Driving down noise
Noise abatement from piling activities is now easier to achieve, finds Dave MacIntyre
Assessments of underwater noise from port construction, particularly pile driving, are becoming increasingly important as statutory bodies tighten their environmental controls to protect fish and marine mammals.
The EU’s Marine Strategy Frameworks Directive requires member states to ensure they achieve Good Environmental Status (GES) in relation to underwater noise. At a local level, underwater noise can form part of the Environmental Impact Assessment (EIA) process for planning approvals. These pressures are driving twin areas of marine engineering innovation, with new noise-monitoring technology and modelling tools being matched by new techniques for quieter pile driving.
Andrew Baker, managing director of UK-based Baker Consultants, says in recent years there has been a realisation that marine noise poses a significant threat to sea life, particularly species that rely on echolocation such as the harbour porpoise, migratory fish, commercial species and whales.
“High levels of noise or continuous exposure to noise at levels greater than ambient can cause behavioural changes and can even be fatal. Pile driving is classed as a loud impulsive source and, as a result, [EU] member states need to monitor and mitigate for this type of source. Pile-driving is required for installation of the large wind farms, the foundations are getting bigger, requiring more piling energy to be applied, meaning unmitigated piling is becoming louder as a source.”
Mr Baker says noise modelling helps gain an understanding of the likely effects of construction work and informs what is required for mitigation; "it can also help in reducing the cost of mitigation, giving the engineers the data they need to choose the most appropriate method. Depending on the outcome of the model, regulators may impose restrictions in the timing of piling (e.g. seasonal bans) or require mitigation tools to be applied." The port of Aberdeen which is currently being expanded is a good example where Marine Scotland has imposed strict mitigation informed by the outputs of the model combined with the knowledge on the local marine mammal populations.
Baker Consultants has developed a noise monitoring system with French company RTSys to provide the piling engineer with a real-time reading of noise levels during operations so that the hammer energy can be adjusted.
The company has also joined forces with HR Wallingford Ltd to offer noise data collection with biological knowledge using a sophisticated hydro-acoustic data model.
The “Hammer” model has been developed in conjunction with Loughborough University and combines a predictive acoustic propagation model (which incorporates seabed bathymetry, sediment type, hydrodynamics, salinity and temperature gradients), with ecological response models for individual marine species.
Canadian company Ocean Sonics confirms that measuring ocean sounds is becoming an important part of meeting new legislation introduced in Europe and North America intended to protect the oceans.
Yet, founder and president Mark Wood says the understanding of acoustic propagation of pile driving sounds is surprisingly complicated: “In addition to the pile emitting sound, the sea bottom conducts sound and does so more effectively as the pile is inserted deeper into the sub-bottom. A lot of progress has been made in understanding and quantifying this, and in parallel with this the mitigation strategies also improve. There is still plenty of potential to improve mitigation approaches for pile driving acoustics.
“With the introduction of the Marine Strategy Framework directive in Europe, and new standards in measuring pile driving and construction sounds, operators can learn best practices and acquire better software tools for collecting consistent field data from these operations." Standardisation of measurements means that manufacturers like Ocean Sonics can provide better and easier-to-learn products for operators.
For example, the company has developed the “Smart Hydrophone”, a further step in the development of monitoring technology. An analogue hydrophone is an underwater microphone that converts sound energy to electrical energy. When connected to an amplifier and speaker it lets humans hear sounds in the water. Digital hydrophones eliminate the need to add preamplifiers, filters and digitisers into the instrument, replacing the output voltage with a binary stream.
Smart hydrophones add a processor and memory to the digital hydrophone. The data can now be processed, acquire meaning, and be streamed or recorded in the instrument. Triggers, based on pre-selected conditions, detect acoustic events, resulting in selectively recorded data, text message or an event history file.
Major innovations are emerging too in noise-attenuation pile technology. For example, bubble curtains reduce sound waves, using a perforated tube around the pile and pressurised air to create a sheath of bubbles. The bubbles break a share of the sound waves generated by the driver head.
A new development by the University of Washington and Marine Construction Technologies PBC in collaboration with Washington State Department of Transportation and Federal Highway Administration is the Reinhall Pile. When a hammer strikes a pile, a bulge travels rapidly down the length of the pile, disturbing the water and sediment and creating resulting sound waves. The Reinhall Pile uses a double wall design and proprietary shoe to encapsulate noise along its full length, blocking noise propagation via the water and seabed. The pile consists of an outer pile, into which a fixed or reusable inner pipe is inserted for driving purposes.
A consistent air gap is maintained between the piles, by spacers and the shoe at the base of the double-walled pile. The driving hammer strikes the inner pipe. Sound waves then travel along the inner pipe while the outer pipe and the air gap between the two piles, mitigate noise transmission into the water. Only a small amount of noise escapes from the pile bottom, tests have confirmed. After installation, the inner pipe can either be left in place or removed and used as a reusable mandrel.
Testing showed that at 10 metres depth, noise was reduced by over 20 dB compared with 6 dB Peak for a bubble curtain.
Julie Hampden, environmental director at Marine Construction Technologies, says that the Reinhall Pile was tested in Puget Sound in 2014 and a seminar held in 2015, followed by a second successful full-scale test in 2015. A research report was issued last year and presentations made to a variety of engineering firms and regulatory agencies such as the US Army Corps of Engineers, National Marine Fisheries Service and the US Fish and Wildlife Service). Pile-driving proofing software adapted for use with the piles is in development and scheduled for completion this summer, after which the product should be available in the marketplace.
LIQUEFACTION A SOLUTION FOR NOISE
In Europe, new piling technology is being developed by innovators Ben Arntz and Nick Noordam of The Hague-based GBM Works. The GBM Pile Drill is a noiseless method for sinking monopiles into the sea bed, particularly applicable to offshore wind turbine foundations, but with potential for port applications.
Mr Arntz became aware of the need for a silent driving technique for foundation piles while working off the Australian coast placing new pipelines for an oil company. To lay these pipelines, a number of piles first need to be driven into the sea floor, generating high noise levels. GBM Works’ idea is to use liquefaction rather than a hammer to drive the monopile into the seabed. Liquefaction is the geological term for the process whereby soil loses its strength and stiffness, temporarily becoming a liquid and allowing the piles to sink in.
Mr Arntz says that the soil directly underneath the pile is weakened by vibrating tip elements: “The oscillations of the tip elements are in a lateral direction, also pushing and compressing the soil to either side. This takes care of the tip resistance. From the tip upwards along the pile shaft, sea water will be injected to form a lubrication layer between the pile shaft and the soil.
“This reduces the shaft resistance. The remaining soil resistance will be lower than the weight of the pile, enabling it to ‘sink’ into the ground. This all happens while the pile is still attached to the crane, controlling the rate of penetration.”
GBM Works is now developing four prototypes, of which the last will be the eventual product. Last year the company tested a first prototype as a proof of principle. A second prototype will install a pile of five meters long and a diameter of 25cm. The third will install a pile the size of a mooring pile, possibly in the Port of Rotterdam or Port of Amsterdam.
The fourth will be an offshore monopole, due to be placed in 2019.
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