Following the UK government's pledge to source at least 10% of all energy generation from renewable sources by 2010
Following the UK government’s pledge to source at least 10% of all energy generation from renewable sources by 2010, the wind farm construction industry is becoming an ever more demanding market. In 2005, in anticipation of this, MENCK began work on developing the MHU 1900S, the world’s most powerful underwater hammer currently available in a rental fleet.
MENCK engineers were instructed to construct a hammer suitable for use from small jackup boats with limited crane capacity. The hammer was to weigh in at less than 300 t but to have the highest energy rating possible.
“It was initially thought that the hammer would only be able to reach a 1500-kJ energy rating,” said Martin Ros, senior sales manager, MENCK. “However, MENCK engineers managed to stretch this to 1900 kJ without adding to the weight.”
The MHU 1900S was developed for shallow water applications to 400 m and with wind farm construction specifically in mind. In achieving a weight of 292 t, the hammer fits the standard 3.75-m-diameter monopiles that are common in offshore wind farm construction.
It was with this feature in mind that, in early 2006, contractor MT Højgaard came to MENCK with a request to hire the new MHU 1900S hammer for use on the Lynn and Inner Dowsing wind farm sites. MENCK is no stranger when it comes to developing effective solutions for uncharted conditions, and this particular project presented several new engineering and geotechnical challenges.
Acquired in 2003 by Centrica, Lynn and Inner Dowsing are two sites 5 km off the UK east coast that host a total of 54 Siemens turbines, each producing 3.6 MW. The major construction work is complete, and commissioning and full operation are anticipated by the end of 2008. It is expected that the two wind farms will generate enough power to meet the annual demand of more than 130,000 homes in the East of England. In the UK, Centrica supplies power and gas through its British Gas business.
It was planned that the foundations would be the new 4.74-m-diameter monopiles. To complicate matters, the contractor was faced with a sub-base consisting of several hard chalk layers: a condition that had not been encountered previously during wind farm construction. It was also important to calculate how much drilling would be required. A decision was taken to install a trial monopile to test the methodology. If this test pile did not penetrate to a minimum depth of 15 m, then Centrica would re-evaluate how to anchor the wind turbines.
In early 2006, a test 2.0-m-diameter monopile was successfully driven to 25 m using a MENCK MHU 800S hammer with a 2.2-m sleeve in order to analyse the soil conditions for monopile stability.
“Until now, as far as we know, monopiles have only been driven in soft chalk,” said Ros.
MENCK has an ongoing driveability calculation scheme, and this, combined with the expertise of its geologists and geotechnicians, enabled it to put together a suite of values to represent the chalk. This data was then used to calculate the drive-power-to-rock ratio for the 4.74-m-diameter monopiles and enabled MENCK to estimate the rock’s behaviour.
With the tests complete, the first MHU 1900S was scheduled for delivery in April 2007. This gave MENCK a year before the hammer needed to arrive, but it still faced one more challenge.
The 4.74-m-diameter monopiles are new to the industry and this was to be their first use. As the hammer is sized for standard 3.75-m monopiles, the piling operation required more fresh thinking.
“We did not have a large adaptor ring for the MHU 1900S,” said Ros. “Such a ring would take over two years to develop, and we needed a solution faster. Our answer was to fit the standard 3.75-m anvil with a 3.75-m pile sleeve and then manufacture a project-specific 4.74-m follower to adapt the hammer to the large pile.”
Piling commenced on 25 April 2007 from the MV Resolution and lasted through to November. The MHU 1900S was equipped with two MHP 1600 power packs to support the high energy requirements and two hose winches.
“We delivered the project on time and without incident,” said Ros. “Because of the flexibility within MENCK, we were able to develop and deliver a powerful hammer adapted for this special situation, thus avoiding lost time and extra expense and without additional drilling and environmental disturbance.
“I think that the performance of the MHU 1900S at Lynn and Inner Dowsing caught Centrica’s attention. They came back to us and said that the hammer was such an important tool that they wanted to secure it for the next project – Lincs offshore wind farm. This was before the project was awarded to a contractor!”
Ross Ovens, Lincs construction manager, Centrica Renewables, said, “The MHU 1900S is an extremely important piece of equipment in today’s market. With the UK government’s target of 10% of all energy generation from renewable sources by 2010, the need for reliable equipment that can drive large monopiles into challenging soil conditions is paramount.
“Both MENCK’s service and consultancy were invaluable for this project, and I look forward to working with the company again in the future.”