Installing mid-water buoys

Abstract

A tethered mid-water buoy is installed in a single-lift operation that lands the buoy and a foundation together on the seabed. The buoy is then released to ascend from the foundation. Elevation of the buoy above the foundation is controlled by applying a controlling force to the buoy through a control line that extends from a surface vessel. In one embodiment, the controlling force is applied to the buoy from above via a ballasting frame that is engaged with the buoy and is suspended from a lifting wire serving as the control line. The frame can be releasably attached to the buoy and / or to the foundation and can encage the buoy. In another embodiment, the controlling force is instead applied to the buoy from beneath via a restraint line that extends between the buoy and the foundation and that forms at least part of the control line.

Description

[0001] Installing mid-water buoys

[0002] This invention relates to the installation of mid-water buoys, as used to provide support to elongate subsea elements at an intermediate depth in the water column between the surface and the seabed.

[0003] Mid-water buoys act on an elongate subsea element such as a dynamic cable used in offshore power generation, or a riser or an umbilical used in offshore hydrocarbon production. By lifting an intermediate portion of the element to form an upwardly-convex hogbend, the buoy imparts an S-configuration to the element. Specifically, the element adopts a configuration known in the art as a lazy-S or a steep-S, the latter being characterised by anchoring the element to the seabed around the touch-down point at the interface between the element and the seabed.

[0004] Beneficially, a buoy-supported S-configuration isolates or decouples the portion of the element around the touch-down point from stresses induced by surface dynamics. Also, in comparison to a free-hanging catenary, top tension is helpfully reduced because the buoy supports part of the weight of the element.

[0005] The invention is concerned with mid-water buoys that are tethered to a foundation on the seabed, with buoyant upthrust of the buoy being balanced by tension in the tethers extending between the buoy and the foundation. An example of such a buoy is a midwater arch or MWA, arranged such that the hogbend of the supported element lies upon and curves across the top of the buoy. MWAs are disclosed in US 6206742, WO 2014 / 062064 and US 9702109.

[0006] Conventionally, installation of mid-water buoys for dynamic risers used in offshore hydrocarbon production has required multiple lifts to install the foundation, the tether system and the buoyancy element separately. This increases costs and operational risks. In US 8893802, for example, a base is lowered onto and anchored to the seabed before anchoring lines are attached to the base and supported by temporary floats. A buoy is then ballasted to sink to a depth required for engagement with the anchoring lines before the anchoring lines are appropriately tensioned. WO 2014 / 081317 proposes the further preliminary complication of assembling an anchor for an MWA from multiple components in separate lifts so that the weight load of each lift is minimised.“Guara-Lula NE Field Development: Mid-Water Arch Installation and Connection of Production System” by Siddiqui, Al, National University of Singapore (https: / / www.academia.edu / 13128206 / Guara_Lula_NE_Field_Development_Mid_Water _Arch_lnstallation_and_Connection_of_Production_System) discusses the steps involved in installing the MWA employed in the Guara-Lula field off Brazil. The MWA was towed to the installation site and then lowered to a depth of 250m ballasted by chains and clump weights, while being guided by construction vessels and ROVs using shackles and winches to control the descent. Once the MWA was in position, tethers were connected to suction pile foundations on the seabed. ROVs then disconnected the ballast, allowing the MWA to achieve positive buoyancy.

[0007] The alternative of a single lift has been proposed but such proposals are overly complex. For example, Ribeiro, EJB, Labanca, EL, Bartz, C, & Iwane, A; "Tubarao Martelo Field Development: Lazy S Riser Configuration Using Mid Water Arch (MWA)" Proceedings of the ASME 201534th International Conference on Offshore Mechanics and Arctic Engineering (https: / / doi.org / 10-1115 / 0 MAE2015-41873) notes that sinking a buoyancy tank of an MWA is conventionally achieved by adding ballast water or by applying an external load using a temporary sinking weight. However, the authors favour an alternative approach, namely overboarding a gravity base that is connected to the tank by permanent tethers and then deploying the tank with the gravity base hanging from it via the tethers to apply a pull-down load to the tank.

