Runwave

WHY

Wind turbines experience significant forces on the foundation, turbine tower, and nacelle during the turbine's life span. These forces are caused by wind, (breaking) waves, and currents. The design of offshore wind turbines is, therefore – amongst others – determined by the expected wave load during moderate or extreme weather conditions.

To determine the loads, approximations need to be made concerning the local extreme wave conditions, the conversion to individual design wave characteristics, and the computation of the forces through empirical formulations. The accumulation of uncertainties can lead to a high uncertainty margin in the design. This may lead to an under-dimensioned design, leading to longer turbine downtime due to a narrower operational window and higher maintenance costs. However, an over-dimensioned design will result in higher costs than necessary.

A significant source of uncertainty in new design of monopiles (e.g. larger monopile and in larger water depth) is the response behaviour of the monopile on wave loads, which can change substantially as the natural periods increase with turbine size. This increases uncertainty in both fatigue and ultimate response loads and stresses in the monopile. Furthermore, the increasing flexibility of the support structure requires a renewed understanding of the response effects of wave nonlinearity and the impact of wave slamming.

WHAT

This project aims to provide a new method to better determine wave loads on fixed offshore wind turbines due to extreme and fatigue sea states. We will expand the current purely deterministic method with a probabilistic model. This model will take into account the probability of wave loads at a specific location in the wind farm and consider local environmental and hydrodynamic conditions. We will update or, if necessary, develop new deterministic empirical formulations to determine wave loads and the resulting response loads and stresses on the structural elements. In this project, we will use the results of the WiFi 1 and WiFi 2 projects (WiFi stands for Wave impacts on Fixed turbines) where a start has already been made on this topic.

To get an impression of these projects, please have a look at a short movie of 9 years ago.

We start by determining the properties necessary to develop the probabilistic model concept. The model will establish a relationship between the probability of wave load occurrence and the final load on the monopile.

Subsequently, physical model tests and numerical simulations will be performed to investigate wave loads at a Deltares facility. The structural response of a monopile at a MARIN facility. The tests will complement the data obtained in the WiFi 1 and WiFi 2 projects. The current state of technology and trends in the field of monopiles/turbines will also be taken into account.

EXPECTED RESULTS

The result of this project will be new or improved design methods for bottom fixed turbines that take into account expected wave loads, including extreme and moderate weather conditions. The new insights will significantly increase the certainty of wave load design and limit the risks that arise from the accumulation of uncertainties within the existing design method.

The project will lead to the following supporting results:

• New design method that takes into account extreme and moderate weather conditions for both wave loading and structural response.
• Newly developed probabilistic model to estimate the probability of occurrence of a given wave load/structural response, including a case for a selected workflow (e.g. limit state function).
• Datasets from small and large-scale physical tests on wave loads and structural response on XXL and 3XL monopiles, in addition to the WiFi 1 and WiFi 2 data sets.
• New and updated empirical formulations to determine wave loading and structural response for current and future monopile foundations.

In addition, recommendations will be made in the following areas:

• Modification of existing design load cases and / or addition of new load cases to capture all relevant conditions for the final response analysis.
• Selection of suitable joint environmental conditions (wind and sea state).
• Suitable wave theories and (empirical) load models.