GB2580187A

GB2580187A - Decommissioning of offshore structures

Info

Publication number: GB2580187A
Application number: GB1821208.4A
Authority: GB
Inventors: Tompkins Charles
Original assignee: Uk Decom Ltd
Current assignee: Uk Decom Ltd
Priority date: 2018-12-24
Filing date: 2018-12-24
Publication date: 2020-07-15
Also published as: GB201821208D0

Abstract

A method of decommissioning an offshore structure 502comprising a topside (504a, Fig 3a) mounted on a jacket (504b, Fig 3a) comprising rotating the offshore structure from a substantially vertical orientation to a substantially horizontal orientation as a single piece, for transportation to shore. Also disclosed is a barge 500 for decommissioning offshore structures comprising a support frame (504, Fig 3a) for supporting an offshore structure comprising a topside and a jacket. The support frame is mounted on the barge and configured to rotate between a loading configuration to facilitate loading of the barge with the offshore structure and a transport configuration to support the offshore structure during transport. The support frame comprises a first portion (506, Fig 3a) disposed at a first end of the support frame, the first portion configured to support the topside of an offshore structure; and a second portion (508a-d, Fig 3a) disposed between the first portion and a second end of the support frame, the second portion configured to support the jacket of the offshore structure.

Description

DECOMMISSIONING OF OFFSHORE STRUCTURES

Field

The present disclosure relates to the decommissioning of offshore structures, such as oil rigs. Disclosed are a barge for decommissioning offshore structures; a system comprising an offshore structure and a barge configured to decommission the offshore structure; methods of decommissioning offshore structures; and methods of configuring a support frame for decommissioning offshore structures.

Background

Offshore structures are facilities used in marine environments, typically for the production and transmission of electricity, oil, gas and other resources.

A commonly used design for offshore structures comprises a topside mounted on a jacket. The topside is a module configured to sit above sea level, and may include living quarters, research equipment, and/or operational equipment. The jacket is a support structure that sits on the seabed, or in some cases floats under the surface, and supports the topside above sea level.

Construction and commissioning of offshore structures is generally performed as much as possible on shore, to minimise the amount of construction and commissioning work that must be done at an offshore location at which an offshore structure is to be used.

When offshore structures reach the end of their operating life, they must be decommissioned. Similarly, when extraction of oil from an offshore oil reserve becomes uneconomical, the associated offshore structure (oil rig) installed at the oil reserve must be decommissioned.

Currently, decommissioning is done by dismantling the offshore structure, and transporting the dismantled structure to shore piece-by-piece. In the case of an offshore structure comprising a topside mounted on a jacket, for example, the topside is removed from the jacket and transported to shore, and the jacket is then transported to shore separately. In other examples, offshore structures are dismantled into even smaller pieces for transportation to shore. For example, the topside may be dismantled and transported to shore piece-by-piece.

Dismantling offshore structures at sea presents significant engineering challenges, necessitating substantial and often labour intensive offshore operations. Existing approaches for decommissioning offshore structures are therefore costly, and hazardous. Furthermore, dismantling offshore structures offshore can cause contamination of the marine environment.

Summary

The aspects and examples described below address the problem outlined above.

In a first aspect there is provided a barge for decommissioning offshore structures. The barge comprises a support frame for supporting an offshore structure comprising a topside mounted on a jacket. It will be understood that the support frame as referred to throughout this description may be of a simple construction, for example a single beam, of a more involved constructions such as a space frame, or of any other suitable construction. The support frame is mounted on the barge and configured to rotate relative to the barge between a loading configuration to facilitate loading of the barge with the offshore structure, and a transport configuration to support the offshore structure during transport. The support frame further comprises a first portion disposed at a first end of the support frame, the first portion configured to support the topside of an offshore structure; and a second portion disposed between the first portion and a second end of the support frame (e.g. a second end distal from the first end), the second portion configured to support the jacket of the offshore structure.

By providing a support frame configured to support both the topside of the offshore structure and the jacket of the offshore structure, it is possible to remove the offshore structure as a single piece without causing damage to the offshore structure. In particular, the support frame eliminates stresses and strains at the interface between the jacket and the topside, which would otherwise become problematic when an offshore structure is rotated out of a vertical orientation, especially when the offshore structure is raised above sea-level. These stresses and strains could otherwise cause damage to the interface between the topside and the jacket, and in some cases could cause the topside to break away from the jacket, which would be dangerous to decommissioning personnel and risk causing substantial environmental pollution. Thus, by supporting the topside and the jacket the barge of the first aspect enables dismantling of the offshore structure on shore.

As used herein, a topside is defined as a module of an offshore structure configured to sit above sea level and a jacket is defined as a structure configured, in use, to sit on the sea bed or float below the surface and support a topside above sea level. The topside may include living quarters, research equipment, and/or operational equipment.