[0008] The authors’ preferred approach requires multiple complex and critical steps, however. Initially, the gravity base and tethers are prepared and interconnected on the deck of an installation vessel. After the vessel transits to an installation site, the gravity base is lifted off the deck, swung outboard and lowered through the splash zone until the tether chains are tensioned. A buoyancy tank is then slid to a launching position on the vessel before the entire system is lifted and swung outboard for the tank to be lowered through the splash zone as the system is deployed to the seabed with the aid of heave compensation.

[0009] Similarly, Fujarra, ALC, Tannuri, EA, Masetti, IQ, & Igreja, H. "Experimental and Numerical Evaluation of the Installation of Sub-Sea Equipments for Risers Support." Proceedings of the ASME 200827th International Conference on OffshoreMechanics and Arctic Engineering (https: / / doi.org / 10-1115 / OMAE2008-57472) proposes deployment of a gravity anchor that is tethered to a buoyancy tank assembly initially floating at the surface. The buoyancy tank assembly is then pulled down by the tethers as the anchor is lowered. Again, these operations involve significant complexity and risk. For example, the tethers must be tensioned correctly to pull the buoyancy tank assembly into the water without risking damage due to dynamic forces.

[0010] GB 2257405 discloses a tether deployment system for a buoyant mid-water support in which tethers are wound onto drums that are rotatably mounted on a gravity base installed on the seabed. The drums are locked against rotation until the tethers are to be deployed, whereupon the tethers unwind as the unlocked drums rotate. The tethers are pulled off the drums by the buoyancy of the mid-water support, which thereby rises above the base to reach a desired elevation in the water column.

[0011] In one embodiment of GB 2257405, the drums are free to rotate once unlocked.

[0012] Problematically, therefore, the tethers would unspool uncontrollably until the mid-water support stops suddenly at its maximum elevation. In another embodiment, hydraulic motors act on the drums to reel the tethers in or out, hence lowering or raising the midwater support in a more controlled manner if required. However, this places considerable demands on the drums and the motors, which must operate reliably in deep water to resist the full upthrust of the mid-water support while controlling deployment of, and tension in, the tethers.

[0013] As a result, the tether deployment system of GB 2257405 is needlessly complex, bulky, heavy and expensive for the purpose of controlled deployment of the mid-water support. Indeed, the bulk and weight of the system could result in a load that is impractical to lift into the water using readily-available installation vessels.

[0014] Additionally, GB 2257405 does not teach how the tether deployment system could be lifted and installed as a unit with the mid-water support. The mid-water support is simply shown floating above the tether deployment system, held only by tension in a flexible tether. It is not apparent how the mid-water support could be lifted into the water and lowered through the water column without potentially clashing with the tether deployment system, if installed in the same operation.Against this background, the invention resides, in one sense, in a method of installing a tethered mid-water buoy. The method comprises: landing the buoy and a foundation together on the seabed in a single-lift operation; releasing the buoy from the foundation to ascend from the foundation; and controlling elevation of the buoy above the foundation by applying a controlling force to the buoy through a control line that extends from a surface vessel.

[0015] Conveniently, the buoy and the foundation can be landed while suspended from the control line. The buoy and the foundation can be landed in fixed mutual relation and then the buoy can be freed for movement away from the foundation.

[0016] The buoy and the foundation can be landed with at least one tether pre-attached between the buoy and the foundation. The or each tether can then be deployed as the buoy ascends from the foundation.

[0017] The controlling force can be applied to the buoy on a load path that enters the buoy from above or from below. In the former case, the controlling force can be applied to the buoy via a frame that is engaged with the buoy and is suspended from the control line, and that can act on the buoy from above. The frame preferably has a weight in water that exceeds buoyant upthrust of the buoy when submerged.

[0018] The buoy, the foundation and the frame can be lifted and landed on the seabed together. The foundation can be suspended from the frame before landing on the seabed, for example directly on a load path that bypasses the buoy and / or indirectly on a load path that passes through the buoy. The frame can then be released from the foundation after landing on the seabed.

