🎓 Lesson 19
D5
Retrofitting Mooring Systems on Operational Devices: Lessons from MeyGen
Retrofitting mooring systems means safely adding or upgrading anchor lines and connectors to an already-deployed marine energy device—like a tidal turbine—without pulling it out of the water.
🎯 Learning Objectives
- ✓ Analyze as-built mooring system performance using measured tension and motion data from MeyGen Phase 1A
- ✓ Design a retrofitted chain-to-rope transition assembly that satisfies DNV-RP-F204 fatigue and termination strength criteria
- ✓ Calculate revised anchor holding capacity for upgraded drag-embedment anchors under updated site-specific current profiles
- ✓ Explain trade-offs between retrofit feasibility, vessel mobilization cost, and operational risk using the MeyGen Lessons Learned Report (2022)
📖 Why This Matters
Over 60% of operational tidal stream arrays globally—including MeyGen in Scotland’s Pentland Firth—have required at least one mooring retrofit within their first 3 years. Why? Because initial designs often underestimate dynamic loading from vortex-induced vibrations, seabed scour evolution, or long-term chain wear. Retrofitting isn’t just maintenance—it’s mission-critical resilience engineering. Getting it wrong risks device loss, insurance penalties, and regulatory non-compliance. This lesson uses MeyGen’s real-world retrofit campaign to teach how engineers diagnose, justify, and execute safe, evidence-based mooring upgrades on live assets.
📘 Core Principles
Retrofitting mooring systems hinges on three interdependent pillars: (1) Condition-based assessment—using ROV inspections, strain gauge histories, and corrosion mapping to quantify degradation; (2) Load-path revalidation—re-running time-domain simulations (e.g., OrcaFlex) with updated boundary conditions (scour depth, sediment stiffness, current spectra) to identify overstressed components; and (3) Interface engineering—designing transitions (e.g., chain-to-fibre rope) that preserve fatigue life, prevent galvanic corrosion, and avoid snagging during deployment. Critically, retrofit design must comply with both original certification basis *and* post-installation verification standards—creating unique compliance pathways not covered in greenfield codes.
📐 Revised Anchor Holding Capacity (Drag-Embedment)
This formula updates anchor capacity when seabed conditions change post-installation (e.g., due to scour or sediment consolidation). It scales the original certified capacity using in-situ geotechnical data and embedment depth correction factors.
Holding Capacity Correction Factor (HCCF)
H_cap,rev = H_cap,orig × HCCF_depth × HCCF_suAdjusts certified anchor holding capacity based on measured changes in embedment depth and soil strength.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| H_cap,rev | Revised holding capacity | kN | Anchor capacity after retrofit assessment |
| H_cap,orig | Original certified holding capacity | kN | Capacity stated in design certification report |
| HCCF_depth | Embedment depth correction factor | dimensionless | Reduction factor from reduced embedment (per DNV-RP-F111) |
| HCCF_su | Undrained shear strength correction factor | dimensionless | Scaling factor for increased su (per ISO 19901-7) |
Typical Ranges:
Scour-induced embedment loss (0.5–1.0 m): 0.65 – 0.85
Consolidated clay (su increase >20%): 1.10 – 1.35
💡 Worked Example
Problem: MeyGen’s original 3.5 m×1.2 m Stevpris Mk VI anchor was certified for 420 kN holding capacity in dense sand (φ = 38°, γ' = 9.2 kN/m³) at 2.1 m embedment. Post-deployment ROV surveys show 0.8 m scour, reducing effective embedment to 1.3 m. In-situ vane shear tests indicate undrained shear strength (su) increased to 45 kPa due to consolidation. Calculate revised holding capacity using HCCF.
1.
Step 1: Compute original embedment ratio (D/B) = 2.1 / 1.2 = 1.75 → original HCCF₀ = 1.0 (baseline)
2.
Step 2: Compute revised embedment ratio = 1.3 / 1.2 = 1.08 → apply DNV-RP-F111 Fig. 5.3: HCCF_depth = 0.72
3.
Step 3: Apply su-based correction per ISO 19901-7 Annex B: HCCF_su = (45 / 32)^0.6 ≈ 1.21 (original su assumed 32 kPa)
4.
Step 4: Revised capacity = 420 kN × 0.72 × 1.21 = 365.7 kN
Answer:
The revised holding capacity is 366 kN, representing an 13% reduction from certified value—justifying retrofit with larger anchor or alternative type.
🏗️ Real-World Application
In 2021, MeyGen retrofitted two 1.5 MW Atlantis AR1500 turbines (Units A1 & A2) after 22 months of operation revealed excessive chain wear (>18% diameter loss) at fairlead contact zones and elevated tension variance (+32% RMS vs. design). The solution: (1) replaced 140 m of Grade 4 chain with 120 m of Dyneema® DSB-12 synthetic rope (with custom stainless steel thimble-and-sleeve termination), (2) installed polymer-coated fairleads to reduce abrasion, and (3) re-embedded the original drag anchors 0.5 m deeper using jetting—validated via post-installation pullout testing. Total downtime: 72 hours; cost: £1.4M (38% less than full retrieval). The retrofit extended predicted service life by 4.2 years (per DNV-RP-F204 fatigue model).
🔧 Interactive Calculator
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