A practical two parameter model of pile-soil gapping for prediction of monopile offshore wind turbine dynamics

Monopile mounted offshore wind turbines (OWTs) are expected to experience a very large number of cyclic loads throughout their operational lifetime, and the existing p-y method of foundation modelling does not fully account for the effects of dynamic cyclic loading, such as pile-soil gapping. In this paper a dynamic model based on the beam on non-linear Winkler foundation scheme with a novel algorithm capable of capturing the effects of pile-soil gapping is presented. It can account for gap cave-in, and the resulting gap size can react dynamically to changing loading amplitudes, using only two calibration parameters. Static and dynamic cyclic loaded model validations are presented, and give very good agreement with experimental results, performing better than existing p-y curves for dynamic loading. The model is also applied to an OWT case study and predictions of natural frequency reduction due to soil erosion agree well with measured results. It is shown that the inclusion of gapping may result in a significant decrease to the natural frequency prediction of OWTs relative to the value predicted without gapping. As such, not to consider gapping could lead to unconservative predictions, and any additional soil degradation throughout the serviceable lifetime could therefore result in unwanted resonance. The method provided in this paper provides a simple and accurate model to predict this behaviour which is crucial to ascertain during the design phase.</p

Modelling the impact of gapping behaviour on monopile mounted offshore wind turbine dynamics

Increased demand for renewable energy production has stimulated interest in the offshore wind turbine (OWT) industry as a viable solution, and with OWTs growing larger in scale, further research is required into the dynamics of these newer structures. The majority of installed OWTs to date are built upon monopile foundations, and it is widely acknowledged that the current design methods for offshore piles are not appropriate for the large diameter piles required. This paper uses a novel pile-soil gapping algorithm to simulate the effects ofdegradation to the soil conditions in the sea bed. Using a 1D Winkler beam spring approach, a dynamic model is validated for prediction of the natural frequencies of several OWT case studies, and the gapping algorithm is shown to reproduce well the reduction in natural frequency likely attributed to soil degradation measured from an OWT in Kentish Flats wind farm. It is found through the simulation of rotor-stop tests that the presence of gapping decreases the measured natural frequency, and this effect is greater for the monopile foundations with a smaller slenderness ratio

Shear strength theories for beams of variable depth

Flexibly formed reinforced concrete beams usually have varying cross sections along their longitudinal axis, capitalising on the fluidity of concrete to create optimised geometries. According to Orr et al. [1], these new shapes have led to challenges for shear design, especially when the depth of the beams is relatively small. It is crucial to be able to accurately determine the shear strength of such beams to maintain structural safety whilst achieving material optimisation.The effective shear force method is adopted for tapering beams in many design codes. Recent work by Paglietti et al. [2] has highlighted concerns over the use of such an approach. In this paper, the theoretical basis for stress distributions in tapered beams built by Timoshenko [3] and Oden [4] in their elastic range is reviewed and then extended to included cracked behaviour.It is found that the effective shear force method used in design codes does not accurately account for the stress distribution in a section both in elastic and cracked stage of concrete, underestimating the peak shear stress for beams with inclined soffits. This is important for flexibly formed beams, and has implications for designersAs a result of this work, a new calculation and design method for shear reinforcement is proposed.Keywords: variable depth beam, shear strength, shear stress distribution, flexible formwork.The authors acknowledge and are grateful for the support of the BRE CICM (www.bath.ac.uk/bre), the University of Bath, and the China Scholarship Council who collectively fund the PhD position that has resulted in this work

Form-Finding approach for flexibly formed concrete elements

There is no reason why concrete elements should be prismatic. Concrete is mouldable and can be cast in efficient forms which follow the stresses varying along the length of a concrete element. One option to achieve this is to use fabric as flexible formwork. Fabric deforms under the hydrostatic pressure exerted by wet concrete during construction, creating the shape of hardened concrete. The final shape needs to be known in advance to be able to perform the analysis and design of structural elements. This paper presents a form-finding approach capable of predicting the shape in cross-section of flexibly formed concrete elements. The approach is shown to predict geometry appropriately, based on the results of an experimental investigation, particularly when applied to complex shapes. The influence of construction tolerances, practical aspects and limitations of the approach are also discussed.</p

