This worked example shows the shape of an ACE FOWT mooring screening deliverable at pre-FEED / early-FEED stage. The intent is to help a wind contractor or developer rank floater-mooring concepts, find first-order red flags, and decide whether the design is mature enough to escalate into coupled aero-hydro-servo-elastic verification with a partner. ACE leads on mooring concept selection, anchor strategy framing, quasi-static stiffness, frequency-domain response trends, and tow-out / hook-up logic transferred from offshore oil-and-gas practice.
ACE does not deliver certification-grade IEC design-load-case execution, controller co-design, or full time-domain coupled simulation in this scope. Those workstreams are explicitly handed off to OpenFAST / WEIS partners or to specialist geotechnical consultants when the project clears the screening gate.
The OC4-DeepCwind semi-submersible (Robertson et al., NREL/TP-5000-60601, 2014) is the canonical public floating-wind reference platform paired with the NREL 5 MW baseline turbine. We use commonly cited public values to demonstrate the screening shape; on a real engagement these are replaced with the project's mass, hydrostatic, and metocean inputs.
| Parameter | Public reference value | Notes |
|---|---|---|
| Reference turbine | NREL 5 MW baseline | Public coupled reference |
| Platform displacement | ~13,470 t | Including ballast water |
| Operating draft | 20.0 m | Below SWL |
| Water depth | 200 m | OC4 reference site |
| Number of columns | 3 outer + 1 main | Semi-submersible |
| Mooring concept | 3-line catenary | 120° spread |
Source: OC4 Phase II definition, NREL/TP-5000-60601. Values are cited to illustrate the screening method; final-design analyses use project-specific inputs.
Three-line catenary spread at 120°, fairleads at the column bases, anchors at 837 m radius (OC4 reference). Each line is a chain–polyester–chain composite to manage axial stiffness and seabed footprint. Pre-tension is set to bring quasi-static surge offset under environmental loading inside the typical 10% water-depth screening band.
| Element | Reference value | Screening intent |
|---|---|---|
| Line count | 3 (120° spread) | Symmetric restoring |
| Top chain (fairlead) | ~120 mm studless, ~80 m | Wear / fairlead interface |
| Mid section | Polyester rope | Compliance, weight relief |
| Ground chain (anchor) | ~120 mm studless, ~100 m | Seabed abrasion margin |
| Anchor radius | ~837 m from platform CG | OC4 baseline |
| Pre-tension (per line) | ~1.1 MN (illustrative) | Replace with project values |
MoorPy quasi-static evaluation produces a restoring matrix the project team can use to sanity-check surge / sway / yaw stiffness against floater displacement. Illustrative public-reference values only — replace with project values before any decision: for an OC4-class catenary, kxx ≈ kyy typically fall in the 70–90 kN/m band and kyaw in the order of 1.0×108 N·m/rad. The screening flags any concept where stiffness is dominated by a single line or where pre-tension drift would push offset outside the operability envelope.
RAFT-based frequency-domain runs give surge / heave / pitch RAOs and natural-period estimates the team can compare against the wave-energy band. Screening looks for surge natural period sitting cleanly above the wind-sea peak, heave and pitch periods avoiding the swell band, and sensitivity of those trends to a ±20% change in mooring stiffness assumptions. A concept whose response trends move materially under that sensitivity sweep is flagged for coupled time-domain follow-on rather than declared acceptable.
Screening also surfaces marine-operations red flags: tow-out draft and stability margin, hook-up sequence on the three lines under expected weather windows, anchor installation method consistent with the pre-tension target, and inspection access for the chain–polyester transitions. Items here are decision-support inputs, not procurement specifications.
The screening above intentionally does not cover, and ACE will not represent it as covering:
If the screening passes the decision gate, ACE hands these scopes to OpenFAST / WEIS partners or specialist consultants and stays engaged as the offshore-mooring and marine-operations technical reviewer.
A real engagement starts with a 1-hour intake to capture floater concept, site metocean, mooring layout candidates, and the decision the screening must support. ACE then runs the 48-hour screening using RAFT + MoorPy with the project's actual inputs and produces a decision-grade memo covering scope, assumptions, screening outputs, red flags, and the explicit gap register. A review session walks the contractor or developer through the result and the recommended escalation path. This follows the prospect-demo SOP already used for other ACE screening deliverables.
Reviewing ACE for early-FEED FOWT mooring work? Start with the one-page capability summary.
Download Capability Summary (PDF, 1 page)