Static Behaviour of Bucket Foundations

One new foundation concept in relation to offshore wind turbines is bucket foundations. The concept is known from the oil and gas industry, though the load conditions here are significantly different. The bucket foundation can be used as monopod or e.g. tripod foundations for offshore wind turbines. The monopod concept is investigated in this thesis, regarding the static behaviour from loads relevant to offshore wind turbines. The main issue in this concept is the rotational stiffness of the foundation and the combined capacity dominated by moments.

The vertical bearing capacity of bucket foundations is of great importance according to this approach. The bearing capacity of bucket foundations including circular surface footings is investigated analytically, numerically and experimentally. A new general expression that describes the bearing capacity factors according to the classical bearing capacity theory is proposed. The proposed expression applies to plane strain as well as axis-symmetric stress conditions for foundations with smooth or rough bases.

A thorough experimental investigation of the static behaviour of bucket foundations subjected to combined loading is carried out. Laboratory tests as well as large-scale tests on bucket foundations subjected to low vertical load are performed during this work.

Numerical simulations of the tests performed are carried out using the Mohr Coulomb material model and the commercial finite element code ABAQUS. Based on the present work, the finite element method is concluded to be a superior method in estimating the post peak behaviour as well as the combined capacity of bucket foundations in relation to the offshore wind turbine problem.