Field of the invention

The present invention relates to flexible risers, anchoring and arrangement thereof. More specifically, the invention relates to midwater arch systems for flexible risers.

Background of the, invention and prior art

Flexible risers are used for oil and gas production and exploration, and they are in general used between floating or stationary vessels of various types and the seabed. Flexible risers are arranged through the water column so as to provide rigidity and flexibility at the same time. The most classical configuration is the so called lazy s, named from the shape of the conftguration as seen from the side. However, numerous arrangements exist, and in general the arrangements comprises at least one portion for which the riser is provided with buoyancy, the parts on either side hanging as inclined catenary Sines between the portion with buoyancy and for example a subsea production system and a FPSG (Floating Production, Storage and Offloading) vessel.

The buoyancy is in general of two types: so called distributed buoyancy, consisting of a number of discrete buoyancy elements, typically based on foam, arranged along a section of the riser; or gas filled tanks as buoyancy elements, typically used for mid water archs (MWA).

A mid water arch -MWA- provides support and buoyancy for the risers. Usually, several risers are arranged together on one MWA. A MWA comprises a buoy, a bottom anchor and an anchoring line or tether, the Sine connecting the buoy to the anchor and holding the buoy at an intended mid water position. A mid water arch (MWA) buoy is the subject matter of the present invention.

A typical MWA buoy comprises a gutter, a loadbearing structure below the gutter, buoyancy eSements, lifting lugs for attachment to a crane, a bridle and anchoring lugs for attachment to an anchor wire or tether. The gutter is a curved plate with grooves or tracks into which risers shall be arranged. The gutter has a curvature feasible in view of the minimum allowable bending radius of the risers and the movements of the risers. The loadbearing structure is arranged below the gutter, interconnected to and supporting the gutter. Two buoyancy tanks are typically arranged in the loadbearing structure. Lifting lugs for hanging up the unit in a crane is typically arranged on top or close to the top of the unit. Anchoring fugs for hanging up the anchor, are typicafly arranged to the lower side of the structure, to which a rigid bridle structure or bridle chains are connected. The structure is dense with limited access, and arranging the lifting lugs high and the anchoring lugs low provides orientation stability.

A typical 1V1WA buoy weights about 70 metric tons whilst the bottom anchor weights about 250 metric tons. MWA's are used mainly for moderate depths, typically 70 ~ 300 m depths. The buoy with anchor, having height about 30-50 m, is lifted and installed in one operation before the risers are arranged onto the gutter.

The buoyancy tanks of a MWA are designed for the operation depth plus a safety factor. However, for redundancy the tanks contain one or more bulkheads, rated to one bar differential pressure, in order to maintain sufficient buoyancy if one tank compartment starts leaking when the IVIWA is installed in submerged position. The tanks are typicaliy filled with gas at a pressure equal to the submerged position seawater pressure plus one atmosphere, The design, fabrication and installation of MWA's are often postponed in a field development project, since decisions with respect to number of and capacity of risers, often are subject to revisions until Sate in the development project.

Selection of main parameters for the MWA's are traditionally dependent on the riser particulars. Thus the IVIWA particulars are decided subsequent of riser design freeze. This places the MWA completion and installation on the critical path of the field development schedule, since the MWA design and fabrication cannot commence prior to completion of the riser design. Currently, a typica! MWA must be installed using a 400 ion lifting vessel, which is expensive. Possibility to use a lower cost lifting vessel would be beneficial.

Currently, design and fabrication of a typical MWA takes about 9 months, A shorter period of time for design and fabrication would be beneficial, allowing later revisions on riser layout.

Currently, a typica! MWA will fail if leakage in one buoyancy tank compartment takes place during installation, even though the MWA is designed to withstand leakage in one buoyancy tank compartment without loosing sufficient buoyancy capacity during normal operation as submerged. This is because the differentia! pressure rating of one atmosphere of the bulkhead easily is exceeded if leakage takes place during installation. More. specifically, the compartments between bulkheads are typically filled with nitrogen at one bar overpressure relative to the intended position as submerged, leakages at elevation far off can not be handled since the bulkhead differentia! pressure rating is one bar. Experience reveal that in substance all damage to the MWA occur during installation, ability to withstand damage better would be beneficial, Currently, some fVlWA's will break down if one of the lifting bridles or slings break, because of instability or overload on the remaining slings.

A typical requirement is that a MWA shall be possible to install in one lifting operation. A typica! requirement is also that the lifting lugs and structure shal! be tested, however this is difficult in practice since the whole buoy must be tested since the lifting forces are taken up through the loadbearing structure .

