Hydrodynamic Response of a Subsea Piling Rig During Offshore Lowering

Abstract

The expansion of floating offshore wind energy into deeper waters requires suitable anchoring solutions, for which IQIP has developed a specialized 170 t subsea piling rig. A critical phase in deploying this system is the lowering operation through the water column to the seabed, which introduces significant operational risks, most notably damaging snap loads resulting from temporary line slack. This occurs when the installation vessel's downward motion exceeds the rig's descent speed. The hydrodynamic behavior of the rig's large mudmat (foundation plate) is complex and creates substantial resistance.
The objective of this thesis was to quantify how the rig's specific shape affects its vertical oscillatory motion and to identify slack-inducing conditions in a given sea state. The hydrodynamic response of the rig and resulting line tensions were investigated using time-domain simulations in OrcaFlex, focusing on the deep submergence and positioning/landing phases. A representative Service Operation Vessel (SOV) was modeled using Response Amplitude Operators (RAOs), and the mudmat was treated using a heave-plate analogy with coefficients derived for low oscillation movements.
Three distinct operational scenarios were analyzed to capture the varying physics of the descent:
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Unbounded Deep-Water Region (Mid-water Transit): In this regime, where the mudmat is sufficiently far from both the surface and the seabed, the rig behaved with stable hydrodynamic forces. Simulations showed that slack events were present but infrequent.
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Near-Seabed Region: The rig's response when positioned close to the seabed was drastically different. Flow confinement amplified both added mass and damping significantly. This caused the slack probability to increase drastically compared to the unbounded case, consistent with the confinement-dominated regime.
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Passive Suction Release: Uncontrolled suction release at the maximum upward vessel movement (heave crest) was found to trigger an immediate, full slack event. The sudden release of stored elastic energy in the lifting line caused the mudmat to shoot upward, leading to a complete loss of line tension even before the vessel began its downward motion.
Based on these findings, operational conclusions emphasize the need for caution. The increased hydrodynamic resistance near the seabed makes the system prone to slack, indicating that the rig should be lifted far away from the seabed before horizontal repositioning. Furthermore, controlled or gradual suction venting is essential, preferably timed near a point of neutral vertical vessel movement, to prevent the severe dynamic snap loads associated with sudden suction failure. Future modeling should incorporate porous boundary conditions using suitable open source potential flow solvers and dedicated experiments to better capture complex flow interactions.