The barge may have a hull comprising a pair of prong portions extending from a base portion to form a fork-shaped hull. The hull may thus be considered as a notched hull -with a notch in the hull defined between the prong portions. The support frame may extend between the prong portions and below the hull in the loading configuration. In some embodiments, a rotation axis on the support frame may be disposed between the prong portions and above a waterline of the hull, for example above a waterline of the hull when the barge is fully loaded to its rated tonnage. Thus, more generally, in some embodiments a plane in which the support frame rotates about its axis lies between the prong portion, for example centred between the prong portions and/or bisecting the hull. The support frame may extend fore and aft sufficiently to support the offshore structure in situ, above and below the waterline as the barge approaches the offshore structure to engage and load the offshore structure. In some embodiments, the first portion extends fore from the axis of rotation and the second portion extends aft of the axis of rotation. In other embodiments, these directions are reversed. In some embodiments, the support frame is disposed relative to the axis of rotation so that it is balanced fore and aft.

In a second aspect there is provided a barge for decommissioning offshore structures. The barge comprises a hull, the hull comprising a pair of prong portions extending from a base portion to form a fork-shape; and the barge further comprising a support frame for supporting an offshore structure. The support frame is mounted on the hull and configured to rotate relative to the hull between a loading configuration to facilitate loading of the barge with the offshore structure and a transport configuration to support the offshore structure during transport. The support frame extends between the prong portions and below the hull in the loading configuration. For example, the offshore structure may have a topside mounted on a jacket and the support frame may have a first portion disposed at a first end of the support frame, the first portion configured to support the topside; and a second portion disposed between the first portion and a second end of the support frame (e.g. a second end distal from the first end), the second portion configured to support the jacket.

Because the support frame extends between the prong portions in the loading configuration, the prong portions provide stabilising buoyancy on either side of the support frame as it rotates and is disposed across the hull of the barge, for example substantially vertically, to accept an offshore structure to be loaded onto the barge. Moreover, when an offshore structure is being supported by the support frame (and when the support frame rotates while supporting the offshore structure), the buoyant prong portions provide buoyancy, front or aft as the case may be, that prevents the barge from capsizing under the weight of the offshore structure. Accordingly, the full weight of the offshore structure can be stably supported by the barge during loading and transport, without the need to dismantle the support structure into smaller parts. Thus, the barge of the second aspect enables safe transport of the integral offshore structure to shore and thus enables dismantling of the offshore structure on shore.

Again, the hull of the barge may be considered as a notched hull -with a notch in the hull defined between the prong portions.

Some optional features common to the first and second aspects will now be set out.

Typically, the first portion of the support frame may be configured to the weight of the topside when in the transport configuration (i.e. when the support beam is in the transport configuration and the offshore structure is supported thereon, the topside may rest on the first portion). And the second portion of the support frame may be configured to support the weight of the jacket when in the transport configuration (i.e. when the support beam is in the transport configuration and the offshore structure is supported thereon, the jacket may rest on the second portion). In the loading configuration, the first portion is positioned above the second portion.

The support frame may comprise engagement portions (e.g. first and second engagement portions) for engaging an offshore structure, for example as described above. The first and second engagement portions may extend laterally from the support frame and may be oriented perpendicularly with respect to the support frame. For example, the first portion of the support frame may comprise a first engagement portion for engaging the topside, and the second portion of the support frame may comprise a second engagement portion for engaging the jacket.

Each engagement portion may comprise a respective friction pad for securely engaging the respective portion of the offshore structure. In some embodiments, each engagement portion may comprise one or more clamps for securely engaging the offshore structure. Each clamp may be hydraulically actuated. Each friction pad and/or clamp may be moveable along its respective engagement portion, e.g. to suit different sizes of offshore structures.

The support frame may comprise first and second parallel support rails on which the offshore structure rests in the transport configuration. Accordingly, it may be possible to slide the offshore structure onto shore along the support rails, e.g. when the barge is positioned adjacent a landing skid (defined herein as a structure on shore configured to receive the offshore structure from the barge). The support rails may be spaced so as to mate with parallel landing rails of a landing skid located on shore. Each support rail may be fitted with a respective first engagement portion and a respective second engagement portion, or a single first engagement portion and a single second engagement portion may each be fitted to both support rails.

The support frame may be configured to rotate about an axis, which may be disposed substantially at a midpoint along the support frame. The axis may be positioned adjacent, above, below, or at a centre of buoyancy of the hull. In some examples, the axis may extend across a gap (e.g. notch) defined between the prong portions. The axis may have a fixed position on the hull and/or a fixed position along the support frame, or the barge may be configurable (e.g. while the barge is on shore) so that the axis can be disposed at a configurable position along the support frame and/or hull. For example, a mount for the support structure may be configurable or moveable to adjust the position of the axis relative to the support frame and/or hull. The mount may comprise a spindle, axle or any other arrangement rotatably securing the support frame to rotate about the axis relative to the hull.

