23-12-2019

Seabed Remediation avoiding critical Spudcan-Boulder Interaction during jack-up vessel installations in offshore wind

L. Kellezi*, S. S. Sundararajan & K. H. Lundvig Geo, Copenhagen, Denmark

Abstract

Jack-up vessels are often used for the installation of wind turbine generators (WTG), or their foundations etc., at different offshore wind farms (OWFs). In this framework, for a six-legged jack-up vessel, to be installed at a relatively small OWF, seabed survey, soil investigation and engineering are carried out. Due to hard seabed conditions and existence of a large number of boulders (at the seabed and sub seabed) in the areas around the WTGs, the locations generally posed a significant risk for spudcan - boulders interaction during vessel installations. Considering the seabed soil conditions (interpreted from the boreholes (BHs) and laboratory tests), consisting of sand till layers with varying thickness, overlying limestone, after removing the large boulders, design and construction of the gravel pads (GPs), (covering the boulders at the spudcan locations), was carried out, remediating the seabed. Based on the spudcan dimensions and rectangular shape, the GPs were designed as truncated pyramids with height = 1.5 m and slope 1:2 for all 6 legs and all WTG locations and top dimensions (11.8 x 13.2) m and (9.8 x 11.2) m, respectively for outer and middle legs. The design was mainly based on Plaxis finite element (FE) 2D plane strain analyses documenting the integrity of the GPs for the preloading phase and elevated conditions.