Here's a complete, self-contained HTML page covering the modular helical mooring system for both scales — parts lists with China ex-works cost estimates, and step-by-step install/remove procedures with timing. ```html Modular Helical Tension-Leg Mooring System — Seastead

Modular Helical Tension-Leg Mooring System
Prototype (½-scale seastead) & Full-Scale Design, Costs, and Procedures

All costs are ex-works China, USD, ±30% (fabricated stainless, small-batch). Sea freight LCL adds roughly $800 (prototype kit) / $2,000 (full-scale kit), plus duties. Soil assumed to be medium dense sand (typical protected Caribbean anchorages). Capacity rule of thumb used throughout: ultimate pull-out capacity ≈ 10 × final installation torque (ft-lb) for helical anchors in sand.

1. System Concept

Permanent anchor string (per screw): a stainless steel lead section with helix plate + bolt-on extensions, installed so ~7 ft (prototype) / ~12 ft (full scale) embeds into the seabed and the top padeye sits ~1 ft above the seabed, just below the surface. The tension leg line shackles to this padeye.
Temporary drive extensions + drive adapter: pinned hex-socket sections stacked on top of the permanent string during installation so the screw top reaches ~1.5–2 ft above the waterline. Removed after each screw is set and reused on the next screw.
Floating torque drive ("drive skiff"): a small two-pontoon catamaran frame carrying an electric motor/gearbox drive head. Crowd (downforce) comes from the frame's buoyancy plus water ballast. Torque reaction is taken by two lines to the seastead's leg cleats — the seastead's GPS/DP thruster system holds heading against this (≈330 lb line load prototype, ≈1,000 lb full scale — well within thruster authority).
One shared drive skiff moves corner-to-corner. Only the drive adapter is "consumed" per screw in the sense of being re-engaged; everything else is permanent or shared.
The drive controller logs motor torque, giving a free in-situ capacity test of every screw at every installation.

Why this stays cheap: no hydraulic power pack, no dedicated barge, no through-hull gear, no coated-steel corrosion schedule. The only powered item is one battery-electric drive skiff; everything else is dumb stainless hardware. The whole kit (except full-scale anchor strings) fits in a fraction of one container corner.

2. Prototype System (½-scale seastead)

Design point: 1,000 lb working load per screw (≈2,500 lb ultimate at FS ≈ 2.5), 8 ft water, 7 ft embedment, 6 in dia. single helix. String sections: 2⅜ in OD × ¼ in wall 316L tube (shaft torque rating ≈ 1,600 ft-lb, comfortably above the ~250–600 ft-lb expected installation torque, with margin for dense sand layers).

2.1 Parts & Costs (per 3-screw set)

#	Item	Spec	Qty	Unit $	Ext $	Notes
A1	Lead section	5 ft, 2⅜"×¼" 316L tube, cast point, 6"×⅜" helix (2" pitch), female socket top	3	190	570	One per corner, permanent
A2	Extension section	3 ft, same tube, male hex stub / female socket	3	95	285	Makes 8 ft permanent string
A3	Anchor head	Welded padeye + 5/8" shackle + retrieval eye	3	45	135	Top of permanent string
A4	Coupler pins & hardware	5/8" 316 cross-pins w/ lanyards, anti-galling paste	set	150	150	Incl. spares
Permanent anchors subtotal					1,140	≈ $380/screw
B1	Temp extension	5 ft, same tube	1	120	120	Shared, reused every screw
B2	Temp extension	3 ft	1	95	95	Shared
B3	Drive adapter	2 ft, 2" hex top + thrust collar	1	190	190	Engages drive head
Temporary install kit subtotal					405	Covers ~6.5–9.5 ft water; add $95–120 per extra section for deeper
C1	Motor + controller	7.5 kW cont / 15 kW peak BLDC-PMSM, 48 V, VFD w/ torque limit & logging	1	650	650
C2	Gearbox	2-speed 30:1 / 90:1 → 100 / 33 RPM; 2,100 ft-lb peak	1	950	950	High gear = fast penetration; low gear = high torque
C3	Drive socket + thrust bearing	2" hex socket, thrust collar ride-on	1	220	220
C4	Pontoons	2 × HDPE 18" dia × 12 ft (≈2,600 lb buoyancy total)	2	190	380
C5	Frame + ballast	316/6061 welded, 2 × 20 gal flood ballast tanks, valves	1	700	700	Net crowd ≈ 2,200 lb
C6	Battery	48 V 100 Ah LiFePO₄ + BMS (~5 kWh)	1	800	800	Recharges from seastead inverter between screws
C7	Wireless pendant + display	Torque readout, auto-stop at target	1	180	180
C8	Charger	48 V 20 A	1	150	150
C9	Misc (cabling, fasteners, anodes)		1	250	250
Drive skiff subtotal					4,280	≈ 500 lb, heaviest single lift ≈ 150 lb
D1	Torque reaction lines	2 × 50 ft ½" nylon + shackles	set	90	90	To leg cleats
D2	Pin-pull pole tool + lanyards	For snorkel-depth pin removal	1	80	80
D3	Marker buoys + retrieval line		set	60	60
D4	Tension leg lines (opt.)	3 × 25 ft ½" HMPE, thimbled	3	50	150	Or use line you already carry
D5	Anti-seize, spares		set	100	100
Accessories subtotal					480
PROTOTYPE KIT TOTAL (ex-works)					≈ 6,300	+ ~$800 freight

