Trenching effects on structural safety assessment of integrated riser/semisubmersible in cohesive soil

• This paper discusses the importance of pipe–soil interaction.
• This study investigates the seabed response for riser/semisubmersible system when subjected to irregular waves.
• Considerable uncertainty in geotechnical parameters is shown.
• Non-linear soil model gives the highest fatigue damage compared to other models.

This paper models an integrated riser/semisubmersible system and subjected to irregular waves. The riser’s pipe is suspended from a semisubmersible and smoothly extending down to the cohesive seabed soils at the touchdown point in a catenary shape. Pipe–soil interaction is modelled using a hysteretic non-linear model in the vertical seabed direction and Coulomb friction model in the lateral direction together with an improved model that includes the breakout soil resistance. Initially, this study discusses the significance of pipe–seabed interaction on the riser response for deepwater applications when subjected to random waves on cohesive clay. In the next step, this study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. The influence of the uncertainty in the geotechnical parameters and the development of the trench in the seabed on the dynamic response are determined. The structural behaviour and the uncertainty of fatigue performance in the touchdown zone are presented. It is noted that the confidence in seabed interaction modelling and geotechnical parameter values is needed in order to have structural safety assurance in the final numerical analysis results.