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Experimental Study of Scour Protection Assessment for Special-shaped Collars Around Monopile Foundations Exposed to Waves

A monopile foundation is a mature, cost-effective and efficient option for offshore wind turbine
foundations (hereinafter referred to as OWTs); over 79% of offshore wind turbines are built with
this type of foundation in Europe. However, Scour hole development may seriously threaten the
performance of the foundation. Traditionally, a riprap scour protection constructed of filter and
amour layers is applied to reduce scour around OWTs in engineering practice. This method has
also been widely investigated and researched. Deltares provided the Handbook of Scour and Cable
Protection Methods (Deltares, 2023), which contains generic design guidance for riprap protection
of offshore support structures. Combining a riprap scour protection with alternative protection
methods (like collars), could lead to a significant performance improvement in resisting scour and
thus a smaller environmental footprint. Using the circular collar as a stand-alone solution was
found to have very limited protection efficiency (Tang et al., 2023; Zhang et al., 2021), but there
is great potential for improvement when combining it with a riprap protection.
This paper aims to quantify the scour protection performance of these innovative structures
through data analysis on small-scale wave experiments and find a more efficient scour protection measure, which will open new possibilities and insights into future scour protection design in
offshore engineering. Our research conducts a comparative analysis of the combination of riprap
and collar protection. Referring to the existing research on bridge pile and OWT collars, 12
different collar designs are proposed, and their protection performances on riprap protection
around monopile foundations are investigated under different wave conditions. They could be
divided into three main groups: hooked collars, conical collars, and inverted conical collars (see
Figure 1).
The experiments were conducted in a wave flume to investigate the damage circumstances of
riprap and wave velocity variation. The flume is located at the Dooge Centre for Water Resources
Research at University College Dublin, Ireland, which has a working section of 20 m long, 1 m
wide and 0.6 m high. The wave design is based on the actual engineering design parameters and
scaled using Froude's similarity laws (Hughes, 1993). Typical extreme events and a stack of storm
tests are considered to simulate the actual extreme wave conditions. To generate irregular waves,
JONSWAP spectra with