[0019] Similarly, the buoy can be held attached to the frame before landing on the seabed, may be sandwiched between the frame and the foundation and / or can be encaged by and within the frame. An upwardly-convex formation of the buoy can be received in a downward ly-concave formation of the frame.

[0020] Where the load path of the controlling force enters the buoy from beneath, that force can be applied to the buoy by a restraint line that extends between the buoy and the foundation and that forms at least part of the control line. An end of the restraint line, joined to the buoy, can be pulled downwardly by pulling an opposed free end portion ofthe restraint line upwardly. Thus, the path of the restraint line can reverse at the foundation.

[0021] The restraint line can be coupled to a further line that is suspended from the surface vessel and that forms part of the control line. That further line could be a lifting line that was used previously to land the buoy and the foundation together on the seabed.

[0022] Alternatively, the restraint line can extend to the surface vessel and so form substantially all of the length of the control line.

[0023] Correspondingly, the inventive concept embraces a mid-water buoy package that can be landed on the seabed by a single-lift operation. The package comprises: a subsea foundation; a mid-water buoy disposed upon the foundation and deployable subsea from the foundation; at least one subsea-deployable tether connected to the buoy and the foundation; and a control line acting on the buoy separately from the or each tether for applying a controlling force to the buoy, thereby to control subsea deployment of the buoy from the foundation. The buoy and the foundation can be releasably coupled together in fixed mutual relation before deployment of the buoy.

[0024] The control line may be a lifting line that is arranged to lift the foundation and the buoy together. A restraint line that extends between the buoy and the foundation can form at least part of the control line.

[0025] The package may further comprise a frame that is engaged with the buoy and that is suspended from the control line. The frame can act on the buoy from above and can have a weight in water that exceeds buoyant upthrust of the buoy when submerged.

[0026] The foundation and / or the buoy may be releasably attached to the frame. The buoy can be sandwiched between the frame and the foundation and / or

[0027] encaged within the frame. An upwardly-convex formation of the buoy can be received in a downwardly-concave formation of the frame.

[0028] Embodiments of the invention involve subsea release of an MWA, for example for dynamic cables used in floating wind applications. These embodiments involve compact single-lift deployment of an MWA system comprising a foundation, a tether system and a buoyancy element, followed by controlled release of the buoyancy element subsea to ascend to an appropriate position in the water column above the foundation.Embodiments of the invention therefore implement a method for installing an MWAto support a dynamic riser or cable, the method comprising: lifting an MWA system, including a buoyancy element, one or more tethers, and a foundation or anchor, in a single operation; deploying the components of the MWA system together onto the seabed; and subsequently releasing the buoyancy element in a controlled manner subsea.

[0029] One embodiment of the invention provides a methodology to install an MWA or buoy in one operation with a lifting frame serving as ballast and encaging the buoy during deployment.

[0030] To recap, an MWA comprises a buoy serving as a buoyant support for a hogbend of a supported riser or power cable. The buoy is connected to the seabed via one or more tethers joined to a subsea foundation. The foundation can lie on the seabed, typically being a dead-man anchor or a peasant anchor. However, a foundation embedded in the seabed, such as a suction anchor, is also possible.

[0031] A frame encages and holds the buoy in a controlled manner from above while the buoy is lifted through the water column away from the foundation after the foundation has been landed on the seabed. The frame serves as a temporary weight to confer negative or neutral buoyancy on the buoy until the tethers hold the buoy and are tensioned.

[0032] After preparing the buoy on a deck or quay, a crane lifts the buoy onto a foundation before the tethers are connected between the buoy and the foundation. The encaging frame is lifted on top of the buoy and connected to the foundation, before or after connecting the tethers. The frame thereby allows the foundation and the buoy to be lifted together in one lifting operation. In this way, the foundation, the buoy and the frame can be lifted together as a unit from a quay onto an installation vessel. Instead, however, the buoy could be lifted onto the foundation and the frame could be lifted onto the buoy aboard the vessel, hence assembling the unit on the deck of the vessel.

