Simple Method for Computing Nonlinear Foundation Stiffness and Damping-Paper

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The paper proposes a simple way to estimate the nonlinear rocking stiffness and damping of skirted offshore foundations from soil stress–strain and damping–strain data, avoiding detailed 3D finite-element analyses during early design.<br /><br />Finite-element simulations examined 15 uniform soil profiles and foundations of two diameters. For uniform profiles, the normalized reduction in foundation rotational stiffness closely followed the soil’s modulus-reduction curve, while foundation damping followed the soil damping–strain curve. In both cases, soil shear strain can be mapped approximately one-to-one to foundation rotation. The relationship was largely independent of foundation diameter. Initial elastic foundation stiffness must be estimated separately, using analytical solutions for relatively uniform soils or finite-element analysis where stiffness varies substantially with depth.<br /><br />Preliminary analyses of layered soils suggest the method works when the relevant soil layers have similar nonlinear response. Where layers have markedly different strength or stiffness, foundation behaviour may reflect an average of their curves or be governed by the softer layer. The general scaling relationship for strongly contrasting layers therefore remains uncertain.<br /><br />The approach offers a rapid, practical basis for estimating foundation response using site-specific soil data and may reduce reliance on time-consuming nonlinear analyses. Further work is needed to validate the method against physical tests, assess strongly layered profiles, and establish scaling relationships for horizontal and vertical response and combined loading. The authors also recommend careful characterization of soil stiffness and damping at the small strains relevant to offshore fatigue and wind-turbine design.

Keywords

nonlinear rocking stiffness

foundation damping

skirted offshore foundations

soil stress-strain

damping-strain data

finite-element simulations

modulus-reduction curve

rotational stiffness

layered soils

offshore wind-turbine design