The objective of the present invention is to provide mid water arch that is favorable with respect to one or more of the issues mentioned above.

Summary of the invention

The objective is met by the invention providing a mid water arch {MWA} buoy for flexible riser arrangements, for lifting and installation, with anchors

connected, in one operation, the mid water arch buoy comprising: a gutter,

a load bearing structure,

buoyancy elements,

lifting lugs for attachment to a crane, and

anchoring lugs for attachment to an anchor wire,

distinguished in that the Sifting lugs and anchoring lugs are common or are arranged above and under horizontally extending elements of the loadbearing structure, so that the loadbearing structure is in substance independent of the lifting forces except of compression components thereof, wherein the buoyancy elements comprise three or more gas filled pressure vessels, said pressure vessels are without bulkheads.

With the terms lifting lugs and anchoring lugs, it is meant any lifting lug, pad eye, hook, hole, fixation point or similar to which the chain, chain hook, twist eye. chain sling or slings, to the crane or anchor, respectively, can be fastened. Common anchoring and lifting lugs, or separation between them by only or in substance only horizontally oriented elements of the loadbearing structure, has a substantial technical effect. More specifically, this means that the loadbearing structure is independent or in substance independent of the lifting forces, so the loadbearing structure oriramework can be dimensioned not for the total weight of anchor plus MWA buoy plus a safety factor, but a significantly reduced load. More .specifically, during Sifting, the Soadbearing structure of the MWA of the invention function merely or substantially as a spreader and can be

dimensioned accordingly. The term arranged above and under horizontally extending elements, means that vertically orientated elements of the

loadbearing structure are not subject to the full lifting forces. An example of a horizontally extending element is a horizontal beam, spreader or yoke in the loadbearing structure. However, the MWA of the invention can comprise relative short rigid vertical elements, with lifting lugs on top and anchoring lugs on bottom, spread around or in between the loadbearing structure, but the full forces of lifting the buoy and anchor must not be carried by the Soadbearing structure, oniy horizontal compression components thereof. In other words, the lifting lugs and anchoring lugs are not separated by elements of the loadbearing structure carrying vertical load components when the buoy with anchor is lifted in a crane, if the lifting and anchoring iugs are separated, they are separated by elements independent of the loadbearing structure or elements of the

loadbearing structure carrying horizontal lifting force components. The number of lifting lugs equa! the number of anchoring lugs, and lifting !ugs and anchoring lugs are in substance vertically aligned.

The weight, size and lead time wif! be substantially reduced by the WWA of the present invention, which will have very favorable effects that will be better understood from the description below. The MWA buoy of the invention, with a bottom anchor connected, can be lifted in one operation by a 250 ton crane vessel, whilst prior art solutions require a 400 ton iifiing vessel, saving about 100 000 USD per day. The MWA buoy of the invention weights about 60 tons, prior art MWA buoys weight about 70 tons. The weight of the achor to be connected to the buoy, can be reduced by about 1 ,4 times the weight saving of the buoy. The lifting height and build height is reduced by about 10 m and 5-6, respectively, allowing reduced crane Siftsng reach out and hesght, which together with the reduced buoy weight allows a far less expensive crane vessel to be used. Furthermore, the lifting and anchoring lugs can easily be tested to three or four times the lifting force without damaging the loadbearing structure, contrary to prior art MWA'.s. Also, the lead time is reduced from about 9 months to 5-6 months....More specifically, the present invention de-couple the main riser design parameters from the sVIWA main parameters by modularizing the MWA design such that the main parts of the MWA can be pre fabricated prior to design freeze of risers. The main benefit of this is that the lead time for fvlWA's can be reduced from 9-12 months to less than 6 months, being on the critical path this will have a direct positive impact on the field development schedule which again yields a significant positive financial impact for the field operators and owners. Terms like vertical, horizontal, top, below and other position or orientation related words, refer to the mid water arch as positioned and oriented as installed, with the gutter on top, unless other meaning is specified. Preferably, all lifting lugs and anchoring lugs are arranged below the center of gravity. This allows a rigid bridle to be omitted, thereby reducing the height and weight and required lifting height of the MWA further. Furthermore, access to the lugs are far easier and safer on a vessel deck, and reduced height and weight facilitates transportation.

The buoyancy elements comprise three or more gas filled pressure vessels, the pressure vessels are without bulkheads, and preferably the pressure vessels are dimensioned so that at least two pressure vessels are sufficient for maintaining buoyancy of the MWA, as installed and during installation.