GFRP durability appraisal: mechanical testing of naturally aged composite panels

The findings of a durability study undertaken on pultruded glass fibre reinforced polymer (GFRP) cladding panels are presented. Sourced at demolition from the Second Severn Crossing Visitors’ Centre building in the UK, the panels offered the rare opportunity to assess the characteristics of naturally aged composite material. Mechanical properties were determined and compared with the properties of new, equivalent material. The phenomenon of polymer hardening, typified by a reduction in the material strain limit over time, was investigated by further mechanical testing procedures. By contrasting the properties as found for panels taken from each of the four external walls of the building, factors concerning environmental exposure and factors relating to the original fabrication conditions were investigated. The results indicate that, regardless of the exposure conditions, in 17 years the mechanical material properties appear not to have significantly diminished, despite aesthetic quality suffering due to lack of maintenance. It has, however, been shown that ultraviolet exposure causes a hardening of the resin component of the composite, resulting in an increase in the compressive elastic modulus, but a reduction in the threshold of the brittle fracture of the matrix in tension. This final result has not been documented before and is significant in understanding the long-term performance of composites

Durability enhancements using fabric formwork

By replacing conventional concrete moulds with flexible sheets of permeable fabric the construction of optimised concrete elements that provide material savings of up to 40% when compared to an equivalent strength prismatic member is possible. This paper details the results of recent tests undertaken at the Building Research Establishment Centre for Innovative Construction Materials at the University of Bath (BRE CICM) that demonstrate significant additional durability advantages for fabric cast concrete.Using accelerated test methods 50% average reductions in both the non-steady state chloride diffusion coefficient and carbonation coefficients were found when comparing concrete samples cast against permeable and impermeable surfaces. Sorptivity, surface hardness, and scanning electron microscopy tests demonstrate further beneficial changes in the fabric cast concrete. The combined results demonstrate that fabric formwork may be used to create structures optimised for strength and durability.The authors gratefully acknowledge the support of the Engineering and Physical Sciences Research Council (EPSRC) and Atkins Ltd in this research

Optimisation and durability in fabric cast ’Double T’ beams

By replacing orthogonal concrete moulds with a system formed of flexible sheets of fabric it is possible to construct optimised, variable cross section concrete elements that can provide material savings of up to 40% when compared to an equivalent strength prismatic member, and thereby offer the potential for significant embodied energy savings in new concrete structures. This paper presents the salient results of two sets of tests recently undertaken at the Building Research Establishment Centre for Innovative Construction Materials (BRE CICM) at the University of Bath that considered 1) the design, optimisation and construction of 4m span double ‘T’ beams and 2) the surface properties of concrete cast into a permeable fabric mould. The results of these tests demonstrate how a fabric formwork construction system may be used to facilitate a sustainable future for concrete construction, providing a design method by which structurally optimised elements may be cast in an economical manner while also providing significant durability and visual benefits that combined provide an advantageous whole-life performance for fabric formed concrete that is unmatched by many other construction systems

VSimulators:A New UK-based Immersive Experimental Facility for Studying Occupant Response to Wind-induced Motion of Tall Buildings

Current vibration serviceability assessment criteria for wind-induced vibrationsin tall buildings are based largely on human ‘perception’ thresholds which is shown not to be directly translatable to human ‘acceptability’ of vibrations. There is also a considerable debate about both the metrics and criteria for vibration acceptability, such as frequency of occurrence or peak vs mean vibration, and how these might vary with the nature of the vibration. Furthermore, the design criteria are necessarily simplified for ease of application so cannot account for a range of environmental, situational and human factors that may enhance or diminish the impact of vibrations on serviceability. The dual-site VSimulatorsfacility was created specifically to provide an experimental platform to address gaps in understanding of human response to building vibration. This paper considers how VSimulators can be used to inform general design guidance and support design of specific buildings for habitability, in terms of vibration, which allow engineers and clients to make informed decisions with regard to sustainable design, in terms of energy and financial cost. This paper first provides a brief overview of current vibration serviceability assessment guidelines, and the current understanding and limitations of occupants’ acceptability of wind-induced motion in tall buildings. It then describes how the dual-site VSimulators facility at the Universities of Bath and Exeter can be used to assess the effects of motion and environment on human comfort, wellbeing and productivity with examples of how the facility capabilities have been used to provide new, human experience based experimental research approaches