Work on a research project aimed at investigating and developing improved design methods for laterally loaded piles specifically tailored to the offshore wind sector has called for some specialist pile testing work.
Ground investigation and testing specialist ESG worked with the Pile Soil Analysis (Pisa) project team to develop cutting-edge testing methods for the large-scale field testing phase of the research.
Pisa is a research project dedicated to improving the design of laterally loaded piles, with the aim of creating a new generation of more cost-effective, yet robust, offshore wind farms.
Pisa is a joint industry initiative run through the UK’s Carbon Trust’s Offshore Wind Accelerator programme (see box) and involves many of the major players in the offshore wind sector including EDF, Iberdrola, RWE, SSE, Statoil, Statkraft, and Vattenfall, all under the direction of Dong Energy as lead partner.
The Pisa project is also supported by an academic working group, led by the University of Oxford, with support from Imperial College London and the University College Dublin.
The most popular method of analysis for laterally loaded piles – and the method adopted in the offshore design codes – is based on the Winkler model and is commonly termed the p-y approach. Despite its relative simplicity, this method ofanalysis has successfully been used in the offshore oil and gas industries for many decades.
However, the current p-y approach is not well suited to predicting the response of piles with the geometry and loading seen in the offshore wind industry.
Cost savings expected to be delivered by the Offshore Wind Accelerator programme
A new design methodology will be developed during the Pisa project to provide greater confidence in the prediction of pile response under the lateral loading seen by offshore wind turbines.
A more accurate method of design may allow monopiles to be installed in deeper waters than is currently possible using the existing standards, and potentially with larger turbines on top.
The UK has been leading the world in offshore wind power for almost a decade now, with as much capacity already installed as the rest of the world combined, which is why much of the research for Pisa has been undertaken by UK-based academics.
Since the first series of licences – known as rounds – were launched in 2001 by the Crown Estate, which owns the seabed off the UK coast, the nation’s offshore windfarms have grown to such an extent that they now generate almost 15TWh of electricity annually – enough energy to power more than 3.5M homes.
The third round of project licensing in 2010 was intended to take offshore wind generation even further in terms of generation capacity and the depth of water in which the turbines will be installed.
Electricity generated annually in the UK from offshore wind
Crown Estates identified up to 33GW of potential additional offshore wind capacity, across nine zones, including a large number in the Dogger Bank area of the North Sea. These locations are a considerable distance from the coast, making them potentially challenging and costly to develop, but once operational, they will bring the UK significantly closer to achieving its government’s target of generating 15% of its energy from renewable sources.
However, to fully develop the zones identified in the third round of licencing, especially those in more remote locations, steps would have to be taken to simplify offshore construction and streamline costs.
Changes in particular needed to be made to the design of the monopiles used to support wind turbines to reduce material use, as well as simplify transport and installation.
All of this would have to be accomplished without compromising on their performance or longevity.
As part of Pisa, the load-bearing capabilities of monopiles of different diameters in varying soil types was to be put under the spotlight in a bid to create new, more cost-effective designs to replace existing models.
“The monopiles currently used in the offshore wind sector are based on designs initially created for groups of long and slender piles used in the offshore oil industry in the 1960s,” explains Dong Energy lead geotechnical engineer Alastair Muir Wood, who is also technical manager for the Pisa project. “In contrast, wind energy monopiles are shorter in length and larger in diameter, with differing loading criteria.”
As a result of these differences, the current design codes do not allow for efficient use of resources, which not only increases the cost of raw materials during manufacture, but also adds to their weight, making them more expensive to transport and install.
Recent measurements of natural frequencies in Dong Energy’s wind farms suggest that monopiles are restrained much closer to the seabed than was previously believed, indicating that wind turbines are, in reality, much stiffer than traditionally assumed by designers. This means that, in theory, monopiles can be designed with smaller diameters without compromising on their stiffness or strength. If this can be achieved then material use and component weight could be minimised, potentially cutting the cost of offshore wind farm construction and, in turn, the cost of renewable electricity.
Muir Wood adds: “We wanted to find out whether shorter and wider monopiles could be created without impacting on the robustness of the finished turbine, but for that we needed accurate site data. Having a long-standing relationship with ESG from a number of similar and successful research projects in the past, we had no doubt the company could deliver precision testing of the new monopile designs.”
DEVISING THE TESTS
ESG began working with Dong to design and create appropriate test rigs to validate the research work done by the academic working group.
“The current design methodology was developed in the US some 50 years ago, in response to the increase in offshore oil exploration at that time,” says ESG operations director for Infrastructure Services Martyn Ellis. “Our brief was to design robust and accurate instrumentation that was repeatable on a range of differing pile sizes at two separate sites to validate the new proposed design standards.
