John M. Harris

The nature of scour development and scour protection at offshore windfarm foundations.

Analysis and interpretation of monitoring data for the seabed bathymetry local to offshore windfarm foundations has shown how the scour develops in time and highlighted variations between sites with different seabed sediment characteristics, i.e. sands and clays. Results from European offshore windfarms have generated a unique dataset for comparison with previously published data. Where surficial sediment is underlain by a marine clay the scour (to date) has been limited, whilst those with unconstrained depths of sandy sediments show scour as deep as 1.38 times the monopile diameter. Scour protection has been installed at some sites for structural stability of the foundation or for cable protection. The flow interaction with the protection causes edge scour or secondary scour in the seabed around the protection. In some cases this scour is deeper than the unprotected case. The analysis has resulted in an improved evidence base for scour in the marine environment.

Marine Scour and Offshore Wind: Lessons Learnt and Future Challenges

The drive for developing marine offshore renewables has led to specific requirements for scour hazard assessment relating to the associated foundation structures and the cabling necessary for in-field transmission and power export. To date within the United Kingdom (UK) a number of demonstrator projects have been constructed covering wind, wave and tidal generation. However, only offshore wind has been developed at large-scale at present as part of two rounds of commercial development of offshore wind farms (OWFs). In June 2008, The Crown Estate, responsible for licensing seabed use, announced proposals for a third round of offshore wind farms to develop an additional 25 GW of energy to the 8 GW already planned for under Rounds 1 and 2. The size of these Round 3 developments will vary, but the largest of these zones will involve the construction of around 2500 seabed foundation structures. Under Round 1 and 2 developments monopile and jacket type foundations have been used, although several other European (non UK) wind farms have been built using gravity base foundations. For a wind turbine the foundations may account for up to 35% of the installed cost. Therefore, one of the future challenges for large volume installation of offshore wind is the control and minimization of these costs. For tidal energy devices one of the principal requirements for many of the devices proposed is their placement in areas of strong tidal energy, and this has implications not only for the stability of the foundation option, but also for the construction methodology. Similarly wave energy devices are designed to be located in shallow, coastal environments as either floating or bottom mounted systems. These devices, by design, are intended to be located in environments with strong wave action. This may be substantial during storm events, which has implications for the integrity of the anchoring system keeping the wave device on station or the design of the device if it is seabed mounted. This paper will explore the lessons learnt from existing offshore wind farm developments as this represents the principal body of collected monitoring data. Using these data the paper will outline some of the challenges facing the offshore renewable industry in respect of the foundation designs and specifically the requirements for scour hazard assessment using the combined experience from those developments currently operational or under construction.

Bed Shear Stress Distribution Around Offshore Gravity Foundations

Offshore gravity foundations are often designed with complex geometries. Such structures interact with the local hydrodynamics and generate enhanced bed shear stresses and flow turbulence capable of scouring the seabed or destabilizing bed armour where deployed. In the present study a novel bed shear stress measurement method has been developed from the camera and laser components of a Particle Image Velocimetry (PIV) system. The bed shear stress amplification was mapped out around six models of gravity foundations with different geometries. Tests were repeated for two bed roughness conditions. The structures tested included uniform cylinders, cylindrical base structures and conical base structures. The flow field around the models was also measured using PIV. The results of this study reveal that the conical base structures generate a different hydrodynamic response compared to the other structures. For uniform cylinders the maximum bed shear stress amplification occurs upstream, at an angle of 45° relative to the flow direction, and measurements were found to agree well with numerical results obtained by Roulund et al. (2005). In the case of the cylindrical base structure the maximum amplification occurs upstream at a similar location to the uniform cylinder case. For the conical base structures the maximum amplification of the bed shear stress occurs on the lee side of the structure, with the magnitude dependent on the side slope of the cone. The bed shear stress results were validated against stresses derived from analysis of the flow fields obtained by the PIV measurements performed under the same test conditions. Conclusions from the study are that the structure with the cylindrical base foundation produces the lowest bed shear stress amplification and that an increase in the bed roughness results in an increase in the amplification of the bed shear stress. These findings have direct implications for design of scour protection. In addition the flow reattachment point behind the foundation is dependent on pile Reynolds number (ReD). This suggests that the results of this study may be extrapolated for higher pile Reynolds using the method described in Roulund et al. (2006).

Scour Prediction Offshore and Soil Erosion Testing

The analysis of the flow mechanisms causing scour in the marine environment combined with a conceptual model for scour in different seabed soils are applied to demonstrate how erosion testing can be used to support detailed assessments of scour. The important role of scour hazard assessment and scour monitoring in the life-cycle management of offshore assets is also assessed.

Scour development through time – modelling scour in layered soils

This paper describes the extension of an engineering model (Harris et al., 2010) developed to predict the time evolution of scour around a monopile in uniform granular soils in the marine environment to layered granular soils. The model is capable of looking at cylindrical, square and rectangular structures. It is capable of representing both the scouring and backfilling process and has been modified to incorporate the ability to predict the effect of scouring in layered soils making use of the results from recent experimental studies into scouring in non-uniform sediment beds. Output from the model is compared with the laboratory results and extended to look at potential implications at field-scale. The model is of use on those occasions when knowledge of the likely variation in depth of scour is important. Examples include predictions of scour development to inform the evaluation of the duration of time after pile driving for a fully developed scour hole to form for installation of scour protection, or to determine the likely exposure of cables entering and exiting offshore turbine foundations.

