NO317001B1 - Stake with special features against vortex-induced vibrations - Google Patents

Description

The invention relates to a heave-resistant, deep-water platform support structure known as a "stake". More particularly, the invention relates to reducing the sensitivity of stakes to drag and vortex induced vibrations ("VIV" = vortex induced vibrations).

Efforts to economically develop offshore oil and gas fields in increasingly deeper waters create many unique engineering challenges. One of these challenges is to provide a suitable surface-accessible construction. Staker provides a promising answer to meet these challenges. Stake structures provide a heave-resistant floating structure characterized by an elongated, vertically placed hull. This hull is usually cylindrical, floating at the top and with ballast at the bottom. The hull is anchored to the seabed via risers, tethers or bar dunes, and/or mooring lines.

Although resistant to heave movement, stakes are not immune to the rigors of the offshore environment. The typical single column profile of the hull is particularly sensitive to VTV problems in the presence of a passing current. These currents cause vortices to be shed from the sides of the hull, causing vibrations that can interfere with normal drilling and/or production operations and lead to failure of anchorages or other critical structural elements.

Helical plateways and linings have been used or proposed for such applications to reduce vortex-induced vibrations. Plate passages and linings can be made to be effective regardless of the orientation of the current towards the marine element, but linings and plate passages usually increase the drag on such large marine elements to a significant extent.

There is thus a need for a low-resistance, VIV-reducing system suitable for deployment when protecting the hull of a stake-type offshore structure.

International patent publication WO 96/14473 discloses a stake platform comprising a deck, a float tank assembly comprising a first float section connected to the deck, and a second float section arranged below the first float section, and a counterweight arranged below the float tank assembly and connected with the flotation tank assembly using a counterweight-space structure.

It is a purpose of the invention to improve the known stake platform so that the vortex-induced vibrations and the drag are reduced.

For this purpose, a stake platform of the above-mentioned type has been provided which, according to the invention, is characterized by the fact that the floating tank assembly further comprises a floating section space construction which connects the first and second floating sections in a way that provides a horizontally extending space between the first and second floating sections.

By providing the space between the first and second flow sections, the correlation of vortex-induced vibrations between the individual sections is essentially interrupted, so that any remaining vortex-induced vibration is significantly reduced compared to the situation in which no space is present between the flow sections .

For completeness, reference should also be made to US Patent No. 3,951,086. This publication discloses a floating support structure comprising a deck having a number of columns rigidly attached to the deck. The columns have an upper, a central and a lower part, where the upper part is filled with air and the lower part is filled with water. The central portion has a reduced cross-section and connects the upper and lower portions of the column in a manner that allows a degree of relative movement between the upper and lower portions.

With the stake platform according to the invention, a further reduction of vortex-induced vibration is achieved if the first and second floating sections have different outer diameters. The different flow sections then have different aspect ratios, and this characteristic further contributes to disturbing any correlation of vortex-induced vibration between the individual flow sections. The difference between the outer diameters of the first and second floating sections can suitably be between approx. 40 and 80% of the larger of the diameters.

The above description, as well as further advantages of the invention will be more fully understood by reference to the following detailed description of the embodiments shown which should be read in conjunction with the accompanying drawings, where

fig. 1 shows a side view of an embodiment of a stake platform with separated buoyancy or buoyancy (English: spaced buoyancy) in accordance with the invention,

fig. 2 shows a cross-sectional view of the stake platform in fig. 1, after the line 2-2

on fig. 1,

fig. 3 shows a side view of an alternative embodiment of a stake platform with separate buoyancy in accordance with the invention,

fig. 4 shows a cross-sectional view of the stake platform in fig. 3, after the line 4-4

on fig. 3,

fig. 5 shows a cross-sectional view of the stake platform in fig. 3, after the line 5-5

on fig. 3,

fig. 6 shows a cross-sectional view of the stake platform in fig. 3, after the line 6-6

on fig. 4,

fig. 7 shows a schematic sectional view of a riser system which is useful in embodiments of the invention,

fig. 8 shows a side view of a riser system deployed in an embodiment of the invention,

fig. 9 shows a side view of another embodiment of the invention, and

fig. 10 shows a side view of a mainly open truss in an embodiment of the invention.

