Chapter

Experimental modelling of the effects of scour on offshore wind turbine monopile foundations

Chapter

Experimental modelling of the effects of scour on offshore wind turbine monopile foundations

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Abstract

Local and global scour around offshore wind turbine monopile foundations can lead to a reduction in system stiffness, and a consequential drop in the natural frequency of the combined monopiletower- nacelle structure. If unchecked this could lead to operational problems such as accelerated fatigue damage and de-rating or decommissioning of the turbine. Research exploring the interaction between scour, foundation stiffness, and structural dynamic behaviour is therefore critical if scour formation is to be properly accounted for in predictions of structural performance, and to guide the implementation of scour remediation strategies. This paper describes experimental work that explores these interactions, conducted on a 1:20 scale driven monopile foundation and tower-nacelle superstructure in a prepared sand test-bed at HR Wallingford’s Fast Flow Facility. The flume allowed realistic scour geometries to be developed, providing a means to explore the effectiveness of different remediation strategies. Measured acceleration and strain caused by harmonic lateral loading are interpreted to deduce changes in structural performance as scour develops.

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... This can also impact the structural behaviour of the foundation, with the impacts being different for static scour protections with larger stone sizes than in dynamic or wide graded scour protections, unless much larger thicknesses are used in the latter. Recently, the works of References [59,60,61] shed light on the structure-protection interaction, scour effects were also analysed. ...
... Moreover, the sand accumulation on the protection's matrix and the further enhancement of the stiffness contribution is diminished if extreme global scour leads to the instability of the protection. The numerical work presented in Reference [59] used a one-dimensional (1D) finite element model developed for the analysis of natural frequencies for monopile-supported turbines with scour and scour protection. The numerical modelling produced consistent results with data from the flume experiments and with the case study of Rigg Offshore Wind Farm, proving that scour protections do influence the soil stiffness and that the choice on the type of scour protection needs to account for such effects. ...
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The offshore wind is the sector of marine renewable energy with the highest commercial development at present. The margin to optimise offshore wind foundations is considerable, thus attracting both the scientific and the industrial community. Due to the complexity of the marine environment, the foundation of an offshore wind turbine represents a considerable portion of the overall investment. An important part of the foundation’s costs relates to the scour protections, which prevent scour effects that can lead the structure to reach the ultimate and service limit states. Presently, the advances in scour protections design and its optimisation for marine environments face many challenges, and the latest findings are often bounded by stakeholder’s strict confidential policies. Therefore, this paper provides a broad overview of the latest improvements acquired on this topic, which would otherwise be difficult to obtain by the scientific and general professional community. In addition, this paper summarises the key challenges and recent advances related to offshore wind turbine scour protections. Knowledge gaps, recent findings and prospective research goals are critically analysed, including the study of potential synergies with other marine renewable energy technologies, as wave and tidal energy. This research shows that scour protections are a field of study quite challenging and still with numerous questions to be answered. Thus, optimisation of scour protections in the marine environment represents a meaningful opportunity to further increase the competitiveness of marine renewable energies.
... Previous experimental tests (Prendergast et al., 2015) provide data on the change in natural frequency with scour for a uniform pile in dry conditions. The current research project includes models of a wind turbine tower and rotor-nacelle assembly (Mayall et al., 2018), which are felt influential on the overall dynamic behaviour of the structure. ...
... Fast Flow Facility experiment layout (fromMayall et al., 2018). ...
... The preparation of the seabed for this test adopted the method of layered preparation, the same method Mayall et al. (2018) adopted. When each layer was laid, water was slowly added to the surface of the sand. ...
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Scour Below Pipelines Scour Around a Single Slender Pile Scour Around a Group of Slender Piles Examples of More Complex Configurations Scour Around Large Piles Scour Around Breakwaters Scour at Seawalls Ship-Propeller Scour Impact of Liquefaction.
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A ″state of the art″ report on the subject of local scour around cylindrical piers is given here. After a description of the scouring process, a critical review of literature on model and field data is presented, and the empirical data are compared with theoretical considerations. The final result is a set of design suggestions together with possibilities for protection against scour. The Report is principally restricted to the following conditions: cylindrical piers (all shapes), noncohesive granular bed material, and one-way current (no tidal influence and waves). The following aspects are presented: the description of the scouring process and an analysis of relevant parameters; a description of model and field data; a comparison of data with theoretical work and a discussion on the influence of various parameters; and the protection against scour and the development of suggestions for design relations.
International Organization for Standardization (ISO) 2007. ISO 19902 Petroleum and natural gas industries -Fixed steel offshore structures
  • M Høgedal
  • T Hald
Høgedal, M. & Hald, T. 2005. Scour assessment and design for scour for monopile foundations for offshore wind turbines. In: Proceedings of the Copenhagen Offshore Wind. Copenhagen, Denmark. International Organization for Standardization (ISO) 2007. ISO 19902 Petroleum and natural gas industries -Fixed steel offshore structures. Geneva: ISO.