A Study of Suction and Scouring of Bottom-Sitting Offshore Structures

Abstract

Two sources of difficulty with offshore submersible structures are suction and scour. It seems reasonable to suppose that the shape of the structure will influence the effect of these forces and the degree of damage they will inflict. Suction-breaker and scour tests have been conducted on models of five different shapes, and the results are presented here. Introduction When a submersible offshore structure sits on the soft, cohesive soil of the sea bed, it penetrates deeply into the soil. When the structure is freed from the bottom, a great suction force is created that not only makes the freeing operation difficult, but also can wreck or capsize the structure. If the foundation soil is sandy, tidal currents or waves may cause scour around the legs of the structure, causing it to turn over, or may cause great local stress upon the structural members. The purposes of this investigation were to study the characteristics of suction and scouring, to develop an efficient suction breaker, and to examine a preventive method against scouring. Suction and scouring were investigated in an experimental tank, and the practical application of the suction breaker was field tested using a large model in the sea. Laboratory Suction-Breaker Test Models and Testing Soil To study the characteristics of suction and the effectiveness of a suction breaker, we designed and manufactured five types of small models (Fig. 1) made of steel and coated with paint, and having a bottom area of 219 sq ft. Sea silt taken from the bed of Hiroshima Bay was reconsolidated and used as the testing soil. First the silt was agitated with water in a holding tank. Then a sheet of canvas was laid over the surface of the silt, and it was weighted down by perforated steel plates for 1 week. The pressure was approximately plates for 1 week. The pressure was approximately 1.5 psi. According to a Vane shear test, the mean cohesion of this soil was as follows. Cohesion of the soil applied to the suction test: C = 0.56 psi. Cohesion of soil applied to the suction-breaker test: C = 0.53 pk Method of Testing Suction Test. After the model penetrated the soil about 8 in. by means of loading, it was unloaded and leg alone for about 5 minutes. Then it was pulled up at a rate of 0.08 in./min; the pulling pulled up at a rate of 0.08 in./min; the pulling force was continuously measured by a tension meter the displacement gauge was read by a displacement gauge every 5 seconds. Suction-Breaker Test. After the model had been sunk into the soil by means of loading as shown in Fig. 2, it was pulled up. When it had been pulled up about 0.8 in. (that is, when the contact pressure under the weight of the model was about zero), pressurized water was jetted by the pumping of the suction breaker until the model was completely pulled out. Items measured were the pulling force, the displacement, the pressure, and the flow rate of the pump. JPT P. 279