Effect of Applying 2nd Order Wave Theory on Pipeline Dynamic Response

Abstract

Ensuring subsea pipelines on-bottom stability by determining the stabilisation requirements which will limit pipelines movement under extreme waves and currents is an essential aspect of subsea pipelines design. These requirements can be a major project cost driver in some locations around the world, where the designer is faced with severe metocean conditions. This is particularly the case when the selected design solution is associated with costly stabilisation requirements such as trenching, anchoring [14], rock dumping, or mattressing. An appreciation of the pipeline structural response, when exposed to waves and steady currents kinematics is fundamental to optimise the stabilisation solution. An advanced approach used to optimise stabilisation requirements is to use transient dynamic finite element analysis. The analysis is used to simulate the dynamic response of subsea pipelines exposed to near-seabed kinematics, due to a combination of steady currents and waves. Wave kinematics at the seabed are therefore an essential input to the analysis and will significantly affect both the hydrodynamic loads on the pipeline and the pipeline response. The typical method for generating the wave kinematics in a dynamic analysis has been based on calculating the near-bed velocities corresponding to a randomly generated seastate, using linear wave theory. It has been acknowledged that this calculation is likely to produce a conservative estimate of the positive wave velocities. An improved prediction of seabed kinematics can be achieved by using higher order wave theories. Application of higher order wave theories, results in changing the velocity magnitude under wave crests and troughs. This change in kinematics may result in a change of pipeline response. This paper investigates the effect of using 2nd order wave theory for predicting the kinematics on the pipeline dynamic response. Dynamic finite elements analysis is used for determining the pipeline response and to compare the pipeline response when using 2nd order wave theory and linear wave theory. The work presented in this paper was commissioned by Woodside and performed by J P Kenny Pty Ltd.