Chapter

Modelling of foundation response to scour and scour protection for offshore wind turbine structures

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... At offshore wind farm sites monopiles are driven into the seabed to create foundations on top of which wind turbine generator towers are installed. When designing a foundation its embedment depth is taken into consideration, since the overburden of the surrounding seabed material will affect the structure's stiffness and, consequently, its dynamic response (Mayall et al., 2019). This overburden can either increase due to the temporary shoaling of bed-levels at the base of a foundation when a bedform crest reaches a structure, or reduce through the removal of material at the base of the structure by scouring or general seabed lowering. ...
... This time series gives an indication of the stiffness of the monopile foundation, which is controlled by the properties of the wind turbine assembly, the soil properties, water depth, and the lateral resistance and displacement of the foundation in the soil. As soil from around the monopile is removed through the process of scouring the monopile becomes less stiff and the natural frequency will reduce (Mayall et al., 2019). Measurements of natural frequency were taken every 10 minutes and were then averaged on a monthly basis. ...
Article
As the number of manmade structures installed on the seafloor is increasing rapidly, we seek to understand the impact of these immobile obstacles on marine geomorphological processes, such as the evolution of bedforms. A 5.8 m diameter monopile foundation was installed at the case study offshore windfarm approximately 30 m ahead of an approaching barchan (crescent‐shaped) dune. The impact of the monopile on the dune’s evolution was analysed using six multibeam bathymetry surveys spanning 20 years. To substantiate this analysis, coupled three‐dimensional numerical modelling of flows and sediment was conducted in which the scour inducing bed shear stresses were calculated from the modelled turbulent kinetic energy. Following the installation of the monopile, the mid‐section of the dune accelerated and stretched in the direction of the wake of the monopile. Four years after the monopile’s installation the rest of the dune had caught up, flattening out the slip face within half the dune’s length downstream of the monopile. Due to the modified flow field, the dune was scoured deeply at the base of the monopile to a depth of 6.8 m (supported by the model results that predicted a scour depth exceeding 2 m over a period of just a few days). The surveyed volume of material scoured amounted to 8% of the total dune volume. Whilst the process of scouring occurs at a timescale of days to weeks, the dune migrated on average by 25 m/yr. The difference in the timescale of the two processes allowed the scouring to occur through the full thickness of the dune. The scoured dune profile recovered rapidly once the dune migrated downstream of the monopile. This paper demonstrates how large geomorphological features can intercept and migrate past a monopile foundation without long‐lasting impacts on the integrity of the feature or the foundation.
Article
Full-text available
The trend for development in the offshore wind sector is towards larger turbines in deeper water. This results in higher wind and wave loads on these dynamically sensitive structures. Monopiles are the preferred foundation solution for offshore wind structures and have a typical expected design life of 20 years. These foundations have strict serviceability tolerances (e.g. mudline rotation of less than 0.25 degrees during operation). Accurate determination of the system frequency is critical in order to ensure satisfactory performance over the design life, yet determination of the system stiffness and in particular the operational soil stiffness remains a significant challenge. Offshore site investigations typically focus on the determination of the soil conditions using Cone Penetration Test (CPT) data. This test gives large volumes of high quality data on the soil conditions at the test location, which can be correlated to soil strength and stiffness parameters and used directly in pile capacity models. However, a combination of factors including; parameter transformation, natural variability, the relatively small volume of the overall sea bed tested and operational effects such as the potential for scour development during turbine operation lead to large uncertainties in the soil stiffness values used in design. In this paper, the effects of scour erosion around unprotected foundations on the design system frequencies of an offshore wind turbine is investigated numerically. To account for the uncertainty in soil-structure interaction stiffness for a given offshore site, a stochastic ground model is developed using the data resulting from CPTs as inputs. Results indicate that the greater the depth of scour, the less certain a frequency-based SHM technique would be in accurately assessing scour magnitude based solely on first natural frequency measurements. However, using Receiver Operating Characteristic (ROC) curve analysis, the chance of detecting the presence of scour from the output frequencies is improved significantly and even modest scour depths of 0.25 pile diameters can be detected.
Article
Full-text available
Severe foundation scour may occur around monopile foundations of offshore wind turbines due to currents and waves. The so-called p-y curves method is suggested in the existing design recommendations to determine the behavior of monopiles unprotected against scour and the reduction of effective soil stress is accounted for by the extreme scour depth. This conservative design approach does not consider the geometry of the scour hole and the effect of pile diameter on the soil resistance. An underestimated foundation stiffness would be obtained, thereby influencing the predicted overall response of the support structure of an offshore wind turbine. In this study, we calculated the load-deformation response and foundation stiffness of a monopile when scour occurred. The influence of pile diameter on the initial modulus of subgrade reaction, and the modification of the ultimate soil resistance of a monopile subject to scour are evaluated. The commercial software BLADED was used to simulate the dynamic response of the reference offshore wind turbine with monopile unprotected against scour at Chang-Bin offshore wind farm in Taiwan Strait. The results showed that when the p-y curve suggested by existing design regulation was used to calculate the load-deformation response, the foundation stiffness was underestimated where the scour depth was greater than the pile diameter, but the foundation stiffness was overestimated when the scour depth was less than the pile diameter.