[0033] After the vessel travels to an offshore installation site, a crane of the vessel or aboard another vessel lifts the foundation, the buoy and the frame together from the deck as a unit. That unit is then lowered through the splash zone and deployed to the seabed in a single lift.At the seabed, the frame is released from the foundation but remains engaged with the buoy. The tethers are then freed for extension or deployment so that the buoy can be deployed upwardly from the foundation, its ascent being controlled by the weight of the frame lifted by a crane or winch of the vessel.

[0034] The weight of the frame acts against the buoyant upthrust of the buoy. Preferably, the weight of the frame in water is slightly greater than the buoyant upthrust, hence ensuring that the crane or winch experiences a small weight load to maintain tension in the lifting line as the buoy ascends. For stability, the assembly of the frame and the buoy should maintain the centre of gravity beneath the centre of buoyancy as the assembly is lifted in the water column. When the tethers are fully extended and tensioned, the frame is lifted slightly above the buoy before rigging that secures the frame to the buoy is released.

[0035] The frame can be equipped with ROV-operable levers or remotely-operable actuators that release only specific identified rigging or connections, like those used to deploy concrete mattresses subsea. In this way, the frame and the buoy can be released from the foundation in a first release operation and the frame can be released from the buoy in a second, subsequent release operation.

[0036] Thus, an embodiment of the invention proposes a deployment frame that serves multiple functions. The frame provides a lifting means for both the buoy and the foundation and then acts as a counterweight to the buoy, holding the buoy in a controlled manner after the foundation has been landed and the buoy is then lifting away from the foundation through the water column. The frame defines a cage that keeps the buoy under control against lateral movements during its ascent, for example as may be driven by currents impinging on the buoy. The frame also enables multiple rigging disconnections to be effected in the same release operation, a first release operation being to release the buoy and the frame from the foundation and a second release operation being to release the frame from the buoy as noted above.

[0037] In summary, a tethered mid-water buoy is installed in a single-lift operation that lands the buoy and a foundation together on the seabed. The buoy is then released to ascend from the foundation. Elevation of the buoy above the foundation is controlled byapplying a controlling force to the buoy through a control line that extends from a surface vessel.

[0038] In some embodiments, the controlling force is applied to the buoy from above via a ballasting frame that is engaged with the buoy and is suspended from a lifting wire serving as the control line. The frame can be releasably attached to the buoy and / or to the foundation and can encage the buoy. In other embodiments, the controlling force is instead applied to the buoy from beneath via a restraint line that extends between the buoy and the foundation and that forms at least part of the control line.

[0039] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0040] Figure 1 is a schematic side view of an MWA in conjunction with a foundation, a lifting frame and other components used in the invention;

[0041] Figures 2a to 2i are a sequence of schematic side views that show steps involved in installation of the MWA using the lifting frame of Figure 1 for supporting a subsea cable on the MWA;

[0042] Figure 3 is a perspective view of an MWA of the invention;

[0043] Figure 4 is a top view of the MWA of Figure 3;

[0044] Figure 5 is a side view of the MWA of Figures 3 and 4;

[0045] Figure 6 is an end view of the MWA of Figures 3 to 5, with buoyancy removed;

[0046] Figure 7 is a schematic side view of an MWA in conjunction with a foundation in a further embodiment of the invention; and

[0047] Figure 8 is a sequence of schematic side views that show steps involved in installation of the MWA of Figure 7.

[0048] Figure 1 shows components of an MWA system of the invention, namely: an MWA 10; a foundation 12 exemplified here by a dead-man anchor; permanent rigging in the form oftethers 14 extending between the MWA 10 and the foundation 12; a lifting frame 16; and lift rigging 18.

[0049] The MWA 10 shown schematically in the drawings comprises a buoy that has an arched, upwardly-convex upper surface for supporting a hogbend of a subsea cable or other elongate element. The MWA 10 further comprises upper tether connections 20 for the respective tethers 14 whereas the foundation 12 comprises corresponding lower tether connections 22 for those tethers 14.

[0050] The lifting frame 16 comprises downwardly-extending legs 24 that terminate in lower fixings 26 to engage the foundation 12. The legs 24 may be rigid or flexible. The weight in water of the lifting frame 16, including its legs 24 and lower fixings 26, is greater than the buoyant upthrust of the MWA 10 when submerged. Thus, the lifting frame 16 can serve as ballast or as a counterweight to the MWA 10.

