, e.g. a boom crane, a portal crane or a gantry crane, e.g. for lifting or lowering a subsea module or a jacket through the stern recess. [0028] In addition, the lifting system may comprise lifting towers. The lifting towers may comprise jacks for connecting and disconnecting a topside to or from an associated jacket, and/or winches with associated tackle for hoisting.

[0029] The lifting towers preferably comprise at least two lifting towers capable of lifting one structure together. In particular, the lifting towers may comprise fore and aft towers on each side of the stern recess. In this embodiment, the fore and aft lifting towers are preferably movable relative to each other in the longitudinal direction of the mother barge such that the arrangement will fit the dimensions and center of gravity requirements for handling different structures. For this, the lifting towers comprise a foundation mounted on the mother barge at the sides of the stern recess that allows the movable towers along the parallel hull elements defining the stern recess. The main part of the lifting towers comprise a structure able to move up and down in the foundation, e.g. rack-and-pinion arrangements similar to those moving the legs in a jack-up rig. The lifting towers preferably also have hydraulically supported forklift arrangements, e.g. for lowering a topside with the desired swiftness as described previously.

[0030] Finally, the feeder barge guided by pins and grooves may form part of the lifting system by controlling the amount of water in ballasting tanks in the feeder barge. Ballasting the feeder barge reduces the critical lift height and load on cranes, e.g. a main crane on the mother barge or a crane on an assisting offshore construction vessel. This enhances the lifting capability and versatility of the system.

[0031] There is no sharp distinction between the various systems. For example, a rotary hydraulic motor with a capstan or chain gear may form a winch transferring forces to a rope, wire or chain. The wire or chain may be arranged via pulleys and/or blocks to pull cargo on the deck or elevated rails, and hence be part of the conveyor system. By arranging wires over different pulleys and blocks, the same winch may be used in a hoist, i.e. form part of a crane. Opposite, a hoist in a lifting tower is by definition also a crane, and any crane with sufficient power may pull a cargo in any direction on the deck by attaching a wire to the cargo, run the wire over appropriate pulleys attached to the deck and connect the wire to the crane's hook. This generally permits using one winch for different purposes. In addition or alternatively, actuators may be dedicated to a specific system, e.g. a winch or hydraulic piston below deck driving a hooked chain in the conveyor system.

[0032] The barge system preferably comprises a dynamic positioning system. As the term is used herein, a dynamic positioning system handles one or more of six degrees of freedom, i.e. heave, surge and sway related to load shifts and roll, pitch and yaw related to vessel motion. DP-systems are commercially available. Familiar examples include systems for keeping a vessel stationary and heave compensation systems for a crane or winch.

[0033] These and other features and benefits will be explained in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be explained with reference to exemplary embodiments and the accompanying drawings, in which:

Fig. 1 is a top view of a barge system according to the invention;

Fig. 2 is a top view of the barge system in Fig. 1 with a feeder barge in transit mode;

Fig. 3 illustrates transport of a jacket using the feeder barge in an inclined position;

Figs. 4a-b illustrate reduction of dynamic load by means of buoyancy;

Figs. 5a-c are side views illustrating three modes of operation for the feeder barge;

Fig. 6 illustrates a floatover operation;

Figs. 7a-g illustrate decommissioning of a platform.

Fig. 8 is a side view illustrating a main crane and transport of a subsea installation; and Figs. 9a-b are stern views illustrating deployment of the subsea module in Fig. 8.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0035] The drawings are schematic and not necessarily to scale. For ease of understanding, numerous details known to the skilled person are omitted from the drawings and following description.

[0036] Figures 1 and 2 are top views illustrating a barge system 100 according to the invention. The main components of the system 100 are a mother barge 101 with a stern recess 102, a feeder barge 110 that fits into the stern recess 102, a conveyor system 130 - 133 for moving cargo 201, 202 on the decks 103, 113 without lifting, a main crane 140 and lifting towers 150.

