Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Lateral bearing capacity of a winged monopile in the scoured sand-bed
Biao Li a,b, Shun-Yi Wang a,b, Wen-Gang Qi a,b , Yifa Wang c, Fu-Ping Gao a,bInstitute of Mechanics, Chinese Academy of Sciences, Beijing, China
School of Engineering Science, University of Chinese Academy of Sciences, Beijing, Chinac
Department of Infrastructure Engineering, Faculty of Engineering and Information Technology, The University of Melbourne, Melbourne, Australia
abwaters at a reasonable cost, several new hybrid monopile foundation
types for OWTs have been proposed recently (Qi & Gao, 2019).

ABSTRACT
Compared with conventional monopile foundations, a hybrid monopile
with wings attached to the perimeter is expected to exhibit better
performance for supporting offshore wind turbines under huge lateralloads. Nevertheless, local scour could compromise the performance of
the winged monopile in a sandy seabed. This hazardous scour effect on
the lateral response of winged monopiles has not been well understoodand adequately evaluated yet. In this study, the local scour process
around a winged monopile in a sandy seabed was firstly physicallymodeled in a water flume. It was observed that both the depth and thewidth of the local scour hole around a winged monopile can be larger
than those of a conventional monopile. Therefore, the wings need to be
mounted below the scour hole to avoid the intensified scour effect that
impedes the effectiveness of the wings. A series of numericalsimulations were then carried out to evaluate the lateral load responseof a winged monopile in the sand under the local scour condition. The
numerical results indicate that the deformation of a winged monopile
can be significantly mitigated compared with conventional monopiles.
Contours of displacement and plastic strain in the soil at the ultimate
limit state are presented to reveal the mechanism of wings forenhancing the lateral bearing capacity of monopiles. The effects of theaspect ratio and the number of wings on the bearing capacity of themonopile are further examined under the premise of the same material
consumption. Finally, the optimized design of wings is proposed totake full advantage of the merit of the winged monopiles under thelocal scour condition.

As an innovative hybrid monopile, the winged monopile is a promisingfoundation solution for withstanding large horizontal and moment loads
from wind, waves and current acting on the turbine. It usually has
wings evenly attached to the monopile perimeter due to the variant
loading directions in the offshore environment. In-situ tests showed thatthe lateral displacement of the pile can be reduced by 70% withproperly designed wings (Murphy et al., 2012), while the centrifuge
model tests demonstrated a reduction of 50% (Bienen et al., 2012). A
number of studies have focused on the optimized design anddeployment of the wings. It was reported that wings mounted near themudline would be beneficial to mobilize the sand resistance (Abongo,2019; Babu & Viswanadham, 2017; Mahdi et al., 2021; Peng, 2006;
Peng et al., 2010). Compared to the triangular and the trapezoidal
wings, rectangular wings can exhibit superior performance inmitigating lateral deformation of the pile (Dührkop & Grabe, 2008;Nasr, 2014). Besides the vertical position and shape of wings, thenumber and the aspect ratio of wings (i.e., the height-to-width ratio ofthe wing) may have a significant influence on the lateral response of awinged monopile. Babu and Viswanadham (2017) suggested that starwing monopiles (i.e., monopiles with eight wings) have higher lateralload capacity than those with four or three wings. Though more wings
with the same size improve the lateral load capacity, three and fourwings are preferable over the star-wing configuration considering
installation efforts. Abongo (2019) further pointed out that monopiles
with four wings show slightly higher lateral capacity than the threewing counterpart. In terms of aspect ratio of wings, Peng (2006) andPeng et al. (2010) found that a larger lateral bearing capacity could be
mobilized with an increased height of wings, which reaches anoptimum wing efficiency when the wing height equals half of the pilelength.

KEYWORDS: scour; winged monopile; sandy seabed; lateral
response.

INTRODUCTION
Offshore wind power has been drawing more attention due to the mass
reserves, high-energy density, and minor disturbance than onshore
wind power. By the end of 2020, the total cumulative installed capacity
had increased up to 35 GW (GWEC, 2021). As the most commonly
adopted foundation for offshore wind turbines (OWTs), monopiles
have supported more than 80% of installed OWTs (Gupta & Basu,2020). The construction and installation cost of the OWT foundations ishigh, accounting for 19% to 36% of the total project cost (Oh et al.,2018). To support larger-capacity wind turbines that operate in deeper

The existing studies regarding the lateral response of winged monopileswere generally limited to the scenario of a pile embedded in a flat
seabed, i.e., the local scour was not taken into account. Nevertheless,severe local scour could be induced by waves and/or current, which
may further reduce the efficient driving length of the pile andcompromise the bearing capacity (Qi & Gao, 2014b). Such scour effecton the lateral response has been intensively studied for the conventionalmonopile without wings (Lin et al., 2014; Qi et al., 2016). Since the1482