AU2013332507B2

AU2013332507B2 - Midwater arch system

Info

Publication number: AU2013332507B2
Application number: AU2013332507A
Authority: AU
Inventors: Geir GRIMSRUD; Jorgen Hogmoe
Original assignee: RISER SUPPORT SYSTEMS
Current assignee: RISER SUPPORT SYSTEMS
Priority date: 2012-10-16
Filing date: 2013-10-14
Publication date: 2017-06-01
Anticipated expiration: 2033-10-14
Also published as: AU2013332507A2; BR112015008443A2; GB201506276D0; NO336936B1; AU2013332507A1; NO20121193A1; GB2520906A; WO2014062064A1

Abstract

The invention provides 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 (MWA) comprising: a gutter, a loadbearing structure, buoyancy elements, lifting lugs for attachment to a crane, and anchoring lugs for attachment to an anchor wire, distinguished in that the lifting 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.

Description

1 2013332507 05 May 2015

MIDWATER ARCH SYSTEM

Field of the invention

The present invention relates to flexible risers, anchoring and arrangement 5 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 10 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 configuration as seen from the side. However, numerous arrangements exist, and in general the arrangements 15 comprises at least one portion for which the riser is provided with buoyancy, the parts on either side hanging as inclined catenary lines between the portion with buoyancy and for example a subsea production system and a FPSO (Floating Production, Storage and Offloading) vessel. 20 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). 25 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 line 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. 30 A typical MWA buoy comprises a gutter, a loadbearing structure below the gutter, buoyancy elements, 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 2013332507 05 May 2015 2 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 5 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 lugs for hanging up the anchor, are typically 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 10 lugs high and the anchoring lugs low provides orientation stability. A typical MWA 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 15 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 20 bulkheads, rated to one bar differential pressure, in order to maintain sufficient buoyancy if one tank compartment starts leaking when the MWA is installed in submerged position. The tanks are typically filled with gas at a pressure equal to the submerged position seawater pressure plus one atmosphere. 25 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 late in the development project. Selection of main parameters for the MWA’s are traditionally dependent on the riser particulars. Thus the MWA particulars are decided subsequent of riser 30 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. 3 2013332507 12 Apr 2017

Currently, a typical MWA must be installed using a 400 ton 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 5 later revisions on riser layout.

Currently, a typical 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 differential 10 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 differential pressure rating is one bar. Experience 15 reveal that in substance all damage to the MWA occur during installation, ability to withstand damage better would be beneficial.

Currently, some MWA’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 20 operation. A typical requirement is also that the lifting lugs and structure shall 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. A preferred objective of the present invention is to provide a mid water arch that 25 is favorable with respect to one or more of the issues mentioned above.

Summary of the invention

In one broad form, the invention provides a mid water arch (MWA) buoy comprising: 30 a gutter, a loadbearing structure, 4 2013332507 12 Apr 2017 buoyancy elements, lifting lugs for attachment to a crane, and anchoring lugs for attachment to an anchor wire, wherein the lifting lugs and anchoring lugs are common or are arranged above 5 and under horizontally extending elements of the loadbearing structure, so that the loadbearing structure is in substance independent of the lifting forces except for compression components thereof; and wherein the mid water arch buoy is configured for lifting and installation, with anchors connected, in one operation. 10

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 15 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 or framework can be dimensioned not for the total weight of anchor plus MWA buoy plus a safety factor, but a significantly reduced load. 20 More specifically, during lifting, the loadbearing structure of the MWA of the invention functions merely or substantially as a spreader and can be dimensioned accordingly.

The term ‘arranged above and under horizontally extending elements’ means 25 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 30 the loadbearing structure, but the full forces of lifting the buoy and anchor must not be carried by the loadbearing structure, only 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 2013332507 05 May 2015 5 anchoring lugs are separated, they are separated by elements independent of the loadbearing structure or elements of the loadbearing structure carrying horizontal lifting force components. 5 Preferably, the number of lifting lugs equals the number of anchoring lugs, and lifting lugs and anchoring lugs are in substance vertically aligned.

Advantageously, the MWA of the invention is adapted for lifting and installation, with anchors connected, in one operation. 10

The weight, size and lead time will be substantially reduced by the MWA of the present invention, which will have very favorable effects that will be better understood from the description below. Typically, the MWA buoy of the invention, with a bottom anchor connected, can be lifted in one operation by a 15 250 ton crane vessel, whilst prior art solutions require a 400 ton lifting vessel, saving about 100 000 USD per day. The MWA buoy of the invention typically weighs about 60 tons, whereas prior art MWA buoys weigh 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 20 reduced by about 10 m and 5-6 m, respectively, allowing reduced crane lifting reach out and height, 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 25 reduced from about 9 months to 5-6 months. More specifically, the present invention permits de-coupling of the main riser design parameters from the MWA 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 MWA’s can be reduced from 9-12 30 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. 6 2013332507 05 May 2015

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. 5 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. 10

