4.1.1. Substructure

The jacket substructure and its appertaining wave loading were modeled using ROSAP (Ramboll Offshore Structural Analysis Programs), version 53 [62]. The jacket substructure considered in this study, which is depicted in Figure 3, has a total height of approximately 75 m. The substructure comprises three legs, each with a diameter ranging between 1.2 and 1.7 m, and four brace bays, each with a diameter ranging between 0.8 and 1.1 m. The substructure model includes, i.a., soil-pile interaction, local joint flexibility, scour, marine growth and appurtenance masses. The water depth is 55 m and the soil conditions are characterized as clay. The substructure includes 50 m grouted piles. The soil-structure interaction is modeled by the use of soil curves linearized according to the API method [63]. The structural damping was modeled according to a Rayleigh model [64] with 0.5% and 1% modal damping in the first and second bending modes, respectively.

**Figure 3.** Substructure model used in the case studies. (**A**) Side view, (**B**) side view with indication of levels (blue circles indicate joints analyzed in the case studies) and (**C**) top view with indication of directions, side and leg names. NB: a wind turbine model is not shown in the figure.

The locations of the selected joints considered in the case studies are indicated in Figure 3B. The joint levels range between 13 (mudline) to 50 (top of the jacket). Results for sides B and C of the jacket, see Figure 3C, are provided. The joints are named in the following way: 50CL, where 50 indicates the level, C is the jacket side, and L indicates the lower element in the joint.
