The driven monopile is currently the preferred foundation type for most offshore wind farms. While the static capacity of the monopile is important, a safe design must also address issues of accumulated rotation and changes in stiffness after long-term cyclic loading. Design guidance on this issue is limited. To address this, a series of laboratory tests were conducted where a stiff pile in drained sand was subjected to between 8000 and 60 000 cycles of combined moment and horizontal loading. A typical design for an offshore wind turbine monopile was used as a basis for the study, to ensure that pile dimensions and loading ranges were realistic. A complete non-dimensional framework for stiff piles in sand is presented, and applied to interpret the test results. The accumulated rotation was found to be dependent on relative density, and was strongly affected by the characteristics of the applied cyclic load. Particular loading characteristics were found to cause a significant increase in the accumulated rotation. The pile stiffness increased with number of cycles, which contrasts with the current methodology where static load-displacement curves are degraded to account for cyclic loading. Methods are presented to predict the change in stiffness and the accumulated rotation of a stiff pile due to long-term cyclic loading. The use of the methods developed is demonstrated for a typical fullscale monopile.KEYWORDS: laboratory tests; piles; repeated loading; sands; settlement; stiffness Le monopieu battu est actuellement le type de fondation préféré pour la plupart des parcs éoliens en mer. Bien que la capacité statique du monopieu soit importante, une conception sans danger doit également tenir compte des problèmes de la rotation accumulée et de variations dans la rigidité, à la suite d'efforts cycliques de longue durée. Les conseils et indications conceptuels relatifs à cette question étant limités, on a procédé à une série d'essais en laboratoire, dans le cadre desquels on a soumis un pieu rigide, enfoncé dans du sable drainé, à un nombre de cycles de moment de charge et de charges horizontales allant de 8000 à 60 000. On a utilisé, comme élément de base de cette étude, un modèle typique de monopieu pour éolienne en mer, afin d'assurer que les dimensions du pieu et les plages de charge étaient réalistes. Pour interpréter les résultats de cet essai, on présente et on applique un cadre non dimensionnel complet pour pieux rigides dans le sable. La rotation cumulée, qui s'est avérée tributaire du poids spécifique, était affectée dans une grande mesure par les caractéris-tiques de la charge cyclique appliquée. On s'est aperçu que certaines caractéristiques particulières de la charge engendraient une augmentaiton significative de la rotation cumulée. La rigidité du pieu augmentait avec le nombre de cycles, contrairement à la méthodologie actuelle, dans laquelle les courbes charge statique/déplace-ment sont dégradées pour tenir compte des charges cycliques. La communication présente des méthodes permettant de prédire les ...

An important engineering challenge of today, and a vital one for the future, is to develop and harvest alternative sources of energy. This is a firm priority in the UK, with the government setting a target of 10% of electricity from renewable sources by 2010. A component central to this commitment will be to harvest electrical power from the vast energy reserves offshore, through wind turbines or current or wave power generators. The most mature of these technologies is that of wind, as much technology transfer can be gained from onshore experience. Onshore wind farms, although supplying 'green energy', tend to provoke some objections on aesthetic grounds. These objections can be countered by locating the turbines offshore, where it will also be possible to install larger capacity turbines, thus maximizing the potential of each wind farm location. This paper explores some civil-engineering problems encountered for offshore wind turbines. A critical component is the connection of the structure to the ground, and in particular how the load applied to the structure is transferred safely to the surrounding soil. We review previous work on the design of offshore foundations, and then present some simple design calculations for sizing foundations and structures appropriate to the wind-turbine problem. We examine the deficiencies in the current design approaches, and the research currently under way to overcome these deficiencies. Designs must be improved so that these alternative energy sources can compete economically with traditional energy suppliers.

The offshore wind industry currently relies on subsidy schemes to be competitive with fossil-fuel-based energy sources. For the wind industry to survive, it is vital that costs are significantly reduced for future projects. This can be partly achieved by introducing new technologies and partly through optimization of existing technologies and design methods. One of the areas where costs can be reduced is in the support structure, where better designs, cheaper fabrication and quicker installation might all be possible. The prevailing support structure design is the monopile structure, where the simple design is well suited to mass-fabrication, and the installation approach, based on conventional impact driving, is relatively low-risk and robust for most soil conditions. The range of application of the monopile for future wind farms can be extended by using more accurate engineering design methods, specifically tailored to offshore wind industry design. This paper describes how state-ofthe-art optimization approaches are applied to the design of current wind farms and monopile support structures and identifies the main drivers where more accurate engineering methods could impact on a next generation of highly optimized monopiles.

Improved design of laterally loaded monopiles is central to the development of current and future generation offshore wind farms. Previously established design methods have demonstrable shortcomings requiring new ideas and approaches to be developed, specific for the offshore wind turbine sector. The Pile Soil Analysis (PISA) Project, established in 2013, addresses this problem through a range of theoretical studies, numerical analysis and medium scale field testing. The project completed in 2016; this paper summarises the principal findings, illustrated through examples incorporating the Cowden stiff clay profile, which represents one of the two soil profiles targeted in the study. The implications for design are discussed.

No abstract