Scour Protection Effects on Laterally Loaded Monopiles-Paper

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The paper investigates how scour and scour protection affect the lateral capacity and stiffness of offshore wind turbine monopiles. Scour can reduce soil support by removing pile–soil contact and by lowering effective stresses in the surrounding soil. Although rock armour is widely used to prevent or remedy scour, its contribution to foundation performance is often omitted from design analyses.<br /><br />Researchers conducted reduced-scale flume tests using a model monopile embedded in dense sand. Five configurations were tested: one with unremediated scour, three with rock fill installed after scour had developed, and one with pre-installed rock armour. Each pile was subjected to monotonic lateral loading. The results were compared with a one-dimensional finite-element model based on the PISA design framework and calibrated for sand.<br /><br />The tests showed that scour protection can increase monopile lateral capacity, including when installed as a remedial measure. Its benefit may be reduced when global scour destabilizes the protection and causes it to slump or become undermined. Sand accumulated within the rock armour in all protected tests, likely adding to its strength and stiffness.<br /><br />Predictions for scour-only conditions were sensitive to assumptions about effective stress beneath a local scour hole. A stress profile calibrated against measurements best matched the test results and suggested that stress recovers with depth more rapidly than implied by the API design approach. The model generally underestimated the performance of protected piles, indicating that rock fill is stronger than the sand-based soil model represents.<br /><br />The authors conclude that rock protection can improve foundation performance, but its stability and extent matter. Designers should consider the risk of global scour undermining protection and provide sufficient rock, while ensuring pre-installed protection is wide enough to limit seabed changes around the pile. The findings also highlight the need for better modelling of rock-fill behaviour and time-varying scour conditions.
Keywords
scour
scour protection
offshore wind turbine
monopile
lateral capacity
stiffness
rock armour
finite-element model
dense sand
PISA framework