Summary of the Joint Industry Project Wave Impact on Fixed Foundations (WiFi JIP)

Abstract

The objective of the Joint Industry Project Wave impact on Fixed foundations (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. In this paper a summary 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. This effort has been reported in WP2 , where design computations were carried out using the embedded stream function approach for several sea states. In this WP Siemens, ECN and Ramboll also calculated the impact response of the monopile to surging and spilling type wave breakers with their engineering tools. In the next phase the designed foundations were tested in MARIN’s shallow water model basin. The foundation for the monopile was modelled as a 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. The model test results were reported in WP3 and 7 and used as validation for WP9 and 10. WP4 delivered more understanding of realistic design conditions for areas typical for OWT, like the North Sea. Particular attention was paid to the probability of occurrence of breaking and steep waves and the associated slamming load. For this an extensive 5 week experimental program was performed from September to October 2013 in the wide wave-current flume at Deltares (Atlantic Basin). During these tests both waves and current were simulated and two bathymetries. WP8 provided analyses of the performed full scale measurements on the response of a OWT. The full-scale measurements were done for a Vestas V90 3MW wind turbine in the Belwind windfarm which is located 46 km off the coast of Zeebrugge on the Bligh Bank. 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 WP11 and 12 this new approach is applied on different case study’s by ECN.

The DeRisk database: Extreme Design Waves for Offshore Wind Turbines

“…This is because in tests 11 and 12, where waves are on average steeper than in tests 22 and 23, slamming loads from breaking waves are more frequent. The ESF method performs better than the WiFi model, which seems to provide conservative estimates, as also observed by de Ridder et al [23]. As for the 1000 year return storms, none of the models is clearly superior to the others.…”

“…We compare the DeRisk database methodology with the Embedded stream function (ESF) method from IEC-61400-1 [1], with the Wi-Fi JIP methodologies [23], and with linear and Second-Order irregular waves from Sharma and Dean [24].…”

“…The WiFi Joint Industry Project (JIP) [23] provides a method to compute the extreme force with a probability of occurrence of 1/1000 in a 1-hr sea state. First, an average steepness parameter is calculated as s P = H S /L P , where L P is the length of the linear wave associated with the spectral peak period.…”

Experimental and numerical investigation of a jacket structure subject to steep and breaking regular waves

“…In terms of the moderately steep waves without the occurrence of wave breaking, the validity of the so-called Morison equation [4] has been investigated, primarily on slender piles [5,6,7,8]. A reasonable approximation of the wave loads using the Morison equation was achieved if correct wave kinematics were available in the context of wave modeling [7,9]. Paulsen et al [7] showed that, for moderately steep irregular waves, the Morison equation combined with a fully nonlinear two-dimensional potential flow solver was a good approximation.…”

“…Although various wave breaking criteria have been developed and recommended by the offshore wind turbine loads standard [2], wave breaking in a realistic ocean environment has large variability in breaking wave conditions, which are typically determined by the site-specific conditions such as bathymetry, current and wind. The uncertainty is more pronounced regarding the wave loads and local stresses induced by the breaking waves as it involves the interaction of water, air and structural dynamics [9,14]. The total breaking wave loads are normally considered as an additional slamming force on top of the Morison force part in an engineering perspective.…”