Current and sea loads

Determination of combined current and sea loads of breaking waves on large diameter monopile offshore structures by computational fluid dynamics (BWLS)

Forschungs- und Entwicklungszentrum Fachhochschule Kiel GmbH
Prof. Dr.-Ing. Kai Graf

The recent development of offshore wind turbines (OWTs) shows a trend towards monopiles with ever-expanding diameters. Monopiles are more cost-effective to build and set up than conventional tripod and jacket structures. However, the ramming of large diameter monopiles has only recently become possible. While the FINO3 platform, built in 2009, has a pile diameter of about 3 meters, wind turbines with 8 meter pile diameters are being erected today. It is clear that in the future, installations with pile diameters of 12 meters and more will be built to meet the quest for ever larger offshore wind turbines.

For the determination of the loads of these large monopiles by sea the same approaches are used so far as for monopiles smaller diameter. These approaches are proven, but their validation for large diameters is still pending.

Schleswig-Holstein's Ministry for Energy Transition, Agriculture, Environment, Nature and Digitization is supporting the project with state funds in accordance with the Directive for granting grants to promote research, development and technology transfer (FIT Directive). The grant applies to the period from 01.01.2019 to 31.12.2021.

Activities and goals

The project described here aims to determine sea loads on monopiles of large diameter by computational fluid dynamics. These flow simulations are performed systematically for increasing monopile diameters and for different wave heights and wave steepness’s. With regard to the design wave and the expected wave spectra, this study draws on existing recent R & D results.

The actual target of the study is the determination of the sea load with a simulation method for viscous flow with free surfaces. Significant progress has recently been made in this area, and the Kiel Institute of Shipbuilding and Maritime Engineering has participated in and contributed to these advances. The method is based on the solution of the time-averaged Navier-Stokes equations (RANS equations) taking into account free surfaces with a volume of fluid method.

As a result of the project, a considerable improvement in the forecast accuracy of the seagoing loads of large monopile structures is expected. Ultimately, this will mean that the structures can be designed to be more targeted and usually lighter in terms of strength, which will lead to cost reductions.