Enhances design of offshore wind energy gravity based structure foundations

Abstract

The offshore wind is set for substantial growth globally in the coming decades. The use of Gravity-based foundations (GBF), although only 3.3% of the current installed fleet is due increase to 8.4% in the coming years. To meet this increased demand, there is a need to examine all areas of design to see where efficiencies can be achieved. The central question to this thesis is to explore if efficiencies can be achieved in the design of gravity-based foundation (GBF) for offshore wind turbines and if a reduction in quantity of material required to ballast the structure is possible. It does this through setting out an alternative approach to calculation the roughness parameter (r), applying this in lab experiments, carrying out a worked example to quantify ballast material savings and carrying out stability checks on the GBF in a 3DFE analysis. The refined approach used was to apply the definition of the roughness coefficient (r) for cohesionless soils was to the problem i.e. the tan of the interface friction angle (tan δ) divided by the tan of the soils internal friction angle (tan φ'). Lab testing was carried to out to obtain tan δ and tan φ' for a range of soil types and interpret an (r) value a smooth and serrated based bottom. An analysis was carried out to compare the pre-2014 DNV method of calculation (r) to the suggested refined approach and quantifies potential ballast material savings. The results showed how a 29% saving in ballast can be achieved; the minimum required ballast weight using this approach is reduced from 26.993 kTonne to 19.081 kTonne (approx. 8 kTonne). It also provides likely savings in ballast required for a range of seabed conditions (fine grained SAND to sandy GRAVEL). It concludes sliding capacity still governs design for friction angles below 43°; above this value overturning is the governing factor and no ballast savings can be achieved. A 3D finite element stability analysis was carried out on the GBF in Plaxis 3D. Analysis showed that with increased loading the vertical displacement increases; a 20kTonne results in a 50mm, 30kTonne equating to 92mm and 40kTonne results in a 104mm within the permitted FoS, thus proving that the bearing capacity is sufficient in Blessington Sand to support the fully ballasted GBF. Tilt was examined and findings showed that only the differential settlement associated with the 20kTonne ballasted GBF (ΔS = 185 mm) was within the tilt tolerance of 0.25°. Ballast weights of 30kTonnes and above would require seabed preparation i.e. installation of coarse material between GBF and the seabed.

The main contributions this thesis offers offshore designers are steps to applying an alternative approach to estimating sliding resistance, and quantifies the amount of ballast that can be saved by using this approach.