The buoyancy elements preferably comprise three or more gas filled pressure vessels, the pressure vessels are optionally 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 15 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 20 anchoring lugs, the number of lifting lugs preferably equals the number of anchoring lugs, the lugs are spread horizontally and lifting 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 25 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. 30 Preferably, the gutter is in substance a loadbearing structure per se, designed to withstand the load of the risers without having to be supported by the framework of the below positioned loadbearing 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 loadbearing structure in three positions, 7 2013332507 05 May 2015 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 5 structure (where the majority of the forces are carried by axial forces) with at least three tanks where one tank is placed in or in immediate vicinity of the buoy centerline. The prior art loadbearing structures are based on plates, obstructing access to lifting lugs. Also the gutter can be a truss structure, facilitating access to anchoring and lifting lugs or points even further in addition to increased 10 strength relative to weight.

The mid water arch (MWA) buoy of the invention can be used in mid water arch systems for flexible riser arrangements. 15 Brief description of the drawings

Embodiments of the invention are illustrated with reference to the accompanying twelve figures, namely

Figure 1 illustrating a MWA buoy of an embodiment of the invention,

Figure 2 illustrating a MWA buoy of an embodiment of the invention, without 20 gutter,

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

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

Figure 5 illustrating a buoyancy tank of a MWA of an embodiment of the 25 invention,

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

Figures 11 and 12, illustrating details of a gutter and how it is fastened or arranged in a MWA of an embodiment of the invention. 30

Detailed description of embodiment(s) of the invention

Reference is made to Fig. 1, illustrating a Mid Water Arch buoy 1 (MWA) of the invention, for flexible riser arrangements. The MWA 1 comprises a gutter 2, a 8 2013332507 05 May 2015 loadbearing structure 3, three buoyancy elements 4, lifting lugs 5 for attachment to a crane, and anchoring lugs 6 for attachment to an anchor wire. As can be seen, the lifting lugs 5 and anchoring lugs 6 are common or are arranged above and under horizontally extending elements of the loadbearing structure, so that 5 the loadbearing structure is in substance independent of the lifting forces except of compression components thereof. In the illustrated embodiment, the lifting 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 10 loadbearing structure. Accordingly, when the MWA buoy with anchor connected is lifted, the loadbearing structure is not taking up any vertical lifting force components directly.

Reference is made to Fig. 2, illustrating the MWA of Fig. 1, without gutter, for 15 increased clarity. The components or elements loadbearing structure 3, gutter 2 and buoyancy tank 4, are illustrated separately on Figures 3, 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 20 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 the invention, so as not or substantially not to transfer lifting loads vertically through the loadbearing structure 3. 25

Reference is made to Figures 11 and 12, illustrating the gutter 2 as a loadbearing structure per se, and how it is fastened to the loadbearing structure in center 11, 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 30 configuration or detailed design of the gutter can be postponed to very late in a development project, reducing the lead 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 2013332507 05 May 2015 9 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. 5 The term “comprise” and variants of that term such as “comprises” or “comprising” are used herein to denote the inclusion of a stated integer or integers but not to exclude any other integer or any other integers, unless in the context or usage an exclusive interpretation of the term is required. 10 Reference to background art or other prior art in this specification is not an admission that such background art or other prior art is common general knowledge in Australia or elsewhere.

Claims

1. A mid water arch buoy (MWA) comprising: a gutter, a loadbearing structure, buoyancy elements, lifting lugs for attachment to a crane, and anchoring lugs for attachment to an anchor wire, wherein the lifting 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; and wherein the mid water arch buoy is configured for lifting and installation, with anchors connected, in one operation.

2. The mid water arch buoy according to claim 1, wherein all lifting lugs and anchoring lugs are arranged below the center of gravity.

3. The mid water arch buoy according to claim 1, wherein the buoyancy elements are at least three pressure vessels, dimensioned so that at least two pressure vessels are sufficient for maintaining buoyancy of the MWA, as installed and during installation.

4. The mid water arch buoy according to any one of claims 1 -3, wherein at least three lifting lugs are arranged, and at least three anchoring lugs, the number of lifting lugs equals the number of anchoring lugs, the lugs are spread horizontally and lifting lugs are common with anchoring lugs or respective lifting and anchoring lugs are in substance vertically aligned.

5. The mid water arch buoy according to any one of claims 1 -4, wherein 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.

6. The mid water arch buoy according to any one of claims 1 -5, wherein the gutter is in substance a loadbearing structure per se, designed to withstand the load of the risers without having to be supported by the framework of the below positioned loadbearing structure.

7. The mid water arch buoy according to claim 1 or 6, wherein the gutter is fastened to the loadbearing structure only, and is independent of the tank and tank stiffener structure.

8. The mid water arch buoy according to any one of claims 1 -7, comprising 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 placed in or in immediate vicinity of the buoy centerline.

9. Use of a mid water arch buoy according to any one of claims 1 -8, for lifting and installation, with anchors connected, in one operation, for flexible riser arrangements.