In the loading configuration, the support frame is oriented to engage the offshore structure prior to and for loading onto the barge. In the transport the support frame is oriented for transporting the support structure to shore. The support frame may be oriented substantially vertical or across a deck of the hull in the loading configuration, and substantially horizontal or along the deck of the hull in the transport configuration.

The engagement portions may be adjustable, for example moveable along the support frame, to fit different offshore structures, e.g. during a configuration step. Where the support frame comprises support rails, the engagement portions may be mounted to the support rails and moveable along the support rails. For example, there may be a pair of first engagement portions (one on each support rail) for supporting the topside. There may also be a pair of second engagement portions (one on each support rail) for supporting the jacket. The pair of first and/or second engagement portions may be moveable towards/away from each other in a direction transverse to the support rails, e.g. based on the dimensions of an offshore structure to be decommissioned. In other words, the spacing between the pair of first engagement portions may be adjustable. And the spacing between the second pair of engagement portions may be adjustable. The first engagement portion may be configured to engage a topside of an offshore structure, and the second engagement portion may be configured to engage a jacket of the offshore structure.

In a third aspect there is provided a system comprising a barge according to the first or second aspect, and an offshore structure comprising a topside mounted on a jacket. In particular, the third aspect comprises a system comprising a barge for decommissioning offshore structures according to the first or second aspect, and an offshore structure comprising a topside mounted on a jacket.

In one example of the third aspect, the barge comprises a support frame for supporting the offshore structure. The support frame is mounted on the barge and configured to rotate relative to the barge between a loading configuration to facilitate loading of the barge with the offshore structure and a transport configuration to support the offshore structure during transport. The support frame further comprises a first portion configured to support the topside and a second portion configured to support the jacket of the offshore structure.

In another example of the third aspect, the barge comprises a hull, the hull comprising a pair of prong portions extending from a base portion to form a fork-shape; and the barge further comprises a support frame configured to support the offshore structure. The support frame is mounted on the hull and configured to rotate between a loading configuration to facilitate loading of the barge with the offshore structure and a transport configuration to support the offshore structure during transport. The support frame extends between the prong portions and below the hull in the loading configuration.

The system of the third aspect may comprise any of the optional features of the first and second aspects as described herein.

The system may be configured such that, when the offshore structure is supported by the support frame and the support frame is in the transport configuration (e.g. when the barge is 'loaded' with the offshore structure), the offshore structure may be positioned at or above sea level, e.g. at or above a waterline of the hull. For example, the offshore structure may be positioned substantially on/across a deck of the hull.

The system may further comprise a landing skid located on shore, the landing skid comprising a pair of parallel landing rails for receiving the offshore structure. The pair of parallel support rails may be configured to mate with the pair of parallel landing rails to facilitate transferring the support structure onto shore. In some embodiments, the landing skid may comprise a first pair of landing rails for receiving the offshore structure from the barge and leading to a jacket processing area; and a second pair of landing rails that intersect with the first pair of lading rails and lead to a jacket processing area.

In a fourth aspect there is provided a method of decommissioning an offshore structure comprising a topside mounted on a jacket, the method comprising rotating the offshore structure from a vertical (e.g. substantially vertical) orientation to a horizontal (e.g. substantially horizontal) orientation as a single piece, for transportation to shore.

By decommissioning the offshore structure with the topside and jacket as a single piece, the need to dismantle the offshore structure at sea (i.e. prior to transportation to shore) is eliminated. Thus, dismantling of the offshore structure can be done on shore.

The method of the fourth aspect may comprise providing a barge comprising a support frame; supporting the topside of the offshore structure with a first portion of the support frame, the first portion disposed at a first end of the support frame; supporting the jacket of the offshore structure with a second portion of the support frame, the second portion disposed between the first portion and a second end of the support frame; and rotating the support frame from a loading configuration to a transport configuration to thereby rotate the offshore structure from the (substantially) vertical orientation to the (substantially) horizontal orientation.

In a fifth aspect there is provided a method of decommissioning an offshore structure comprising a topside mounted on a jacket using a barge according to the first aspect. The method comprises: supporting the topside of the offshore structure with the first portion of the support frame, supporting the jacket of the offshore structure with the second portion of the support frame, and rotating the support frame from the loading configuration to the transport configuration to thereby rotate the offshore structure from a vertical (e.g. substantially vertical) orientation to a horizontal (e.g. substantially horizontal) orientation as a single piece, for transportation to shore.

In a sixth aspect there is provided a method of decommissioning an offshore structure using the barge of the second aspect, the method comprising supporting the offshore structure with the support frame, and rotating the support frame from the loading configuration to the transport configuration to thereby rotate the offshore structure from a vertical (e.g. substantially vertical) orientation to a horizontal (e.g. substantially horizontal) orientation.

