Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5mand 6mare routinely used, with diameters of up to 10 m, or more, being considered for future designs. There are concerns that current design approaches, such as the p − y method, which were developed for piles with a relatively large length to diameter ratio, may not be appropriate for large diameter monopiles. A joint industry project, PISA (PIle Soil Analysis), has been established to develop new design methods for large diameter monopiles under lateral loading. The project involves three strands of work; (i) numerical modelling; (ii) development of a new design method; (iii) field testing. This paper describes the framework on which the new design method is based. Analyses conducted using the new design method are compared with methods used in current practice.

There is currently a significant focus on developing offshore wind power in the UK and Europe. The most common foundation type for wind turbines is a single large diameter pile, termed a monopile, on which the turbine is located. As the diameter of such piles is envisaged to increase in future installations, there are concerns that current design methods are not applicable. To explore this problem, the joint industry project PISA has been established, with the aim to develop a new design framework for laterally loaded piles utilised in the offshore wind industry, based on new theoretical developments, numerical modelling and large scale field pile testing. This paper presents an overview of numerical modelling undertaken as part of the project.

Offshore wind power in the UK, and around Europe, has the potential to deliver significant quantities of renewable energy. The foundation is a critical element in the design. The most common foundation design is a single large diameter pile, termed a monopile. Pile diameters of between 5m and 6m are routinely used, with diameters up to 10m or more, being considered for future designs. Questions have been raised as to whether current design methods for lateral loading are relevant to these very large diameter piles. To explore this problem a joint industry project, PISA, co-ordinated by DONG Energy and the Carbon Trust, has been established. The aim of the project is to develop a new design framework for laterally loaded piles based on new theoretical developments, numerical modelling and bench- marked against a suite of large scale field pile tests. The project began in August 2013 and is scheduled to complete during 2015. This paper briefly outlines the project, focusing on the design of the field testing. The testing involves three sizes of pile, from 0.27m in diameter through to 2.0m in diameter. Two sites will be used; a stiff clay site and a dense sand site. Tests will include monotonic loading and cyclic loading. A suite of site investigation will be carried out to aid interpretation of the field tests, and will involve in-situ testing, standard laboratory testing and more advanced laboratory testing.

In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

Rapid expansion of the offshore wind industry has stimulated a renewed interest in the behaviour of offshore piles. There is widespread acceptance in practice that pile design methods developed for the offshore oil and gas industry may not be appropriate for designing wind turbine foundations. To date, the majority of offshore wind turbines are supported by large diameter monopiles. These foundations are sensitive to scour which can reduce their ultimate capacity and alter their dynamic response. In this paper, the use of a vibration-based method to monitor scour is investigated. The effect of scour on the natural frequency of a model monopile was measured in a scale model test. A spring-beam finite element numerical model was developed to examine the foundation response. The model, which used springs tuned to the small-strain stiffness of the sand, was shown to be capable of capturing the change in frequency observed in the scale test. This numerical procedure was extended to investigate the response of a full-scale wind turbine over a range of soil densities, which might be experienced at offshore development sites. Results suggest that wind turbines founded in loose sand would exhibit the largest relative reductions in natural frequency resulting from scour.

This paper presents the results of compression and tension load tests performed on a single helical pile installed in dense sand. The pile was instrumented using strain gauges that allowed the shaft and base load resistance to be separated and the distribution of shaft resistance along the pile during the test to be determined. The pile was loaded first in compression, with a maintained load test, followed by a constant rate of penetration load test being performed to assess the effects of creep on the pile’s response to compression loading. The pile was then loaded in tension using a maintained load test procedure. Finite element analyses were performed using Abaqus and these helped to provide additional insights to explain the response of the instrumented pile during loading. The test showed that during compression loading, substantial bearing pressures developed beneath the pile helix, which provided the majority of axial load resistance. During tension loading, uplift pressure mobilized on the helix again provided the majority of axial resistance. The strain gauges suggested that the pile load response to compression loading was ductile. During tension loading, the pile response was brittle. Whilst load tests performed on only one instrumented pile test are presented, the use of instrumentation and finite element analyses allowed important insights into the load–displacement response of helical piles.

