Stability Analysis Of The North Sea Semi-submersible Drilling Rig

With the continuous exploration of deepwater oil and gas field,As semi-submersible drilling rig has some advantages,such as smaller motion response,deeper water depth,more ability to anti wind and wave,larger deck area and more onboard load,it has been widely used in deepwater oil and gas exploration.The North Sea is now one of the main oil and gas fields and semi-submersible drilling rig is widely used in this area.Because of the hostile environment in the North Sea,sufficient stability is necessary for safety operation of semi-submersible drilling rig.At the moment,Norwegian Maritime Directorate,International Maritime Organization and Det Norske Veritas all have set strict regulationson the stability of semi-submersible drilling rig.Regulations of the three organizations are compared in three aspects as initial stability,intact stability and damaged stability,to study the similarity and difference between each other,and it is found that the regulations of NMD are more strict than that of IMO and DNV.In general,there are two ways to acquire wind heeling moment,one is to calculate based on the formula recommend by IMO,the other is to acquire from wind model test,both methods are acknowledged by class.To guarantee the accuracy of wind heeling moment,the wind model test for the North Sea semi-submersible rig was carried out and the results of two methods were compared.The stability analysis procedure was summarized according to stability regulations.As the intact stability analysis and damaged stability analysis for transit condition,survival condition,operation condition and temporary condition is completed,the curve of allowable vertical center of gravity is extracted.At last,Classify the loads of semi-submersible drilling rig,and design load condition of semi-submersible drilling rig according to loading conditions philosophy and ballasting requirements,all the designed load conditions is checked to fulfilled the requirement of regulations.