Hydraulic and earth engineering – Marine structure or fabrication thereof – Cast in situ
Patent
1984-06-05
1987-08-25
Taylor, Dennis L.
Hydraulic and earth engineering
Marine structure or fabrication thereof
Cast in situ
405223, 405203, 405195, E02B 1700
Patent
active
046889671
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an offshore platform structure comprising a deck superstructure, a substructure and a base structure having in general a polygonal, preferable a triangular shape. The substructure consists of at least three lower supporting columns which are positioned apart and extend upwardly and inwardly from the base structure to an intermediate structure. The substructure consists further of one or more upper columns extending upwardly from the intermediate structure, supporting the deck superstructure. At least the base structure and the substructure are made of reinforced concrete.
The present invention relates further to means for slip-forming, intended for slipforming the supporting columns as described above, comprising a slipform platform provided with slipforms which is supported by and may be lifted or lowered by means of a system of climbing rods rigidly supported by the cured portions of the previously cast column, a downwardly extending steering tower which is pivotally/rotatably fixed to the slipform platform, extending in general parallel to the previously cured portions of the column and a steering wagon fixed to the steering tower. The steering wagon is provided with roller or sliding means equally spaced in vertical and circumferential direction, said roller or sliding means being in contact with inner wall of the column and means for adjusting the inclination of the steering tower with respect to the steering wagon and consequently with respect to the previously cured portions of the column. The slipform is suspended from the slipform platform by means of a plurality of yokes and comprises an inner and outer, in general cylindrical or elliptical wall formed of form sheets arranged around the periphery of the slipform platform.
Offshore platform structures as described above are well known in the art, for instance from Norwegian Patent Specification No. 135,677. Means for slipforming inclined, hollow columns of concrete for such offshore platform structures is for instance known from Norwegian Patent Specification No. 137,559 and British Patent Specification No. 1,512,078.
Construction and towing of an offshore platform structure to the offshore location is performed with the platform structure in an upright vertical position. During the towing phase it is desirable with as large draught as possible, since such large draught from a buoyancy and stability point of view is beneficial for the design of the platform structure. From the stability point of view it is advantageous having large buoyant volumes arranged high up in the structure. The optimal towing draught would be a draught corresponding to the water depth at the offshore location less the required practical clearance between the sea bed and the structure. Due to limited water depths along the towing route the towing draught will be substantially less than the water depth at the offshore location, for locations with depths exceeding approximately 300 meters. For a platform structure, for instance intended to be installed on locations having 400 meters depths, more than half the platform structure will project above the sea level during the towing phase. If, in addition to the concrete substructure, a deck superstructure is installed on top of the columns, the stability of the entire structure will be seriously affected. Norwegian Patent Application No. 80,2268 discloses a method of improving the stability by means of incorporated secondary buoyancy tanks.
For an offshore platform structure described above, intended to be installed at locations having large water depths, for instance 400 meters, the intermediate structure will be positioned above the towing water line during the towing phase. This implies that only the lower supporting columns provide the required buoyancy and stability.
The object of the present invention is to arrange a larger part of the buoyancy at a higher level in the structure, in order to improve the stability. The object is achieved by providing the lower supporting columns of the platfo
REFERENCES:
patent: 3456447 (1969-07-01), MacKintosh
patent: 4002038 (1977-01-01), Phares et al.
patent: 4043138 (1977-08-01), Stageboe et al.
patent: 4256417 (1981-03-01), Bohannon
Einstabland Tomas
Opedal Torlak
Ingenior F. Selmer A/S Norway
Taylor Dennis L.
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