A STUDY ON THE VALIDITY OF A ONE DEGREE OF FREEDOM MODEL FOR DEEPWATER SUBSEA LIFTS

The lowering into deepwater phase of the installation of subsea equipment is analyzed by a one degree offreedom model and also by a multiple degree of freedom model in order to determine the scenarios where the simplified model can be used accurately. The one degree offreedom system is based on the assumption that the lifting cable has a length that varies throughout the time as a function of the payout speed. On the other hand, the multiple degree of freedom system is constructed using the commercial software Orcaflex which uses the line feeding feature to evaluate the continuous lowering of the equipment during the operation. Firstly, possible critical scenarios for the use of the simplified model are obtained by evaluating the total time for longitudinal pressure waves to travel inside the cable and by analyzing the resonance frequencies of the system for different water depths, types of cables, and equipment. Secondly, the one degree and the multiple degree offreedom systems are compared. The results show that high water depth and high mass ratio tends to lead to more critical scenarios for the use of the simplified model, especially when the excitation period is low. Further, the dynamics of the system for different payout speeds is assessed considering the most critical scenario for the use of the simplified model. In this case, the differences between the models are not affected by the speed. It is concluded that the one degree offreedom model should not be considered as an accurate method to analyze subsea lifts for scenarios of high mass ratio, high water depth and low excitation periods, independent of the payout speed considered for the operation. Finally, guidance for the selection between both models is provided.