Submerged Towing and Upending of TLP Thethers - Numerical Simulations and Assessment of Operational Limits
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- Institutt for marin teknikk 
In the event of transport, installation and operation of subsea oil and gas factories, marine operations are often a huge part of the total capital expenditure. As a result of this, oil and gas companies are looking for new or improved methods for transport and installation of offshore installations in order to reduce cost and minimize risk. The focus of this thesis is a study of the challenges related to transport and installation of long slender structures such as risers and TLP tethers. The scope of this thesis is to evaluate a submerged tow operation of replacement tethers for Heidrun TLP and propose an efficient method to conduct the upending at site. The thesis work include a further development of Riflex simulation models for numerical simulations of a tow and upending operation. Time domain model simulations are used to conduct parametric study in order to study the behaviour of the system, evaluate the importance of different load effects, establish operational limitations and compare different solutions for the tow and upending. The simulation results indicate that the limiting factor for the tow operation will be with respect to fatigue damage in tether welds. With a operational limit of maximum 10% total fatigue utilization, the results indicate that the tow solution is not feasible for any of the simulatedconditions.ThissuggestsanoperationweatherforecastlimitofOPwf =HS < 1.8m. The main contribution to both ultimate stress and fatigue damage are identified as bending stress and dynamic loading from the vessel motions. It is recommended that measurements should be taken in order to increase operability. Based on simulation comparison of four possible upending solutions, the study suggest a free-drop upending method, with clamp-on weights still attached. The solution appeared to be the most efficient of the simulated cases. The tether seemed to behave in a stable manner and with induced stress well within the operational ULS limit for the simulated weather conditions. The results suggests that the effect of currents is negligible compared to the effect of wave and vessel motion induced loads.