Analysing Scour Interaction Between Submarine Pipelines, Valve Stations And Mechanical Protection Structures

Subsea pipeline networks with components such as Wye-pieces and Pipeline End Manifolds (PLEMs) require protection from mechanical impacts. Pipeline scour is well understood and scour at solid caissons/gravity base foundations and piled foundations has been studied (Whitehouse, 1998; Whitehouse, et al., 2011; Sumer and Fredsoe, 2002). The scouring at small subsea gravity protection structures has received less attention than those with piles (Ottesen Hansen, 1997; Fog and Krogh, 2001) and hence there is uncertainty about predicting scour at such structures (Whitehouse, et al., 2010). The design of subsea structures requires a composite assessment of scour and stability for the protected asset and the protection structure itself. A closed structure may lead to the formation of a scour footprint that is quite similar to a solid caisson or gravity base. For an open structure, the contribution to scour interaction from the different components is more difficult to assess. Large-scale mobile bed laboratory experiments evaluated scour for a surface laid pipeline, with a spur and valves, and a generic subsea protection structure with mudmats and a perimeter skirt. Bathymetry mapping was achieved with a high precision underwater laser scanner. The experiments evaluated the contributions to scour arising from the different subsea components. Sediment mobility was a key factor, with the perimeter skirt able to resist scour over part of its length when the mobility was low/moderate, whereas in a high mobility environment the skirt was completely undermined. The foundation beams were underscoured quite quickly and the eroded sediment was flushed out through the perforated mudmats, highlighting the importance of mudmat design. The scour pattern and rate was similar with or without the pipeline in place. The underscouring of the beams caused settlement and tilting of the structure, highlighting the importance of assessing scour and scour countermeasures for these types of structure. The scour assessment needs to be integrated into the structural, geotechnical and functional performance study for any structure and the new results illustrate the importance of understanding effects related to sediment mobility, the prevailing direction of currents, the design of mudmats and skirts, and the need for additional scour countermeasures.

Equilibrium scour prediction for uniform and non-uniform cylindrical structures under clear water conditions

Offshore Gravity Base Foundations (GBFs) are often designed with non-uniform cylindrical geometries. Such structures interact with the local hydrodynamics which amplify the adverse dynamic pressure gradient, which is responsible for all flow and scour phenomena including the bed shear stress amplification. In this study a method for predicting the effect non-uniform cylindrical structure geometries have on local scour around offshore structures under the forcing of a unidirectional current is presented. The interaction of the flow field with the sediment around these complex structures is described in terms of non-dimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights in the influence a form of the Euler number has on the equilibrium scour around uniform and non-uniform cylindrical structures. Here the Euler number is defined as the depth averaged pressure gradient (calculated using potential flow theory) divided by the product of the square of mean flow velocity and the fluid density. The insights are confirmed through a series of experiments where the equilibrium scour was monitored for different types of structures and flow conditions. The results of this study show that the Euler number is a more appropriate parameter for describing the scour potential of a structure compared to using the equivalent pile diameter. The experimental data show that an increasing Euler number yields an increase in the non-dimensional equilibrium scour. The results of this study also suggest that an increase in the water depth yields a decrease in the equilibrium scour depth for the conical, cylindrical base structures and truncated cylinders and an increase in the equilibrium scour depth for the uniform cylinders which can also be explained in terms of changes in the Euler number. Finally, the Buckingham π theorem in conjunction with the experimental data was used to derive a simple shape correction factor that could be used to determine the scour depth of a non-uniform cylindrical structure based on the equilibrium scour produced for the same flow conditions by a uniform cylinder.

Scour Development around Large-Diameter Monopiles in Cohesive Soils: Evidence from the Field

Despite the progress in scour research over the last three decades, seabed scour development in cohesive and nonuniform soils is still an area of great uncertainty and remains a challenge for designing structurally efficient and effective foundations in the offshore marine environment. The uncertainty is made greater by the timescale required for scouring and effects, such as sediment abrasion, pile installation impacts, and operationally and environmentally induced dynamic motions. The rapid growth in offshore wind, particularly in European waters, has led to a requirement for estimates for scour development in such soils. This becomes very pertinent for large-volume serial installation of foundations, such as those required for offshore wind farm developments, given that there is a limit to the amount of detailed geotechnical information that can be collected as part of a project, and soil erosion testing is not standard. There is a reliance in geotechnical data, such as undrained shear strength, derived from cone penetration tests, supplemented with borehole data collected at a limited number of locations across the wind farm site combined with laboratory analysis of soil samples. This paper reviews the present evidence from both field and laboratory measurements of scour potential and looks at possible approaches for determining scour magnitude in cohesive soils including hydraulic and mechanical effects.

Scour and Erosion : Proceedings of the 8th International Conference on Scour and Erosion (Oxford, UK, 12-15 September 2016)

The 8th International Conference on Scour and Erosion (ICSE 2016) was organised by HR Wallingford, Oxfordshire, UK and held at the Mathematical Institute, Andrew Wiles Building, The University of Oxford from 12 to 15 September 2016. Contributions were received across eight principal themes: internal erosion, sediment transport, grain scale to continuum scale, advanced numerical modelling of scour and erosion, terrestrial scour and erosion, river and estuarine erosion including scour around structures, coastal and offshore scour and erosion and, management of scour/erosion and sediment (including hazard management and sedimentation in dams and reservoirs). The conference included four keynote lectures from world leading academics and practitioners cutting across the themes of scour and erosion, together with 132 peer-reviewed papers from 34 countries.

Scour development around structures with non-uniform cylindrical geometries

Many offshore foundations are composed of non-uniform cylindrical geometries such as cones and different diameter composite cylinders. However, little is known about how these types of structure respond with regards to scour under the forcing of a unidirectional current. The present paper describes a series of laboratory experiments that have been performed to examine the effect that the geometry of a marine structure has on the evolution and equilibrium depth of scour under different hydrodynamic conditions. It was found that the hydrodynamic scour response of the non-uniform cylindrical structures is fundamentally different to that of a uniform cylinder.