[0051] The lift rigging 18 comprises connectors 28 by which the lift rigging 18 can be releasably coupled to the MWA 10.

[0052] Figures 2a to 2i show steps involved in installation of the MWA 10 to support a hogbend of an elongate subsea element such as a cable.

[0053] In Figure 2a, the lift rigging 18 has been attached to the MWA 10, conveniently by using the upper tether connections 20 as in this example, and to a lifting wire 30 that extends upwardly from the lift rigging 18 to a crane, not shown. The MWA 10 can thereby be lifted onto the foundation 12, for example on a quay 32, before the lift rigging 18 is detached from the MWA 10 as shown in Figure 2b. Figure 2b also shows the tethers 14 being attached to the upper tether connections 20 of the MWA 10 and to the lower tether connections 22 of the foundation 12. The tethers 14 are slack at this stage.

[0054] The MWA 10 and the foundation 12 may have complementary interface surfaces, exemplified here by flat lower and upper surfaces respectively, whereby the MWA 10 and the foundation 12 can abut in mutual contact.

[0055] Figure 2c shows the lifting wire 30 having lowered the lifting frame 16 onto the top of the MWA 10 to form a unit, system or package that further comprises the foundation 12 and the tethers 14. It will be noted that the lifting frame 16 has a downwardly-concavelower surface that complements the convex curvature of the upper surface of the MWA 10. These complementary contours interlock to fix the MWA 10 against lateral movement relative to the lifting frame 16.

[0056] The legs 24 of the lifting frame 16 embrace and encage the MWA 10 and the foundation 12 and are coupled to the foundation 12 via the lower fixings 26, which can engage the underside of the foundation 12 as shown. The MWA 10 is then sandwiched between the foundation 12 and the lifting frame 16 although the load path of the weight of the foundation 12 can bypass the MWA 10. The legs 24 may be under tension to hold the foundation 12 against the MWA 10. If the legs 24 are flexible, they can behave substantially rigidly by virtue of that tension. The foundation 12 can additionally, or instead, be releasably fixed to the MWA 10 directly.

[0057] Separately, the lift rigging 18 is attached at an upper end to the lifting frame 16 and is coupled at a lower end to the MWA 10. The lift rigging 18 hangs down from the lifting frame 16 to embrace and encage the MWA 10. Thus, the lift rigging 18 cooperates with the legs 24 of the lifting frame 16 to fix the MWA 10 against lateral movement relative to the lifting frame 16.

[0058] With reference to Figure 2d, the lifting wire 30 and the lifting frame 16 can then be used in a single-lift load-out operation to lift the MWA 10 and the foundation 12 together from the quay 32 onto a deck of an installation vessel 34. Links 36 then lash the MWA 10 and the foundation 12 to the deck of the vessel 34 to sea fasten the load for transit to an installation site. The lifting frame 16 can be removed temporarily from the MWA 10 and the foundation 12 during transit, as shown in Figure 2d, or can instead remain in place during transit. In the latter case, the legs 24 of the lifting frame 16 can rest on the deck of the vessel 34 to support the top structure of the lifting frame 16 above the MWA 10.

[0059] Figure 2e shows the lifting wire 30 and the lifting frame 16 being used in a single-lift installation operation to lift the MWA 10 and the foundation 12 together from the deck of the installation vessel 34 when at the installation site. The MWA 10, the foundation 12 and the lifting frame 16 are then lowered through the splash zone beneath the surface 38, as shown in Figure 2f, before being landed at a target location on the seabed.

[0060] Beneficially, the buoyant upthrust acting on the MWA 10 greatly reduces the weight load borne by the lifting wire 30 when the assembly of the MWA 10, the foundation 12and the lifting frame 16 is submerged. For example, the weight in air of that assembly may be approximately eighty tons but its weight in water may be approximately thirty tons.

