GB2106164A - Multi-body offshore arrangement - Google Patents

Abstract

This specification describes a compliant multi-body offshore concept suitable for production and/or drilling purposes wherein the buoyant mooring 1 and load carrying elements 4 are separated. The buoyant mooring elements 1 are attached to the sea floor by means of tensioned vertical moorings 3. Because of the separation of the load carrying and tension leg mooring elements the invention is less susceptible to the effect of increasing payload reducing mooring tension which occurs in the conventional tension leg platform. The central process facility may be easily moved to other locations, and the comparatively small size of the tension leg mooring buoys eases their fabrication and installation. The invention may also be used for the deep water or hostile environment mooring of conventional semi- submersibles where the spread of a catenary mooring would prove to be excessive. <IMAGE>

Description

SPECIFICATION A compliant multi-body offshore production/drilling facility 2,1 Technical Field The technical fields to which the invention described in section 2,3 relates are those of offshore facilities and marine structures.

2,2 Background Technology The background technology and experience considered useful for a full appreciation of the description of the invention can be found within the fields of: 2,2,1 Naval architecture.

2,2,2 Marine engineering.

2,2,3 Ocean engineering.

2,3 Disclosure and Example The facility consists in its full form of a number of compliantly moored vertical cylindrically shaped bodies or buoys, to which are moored a semisubmersible, bargelike or shipform structure on or in which is carried the facility's process train. The general arrangement is shown in Figures 1(A) and (B), where the central facility is depicted as a semi-submersible.

The vertical cylindrical buoys (1) are moored to the sea bed via a one piece anchor template (2) and a number of vertical mooring members or tethers (3). These mooring members consist of cable, chain or pipe like lements which have the characteristics of high axial stiffness coupled with very low lateral stiffness. Each group of these moorings is held in permanent tension by means of an excess of buoyancy over weight at the buoy.

This excess buoyancy is such as to cause a tensile load in the vertical mooring members for all combinations of wave, wind, tide and attached facility loadings. Tension variations in the mooring elements due to heave forcing of the buoys by wave action will be minimised by arranging the axial distribution of buoy cross sectional area with its associated height above keel in such a way as to take advantage of the decay of wave amplitude with depth of immersion. Ground reaction against this tension is provided at the buoy mooring template either by pinning it in position with piles or by ensuring that its immersed weight is sufficient to both balance the upward load and provide adequate foundation loads on the seabed. In the latter case the template would be permanently ballasted after installation.Each buoy is free to take up an arbitrary lateral offset from its static position depending upon the environmental and mooring forces applied to it. The arrangement is thus laterally compliant.

The construction of the actual buoyancy member can, because of its cylindrical form, be easily realised in either steel or concrete.

As shown in Figure 1 (A), a number of these vertically moored tension leg buoys are anchored in a symmetrical pattern. Figure 1(A) shows a particular arrangement consisting of three such moored buoys. The actual number required in practice will depend upon the size of the buoys being used and the total expected environment and attached facility loadings, and will be the subject of detailed calculations for each proposed installation. Within this pattern the floating process/production facility (4) is constrained via multiple tensioned moorings (5), which are capable of disconnection, to each of the outer moored tension leg buoys. The point of attachment of these moorings to the outer buoys is positioned close to the mean position of the centre of lateral resistance of the buoy.The centrally moored facility is not at a constant elevation above the sea floor, but may have a variable draught depending upon its time to time payload requirements. This central facility need not be a purpose built structure. For marginal fields it is envisaged that a converted semisubmersible of a suitable size would provide an economic solution.

There may be more than one mooring pattern installed at different places for the same central facility.

Drilling and or production risers (6) are run from the central facility to a pre-positioned sub-sea wellhead template (7) situated vertically beneath the static position of the central facility. Accurate initial positioning of the central complex over the wellhead template is assured by making small adjustments in the mooring tensions to the mooring buoys at the time of installation. Where the field logistics are such that product export from the facility is to be accomplished by sub-sea pipeline then the export risers will also be run from the central facility to the wellhead template and from thence tied into the pipeline.Where this is not feasible, or where the resource is judged to be a marginal one, then the export products will be conveyed to one or more of the tension leg mooring buoys from the central facility via flexible pipelines which do not transmit, or are subjected to, significant mooring loads. From these buoys the products will be transferred into shuttle tankers temporarily held on station with auxiliary mooring buoys.

2,4 Exploitation The purpose of the invention is to facilitate the installation and operation of a buoyant production/process facility offshore over a previously determined resource.

This resource can take the form of an oil field, although the application is not solely limited to this. The invention provides a stable marine structure suitable for ant operations conventionally carried out from a fixed jacket or moored semi-submersible rig, particularly in deep or hostile waters.

3) CLAIMS

3,1 The Claim The invention comprises a vertically moored buoyant tension leg offshore platform wherein the improvement is characterized by the separation of the mooring restraint function and the deadweight

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   SPECIFICATION A compliant multi-body offshore production/drilling facility 2,1 Technical Field The technical fields to which the invention described in section 2,3 relates are those of offshore facilities and marine structures.

   2,2 Background Technology The background technology and experience considered useful for a full appreciation of the description of the invention can be found within the fields of: 2,2,1 Naval architecture.

   2,2,2 Marine engineering.

   2,2,3 Ocean engineering.