[0037] The mother barge 101 is a self-contained vessel, i.e. a vessel capable of operating without assistance from other vessels, e.g. tugs. An engine room 120 contains equipment for supplying power to the various subsystems, e.g. an engine, an electric power generator, etc. Electrically powered azimuth thrusters 121 shown in Figs. 3 - 9 are provided for propulsion, steering and dynamic positioning. A superstructure 122 with an antenna 124 represents means for control and communication. The superstructure 122 may contain, for example, a bridge, a control room and accommodations for a crew. The antenna 124 provide communication to and from the barge system, and comprises a receiver for a navigation system such as GPS. A dynamic positioning system requires positioning data from the navigation system.

[0038] Two parallel hull elements 101a and 101b extend rearward on the mother barge 101 and form a stern recess 102 between them. A hinge mechanism 105 extends across the fore end of the stern recess 102. The rear end of a feeder barge 110 or a support arm 111 attached to the hinge mechanism 105 at a fore end is able to swing up and down in the water.

[0039] In Figure 1, the feeder barge 110 is locked in the stern recess 102 by the hinge mechanism 105, guide pins 106 and grooves 107. The guide pins 106 and grooves 107 allow a vertical displacement of the feeder barge 110 with respect to the mother barge 101, and may operate independent of the hinge mechanism 105. For example, the feeder barge 110 may be submersible, and the guide pins 106 may form part of a lifting system similar to the platform used in the Sea Bee system described above. In addition or alternatively, the feeder barge 110 may lift or lower cargo by ballasting while the guide pins 106 slide in the grooves 107. The guide pins 106 are retractable from the grooves 107 to release the feeder barge 110 to the state shown in Fig. 2.

[0040] Movable parts for the hinge mechanism 105, retractable guide pins 106, a ballasting system etc., may be controlled from the mother barge 101. Subsystems on the feeder barge 110, e.g. retractable guide pins 106 or a separate ballasting system, can be connected to a control system on the mother barge 101 through any suitable contact for power supply and a wired or wireless communication channel.

[0041] Apart from the engine room 120, ballasting tanks 108 occupy most of the space below deck 103. The ballasting tanks 108 represent a ballasting system to compensate for slowly varying load shifts. For example, a heavy load resting on the feeder barge 110, suspended from the main crane 140 or held by the lifting towers 150 at the stern of the mother barge 101 may be compensated by pumping water into ballasting tanks 108 near the bow of the main barge 101. The heavy lift equipment 110, 140, 150 is located near the stern to provide a long moment arm to the fore ballasting tanks, and thereby minimize the amount of ballasting water required for counter ballasting. This is similar to the design of other heavy lift vessels, and appropriate ballasting systems are known in the art.

[0042] The engine room 120 may comprise combustion engines, gas turbines, electric generators, hydraulic systems and other means to power the various equipment and subsystems on the mother barge 101. For example, a generator driven by a combustion engine or gas turbine may provide electric power to thrusters 121 (see e.g. Fig 3) for propulsion and steering. Suitable thrusters 121 include azimuth thrusters or 'pods' with electric motors directly coupled to the propellers. Such thrusters are commercially available, and able to provide thrust in any azimuth angle. They typically also form part of a DP-system. The use of azimuth thrusters 121 does not exclude conventional propellers and rudders from propulsion and steering. Engines and systems in the engine room 120 are not part of the invention as such, and thus need no further explanation herein.

[0043] The fixed superstructure 122 at the bow of the mother barge 101 typically contains a bridge and/or a control room. An antenna 124 represent communication in general. For example, the antenna 124 may receive signals from the American GPS-system and/or its Russian counterpart GLONASS as input to a DP-system. The antenna 124 may also transmit and receive radio signals for communication with other vessels or an onshore control center. The various control and communication systems are not part of the invention as such, and hence need no further explanation herein.

[0044] Figure 2 shows the barge system in Fig. 1 with the feeder barge 110 separated from the mother barge 101, e.g. in transit to or from a shallow port with a load or cargo 202. The cargo 202 may be transferred to the deck 103 of the mother barge 101 before transport to or from an offshore field. Alternatively, the cargo 202 may remain on the feeder barge 110 during the transport to or from the offshore field.