Fig. 1 shows a stake 10 in accordance with the invention. Stakers are a broad class of floating, moored offshore structures characterized by being resistant to heave movements and exhibiting an elongated, vertically oriented hull 14 which is floating at its top, here a floating tank assembly 15, and is ballasted at its bottom, here a counterweight 18, which is separated from the top via an intermediate structure or counterweight-space structure 20.

Such stakes can be deployed in many different sizes and configurations suitable for their intended purposes varying from drilling only, drilling and production, or production only. Figs. 1 and 2 show a drill stake, but those skilled in the art can readily adapt suitable stake configurations in accordance with the invention for production-only operations or for combined drilling and production operations as well as in the development of offshore hydrocarbon reserves.

In the illustrative example of fig. 1 and 2, the stake 10 supports a deck 12 with a hull 14 which has a number of mutually separated buoyancy or floating sections, here a first or upper floating section 14A and a second or lower floating section 14B. These float sections are separated by a float section spacer structure 28 to provide a substantially open, horizontally extending, vertical space 30 between adjacent float sections. The floating sections here have equal diameters and divide the floating tank assembly 15 into sections of essentially equal length below the water line 16. Furthermore, the height of the space 30 is largely equal to 10% of the diameter of the floating sections 14A and 14B.

A counterweight 18 is provided at the bottom of the stack and the counterweight is separated from the floating sections by a counterweight spacer structure 20. The counterweight 18 can be in any number of configurations, e.g. cylindrical, hexagonal, square, etc, as long as the geometry is suitable for connection to the counterweight space structure 20.1 this embodiment, the counterweight is rectangular, and the counterweight space structure is provided by means of a substantially open truss framework 20A.

Mooring lines 26 secure the stake platform above the well site to the seabed 22. In this embodiment, the mooring lines are bundled (see Fig. 2) and provide the characteristics of both tight and catenary mooring lines, with buoys 24 included in the mooring system (see Fig. 1). The mooring lines are terminated at their lower ends in an anchor system 32, here piles 32A. The upper end of the mooring lines may extend upwards via shoes, sheaves, etc. to winch equipment on the deck 12, or the mooring lines may be more permanently attached at their exit from the hull 14 at the bottom of the float tank assembly 15.

In fig. 1, a drill riser 34 is brought into position under a derrick 36 on deck 12 of the stake platform 10. The drill riser connects drilling equipment at the surface with a well 37 on the seabed 22 via a central underdeck opening (moonpool) 38, see fig. 2.

A fundamental characteristic of the stake construction is its heave resistance. However, the typical elongate cylindrical hull members, whether the "classic" stake's single-sinking case or the float tank assembly 15 in a truss-type stake, are very sensitive to vortex-induced vibration ("VIV") in the presence of a passing current. These currents cause vortices to be shed from the sides of the hull 14, thereby causing vibrations that can impede normal drilling and/or production operations and lead to failure of the risers, mooring line connections, or other critical structural elements. Premature fatigue failure is a particular concern.

Previous efforts to suppress VIV in stake hulls have concentrated on plating and cladding. However, both of these efforts have tended to produce designs with high drag or drag coefficients, making the hull more sensitive to drift. This entails significant increases in the robustness required in the anchoring system. Furthermore, this is a significant expense for structures that may have many elements that extend from near the surface to the seabed, and which typically comes into consideration for water depths of over 800 m or thereabouts.

The present invention reduces VIV due to currents, regardless of their angle of attack, by dividing the aspect ratio of the cylindrical elements in the stack with essentially open, horizontally extending, vertical spaces 30 with selected spaces along the length of the cylindrical hull. A space with a height of approx. 10% of the diameter of the cylindrical element is sufficient to substantially break the correlation of current around the combined cylindrical elements, and this advantage can be maximized with the fewest possible such gaps by dividing the combined cylindrical elements into sections of substantially equivalent aspect ratios . For typically sized truss-type stakes, such a gap through the float tank assembly can be sufficient relief, as the truss framework 20A which forms the counterweight-gap structure 20 contributes little to the stake's VW response.