[0061] Figure 2g shows the foundation 12 resting on or embedded into the seabed 40 at the target location. The lower fixings 26 of the legs 24 have been released from the foundation 12 to allow the MWA 10 and the lifting frame 16 to ascend together, hence pulling on and extending the tethers 14. Elevation of the MWA 10 and the lifting frame 16 above the foundation 12 continues until the tethers 14 reach their full extension and tauten to limit further elevation.

[0062] As the MWA 10 lifts away from the foundation 12, the aforementioned interengagement of the MWA 10 with the lifting frame 16 holds the MWA 10 against lateral movement due to currents. The negative buoyancy of the lifting frame 16 also acts against, and slightly exceeds, the positive buoyancy of the MWA 10. As a result, the lifting wire 30 is kept in tension by experiencing the modest apparent weight of the negatively-buoyant combination of the MWA 10 and the lifting frame 16. Consequently, a crane or winch of the vessel 34 acting on the lifting wire 30 can easily control the speed and elevation of the MWA 10 and the lifting frame 16 to prevent shock loads or excessively rapid deployment of the tethers 14. It is even possible to lower the MWA 10 and the lifting frame 16 back toward the foundation 12 to adjust the degree of elevation in either direction if required.

[0063] Thus, the lifting wire 30 extending from the vessel 34 can be regarded as a control line that controls elevation of the MWA 10 above the foundation 12 by applying a controlling force to the MWA 10 via the lifting frame 16. In this instance, the controlling force is applied on a load path that enters the MWA 10 from above.

[0064] Finally, as shown in Figure 2h, the lifting frame 16 is detached from the MWA 10 by disconnecting the connectors 28 to free the lift rigging 18 from the MWA 10. The lifting frame 16 and the lift rigging 18 are then recovered to the surface. Tension in the tethers 14 can be adjusted and balanced at this stage or afterwards.

[0065] Once the MWA 10 has been installed as shown in Figure 2h, an elongate subsea element such as a cable 42 can be installed across the convex top of the MWA 10 asshown in Figure 2i, thereby adopting an S-configuration comprising a hogbend supported by the MWA 10.

[0066] Figures 3 to 6 show a practical embodiment of an MWA 10 that can be used in the invention and can be surmounted by a lifting frame of the invention. The MWA 10 comprises a steel frame 44 that supports, and is buoyantly supported when underwater by, various buoyancy elements 46 such as blocks of syntactic foam. The frame 44 is shown without the buoyancy elements 46 in Figure 6.

[0067] The frame 44 of the MWA 10 has upper tether connections 20 for respective tethers 14 and is surmounted by upwardly-convex channels 48 that cross the MWA 10. The channels 48 can receive and support the hogbend of S-configuration elements such as cables 42 when the MWA 10 has been installed in the water column as shown in Figure 2i.

[0068] In this example, the frame 44 has downwardly-extending spigots 50 that effect a releasable direct connection with a foundation 12, not shown. Thus, a lifting frame surmounting the MWA 10 need not be coupled only directly to the foundation 12 but could instead, or additionally, support the weight of the foundation 12 via the MWA 10.

[0069] For this purpose, as shown in the enlarged detail of Figure 5, tubular sockets 52 extending upwardly from the foundation 12 cooperate telescopically with the spigots 50 that depend from the frame 44. The spigots 50 are received by the sockets 52 and are then locked in the sockets 52 by transverse insertion of locking pins 54. An ROV can remove the locking pins 54 to free the MWA 10 from the foundation 12 after the MWA 10 and the foundation 12 have been landed together on the seabed 40.

[0070] Finally, Figures 7 and 8 show another embodiment of the invention in which, again, like numerals are used for like parts. Here, connectors 56 releasably fix an MWA 10 to a foundation 12 such as a dead-man anchor. For example, the connectors 56 could work in the manner of the pin-secured, spigot-socket connection shown in Figure 5.

[0071] When the MWA 10 is attached to the foundation 12 in fixed relation as shown in Figure 7, the MWA 10 and the foundation 12 are lifted together in combination onto an installation vessel during load-out and thereafter from the vessel during installation offshore. For this purpose, Figures 7 and 8 show a lifting wire 30 coupled to thefoundation 12, which in turn bears the weight of the MWA 10. However, it would instead, or additionally, be possible to couple the lifting wire 30 to the MWA 10 to suspend at least some of the weight of the foundation 12 via the MWA 10.