[0045] In order to access yards and storage areas with limited water depth and/or quay facilities, the draft, i.e. the distance from the waterline 1 to the lowest part of the hull when loaded, of the feeder barge 110 may be less than the draft of the mother barge 101. The length of the feeder barge 110 depends on an intended cargo, and may be greater than the length of the stern recess 102, i.e. such that a feeder barge 110 connected to the hinge mechanism 105 may extend behind the aft end of the hull elements 101a, 101b.

[0046] One or more feeder barge(s) 110s can be used as the feeder barge 110 spends time to and from other vessels, fixed or floating installations in the offshore field. Due to less displacement and draft, the feeder barge 110 has less fender and support needs than the mother barge 101.

[0047] The mother barge 101 has ballast and power systems permanently installed onboard. These systems may serve the feeder barge 110 when they work together. The feeder barge 110 may also be ballasted using temporary portable pumps and a hydraulic power unit and/or a permanent ballast system while loading and unloading onshore.

[0048] Typically, one or more tugs (not shown) will tow and/or push the feeder barge 110 in transit, e.g. on a river or in a harbor. If desired, the feeder barge 110 may be provided with propulsion and steering similar to the propulsion and steering of the mother barge 101. [0049] A conveyor system 130 - 133 enables transfer of cargo 201, 202 on the decks 103, 113 of the barges 101 and 102 without lifting. For example, reference numeral 130 represent a winch with a capstan or chain gear, and numeral 131 represent a wire, rope or chain. These elements with associated pulleys and blocks are known as 'tackle', and has been used to pull and hoist cargo since the antiquity. In current applications, the winch is rotated by a hydraulic or electric motor, otherwise the tackle has been essentially unchanged for centuries. Linear actuators, typically hydraulic pistons, provide a known alter rotary actuators such as the winch 130.

[0050] Reference numerals 132 and 133 represent rails, skid beams, rollers etc. provided to facilitate moving cargo 201, 202 on the decks 103, 113 without lifting. The rails may be elevated to form support structures as described with reference to Fig. 7.

[0051] The main crane 140 illustrated in Figs. 1 and 2 comprises a latticed boom 141, and is rotatable about a vertical axis. An alternative main crane 145 is shown in Fig. 8. Yet another alternative would be a gantry crane, possibly with a trolley running along the main beam to access any point on the decks 103 and 1 13. The choice of main crane 140, 145 depends on the intended use of the barge system, e.g. required load capacity, lifting height, radius of operation, etc. The barge system 100 may also have one or more auxiliary cranes 142.

[0052] Four lifting towers 150 are mounted at the stern of the mother barge 101, two on the starboard hull element 101a and two on the port hull element 101b. The lifting towers 150 are preferably mounted on rails or racks (not shown) extending along the hull elements 101a, 101b such that the distance between the fore and aft towers 150 can be varied, and such that the center of gravity of a load carried by the lifting towers 150 can be shifted or adapted. The lifting towers 150 also comprise support frames 151 for lifting and lowering a load. The support frames 151 are further explained with reference to Fig. 8.

[0053] In Fig. 3, a jacket 210 fitting into the stern recess 102 rests on the feeder barge 110. To avoid damage to the jacket 210, a rocker arm similar to the support arm 111 in Figs 4a and 4b distributes the weight of the jacket 210. The rocker arm on feeder barge 110 is omitted from Fig. 3 for clarity.

[0054] In Fig. 3, the feeder barge 110 is hinged to the mother barge 101 through the hinge mechanism 105, which has a pivot at the top, fore end of the stern recess 102. When the stern of feeder barge 110 sinks, a buoyancy provided by air-filled buoyancy tanks in the feeder barge 110 increases. Opposite, when the stern of feeder barge 110 raises out of the water, the buoyancy decreases. Thus, the hinged feeder barge 110 tends to even out vertical motion of the load 210, i.e. the heave. In addition, the center of gravity of the jacket 210 may be kept stationary with respect to the mother barge 101 by varying the length of wire 131, and thereby reduce surge. A heave compensating winch 130 may be sufficient to handle the resulting slowly varying heave and surge, i.e. the dynamic loads imposed on the mother barge from the load during transport.

