Electro-thermo-mechanical Coupling Analysis Of Deep-water Strong-current Composite Umbilical Cable

Umbilical cable is the “nerve lifeline” connecting subsea production system and floating structure which plays an important role.With the advancement of deep-water marine oil and gas exploitation and the use of subsea high-voltage equipment,strong-current composite umbilical cable has emerged.When strong electric energy is transmitted through the umbilical cable,it will generate a large amount of heat,which will increase the internal temperature of the umbilical cable and cause thermal stress.Meanwhile,the mechanical properties of the umbilical cable will be further affected by the coupling effects of environmental load and functional load.Therefore,it is necessary to analyze the electro-thermo-mechanical coupling effects of deep-water strong-current composite umbilical cable.This thesis first takes the submarine single-core cable and three-core cable as examples to introduce the calculation methodology of the temperature field and the cable carrying capacity,establishes the thermal equivalent circuit model of the cable,and then deduces the temperature formula of the insulating layer of single-core cable and the temperature formula of the copper conductor of three-core cable.To verify the accuracy of the formula,finite element software COMSOL Multiphysics is used to establish the two-dimensional cable model,and it turns out that the temperature distribution under the formula is highly coincided with that under the simulation model.Then,the electro-thermal coupling effects of the high-voltage composite umbilical cable under three circumstances(i.e.,one being I-tube area,one being submarine buried area and another being seawater area)are analyzed.Under the first circumstance,the temperature field and current-carrying capacity of the umbilical cable in the closed and ventilated I-tube is compared,which shows that the current-carrying capacity of umbilical in the ventilated I-tube can be strengthened significantly.Under the second circumstance,the electro-thermal coupling effects of the umbilical cable in the submarine buried area is evaluated,which shows that the change of the cable temperature is linearly related to soil thermal conductivity and current while the current carrying capacity increases linearly with the soil thermal conductivity.Under the third circumstance,the results show that the seawater depth has little or no effect on cable temperature and current carrying capacity.Finally,the mechanical properties of the umbilical cable under mechanical separate load and thermo-mechanical coupling load respectively are compared,the stress of the umbilical cable under the coupling of temperature field load and tensile load is mainly contributed by the thermal stress caused by the temperature field generated by electro-thermal coupling,while the stress subjected to temperature field load and bending and torsional load is the coupling of thermal stress caused by the temperature field and bending stress and torsional stress.After the coupling analysis,the tensile stiffness of the umbilical cable reduces substantially,the bending stiffness increases slightly,the torsional stiffness remains the same,and the mechanical properties of the umbilical cable are reduced compared to the mechanical properties under mechanical separate load.Therefore,the electro-thermal-force coupling analysis of umbilical cable has a great influence on the temperature field distribution,stress analysis and stiffness analysis of umbilical cable.Multi-field coupling analysis is an important part of umbilical cable design.