IN DEPTH: Cost imperative drives monopiles to new depths

WORLDWIDE: Monopile foundations, by far the most popular foundations to date for offshore turbines, with 66% of the market, are having to adapt to survive in a future of wind farms set in ever-deeper waters and increasing turbine size.

Pillar of strength…  Monopiles have been the mainstay of offshore foundations (pic: Dong Energy)
Pillar of strength… Monopiles have been the mainstay of offshore foundations (pic: Dong Energy)

While monopiles have been written off by many as shallow-water foundations, the XL (extra long) monopile, designed for water depths of up to 30 metres, is offering hope for this type of foundation.

Danish civil-engineering firm MT Hojgaard has analysed a monopile foundation for water depths of up to 35 metres. The company considered monopile and jacket foundation concepts suitable for a generic 6MW turbine, looking at cost, risk and the time taken for design, manufacture, installation, and operations and maintenance. XL monopiles compared favourably.

Untapped potential

"During this work we discovered a huge untapped potential for XL monopiles," says offshore director Kim Andersen. One significant advance was the production of a massive ten-metre diameter cylindrical steel section, manufactured from a single 95mm-thick plate that is 31.4 metres long and three metres wide.

The company that achieved this feat, EEW SPC, is expecting to receive enquiries this year for XL monopile cylinders of up to nine metres in diameter.

Soren Juel Petersen, director of business for offshore wind at engineering consultancy Ramboll Wind, is thinking further ahead. Although Ramboll, which designs offshore foundations, currently has monopile foundations installed in depths of 32 metres, Petersen believes that in as little as five years, XL monopiles could be used in water depths of up to 60 metres.

But with size new difficulties arise, and the logistics of transportation, storage and installation can make larger components commercially unviable. Petersen admits the current limitations. "As the ratio of pile diameter and plate thickness increases there is a larger risk of plates buckling during driving," he says.

Joachim Dahms, structural engineer at Bard, agrees, saying that it is theoretically possible to design XL monopiles with a diameter of eight metres and plates with a thicknesses of 85mm. But the risk of buckling will be greater both during installation by the sea fastening and lifting gear, and after installation when the monopile will be under greater residual stress.

EEW, which is already tendering for projects with nine-metre diameter piles, believes that thicker plates will increase stability. Furthermore stiffeners could be used for critical points. It has consulted with various engineering, procurement and construction contractors, who think it is a proven technology. EEW claims to be able to bend plates with a wall thickness up to 160mm. It will make the nine-metre diameter pipes using plates between 115 and 135mm thickness.

Logistics

For Peter Burton, head of foundations at Centrica, the problems with XL monopiles start before the foundation gets offshore. "The main problems are the logistics of transporting monopiles of that size from manufacturer to storage facility, as well as then taking them offshore, upending them and driving them into the seabed."

Vessels, too, become a major issue for installation. According to Lars Andersen, deputy head of structural mechanics in the civil engineering department of Denmark's Aalborg University, installation vessels and driving equipment have yet to be developed to cope with monopile diameters much beyond seven metres.

Petersen explains that while the advantage of monopiles up to now has been that they could be installed using relatively small vessels, the limited availability of vessels and hammers equipped to install XL monopiles is driving up costs. However, he is optimistic that the problem will be short-lived. "Once service providers realise XL monopiles provide vessels with larger payloads, new vessels will be built," he says.

A further installation challenge facing XL monopiles is that the tremendous power required to drive them into the seabed can cause damage to fish and seals. "Even smaller piles cannot be installed without risk to marine life," says Andersen. "Other installation methods, such as drilling, can be used to reduce some of the environmental impact, but this will require a drill that is as large as the ones used for bored tunnels."

The German interpretation of EU guidance on underwater noise is strict, Petersen notes. This could prove an obstacle to installing large-diameter piles if other countries take a similar stance. As yet, there are no proven concepts for reducing noise emissions. Petersen says it has not yet prevented any projects from going ahead as far as he knows, but admits it is an area of considerable concern.

Design factors

Before getting as far as installation, problems need to be tackled at the design stage. To realise the full potential of XL monopiles, design methodologies need to be updated because those used for conventional monopiles are not sufficiently accurate, Andersen explains.

Unlike gravity foundations, which are designed according to bearing capacity at extreme loading, the design of large monopiles needs to take into account how much they will deform in operation, he says. The main concern is to ensure sufficient stiffness to withstand the dynamic force of wind and waves.

Slender piles are typically designed by analysing data on a graph showing the ability of deep foundations to resist loads applied in the lateral direction. The so-called p-y method results in a curve that can be used to assess the force applied to the soil by the pile against the lateral deflection of the soil. However, existing p-y curves are not able to predict the stiffness of XL monopiles, which have a larger diameter-to-length ratio, Andersen points out. Tests suggest that some calculations may be out by as much as 30%, leading to conservative designs, he warns. "P-y curves could be 'updated' by rigorous numerical models or prototype tests of representative soil profiles and pile geometries," he says.

A more reliable and cost-efficient design will call for better site-assessment methodologies, Andersen says. Current geotechnical site assessment techniques do not focus on XL monopiles, which require a certain strength of soil to ensure they are not distorted by wind and waves below, or the weight of larger turbines and the frequency of the rotor above.

Petersen contends that new investigation methods are needed to assess the quality of the soil's stiffness - the ability to withstand deformation — and its strength — the ability to carry load without collapsing. "While it is true that there is a correlation between soil stiffness and strength, it is too general to say that the softer the soil, the lower the strength," he says.

"Clay, sand, rock and chalk all respond in different ways to loading, and cyclic-loading - typical of wind turbines — causes different soil to degrade in different ways. As piles get longer there needs to be better analysis of soil geology as well as strength. In Germany they use more advanced laboratory testing on samples extracted from the site and this practice should increase and become more widespread."

Petersen does not foresee a huge difference in cost between conventional and XL monopiles. He thinks that from an operations-and-maintenance perspective there will be benefits of scale: "If you have a 600MW project, using 6MW turbines will mean you only have 100 turbines to service as opposed to 150 4MW machines.

"Monopiles are nearly always the cost-optimal foundation solution," he says. "So it makes good economic sense to extend the conditions in which they can be used. We have all the knowledge already. When everyone starts to realise the benefits of investing in XL monopiles, 60-metre water depths will cease to be only possible in theory, and become widely used."

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