Irregular Wave Simulation and its Impact on Riser Extreme Response for a Production Semi

Abstract

Steel Catenary Risers (SCRs) are widely used in deepwater and ultra-deepwater field developments. The dynamic strength of SCRs is a concern in terms of the global performance. The analysis results are quite scattered in many cases due to the nature of the irregular wave stochastic properties. The widely accepted approach to predict the riser dynamic response in the irregular seas is to run the multiple time domain simulations based on different random seeds. This paper will address the impacts on the predicted riser dynamic response due to the random seeds selection. The discussion is based on the independent engineering verification work for a production Semi project in South China Sea. The site specific irregular waves are usually defined by not only the wave spectrum, but also the properties of individual waves, such as maximum wave height and minimum wave trough, which have big impacts on the riser extreme response. The code recommended approach for irregular wave simulation is based on the linear wave theory, which can ensure the match of the target wave spectrum, for example, Hs, Tp (or Tz), wave peakness for JONSWAP spectrum. But the variation of simulated individual wave properties to the specified value can be significant or there is no specified value to match. The simulated irregular waves based on linear theory is also a distortion to the real wave elevation time trace, such as the asymmetry of the wave crest and trough, especially for the tropical cyclone sea states. Some riser response, such as the compression load at riser touch down zone, can be significantly impacted by the nonlinear nature of the waves and the variation to the target individual wave properties. This paper will discuss the random wave simulation and its impacts on riser dynamic response. A SCR strength design case is presented for illustration in this paper. Key parameters are identified to show the correlation with the SCR dynamic response. The conclusion is finally drawn from the work presented in this paper.