A Reliability based Design Method for Offshore Wind Turbine Tower Foundations

Foundations for offshore wind turbine towers can be grouped into two types: (a) Grounded systems and (b) Floating systems. Both are subjected to complex loading regimes, including static forces from self- weights of structural components, cyclic and dynamic wind loads, forces due to aerodynamic imbalances, blade shadowing effects, wave impact forces, and seismic actions. Improper consideration of the dynamic soil-structure interaction in designing the foundations can lead to structural and geotechnical failure.

For monopile grounded systems, Carter and Kulhawy (1992) presented closed-form equations that determine whether, under the lateral loads, the pile would fail in the ground as a rigid body or in plastic hinge mode. For a suction caisson foundation, Randolph and Gourvenec (2011) and Supachawarote et al. (2004) presented a failure criterion involving both the lateral and vertical load capacity and soil-pile properties. For an anchor foundation incorporating a catenary support of the turbine tower, Randolph and Neubecker (1995) developed a set of equations for determining the anchor pad-eye tension and the catenary tangent slope at the anchor point. These equations allow load transfer from the mud line to the anchor to be calculated. The resulting anchor load considers load reduction on the inverse catenary forming at the anchor, due to both the load magnitude and the angle at which the load is applied changing through the inverse catenary shape at the anchor. The above theories and methods, although helpful in offshore foundation design, do not include reliability analysis. Hence there is need for a new approach which can establish a series of partial safety factors appropriate for a particular design for a target reliability level of a structure. This is the goal of the proposed research. The deliverable objectives of the work are:

(i)     To review the current methods of structural design for offshore wind turbine structures

(ii)    To analyse the geotechnical aspects of the design methods, with respect to both soft and stiff marine strata.

(iii)   To develop a reliability design-based method for offshore wind turbine foundations, including both structural and geotechnical performance.

The methodology of the research involves collecting data from case records of offshore structures or using laboratory tests on physical model foundations and applying the COSSAN software to carry out structural reliability analysis by considering the uncertainties of the design variables which a deterministic analysis cannot do. The following outcomes of the reliability analysis are envisaged: safety index, probability of failure, sensitivity factor and partial safety factors.

The research will lead to development of a guidance note for use by engineers to optimize design through performing reliability analysis of wind turbine tower foundations.