Innovation meets fast-moving market

Scott Whyte is a geotechnical engineering consultant with global geo-intelligence and asset integrity solutions provider, Fugro, and a DEng student at the University of Oxford within the Renewable Energy Marine Structures (REMS) centre group. His joint industry consultancy and academic activities focus on the science behind designing cost-effective foundations for large-scale wind turbines with the aim of reducing the levelised cost of electricity (LCOE) for offshore wind in a fast-moving market.

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Whyte recently delivered a thought-provoking presentation, as part of the KIVI engineering society lecture series, titled: “Foundation Design Optimisation for Ever Larger Offshore Wind Turbines: a Geotechnical Perspective”. This article presents a brief overview of some the interesting components of the presentation.

In recent years, Whyte’s work has concentrated on geotechnical design aspects linked to the market-leading solution: monopile foundations. These substructures are now being utilised for the latest 8 MW – 9.5MW wind turbine generators (WTG), with monopile diameters ranging from 7.5 m to 9 m, utilised in water depths of up to 40 m. Whyte: “This is significantly beyond what was thought possible in terms of turbine size and water depth even five years ago. Dedicated ongoing research efforts by industry and academia have been instrumental in pushing the boundaries of monopile design, further enabled by continuous technology design advances and newly developed procedures being put into practice.” Whyte added that foundation design optimisation has been a significant component in reducing overall windfarm capital expenditure (CAPEX) which has helped contribute to the overall dramatic drop in costs of offshore wind projects in recent years.

A major challenge facing the offshore wind industry is the continued pressure to increase the wind turbine size and average array capacity coupled with the increasing development of sites further from shore with increased water depths. The 174-turbine 1.2 GW Hornsea One project along the UK east coast is planned for commissioning in 2022 and currently holds the size record for windfarms. This is only a temporary record, as larger projects are already in planning. Hornsea One covers an area of 407 km², is built in water depths ranging from 23 m to 37 m, and the distance from shore is reported as 120 km. Such large-scale windfarms covering large sea areas show increased potential for considerable soil variability between individual turbine locations. Whyte: “Subsurface conditions are generally very complex and challenging; at large windfarm sites, soil stratigraphy is often highly variable across turbine locations. Tackling this major site-characterisation challenge requires broad-scale thinking and fresh engineering considerations.”

“A major challenge facing the offshore wind industry”

Whyte believes that the best solution to overcome some of the issues associated with developing large windfarm sites is by building ‘fully informed’ three-dimensional (3D) ground models: “The development of 3D ground models as a site characterisation tool, hosted as digital GIS databases, combining a large amount of multi-disciplinary data (e.g. metocean, geological, geophysical, geotechnical) allow the project team live access to the latest datasets from a single, intuitive, web-hosted map-based system. The use of a holistic digital database to host and store all the geo-data associated with the site is pivotal to help mitigate potential geo-risks across large wind farm sites and develop the most optimised design solutions.” Whyte continued to explain that across a large windfarm site, there will be many different geotechnical constraints, some of which are not immediately obvious, such as: pile buckling risks, failure to install suction buckets, shallow (insufficient) pile penetration length, gravity-based foundations not being suitable due to local bathymetry, and dynamic seabed/scour conditions. Such geotechnical constraints can be identified and mitigated at an early stage with the use of a 3D ground model.

Whyte added: “The days of using many individual uncoupled site investigation reports are behind us and the importance of a holistic integrated digital ground modelling approach is now widely recognised as essential by windfarm developers. The 3D ground models are often coupled with other spatial drivers, such as wake turbulence modelling and cable connection least cost maps, to inform developers on the most economical wind turbine layout.”

Another recent development in site characterisation for offshore wind farm sites, Whyte discussed, was closer co-operation between the geotechnical contractor and the foundation designer at an early stage of the site investigation process. Historically the geotechnical design consultant would inherit data collected by a site investigation contractor. Unfortunately, this was rather inefficient and often led to incorrect data being collected for advanced design methods, which often requires highly specialised testing. However, more recently, geotechnical site investigation contractors and consultants are working together at the inception of a project to maximise efficiency. Whyte stated: “Fugro, recognising this, have incorporated their site investigation and consulting divisions, into a harmonious site characterisation service line, which enables consultants and contractors to work collaboratively from an early stage in the project development cycle. This subtle, but significant change in the site characterisation process doesn’t seem like a seismic shift; however, coupled with advanced analysis methods, this has already started to yield further cost savings”.

“As the project evolves from an early concept design”

As the project evolves from an early concept design and data collection stage to a detailed design phase the complexity of the analysis methods used also increase with highly advanced numerical tools being employed more regularly with the aim of reducing costs. Whyte stated: “Although design standards play a very important role in the design of offshore wind turbine foundations, the use of niche and bespoke numerical analysis techniques are being employed increasingly frequently to refine the foundation design.”

Recent publication of the PISA joint industry research project, which was concerned with the development of improved design methods for offshore monopiles, presents a good example of the way the design process for offshore wind turbines is rapidly progressing. Previously, offshore wind monopile foundations were being designed according to Oil and Gas design standards, however, monitoring data of installed wind turbines highlighted these foundations were often significantly over-designed. As a result, the PISA project, which was led by the University of Oxford, Imperial College London and Ørsted Wind Power, focused on developing a new design method which the industry could utilise for the design of offshore wind turbine foundations. The PISA approach outlined a site-specific methodology for the design of offshore monopiles, as opposed to a rigid codified approach. This methodology involves performing a small number of 3D Finite Element Analysis (FEA) calculations, which can often take extensive periods, to develop site-specific reaction curves to use in 1D pile analysis calculations that can then be performed rapidly to dramatically reduce the effort required for foundation optimisation across a site. As a result, 3D FEA calculations are now being employed more routinely to assist in optimising the design offshore wind turbine foundations.

Whyte went on to show several examples of bespoke soil models which have been developed by Fugro to better capture the salient features of soil response in 3D FEA calculations. Such soil models, termed constitutive models, are a mathematical representation of soils mechanical behaviour, and a fundamental building block of a geotechnical FEA calculation. Previously, the use of such advanced numerical models were confined to the realms of academia; however, they are now being developed and implemented by specialised geotechnical consultants to better predict the foundation response and hence allow for more optimised design. Whyte added that: “The use of such advanced soil models and numerical analysis techniques must be coupled with suitably advanced laboratory testing at the site investigation stage to maximise potential savings.” Whyte also remarked on the complexity of monopile installation, in particular, highlighting the need for pile buckling to be reviewed in detail. Unfortunately, Whyte felt this was, until recently, often being overlooked by many windfarm developers.

Finally, Whyte highlighted the need for the industry to concentrate on developing cloud-based engineering design applications to develop complex foundation optimisation tools: “Fugro, in collaboration with Dr. James Doherty from University of Western Australia have focused on developing cloud-based applications that allow for rapid and rigorous foundation optimisation to be performed. The scalability of these numerical tools is truly incredible with the potential to run a huge number of calculations simultaneously across a virtually unlimited number of processors. Having access to an almost unlimited computer resources means that reliability-based design optimisation approaches (i.e. accounting for the uncertainties in the design process) become more feasible.”

This article was previously published in the Offshore WIND magazine, issue 3, 2018.