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Modular Helical Mooring System for Seastead
Modular Helical Mooring System — Prototype & Full Scale
This is a modular, above-water-torqued helical screw mooring system for a tension-leg seastead.
A single reusable powered torque head drives one screw at a time via a removable drive-coupling section.
The torque head is detached and moved to the next leg after each screw is installed.
Key design choices:
- 316 stainless steel screws (solid helix + solid shaft) for repeated installation/removal without coating wear.
- Electric hydraulic or direct electric torque head — the seastead has ample solar/battery power.
- Modular drive adapter ("top section") so the motor/gearbox lives topside, not buried.
- Seastead holds position via GPS + thrusters; its mass resists reaction torque.
- Tension-leg tendon attaches to a permanent eye/padeye on the screw top after the drive adapter is removed.
Part 1 — Prototype (½-scale, ⅛ weight)
Target: 1000 lbf working load per screw, 8 ft water depth, 6" helix × 8 ft shaft (≈7 ft embedment into sand).
1) Parts and Costs (Prototype)
| # | Part | Description / Spec | Qty | Unit Cost (USD, China-sourced) | Subtotal |
|---|
| 1 | Helical screw anchor |
316 SS, 1.5" solid shaft, single 6" Ø helix welded at bottom, 3/8" pitch helix plate, 8 ft overall.
Top end: square drive socket (1" internal square) + cross pin hole, plus a welded padeye / shackle eye for the tendon. |
3 | $180 | $540 |
| 2 | Drive-adapter ("top section") |
316 SS, ~3 ft long, 1.5" shaft with 1" square male drive on the bottom (engages screw), keyed/splined top to match torque head. Quick-release shear pin through the bottom coupling. |
1 | $120 | $120 |
| 3 | Electric torque head |
Brushless DC or 48V industrial right-angle gearmotor, ~800–1500 N·m continuous (≈600–1100 ft·lbf), ~20–40 RPM output. Chinese planetary-gear BLDC or scaffold-jack-style motor. Includes handle frame and reaction arm. |
1 | $450 | $450 |
| 4 | Reaction-arm / guide frame |
Aluminum or stainless tube frame that hangs over the seastead rail; keeps the torque head vertical, transfers reaction torque to the seastead structure, includes roller guides for shaft. |
1 | $200 | $200 |
| 5 | Electric winch (install/retrieve assist) |
12V or 48V 2000 lbf winch for lowering/lifting the screw+adapter assembly (screw weighs ~30 lb, but pullout can need 1000+ lb). |
1 | $150 | $150 |
| 6 | Load cell / torque indicator |
Inline shaft torque sensor or motor-current readout with calibration; used to verify installation torque → holding capacity (rule of thumb: holding lbf ≈ 10× install torque ft·lbf for small helices). |
1 | $120 | $120 |
| 7 | Tendons (tension leg lines) |
1/2" Dyneema or SS wire rope with thimble + shackle, ~12 ft working length, adjustable turnbuckle (½" SS) for pre-tensioning. |
3 sets | $80 | $240 |
| 8 | Shackles, pins, misc hardware | 316 SS shackles, R-clips, shear pins, grease. | — | — | $80 |
| 9 | Cables, connectors, controller | Power cable to torque head, trigger switch, soft-start ESC. | 1 | $100 | $100 |
| Prototype total (approx., ex-works China) | ≈ $2,000 |
Add roughly 20–30% for shipping, duties, and spares (extra shear pins, one spare screw) → budget about $2,500–$2,700.
