Wind Energy Update speaks to independent geotechnical and engineering geology consultant, Dr. Chris Golightly, Proserv Offhore's Renewable Energy Manager Mike Yeomans, and Det Norske Veritas' engineering experts to learn which retrofit and future design solutions are best for monopile foundations.
By Rikki Stancich
Roughly 600 of Europe’s installed 948 offshore turbines have been prone to grout connection failure, causing turbines to tilt within their monopile foundations. But according to some, the problem lies less with the grout itself, than with a design that relies on grout to hold the foundation and transition piece together.
Independent geotechnical and engineering geology consultant, Dr. Chris Golightly, explains that, unlike oil and gas platforms that are more stable due to their sheer weight, wind turbines are relatively light and experience a lot of bending moments over their lifecycle. “The constant bending results in the grout crushing at the top and bottom; the progressive failure of the grout causes the structure to drop and tilt slightly”, he says.
An emerging consensus in the industry is that monopile foundations are failing primarily due to the widespread non-use of shear keys.
“In the oil and gas sector it is common practice to use shear keys. But the DNV J101 2007 design code left it open for designers not to use them – which is what [designers] did”, says Dr. Golightly.
“So the grout is failing, but ultimately, the failure is down to the design,” he adds.
Dr Golightly points out that during offshore installation of the transition pieces, there is limited, accurate knowledge of the actual thicknesses of the grout sheaths injected into the gap, or of the variation in thickness of the grout annulus itself.
“This is a fairly critical parameter required in analysing the capacity of any grouted connection, with or without shear keys. A wind turbine subject to cyclic wave and wind loads over a 20-25 year design life, being held by a hollow cylinder of brittle rock of unknown and variable thickness sandwiched between two steel tubes, is not a good situation.”
If this is the case, why did designers opt for grouted connections? According to Dr Golightly, the original Danish practice of abandoning shear keys in favour of a well-tested appropriate underwater grout was developed as a means of grouting piles into bedrock at Danish sites.
On the basis of the techniques relative success at the Danish sites, the technique and the grouts were widely adopted for the grouting together of structural steel piles and transition pieces to wind turbine towers - “basically to save time and money”.
Referring to the Scroby sands project, which opted for the more expensive process of using a bolted flanged connection, and the Beatrice offshore Scotland (DownVind Project), where an oil and gas technique known as “Hydralok” was used, Dr Golightly argues that the only monopile foundations that haven’t failed are the ones using shear keys.
“The industry should be looking at steel-to-steel connections using flanges and bolts, the old-fashioned way”, he says.
Retrofits: expensive, but promising
Several solutions to remedy ailing monopiles have come to the fore. A promising solution put forward by cutting specialist and offshore contractor and engineering firm, Proserv Offshore, involves inserting a series of pins through the walls of the transition piece, the grout and the pile wall and securing them. The pins effectively act as shear keys.
“Once in, [the pin’s] precision fitting precludes any movement between the transition piece and the monopile”, explains Mike Yeomans, Proserv Offshore’s renewable energy manager. A major developer plagued by failing grout connections in their monopiles has already expressed an interest in Proserv’s solution.
Looking ahead, the best solution, says Dr Golightly, would be “to abandon grouted pile to TP connections completely and devise a working bolted flange connection in conjunction with in tower seismic industry style dampers”.
In lieu of this, he suggests the adoption of a configuration where the structural installation and fabrication tolerances are sufficiently well defined and tightened up, the annulus thickness is better known, shear keys are proved to be able to function properly at correct spacings, possibly also adopting vertical reinforcement strips or lines orthogonal to the circumferential weld bead.
“In addition there should be development of an accepted spring-loaded supports, as in Statoil’s Sheringham Shoal solution”.
Cracks in the solution
While the DNV has initiated a new joint industry project that looks at design practices for grouted connections with shear keys, it is also recommending a design practice using conical shaped connections, where the monopile and transition piece are fabricated with a small cone angle in the grouted section.
Dr Golightly warns that such a solution is questionable. “The philosophy of the design seems to be to allow, or even expect, failure and for the cone to permit the joint to recompress to some sort of stable condition. This would not be permitted say, in the reinforced concrete design of large-scale buildings.”
DNV’s engineering experts defended the design in an email to Wind Energy Update, saying: “in the design of connections with shear shear keys in pile sleeves the axial load is absorbed mainly by the shear keys alone. Based on tests it is observed that the bonding between the grout and the steel wall is also broken here before the shear keys are fully activated.”
As for the comparison with multi-storey structures and bridges, DNV adds: “the reinforcement bars used in these concrete [multi-storey building and bridge] structures will also not be activated before small cracks occurs in the concrete used.
“A similar principle is applied when using a conical connection that the resistance from the cone geometry is not fully activated before the bonds between the steel and the grout is broken.”
Mike Yeomans, Renewable energy manager at offshore Contracting and engineering firm Proserv Offshore, concurs that DNV’s conical concept is based upon sound engineering principles. “A tapered design is always a good way of locking two components together”.
But he warns that product quality will be an issue. “The piles and transition pieces are large cylindrical structures which will require close cylindrical tolerancing; any ovality of either component will affect the locking together of the two.”
To respond to this article, please write to the editor: Rikki Stancich
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