RAO-driven dynamic assessment of a subsea-manifold installation at 1526 m water depth

Installing a large asymmetric subsea structure in ultra-deep water requires coordinated control of vessel motion, long-wire compliance, splash-zone hydrodynamics, crane and rigging capacity, resonance, and final-approach velocity. This study develops an RAO-driven one-way coupled time-domain assessment of the Normand Oceanic crane-wire-rigging-Manifold E3 system in OrcaFlex. Vessel and crane-tip motions are prescribed from displacement response-amplitude operators, while the nonlinear hoisting lines, rigging, and six-degree-of-freedom payload respond dynamically without feedback to the vessel solution. The analysis covers six installation stages from half-submerged entry to a controlled approach 10 m above the seabed at 1526 m water depth. Stage-specific calculations quantify crane-tip, hook, and sling forces, dynamic amplification, natural-period migration, and approach velocity for spectral peak periods of 4–11 s and three project headings. The hydrodynamic formulation distinguishes the 43.51 t displaced-water reference mass from the 1874.90 t vertical added mass used in the dynamic model. At a 165° wave heading, the most restrictive condition occurs during the final approach when active heave compensation is off: the allowable significant wave height decreases to 0.6 m at a peak period of 11 s, whereas all investigated AHC-on cases satisfy the acceptance criteria at H s = 3.0 m. The resulting stage-resolved workflow links hydrodynamic loading, hoisting-system dynamics, equipment capacity, clearance, and final-approach control in a traceable operational framework that supported engineering planning for the field installation.