Five steps from the site read to the anchor taking load. Every anchor gets a torque record and the capacity calculated from it before our crew leaves your site; where your project calls for it, a pull test on selected anchors confirms the estimate.
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The same five steps on every job, from the first site read to the anchor taking load.
Step 1
We read the site: water depth, tidal range, wave and current exposure, and the seabed. Soil data (CPT, SPT or samples, where you have them) sets the helix size and how deep the anchor goes.
Soil data sets the plate size and how deep the anchor goes.
Step 2
From the load and the soil we choose the shaft, the helix plate size and the number of plates. In soft mud or silt, shaft extensions carry the plates past the weak surface layers to firmer soil.
More plates spread the load through more soil in weak ground.
Step 3
A compact hydraulic torque motor turns the anchor in from a work vessel or barge sized to the job. Nothing is dug out, there is no hammer, and vibration is near zero.
Run from the surface, with no divers. Mooring anchors need no crane barge.
Step 4
A digital indicator records torque as the anchor advances, and the final torque is converted to a calculated capacity for that anchor. How the conversion works is set out below.
A torque record for every anchor before the crew leaves.
Step 5
At target torque the anchor can take its design load, so mooring hardware, docks or foundation connections go on while the crew is still on site.
No concrete, no curing: loaded the same day it goes in.
Torque-to-Capacity Correlation
Each anchor’s capacity comes from a torque reading taken as it goes in. A block or a mushroom anchor gives no such reading: its capacity is estimated from its weight and seabed friction.
Step 1
A calibrated digital indicator records torque continuously as the anchor goes in. T is the average over the final three helix diameters of penetration, so it reflects the soil the plates bear on, not the softer layers above.
Step 2
Kt is an empirical factor set mainly by the shaft size, with helix geometry and soil as secondary factors. Published values come from full-scale load tests; we apply them conservatively.
| Round shaft (OD) | Kt | Basis |
|---|---|---|
| 73 mm (2⅞ in) | 30 m⁻¹ | AC358 default |
| 89 mm (3½ in) | 23 m⁻¹ | AC358 default |
| 114 mm (4½ in) | ≈ 18 m⁻¹ | Perko (2009) |
| 141 mm (5.56 in) | ≈ 15 m⁻¹ | Perko (2009) |
Published values: Kt depends mainly on shaft size. The project Kt is set by the engineer of record, and a load test can confirm it.
Step 3
Multiplying T by Kt gives the ultimate axial holding capacity, Qult. It goes into the anchor's record, listed below, with the factor of safety your engineer sets. Where the project calls for it, selected anchors are pull-tested to confirm the torque estimate.
Ready to go into structural engineering packages, permit applications and owner project records.
Waves, tides and storm surge load a mooring over and over. Screw anchors recover from repeated loading when it stays well below their ultimate capacity (Qult), so we size exposed installations that way:
How a screw anchor fails depends on how deep its top plate sits (H) relative to the plate diameter (D). At shallow ratios, below about H/D = 4, failure can run up to the surface as a general shear break, with a sharp drop in resistance.
From about H/D = 4–5 down, depending on the soil, the anchor behaves as a deep foundation: failure stays local around the plates, capacity is predictable, and the torque correlation applies. Deep embedment is the design target wherever the seabed allows it. Where a job uses shallower anchors, as the 2 m anchors at Erdek did, the shallow failure mode governs the design; their capacity was established by pull testing on site.
Very soft mud, organic silt and loose saturated sediment are common in harbour bottoms and estuaries. Three measures reach the target capacity there:
Together they reach the required capacity in most marine soils, including soft harbour mud, as long as firmer soil can be reached at depth.