This paper presents the results of a field investigation into pile aging in soft clay that was conducted over a period of 10 years. Static load tests were conducted after the excess pore pressure generated by the installation of 6-m-long, driven concrete piles were fully equalized. These tests allowed the time-capacity aging profile to be established. Anormalized capacity-time trend established for the case history is seen to be consistent with the response observed from a wider database of pile tests in clay compiled from the literature. A simple reliability-based design example is provided to highlight the positive impact that pile aging could have for industrial practice.

Expansion of the offshore UK wind energy sector has stimulated renewed interest in the response of piles to lateral and moment loads. This paper compares the state of the art in foundation design with current industry trends in offshore wind turbine construction. The historical evolution of pile design for lateral loading is described in detail, focusing on the American Petroleum Institute guidelines used by the offshore sector. The limitations of these design codes are discussed in light of the specific requirements for the wind sector. Recent research efforts attempting to bridge the gap between practice and industry are highlighted and further research needs are identified.

The paper considers the current state of the art for estimating the pull-out capacity of driven open-ended piles used to support wind turbine foundations founded on sand. The latest edition of the American Petroleum Institute guidelines for pile design includes a conventional earth pressure approach and four alternative cone penetration test (CPT) methods for estimating pile shaft resistance in sand. A database of open-ended pile tests was used to assess the predictive reliability of the design approaches. While the earth pressure approach was unreliable, exhibiting bias with pile slenderness and sand relative density, the CPT methods were shown to provide improved and relatively consistent estimates of pile capacity. However, the tension loads experienced by wind turbine foundations are significantly higher than those applied to piles in the database. When the CPT methods were used to estimate the pile length required to support a 5 MW turbine installed in typical offshore soil conditions, the CPT methods provided a wide range of predicted pile lengths. The reasons for this divergence are discussed and an alternative framework for considering driven pile shaft resistance is put forward.

This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging.

The offshore wind sector is undergoing rapid expansion across Europe, driven by the demand for renewable energy and uncertainties regarding fossil fuel supplies. The proposed wind farms are creating significant geotechnical challenges, particularly in terms of efficient foundation design. The majority of wind farms constructed to date have been founded in water depths of less than 30 m. However 70 of wind farm developments to be undertaken over the next 10 years will be located in water depths of between 30 and 70 m. In addition, because of developments in turbine technology, larger 5 MW turbines are coming into operation. The combined effect of larger structures and greater water depths leads to significantly increased vertical, lateral and moment loading on turbine foundations. This paper considers some aspects relating to the reliability of design methods for monopile and jacket structure foundations used to support the next generation of wind turbines.

This paper presents the results of a series of field experiments performed to study the effect of installation method on the shaft resistance developed by a pile installed in soft clayey silt. Tests were performed on piles that experienced different levels of cyclic loading during installation. The test results indicate that the radial total stress, pore-water pressure, and shear stress on the pile shaft during installation were strongly affected by the installation procedure; all three were found to increase when the jacking stroke length used during installation increased (or the number of cyclic load applications decreased). However, equalized radial effective stresses that control the long-term pile shaft capacity were found to be insensitive to the installation method. A simple expression that requires the results of a cone penetration test, laboratory measurements of the interface friction angle, and the pile geometry is proposed to calculate the shaft resistance.

This paper describes the development of a model instrumented open-ended (pipe) pile. The importance of model geometry and separating the shaft, annular and plug load, and horizontal effective stresses is discussed.A detailed description of the construction of the twin-walled open-ended pile is presented. Particular attention was given to protecting the fragile instrumentation from the rigours of installation and the effects of water ingress. Calibration procedures, which were used to verify the instrument reliability, are also discussed. The final section describes field tests conducted in both loose sand and medium-dense sand deposits, which are used to validate the instrument performance.

This paper presents the results of a series of field tests performed to study the effect of soil plugging during installation on the base resistance developed by an openended (pipe) pile in clay. Three instrumented pipe piles were installed in soft clay, and the installation resistance and soil plug development were recorded. The tests revealed that the annular base resistance was equal to the cone penetration test qc resistance and was independent of the soil plug development. In contrast, the soil plug resistance increased from a minimum when the pile was fully coring to a maximum when the pile was fully plugged. A simple expression is proposed that links the plug resistance to the cone penetration test qc value at the specified depth and the incremental filling ratio value developed during installation. This expression is shown to provide reasonable estimates of the plug resistance developed by a large-diameter pipe pile driven into stiff to hard clay in the North Sea. The proposed relationship will be particularly useful for modelling the installation resistance of monopile foundations.