Offshore structures under extreme loads: a methodology to determine design loads

Abstract

The majority of the Earth's surface is covered by oceans. Waves, currents, and winds are phenomena that act as load requirements in the design process of any offshore structure. While there are many types of offshore structures such as bridges, oil platforms, or wave energy converters, one type of structure currently gets the most attention: offshore wind turbines, which are expected to become one of the main sources of future energy supply. They are typically designed for a design life of 20 or 25 years and therefore must withstand all environmental conditions that can reasonably be expected during this time. To evaluate a design, one estimates its structural response under given environmental loading. This requires a description of the expected environmental conditions and a method to decide which environmental conditions should be considered as design requirements.

This thesis addresses the design process of offshore structures. Engineering standards and guidelines describe the state of the art of this process and recommend models that shall be used to describe the environment and to estimate the extreme structural response. In particular, three design process steps where current methods can lead to problems are addressed: (1) Modeling the probability distribution of significant wave height; (2) modeling the joint distribution of wind speed and wave height; and (3) determining 50-year joint environmental extremes. New methods to deal with these three steps are proposed and evaluated. Finally, a case study on a 5 MW wind turbine is conducted.