Abstract:
A semi-submersible vessel is disclosed in which a side wall which is continuous in the horizontal direction connects an underwater buoyant pontoon hull to an operating deck located above the water surface. In operation the side wall is subject to moderate cyclic fatigue stresses only, thereby increasing the useful life of the vessel.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a semi-submersible vessel. 
     Many designs of semi-submersible floating platforms used for drilling or other offshore operations are based on the use of three or more vertical buoyant columns to support a deck at a safe distance above sea level, their bottom ends being attached to underwater chambers forming pontoons to provide additional buoyancy. 
     The operating deck is designed for carrying equipment and living quarters and may contribute to the rigidity of structure. The deck then forms a strong box structure enclosing a number of tween decks and integrated with the tops of the columns. Columns and bracings between columns are subject to wave and current forces and their points of attachment are susceptible to material fatigue. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide an offshore platform which does not rely on columns and bracings to support an operating deck and which displays superior strength and fatigue characteristics. 
     A vessel according to the present invention comprises a side wall which is continuous in a horizontal direction, which wall is attached to an underwater pontoon hull at its lower end and supports an operating deck at its upper end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 is an elevation view of a semi-submersible vessel according to the invention; 
     FIG. 2 is a horizontal section of the side wall of the semi-submersible vessel of FIG. 1, taken at line 2--2 therein; and 
     FIG. 3 is a horizontal cross-sectional view of the configuration of the operating deck of the vessel shown in FIG. 1 taken at line 3--3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The vessel shown in the drawing comprises a horizontally continuous wall 1 to support the operating deck 2. The wall 1 may be circular in plan view or multifaceted. The bottom of wall 1 is attached to a pontoon hull 3 of generally circular or multifaceted cross-section and with a similar plan shape to the wall. The operating deck 2, side wall 2 and pontoon hull 3 preferably have the shape of ring-like polygons surrounding a substantially prismatic central opening 15. Said polygons and prismatic central opening 15 preferably have a common substantially vertical axis of symmetry 14. 
     The wall 2 comprises a series of buoyant compartments 4. Between the buoyant compartments are perforations 5 passing through the wall to allow passage of water and waves into and from the central opening 15. These perforations 5 are necessary to reduce the wave loadings and impact forces acting on the wall and assist in reducing hull motions. The number, shape and spacing of the perforations will be designed to suit operational and constructional requirements but a permeability of 30% is typical. 
     The wall 2 is self supporting and does not rely on the deck structure to provide rigidity or require cross bracings. The design of the operating deck may therefore be designed to suit operational requirements and one arrangement shown in FIG. 3 comprises a flat annular deck 6 supported on cantilever beams 7 fixed to the top of the wall. Other deck configurations are possible to suit particular operations, including a fully plated deck spanning the entire central area from wall to wall. A strong box deck can be used to contribute to the overall structural strength of the vessel. 
     The underwater pontoon hull 3 and side wall are divided into ballast tanks 8 connected to pumping machinery to allow the draft to be altered. Internal access to these tanks is provided :or inspection purposes. Tanks in the walls 9 are also used for fuel, potable water and other consumable supplies. The vessel is held in position by catenary mooring lines 10 attached to the side wall and secured to seabed anchors (not shown). 
     The height of the wall is such that the waterline 11 is located approximately half the height of the wall guaranteeing that the operating deck is located well above the waves and that the pontoon is immersed to such an extent that it does not emerge during the passage of the largest waves. 
     The essential feature of all semi-submersibles is a small water plane area symmetrically placed in relation to the axis of symmetry of the vessel. In the semi-submersible vessel according to the invention, the average distance from the side wall 2 to the vertical axis of symmetry 14 is chosen large enough to provide the large water plan inertia necessary t0 produce good stabiliy characteristics. 
     The vessel shown is intended to be used as an oil production vessel and is provided with flexible riser pipes 12 to conduct produced fluids from wellhead to vessel. A hydrocarbon processing plant 13 is situated on the operating deck and separated oil and gas are removed from the vessel either by flexible risers to a seabed pipeline or by tanker. Accommodation for personnel is provided in a deck house 15 separated from the hazardous areas associated with hydrocarbon operations. Surplus gas can be flared using special burners mounted on a tower or boom and a helicopter platform is provided for personnel transfer. Facilities are provided to store the vessel from a supply boat stationed adjacent to the vessel. 
     The embodiment described above and shown on the drawings is merely an example of the invention. The components forming part of the platform may be varied within the scope of the appended claims, which define a structure which is safe with respect to strength and stability, even in damaged conditions. The design permits easy inspection of the underwater parts and is especially suited for use in waters where icedrift is expected. 
     Other modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that this appended claim be construed broadly and in a manner consistent with the spirit and scope of the invention herein.