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Under water noise mitigation during pile driving

Project description

Project summary 

Pile-driving in Europe is facing restrictions caused by “precautionary regulations” for underwater sound and its effect on the environment. At the moment, anti-noise legislation in The Netherlands prohibits pile-driving from 1 January to 1 July, with a maximum of one offshore wind farm project per year. This limitation during construction increases the cost price for the offshore wind industry. Successful noise mitigation measures during pile-driving would double the period for construction and reduce costs.
The knowledge about underwater noise and its effect on the environment is limited. In general the objectives are to get more knowledge about the source and distribution of underwater noise generated by pile driving and its effect on the environment as well as to generate solutions to mitigate the underwater noise during pile driving operations. The main objective of this project is to develop and test a noise mitigation method in offshore environment, which gives a significant mitigation of underwater noise and is fulfilling the requirements of offshore operation.
The second objective is to provide measurement data from an offshore test to validate the research of the project ‘Silence’, which is done at the Delft University of Technology (DUT) under the Far and Large Offshore Wind innovation program (FLOW). The project is divided in two phases and a separate described extension of the second phase:

Phase 1:
IHC Hydrohammer B.V. (IHC HH) has carried out desk research, laboratory tests and prototype tests to determine the most efficient mitigations method for underwater noise, which is useful in an offshore environment. The results of this research is used to determine the base of design and method statement for a Noise Mitigation System (NMS) to be used for the installation of a monopile in offshore environment.

Phase 2:
RWE Offshore Wind Netherlands (RWE) has facilitated the test of a prototype NMS during the installation of two identical monopile foundations. These monopiles (diameter of 3.35 meter) are the foundations for meteorological masts for the offshore windfarms ‘Ijmuiden Buiten’ (MM-IJM) and ‘Nordsee Ost’ (MM-NO). The monopiles are installed with a Hydrohammer S-800 at a water depth of 25 meter. As part of this project, IHC HH has specially developed a NMS for the project specific parameters of this installation. An Extensive test setup was made together with DUT to facilitate the research (‘Silence’) done on underwater noise.

Extension Phase 2:
In the summer of 2012, Seaway Heavy Lifting has carried out the contract for installation of 30 monopiles for the Riffgat offshore wind farm. The Riffgat offshore windfarm operates in water depths ranging from 18 up to 25 meters. The monopiles, varying from a diameter of 5.7 meter up to a diameter of 6.5 meter, were first preinstalled with the use of a vibratory hammer and, after installation of a NMS, driven to target depth with a Hydrohammer S-1800. As part of extension of phase 2, an extensive test up was made together with DUT to facilitate the research (‘Silence’) done on underwater noise. The measurements were undertaken at 5 of 30 monopiles.

From the tests executed at MM-IJM, MM-NO and Riffgat the following conclusions can be drawn in relation to the project objectives:

1. Design criteria issued by IHC Hydrohammer
The design of the NMS as used in the Riffgat project showed a significant noise reduction and complied to the German regulation for most of the monitored piles. It showed very good efficiency at mid and higher frequency ranges. In the lower frequency ranges the efficiency is lower. Special measures for ground borne noise should be undertaken to effectively increase the efficiency at lower frequency ranges.

2. Design criteria issued by installation contractors
The NMS used at the Riffgat project installed 30 monopiles without major problems. The use of the NMS did not affect the weather window of installation. The method statement for installation was efficient and the increase in installation time was limited (approximate 1 – 2 hour additional installation time per monopile).

3. Further research and testing
The ‘Silence’ project at DUT advised on the test set-up for the MM-NO, MM-IJM and Riffgat project. All measurement results are handed over to the ‘Silence’ project for validation of their research. The measurement results are directly used by IHC HH to further optimize the design of the NMS.

4. Test at offshore locations
RWE gave the opportunity to test at the installation of MM-IJM and MM-NO. A full scaled NMS is tested at offshore location.

5. Demonstration project 
The installation of the Riffgat project is considered as a demonstration project of the NMS.

The Dutch legislation on underwater noise has the following legislation on impact piling of offshore wind foundations to meet the requirements of the Marine Strategy Frame Directive:

• Pile driving is only allowed from the 1st of July until the 31st of December;
• Not more than one offshore wind farm shall be installed by means of impact pile driving at the same time.

An effective mitigation measure reduces amount for under water noise due to pile driving piling to a level that it does not considered to be harmful for sea life in general. Therefore it is most likely to say that an effective noise mitigation measure, which is most probably pointed out by the test results of this project for the NMS, will eliminate the current legislation for pile driving. This will lead to the following contributions to the FLOW targets:

Acceleration

Elimination of the legislation will make it possible to install more offshore wind farms at the same time. Next to this, it makes it possible to install foundations the whole year around, which leads to more installation time. This will speed up the process of installation and shall make it possible to meet the 2020 target. 

Cost reduction

The elimination of current legislation opens up the possibility to use foundations which are installed by impact pile driving, herewith reducing the Levelised Cost of Energy (LCoE) with approximate 1.5%.

Furthermore, eliminating legislation for the installation window makes it possible to install in months with the best weather window. This will speed up the installation process, reducing the down time by weather and decreasing the risk for the marine contractor. Calculations with the FLOW-TKI offshore wind cost model pointed out that this will result in a reduction of the LCoE between the 0.66 and 0.81%. Cost reduction could be further increased by further optimization of the installation method. This will be further research in the FLOW project P201101-013-IHC ‘Installation methods for use of an underwater noise mitigation screen in conjunction with pile driving from a floating vessel and from jack-up’.