A number of cases have been reported worldwide, where offshore mobile jackup 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 which finally led to the successful 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 cane), 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 cafe execution of jacking up and preloading operation. The LPMS allows for monitoring of jacking operations also by other personnel than the jacking operator, through the CRT screens located in various places aboard the rig. The data acquired will serve as backup material for subsequent review of rig operations as well as for the purposes of simulation training of operational crew. In parallel with the system development, scale model experiments were conducted in a laboratory for the purpose of evaluating the occurrence and the effects of punch-through phenomena.
It is reported that some 20% of major jackup rig accidents occurred have been caused by some sorts of leg troubles including punch-throughs (P-T) of footings. Once a P-T happens, it likely causes damage to the leg structure and/or jacking units, or it even threatens total loss of the rig, which directly relates to the safety of human life, inasmuch as the average accommodation capacity of an offshore jackup rig today totals nearly 100 people. Not only that, for the oil company and drilling contractor, the resultant suspension or eventual abandonment of the drilling operation poses even more serious problem than just the physical damages of the rig.
Moreover, jackup rigs are becoming each time larger and heavier according as the offshore oil and gas development goes into deeper and severer waters. Some jackups have found application as production platforms, staying on one specific point for a prolonged time period.