Alternative buoyancy elements can be used, such as elements based on syntactic foam or other foam, which can be feasible for larger depths than about 300 m. The MWA buoy comprises at least three lifting lugs, and at least three anchoring lugs, the number of lifting lugs preferably equals the number of anchoring lugs, the lugs are spread horizontally and Sifting lugs are common with anchoring lugs or respective lifting and anchoring lugs are in substance vertically aligned. Preferably, the MWA comprises four lifting lugs and four anchoring lugs, the lifting lugs are arranged on top of a horizontal spreader bar and the anchoring lugs are arranged below the horizontal spreader bar, which means on the underside, below the lifting lug. The MWA and associated lifting equipment are preferably designed so that failure in one sling will not result in loss of stability or loss of control.

Preferably, the gutter is in substance a ioadbearing structure per se, designed to withstand the load of the risers without having to be supported by the framework of the below positioned Ioadbearing structure. This has the benefit of allowing the gutter to be modified until very late in a development project.

Preferably the gutter is fastened to the ioadbearing structure in three positions, on top, at a central position, and at or close to either end. Thereby the gutter is independent of the tank and tank stiffener structure Preferably, the mid water arch buoy comprises a truss type loadbearing structure (where the majority of the forces are carried by axial forces) with at least three tanks where one tank is piaced in or in immediate vicinity of the buoy centerline. The _. prior art loadbearing structures are based on plates, obstructing access to iifting iugs. Also the gutter can be a truss structure, facilitating access to anchoring and Sifting lugs or points even further in addition to increased strength relative to weight. The invention also provides use of the mid water arch (MWA) buoy of the invention, for use in mid water arch systems for flexible riser arrangements.

Figures

The invention, is illustrated with twelve figures, namely

Figure 1 illustrating a MWA buoy of the invention,

Figure 2 illustrating a MWA buoy of the invention, without gutter,

Figure 3 illustrating a gutter of a MWA of the invention,

Figure 4 illustrating a loadbearing structure of a MWA of the invention,

Figure 5 illustrating a buoyancy tank of a MWA of the invention,

Figures 6-10 _S illustrating details of a MWA of the invention, and

Figures 11 and 12, illustrating details of a gutter and how it is fastened or arranged in a MWA of the invention.

Deta i led desert ption

Reference is made to Fig.1 _f illustrating a Mid Water Arch buoy 1 (MWA) of the invention, for flexible riser arrangements. The MWA 1 comprises a gutter 2, a loadbearing structure 3, three buoyancy elements 4, iifting lugs 5 for attachment to a crane, and anchoring lugs 6 for attachment to an anchor wire. As can be seen, the iifting lugs 5 and anchoring lugs 6 are common or are arranged above and under horizontally extending elements of the loadbearing structure, so that the loadbearing structure is in substance independent of the Iifting forces except of compression components thereof. In the illustrated embodiment, the Iifting lugs 5, of which only one can be seen clearly, is arranged on top of a horizontal member of the loadbearing structure 3, and the anchoring lugs 6, of which only one can be seen clearly, are arranged under said horizontal member of the loadbearing structure. Accordingly, when the MWA buoy with anchor connected is Sifted, the loadbearing structure is not taking up any vertical Sifting force components directly.

Reference is made to Fig. 2, illustrating the MWA of Fig, 1 , without gutter, for increased clarity. The components or elements loadbearing structure 3, gutter 2 and buoyancy tank 4, are illustrated separately on Figures 3 _S 4 and 5, respectively. Further details, with only some components illustrated for clarity, are shown on Figures 6, 7 and 8, ^' illustrating the loadbearing structure and one central buoyancy tank, with lifting slings attached and anchoring slings or bridle attached, as seen from different points of view. Figures 9 and 10 are provided in order to illustrate clearly how the lifting lugs 5 and anchoring lugs 6 are arranged for the illustrated embodiment of a MWA ^' buoy of ihe. invention, so as not or substantially not to transfer lifting loads vertically through the loadbearing structure 3.

Reference is made to Figures 1 1 and 12, illustrating the gutter 2 as a

loadbearing structure per se, and how it is fastened to the loadbearing structure in center 1 1 , which is on top, and at or towards the ends 12. Since the gutter is loadbearing per se, it is not dependent on the loadbearing structure, and the configuration or detailed design of the gutter can be postponed to very late in a development project, reducing the iead time.

Only one specific embodiment of a MWA buoy of the invention is illustrated, for the sake of clarity. Numerous variations are possible, as is clear from the description. The MWA buoy of the invention can comprise any feature as here described or illustrated, in any operative combination, and each such operative combination is an embodiment of a MWA buoy of the invention.