Provision of one or more spaces 30 also helps to reduce the drag effects of current on the stake hull 14.

Fig. 3-5 show a stake 10 in accordance with another embodiment of the invention. In this illustration, stack 10 is a production stack with a derrick 36 for overhaul operations. A float tank assembly 15 supports a deck 12 with a hull 14 having two separate buoyancy sections 14A and 14B, of different diameters. A counterweight 18 is arranged at the bottom of the stake, and the counterweight is separated from the buoyancy sections by means of a truss framework 20A which is open in the main case. Mooring lines 19 secure the stake platform over the well site.

Production risers 34A connect wells or manifolds on the seabed (not shown) to surface equipment on deck 12 to provide a flow line for production of hydrocarbons from subsea reservoirs. The riser pipes 34A here extend through an inner or central lower deck opening 38 which is shown in the cross-sectional drawings in fig. 4 and 5.

Stake platforms characteristically resist, but do not eliminate, heave and pitch movements. Furthermore, other dynamic responses to environmental forces also contribute to relative movement between the risers 34A and the stake platform 10. Effective support for the risers that can accommodate this relative movement is critical because a net compressive load can crack the riser and collapse the passage inside the riser that is necessary to conduct well fluids to the surface. Similarly, excessive tensile force from an uncompensated, direct support can seriously damage the riser. Fig. 7 and 8 show a deep-water riser system 40 which can support the risers without the need for active, movement compensating, riser tightening systems.

Fig. 7 shows a schematic sectional view of a deep-water riser system 40 which is constructed in accordance with the invention. Inside the stake structure, production risers 34A run concentrically inside buoyancy can tubes 42. One or more centralizing devices 44 ensure this location. A centralizing device 44 is here attached at the lower edge of the buoyancy casing tube and is provided with a load transfer connection of an elastomeric, flexible joint which absorbs axial load but transmits some bending deformation and thus serves to protect the riser tube 34A against extreme bending moments which would be a result of a fixed riser-stake connection at the bottom of the stake 10.1 this embodiment, the bottom of the buoyancy casing pipe is otherwise open to the sea.

However, the top of the buoyancy casing tube is provided with an upper seal 48 and a load transfer connection 50. In this embodiment, the sealing and load transfer functions are separate, provided by an inflatable gasket 48A and a spider 50A, respectively. However, these functions could be combined in a hanger/seal assembly or provided in another way. Riser 34A extends through seal 48 and connector 50 to present a valve tree 52 close to production equipment not shown. This equipment is connected by a flexible conduit which is also not shown. In this embodiment, the upper load transfer connection 50 takes a smaller axial load than the lower load transfer connection 46 which takes the load of the production riser therebetween. In contrast, the upper load transfer connection only accommodates the riser load through the length of the stack and this is only necessary to increase the riser side support provided to the production riser by the concentric buoyancy casing surrounding the riser.

External buoyancy tanks, here provided by means of hard tanks 54, are arranged around the circumference of the relatively large diameter buoyancy casing pipe 42, and provide sufficient buoyancy to at least keep an unloaded buoyancy casing pipe afloat. In some applications, it may be desirable that the hard tanks or other form of external buoyancy tanks 54 provide a certain redundancy in the total riser support.

In addition, load-bearing buoyancy is provided to the buoyancy cap assembly 41 in the presence of a gas 56, e.g. air or nitrogen, in the annulus 58 between the buoyancy jacket 42 and the riser 34A below the seal 48. A pressure charging system 60 supplies this gas and drives hot out from the bottom of the buoyancy jacket 42 to establish the load-carrying buoyancy force in the riser system.

