Equivalent linear soil stiffness in fatigue design of steel catenary risers

For the development of the oil and gas fields in deep water, steel catenary risers (SCRs) are considered a versatile riser system option. However, fatigue design of SCRs in the touch down zone is an engineering challenge considering the complexity of the seabed-riser interactions. Advanced nonlinear interaction models have recently been developed for fatigue design of SCRs. However, the use of these nonlinear models tremendously increases the computational costs. Traditional fatigue design procedures for SCRs are based on linear soil models for the seabed and though these are simple, they usually lead to conservative results. The main aim of this study is to investigate the concept of using “equivalent linear soil stiffness” instead of the complex nonlinear seabed-riser interaction model, for the early stages of fatigue design of SCRs. The procedure is examined using two example SCRs in different water depths. The structural responses of the systems with nonlinear and equivalent seabed models due to regular vessel motions are compared first and then the contributions of some key input parameters for evaluation of the equivalent linear stiffness are investigated. In addition, an attempt is also made to propose a simplified formula for the equivalent linear soil stiffness for the SCR systems.