The offshore structure of the sixth aspect may comprise a topside mounted on a jacket.

Optional features common to the fourth, fifth, and/or sixth aspects will now be set out. For example, the method may comprise attaching the barge to a control vessel, moving the barge to a position adjacent to the offshore structure using the control vessel, the control vessel optionally supplying power to the barge, the power to be used in rotating the support frame. In other words, the control vessel may provide power to machinery on the barge configured to rotate the support beam. Transporting the barge to shore may be done with the offshore structure in the (substantially) horizontal orientation, and removing the topside from the jacket on shore. The method may further comprise transferring the offshore structure to shore along a landing skid comprising at least one landing rail (e.g. a pair of parallel landing rails) for receiving the offshore structure. In particular, the method may comprise sliding the offshore structure from the support frame onto the pair of parallel landing rails. In some examples, the method may comprise waiting for a desired tidal height before transferring the offshore structure to shore. Typically, the desired tidal height will not be high tide. As an example, the desired tidal height may be low tide.

Moreover, the method may comprise transferring the offshore structure onto a first pair of parallel landing rails located on shore; detaching the topside from the jacket; transporting the jacket to a jacket processing area along the first pair of parallel lending rails; and transporting the topside to a topside processing area along a second pair of parallel landing rails that intersect with the first pair of parallel landing rails.

The method may comprise configuring the support frame on shore, prior to deployment, for supporting the offshore structure when decommissioning.

The method may comprise using an increase in tidal height to lift the offshore structure prior to rotating the offshore structure, specifically, in some embodiments, securing the offshore structure to the support frame, for example at low tide, and rotating the support frame once a rise in tide has lifted the offshore structure off the sea floor. Additionally, or alternatively, ballast may be removed/dumped from the barge in order to lift the offshore structure. To this end, a ballast tank may be provided on the deck of the barge and/or within the hull of the barge. In some examples, there may be a plurality of ballast tanks on the deck of the barge. Further, there may be a plurality of ballast tanks in the hull of the barge. The ballast tanks may be distributed along the barge in the fore-aft direction of the barge. During a decommissioning operation, the ballast tanks may be de-ballasted in a pre-determined sequence. The predetermined sequence may be configured to maintain the trim of the barge within acceptable limits during the decommissioning operation.

The method may comprise supporting the offshore structure in the horizontal orientation, at a position that is at or above sea level. For example, when the support frame is in the horizontal orientation, it may support the offshore structure at a position that is at or above a waterline on a hull of the barge.

In a seventh aspect there is provided method of configuring a support frame for decommissioning an offshore structure comprising a topside mounted on a jacket, the method comprising: configuring a first portion of the support frame disposed at a first end of the support frame for supporting the topside of the offshore structure, and configuring a second portion of the support frame disposed between the first portion and a second end of the support frame for supporting the jacket of the offshore structure.

The method of the seventh aspect may be used to configure a barge comprising the support frame. For example, the seventh aspect may be used to configure the barge of the first aspect or the barge of the second aspect (i.e. to configure the support frame of the barge in the first/second aspect).

By configuring the support frame to support both the topside of the offshore structure and the jacket of the offshore structure, it is possible to use the support frame to decommission the offshore structure as a single piece, without causing damage to the offshore structure. Thus, dismantling of the offshore structure can be done on shore.

Configuring the first and second portions of the support frame may comprise adjusting a first engagement portion on the first portion of the support frame for engaging the topside, and adjusting a second engagement portion on the second portion of the support frame for engaging the jacket. Adjusting the first and second engagement portions may comprise moving the first engagement portion and/or the second engagement portion along the support frame. This may be done to ensure that the trim of the barge in the water is within acceptable limits when an offshore structure is supported by the support frame and the frame is in the transport configuration, e.g. so that the centre of mass of the offshore structure is positioned on the barge as required to ensure that the trim of the barge is within acceptable limits.

In an eighth aspect there is provided a method of configuring the barge of the first aspect for decommissioning an offshore structure comprising a topside mounted on a jacket, the method comprising: configuring the first portion of the support frame for supporting the topside of the offshore structure, and configuring the second portion of the support frame for supporting the jacket of the offshore structure.

Configuring the first portion of the support frame for supporting the topside of the offshore structure may comprise adjusting a first engagement portion for engaging the topside, and configuring the second portion of the support frame for supporting the jacket of the offshore structure may comprise adjusting a second engagement portion for engaging the jacket, wherein the first portion of the support frame comprises the first engagement portion, and the second portion of the support frame comprises the second engagement portion.

In a ninth aspect there is provided a method of configuring the barge of the second aspect for decommissioning an offshore structure, wherein the barge of the second aspect comprises a first engagement portion configured to engage a topside of an offshore structure, and a second engagement portion configured to engage a jacket of the offshore structure, the method of the ninth aspect comprising adjusting the first engagement portion for engagement with the topside of the offshore structure, and adjusting the second engagement portion for engagement with the jacket of the offshore structure.