[0072] Figure 7 shows the tethers 14 initially hanging slackly between upper tether connections 20 on the MWA 10 and lower tether connections 22 on the foundation 12. Figure 7 also shows a restraint line 58 attached to the underside of the MWA 10 and terminating at its free end in a subsea connector 60. The restraint line 58 extends downwardly from the MWA 10 and then reverses through a shackle 62 positioned on the upper side of the foundation 12. The restraint line 58 could instead reverse around a pulley. A free end portion of the restraint line 58 is initially stored compactly on the foundation 12 in a rolled or folded configuration.

[0073] Figure 8 shows the MWA 10 and the foundation 12 initially locked together in the configuration shown in Figure 7, being lifted together during installation offshore as shown at Step A and then being lowered beneath the surface 38 at Step B before being landed together on the seabed 40 at Step C. The lifting wire 30 is then detached from the MWA 10 and the foundation 12 at Step D before being coupled to the restraint line 58 via the connector 60 at Step E.

[0074] The lifting wire 30 is then pulled upwardly by a crane or winch 64 of the installation vessel 34 to extend the restraint line 58 fully and so to apply tension to the restraint line 58 at Step F. By virtue of the restraint line 58 reversing direction through the shackle 62, upward tension applied to the restraint line 58 applies downward force to the MWA 10. The connectors 56 can then be released, as also shown at Step F, provided that the downward force exerted by the restraint line 58 is sufficient to restrain the MWA 10 against upward movement driven by buoyant upthrust.

[0075] With the MWA 10 thereby restrained against its buoyant upthrust, the portion of the restraint line 58 between the shackle 62 and the MWA 10 is extended by paying out the lifting wire 30 that remains connected to the free end of the restraint line 58. The MWA 10 thereby lifts away from foundation as shown at Step G, hence extending the tethers 14 in the process.

[0076] Controlled ascent of the MWA 10 in the water column continues as the lifting wire 30 is paid out further until the tethers 14 reach their maximum extension and become taut asshown at Step H. The restraint line 58 then becomes slack and can be disconnected from the lifting wire 30 as shown at Step I. As in preceding embodiments, ascent and elevation of the MWA 10 is controlled easily and conveniently by raising or lowering the lifting wire 30 extending to the surface vessel 34.

[0077] Again, therefore, the lifting wire 30 extending from the vessel 34 can be regarded as part of a control line that controls elevation of the MWA 10 above the foundation 12 by applying a controlling force to the MWA 10, in this case via the restraint line 58. Thus, the controlling force is applied on a load path that enters the MWA 10 from beneath. All embodiments of the invention share the inventive concept of installing a buoy, such as an MWA, tethered to a foundation, such as a dead-man anchor, in a single lift and then using a line extending to the surface to control ascent of the buoy from the foundation.

[0078] Many other variations are possible within the inventive concept. For example, the restraint line could extend to the surface to serve as the entire control line rather than being coupled to another line, such as the lifting wire, to serve as only part of the control line.

[0079] The telescopic arrangement shown in the enlarged detail of Figure 5 could be reversed, with upwardly-extending spigots of the foundation instead being received within tubular parts of the frame serving as sockets for those spigots.

Claims

Claims1. A method of installing a tethered mid-water buoy, the method comprising:in a single-lift operation, landing the buoy and a foundation together on a seabed;releasing the buoy from the foundation to ascend from the foundation; andcontrolling elevation of the buoy above the foundation by applying a controlling force to the buoy through a control line extending from a surface vessel.

2. The method of Claim 1 , comprising landing the buoy and the foundation suspended from the control line.

3. The method of Claim 1 or Claim 2, comprising landing the buoy and the foundation in fixed mutual relation and then freeing the buoy for movement away from the foundation.

4. The method of any preceding claim, comprising landing the buoy and the foundation with at least one tether pre-attached between the buoy and the foundation, and deploying the at least one tether as the buoy ascends from the foundation.