Article
Full-text available
Soil scour around a shallowly embedded pile can significantly compromise its lateral response, reducing both stiffness and capacity. Estimation of the lateral pile response must take into account both the scour-hole geometry and the overconsolidation effects on the remaining soil. A series of centrifuge model tests with various scour profiles were conducted at a scale of 1:250 to investigate the effects of both local and general scour on the response of a laterally loaded pile. Measured pile moment distributions and force–displacement data at the pile head were used to derive p–y curves quantifying the lateral pile–soil interaction. The p–y curves derived from various scour profiles were compared for equivalent depths below the new scour base, and below the original soil surface. For the general scour cases, the p–y curves for given depths below the post-scour surface are essentially identical to those at the same depths without scour. In contrast, for the local scour cases, the p–y response at a given depth below the scour-hole base is much stiffer than at the same depth below the original soil surface. As a practical approach to evaluate the effects of scour, the concept of an effective soil depth is introduced to determine the corresponding p–y curves for shallowly embedded piles in sand.
Conference Paper
Full-text available
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.
Conference Paper
Full-text available
Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5m and 6m are routinely used, with diameters of up to 10m, 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.
Article
Full-text available
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.
Article
Full-text available
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-of-the-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. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
Chapter
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.
Article
Offshore gravity base foundations (GBFs) are often designed with complex geometries. Such structures interact with local hydrodynamics, creating an adverse pressure gradient that is responsible for flow and scour phenomena, including the bed shear stress amplification. In this study, a method is presented for predicting clear-water scour around cylindrical structures with nonuniform geometries under the force of a unidirectional current. The interaction of the flow field with the sediment around these complex structures is described in terms of nondimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights into the influence the streamwise depth-averaged Euler number has on the equilibrium scour around uniform and nonuniform cylindrical structures. Here, the Euler number is based on the depth-averaged streamwise pressure gradient (calculated using potential flow theory), the mean flow velocity, and the fluid density. Following a dimensional analysis, the controlling parameters were found to be the Euler number, pile Reynolds number, Froude number, sediment mobility number, and nondimensional flow depth. Based on this finding, a new scour-prediction equation was developed. This new method shows good agreement with the database of scour depths acquired in this study (R2=0.91)(R2=0.91). Measurements of the equilibrium scour depth around nonuniform cylindrical structures were used to show the importance of the Euler number in the scour process. Finally, the importance of the remaining nondimensional quantities with respect to scour was also investigated in this study.
Article
This paper provides the background to the 50th Rankine Lecture. It considers the growth in emphasis of the prediction of ground displacements during design in the past two decades of the 20th century, as a result of the lessons learnt from field observations. The historical development of the theory of elasticity is then described, as are the constitutive frameworks within which it has been proposed that geotechnical predictions of deformation should be carried out. Factors affecting the stiffness of soils and weak rocks are reviewed, and the results of a numerical experiment, assessing the impact of a number of stiffness parameters on the displacements around a retaining structure, are described. Some field and laboratory methods of obtaining stiffness parameters are considered and critically discussed, and the paper concludes with a suggested strategy for the measurement and integration of stiffness data, and the developments necessary to improve the existing state of the art.
Article
According to the Danish wind turbine industry cross-wind vibrations due to wave loading misaligned with wind turbulence often have a significant influence on the fatigue lifespan of offshore wind turbine foundations. The phenomenon is characterised by increasing fatigue loads compared to the fore-aft fatigue and a small amount of system damping since almost no aerodynamic damping from the blades takes place. In addition, modern offshore wind turbines are flexible structures with resonance frequencies close to environmental loads and turbine blades passing the tower. Therefore, in order to avoid conservatism leading to additional costs during the load calculation and the design phase, the structural response must be analysed using reliable estimations of the dynamic properties of the wind turbines. Based on a thorough investigation of “rotor-stop” tests performed on offshore wind turbines supported by a monopile foundation for different wind parks in the period 2006–2011, the paper evaluates the first natural frequency and modal damping of the structures. In addition, fitting of theoretical energy spectra to measured response spectra of operating turbines is presented as an alternative method of determining the system damping. Analyses show distinctly time-dependent cross-wind dynamic properties. Based on numerical analysis, the variation is believed to be caused by sediment transportation at seabed level and varying performance of tower oscillation dampers.
Article
When designing offshore monopiles without scour protection, the stiffness of the foundation will vary with time due to the dependency of the scour depth on current and sea conditions. Currently, design regulations of organizations such as Det Norske Veritas (DNV) and the International Organization for Standardization (ISO) recommend the use of the most extreme local scour depth as the design scour depth. This is a conservative approach, because the scour depth depends on the sea conditions and because the equilibrium scour depth is low during moderate to extreme wave loading. In this paper the effect of using expected scour depths when designing for the ultimate limit state and the fatigue limit state is illustrated by means of a desk study.
Article
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.
RP 2GEO. Recommended Practice for Geotechnical Foundation Design Consideration
API, 2011. RP 2GEO. Recommended Practice for Geotechnical Foundation Design Consideration.
Standard DNVGL-ST-0126
DNVGL, 2016. Standard DNVGL-ST-0126, Support structures for wind turbines, Edition April 2016.
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.
The effects of Scour on the design of Offshore Wind Turbines
  • J Tempel
  • M B Zaaijer
  • H Subroto
Tempel, J., Zaaijer, M.B. & Subroto, H., 2004. The effects of Scour on the design of Offshore Wind Turbines.