Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
  • B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
  • B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
  • B63B2021/505—Methods for installation or mooring of floating offshore platforms on site

Definitions

• the present invention relates to the installation of structures on a seabed. More specifically, the invention relates the positioning of buoyant structures in a body of water in relation to fixed anchors. In particular, the invention relates to offshore structures provided with buoyancy for towage to the installation site.
• buoyant structure is positioned in the water by two or three vessels connected by tow lines to the structure.
• the vessels are able to maneuver the structure to the target area, keep it positioned and in a desired orientation until the structure is installed on the seabed.
• the achievable positioning accuracy depends on the waves and currents occurring during the positioning operation. Consequently, the positioning and installation of the structure can only be executed in sea conditions which are less severe than the operability of the vessel otherwise would allow.
• a system for positioning a structure, floating in a body of water, at a desired position characterised by at least two anchoring means positioned on the seabed, each anchoring means comprising respective connection means for attachment to the structure, and further comprising manoeuvring means connected to the structure.
• the respective connection means comprise individual adjustment means.
• the manoeuvring means comprises a propulsive unit by which a pulling force is applied to the structure.
• the adjustment means is adjustable while the propulsive unit is applying the pulling force.
• the connection means preferably comprise individual lines.
• the length of the individual positioning lines are adjusted individually and selectively during the application of tension by the vessel.
• Figure 1 is a top view of a floating structure attached to the positioning arrangement comprising positioning anchors;
• Figure 2 is a side view of the floating structure attached to the positioning arrangement comprising positioning anchors shown in figure 1 ;
• Figure 3 is a sketch showing the composition of the positioning line
• Figure 4 is a side view of an installed positioning anchor with a lower segment of the positioning line buoyed off and ready for connection of the remaining part of the positioning line;
• Figure 5 is a side view of the positioning anchor shown in fig. 4 with lower section of the positioning line exposed to loads from a structure (not shown) being positioned;
• Figure 6 is a top view of the positioning arrangement in combination with a number of already installed structures. Detailed description of a preferential embodiment
• a floating structure 1 is schematically illustrated being positioned at a target position P, attached to two positioning lines 2, 3 and to a line 4 connected to a vessel 5 (e.g. a tug, or similar).
• a first anchor 6, a second anchor 7, and a third anchor 8 are pre-installed on the seabed.
• the positioning lines 2, 3 are attached to respective fixed second and third anchors 7, 8, and to respective points on the structure, the position P and orientation of the structure 1 is determined by the physical parameters involved; i.e. the position of the second and third anchors 7, 8, lengths and stiffness of the positioning lines 2, 3, tension in the positioning lines caused by the vessel 5, and finally by the geometrical shape of the positioning lines in the vertical plane, often resembling a catenary.
• each of the positioning lines 2, 3 may be calculated from known as-installed positions of the second and third anchors 7, 8; accounting for the lines' properties and tension from the positioning vessel 5, such that very accurate position of the installed structure 1 can be achieved.
• the intention is to enable a positioning method and equipment for performing the positioning is as high a seastate as possible, and the liberty of approaching the installation site from an arbitrary direction, it may be advantageous to preinstall the three fixed anchors 6, 7, 8 with an approximate separation of 120° in relation to the structure's target position P.
• Which pair of the three anchors 6, 7, 8 should be used for the positioning would be determined from the actual weather and seastate direction. For example, referring to figure 1, if waves are approaching mostly in direction within the sector a, then the second and third anchors 7, 8 are used. If the approach direction is within sectors b, then the first and second anchors 6, 7 are used, and if the approach direction is with sectors c, then the first and third anchors 6, 8 are applicable.
• FIG. 2 is a schematic side view of the positioning arrangement for the structure 1.
• the structure moves in six degrees of freedom.
• the invented attachment of the structure 1 to the anchors 6, 7, 8 ensures that the structure's motions in the water are kept to a minimum, thus allowing accurate positioning in the water, even in large waves.