Adjusting the first and second engagement portions in the eighth/ninth aspect(s) may include moving the first engagement portion and/or the second engagement portion along the support frame so that a centre of gravity of the offshore structure coincides (e.g. substantially coincides) with a midpoint of the support frame, when the first engagement portion engages the topside and the second engagement portion engages the jacket.

The configuring of the support frame in the seventh, eighth, and/or ninth aspects may be done on shore prior to deployment.

Brief description of the figures

Examples of the present disclosure will now be described, by way of example, with reference to the accompanying drawings in which: Figure la is a side-view of a barge comprising a notched hull, and a support frame in a loading configuration; Figure lb is a side-view of the barge of Fig. la, with the support frame in a transport configuration; Figure lc is a plan view of the barge of Figures la and lb, with the support frame in the loading configuration; Figure ld is a plan view of the barge of Figures la to lc, with the support frame in the transport configuration; Figures 2a-2g are illustrations of stages in a decommissioning operation using the barge of Figures la-id; Figure 3a shows a side view of a barge in accordance with an example of the invention, before engaging an offshore structure; Figure 3b shows the barge of Figure 3a, positioned adjacent the offshore structure; Figure 3c shows the barge of Figure 3b, de-ballasted to support the weight of the offshore structure, Figure 3d shows the barge of Figure 3c, further de-ballasted to lift the offshore structure; Figure 3e shows the barge of Figure 3d, with the support beam partially rotated while supporting the offshore structure; Figure 3f shows the barge of Figure 3e, fully rotated to support the offshore structure in a horizontal orientation.

Figure 4 is a flowchart showing a method of decommissioning an offshore structure; and Figure 5 is a flowchart showing a method of configuring a support frame for supporting an offshore structure.

Detailed description

Figures la and lb show a side-view of a barge 100 comprising a notched hull 102 and a support frame 104. Figures lc and id show a plan view of the barge 100. The notched hull comprises a base portion 106 and two prong portions 108a 108b. The prong portions 108a 108b define the notch of the notched hull between them. The notched hull can also be described as a forked hull with the prong portions extending from the base like prongs of a fork.

Figures la and lc show the barge 100 in a loading configuration (explained in more detail below). Figures lb and ld show the barge 100 in a transport configuration (also explained in more detail below).

An axis 110 extends across the notch, from first prong portion 108a to second prong portion 108b. Support frame 104 is mounted on an axle (or a set of spindles) located on the axis 110. The axle passes through a midpoint along the support frame (thereby enabling an offshore structure to be rotated approximately about its centre of gravity, as will become clear below). Accordingly, the support frame 104 can rotate between a transport configuration in which the support frame has a horizontal orientation and lies along the deck 112 of the notched hull 102 (Figures lb, 1d), and a loading configuration in which the support frame has a vertical orientation (Figures la and 1c), with a first portion 104-1 of the support frame positioned above the deck 112, and a second portion 104-2 of the support frame passing between the prong portions 108a, 108b and below the notched hull 102.

As shown, the support frame 104 comprises two parallel support raill 14, each support rail having a first engagement portion 116a and a second engagement portion 116b mounted to it. The engagement portions are moveable along their respective support rails. They are oriented perpendicular to the support rails to which they are mounted (and therefore perpendicular to the support frame).

Each engagement portion comprises at least one friction pad 122 and/or at least one hydraulically operated clamp 124.

Cable or chain 118 is attached to the first portion 104-1 of the support frame, for rotating the support frame 104 from the loading configuration (Figure la) to the transport configuration (Figure lb). A chain may be preferred where the barge is to be used in decommissioning particularly large offshore structures.

Operating machinery 125 is shown on the base portion of the deck. The operating machinery may comprise a winch for rotating the support frame by pulling on the cable or chain 118, and hydraulics for actuation of the clamps 124 Any suitable mooring system can be used to attach the barge to a control vessel. An 8-point mooring system (not shown) may also be provided at an edge of the notched hull 102, for securing the barge in place once in position adjacent the offshore structure.

Figures 2a-2g are illustrations showing a decommissioning operation performed using the barge 100 of Figures la-id.

Figure 2a is a plan view illustration showing a control vessel 200 attached to the barge 100The support frame is in the transport configuration, so that the barge 100 can be transported to an offshore structure to be decommissioned (not shown) using transport vessels (also not shown).

Figure 2b is a side-view illustration showing the barge 100 and control vessel 200 positioned near a vertically oriented offshore structure 202 (e.g. oil rig) comprising a topside 204 mounted on a jacket 206. The support frame 104 of the barge 100 has been moved into the loading configuration, ready for engaging the offshore structure 202. Because the barge 100 is unloaded, it sits high in the water. Waterline 210 on the hull 102 is shown. Offshore structure 202 is also shown as resting on the sea bed 212.