“Taking readings of the load at or below ground, however, as is done in traditional testing methods, doesn’t reflect the real world loading of wind turbine monopiles.We had to devise systems that would identify how piles performed under lateral loading at the point where the load is greatest to understand both pile and soil stiffness responses and failure criteria at two sites with different soil types. This meant not only carrying out lateral loading above ground, but also applying cyclic loading for up to five days on one pile ateach location.”
ESG spent six months developing new methodology that would deliver accurate, traceable results, as well as test rigs that were not only appropriate for the readings required, but would also be practical in the field. The result was a testing method for load application that used standard instrumentation, such as fibre optic wires, retrievable extensometers and in-place inclinometers carefully installed in the correct locations to collect the necessary data.
Dong and ESG took the new testing method to two of Pisa’s test sites – one is located in northern England at Cowden near Hull and the other is in northern France at Dunkirk. The two sites were chosen as they represented the extremes of the ground conditions under which monopiles are installed, with cohesive clay at Cowden, and non-cohesive sand strata at Dunkirk.
Testing was carried out in two phases, first at Cowden, then at Dunkirk, with 28 tests undertaken in total – 14 at each site – to validate the new design codes created by the Pisa academic working group.
Readings were taken for piles with diameters of 0.273m, 0.762m, and 2m, as well as different wall thicknesses and penetrations, to allow for the validation to be scaled up to the current 10m diameter design proposals.
As in traditional testing, piles of the same diameter and foundation depth were installed in the ground, each 28m apart from each other, and a hydraulic rig designed to pull the monopiles together was fitted to each pile 10m above the ground. Where the new test differed from the old approach was in the number of instruments used and where they were located.
“On the largest test, involving two 2m diameter piles, we fitted some 220 individual instruments, both on and in, the piles,” says Ellis.
“Many of these were for control purposes and validation of measurements. The similar results obtained from both the fibre optic and extensometer measurements meant we could be confident that they were accurate.”
The test rigs presented a number of challenges that had to be overcome to ensure the success of the project. Measuring instruments from a number of suppliers were used and needed to be connected to synchronise with each other and transmit data to a single centralised control hub. The hydraulic systems employed to pull the piles together also had to be responsive and operate at high speeds for long periods of time to ensure an exact load was applied and controlled at the right frequency.
The biggest challenge, however, came from setting up and monitoring the equipment so high up in the air.
Ellis says: “The team developed a safe system of working at 5m and 10m above the ground, minimising the amount of work at height required, as well as adaptability from one pile size to another. This effort was vital though, to ensure the test readings offered the necessary precision and repeatability, while upholding worker health and safety.”
The information collected during the Pisa tests, at both sites, are still being analysed by the programme’s academic work group, but preliminary findings are positive for the project.
Testing in the clay soils at Cowden showed that the monopiles behaved as predicted. The piles tested at Dunkirk, however, were shown to be even stiffer than initial computer models suggested, creating an opportunity to refine the soil modelling used.
“While we’re still studying the findings, the implications of the tests at Cowden and Dunkirk are enormous,” says Muir Wood. “If we can reduce the thickness or length of the monopiles by even a small fraction, the cost savings can be considerable. The next challenge will be applying what we’ve discovered here to pile foundation designs.”
Ellis adds: “The testing has shown that lateral loading and cyclic loading doesn’t have to be confined to ground level testing.”
The Pisa research project is still ongoing, but its work has already identified a host of design improvements that could enhance performance and streamline the cost of offshore construction.
For more information, visit www.eng.ox.ac.uk/geotech/research/PISA or www.carbontrust.com/our-clients/o/offshore-wind-accelerator
Under the Offshore Wind Accelerator (OWA) programme, the Carbon Trust has brought together nine offshore wind developers in a joint industry project to work towards reducing the cost of offshore wind by at least 10% ahead of the third round of offshore projects in the UK.
The Carbon Trust has described the OWA, which was set up in 2008, as its flagship collaborative research and development programme.
“Technology challenges are identified and prioritised by the OWA members based on the likely savings and the potential for the OWA to influence the outcomes,” says the Carbon Trust in its mission statement for the OWA.
“Projects are carried out to address these challenges, often using international competitions to inspire innovation and identify the best new ideas. The most promising concepts are developed, de-risked and commercialised as the OWA works closely with the supply chain throughout the process. The OWA model brings together Carbon Trust’s expertise in delivering innovation and convening industry consortiums with the industrial partners’ technical knowledge and resources.
“The OWA is two-thirds funded by industry and one-third funded by the UK Department of Energy and Climate Change (DECC) and the Scottish government.”
Pisa is just one part of the foundations issues being considered by OWA.
Other projects are looking at the potential offered by suction bucket jacket foundations and vibro driving techniques for the turbines themselves, as well as new techniques for constructing the foundations of the met masts.
In addition to issues relating to the foundations of future offshore wind farms, the OWA has also looked at the challenges faced in the areas of cable installation, electrical systems and access.