[0055] Figs. 4a and 4b illustrate a variant of the same principle to reduce dynamic loads. The mother barge 101 is partially sectioned to show the hinge mechanism 105 and one of several ballasting tanks 108. The feeder barge 110 in Fig. 3 is replaced with a support arm 111 attached to the hinge mechanism 105. An elongated load, e.g. a jacket 210 as shown or a tower for a wind turbine, is attached to the support arm 111, e.g. by clamps. One purpose of the support arm 111 is to distribute the weight of the load over a large area to avoid damage from excessive pressure, i.e. too large weight on a small area. For this, the support arm 111 shown in Figs 4a and 4b extends in opposite directions from the hinge element or pivot 105. The hinge element 105 is moved located aft of the position shown in Fig. 3 such that the fore part of the support arm 111, i.e. the part to the left of the hinge element 105 in Figs. 4a and 4b, can pivot about the hinge 105 without colliding with the deck 103. In general, the hinge element 105 has an axis of rotation that extends parallel to the deck 103 and across the stern recess 102, i.e. parallel to the pitch axis. This axis or rotation or pivot may be located in any suitable location within the stern recess 102, e.g. in the top front as shown in Fig. 3 or slightly lower and further back as in Figs. 4a and 4b.

[0056] A partially submerged buoyancy element 112 is attached to the aft end of the jacket 210. Only the part of the buoyancy element 112 below the sea surface 1 contributes to the buoyancy. The location at the end of the jacket 210 provides a long moment arm to the hinge mechanism 105. This reduces the required buoyancy force provided by the element 112. In addition, the center of gravity of the jacket 210 as shown is behind the center of gravity of a similar jacket resting on the deck 103 of the mother barge. This extends the moment arm from the center of gravity of the jacket 210 to the bow of the mother barge 101, and hence reduces the amount of water needed for counter ballasting.

[0057] In Fig. 4a, most of the buoyancy element 112 is above the waterline 1, so the buoyancy element 112 provides a relatively small buoyancy. In Fig. 4b, the support arm 111 has pivoted about the hinge mechanism 105 such that most of the buoyancy element 112 is under water, i.e. below the waterline 1. Thus, the buoyancy provided by the element 112 in Fig 4b is greater than in Fig. 4a. The oscillation of the support arm 111 may be damped further as known in the art, and the remaining load shifts reduces the requirements for a heave compensating winch 130 as shown in Fig. 3. Alternatively, a heave compensating winch (not shown) may be placed on the protruding element 101a with a wire attached to the stern end of the support arm 111. The performance requirements for such a winch is reduced by the buoyancy element 112 opposing the weight of the jacket 210.

[0058] During installation of the jacket 210, the support arm 111 may extend vertically from the hinge mechanism 105. Alternatively, the jacket 210 may rotate about a fulcrum at the stern end of the feeder barge 110 as indicated in Fig. 3. In both cases, buoyancy may reduce the dynamic load imposed on a hoist used to lower the jacket 210 to the seabed.

[0059] Figs. 5a-c illustrate three uses of the feeder barge 110 in the barge system 100.

[0060] Fig. 5a shows guide pins 106 on the feeder barge 110 engaging grooves 107 in the mother barge 101 as if the stern of the mother barge 101 was transparent. The deck 113 of the feeder barge 110 is essentially flush with and parallel with the deck 103 of the mother barge 101. The distance from a waterline 1 to the lowest part of the hull of the feeder barge 110 is less than the distance from the waterline 1 to the lowest part of the hull of the mother barge 101 to illustrate that the draft of the feeder barge 110 may be less than the draft of the mother barge 101.

[0061] Thrusters 121 represent means for propulsion and steering, and are typically part of a DP-system. Azimuthal stern thrusters, optional rudders, a skeg etc. are omitted from Figs. 5a- 5c for clarity. However, any means for propulsion, steering and dynamic positioning known in the art may be used with the invention.

[0062] The superstructure 122 typically comprises a bridge and/or a control room. The antenna 124 extends from the superstructure 122, and may carry any suitable means for transmitting and receiving signals. This includes, for example, a rotating radar transceiver in addition to the satellite receivers and communication transceivers mentioned above.

[0063] The locked position shown in Fig. 5a facilitates transfer of cargo between the decks 103, 113, for example by means of the conveyor system 130 - 133 described above. The large continuous deck 103, 113 is also useful for transporting much or large cargo at sea, and provide a large and stable platform for offshore operations.