Figure 2c is a plan view illustration showing the barge 100 and control vessel 200 positioned such that the offshore structure 202 is located between the prong portions 108a, 108b. In this position, the friction pads 122 and clamps 124 of the first engagement portions 116a (not shown in Figure 2c) securely engage the topside 204 of the offshore structure, and the friction pads 122 and clamps 124 of the second engagement portions 116b (also not shown in Figure 2c) securely engage the jacket 206 of the offshore structure. The barge 100 is thus ready to rotate the offshore structure 202 into the horizontal transport configuration. In some embodiments, the offshore structure 202 is rotated once an increase in tidal height lifts the offshore structure 202 away from the sea bed, the additional clearance facilitating the rotation and removal of the offshore structure. For example, the offshore structure 202 may be secured to the support frame 104 at low tide. Other means of lifting the secured support frame 104 are equally envisaged, for example the height of the barge 100 in the water may be adjusted by flooding hull compartments prior to engagement of the offshore structure 202 and then de-ballasting the water 0.e. by opening valves to dump water from ballast tanks on the deck of the barge) to lift the offshore structure 202. Equally, the offshore structure 202 may be rotated in combination with fore or aft (as the case may be) movement of the barge 100 to provide space for the rotating offshore structure.

Figure 2d is a side view illustration showing the offshore structure 202 supported by the barge 100 in the horizontal transport configuration (i.e. after the support frame 104 has been rotated to the horizontal transport configuration while securely engaging the offshore structure). The barge 100 is thus ready to be transported to shore by being towed by transport vessels (not shown), with the offshore structure supported on the barge 100 as a single piece. Because of the increased weight of the barge in this 'loaded' configuration, the barge 100 may sit lower in the water than it did in the 'unloaded' configuration. As shown, the waterline 210 may thus be closer to the deck than in the unloaded configuration. Nonetheless, because the offshore structure 202 is supported along or above the deck, it is also supported above sea level (e.g. above the waterline). However, in examples where de-ballasting is performed to help with lifting of the barge, the waterline in the loaded configuration (i.e. after de-ballasting has been performed) may be substantially the same as the waterline in the unloaded configuration with the ballast tanks filled (if the weight of the ballast water dumped from the ballast tanks is approximately equal to the weight of the offshore structure).

Figure 2e is a side view illustration showing the offshore structure 202 being transferred from the barge 100 to shore along a landing skid 208 comprising parallel landing rails (not shown). The support rails 114 of the support frame 104 are spaced to mate with the parallel landing rails of the landing skid 208, such that the offshore structure 202 can slide off the barge and onto shore with ease.

In an alternative arrangement to Figure 2e, the offshore structure could be transferred from the barge 100 to shore in the opposite direction to that illustrated in Figure 2e, i.e. by skidding the offshore structure on to shore over the bow of the barge (rather than skidding over the stern of the barge as illustrated in Figure 2e). This is because the bow of the barge provides more buoyancy than the stern (the fork-shape of the stern reduces the buoyancy provided by the stern).

Figure 2f is a side view showing the topside 206 and jacket 204 after being dismantled on shore.

Figure 2g shows a plan view illustration of the barge 100, landing skid 208, topside 204 and jacket 206. As in Figure 2f, the topside has been detached from the jacket. Moreover, the jacket 206 is positioned on a first set of parallel landing rails 209. And the topside is positioned on a junction between the first set of parallel landing rails and a second set of parallel landing rails 211. The second set of parallel landing rails 211 has a perpendicular orientation relative to the first set of parallel landing rails 209. Accordingly, the jacket can be moved to a jacket processing area along the first set of parallel landing rails 209, and the topside can be moved to a topside processing area along the second set of parallel landing rails 211.

As can also be seen from Figure 2g, the notch in the hull of the barge enables the first set of parallel landing rails 209 to mate with the support rails 114 of the barge 100. Accordingly, the offshore structure can slide onto shore in a single motion.

Mooring lines 214 for securing the barge in place are also shown. The mooring lines are used to maintain alignment between the landing skid 208 and the support rails 114.

Figure 3a shows a barge 500 according to another example of the present disclosure. The barge 500 is positioned near to a vertically oriented offshore structure 502 in Figure 3a. Support beam 504 of the barge is in the vertical loading configuration, ready for engaging with the offshore structure 502. As shown, support beam 504 comprises a first engagement portion 506 configured for engaging the topside 502a of the offshore structure. And second engagement portions 508a, 508b, 508c and 508d for engaging the jacket 502b of the offshore structure. First engagement portion 506 includes a reinforcement beam 506a extending obliquely between an end of the engagement portion 506 and the support beam 405. The reinforcement beam 506a provides reinforcement. The reinforcement is necessary because when the offshore structure is supported in the vertical configuration, the first engagement portion 506 supports the weight of both the topside (which is significantly heavier than the jacket), and of the jacket.

