Gravity base foundations in offshore wind – design drivers
21 April 2015
Authors: Dr Azadeh Attari, design engineer and Dr Paul Doherty, managing director – both Gavin & Doherty Geosolutions Ltd
The gravity concept, originally implemented in the oil and gas sector, is based on utilising the dead weight of the foundation material (typically concrete) to generate the restoring forces required to resist the high lateral loads and overturning moments resulting from the service loads.
However, the significant dead-weight of the foundation usually results in costly transportation and installation operations, given the high charter rates of the required vessels, lifting cranes and infrastructures. Achieving the EU targets for the levelised cost of energy (LCOE) of offshore wind encourages development of alternative approaches with cost-reduction potentials.
Several gravity concepts have been proposed in recent years to attain a self-buoyant gravity base foundation, thereby minimising the need for costly marine operations. This article reports a parametric study that investigates the feasibility and cost-benefit of such concepts, in terms of performance, inter-mediate stability and their impact on the overall cost of foundations.
Often pitched as an unconventional substructure, gravity-base foundations (GBFs) are in fact one of the most common foundation types employed in the offshore wind industry to date (Figure 1). At the end of 2013, some 12% of the total number of fully-installed substructures in the European waters were gravity bases.
Considering that the foundation self-weight provides resistance against the overturning moments, determining this parameter leads to a trade-off between the manufacture, transportation and installation costs on one hand, and the structure’s reliability to resist the service loads on the other hand. Therefore, a common approach taken by industry is to minimise the dead weight of the foundation during transportation and installation, and increase it once the foundation is installed in the offshore site, by means of in-place ballasting.
Cost reduction potentials and costly marine operations
Concrete gravity-base foundations introduce several benefits to the industry, including a reduced dependence on the volatile steel prices and also eliminating the need for piling the seabed and the assosciated noise concerns. GBFs are promising concepts in terms of potential cost reduction, especially as the industry is moving towards deeper water sites. However, reaching the EU cost-reduction targets for offshore wind energy requires further improved efficiencies across the supply chain of GBFs.
The relatively large weights, in the order of several thousand tonnes depending on the water depth and the service loads, imply that transporting GBFs from the construction site to the offshore windfarm site and the subsequent installation phase requires heavy-lift vessels (HLVs) and cranes with high lifting capacities.
As such, the cost of transportation and installation usually comprises a considerable share of the foundation cost. Considering that future windfarm developments are targeting sites located further offshore, and in harsher sea-states, this is an important issue to address to reduce the cost of offshore wind energy.
To address the concerns over heavy lift infrastructure, vessel costs and crane capacities, a range of GBF geometres have been proposed in recent years, with the aim of achieving a self-buoyant foundation that can be floated out and towed to the installation site using tug-boats – hence eradicating the need for costly vessel spreads. Once in position at the installation site, the foundation will be lowered to the seabed and installed by ballasting. Succesful implemention of these sensitive marine operations requires detailed analysis of the hydrodynamic stability required for float-out, transit and ballasting.
The overall performance of the GBF in terms of buoyancy and intermediate stability during these phases are determined by various geometrical attributes such as the shape of the substructure, the overall height of the structure and width of the bearing area, and the arrangement of internal ballasting chambers in the base.
To achieve the required levels of hydrodynamic stability in the interim transport and ballasting phases, the structure may require additional volume, resulting in higher material costs than may be otherwise requried for the in-place condition. The cost trade-off between material, heavy lift infrastructure and vessel costs is further complicated by additional factors such as the availability of light-weight aggregrates.
The overall economical merits of floated versus lifted GBF needs to be determined by giving consideration to the inter-dependence of the increased weight of foundation, and the reduced cost of vessels and equipment. The constraints and bottlenecks involved in the various stages of implementation of each solution should be investigated, in terms of availability and cost of the associated logistics, e.g. port infrastructure and load-out requirements. These factors are highly site dependent, varying for each windfarm with distance to shore, proximity to key ports, availability of materials and regulatory requirements for each country.
The relative impact of the structural dead-weight for improving the stability in comparison to its effect on the bearing capacity should also be evaluated. The relative cost-benefit of the two concepts (self-buoyant vs lifted) is a highly debated issue that many developers feel requires to be addressed through cost assessment of the holistic supply chain network.
A reliable cost-benefit analysis mandates conducting a comprehensive and multi-disciplinary study. It also requires insight and feedback from various key players across the supply-chain network. Relative cost and availability of suitable ports, aggregates, construction sites, capable vessels etc are the important parameters to be considered, for an informed evaluation of lifted versus floated gravity bases.
This will be addressed as part of the scope of work of LEANWIND, an EU-funded research project that started in late 2013. LEANWIND’s consortium is comprised of 31 industrial and academic European partners, specialising in various fields of the offshore wind industry. The main objective of the project is reducing the levelised cost of energy (LCOE) from offshore wind, by facilitating the industrial application of the innovative concepts with cost-saving potentials.
As part of the scope of the LEANWIND project, Gavin and Doherty Geosolutions Ltd (GDG) and ACCIONA Energy are conducting a parametric study considering the potential advanatages of different GBF geometries and modes of construction. This study represents the first step in establishing the cost benefits that underpin the technical issues surrounding the hydrodynamic stability required for self buoyancy of the gravity-based foundations.
Apart from the hydrodynamic stability, the in-place analysis should consider the impact of increased foundation loads on the geotechnical capacity of the seabed deposits (Figure 2). Seabed competency is of prominent importance when deciding on the feasibility of GBF for a specific offshore site. Therefore, an optimal design should address both stability during transport and most efficient soil-strucutre interaction models.
The preliminary results of the LEANWIND GBF study were presented to the stakeholders during the EWEA Offshore 2015 conference last month.
Dr Azadeh Attari is a design engineer at GDG, working on geotechnical and foundation design projects. She completed her PhD on probabilistic modelling of reinforcement corrosion in sustainable cement combinations, and now applies this expert knowledge to concrete marine structures. The focus of her research is on offshore wind foundation optimisation.
Dr Paul Doherty is the managing director at GDG and has successfully delivered a wide range of consultancy projects, including offshore foundation design, piled foundations, slope stability and geotechnical instrumentation, on time and in budget. He has published over 40 technical articles and is an active member of the Deep Foundation Institute technical committees.
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