• This is achieved by connecting the positioning lines 2, 3 to the structure 1 by means of respective bridles 9, 10, which in turn are connected to the structure at respective points 1 la,b and 12a,b.
• the elevations on the structure at which the connections points are positioned, and the vertical distance between them, are calculated from a dynamic analysis targeting minimum motions of the structure at acceptable loads induced in the positioning lines 2, 3 and in the towing line 4.
• Figure 3 identifies main components in the invented positioning line 2, 3, i.e., sequentially from left: a lower segment 13, a main segment 14, a length adjustment segment 15 and a tension release device 16.
• the function of the lower segment 13 is explained below, with reference to figure 4.
• the length adjustment segment 15 allows rapid adjustments of the positioning line length to reflect as-installed positions of the positioning lines.
• the length adjustment segment 15 comprises a chain (indicated as a dotted line in figure 3) that allows any point of the adjustment segment to be connected to an adjacent segment.
• the length adjustment segment 15 may also comprise a winch unit, or similar (not shown), which for example may be placed on a floating vessel (not shown) tethered to the anchors 6, 7.
• the length of the individual positioning lines may be adjusted individually and selectively during the positioning operation.
• the adjacent segment is the tension release device 16.
• the tension release device 16 is used upon completion of the positioning and installation procedure of the structure, when the tensioned positioning lines 2, 3 are to be slackened for easy disconnection and subsequent retrieval.
• the positioning lines 2, 3 are designed with the appropriate stiffness, determined by dynamic analysis. This stiffness is mainly achieved by the design and composition of the main segment 14.
• the main segment 14 comprises fiber ropes that have lower elasticity modules than steel wires.
• FIG 4 shows an anchor 7 for use during positioning and installation of the structure once it has been installed.
• the anchor 7 - which is similar to the anchors 6, 8 described above - is made as a hollow cylinder which is embedded
• the minimum embedment depth for the anchor 7 is given by rules applicable for the particular location.
• the embedment is in this embodiment achieved by an embedment device 18, in the figure shown suspended in the water (dotted line indicating lifting wires connected to installation vessel (not shown)) under repositioning to next preinstalled anchor or under retrieval to surface.
• the embedment device 18 is designed to provide sufficient weight for embedding the anchor into the seabed.
• the embedment device is shaped so that it does not produce unnecessary penetration resistance, and is provided with mating guides 19 that facilitates subsea docking onto the pre-installed anchor 7.
• the embedment device is adapted for addition of weight, by e.g. clump weights or ballast chain (not shown) onto an upper surface.
• each positioning line Before installation of the anchor 7, the respective lower segment 13 of each positioning lines is attached to its respective anchor in air (before it is lowered into the water) in order to avoid subsea work.
• the lower segment 13 shown in figure 4 is buoyed off to a surface buoy 20, thus allowing connection to the remaining part (e.g. main segment 14) of the positioning line above water.
• Figure 5 shows an installed positioning anchor 7 and positioning line 2 exposed to tension loads from the floating structure (not shown) during positioning.
• FIG. 6 shows schematically in figure 6, where reference numbers la-f indicate structures 1 already installed to fixed positions on the seabed. Two of these already installed structures (le, If) are being used for placing a floating structure lg at a desired position P. Thus, the two installed structures (le and If in figure 6) serve the similar functions as the fixed anchors 6, 7 as described above with reference to figure 1.
• already installed structures le,f are used for positioning in this fashion, they are preferably furnished with individual lower segments 13 (not shown in figure 6) before installation, as described above with reference to figure 4.
• the floating structure lg (to be positioned and installed) is connected to positioning lines 2, 3 via respective bridles 9, 10 to the already installed structures le, If.
• the vessel 5 exerts the required tension in the positioning system via the tow line 4, as described above.
• the positioning of a floating structure may typically be performed by:

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• Ocean & Marine Engineering (AREA)
• Earth Drilling (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Family Cites Families (4)

• 2011
  • 2011-07-13 EP EP11749264.5A patent/EP2593355A1/de not_active Withdrawn
  • 2011-07-13 WO PCT/NO2011/000200 patent/WO2012008848A1/en active Application Filing

Non-Patent Citations (1)

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