Offshore windfarm dredging - Managing mobile seabeds and dynamic dredging in foundation and trench preparation
Type:
Presented during:
CEDA Dredging Days 2026
Authors:
S. Nanayakkara, S. Burgmans and N. Dalziel
Abstract
Offshore windfarms have expanded rapidly along Taiwan's west coast in the Taiwan Strait, an area characterised by strong tidal currents and mobile sand waves up to 10 m high. These dynamic seabed conditions pose significant challenges for dredging foundation pockets and cable trenches, as freshly prepared sites are prone to rapid infill and deformation. This paper examines the strategies adopted to address these challenges on the Hai Long Windfarm project. To optimise the construction schedule, bulk dredging was scheduled in winter, when conditions were unsuitable for jacket or cable installation. Nevertheless, operations were initially constrained by weather delays, reflecting the region's well-known wave climate. In terms of methodology, foundation pockets allowed relatively flexible dredging approaches, whereas trenching required precise directional control. Longitudinal dredging proved most efficient, minimising turning and back-sailing, though perpendicular dredging, slower and less adaptable, was often necessary due to the strong cross-trench tidal currents. Synchronising dredging with cable installation proved critical, as delays between the two activities created a "cat-and-mouse" dynamic that demanded adaptive planning. Dynamic design approaches and alternative trenching techniques were therefore adopted to improved outcomes. Pre-trenching with a Trailer Suction Hopper Dredger enabled early removal of bulk volumes, while post-trenching with trencher or controlled flow excavation provided final burial. However, post-trenching was less effective in very fine, loose or clay-rich sediments, underscoring the importance of proactive pre-trenching. Heavier cables encouraged natural self-burial, reducing exposure to environmental loading and fatigue damage. Nevertheless, design practices adopted from pipeline engineering may not have fully translated to power cable applications and require further validation. Although not explored in detail here, it remains a critical consideration for holistic cable protection design. Overall, the experience demonstrates how adaptive dredging methods and installation planning can reduce risk and support the reliable delivery of offshore windfarms in highly mobile seabed conditions.