2.2 String Makeup vs. Water Depth (prototype, 7 ft embedment)

Water depth	Permanent string	Temp stack on top	Adapter top height
6 ft	lead 5 ft + ext 3 ft (top 5 ft down)	5 ft ext + adapter	≈ 2 ft above WL
8 ft	lead 5 ft + ext 3 ft (top 7 ft down)	5 ft + 3 ft exts + adapter	≈ 3 ft above WL
10 ft	add one more 3 ft ext to permanent	5 ft + 3 ft exts + adapter	≈ 2 ft above WL

3. Full-Scale System

Design point: 5,000–8,000 lb working load per screw (≈17,000 lb ultimate at FS ≈ 2.5), 16 ft water, 12 ft embedment, 12 in dia. helix. String sections: 4½ in OD × ⅜ in wall 316L (shaft torque rating ≈ 9,000 ft-lb). Optional upgrade: 2205 duplex (+≈$600/set) for higher strength and better crevice-corrosion resistance on long stays in warm water.

3.1 Parts & Costs (per 3-screw set)

#	Item	Spec	Qty	Unit $	Ext $	Notes
A1	Lead section	6 ft, 4½"×⅜" 316L tube, point, 12"×⅝" helix (4" pitch), socket top	3	600	1,800	≈140 lb each
A2	Extensions	3 × 6 ft + 1 × 3.5 ft per string (mix to suit depth)	4/string	350 avg	1,400	Makes ~27.5 ft at 16 ft depth
A3	Anchor head	Padeye + ¾" shackle + retrieval eye	3	70	210
A4	Coupler pins & hardware	¾" pins, lanyards, anti-galling paste	set	250	250	Incl. spares
Permanent anchors subtotal					3,660	≈ $1,220/screw
B1	Temp extensions	3 × 6 ft (316L)	3	260	780	Shared; covers 16 ft water
B2	Drive adapter	2.5 ft, 3" hex top + thrust collar	1	300	300
Temporary install kit subtotal					1,080	Add ~$260 per 6 ft section for deeper water
C1	Motor + controller	15 kW cont / 30 kW peak PMSM, 96 V, torque limit & logging	1	1,400	1,400
C2	Gearbox	2-speed 25:1 / 75:1 → 96 / 32 RPM; 8,000 ft-lb peak	1	3,200	3,200	~$2,200 if capped at 5,000 ft-lb
C3	Drive socket + thrust bearing	3" hex socket	1	450	450
C4	Pontoons	2 × HDPE 30" × 14 ft (≈8,600 lb buoyancy)	2	500	1,000
C5	Frame + ballast	Welded frame, 200 gal flood ballast, valves	1	1,800	1,800	Net crowd ≈ 8,500 lb
C6	Battery	51.2 V 200 Ah LiFePO₄ (~10 kWh)	1	1,500	1,500
C7	Pendant + controls		1	300	300
C8	Charger		1	250	250
C9	Misc		1	400	400
Drive skiff subtotal					10,300	Assembled ≈ 2,500 lb; launch pontoons separately, assemble afloat
D1	Torque reaction lines	2 × 60 ft ⅝" nylon + shackles	set	150	150
D2	Pin tools, lanyards		set	120	120	Diver (16 ft — easy recreational depth)
D3	Buoys + retrieval line		set	80	80
D4	Tension leg lines (opt.)	3 × 30 ft ¾" HMPE	3	130	390
D5	Anti-seize, spares		set	150	150
Accessories subtotal					890
FULL-SCALE KIT TOTAL (ex-works)					≈ 15,900	+ ~$2,000 freight

Optional handling gear: prototype kit is manageable with 2 people plus the dinghy davit. Full scale: a small knuckle-boom crane (1,000 kg) on the walkway corner, ≈ $3,500–6,000, or a portable aluminum gantry ≈ $800. Recommended if relocating often.

4. Installation Procedure

Crew: 2 on the seastead/deck + 1 snorkel diver (prototype) or 1 recreational diver (full scale). The seastead holds station on GPS/thrusters throughout; torque reaction lines keep the skiff aligned with the leg.

