Simulation stage-based seabed pre-trenching technique for steel catenary riser touchdown fatigue analysis

The development of seabed trench by the steel catenary riser (SCR) touch down zone (TDZ) in its early life can be caused by installation loads, direct hydrodynamic loads and vessel first and second-order motion imposed on the SCR during and after its installation. Several studies have been conducted to investigate the SCR TDZ fatigue response as the excited SCR TDZ progressively trench itself into the seabed, while other studies have investigated the impact of existing trench or pre-trench on the SCR fatigue response. However, most of these investigations were conducted using a series of regular wave loads through quasi-static simulations. Also, though important information on the trench effect on SCR TDZ fatigue response is known in the research domain, little has been said about how to incorporate them in the actual riser design process. This paper (part 1) presents a numerical technique by which pre-trench can be initiated for fatigue response calculations during SCR detailed design analysis. Examples are presented to demonstrate the new approach and how the SCR fatigue response can be calculated in the presence of the created pre-trench. The SCR (after the pre-trenching process) is allowed to respond to the vessel first order six degrees of freedom motions about its nominal position in the presence of the created pre-trench. As demonstrated in this paper, the pre-trenching technique makes it possible to conduct a full time-domain, irregular wave simulations of the SCR in the presence of a pre-trench created using the hysteretic non-linear pipe soil interaction model.