2) Installation Procedure (Prototype)
Assume calm conditions, sandy bottom, 8 ft water. Two people on deck.
| Step | Action | Time |
|---|
| 1 | Hold seastead on station with GPS + thrusters over first mooring point. | 2 min |
| 2 | Mount reaction-arm frame to seastead rail above the chosen screw location. | 5 min (once per deployment) |
| 3 | Attach drive-adapter to top of screw (drop into square socket, insert shear pin). Hang off winch hook. | 2 min |
| 4 | Lower screw+adapter through guide frame using winch until helix touches seabed. | 1 min |
| 5 | Engage torque head onto adapter top. Begin powered rotation. Screw advances ~3/8" per revolution → ~7 ft embedment ≈ 225 revolutions. At 30 RPM ≈ 7.5 min of drive time. | 8–10 min |
| 6 | Stop when target install torque reached (≈100 ft·lbf for 1000 lbf holding, confirm with sensor). | 1 min |
| 7 | Detach torque head. Pull shear pin, lift drive-adapter off screw. Screw padeye now sits just below/at waterline. | 2 min |
| 8 | Connect tendon + turnbuckle between screw padeye and seastead leg attachment point. | 3 min |
| 9 | Move torque head to next of the 3 screw locations; repeat steps 3–8. | per screw |
| 10 | Pre-tension all three tendons evenly using turnbuckles; check load cells if used. | 5 min |
| Total install time, 3 screws | ~45–60 minutes |
Removal Procedure (Prototype)
| Step | Action | Time |
|---|
| 1 | Release tendon turnbuckles, disconnect tendons from screws. | 3 min per screw |
| 2 | Re-attach drive-adapter + torque head to first screw. | 2 min |
| 3 | Reverse rotation. Removal torque typically 1.2–1.5× install torque. Back out ~225 revs → ~8 min. | 8–10 min |
| 4 | Winch screw back aboard, stow. | 2 min |
| 5 | Repeat for remaining screws. | — |
| Total removal time, 3 screws | ~45 minutes |
Part 2 — Full Scale
Target: 5,000–8,000 lbf working load per screw, up to 16 ft water depth, 12" helix × 12 ft shaft (≈10–11 ft embedment).
1) Parts and Costs (Full Scale)
| # | Part | Description / Spec | Qty | Unit Cost (USD, China-sourced) | Subtotal |
|---|
| 1 | Helical screw anchor |
316 SS, 2.5" solid shaft (or 3" schedule-80 SS pipe), single 12" Ø helix, 3" pitch, 12 ft overall. Top: 1.5" square drive socket + cross pin + welded padeye (½" plate, 1" hole) for shackle/tendon. |
3 | $650 | $1,950 |
| 2 | Drive-adapter ("top section") |
316 SS, ~6–8 ft long (spans from deck down to screw top at waterline + margin), 2.5" shaft, 1.5" square male bottom drive, splined/keyed top. Shear pin coupling. |
1 | $400 | $400 |
| 3 | Electric torque head (high torque) |
Industrial electric-hydraulic torque unit OR large BLDC planetary gearmotor. ~6,000–10,000 N·m (≈4,500–7,400 ft·lbf) continuous, ~10–20 RPM output. 48–96V, ~3–5 kW peak. Chinese brands (e.g., anchor-post / utility-pole drive heads) are common. |
1 | $2,800 | $2,800 |
| 4 | Reaction-arm / guide frame |
Steel or aluminum frame integrated with seastead rail, rated to ~8,000 ft·lbf reaction torque. Includes roller guides centering the adapter shaft, quick-pin mounts at each of the 3 screw stations. |
1 | $700 | $700 |
| 5 | Electric winch |
48V 5,000–8,000 lbf winch for deploying/retrieving the ~250 lb screw and for breakout assistance. |
1 | $450 | $450 |
| 6 | Torque / load instrumentation |
Inline rotary torque sensor (up to 10 kN·m), digital readout with log. Confirms install torque → holding capacity. |
1 | $450 | $450 |
| 7 | Tendons (tension-leg lines) |
7/8"–1" Dyneema (SK-78) with SS thimbles, or SS wire rope, length ~22 ft, plus heavy ¾"–1" SS turnbuckle / load-adjuster per leg. |
3 sets | $280 | $840 |
| 8 | Shackles, pins, hardware | SS bow shackles rated 10,000+ lbf, shear pins, grease. | — | — | $250 |
| 9 | Power / control | Heavy cable, contactor, soft-start controller, emergency stop, dead-man switch. | 1 | $300 | $300 |
| Full-scale total (approx., ex-works China) | ≈ $8,100 |
With shipping, duties, spares and one spare screw, budget about $10,000–$11,000.
