Complete site conditions integration
The atmospheric and oceanographic site conditions across a wind farm site interact with one-another to define the loading on the support structure foundation, both via the wind turbine and directly on the support structure itself. The geotechnical site conditions then define the response of the support structure, the dynamic characteristics of which feed back to further influence the loading. Hence the overall successful design of an offshore wind farm must not only capture the individual site conditions, but also their complex and collective interaction.
The challenge is further compounded by the governing site conditions being spatially variable across a site, or in some cases variable over the design lifetime, for example, seabed levels in environments subject to sediment mobility. This brings a further challenge of needing to generalise, or cluster, site conditions appropriately and sufficiently to enable practical design, but whilst retaining sufficient resolution to deliver a design which is optimised for each individual location.
Wood Thilsted’s approach to addressing this challenge and the underlying site conditions integration is founded on deep, discipline-specific understanding of the many different site conditions, and also their interaction. This is captured in our workflow using innovative software twinned with collaborative, inter-disciplinary working practices.
Pile soil analysis
Our 3D FE analyses of the soil structure interaction is fully integrated and controlled by our in-house monopile design software (Morpheus) to ensure consistency between data assumed for foundation design derivation and the 3D FE model. We have undertaken an extensive FE benchmark test toward the PISA pile load tests in Cowden and Dunkirk to prove accuracy and integrity of the models we use, to ensure that the simulations provide trustworthy and accurate results. Our models are calibrated solely to soil test data without application of post-fitting parameters. Therefore, when properly calibrated, our 3D FE models are highly reliable and capable of evaluating the impact of changing modelling conditions, such as changes of pile diameter, soil layering and parameters, etc.
The calibration of the constitutive models follow an innovative approach where the calibration parameters are automatically calibrated against relevant high-quality laboratory data using the least mean square method to find the most optimum fit to each individual test. This implies that up to hundreds of simulations are performed for each test leading to optimised calibration parameters for each governing geotechnical unit.
Wood Thilsted has developed a novel cyclic degradation methodology for assessing the effects of cyclic loading arising from wave and wind actions on offshore foundations. For laterally loaded foundations this method is calibrated and benchmarked against the PISA cyclic pile load tests and is considered to reflect a rather high degree of accuracy given the uncertainties and complexity of modelling this problem statement.
Cyclic loading can lead to considerable strength degradation and cyclic mobility which affects the effective geotechnical properties of the soil volume supporting the foundation. This can result in excessive deformations, collapse and/or fatigue damage reducing the design life or the operational constraints of the structure if not carefully accounted for. Thus, accurate modelling of cyclic effects is vital to achieve an optimised and safe design. Applying soil reaction curves derived through 3D FEA modelling requires a efficient cyclic framework in order to be applicable for substructure and foundation design derivation as these reactions offer a more accurate and less conservative prediction of design stiffness and capacity. Therefore, capturing effects of cyclic loads is of high importance.