   2,3 Disclosure and Example The facility consists in its full form of a number of compliantly moored vertical cylindrically shaped bodies or buoys, to which are moored a semisubmersible, bargelike or shipform structure on or in which is carried the facility's process train. The general arrangement is shown in Figures 1(A) and (B), where the central facility is depicted as a semi-submersible.

   The vertical cylindrical buoys (1) are moored to the sea bed via a one piece anchor template (2) and a number of vertical mooring members or tethers (3). These mooring members consist of cable, chain or pipe like lements which have the characteristics of high axial stiffness coupled with very low lateral stiffness. Each group of these moorings is held in permanent tension by means of an excess of buoyancy over weight at the buoy.

   This excess buoyancy is such as to cause a tensile load in the vertical mooring members for all combinations of wave, wind, tide and attached facility loadings. Tension variations in the mooring elements due to heave forcing of the buoys by wave action will be minimised by arranging the axial distribution of buoy cross sectional area with its associated height above keel in such a way as to take advantage of the decay of wave amplitude with depth of immersion. Ground reaction against this tension is provided at the buoy mooring template either by pinning it in position with piles or by ensuring that its immersed weight is sufficient to both balance the upward load and provide adequate foundation loads on the seabed. In the latter case the template would be permanently ballasted after installation.Each buoy is free to take up an arbitrary lateral offset from its static position depending upon the environmental and mooring forces applied to it. The arrangement is thus laterally compliant.

   The construction of the actual buoyancy member can, because of its cylindrical form, be easily realised in either steel or concrete.

   As shown in Figure 1 (A), a number of these vertically moored tension leg buoys are anchored in a symmetrical pattern. Figure 1(A) shows a particular arrangement consisting of three such moored buoys. The actual number required in practice will depend upon the size of the buoys being used and the total expected environment and attached facility loadings, and will be the subject of detailed calculations for each proposed installation. Within this pattern the floating process/production facility (4) is constrained via multiple tensioned moorings (5), which are capable of disconnection, to each of the outer moored tension leg buoys. The point of attachment of these moorings to the outer buoys is positioned close to the mean position of the centre of lateral resistance of the buoy.The centrally moored facility is not at a constant elevation above the sea floor, but may have a variable draught depending upon its time to time payload requirements. This central facility need not be a purpose built structure. For marginal fields it is envisaged that a converted semisubmersible of a suitable size would provide an economic solution.

   There may be more than one mooring pattern installed at different places for the same central facility.

   Drilling and or production risers (6) are run from the central facility to a pre-positioned sub-sea wellhead template (7) situated vertically beneath the static position of the central facility. Accurate initial positioning of the central complex over the wellhead template is assured by making small adjustments in the mooring tensions to the mooring buoys at the time of installation. Where the field logistics are such that product export from the facility is to be accomplished by sub-sea pipeline then the export risers will also be run from the central facility to the wellhead template and from thence tied into the pipeline.Where this is not feasible, or where the resource is judged to be a marginal one, then the export products will be conveyed to one or more of the tension leg mooring buoys from the central facility via flexible pipelines which do not transmit, or are subjected to, significant mooring loads. From these buoys the products will be transferred into shuttle tankers temporarily held on station with auxiliary mooring buoys.
2. 2,4 Exploitation The purpose of the invention is to facilitate the installation and operation of a buoyant production/process facility offshore over a previously determined resource.

   This resource can take the form of an oil field, although the application is not solely limited to this. The invention provides a stable marine structure suitable for ant operations conventionally carried out from a fixed jacket or moored semi-submersible rig, particularly in deep or hostile waters.
3. 3) CLAIMS

   3,1 The Claim The invention comprises a vertically moored buoyant tension leg offshore platform wherein the improvement is characterized by the separation of the mooring restraint function and the deadweight (or payload) carrying functions of the platform into independent and discrete elements.

   3,2 Dependent Claims The dependent claims for the invention which arise as a result of the Claim 3,1 are: 3,2,1 The invention can be applied directly to semi-submersible type installations when the mooring spread required for deepwater applications, compared to a conventional catenary mooring of the central load carrying facility, is greatly reduced.

   3,2,2 The central load carrying facility may easily be removed from the mooring arrangements described for maintenance or refitting purposes.

   3,2,3 In the case of the exploitation of marginal resources the central load carrying facility may be transferred to other areas of operation within its operating lifetime, thereby maximising the use of investment capital.

   3,2,4 The loads arising in the vertical mooring elements as a consequence of misalignment and mispositioning are reduced. Each tension leg buoy is capable of independent movement in yaw, pitch and roll, thereby allowing the equalisation of loads to take place within the group of tensioned anchor members attached to any particular buoy.

   3,2,5 The dependence of the vertical mooring tethers' pretension upon the facility payload occurring in conventional tension leg platforms is removed, thereby allowing greater flexibility in design and operation.

   3,2,6 Installation of the invention on site is easy and uncomplicated, each independent tension leg buoy being installed separately prior to the arrival of the main facility. Small errors in the positioning of the tension leg buoy mooring templates will be taken up by adjusting the tensioned moorings from the central facility to the mooring buoys at final installation.

   3,2,7 The individual tension leg mooring buoys are small when compared to a complete platform having the same aggregate buoyancy and thus pose less building and handling problems.

   3,2,8 Because of the simple cylindrical shape of the mooring buoys they may be fabricated in either steel or in concrete using pre or post tensioning techniques.