Sanity check on torque: Rule of thumb — holding capacity (lbf) ≈ 10 × installation torque (ft·lbf) for helical anchors of this size in medium sand.
For 8,000 lbf holding you need ~800 ft·lbf install torque, well within the 4,500+ ft·lbf rating chosen above (the extra capacity is for harder bottoms, shells, or obstructions).
2) Installation Procedure (Full Scale)
| Step | Action | Time |
|---|
| 1 | Hold seastead on station with GPS + thrusters over first mooring point. | 2 min |
| 2 | Pin reaction-arm frame to the first screw station on the seastead rail. | 5 min (first station only) |
| 3 | Use davit/winch to lift a 12 ft, ~250 lb screw from storage rack; lower to vertical through guide frame. | 4 min |
| 4 | Attach drive-adapter (6–8 ft) to screw top, insert shear pin. | 2 min |
| 5 | Lower assembly until helix touches seabed. | 2 min |
| 6 | Engage torque head onto adapter top. Begin powered rotation. Advance ~3" per rev → ~10 ft embedment ≈ 40 revs. At 15 RPM ≈ 3 min under load. | 5 min (incl. start/stop/torque checks) |
| 7 | Stop at target install torque (~800–1000 ft·lbf), log value. | 1 min |
| 8 | Detach torque head; remove drive-adapter. | 3 min |
| 9 | Connect tendon + turnbuckle to screw padeye and seastead attachment. | 4 min |
| 10 | Move torque head / reaction-arm to next screw station; repeat 3–9. | per screw |
| 11 | Even out tendon pretension across all 3 tendons (use thrusters or ballast trim). | 10 min |
| Total install time, 3 screws | ~75–90 minutes |
Removal Procedure (Full Scale)
| Step | Action | Time |
|---|
| 1 | Slacken and disconnect all three tendons. | 10 min total |
| 2 | At first screw: install reaction frame, attach drive-adapter + torque head. | 6 min |
| 3 | Reverse rotation. Breakout torque may be 1.3–1.5× install; soft-start then 40 revs out ≈ 3 min. | 6 min |
| 4 | Winch screw aboard, stow in rack. Move to next station. | 5 min |
| 5 | Repeat for remaining two screws. | — |
| Total removal time, 3 screws | ~60–75 minutes |
Summary Comparison
| Item | Prototype (½ scale) | Full scale |
|---|
| Working load / screw | 1,000 lbf | 5,000–8,000 lbf |
| Water depth | ≤ 8 ft | ≤ 16 ft |
| Screw | 6" Ø helix × 8 ft, 316 SS | 12" Ø helix × 12 ft, 316 SS |
| Install torque target | ~100 ft·lbf | ~800–1000 ft·lbf |
| Torque head rating | ~600 ft·lbf | ~4,500+ ft·lbf |
| System cost (China, ex-works) | ≈ $2,000 | ≈ $8,100 |
| Landed + spares budget | $2,500–$2,700 | $10,000–$11,000 |
| Install time (3 screws) | ~45–60 min | ~75–90 min |
| Removal time (3 screws) | ~45 min | ~60–75 min |
Practical notes for either scale:
- A modular extension shaft (a 3–6 ft SS tube with the same drive couplings at each end) lets you reach deeper water without a longer adapter, while keeping the torque head above water.
- Use a shear pin at the adapter-to-screw joint sized to fail just above the motor's peak torque — protects the drivetrain if the screw hits a rock.
- For very frequent relocations (weekly or daily), conventional Danforth or fortress anchoring will be faster (5–10 min total) — reserve the helical tension-leg setup for stays ≥ a few weeks.
- Grease the drive sockets and shear-pin holes before each install; seawater + 316 SS still benefits from anti-galling lubricant (e.g., marine Tef-Gel).
- Keep a spare screw aboard — if one gets stuck or bent, you want to finish mooring without a shore trip.
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