The objectives and proposed approach for a joint industry study to investigate the problems associated with jack-ups manoeuvring on and off location are described. The study involves assessments of appropriate wave statistics, jack-up motions, seabed response, dynamic impact analyses, and structural evacuation. The results are expected to be harnessed through an on-board PC-based unit.
Results are reported from the motions analyses, soil-structure interaction, end 2-D impacts of the forward leg of a widely used rig onto clay, sand and rock. Conclusions drawn from these individual studies are presented together with a summary of progress on the remaining parts of the project.
Currently, the responsibility for deciding whether or not to move a jack-up onto or off a particular location is taken by the drilling contactor's marine supervisor on board the rig. Undoubtedly, in reaching a decision, the opinions of the barge engineer, tool pusher, operator representative and marine surveyor are all taken into consideration. The decision is based on the practical experience of the personnel and on the limited technical guidance given in the operating manual.
Typically, the operating manual states that the limiting environmental condition is a 5 ft sea. A critical roll/pitch motion curve is sometimes given as a function of water depth. Such a curve is of little use when moving off location. It is also of limited use when moving onto location since it ignores other environmental factors such as current and wind but, more particularly, takes no account of the nature of the seabed, nor of any heave motion. Clearly, different consequences for the rig will result from moving onto soft clay compared with dense sand and hard clay. Touching down on rock will be even more onerous but this aspect is ignored completely in the advice formally offered to the supervisor.
The reason for providing advice when moving onto and off Iocatlon is to minimize the possibility of damage occurring to the legs or jack-houses. Structural damage can occur to these in the event of an unexpectedly severe touch-down.
In an effort to provide more appropriate guidance in such circumstances, the joint industry project described in the next section was initiated. It is now complete although only part of the results can be presented at this time. The study is being complemented by a further investigation concerned with jack- UPS manoeuvring onto and off location in close proximity to a fixed installation. This is receiving separate funding but the results from both studies will be exchanged.
The basic criterion which is to be satisfied is that the risk of exceeding the structural capability of any part of the rig during initial touch-down or lift-off is not unacceptably high. To satisfy this criterion objectively, it is necessary to model these events in some detail. The elements of the approach are illustrated in Fig. 1 and are as follows.