5. The method of any preceding claim, comprising applying the controlling force to the buoy on a load path that enters the buoy from above.

6. The method of Claim 5, comprising applying the controlling force to the buoy via a frame that is engaged with the buoy and is suspended from the control line.

7. The method of Claim 6, wherein the frame acts on the buoy from above.

8. The method of Claim 6 or Claim 7, wherein the frame has a weight in water that exceeds buoyant upthrust of the buoy when submerged.

9. The method of any of Claims 6 to 8, comprising landing the buoy, the foundation and the frame on the seabed together.

10. The method of Claim 9, preceded by lifting the buoy, the foundation and the frame together.

11. The method of Claim 9 or Claim 10, comprising suspending the foundation from the frame before landing on the seabed.

12. The method of Claim 11 , comprising suspending the foundation directly from the frame on a load path that bypasses the buoy.

13. The method of Claim 11 or Claim 12, comprising releasing the frame from the foundation after landing on the seabed.

14. The method of any of Claims 6 to 13, comprising holding the buoy attached to the frame before landing on the seabed.

15. The method of any of Claims 6 to 14, comprising sandwiching the buoy between the frame and the foundation.

16. The method of any of Claims 6 to 15, comprising encaging the buoy within the frame.

17. The method of any of Claims 6 to 16, comprising receiving an upwardly-convex formation of the buoy in a downwardly-concave formation of the frame.

18. The method of any of Claims 1 to 4, comprising applying the controlling force to the buoy on a load path that enters the buoy from beneath.

19. The method of Claim 18, comprising applying the controlling force to the buoy by a restraint line that extends between the buoy and the foundation and that forms at least part of the control line.

20. The method of Claim 19, comprising pulling an end of the restraint line, joined to the buoy, downwardly by pulling an opposed free end portion of the restraint line upwardly.

21. The method of Claim 20, comprising reversing a path of the restraint line at the foundation.

22. The method of any of Claims 19 to 21 , comprising coupling the restraint line to a further line that is suspended from the surface vessel and that forms part of the control line.

23. The method of Claim 22, wherein the further line is a lifting line used previously to land the buoy and the foundation together on the seabed.

24. The method of any of Claims 19 to 21 , wherein the restraint line extends to the surface vessel and forms substantially all of the control line.

25. A mid-water buoy package that can be landed on a seabed by a single-lift operation, the package comprising:a subsea foundation;a mid-water buoy disposed upon the foundation and deployable subsea from the foundation;at least one subsea-deployable tether connected to the buoy and the foundation; anda control line acting on the buoy separately from the at least one tether for applying a controlling force to the buoy to control subsea deployment of the buoy from the foundation.

26. The package of Claim 25, wherein the control line is a lifting line that is arranged to lift the foundation and the buoy together.

27. The package of Claim 25 or Claim 26, wherein the buoy and the foundation are releasably coupled in fixed mutual relation.

28. The package of any of Claims 25 to 27, further comprising a frame that is engaged with the buoy and is suspended from the control line.

29. The package of Claim 28, wherein the frame acts on the buoy from above.

30. The package of Claim 28 or Claim 29, wherein the frame has a weight in water that exceeds buoyant upthrust of the buoy when submerged.

31. The package of any of Claims 28 to 30, wherein the foundation is releasably attached to the frame.

32. The package of Claim 31 , wherein the foundation is attached directly to the frame on a load path that bypasses the buoy.

33. The package of any of Claims 28 to 32, wherein the buoy is releasably attached to the frame.

34. The package of any of Claims 28 to 33, wherein the buoy is sandwiched between the frame and the foundation.

35. The package of any of Claims 28 to 34, wherein the buoy is encaged within the frame.

36. The package of any of Claims 28 to 35, wherein an upwardly-convex formation of the buoy is received in a downward ly-concave formation of the frame.

37. The package of any of Claims 25 to 27, further comprising a restraint line that extends between the buoy and the foundation and that forms at least part of the control line.

38. The package of Claim 37, wherein the restraint line follows a path that reverses at the foundation.

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