Abstract: An earthquake-resistant off-shore gravity structure which can stand on the sea bed under its own weight and the weight of any ballast carried by the structure has coupled to its undersurface a separately formed substantially flat concrete foundation grid which underlies at least a major part of the area of the undersurface of the structure, which is of a thickness substantially less than the height of the base and which has a downwardly extending projection for penetrating the sea bed. The coupling between the structure and the foundation grid is such that vertical loads on the structure are transferred through the foundation grid to the sea bed and is such that, for horizontal forces on the structure less than a predetermined maximum value, relative movement between the foundation grid and the structure is substantially prevented and, for a horizontal force equal to or in excess of said maximum value, limited relative movement between the foundation grid and the structure is permitted.

Abstract: The base of an offshore gravity platform consists of at least two separately formed caissons, preferably of concrete, which are mounted one on top of the other or another. The roof of the lower of adjacent caissons and/or the bottom of the adjacent upper caisson is or are so shaped as to form between the caissons a space which is rendered substantially fluid-tight and wherein, when the platform is floating on the sea and when the platform is standing on the sea bed, fluid pressure in the space between the caissons is such that the two caissons are urged tightly together by hydrostatic pressure. Adjacent caissons may also be mechanically secured together.