Sandwaves and megaripples at Race Bank (UK) Offshore Wind Farm

The Race Bank Offshore Windfarm is part of the UK Round 2 extension. The windfarm comprises 91 6.3 MW turbines and two substations with total installed power of 573 MW. Construction work is scheduled for 2016/17. The windfarm is located in UK waters in the southern part of the North Sea about 30 km north of the Norfolk coastline and 30 km east of Lincolnshire. The Race Bank site has a diverse morphology located on large sandbanks with mobile sandwaves and megaripples and intersected by deeper featureless swales where clay soil formations are found close to or at the sea floor. The morphological complexity of the site was recognised early in the project development and numerous bathymetric surveys have been undertaken to help quantify sandwave migration rates and expected seabed variability. Morphological seabed assessments have been conducted by ABPmer throughout the project development. In the detailed design phase the ABPmer assessments have been combined with in-house DONG Energy analysis to provide design seabed levels and anticipated ranges of natural seabed changes within the lifetime of the windfarm. The paper presents an overall morphological characterisation of the windfarm site and provides examples of the very diverse seabed morphology encountered. When quantified, the morphological diversity can be absorbed in the monopile foundation design and cable installation contractors can select appropriate installation tools. The paper thus comes with a recommendation to ensure that bathymetric surveys in morphological active sites are carried out early and repeatedly during the project development stages

Parametric equations for Shields parameter and wave orbital velocity in combined current and irregular waves

A fundamental requirement for any scour assessment and scour protection design is the ability to determine the Shields parameter for combined wave and current conditions. The Shields parameter can be calculated for current combined with monochromatic waves using the approach of Soulsby (1997) in combination with the wave friction factor concept. For current in combination with irregular waves, the same approach is suggested using a wave orbital velocity, Um, for representation of the irregular sea state. Um is defined as 1.41 times the standard deviation of the near bed wave orbital velocity. The Soulsby (2006) expression for Um is compared with a hyperbolic expression and validated using numerical methods and laboratory measurements. A large number of expressions exist for the wave friction factor as a function of relative bed roughness. From a literature study, the paper proposes a combination of existing expressions to cover relative bed roughnesses from sand over gravel to coarse armour rock

Scour and seabed changes at cable protection rock berms – Field observations

United Kingdom waters have morphologically dynamic bays and estuaries through which offshore windfarm export cables are laid. The export cables must cross existing cable and pipeline infrastructure before making landfall. Rock berms have traditionally been installed at such crossings and at locations of shallow cable burial for cable separation and protection. In some instances, the rock berms have caused scour and been affected by general seabed changes. Recent field observations of scour and seabed change at cable protection rock berms is presented. In one location two separate rock berms are exposed to 1.0 m to 1.5 m general seabed lowering. One berm is aligned oblique to the current and has experienced excessive scour, while the other berm is aligned more parallel with the current flow direction and has experienced only minor scour. A second location comprise a cable crossing with double rock berms aligned oblique to the current direction. The current speed was asymmetric being largest during flood and smaller during ebb tide. Excessive scour was observed in-between the two berms and on the in-shore side, while only limited scour was observed at the off-shore side of the berms. The asymmetry in scour is linked to the asymmetry in current speed indicating a strong scour dependence on the currents speed. In non-dimensional form, this dependence is linked to the so-called Shields parameter governing magnitude and mode of sediment transport

Scour at cable protection rock berms - Model test observations

Rock berms or concrete mattresses have traditionally been applied for protection of shallow buried, surface laid or crossing pipelines and cables. In most cases the installed protection has worked satisfactorily, but under certain adverse conditions, field evidence has shown examples of severe scour both at mattresses and rock berms. The paper presents and discusses physical model tests of rock berm scour in combined waves and current. Both single and double berm configurations were studied. The conclusions from the tests were two-fold addressing: 1) Scour development caused by rock berm(s), and 2) Structural robustness of the rock berm when subject to wave action and seabed model scale effects. The model tests support field evidence that scour is prone to develop in strong current environments and when the protection is aligned oblique or perpendicular to the prevailing currents. The tests also demonstrate larger structural integrity of wide crested berms as opposed to sharp crested berms