The Impact of a Noise Mitigation Screen on the Scour Protection

Abstract

Current trends show that the offshore wind industry is evolving into deeper waters, with larger turbines and consequently larger foundations. In Europe 80% of the wind turbines are installed on monopile foundations and this is expected to be the dominant choice in the future. The shallow geology of most of the wind farms is characterized as sand. A known phenomenon around foundations in sandy soils is the occurrence of scour, due to a disturbance in the flow caused by the waves and currents. Scour affects the embedment depth and natural frequency of the turbine. In order to prevent these effects a commonly used technique is to install a rock blanket around the monopile, to act as scour protection. Monopiles are mostly driven into the soil with a hydraulic impact hammer. The blows of the hammer result in an elevated noise level. Within the German EEZ strict rules are set regarding the allowable noise level. Noise mitigation measures, such as a noise mitigation screen, are used to prevent exceedance of the sound exposure level, see figure 1.

In an attempt to reduce the Levelised Costs of Energy (LCOE) of offshore wind, cost-saving optimizations are considered such as the installation of a single layer scour protection before foundation installation. The conventional two layer system requires two mobilization campaigns one for the filter and the second for the armour layer, after monopile installation. The single layer solution results in a reduced installation time and no risk of collision with the monopile. The NMS is lowered on the scour protection and the monopile is driven through the NMS. During the installation the NMS penetrates in the scour protection and deformations are observed, see the area close to the monopile in figure 2. Remedial works are very expensive and therefore one would like to know what the impact is on the performance of the scour protection during the lifetime of the wind farm.

The project is based on a review of literature regarding relevant topics such as: soil mechanics, scour protection, noise mitigation and installation processes. Because of the problem’s novelty, the available research is limited. In order to acquire insight in to the problem there is a need to investigate which processes are involved and relevant. This is done by collecting all available field data from selected offshore wind farms installed with a NMS, such as CPT data, survey data and hammer logs.

The analysis of the available data shows that the main factor influencing the NMS penetration is the combination of the use of a NMS together with the hammering procedure. Based on the available soil, hammer log, NMS and pile penetration data it is believed that the NMS imprint is a consequence of compaction of the underlying soils and scour protection. Compaction of underlaying soils is seen as having a beneficial effect on the scour protection system. Further it is investigated whether a large NMS penetration combined with a high relative mobility for the scour protection during storm conditions can lead to undesirable effects. Time series of environmental conditions were reconstructed from field measurements and numerical model hind casts. These time series serve as input to assess the relative rock mobility between installation and most recent bathymetry survey. Locations are verified for the imprint depth and relative mobility and a potential worst case is identified. The identified worst case is evaluated and validated against model tests of similar conditions. The outcome of the study is that limited rock movement has occurred from the edges of the imprint into it, but that the scour protection is still performing well after a 5yr design storm. This is also confirmed by model tests. A potential weak-link is identified in the slope of the imprint where the scour protection thickness is reduced. Practical mitigation and monitoring measures are proposed, but further research is needed to verify whether the risk of winnowing can appear within the slope of the deformed part of the imprint.