[0064] Fig 5b shows the feeder barge 110 connected to the mother barge 101 through the hinge mechanism 105. In this state, the inclined feeder barge 110 may support an elongated structure as illustrated in Fig. 3. The stern wall 102a of the stern recess 102 is inclined such that the feeder barge 110 may pivot about the hinge mechanism 105. For the same reason, at least some grooves 107 are arcs of circles that are concentric with the hinge mechanism 105. Guide pins 106 are not shown explicitly in Fig. 5b, but prevent excessive loads on the hinge mechanism 105 in a real embodiment. [0065] Fig. 5c shows the feeder barge 110 displaced vertically relative to the mother barge 101. The vertical displacement does not exclude a horizontal displacement. The feeder barge 110 may be submersible, and double as a platform in a lifting system. When the feeder barge 110 is submerged, buoyancy working on a cargo may reduce the requirement for a main crane, e.g. for lowering a subsea module to the seabed. The lift may be provided by pumping water into or out of ballast tanks in the feeder barge 110, by separate jacks or by other means, e.g. the lifting towers 150 described above. The guide pins 106 and grooves 107 prevent the feeder barge from leaving the stern recess 102.

[0066] While the decks 103, 113 are parallel in Fig. 5c, they are not parallel in all embodiments. For example, several grooves 107 concentric with the hinge mechanism 105 cause a rotation. Accordingly, the deck 113 may be allowed to tilt with respect to the deck 103 during the vertical displacement.

[0067] Fig. 6 illustrates a floatover operation. The jacket 210 is installed on a seabed, and is small enough to fit into the stern recess 102. Thrusters 121 controlled by a DP-system keep the mother barge 101 in position. Tugs (not shown) may also assist to keep the mother barge 101 stationary. In theory, the mother barge 101 could also be kept in place by anchors.

However, in many or most cases mooring would be too time consuming and expensive.

[0068] A topside 220 with a derrick 221 is lifted by means of lifting towers 150 and associated support frames 151. The support frames 151 may be actuated by hydraulic pistons for rapid mating of the topside 220 to the jacket 210 as discussed in the introduction. In addition or alternatively, the support frames 151 may comprise pinions to climb on vertical racks similar to the rack and pinion arrangements used on the legs of jackup rigs.

[0069] Figures 7a-e illustrates removing a topside 220.

[0070] In Fig. 7a, the jacket 210 is surrounded on three sides by the stern recess 102 as discussed in connection with Fig. 6. In contrast to the state shown in Fig. 6, Fig. 7a shows the topside 220 attached to the jacket 210, and the support frames 151 do not yet engage the topside 220.

[0071] In Fig. 7b the support frames 151 engage the topside 220, and the topside 220 is still attached to the jacket 210. If the support frames 151 are fixed to the lifting towers 150, the mother barge 101 is essentially fixed to the seabed. This may be perfectly acceptable in sheltered waters. In other locations, swell and/or wind generated waves may exert unacceptable loads on the mother barge 101 and hence on the platform 210, 220. In these cases, heave compensating support frames 151 may compensate for vertical motion of the mother barge 101 while exerting a suitable lifting force on the topside 220. [0072] In Fig. 7c, the connection between the jacket 210 and the topside 220 is cut, and the topside 220 is lifted off from the jacket 210. The mother barge 101 is ballasted accordingly.

[0073] In Fig. 7d, the mother barge 101 has moved away from the jacket 210. The support frames 151 carry the topside 220, and the stern recess 102 is empty.

[0074] In Fig. 7e, a feeder barge 110 with transport supports 152 has entered the stern recess 102, and the jacks 150, 151 lower the topside 220 onto the transport supports 152. The transport supports 152 may be permanently mounted on the deck 113 of the feeder barge 110.

[0075] Similar transport supports may be mounted on the deck 103 of the mother barge 101, and the topside 220 may be transferred to the mother barge 101 for transport from the offshore field. In this case, the stern recess 102 is available for transporting the jacket 210 using the feeder barge as support arm as described with reference to Fig. 3.

