Can a screw anchor replace a steel anchor on a fish farm?

Yes, sized from your site’s wave, current and grid loads, wherever the seabed can take one. It goes in at the coordinates you give and gets a torque record like every anchor we set. On rock, it is not the right choice.

Can screw anchors handle the extreme wave and current loads of exposed offshore sites?

Yes, when they are sized for the site. Each anchor is specified so design storm loads stay well below its ultimate capacity, the range in which screw anchors recover from cyclic wave loading, and its installation torque gives that capacity anchor by anchor. Send us your wave and current data and we will size them.

How do screw anchors compare with concrete blocks on a mussel farm?

A block holds by its weight and friction on the bottom, and loses about 43% of its weight in seawater. A screw anchor holds by plates bearing on the soil at depth, on a much shorter scope, so more growing line fits in the lease. A block can go on rock; a screw anchor needs a seabed it can be screwed into. The table above compares them row by row.

Will installation disturb my existing farm infrastructure?

Very little. The rig works from a work vessel at near-zero vibration, with no crane barge, no chain dragged across the bottom and no excavation, beside the lines already in the water.

Aquaculture moorings

Screw Anchor Moorings for Fish and Mussel Farms

Fish-cage grids are usually moored on steel anchors dragged in on chain, and mussel long-lines on concrete blocks. A screw anchor, also called a helical anchor, is screwed into the seabed instead and takes load the same day.

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The same cage grid, moored two ways

Steel anchor and chainLegs 147 m · 22.1 ha

Screw anchorLegs 52 m · 7.7 ha

Sea area inside the anchors, in this example65% less

See how it is drawn

Moored on steel anchors

Where the pull meets the seabed

After every storm, someone on a steel-anchored farm takes the boat out to check whether an anchor has shifted or a line has gone slack. Here is why an anchor can shift.

A steel anchor holds by digging in as it is dragged, so the pull has to reach it almost flat along the bottom. That is what the heavy ground chain is for. Lift the pull and the anchor can break out. A screw anchor is already set at depth, so the line can run straight to it at an angle, and upward pull is taken by the soil above its plates.

Illustrative, not to scale. Your site survey and grid loads set the real layout.

How this is drawn

One mooring leg from each system to the same surface float, at the same water depth. Line angles and chain length are drawn for clarity, not taken from a design. A real cage grid has several legs at every corner.

Steel anchor and ground chainPull reaches the anchor flat along the bottom

Screw anchorLine runs straight to the anchor

Same cages. Less sea.

Each mooring leg runs out past the grid to its anchor. On a steel anchor and its chain the leg runs at about 5:1; on a screw anchor, at about 2:1, so the anchor sits about a third as far out. Around a whole grid, it adds up.

Sea area inside the anchors, same grid, in this example65% less

Illustrative geometry, not a farm design. Your site survey and grid loads set the real layout.

How this is drawn

A 2 × 4 grid of 60 m cells at 30 m depth, with one leg from each node on its edge and two from each corner. Leg reach is √(scope² − 1) × depth, at 5:1 for a steel anchor on chain and 2:1 for a screw anchor. The sea area is the rectangle the anchors sit in; the cages show the grid and are not to size. A real grid’s legs, bridles and anchors are sized to its loads.

Steel anchor and chainLegs 147 m · 22.1 ha

Screw anchorLegs 52 m · 7.7 ha

CageMooring legSteel anchorScrew anchorSea area inside the anchors

On a cage grid

Steel anchors vs screw anchors

Steel anchor (500–5,000 kg) compared with a screw anchor
Aspect	Steel anchor (500–5,000 kg)	Screw anchor
When the load direction swings	Set facing one way. If the pull swings round, it can break out and drag before it re-sets.	Holds the same whichever way the line pulls. There is no fluke to turn.
Where it ends up	Wherever it sets after dragging in, so its final position varies.	At the coordinates you give us.
Getting it there	0.5 to 5 tonnes of steel per anchor, plus heavy chain for every leg.	Compact steel parts, turned in from a work boat sized to the job. It takes load on the day.
Capacity known at install	Only if it is tensioned to a test load after it sets.	Installation torque is recorded for every anchor and converted to a calculated capacity. A pull test confirms it where the project calls for it.
Where each one fits	Standard equipment that any anchor-handling vessel can lay, recover and re-lay.	Sand, silt, mud and mixed seabeds, not rock. It needs specialist installation equipment.

Moored on concrete blocks

Concrete blocks vs screw anchors

Long-line mussel and shellfish farms are usually moored on concrete blocks. A block can go on rock, where a screw anchor cannot; on any other seabed, here is how the two compare.

Concrete block compared with a screw anchor
Aspect	Concrete block	Screw anchor
How it holds	By its weight in water and its friction on the seabed.	It is screwed to a set depth, where its helical plates bear against the soil.
Holding for its weight	Low. It loses about 43% of its weight in seawater.	Its own weight plays no part in holding.
In a storm	Can slide or shift once the pull exceeds its friction on the bottom.	Holds as a fixed point. There is nothing resting on the bottom to drag.
Scope (line length ÷ depth) and ground chain	About 5:1, with chain lying on the bottom.	About 2:1, with no ground chain.

Same lease. Longer lines.

Mooring legs run out past both ends of every line, and every meter of leg is lease that grows nothing. A block needs a long, low leg to hold; a screw anchor holds on a steeper one, so the lines reach further.

Growing line per row, same lease, in this example+62%

Illustrative geometry, not a farm design. Your site survey sets the real scope.

How this is drawn

A 400 × 120 m lease at 20 m depth, end moorings only. Leg reach is √(scope² − 1) × depth, at 5:1 for block and chain and 2:1 for a screw anchor. A real layout also needs side moorings.

Block and chainLegs 98 m each end · line 204 m

Screw anchorLegs 35 m each end · line 331 m

Concrete blockScrew anchorMooring legLong-line with floats

Eight years on a mussel-farm seabed

Two of our anchors at the Misya Seafood mussel farm off Koyun Island, Türkiye. The first went in at 15 m in 2014 and was photographed in 2022, its drive head and mooring eye grown over. The second was set at 35 m in 2016.

Screw anchor on the seabed at the Misya Seafood mussel farm off Koyun Island, its drive head and mooring eye colonised by marine growth eight years after installation. — Misya Seafood mussel farm · Koyun Island, BalıkesirInstalled 2014 · Photographed 2022 · 15 m deep

Screw anchor installed on the seabed at the Misya Seafood mussel farm off Koyun Island, showing the drive head used to turn it and the revolving eye where the mooring rope is tied. — Misya Seafood Mussel Farm · Koyun Island, BalıkesirInstalled 2016 · 35 m deep · 4 m anchor · 600 mm plate

Specified for your farm

What farm operators ask before they commit

Plan your next farm mooring

Send your farm coordinates, water depths and deployment window. An engineer replies within 24 hours with questions and a first read; the full proposal follows within two business days.

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