Finding The Right Combination

Two-hammer method reduces piledriving noise.

Subsea noise associated with piledriving is a major issue on installation projects for offshore wind farms. MENCK is taking part in a pioneering project in the German North Sea that combines two hammer systems: a MENCK impact hammer and a vibratory hammer. The combined approach means that noise levels for much of the piledriving work is considerably lower and that the use of additional noise reduction methods, such as bubble curtains, is reduced.

The Global Tech I offshore wind farm array will consist of 80 wind turbines and occupy an area of roughly 41 km2. Each turbine will have a tripod foundation, so 240 piles need driving. HOCHTIEF Solutions is managing the erection of the tripod foundations and turbines. Various systems are being used to minimise noise and the associated disturbance to wildlife, particularly marine mammals.

Christoph Dytert, director of sales and marketing, MENCK, explains: “Thirty years ago, oil and gas platforms were installed in the North Sea with little thought for how the installation process might affect marine wildlife. New regulations place an onus on operators to install piles and their associated infrastructure with careful regard for the environment.”

The pile installation work combines two hammer systems and a large bubble curtain around the installation site. The two hammer processes run in series. An upending vibratory hammer (a highfrequency device that drives piles smoothly into the ground) drives the pile about two-thirds of the way into the ground then a MENCK hydraulic underwater hammer takes over to complete the job and establish the load capacity of the pile.

Using the much quieter vibratory hammer for the first phase means that the bubble curtain only operates when the conventional hammer is running. Consequently, the compressor that generates the bubble curtain runs for only a fraction of the time required for a conventional piling operation: typically, 15 minutes rather than 90 minutes. This also means that significantly less fuel is needed to power the air compressor, so less carbon dioxide is emitted.

Changing between the two hammers extends the piling operation by about 30 minutes per pile, but the environmental benefits in terms of reduced noise and air pollution are significant.

In this project, the approach has been to use a different crew and separate power packs for each hammer. As part of wider research into noise reduction, MENCK is investigating ways to combine the crews and operate both hammer systems from a single power pack, which will save time for the installation contractor and space on the installation vessel. Dytert says, “This is a new way of working and there will be improvements and enhancements over time. For the past 10 years, MENCK has taken an active role in learning about the causes and effects of underwater noise. We have developed various systems and methods, including a highly effective system called the MNRS-U, which is a small-bubble curtain placed around the pile. The MNRS-U is a unique combination of effective noise mitigation and fast, easy operation that can easily be combined with the two-hammer method.

“Participating in this project underlines our commitment to helping offshore operators to achieve noise reductions through proven new technology and new working methods,” he concludes.

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