Punch-through and Suction of Jack-up Rig Footing

Abstract

The jack-up rig is set on the seabed and supported on the seabed ground. There are many problems arising from the seabed ground (such as penetration and punch-through of footing, liquefaction of supporting sand foundation due to earthquake and wave, scouring due to wave and current, suction force on bottom being freed, etc.), which are extremely important in connection with the basic planning and the operation of a jack-up rig and might have been the major cause of its accidents.
Of these problems, in this paper, an analysis method of punch-through of footing and a method for reducing suction force are discussed.
The analysis method of punch-through of footing is first described. It was used in analyzing an actual example in certain waters based on the local soil and load conditions and the results were compared with the actual values. The main conclusions reached are: (1) an analysis method of punch-through has been proposed, and (2) an actual example of punch-through was analyzed by this method based on the actual soil and load conditions, resulting in good agreement with the actual values.
Next, a natural inflow suction breaker was used as a means for reducing suction force. The footing is installed with connection pipes that are allowed to naturally run sea water existing over the footing top therethrough under the suction produced on the bottom being freed, thereby reducing such suction force. A reconsolidated sea-silt ground was prepared in a soil tank. An about 1/20-scaled footing model was placed thereon to penetrate into it for a bottom freeing test. In this test, the suction force, the amount of pullout of footing, the amount of water flow through the suction breaker and the suction on the footing bottom were measured as changing the location of the suction breaker nozzles, water depth and footing pullout speed. The main conclusions obtained in this test are: (1) it is desirable to arrange the suction breaker nozzles evenly around the area close to the footing edge, (2) the relation between the efficiency of the suction breaker and the inflow rate of water into the void under the footing is almost proportional, and (3) the efficiency of the suction breaker has an optimum pullout speed.