Parameters influencing seismic assessment of subsea structural systems

Significant improvements in technology include the development of subsea systems capable of retrieving and pre-processing oil and gas at sites located miles away from the host platform. The use of subsea systems at deep water sites in seismic regions has become increasingly common in recent years due to the growing exploration and continuing advances in the construction, placement and maintenance of subsea systems. Seismic events may impact the operability, stability and safety of subsea structural systems. Thus, the focus of this study is to evaluate the response and vulnerability of subsea structures during a seismic event and provide a quantitative sensitivity assessment for various parameters affecting the analysis. An analytical parametric study for a subsea structure and connecting pipe system was performed. The structural configurations of the system, surrounding soil conditions and the expected seismic demands along with the intensity and frequency characteristics of the ground motions are crucial for accurately modeling and predicting the expected structural response during earthquakes. Computer models using non-linear finite element method analysis procedures are developed to assess subsea systems under seismic loads. Sensitivity studies are conducted on parameters such as soil damping, soil-pile interface gapping, and multilevel excitation to simulate the response of subsea structural system. Computer model was generated, and nonlinear seismic response was simulated in CAPFOS software. In the case study, it was observed that the soil radiation damping effects, pile-soil interface gapping and uniform versus depth-varying seismic load application can have considerable effect on the response of the subsea structural systems. The results of this study show that the response of the subsea structural system is sensitive to parameters considered. Therefore, specific design information for a subsea system is critical in making a final assessment of the expected performance under site specific ground motions. This study may help offshore engineers to gain a quantitative understanding of how different parameters influence the results of seismic response of subsea structural systems.