The load transfer connections 46 and 50 provide a relatively firm support from the buoyancy cap assembly 41 to the riser 34A. Relative movement between the stake 10 and the connected riser/buoyancy assembly is accommodated by riser liner constructions 62 which include wear-resistant liners or bushings inside riser guide pipes 64. The wear interface is located between the guide pipes and the relatively large diameter buoyancy cap pipes, and the riser pipes 34A are protected.

Fig. 8 shows a side view of a deep water riser system 40 in a pile 10 shown in partial section with two buoyancy or floating sections 14A and 14B which have different diameters and are separated by a space 30. A counterweight 18 is arranged at the bottom of the pile, separated from the floating sections by means of a mainly open truss framework 20A.

The relatively small diameter production riser 34A extends through the buoyancy casing pipe 42 which has a relatively large diameter. Hard tanks 54 are attached around the buoyancy casing tube 42, and a gas injected into the annulus 58 drives the water/gas interface 66 inside the buoyancy casing tube 42 far down through the buoyancy casing assembly 41.

The buoyancy cap assembly 41 is slidably engaged through a number of riser guides 62. The riser guide construction provides a guide pipe 64 for each deepwater riser system 40, all of which are interconnected in a structural framework which is connected to the stack hull 14. In this embodiment, a substantial density of structural guide pipe framework is further provided on such levels that the conductor pipe guide constructions 62 for the entire riser arrangement are tied to the stake hull. Furthermore, this may include a plate 68 across the lower deck opening 38.

The density of conduit framework and/or horizontal plates 68 serves to dampen heaving movement of the stake. Furthermore, the enclosed mass of water affected by this horizontal construction is useful for different tuning of the stake's dynamics, both for more detailed determination of over-oscillations and inertial response. Nevertheless, this virtual mass is provided with minimal steel and without a significant increase in the pile's buoyancy requirements.

Horizontal obstructions across the lower deck opening in a stack with separated buoyancy sections can also improve dynamic response by inhibiting the passage of dynamic wave pressures through the gap 30, up through the lower deck opening 38. Other placement levels of the conduit guide framework, horizontal plates, or other horizontal impact structure 11 (see Fig. 10 ) can be useful, whether it is over the lower deck opening, across the essentially open truss 20A, as an outer projection from the stake, or also as a component of the relative sizes of the upper and lower floating sections 14A respectively. 14B. See fig. 7.

Furthermore, vertical impact surfaces, such as the additional vertical plates 69 at various limited levels in the open truss framework 20A, can similarly improve tamping dynamics for the stack with effective entrapped mass. Such vertical plates may, on a limited basis, close the perimeter of the truss 20A, criss-cross within the truss, or be designed in some other multi-way configuration.

Returning to fig. 6, another optional feature of this embodiment is the absence of hard tanks 54 close to the space 30.

The gap 30 in this stake construction controls vortex-induced vibration ("VIV") on the cylindrical buoyancy or float sections 14 by dividing the aspect ratio (diameter in relation to height below the waterline) by two, the separate float sections 14A and 14B having similar volumes and e.g. e.g. a separation of approx. 10% of the diameter of the upper float section. Furthermore, the gap reduces drag on the stake, regardless of the direction of flow. Both of these advantages require that the current has the ability to pass through the stake at the gap. A reduction of the outer diameter of a number of deepwater riser systems at this gap can therefore enable these benefits.

Another advantage of the space 30 is that it allows the passage of steel import and export catenary risers 70 mounted outside the lower float section 14B, to the lower deck opening 38. See FIG. 6 and also fig. 3-5. This provides the advantages and convenience of hanging these risers outside the hull of the stake, but provides the protection of having these risers inside the lower deck opening near the waterline 16 where collision damage presents the greatest danger and provides a concentration of wiring that facilitates efficient treatment equipment. The import and export seat tubes 70 are attached by means of spacers and clamps above their main load connection with the stake. During this connection, they descend in a chain line bearing to the seabed in a manner that accepts vertical movement at the surface more readily than the vertical access production risers 34A.

