Noise mitigation during pile driving has long been an issue in the offshore wind industry, as has the growing size of monopiles – the Blue Hammer departs from convention and could provide a solution
Developed by a Fistuca, a Dutch company that started life as a spin-off from the faculty of mechanical engineering at Eindhoven University of Technology in which crane manufacturer Huisman has a stake, the Blue Hammer is a pile-driving solution unlike any other.
Rather than hammering a monopile into the ground, the Blue Hammer concept developed by Fistuca uses acceleration of a water column by a gas mixture to provide the driving force – a mechanism that can deliver a large amount of energy without exciting undue vibration in a monopile.
As Jasper Winkes, founder and director of Fistuca, and Henrik Bisgaard Clausen, a senior consultant at Ramboll in Denmark, told the Offshore Wind Energy 2017 conference and exhibition in London in June, a prototype of the new concept has been successfully tested.
“The combustion of a gas mixture leads to an increase in pressure. This pressure creates an upward acceleration of the water and a downward force that pushes the pile into the seabed,” Mr Winkes explained. “Another powerful downward blow is delivered to the pile when the water mass falls. After the exhaust gases have been released, the cycle is repeated.”
Mr Winkes believes that ‘blue piling’ has a number of advantages compared to conventional impact hammers. Firstly, the hammer produces very low levels of noise compared to conventional hammers. This means that noise reduction measures – which can add significantly to the cost of piling operations – are much less likely to be required. The underwater noise level produced by the Blue Hammer is approximately 20 dB lower than that produced by conventional hydraulic hammers. “This means that noise mitigation is unnecessary in most conditions,” Mr Winkes said.
Secondly, compared to the impact of a conventional hammer, the duration of the impact from the Blue Hammer is much longer, which minimises fatigue damage to the monopile. “Conventional hammers exert force over a short period of time,” Mr Winkes explained. “The Blue Hammer delivers a blow over a period of more than 100 milliseconds. This makes the force delivered by the system more efficient. The water-based blows ensure a gradual build-up of force on the monopile, reducing tensile stresses in the pile and reducing acceleration levels. Fewer blows and lower stress levels reduce installation fatigue.”
A third and increasingly important benefit of the Blue Hammer, given the rapid growth in the size of offshore wind turbines and in their foundations, is that it is able to drive large-diameter piles. Another advantage is that the concept is compatible with having secondary attachments mounted on the monopile before pile driving is undertaken. Lower acceleration levels allow piles to be partially pre-assembled prior to piling, reducing the loads on components such as flanges. Based on a generic monopile design and a typical North Sea site, Ramboll has demonstrated that monopiles can be driven into the seabed using the Blue Hammer and that this can be done with pre-mounted attachments such as boat landings, internal platforms and cathodic protection equipment installed. This leads to a reduction in the amount of work needed on a monopile once it has been installed offshore.
Apart from being able to drive larger-diameter monopiles, Blue Hammer is also well suited to doing so in deeper water. “It is suitable for all types of conventional piling and foundation work using jacket (pin) piles as well as so-called XL monopiles. It interfaces with the pile in the same way as conventional piling technology, so the pile itself doesn’t need to be adapted in any way,” Mr Winkes concluded, noting that Huisman Equipment is currently building a Blue 25M, which Mr Winkes believes will be the largest and most powerful hammer in the world, on behalf of Fistuca. It is due to be delivered by the end of 2017 and will be tested at Port of Rotterdam in the first quarter of 2018 and then offshore in the second quarter of 2018. Several major players in the offshore wind industry have shown interest in supporting the demonstration.