Small-Scale Testing of Cyclic Laterally Loaded Monopiles in Dense Saturated Sand

Monopiles are currently the most common foundations for offshore wind turbines, which are subjected to millions of cyclic loads that are still not well-interpreted in the design guidelines. The accumulated rotation of the turbine and the change of foundation stiffness due to the long-term cyclic loading are issues that should be investigated. In the present work, a small-scale test campaign of a stiff pile was performed in order to validate methodologies proposed by recent studies. Cyclic loading was not found to degrade the ultimate static resistance of the pile, which contrasts with current design guidelines. Wind energy capacity has increased more than four-fold in the last decade. The offshore wind energy sector has widely developed, especially in northern Europe. High mean wind speeds and large space availabilities are the main advantages in comparison with the onshore converters. Offshore wind energy is a technology that is still more expensive than onshore wind energy, and therefore a reduction of the costs is required. The support structure is a significant proportion of the total cost of offshore wind turbines and still has large potential for cost reduction. The monopile foundation is the most popular support structure for offshore wind turbines. Such foundations are stiff steel tubes, often with diameters of 4 to 6 m and embedded lengths of 20 to 30 m. The design of monopiles relies on standards and empirical data originating from the oil and gas sector, which actually were based on works performed with flexible piles with diameters less than 1 m. However, the substructures of offshore wind turbines are exposed to different loading conditions that mostly originate from waves and wind. Such loads are naturally cyclic, and therefore an offshore wind structure is subjected to millions of load cycles over its lifetime. This causes the offshore wind energy structure to accumulate a rotation that may damage the wind turbine. The main current design standards, DNV (2010) and API (2010), provide guidelines that are not capable of properly accounting for cyclic loading and consequent accumulated rotations. Moreover, the behavior of non-slender piles used in the offshore wind energy sector might be different from the slender piles used in the oil and gas sector. Therefore, a proper design guideline is required and further investigations are necessary. Previous Evaluation of Lateral Deflections The most common approach for predicting lateral deflections of monopiles deals with the use of the p-y curve method. The p-y curves provide a relationship between the soil resistance p and the lateral displacement of the pile y. The p-y approach was proposed by Reese et al. (1974) and O'Neill and Murchison (1983) through research promoted by the oil and gas sector. Such an approach is based on full-scale tests of slender piles with no more than 100 load cycles. Hence, the long-term response of the piles is not accounted for. Cyclic loading is taken into account by reducing the soil resistance by means of empirical factors. Long and Vanneste (1994) reviewed previous works and proposed to reduce the soil resistance with a power law as a function of the number of cycles through an analysis of 34 field tests with 5 to 500 load cycles. Lin and Liao (1999) performed tests on flexible piles with up to 100 cycles, proposing a relationship in which the lateral displacement of the pile increases with the number of cycles through a logarithmic evolution. Peng et al. (2006), Peralta and Achmus (2010), and LeBlanc et al. (2010) investigated the behavior of piles under cyclic lateral loading through small-scale testing at 1 g. Peng et al. (2006) found that the lateral displacement of a pile is larger, increasing the frequency and the magnitude of the cyclic load. Peralta and Achmus (2010) concluded their research by stating that the lateral displacement of a pile increases with the number of cycles through a power law in the case of a rigid pile and through a logarithmic law in the case of a flexible pile. LeBlanc et al. (2010) proposed a method to predict the long- term accumulated rotation of a monopile, performing cyclic lateral loading tests of a model stiff pile in dry loose sand. Tests with 8,000 to 65,000 load cycles were performed in a small-scale rig. The tested pile had a slenderness ratio of 4.5 and a diameter of 80 mm. The proposed approach yielded a relationship in which the accumulated rotation of the pile is related to the number of cycles as a function of a power law. Furthermore, it was found that the pile stiffness always increases during cyclic loading, proportionally to the number of cycles. A significant change of stiffness of the structure may lead to a change of the natural frequency during its lifetime, which may be critical in case of resonance. Therefore, further works aimed at investigating possible changes of the foundation stiffness are required. Kirkwood and Haigh (2013) performed an investigation similar to LeBlanc et al. (2010) but by means of centrifuge testing, confirming the results achieved by Leblanc et al. (2010). Purpose of the Paper