Fatigue And Crack Propagation Research Of Deepwater Top Tensioned Risers

With the dramatic increase in human society’s demand for energy, the exploration, development and utilization of marine resources is rapidly going into to the deep sea region. Top Tensioned Risers are essentially conductor pipes connected between floaters on the surface and the subsea facilities at the seabed that can be used for drilling, well completion and production. They are important parts in the deep-sea oil and gas resource development system. The marine environment in which risers are working is very complex, affected by wind, waves, currents, tides, earthquakes, sea ice and the floating movement. Therefore, the changing load easily lead to fatigue damage for risers. In addition, a considerable part of the riser is welded structure with welding defects, hence the crack propagation is an important factor to consider in the riser fatigue life analysis.This paper studies the fatigue and crack propagation of deep-water Top Tensioned Risers, including the following work:1. Commonly used methods for marine engineering structure analysis are described:The method based on the S-N curve and the theory of Miner linear cumulative damage and the method of fracture mechanics based on the Paris formula.2. Commonly used linear wave theories and the basic equations of the Stokes wave theory and the derivation process are described, and then the Morrison equation was used to calculate the wave load on the riser. The selection of the wave theory, the determination of the drag coefficient and quality coefficient are discussed.3. Dynamic responses of Top Tensioned Risers under the waves, currents, the platform movement and other complex marine environmental load and boundary conditions are researched. Further more, the fatigue life of the riser is calculated with different SN curves based on the Miner linear cumulative damage theory. The location with the largest fatigue damage rate of the riser is4meters below sea level.4. The crack propagation for defects lack of fusion with oval shape and circular shape are researched based on the Paris crack extension model. In detail, the crack length and propagation length, the crack area and the corresponding crack propagation life are given.