Overview of the Joint Industry Project Wave impact on Fixed foundations I and II (WiFi JIP)
Erik-Jan de Ridder, Tim Bunnik
MARIN, Wageningen, The Netherlands
The objective of the Joint Industry Project Wave impact on Fixed foundations I/II (WiFi JIP) was to increase the understanding of breaking and steep wave impact's on fixed foundations of offshore wind turbines (OWT). The project was set-up as a Joint Industry Project (JIP) and in total 20 companies and research institutes participated in the project.
An overview of the complete WiFi JIP project will be presented. At the start of the project the state of the art design methods and guidelines were reviewed (WP1). Thereafter a jacket and a monopile foundation were designed using these state-of-the-art tools that were available at the start of the project. In the next phase the designed foundations were tested in MARIN's shallow water model basin. The model test results were used as validation for numerical simulations. WP4 delivered more understanding of the wave climate in OWT areas, like the North Sea, in terms of the probability of occurrence of breaking and steep waves. WP 8 provided analyses of the performed full scale measurements on the response of a OWT. The CFD simulations performed in WP 9 showed that a good agreement is obtained between the CFD simulations and the model and full scale measurement. In work package 10, an improved methodology was developed based on the outcome of the previous WP's to model the breaking wave impact of plunging type breakers. In WP 11 and 12 this new approach is applied on different case study's by ECN.
The method used in the WIFI JIP project, was by using the model tests results to validate the numerical CFD (computation fluid dynamic) models. Thereafter the CFD models were used to understand the complex loading of steep an breaking waves on a wind turbine foundation. This knowledge was then transformed in improved design methods and guidelines for modelling steep and breaking wave impacts on offshore wind turbine foundations.
The foundations for the monopile was modelled as an rigid and flexible foundation. The foundations were tested in regular waves, irregular sea states and so called focused waves. During the model tests the wave heights, wave run-up, accelerations, impact pressures and loads on the foundation and boat landing were measured. For better understanding of the wave climate in OWT areas, like the North Sea, in terms of the probability of occurrence of breaking and steep waves, an extensive 5 week experimental program was for different water depths, currents and seabed shapes. The results were summarized in wave almanac which gives a overview of the measured loads for the different environmental combinations.
From the complete WIFI JIP it followed that for the existing design approach using streamfunction it is difficult to accurately capture the maximum wave load on a OWT foundation for steep and breaking waves. Within the JIP an improved design method was delivered in which the wave kinematics were obtained with a non-linear wave model instead of stream function. Furthermore and improved wave slamming formulation was developed which could be added to the guidelines.
The knowledge developed in the WIFI JIP can be used to improve the design of offshore wind turbine foundations from a wave loading point of view. The objective to share a part of our acquired knowledge is to make designers, operators and developers aware of the developed knowledge within the project.