Onset of Motion of Sediment underneath Scour Protection around a Monopile

Abstract

The stability of scour protections is, potentially, an important issue during the design of fixed foundations for offshore wind turbines. One of the failure mechanisms observed at placed scour protection around offshore foundations is suction of sediment through the scour protection and subsequent sinking of the scour protection. Incipient motion of sediment and the initiation of suction underneath scour protections around piles in the marine environment were studied under waves, current and combined waves and current conditions. The motion of a thin layer of sediment underneath the scour protection was studied through the glass bottom of the test flume, which provided a clear view of the initiation of the motion of the sediment. The results show that the mobility depends on the Keulegan–Carpenter ( K C ) number for the pile, the ratio between waves and current flow and the ratio between the thickness of the scour protection and the base sediment. The critical mobility number is smaller for the wave-dominated situation compared to current-dominated conditions, which again are smaller than for combined waves and current conditions. Consequently, larger K C -numbers cause larger critical mobility numbers than smaller K C -numbers. Design diagrams are presented for the threshold of incipient motion of sediment underneath a scour protection in waves, current and combined waves and current.

Scour Protection Measures for Offshore Wind Turbines: A Systematic Literature Review on Recent Developments

“…In addition, a set of non-dimensional parameters was proposed by researchers [52] for formulating the initiation of the motion beneath the scour protection and the sinking of it. Furthermore, Nielsen et al [53] studied the motion process of the former to quantify and compare the critical mobility number of the sediment beneath scour protection under different hydrological conditions and provided design diagrams for the threshold of the sediment incipient motion.…”

Quantification of Measurement and Model Effects in Monopile Foundation Scour Protection Experiments

“…Other relevant works are seen in [17][18][19][20][21][22]. Failure modes (2) and (3) are addressed in [23][24][25][26]. In addition to the laboratory experiments, field surveys provide more details about how monopile scour protection behaves in real life sea conditions, such field data is available in [27].…”

Large Scale Experimental Study of the Scour Protection Damage Around a Monopile Foundation Under Combined Wave and Current Conditions

“…Later on, various design methodologies have been developed and validated by means of wave flume experiments [17][18][19][20][21][22][23][24][25]. Secondly, the sinking of protection materials in the scour protection layer is described in [26][27][28][29]. In third place, field and laboratory studies are seen in [30][31][32] for the edge scour failure mode.…”