Arrangement in or relating to drainage

Claim 1

In a fully positioned marine gravity-type structure surrounded by the sea so as to be subject to cyclic loads from environmental forces and resting firmly on the seabed at a predetermined ultimate depth and orientation and at a location where the seabed material is of substantially uniform grain size distribution, continuously available means for counteracting liquefaction in the supporting soil beneath the structure at any desired time by relieving excess pore water pressure arising at any time from cyclic shearload in the supporting soil, said means comprising one or more permeable wells penetrating said supporting soil, a water-tight open-topped tower extending from the seabed to above the water surface, means communicating said wells with the lower portion of said tower, and continuously available means for selectively maintaining the water level inside said tower at a lower level than the water surface so as to maintain a lower hydrostatic pressure inside said wells than outside, and hence relieve excess pore water pressure occuring at any time in the supporting soil about said wells.