**1. Introduction**

Offshore wind power is developing into a major energy source globally due to the abundant reserves, high-energy density, and fewer civil complaints compared to onshore wind power. The global installed capacity reached more than 14,300 MW at the end of 2016 [1], and an increased potential for growth in future was estimated [2, 3]. To achieve high energy harvesting efficiency, the industry is currently advancing into deeper waters accompanied by upscaling the offshore wind turbines (OWTs) from 5 to 8 MW, 10 MW and then 12 MW [4] (see **Figure 1**). The steady upward trends in both water depth and size of OWTs have led to consequential growth in the loading, further aggravating the deformation of the foundations and potentially jeopardizing the operation of the OWTs.

**2. Local scour around a monopile**

*DOI: http://dx.doi.org/10.5772/intechopen.88591*

been given in Refs. [12, 13].

flow conditions.

**Figure 2.**

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*General flow patterns involving local scour around a monopile.*

**2.1 General description and scour mechanisms**

In the river hydraulics, the local scour at the bridge piers has been widely recognized to be one of the main causes for the structure failure [11]. Hence, the phenomenon of scouring at bridge piers has ever been studied extensively over the past half century from various aspects, including local scour mechanisms associated with flow characteristics, prediction of the maximum scour depth, etc. Comprehensive descriptions of scour around a pile or bridge pier in steady currents have

*Local Scour around a Monopile Foundation for Offshore Wind Turbines and Scour Effects…*

For a pile foundation installed in the marine environment, several phenomena with regard to the flow pattern would arise including a horseshoe vortex (HSV, or sometimes necklace vortex due to its shape) in front of the pile, wave vortices convected through the pile's lee-side, and the contraction streamlines at the side edges of the pile. These changes induce an increased sediment transport capacity. If the scouring potential created by the large-scale eddy structures is strong enough to overcome the particles' resistance to motion, local scour will be initiated around the pile (see **Figure 2**). As scour hole develops, the strength of the large-scale eddy structures will gradually be weakened, thereby reducing the transport rate from the local scour region. Equilibrium is reestablished if the sediment inflow of the scour hole is equal to the outflow for live-bed scour, or the shear stress caused by vortices is equal to the critical shear stress of incipient sediment motion at the bottom of the scour hole for clear-water scour. Two major issues concerning the local scouring process, namely the equilibrium scour depth and the time scale, usually need to be quantitatively characterized and are detailed in Sections 2.2 and 2.3, respectively. We herein focus on reexamining the general scouring mechanisms, which mainly involve characterization of the flow pattern under different incoming

A surface-mounted cylinder induces the boundary layer separation from the upstream bed in the incoming flow, resulting in a HSV system at the upstream junction corner of the cylinder. The existing test observations indicated that the HSV (including the down-flow) upstream of a cylinder is the most evident and significant contributor to the scour process in currents alone. Much work has been done for the HSV at a cylinder in steady currents. Many researchers investigated the HSV at a flatbed for the laminar case [14–16]. However, most of the practical HSVs occur with turbulent approach boundary layers [17]. Baker [18] used smoke- and oil-flow visualization to observe the turbulent HSV system. The variation in the

**Figure 1.** *Growth in size and power of horizontal axis wind turbine [4].*

As the most commonly adopted foundation for OWTs, the monopile had been used as the foundation of 2653 OWTs by the end of 2016, which accounts for over 80% [5], due to their ease of design and manufacture in contrast to other foundation types. A typical monopile is composed of a hollow steel cylinder with a diameter typically spanning 4 to 8 m and a slenderness ratio (driving depth over diameter) less than 10 [5]. A special aspect to consider with monopile foundations for OWTs is that the vertical load mainly comprised of gravity load is relatively small compared with other offshore structures like oil and gas platforms. The horizontal loads generated by wind and wave actions, which are of similar magnitude to the gravity loads, are therefore of particular importance for the design [6]. A typical laterally loaded offshore monopile foundation generally behaves rigidly due to their large bending stiffness and low slenderness ratio [7]. The serviceability requirements including lateral deformation and stiffness rather than ultimate lateral resistance tend to govern design due to strict rotational tolerance specifications for monopile foundations [8].

Under the current and/or wave loadings, severe local scour can be generated around a monopile situated in cohesionless sediments. The maximum scour depth is generally predicted to be proportional to the pile diameter, and scour depths of over 1.5 times pile diameter were observed in many practical installations [9]. Considering the lower slenderness ratio compared with more conventional offshore pile for oil and gas industries, the maximum scour depth at a monopile may account for over 25% of the driving length [10]. This scour-induced driving length reduction has significant influence on both quasi-static lateral responses (e.g., ultimate lateral resistance and deformation stiffness) and dynamic responses (e.g. natural frequency) of monopile foundations. Therefore, proper assessments of local scour and its influence on structural responses are essential to the design of monopile foundation for OWTs.

In view of the complexity of local scour phenomenon around a monopile and scour effects on manifold monopile responses, this chapter aims to systematically summarize the predicting methods involving the maximum scour depth and the time scale under various flow conditions. The controlling mechanisms of local scour around a monopile are also revisited. Subsequently, we examine existing research on the effects of local scour on the responses of the monopile foundation.

*Local Scour around a Monopile Foundation for Offshore Wind Turbines and Scour Effects… DOI: http://dx.doi.org/10.5772/intechopen.88591*