Supported by only hard tanks 54 (without any pressurized source of annulus buoyancy), the unsealed and open-top buoyancy casing pipes 42 can serve much the same way as well casings on traditional, fixed platforms. The large diameter buoyancy casing thus allows the passage of equipment, such as a guide funnel and compact mud mat in preparation for drilling, a drill riser with an integrated tieback connector for drilling, surface casing with connection plug, a compact subsea valve tree or other valve assemblies, a compact wireline sluice pipe ( wireline lubricator) for overhaul operations, etc, as well as the production riser and its platform connector. Such other tools can be conveniently supported from a derrick, a gantry crane or the like throughout the operations, as well as the production riser itself during installation operations.

After the production riser 34A is run down (with the centralizing device 44 attached) and forms a connection with the well, the seal 48 is established, the annulus is charged with gas and seawater is drained, and the production riser load is transferred to the buoyancy cap assembly 41 as the deballasted assembly rises and the load transfer connections at the top and bottom of the assembly 41 forms an engagement to support the riser 34A.

One must be aware that although most of the illustrative embodiments described herein utilize the invention in stakes with inner underdeck openings 38 and a substantially open truss 20A separating the floating sections from the counterweight 18, it is clear that the VTV suppression and drag reduction according to the present invention is not limited to this type of stake design. Such arrangements may be utilized for stacks that have no underdeck opening and externally run vertical access production risers 34A, or they may be utilized in "classic stacks" 10 in which the float tank assembly 15, counterweight space structure 20, and counterweight 18 are all arranged in the profile of a single elongated cylindrical hull interrupted only by the spaces according to the invention. See, for example, fig. 9 which illustrates both of these design aspects. One must also be aware that dividing the float tank assembly into several float sections facilitates a modular method of stack construction whereby equipment requirements and accompanying deck loads can be taken into account by adding or changing one or more of the float sections rather than redesigning the entire stack as an integral cylindrical unit such as a "classic" stake.

Claims (10)

1. Stake platform comprising a deck (12), a float tank assembly (15) comprising a first float section (14A) which is connected to the deck, and a second float section (14B) which is arranged below the first float section, and a counterweight (18) which is arranged below the float tank assembly (15) and which is connected with the float tank assembly by means of a counterweight intermediate rudder structure (20), characterized in that the float tank assembly further comprises a float section spacer structure (28) which interconnects the first and second float sections (14A, 14B) in a manner that provides a horizontally extending gap (30) between the first and second flow sections. 2. Stake platform according to claim 1, characterized in that the height of the space (30) is approx. 10% of the width of the first floating section (14A). 3. Stake platform according to claim 1 or 2, characterized in that the first and second floating sections (14A, 14B) have different outer diameters. 4. Stake platform according to claim 3, characterized in that the difference between the outer diameters of the first and second floating sections (14A, 14B) is between approx. 40 and 80% of the largest of the diameters. 5. Stake platform according to one of claims 1-4, characterized in that an open lower deck opening (38) extends mainly vertically through the first floating section (14A), and that at least one riser (34A) which has an upper end part, extends through the lower deck opening up to the stake platform deck (12). 6. Stake platform according to claim 5, characterized in that the open lower deck opening (38) further extends mainly vertically through the second floating section (14B). 7. Stake platform according to claim 5 or 6, characterized in that each riser (34A) is connected to the stake platform (10) by means of a buoyancy cap assembly comprising an open-ended buoyancy cap pipe (42) through which the upper end part of the riser (34A) extends, a seal (48) which is arranged in an upper part of the buoyancy casing pipe and closes the annular space between the riser pipe (34A) and the buoyancy casing pipe (42), a load transfer connection (46 or 50) between the riser pipe (34A) and the buoyancy casing pipe (42), and a pressure charging system (60) in fluid communication with the annulus (58) below the seal. 8. Stake platform according to one of claims 1-7, characterized in that the counterweight-space structure (20) is a mainly open truss framework (20A). 9. Stake platform according to claim 8, characterized in that it comprises a number of horizontal collision structures (11) across the truss framework (20A). 10. Stake platform according to claim 9, characterized in that the horizontal impact structures (11) are formed by riser guide structures (62).