Noise mitigation system: First commercial operation…
There are different restrictions for maritime pile driving operations in place in the North European countries to protect the local sea life. It is quite right and proper that there should be restrictions. However, it could be argued that it is probably not so right and proper that the individual legislation for each country for the restrictions is not all based on the same idea and that this does not make it any easier to work within the restrictions.
For example in Germany there is a measured noise limit 160dB sound exposure level at a distance of 750m from the pile and in the Netherlands it is limited to only one operation at a time within the period between July and December. However work is in progress that could ultimately persuade the various governments to accept a common set of conditions.
IHC Hydrohammer started research on their Noise Mitigation System (NMS) in 2007. This early research and testing was described in Offshore WIND in October 2011. Between October 2011 and September 2012 IHC Offshore Systems and IHC Hydrohammer had carried out tests on larger and ever more advanced trial versions, culminating in the first commercial operation of the system. We return now with the results of this first commercial operation of the NMS-6900 at the Riffgat offshore wind farm carried out in the summer of 2012.
The HLV Oleg Strashnov from Seaway Heavy Lifting had been contracted to carry out the foundation piling work and the IHC Hydrohammer S-1800 working with the NMS-6900 had been selected to carry out the work. Riffgat consists of 30 turbines placed on monopile foundations, up to 70m long and 750t heavy in waters up to 30m deep, 15km North West of the German island of Borkum.
For all tests there must be a reference line or level. The results are then calculated as being higher or lower than this reference level. The following variables determine the reference level:
- Water depth Location
- Sea floor conditions
- Weather conditions
- Diameter pile
- Hammer size/force
- Running/fixed pile
- Sea level (waves)
Background noise in the North Sea, created by shipping for example, can be between 100-110 dB depending on traffic. When the vessel carrying out the piling is included the noise level increases to between 130-140dB.
Due to contractual limitations the reference level at Riffgat had to be set by ITAP, the German Institut fur Technische und Angewandte Physik GmbH.
ITAP made their estimations for the noise level for operations using a maximum pile diameter of 6.5m, in a water depth between 25 and 30m with a S-1800 hammer. They calculated that these operations without the NMS-6900 being used would have a total noise level of 181.5dB.
Previous tests off IJmuiden, with the FLOW initiative, had shown that the difference in noise levels, compared with and without an NMS, was over lldB. With the ITAP calculated noise level of 181.5dB, a reduction of only lldB achieved the result would still be too high even with the NMS system. Between the FLOW tests and Riffgat IHC made certain modifications to the system.
The NMS system and the S-1800 were installed on the Oleg Strashnov. The test to be made during the project included:
- Acceleration sensors on NMS
- Near-field sensor array (<10m)
- Far-field sensor array (750m)
- Soil impedance test
- Kinetic energy Hydrohammer
- Load measurement flexible guidance system
- Bubble test
- Floating test
- Impulse excitation test
Hydrophones were placed underwater at <10m and at 750m from the pile. The piling and testing commenced.
The results that were produced were good, much better than predicted. The graph, figure 1, shows the calculated levels prior to testing and the results from the FLOW tests, with and without NMS, and the impressive results from the Riffgat project. However this is still a learning curve and the fact that the noise level was below 153dB, does not mean that they have achieved 100% success. They will continue tweaking the system to improve levels.
They have found where the levels increased under certain settings of the hammer and the system and more importantly settings where the noise levels were at a minimum. But they are still learning where to go and what to do. This is still the learning phase and they know that more modifications that are planned will result in even greater reductions in noise level.
Even today, long after the project at Riffgat is over, IHC Hydrohammer is reluctant to disclose any details of the modifications that have been made. The current equipment is 10 metres in diameter, 30 metres high and weighs 360 tonnes. They told Offshore WIND that the basic principles of the NMS were an outer screen and inner screen with, between the two, an air gap. There is also a bubble screen injected in the water inside. They have also explained that the extra features included a Multi level bubble injection system, injecting differing sized bubbles, and also that the inner and outer screen were isolated.
Within the industry there are a lot of wild tales in relation to underwater noise. With the NMS IHC is not telling fairy tales but focussing on facts. These results are facts. Riffgat are pleased with the results.
With regards to noise mitigation, jackets and tripods face the same challenges as monopiles. However apart from that, factors as safety, accuracy, flexibility, efficiency, inclination, and stick up length of the piles play an important role.
IHC Hydrohammer is working on a guiding frame for three or four piles that will be used with the jackets and tripods. This IHC TiNS template system will also be able to accommodate an NMS variation. There will be more facts to write about in future issues of Offshore WIND.