07 Jun 2011
To ensure offshore wind turbines are still upright 20 years down the track, more research is needed into the behaviour of the soil structures underlying them, says the latest report from Wind Energy Update.
A distinguishing feature of offshore wind turbine structures is that the turbines impose a large horizontal load, at a great height, above the turbine’s foundation. The over-turning forces at foundation level resulting from this high ratio of lateral-to-vertical loading implies a loading regime that is very different from that of existing offshore oil and gas installations.
“The behaviour of the soil / foundation system at these high ratios, particularly over the long-term under dynamic loading, is not as well researched and understood as for lower ratios of horizontal to vertical stressing,” says Alan Tricklebank, author of the Offshore Wind Installations and Construction Report.
“There is a case for further basic research on this aspect of foundation design given the importance of achieving a long and reliable life for turbine support structures,” he adds.
Water depth, exposure, and soil properties
The UK’s Round 3 sites are situated predominantly in depths beyond 20m, with 54% in the 20-40m range; 17% in 40-50m; and 25% beyond 50m depths. For the transitional depths (20-50m), the major lateral load of the turbine on an otherwise relatively modestly loaded structure is further compounded by additional loadings from waves and currents, which increase significantly as water depth and exposure increases.
How these forces are resolved into geotechnical loadings and how they impact the foundation depends on the foundation configuration and soil properties. Some configurations, particularly monopiles, put much greater reliance than others on lateral resistance of the soil to provide stability, notes the report.
“Of course, all foundations need to provide substantial resistance to lateral forces, whether through friction and shallow penetration in the case of gravity bases, or through deeper mobilisation of passive pressure in the soil in the case of piled foundations”, explains Tricklebank.
The problem is that designers are currently working with limited test data, which has led to problems such as grout failure in monopiles, and wide variations in calculations. “A comparison of designs from three different bidders for monopile-supported met masts revealed that on some sites, up to a 300% difference in the weight of piles was calculated”, notes the report.
For jacket structures, axial tension in the foundation piles normally controls the design. But for typical offshore conditions in very dense sand, different calculation methods have yielded results for lengths that vary by a factor of 2, with considerable inconsistencies between methods in the load capacity predicted, with different embedment depths. “Actual offshore experience of relying on these methods is either limited, or does not exist,” notes the report.
Reliable, affordable designs
At the end of the day, the detailed engineering design is not simply a matter of overturning moments.
“There is structural modeling and analysis, and matching the structural resistance to the loading envelope to consider. Attention must also be given to dynamic tuning of the structure, fatigue strength, and to achieving fatigue–robust, corrosion resistant and environment-robust details, particularly at nodes, or connections, among other things” explains Tricklebank.
Furthermore, given site variability, competitive foundation designs need to be suitable and adaptable for a wide range of site conditions. They need to be straightforward to fabricate, and amenable to series production and rapid offshore installation.
“Round 3 will be a catalyst for investigations leading to optimised design procedures that yield more reliable and economical designs”, predicts Tricklebank.
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