Also included (but not shown) in the barge 500 are hydraulic cylinders configured to effect rotation of the support beam between the vertical loading configuration and a horizontal transport configuration. These provide a rigid rotatable link between the barge and the support beam.

Also shown in Figure 3a are four ballast tanks 510a, 510b, 510c and 510d located on the deck of the barge 500; and five ballast tanks 512a, 512b, 512c, 512d, 512e located within the hull 514 of the barge 500. The four ballast tanks 510a, 510b, 510c and 510d positioned on the deck are filled with seawater prior to initiation of the decommissioning operation, for reasons that will become clear below. Ballast tank 512e in the bow of the hull is also filled with sea water, in order to maintain the trim of the barge within acceptable limits.

The reference numerals of Figure 3a apply equally to Figures 5b-5g, but are not shown in Figures 5b-5g for reasons of image clarity.

In Figure 3b, the barge has been moved to a position adjacent the offshore structure, with the first engagement portion 506 engaging the topside 502a, and the second engagement portions 508a, 508b, 508c and 508d engaging the jacket 502b.

Ballast tanks 510a, 510b, 510c and 510d on the deck are then de-ballasted (by dumping the seawater contained therein) one-by-one, until the weight of the offshore structure is supported by the barge. Because the ballast tanks 510a, 510b, 510c and 510d are positioned above a waterline on the barge, they can be quickly de-ballasted by dumping seawater, rather than having to pump water out of the ballast tanks. The de-ballasting can therefore be performed quickly.

As shown in the specific example of Figure 3c, three of the four ballast tanks on the deck have been emptied to support the weight of the offshore structure. But in other examples (e.g. in the case of a smaller offshore structure), it may only be necessary to de-ballast two of the ballast tanks in order for the barge to support the weight of the offshore structure. Preferably, the ballast tanks are emptied in a pre-defined sequence, starting with the fore-most ballast tank and the aft-most ballast tank. Accordingly, the trim of the barge is maintained within acceptable limits while the de-ballasting is performed.

Next, the final ballast tank 510c on the deck is emptied, in order to lift the offshore structure away from the sea bed, as shown in Figure 3d.

Next, the hydraulic cylinders are operated to rotate the support beam from the vertical loading configuration to the horizontal transport configuration. As the offshore structure is rotated from the loading configuration to the transport configuration, the effective centre of mass of the barge changes. In order to account for this changing centre of mass, the amount of ballast water in each of the fore-most ballast tank 512a in the hull; and the aft-most ballast tank 512e in the hull is gradually changed, in order to keep the trim of the barge to within acceptable limits.

As shown in Figure 3e, by the time the offshore structure has been partially rotated from the vertical orientation to the horizontal orientation, some ballast water has been added to the foremost ballast tank 512a and some ballast water has been removed from the aft-most ballast tank 512e. This may be done by transferring ballast water directly from the aft-most ballast tank 512e to the fore-most ballast tank 512a using a pump. Alternatively, this can be done by using a pump to dump seawater from the aft-most ballast tank 512e to sea; and by opening a valve in the fore-most ballast tank 512a in order to allow seawater into the fore-most ballast tank 512a.

As shown in Figure 3f, by the time the offshore structure has been fully rotated into the horizontal orientation so that it lies along the deck of the barge, this process of transferring ballast water has continued so that the fore-most ballast tank 512a contains more ballast water than the aft-most ballast tank 512e. Accordingly, the trim of the barge is controlled so that the barge sits substantially level in the water when the offshore structure is supported along the deck of the barge. The barge is therefore ready for transporting the offshore structure to land.

Figure 4 a flowchart showing a method of decommissioning an offshore structure.

At step 300, barge 100 is attached to a control vessel 200. In particular, it is moored to the control vessel. The decommissioning operation can thereby be operated from the control vessel 200.

At step 302 the barge 100 is coupled to the offshore structure. In particular, the barge is moved to a position adjacent an offshore structure to be decommissioned using towing vessels, and the support frame 104 is moved from the horizontal transport configuration to the vertical loading configuration. The offshore structure comprises a topside mounted to a jacket. At low tide, the barge is then gradually moved towards the offshore structure, until the offshore structure is located between the prong portions 108a 108b of the barge 100, with the friction pads 122 of the first engagement portions 116a engaging the topside of the offshore structure, and the friction pads 122 of the second engagement portions 116b engaging the jacket of the offshore structure. Once the barge is in place, the clamps 124 are then operated to securely engage the offshore structure.

At step 304, the system remains idle while an increase in tidal height and/or de-ballasting gradually lifts the offshore structure away from the seabed.

At step 306, the support frame is then rotated from the vertical loading configuration to the horizontal transport configuration, thereby rotating the offshore structure to a horizontal orientation, in which it is supported above sea level (e.g. above a water line on the hull) by the barge.

