A Study on the Developmnet of Leg Penetration Monitor System of Jackup Rigs

Abstract

A number of cases have been reported worldwide, where offshore mobile jack-up rigs, whose platform is brought up and down along the legs resting on the seabed, had their decks sharply inclined due to unexpected penetration, viz, punch-through, of the legs into the sea floor. Particularly, the recent marked trend toward heavier jackups being built to cope with deeper and harsher seas of operation calls for effective countermeasures against such sudden penetration of jackup rig legs.
This paper reports the results of a study on the development of two preventive measures against offshore disasters likely to take place from sudden punch-through of legs. These countermeasures conceived are Strata Data Monitor System (SDMS) and Vertical Position Monitor System (VPMS). The SDMS permits to precisely know the properties of seabed soil by means of a device, which, attached to the rig in itself, surveys the seabed nearby or immediately beneath the leg footings (spud cans), to evaluate the safety of the sea floor where to place the legs, and further to suggest safe preloading procedures. The VPMS acquires, monitors and records data on the vertical position of the platform with respect to the sea level and that of the footings with reference to the sea floor, during the operations of jacking up and down and preloading.
The two systems newly developed are altogether referred to in this paper as Leg Penetration Monitor System (LPMS), which is intended for safe execution of jacking up and preloading operation.
In conjunction with the System development, scale model experiments of punch-through were carried out in a laboratory for the purpose of evaluating the occurrence and the effects of punch-through phenomena. Prototype experiments of the SDMS and the VPMS were also conducted on land and in waters to evaluate their performance, respectively.