[0076] Fig. 7f, illustrates transport from the field. The topside 220 rests on the transport supports 152 on the feeder barge 110, which in turn is locked to the main barge 101. The jacks 150, 151 carry no weight.

[0077] As noted, the topside 220 could be transferred to the mother barge 101 during the transport from the offshore field, and the jacket 210 could be transported in the stern recess 102 on the same trip.

[0078] Fig. 7g illustrate the final stretch of transport to a yard for decommissioning or maintenance. The feeder barge 110 has left the stern recess 102, and carries the topside 220 on the transport supports 152.

[0079] The barge system 100 is also used to remove the jacket 210 in Figs. 6 and 7.

[0080] In general, the jacket 210 is firmly attached to the seabed, and the legs must be cut at the seabed for removal. The lifting towers 150 and/or the main crane 140, 145 provide a lift during the cutting. If a mechanical tool is used, the lift prevent that the cutting tool gets stuck. If an underwater torch is used, the lift prevents that the jacket legs sinks back over the cut and become re-welded.

[0081] After the legs have been successfully cut, the jacket 210 is moved to a location nearby to avoid risk of damage to a well or other seabed installation associated with the platform jacket to be removed. At the nearby location, the jacket 210 may be provided with appropriate buoyancy, and winches 130 with associated tackle 131 pulls the jacket 210 over the feeder barge 110 or support arm 111 for transport as shown in Figs. 3, 4a and 4b.

[0082] In an alternative lifting operation at the nearby location, a rocker arm similar to the support arm 111 in Figs 4a and 4b is hinged at the stern of a feeder barge 110. The stern of the feeder barge 110 is then submerged by ballasting, and the rocker arm attached to the jacket 210. Then, the jacket is lifted from the seafloor as above, and the feeder barge is attached to the hinge mechanism 105 for transport to an onshore facility.

[0083] In the two previous examples, air-filled legs and separate buoyancy elements 112 may increase the lift and stability during the operation. Furthermore, a skilled person knowing the application at hand must determine the required steps as well as the number and type of winches, ropes, pulleys, blocks and attachment points on the legs of the jacket 210. Similar consideration apply to similar support structures, e.g. a tower for a wind turbine.

[0084] Figs. 8, 9a and 9b illustrate unloading of a subsea module 230.

[0085] Fig. 8 shows the subsea module 230 resting on the deck 113 of the feeder barge 110. A main crane in the form of a portal crane 145 is tilted by means of a hydraulic piston 146 such that a top beam is positioned over the subsea module 230. The portal crane 145 is fixed to the deck 103. If desired, the portal crane 145 may be replaced by a gantry crane running on rails along the mother barge 101. A trolley may run along the main beam of the gantry crane, such that a block and hook carried by the trolley may be placed anywhere over the stern recess 102 or deck 103.

[0086] The wire 131 connects the winch 130 with a bridle to form a hoist for the subsea module 230. The winch 130, wire 131 and/or other parts of a tackle may also form part of the conveyor system 130 - 133 described above.

[0087] Fig. 5a is a stern view showing the subsea module 230 suspended from the main crane 145. The feeder barge 110 has left the stern recess 102. Fig. 5b is a stern view similar to Fig. 5a. In Fig. 5b, the subsea module 230 has been lowered through the stern recess 102. If required, e.g. for deployment on large depths, lowering the subsea module 230 to the seabed may involve additional vessels, e.g. an OCV and/or a WROV.

[0088] Ballasting a submersible feeder barge 110 may assist in lowering or lifting the subsea module 230. In this case, the guide pins 106 and associated grooves 107 prevent the feeder barge 110 from leaving the stern recess 102. The submerged feeder barge reduces the required lifting height for the main crane 145. In addition, the subsea module may be provided with temporary floating elements to reduce the required lifting capacity for the main crane 145 and/or a crane aboard an OCV used for the installation.

[0089] Some of the examples above regard installation and other examples regard to removal. The procedures generally involves steps in the opposite order and directions with obvious modifications. For example, a jacket 210 may be installed by performing the steps for removal in the opposite order, except that pylons might be hammered into seabed sediments instead of cutting the legs of the jacket 210 near the seabed. [0090] While the system has been explained by way of examples, the scope of the invention is defined by the accompanying claims.