At step 308, the barge is detached from the control vessel, and the towing vessels are used to transport the barge to shore. Once near shore, the barge is positioned so that the support frame is located above the landing skid. The barge is then maintained in position while the tidal height reduces to a required tidal height, i.e. until that the support rails 114 of the support frame mate with landing rails of the landing skid. The offshore structure is then transferred to shore along the landing rails of the landing skid.

Finally, at step 310 the offshore structure is dismantled by removing the topside from the jacket.

Figure 5 shows a method of configuring a support frame (e.g. support frame 104 of barge 100) for decommissioning an offshore structure. The method shown in Figure 5 may be considered as preceding step 300 in Figure 4.

At step 400, measurements of an offshore structure to be commissioned are determined. In particular, a height, total weight and weight distribution of a jacket of the offshore structure are determined. Additionally, a height, total weight and weight distribution of a topside mounted to the jacket are determined. This information may be determined from known specifications for the offshore structure.

At step 402, a required configuration for support frame 104 is calculated, based on the measurements determined at step 400. In particular, positions along the support frame 104 for first engagement portions 116a, and for second engagement portions 116b, are calculated. The calculation is made to ensure that the trim of the barge in water is within acceptable limits when the offshore structure is supported on the barge in the horizontal orientation. Accordingly, the support frame is configured such that the offshore structure is evenly balanced on the support frame in the transport configuration.

At step 404, the first engagement portions 116a and second engagement portions 116b are moved along the support frame 104 to the calculated positions.

Finally, at step 406, the positioning of the friction pads 122 and clamps 124 along the engagement portions is fine-tuned for secure engagement of the offshore structure.

It will be appreciated that the above description is made by way of example and not limitation of the scope of the appended claims, including any equivalents as included within the scope of the claims. Various modifications are possible and will be readily apparent to the skilled person in the art. Likewise, features of the described embodiments can be combined with any appropriate aspect described above and optional features of any one aspect can be combined with any other appropriate aspect.

Further embodiments are disclosed in the following numbered clauses.

Clause 1. A barge for decommissioning offshore structures, the barge comprising: a hull, the hull comprising a pair of prong portions extending from a base portion to form a fork-shape; and a support frame for supporting an offshore structure, the support frame mounted on the hull and configured to rotate relative to the hull between a loading configuration to facilitate loading of the barge with the offshore structure and a transport configuration to support the offshore structure during transport, wherein the support frame extends between the prong portions and below the barge in the loading configuration.

Clause 2. The barge of clause 1, wherein the support frame is configured to rotate about an axis positioned substantially at a midpoint along the support frame.

Clause 3. The barge of clause 2, wherein the axis is positioned adjacent a centre of buoyancy of the hull.

Clause 4. The barge of any preceding clause, wherein the support frame is substantially vertically oriented in the loading configuration, and substantially horizontally oriented in the transport configuration.

Clause 5. The barge of any preceding clause, wherein the support frame comprises engagement portions for engaging the offshore structure, the engagement portions extending laterally from the support frame.

Clause 6. The barge of clause 5, wherein the engagement portions are moveable along the support frame.

Clause 7. The barge of clause 5 or clause 6, wherein the engagement portions comprise a first engagement portion configured to engage a topside of an offshore structure, and a second engagement portion configured to engage a jacket of the offshore structure.

Clause 8. The barge according to any preceding clause, wherein the support frame comprises a pair of parallel support rails on which the offshore structure is configured to rest in the transport configuration.

Clause 9. The barge of clause 8 as dependent upon clause 7, wherein the engagement portions are secured to the rails so as to be moveable along the rails.

Clause 10. A method of configuring the barge of clause 7 for decommissioning an offshore structure, the method comprising adjusting the first engagement portion for engagement with the topside of the offshore structure, and adjusting the second engagement portion for engagement with the jacket of the offshore structure.

Clause 11. A method of decommissioning an offshore structure using the barge of any of claims 1 to 8, the method comprising supporting the offshore structure with the support frame, and rotating the support frame from the loading configuration to the transport configuration to thereby rotate the offshore structure from a substantially vertical orientation to a substantially horizontal orientation.

Clause 12. The method of clause 11, further comprising using an increase in tidal height, and/or dumping ballast water from the barge, to lift the offshore structure prior to rotating the support frame Clause 13. The method of clause 11 or clause 12, further comprising transporting the offshore structure to shore and removing the topside from the jacket on shore.

Clause 14. The method of any of clause 11 to 13, further comprising transferring the offshore structure from the barge to shore along a first pair of parallel landing rails located on shore; detaching the topside from the jacket; transporting the jacket to a jacket processing area along the first pair of parallel lending rails; and transporting the topside to a topside processing area along a second pair of parallel landing rails that intersect with the first pair of parallel landing rails.