Methodology
This screening evaluates whether a candidate floater-mooring concept is fit-for-purpose at a pre-FEED / early-FEED decision gate. The workflow uses MoorPy quasi-static stiffness analysis to characterise restoring behaviour, then RAFT-based frequency-domain response screening to flag concepts whose surge, heave, or pitch periods sit too close to the wave-energy band. The standards basis is API RP 2SK (stationkeeping system design and analysis) and DNV-OS-E301 (position mooring), which set the safety-factor and analysis-tier expectations the screening is meant to feed.
Each candidate concept (catenary, taut, hybrid) is screened for restoring stiffness, line tension margin against minimum breaking load (MBL), vessel offset against the typical 10% water-depth band, and sensitivity of those trends to a ±20% sweep on mooring stiffness assumptions. A concept whose ranking flips under that sweep is flagged for coupled time-domain follow-on rather than declared acceptable. The tier is intentionally screening-grade — quasi-static plus frequency-domain — and is the layer that decides whether the more expensive coupled analysis is worth running.
Applicable Codes & Standards
- API RP 2SK — Stationkeeping Systems for Floating Structures
- DNV-OS-E301 — Position Mooring
- DNV-OS-E302 / E304 — Offshore Mooring Chain & Steel-wire reference
- RAFT (NREL) — frequency-domain floating response, screening-tier
- MoorPy — quasi-static mooring analysis, screening-tier
Parameter Matrix
Illustrative parametric envelope: 3 platforms × 4 mooring concepts × 5 water depths × 2 sea states = 120 cases. The matrix is sized for an overnight screening sweep; on a real engagement, axes are replaced with the project's actual platform, candidate spreads, site water depth, and operating / survival metocean.
| Axis | Range | Notes |
|---|---|---|
| Platform | Semi-submersible, spar, FPSO-class | Public-reference floater archetypes |
| Mooring concept | 3-line catenary, 4-line catenary, taut polyester, chain-polyester hybrid | 120° or 90° spread depending on concept |
| Water depth | 200, 500, 1000, 1500, 2000 m | Spans intermediate to deepwater |
| Sea state | Operating (1-yr), Survival (100-yr) | Hs / Tp pair per state |
| Anchor strategy | Drag-embedment, suction pile (downstream of concept) | Geotechnical handoff at next gate |
| Pre-tension | Set to bring quasi-static surge offset under environmental loading inside the 10% water-depth band | Per-line pretension is a result, not a free axis |
Screening Results Summary
PASS / MARGIN / FAIL distribution by concept and condition. Pass = quasi-static offset inside 10% water-depth band, line tension below API RP 2SK / DNV-OS-E301 intact safety factor, and surge / heave / pitch natural periods clear of the wave-energy band. Margin = at least one of those checks within 10% of the limit or sensitive to the ±20% stiffness sweep. Fail = limit exceeded or rank-flip under sensitivity.
| Concept | Cases | PASS | MARGIN | FAIL | Acceptable % | Dominant flag |
|---|---|---|---|---|---|---|
| 3-line catenary | 30 | 21 | 6 | 3 | 90% | Offset margin at 2000 m, survival |
| 4-line catenary | 30 | 25 | 4 | 1 | 97% | Tension margin in survival sea state |
| Taut polyester | 30 | 18 | 8 | 4 | 87% | Surge period drift under stiffness sweep |
| Chain-polyester hybrid | 30 | 22 | 6 | 2 | 93% | Anchor footprint at shallow depths |
Pass-rate column includes PASS only; MARGIN cases require either tightened assumptions or escalation into coupled time-domain analysis before they can be cleared.
Concept Comparison Chart
Quasi-static restoring stiffness kxx by concept (kN/m, illustrative public-reference order of magnitude only):
| Concept | kxx (kN/m, illustrative) | Restoring character |
|---|---|---|
| 3-line catenary | ~80 | Soft, geometry-driven; gentle offset growth, large footprint |
| 4-line catenary | ~110 | Symmetric restoring, redundancy on line failure |
| Chain-polyester hybrid | ~140 | Tunable via polyester axial stiffness, smaller footprint |
| Taut polyester | ~220 | Stiff, axial-driven; small footprint, sensitive to pre-tension |
Where this stops
This is a screening-tier deliverable. It is intentionally not, and ACE will not represent it as:
- Detailed time-domain coupled analysis (OrcaFlex / OpenFAST coupled runs across the full DLC matrix).
- Full IEC 61400-3-2 design load case execution for a floating wind turbine, including controller-coupled DLCs.
- Geotechnical anchor capacity design or site-specific foundation engineering — those scopes go to specialist consultants once the screening clears the gate.
- Final fatigue verification of mooring components for life-cycle signoff or class / certifier submission.
GTM Value
| Activity | Before | After |
|---|---|---|
| Mooring concept selection | Weeks of bespoke setup before any concept is ranked | ~120 screened cases in 24 hours, decision-grade memo in 48 |
| Sensitivity check | Manual rerun of bespoke models per axis | ±20% stiffness sweep baked into the workflow |
| Anchor strategy framing | Late-stage hand-off, often after concept locked | Anchor implications surfaced at concept gate |
| Escalation gate | Implicit, often missed | Explicit gap register: which concepts need coupled time-domain follow-on, which do not |
| Reviewer audit trail | Spreadsheet provenance | Git-tracked Python + YAML, reviewable by a third party |
Reviewing ACE for early-stage mooring or station-keeping work? Start with the one-page capability summary.
Download Capability Summary (PDF, 1 page)