Stability of Gravity-Type Platforms by Filling Under the Raft

INTRODUCTION The foundation principle of the gravity-type platform lies in the setting of their base directly onto the sea floor. From the beginning of the studies of these platforms, one is faced with the problem of the contact between raft and sea bed. Moreover, the stability studies pointed out that it was necessary to backfill the water-filled spaces between the raft and the sea bed. In conjunction with Sea Tank Co. (France) we, therefore, developed a special under raft filling method to be used on the Elf Norge operated FRIGG field platform TP 1 in June 1976. THE NECESSITY FUR GROUT FILLING From the beginning, the necessity for grout filling had been obvious, due to the topography of the sea bed. Indeed, it was difficult to admit that the rafts of gravity-base platforms should lie on an uneven sea bed made of hollows and bumps, or "ripple marks". These irregularities could have as consequence that the raft could lie on some too-localized contact zones. The grout filling ensures a better distribution f the loads under the whole surface of the raft and avoids a rearrangement of the upper layers of the ground, under cyclic loading, since they will be blocked up by the grout. A second approach to the stability problem confirmed the necessity for grout filling. The horizontal stability study of the gravity-base platform has shown that skirts under the raft were essential in order to improve the security against sliding of these structures. The skirts, by penetrating into the soil, increase considerably the stability of the gravity-base platforms against the horizontal loading from waves, currents, etc. The skirts girdle the raft base and partition it into compartments, which will be called "skirt compartments". It is these skirts, which by penetrating into the soil, initially secure the platform onto the sea bed. Since their penetration into the soil is not always complete, the raft at the end of the ballasting of the platform is not necessarily in contact with the soil. Therefore, even if the sea floor is level, there can subsist a space between the under face of the raft and the sea bed. This space has to be filled with grout in order to avoid any further sinking of the platform under storm action. There is a third advantage of the grout filling when the sea floor has a slope of approximately 1 to 2 percent. By differential ballasting, the platform can be set down more or less horizontally and can be blocked in this position by the grout. THE GROUT FILLING PROCESS The filling process was submitted to one main constraint. It was essential not to generate any high overpressures in the skirt compartments during the filling.