Design of monopile supported offshore wind turbine in clay considering dynamic soil–structure-interaction

• Soft–soft design is likely for 2 MW turbine subjected to rated wind speed.
• The serviceability and fatigue criteria govern the design of 5 MW turbine.
• Small diameter monopile is economical at soft–soft design.
• Large diameter monopile is economical at soft–stiff design.
• Fatigue limit governs the design of offshore wind turbine installed at stiff clay.

This paper addresses the feasibility of soft–soft and soft–stiff design approaches considering a 2 MW and 5 MW monopile supported three bladed offshore wind turbine (OWT) founded in clay. The serviceability limit state and fatigue life of the structure are checked in order to assess the safety. Resonance condition is also avoided keeping the fundamental frequency of the system away from the rotor frequencies. The OWT system is modeled using linear beam and soil–structure interaction is accounted for incorporating American Petroleum Institute based cyclic p–y springs attached to the monopile. Aerodynamic and hydrodynamic loads are applied on the structure and dynamic analysis is carried out using a finite element method in time domain. Overall mass of the structure is examined considering two design approaches in order to obtain an economical design solution. The study shows that soft–soft design is possible for 2 MW OWT subjected to rated wind speed for long tower. Rotor nacelle assembly mass and tower height is found to have governing role in soft–stiff design and on the material consumption. Embedded depth of monopile beyond critical depth has marginal impact on design. Fatigue life is observed to be governing design criteria for OWT at stiff clay.