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

The performance of the plate anchor to adapt the trenching seabed
Congcong Han, Jun Liu
State State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology
Dalian, Liaoning, China

width, enlarges with time and gradually approaches to the anchor. Some
attempts have been made to investigate the progressive development ofseabed trenching (Sassi et al., 2017; Sun et al., 2020; Wang et al., 2020),
while the evolution mechanism of the trench is still obscure. However,there is no doubt that the seabed trenching is disadvantageous for thestability of the anchor embedded within the seabed. For fixed suction
anchors, the trench may change the failure mode of the anchor, resulting
in significant reduction of the holding capacity (Arslan et al., 2015;
Alderlieste et al., 2016; Sassi et al., 2018; Feng et al., 2019). For instance,
the horizontal capacity of the suction anchor reduces by 40%~70% whenthe trench depth reaches the padeye (Feng et al., 2019). Therefore, theseabed trenching threatens the safety of the floating system. It isnecessary to propose methods to restrain the effect of seabed trenchingon the holding capacity of anchoring systems.

ABSTRACTThe seabed trenching is prevalent in many floating moorings. The seabedaround the dip-down point of the mooring line is tunnelled due to thechain-soil-water interaction. The trenching presence may significantly
reduce the holding capacity of the anchoring foundations, which
threatens the safety of the floating system. The plate anchor is relatively
mobile in comparison with a fixed suction caisson. With appropriate
padeye offset, the plate anchor can dive deeper into stronger soils withhigher strength under extreme conditions. Therefore, it is reasonable toassume that plate anchors provide an alternative to adapt the seabedtrenching. Once the seabed trenching is severely developed, the mooringline connected to the plate anchor is further tensioned by a tug, forcing
the anchor to dive deeper to gain additional holding capacity. Two issues
should be addressed to illustrate the performance of plate anchors intrenched seabed. The first is the anchor stability in trenched seabed, i.e.
how to determine the critical trench depth for the anchor to maintainstability within the seabed? The second is the further keying property ofthe plate anchor in trenched seabed, i.e. does the anchor still exhibit
diving behaviour in the trenched seabed? The present study performedmodel tests to address the above issues. First, the progressive
development of the seabed trenching was simulated, which aimed toinvestigate the stability of the plate anchor in the trenched seabed.
Second, the plate anchor was further keyed in the trenched seabed, which
aimed to investigate the further keying behaviour of the anchor. The test
results indicate that the plate anchor does not fail with the trench depth
approaching 0.90 times the padeye embedment depth, and still exhibits
diving behaviour in the trenched seabed. Overall, plate anchors arerelatively mobile and are useful to restrain the effect of seabed trenchingon the holding capacity of the anchoring system.

Fig. 1 Seabed trench induced by periodic motion of the mooring line
Plate anchors are relatively mobile/flexible compared with a fixedsuction anchor. The plate anchor is usually vertically installed. Then theanchor is forced to move and rotate by tensioning and dragging themooring line as shown in Fig. 2. This process is termed as ‘keying’. By
setting an appropriate padeye offset, the plate anchor exhibits diving
behaviour under extreme loading conditions (Fig. 2), during which theanchor dives deeper into stronger soils and will gain higher capacity(Cassidy et al., 2012; Liu et al., 2016; Tong et al., 2020). In the trenchedseabed, the plate anchor may be forced to move forward and dive deeper
by untying the mooring line from the fairlead and tensioning the mooringline using a tug. Therefore, it is assumed that the movable plate anchor
is effective to restrain the seabed trenching effect. Two aspects should
be addressed to validate the effectiveness of the plate anchor in the

KEY WORDS: plate anchor; trenching; chain; mooring; offshore
anchoring system & foundation.

INTRODUCTION
Offshore floating structures are secured in position using anchors
through mooring lines. A section of mooring line periodically cuts
through the seabed due to the cyclic movement of the floating structure.
Due to the mooring line–soil–water interaction, a tunnel/trench (Fig. 1)
is gradually formed as indicated from subsea inspections (Bhattacharjee
et al., 2014; Colliat et al., 2018). The trench size, including the depth and1598