Nonlinear Time-domain Analysis Of Tube-in-tube Structure

The marine riser is the key component of offshore oil drilling system. The tube-in-tube structure is widely used in TTR system for its special features. In the present specification, the equivalent pipe model is used for the global strength analysis of tube-in-tube structure, then individual tube strength is checked based on the global strength analysis. But there are some flaws in this analysis, in the tube-in-tube structure, when the deformation of tubes are different, the equivalent model can't simulate the interaction between tubes, which lead the simulation result and the real situation are quite different. Based on the National 863 project "Top Tensioned Riser design and installation of key technology research" project, this thesis evaluated the dynamic response of the tube-in-tube structure, which is used for TLP, in a specialized environment.In this thesis, tube-in-tube structure is simulated in ABAQUS/AQUA, in a specialized environment, a reasonable centralizer distribution is determined, then four different top tension distributions are calculated, and the equivalent stress, bending moment, displacement of each tube and the centralizer force are compared. A new allocation plan of effective tension of inner and outer tube has been presented after the calculation. Then the effect of TTF, clearance of tubes, current and wave, and so on has been evaluated.According to the analysis, the distribution of top tension and TTF has a distinct effect on the dynamic response of tube-in-tube structure. For inner and outer tubes, the equivalent stresses are sensitive to the distribution of top tension. The lateral deformation of riser can be diminished, and the interaction of tubes can also grow down with the increment of TTF, at the cost of a high stress level. An adaptive allocation scheme of TTF and distribution of centralizer should be considered in a real project. Although the clearance of tubes, current and wave all have some effect on the dynamic response of tube-in-tube structure, the effect of wave is limited to the wave splash zone, and the current has an overall influence along the riser, while clearance of tubes has the tiniest influence.