## The soil profile considered in this paper

The soil profile considered in this paper was divided into three layers (soft clay, stiff clay, and hard clay) to account for changes in soil parameters with depth (Fig. 6). Input parameters were based on a specific North Sea offshore site as shown in Fig. 6. Based on the established soil profile and a loading frequency of 0.3 Hz, curves for shear modulus GCK 1026 and damping versus shear strain were established based on equations given in Ref. [23] assuming a density of 2000 kg/m3, over consolidation ratio of 10, and plasticity index of 20 for all layers. In principal, different modulus reduction and damping curves should be used for each layer since modulus reduction depends on confining stress and depth below the mudline. Since the effect of confinement on the modulus and confinement curves is small compared the changes in the shear modulus and shear strength themselves, the same modulus and damping reduction curves have been used for all three layers (Fig. 3). The resulting stress strain curves for the three layers are shown in Fig. 7.