Patent GB-849588-A (GB)
Title: Method and apparatus for anchoring marine structures
IPC: B63B21/50
Filed: 1959-06-19
Abstract: 849,588. Marine platforms for oil-well drilling. CALIFORNIA RESEARCH CORPORATION. June 19, 1959 [June 25, 1958], No. 21153/59. Classes 68(2) and 85. [Also in Group XXXIII] Apparatus for anchoring a marine structure, particularly for off-shore oil-well drilling, comprises a number of floats 26 which are spaced around the structure and are restrained in submerged state by anchor lines 28, 30, the floats being connected by lines 36 to the structure. Lateral movement of the structure during extreme weather conditions, forces the floats, on the side away from the movement, further down in the water and the restoring force is increased. Also, the tendency of the system to develop resonant oscillations is minimised. The anchor lines 28, 30, 36 are buoyed, or made, to be of neutral buoyancy, the lines thus becoming straight in operation, as opposed to forming catenaries. To effect this, the lines may each be a continuous metal tube sealed at its ends and having a wall thickness varying with the depth, or a steel cable may be covered in buoyant material 37, or a cable may be buoyed at intervals along its length. The structure 22 is a hollow column which is floated horizontally into position, up-ended by flooding the lower compartments of the column, sunk in the desired position and weighted by filling the lower compartments with sand or cement. As an example, a column for use in 500 feet of water is about 28 feet in diameter and the anchor lines 36 are attached to it at about 80 feet below the surface. The lines 36 are tightened by winches 42 on the drilling platform 20. The anchors 32 may be of any type, but as shown are sinkable buoyancy chambers which may have bottom projections to sink into the sea bed. The chambers are filled with sand which is pumped out when the anchors are to be recovered for use elsewhere, the sand and air lines being buoyed for this purpose. The lines 30 are attached to the bottom of the column 22 and to eyes 68 on the floats 26, the lines 36 being attached to eyes 66. Each float is spherical and is positioned by hydraulic means controlled from the surface, the anchor line 28 passing through a diametral conduit 70 in which slides a carriage 80 housing gripping jaws 72, 74 pressed down tapered stationary surfaces by springs 82, 84, anti-friction rollers 90 being provided. The jaws 72, 74 are connected by rods 92 to simultaneously acting pistons 94 which are controlled from the surface or by a diver through valves 106, 114. When the pistons are actuated to move the carriage 80 outwardly from the centre of the float, the grippers 72, 74 bite on the cable and move the float to a new position along the cable. The float is held in this position by grippers 118, 120 actuated by hydraulic pistons 122 controlled by the valve 146, or from the surface. The pistons 94 are now moved in the opposite direction to pull in the carriage and release the grippers 72, 74 to prepare them for another grip on the cable and another movement of the float on the cable, the grippers 118, 120 being released to permit this. When the float is correctly positioned, the carriage 80 rests against the abutment 154 and the tension in the line 28 keeps the grippers functioning. When the float is adjusted by a diver within the float, air is supplied through a line 156 and access is obtained through a door 158. Packings 160, 162 prevent water from entering the conduit 70. A line 168 supports the float from a derrick barge during the installation. The sequence of operations during the installation is described. In Fig. 9, a conventional positive buoyancy tied-down floating structure 202 is anchored by a number of radiating systems each comprising two anchors 220, 222 joined by lines 216, 218 to a member 238 on the float 212, the line 232 being trained around a pulley 234 for tensioning by a winch 236. Buoyed lines 240, 242, 244 serve for the recovery of the anchors 220, 222 and the floats.