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Scour protection study

Public Summary

When a foundation pile is installed offshore in a mobile seabed (e.g. consisting of sand, silt or weak clay) and the pile experiences a hydraulic load consisting of a current, waves or a combination of both, scour can occur. Scour is local erosion of the soil around the foundation pile, caused by the acceleration of the current and waves flowing around the pile. The depth of a scour hole can be significant and is mainly governed by the hydrodynamic climate (currents, waves, water depth) and the seabed conditions. The formation of a scour hole results in an increased unsupported length of the structure and reduced embedded length. In turn these effects lead to a lower natural frequency, increased loads and lower lateral bearing capacity of the foundation. In general, to avoid these consequences of scour, scour protection around foundation piles is applied. However, the application of scour protection around a foundation pile is expensive due to the additional offshore operations and the large quantity of rocks that needs to be purchased and transported to the wind farm site. Furthermore accurate placement of the scour protection material takes time and requires expensive vessels. Another point of attention is to make sure the scour protection will remain intact throughout its intended design life of 20 or 25 years.

The objectives of this project were as follows:

1. Development of a Scour Prediction Model (SPM) to predict scour around monopiles
2. Development of a Scour Protection Design Model (SPDM) to design an optimized scour protection without unnecessary conservatism
3. Development and application of a continuous Scour Measurement System (SMS)

Objective A. Development of a Scour Prediction Model (SPM) to predict scour around monopiles
The first main objective was to improve the predictability of scour development around an unprotected monopile during the complete lifetime of an offshore wind farm. Based on the criterion as defined in standards, it is often considered to be more cost-efficient to install a scour protection instead of fabricating longer and thicker foundation piles (and as such increase the amount of primary steel).
Existing conceptual scour prediction models (e.g. by Deltares) have been able to hindcast scour development around unprotected piles with reasonable accuracy. However, these models lack validation against continuous field measurements.
In order to improve on existing models, a scour prediction model was developed which was validated against a continuous field measurement campaign (Objective C).

Objective B. Development of a Scour Protection Design Model (SPDM) to design an optimized scour protection without unnecessary conservatism
In order to be able to decide whether or not scour protection needs to be installed, the above mentioned Scour Prediction Model can be applied to predict scour development throughout the lifetime of the wind farm. When the predicted scour depth is considered to be unacceptable, scour protection needs to be designed. There is significant cost optimisation potential by optimising the scour protection volume, especially for wind farms that are located farther offshore or in areas with less scour development. Other optimisation possibilities are related to reducing the number of different rock gradings (one wide grading instead of filter and armour layer) and extending the range of possible installation methods (smaller rocks can for instance also be installed with fall pipe vessels).

The objective of the project was to perform model testing for different scour protection layouts, and deriving design formulae which take into account current, wave conditions, water depth and structure dimensions. These design formulae could subsequently be integrated in the scour prediction model.

Objective C. Development and application of a continuous Scour Measurement System (SMS)
Until early 2014 the seabed in offshore wind farms has only been monitored on a discontinuous basis (e.g. by annual multi-beam surveys). To improve the understanding of the dynamic processes at the seabed around an offshore foundation and to obtain reliable input for the development of the improved scour prediction model, continuous scour measurements were required.
Within the offshore wind farm Eneco Luchterduinen, two monopiles without scour protection were installed, to create the opportunity to apply scour development measurement in the offshore environment at full scale.
The objective was to design and install the scour measurement system at these foundations, and carry out for an envisaged period of one year (October 2014-October 2015).

Results
The dynamic scour prediction model has proven to be a useful tool to translate scale model results into long term scour predictions in the field. The tool is valuable in decision making in order to assess whether scour protection is beneficial or not. The optimal solution for scour protection is found to depend on location (water depth, hydrodynamic climate, currents vs. waves), pile diameter (defined by turbine type and size), soil conditions and the developer and contractor of the project (e.g. the type of “in-house” equipment available such as subsea rock installation vessels).
Depending on the steel price, the wind park location and the turbine size a well-founded decision can be made to install or omit scour protection. For sites with severe currents and smaller waves applying a scour protection often turns out to be cost-efficient, whereas for sites with lower current velocities and larger waves, omitting protection appears to be the better solution. In view of the massive developments in areas with relatively small current velocities, such as Doggerbank and the Baltic Sea, tens of millions of euros can be saved by adopting the presented integrated approach. Figure 1 presents an example output of the model showing the opportunity map for the North Sea region, showing whether it is beneficial to leave out scour protection.