Step	Prototype (per screw)	Min	Full scale (per screw)	Min
1	Pre-assemble 8 ft permanent string on deck; shackle tension line to padeye, coil line	10*	Stage string sections on deck; shackle tension line	10*
2	DP on station at corner; hold heading	4	Same	5
3	Launch drive skiff; connect 2 torque lines to leg cleats	9	Launch pontoons + frame (crane/davit); assemble afloat; connect torque lines	14
4	Stand string on bottom; stack temp exts + adapter; pin each joint	11	Lower lead section; pin on extensions one at a time to full 27.5 ft; add temp exts + adapter	17
5	Engage drive; high gear (100 RPM) until torque rises, then low gear to target ≈ 500 ft-lb (auto-stop)	5	High gear until load, low gear to 4,000–6,000 ft-lb target	10
6	Hold 60 s at target torque; log value (capacity QA)	2	Same	3
7	Diver pulls 2 coupler pins; hoist temp exts + adapter onto skiff	9	Diver pulls 3–4 pins; hoist temp stack	14
8	Detorque lines; skiff to next corner	6	Same	8
9	Winch tension line to ≈ 400 lb pretension	3	Pretension ≈ 2,000–3,000 lb per leg (pulls seastead down ~3 ft)	6
Total per screw (first attempts)		≈ 55		≈ 75
3 screws, practiced (incl. setup & final tensioning)		1.5–2 h		2.5–3.5 h

* Steps 1–2 can be done before arriving on station, so they don't add to on-station time.

5. Removal Procedure

Step	Prototype (per screw)	Min	Full scale (per screw)	Min
1	Slack tension line; DP over screw (find via marker buoy / tension line)	4	Same	5
2	Launch skiff; torque lines on	8	Same	12
3	Diver pins temp exts + adapter onto anchor head (led to it by the tension line)	9	Same, 3–4 joints	14
4	Hang string from drive thrust collar (skiff buoyancy = uplift); reverse-rotate in low gear — extraction torque typically 60–80% of install	7	Same	12
5	Hoist string, wash with freshwater, stow on deck	10	Hoist section by section	15
6	Skiff to next corner	5	Same	7
Per screw		≈ 40		≈ 60
3 screws, practiced		≈ 2 h		≈ 3 h

6. Operational Notes

Relocation frequency: for weekly/daily moves, set 3 conventional anchors (e.g., 3 × 15–35 lb claw anchors + rope/chain, ~$700–1,000, 15–20 min to set) and save the tension-leg system for stays of weeks to months. The helix kit then pays for itself in holding precision (near-zero excursion) rather than speed.
Pair-per-corner option: your layout sketch shows a pair of screws per corner. That doubles anchors (+$1,140 prototype / +$3,660 full scale) but shares the same drive skiff and temp kit, and gives redundancy and easier per-screw loads.
Torque QA: log final torque every install. If a screw hits target torque early (shallow refusal), capacity is still ≈ 10 × torque — you may accept it or re-site a few feet away. If torque spikes above 80% of drive peak: back off 1 ft, re-attempt; if it repeats, move the hole (boulder/coral).
Corrosion: bare 316L is fine with rinse-downs every few months. For stays > 6 months or always-warm water, step up to 2205 duplex (+≈10–15% on anchor cost). Never substitute 304. Anti-galling paste on every pin and thread, every cycle.
Deeper water later: the system is modular — each extra 6 ft extension section extends depth range for ~$120 (prototype) / ~$260 (full scale). The practical ceiling is diver-friendly depth (~30–50 ft) for pin work.
Speed tuning: the 2-speed gearbox is your "finish faster" feature — high gear penetrates soft sand at up to ~8–10 ft/min (prototype); low gear is only for the last few feet and dense layers. Typical pure spin time for 7 ft at 2" pitch is under a minute at 100 RPM; soil and pin work dominate the schedule, not the motor.

Engineering disclaimer: torque/capacity figures use the standard helical-anchor rule of thumb for sand and should be confirmed with a pull test on the first prototype installation (the drive skiff's thrust collar + a load cell on the tension line makes this a 30-minute test). Confirm shaft torque ratings against your final section wall thicknesses, and have the seastead leg cleats and DP authority checked against the torque line loads shown.

Prepared for the seastead project — modular tension-leg mooring, prototype & full scale. Cost basis: small-batch fabrication, South China, ex-works, ±30%.

``` A few key design decisions worth flagging: 1. **Crowd force from buoyancy + water ballast** — this is the trick that keeps the system cheap. The drive skiff's flooded ballast tanks give ~2,200 lb (prototype) / ~8,500 lb (full scale) of downforce with no hydraulics or winches. 2. **Torque reaction via two lines to the leg cleats**, as you suggested — the loads are modest (~330 / ~1,000 lb) and well within your thruster authority while DP holds heading. 3. **Pinned hex-socket couplers** so a snorkel diver can pull one pin per joint to strip the temporary extensions — no underwater torque tools needed. The tension line leads the diver right to the anchor head during removal. 4. **The 2-speed gearbox** is what buys you the "finish faster" RPM you wanted: high gear spins at ~100 RPM (prototype) in soft sand, low gear provides peak torque only for the final feet. 5. **One arithmetic note:** since the drive is above water, the temporary stack must span the water column, so the temp kit scales with depth (~$120 per extra 3 ft section prototype, ~$260 per 6 ft section full scale). The tables include a depth-vs-makeup guide.

Modular Helical Tension-Leg Mooring System Prototype (½-scale seastead) & Full-Scale Design, Costs, and Procedures