Construction method for fixing hollow column for supporting marine structures and offshore platforms to a seabed

Abstract

A construction method for fixing a foundation of a waterborne structure including an offshore platform to a seabed ( 2 ) is described. The foundation includes at least one hollow column ( 108 ) which incorporates a pile cap ( 43 ) at its bottom. In the method, piles ( 49 ) are driven into the seabed ( 2 ) to a design depth, and the hollow column ( 108 ) is then lowered to the seabed ( 2 ) so as to surround the pile group and be supported thereby. Concrete is cast into the bottom of the hollow column ( 108 ), wherein reinforcement steel bar ( 45 ) has been pre-installed to translate the conventional mass concrete plug into a reinforced concrete small pile cap ( 43 ) adapted to support the hollow column ( 108 ) thereon during construction. The hollow column ( 108 ) is de-watered, excessive pile length is cut, and pile cap reinforcement ( 47 ) is fixed followed by concrete casting of the pile cap ( 44 ). A platform deck is then installed and constructed on top of the now-fixed hollow column ( 108 ).

1 . A construction method for fixing a foundation having a hollow column adapted to support a waterborne structure thereon to a seabed, comprising:
 driving a plurality of steel tubes into the seabed at an installation location for the hollow column, each of the steel tubes having a free length above sea level,   lowering the hollow column down to the seabed so as to be supported on the steel tubes, the steel tubes arranged within the hollow column,   injecting underwater concrete between the seabed and the hollow column bottom to form a concrete plug,   dewatering the interior of the hollow column,   cutting the steel tubes above a level of a pile cap to be formed or at a determined design level,   modifying the pile head so as to be fitted with lapping steel bars integrated into a pile cap reinforcement cage,   fixing the steel reinforcement of the pile cap, and   injecting concrete to cast the pile cap and complete the foundation.   
     
     
         2 . A construction method for fixing a foundation having a precast, pre-stressed, reinforced concrete hollow cylindrical column adapted to support an offshore marine platform thereon to a seabed, the offshore platform adapted to support wind turbines, bridges and marine buildings thereon, the method comprising the steps of  claim 1 . 
     
     
         3 . The method of  claim 1 , wherein the steel tube is embodied as a steel pipe pile or a steel casing if the pile is a concrete bored pile. 
     
     
         4 . The method of  claim 1 , further comprising:
 fitting the top of the hollow column with a steel frame resting on a temporary pile head jacking prop fixed to the top of a corresponding steel tube, each prop having one or more level adjusting jacks.   
     
     
         5 . The method of  claim 4 , wherein, prior to injecting concrete into the bottom of the hollow column, the method further comprises:
 adjusting level and verticality within a tolerance using the jacks installed at the top of each steel tube.   
     
     
         6 . The method of  claim 1 , wherein, prior to injecting concrete into the bottom of the hollow column, the method further comprises:
 inspecting between the bottom end of the hollow column and the seabed to look for gaps, and   filling any discovered gaps with sand gravel if the gap size exceeds a pre-determined limit   
     
     
         7 . The method of  claim 1 , further comprising:
 fixing reinforcement steel bars exposed from the steel tubes to the bottom of the hollow column at an expected concrete plug formation zone, the reinforcement steel bars serving as reinforcement in the to-be-formed concrete plug after concreting the bottom of the hollow column.   
     
     
         8 . The method of  claim 1 , wherein a diameter of the bottom part of the concrete plug is larger than that of the hollow column. 
     
     
         9 . The method of  claim 1 , further comprising:
 employing a plurality of shear keys to increase a contact area and bonding between the steel tubes and concrete plug.   
     
     
         10 . The method of  claim 9 , wherein the shear key is configured as an upside down, triangular-shaped steel plate. 
     
     
         11 . The method of  claim 1 , further comprising:
 adding skirting to an end of the bottom of the hollow column to increase bearing area and prevent the underwater concrete from escaping sideways.   
     
     
         12 . The method of  claim 11 , wherein the skirting includes a cantilever plate and a downward skirting board with a plurality of stiffeners adapted to strengthen the cantilever plate, the skirting bolted to the end of the hollow column and having a diameter greater than that of the hollow column. 
     
     
         13 . The method of  claim 11 , wherein the skirting includes a steel ring with teething that is bolted to the end of the hollow column and with a diameter equal to the hollow column. 
     
     
         14 . The method of  claim 11 , wherein the skirting includes a plurality of L-shaped skirting plates fixed by bolts to the end of the hollow column and having a diameter greater than that of the hollow column, the short arm of the L in each plate pointing downward and the long arm of the L in each plate level. 
     
     
         15 . The method of  claim 1 , wherein the plurality of steel tubes consist of four tubes. 
     
     
         16 . The method of  claim 1 , wherein the hollow column serves as a cofferdam. 
     
     
         17 . A foundation having a hollow column adapted to support a waterborne structure thereon, the hollow column to be fixed to a seabed according to the method of  claim 1 . 
     
     
         18 . A method for fixing a foundation adapted to support waterborne structures such as an offshore marine platform, the foundation having one or more hollow columns for supporting the offshore platform thereon that are to be fixed to the seabed, comprising:
 driving a plurality of piles into the seabed to the founding level,   lowering a hollow column down to the seabed so as to surround the piles,   forming a reinforced concrete plug in the bottom of the hollow column, the hollow column serving as a cofferdam,   dewatering the interior of the hollow column,   fixing pile cap reinforcement, and   casting concrete to form a pile cap for the upper ends of the piles.   
     
     
         19 . The method of  claim 18 , wherein the piles are configured as steel pipe piles. 
     
     
         20 . The method of  claim 18 , wherein a diameter of the bottom part of the concrete plug is larger than that of the hollow column. 
     
     
         21 . The method of  claim 18 , further comprising:
 employing a plurality of shear keys to increase a contact area and bonding between the steel tubes and concrete plug.   
     
     
         22 . The method of  claim 21 , wherein the shear key is configured as an upside down, triangular-shaped steel plate. 
     
     
         23 . The method of  claim 18 , further comprising:
 adding skirting to an end of the bottom of the hollow column to increase bearing area and prevent the underwater concrete from escaping sideways.   
     
     
         24 . A construction method for waterborne structures including offshore platforms supported by at least one hollow columns adapted to support a beam-and-slab platform deck as part of the offshore platform, comprising:
 fixing the hollow column to the seabed as recited in  claim 18 , and   continuing installation and construction of the offshore platform from the already fixed hollow columns.