Almost everything that goes down to the seabed or comes back up is hooked on by a diver. Underwater rigging is no different from rigging on land in its physics, but it is different in its consequences: you cannot see well, you cannot hear well, the load changes weight as it leaves the water, and whoever is handling it is right underneath it. Understanding angles, anchor points and communication with the crane operator is what separates a routine lift from an accident.
El commercial diving has a side almost nobody associates with diving: stevedoring. Concrete blocks, lengths of pipe, pumps, gates, heavy tools, debris that has to be taken away. All of it goes up and down hanging from a crane, and the person hooking or unhooking it on the seabed is a diver.
The physics of lifting is the same as on land. What changes is everything else: visibility, communication, the apparent weight of things, and the position of the person handling the load, who is underneath it.
First: the gear
Lifting gear is not generic hardware, nor is it part of the personal equipment diver's own kit. Every item has a safe working load value certified by the manufacturer, marked on the piece itself, with its own documentation and inspection history.
Slings can be wire rope, chain or textile. Wire rope stands up well to abrasion and contact with edges; textile slings are light and do not mark the surface of whatever they are wrapped around, but they cut easily against a sharp edge. The choice depends on the load, not on whatever happens to be closest to hand.
Shackles connect items to each other and are the point where the most can go missing: a pin that works loose, a bolt that is missing, a cotter nobody replaced. They are checked before every use, and it is also checked that the pin actually belongs to that shackle and not to a similar-looking one.
The general rule is hard and simple: an item with no legible marking or no valid certificate is an item that does not get used. Underwater, where nobody can inspect the lift in real time, that discipline is the only safeguard left.
Angles and load sharing
The concept that prevents the most accidents, and that is worst understood, is the angle between sling legs.
When a load hangs from two vertical slings, each one carries roughly half the weight. But as soon as the legs spread apart, the tension in each one stops being half and starts to climb. The wider the angle, the higher the tension, and that increase shoots up at very wide angles.
The practical consequence is that a load that looks light can push its slings to their limit simply from being hooked up badly. And that happens easily underwater, where the diver is working right up against the piece and has no perspective to judge the geometry of the whole rig.
The second concept is actual load sharing. Three or four legs do not share the load equally unless the geometry is perfect: in practice, two of the legs usually end up carrying nearly all of it. That is why the calculation is done assuming the worst case, not the ideal one.
The third is the centre of gravity. A piece hooked up off its centre of gravity does not come up straight: it rotates until it finds balance, and that rotation happens suddenly the moment the load lifts clear of the seabed. A diver standing nearby at that moment has a problem.
What is different about water
There are three effects that do not exist on land and that change the numbers.
The first is buoyancy. Submerged, a piece weighs less than it does in air, and how much less depends on the material: steel loses relatively little, concrete a lot more. The result is that a load handled easily on the seabed gets all its weight back the moment it breaks the surface. Leaving the water is, almost always, the most critical moment of the lift.
The second is trapped water. A tank, a capped length of pipe, or any enclosed volume can come up full, and then the actual load is far higher than what was calculated. Before lifting, a check is made for whether the piece can hold water and, if possible, it is vented.
The third is seabed suction. A piece that has sat in mud for a while sticks to it, and the seabed resists letting go. The crane pulls, the tension climbs well above the actual weight of the load, and when the mud finally gives way, the piece shoots upward carrying all that stored energy with it. It is one of the most dangerous scenarios there is, and the only way to handle it is to pull slowly, with the diver well clear and the lift planned in advance.
Communication
The diver does not talk to the crane operator. They talk to the supervisor, who translates and gives the lifting order. That chain exists for a reason: a misunderstanding with a crane running costs dearly, and it takes someone with the full picture to filter and confirm.
The language is deliberately sparse and repetitive. Up slowly, stop, down a hand's width, take up tension, hold. Every order is confirmed before it is carried out, and its execution is confirmed too. The hardwired communication is what makes this work possible, and the moment it fails, the lift stops: nothing gets hoisted with a diver cut off from communications in the water.
Before any movement of the crane, the supervisor confirms the diver is clear and in a safe position. It is never assumed, and it is never inferred from silence: it is asked, and it is answered.
The mistakes that keep repeating
Lifting accidents on underwater jobs come from a short, well-known list.
Being under the load or in its path. It is mistake number one, and the one with the most serious consequences. The response is always the same: get clear before anything moves.
Improvising the hook-up. Using whatever is to hand, bending a sling over a sharp edge, fitting a shackle the wrong way round. All of it is done to save time, and all of it is what fails.
Leaving the umbilical inside the rigging. An umbilical running underneath the load or between the sling legs gets trapped the moment the crane takes up tension. Before the order is given, the umbilical's path is confirmed, and that confirmation comes from the diver.
Not stopping. When something does not add up — the load is not moving the way it should, a strange noise, a sling shifting — the right response is to stop the lift and take another look. The pressure to get it finished is real, and it is exactly what has to be resisted.
Where you learn it
Rigging is part of the syllabus at commercial diving schools, but what you get there is the basics. Real competence is built by repeating lifts alongside people who know what they are doing, and in many markets there are specific rigger qualifications that carry real weight when hiring.
For a young diver it is one of the most worthwhile investments they can make: underwater work is full of lifts, and whoever hooks up well and communicates clearly quickly becomes the person sent down when something matters. It is also one of the natural routes toward the job of supervisor, because it forces you to think about the whole operation and not just your own task. In the job market, that reputation is worth more than any certificate.
Frequently asked questions
What is rigging in commercial diving?
The set of techniques and gear for hooking up, securing and lifting loads: slings, shackles, strops, hooks, and the way they are arranged so the load comes up under control and without damaging anything.
Why does the sling angle matter so much?
Because as the angle between the legs opens up, the tension in each one grows beyond the weight the two of them are sharing. A very wide angle can push each sling close to its limit even when the load looks light.
Does a load weigh the same in and out of the water?
No. Submerged it weighs less because of buoyancy, and once it crosses the surface it gets its real weight back. On top of that, if it is holding water inside it, it comes out heavier than expected. That is why the critical moment of a lift is usually when the load leaves the water.
How does the diver communicate with the crane operator?
Through the supervisor, over hardwired communications in the umbilical. The diver describes, the supervisor translates that into lifting orders, and the crane operator carries them out. Direct orders are never given, and shortcuts are never improvised in that chain.
What is an item's safe working load?
The maximum load value the manufacturer certifies for that item under its intended conditions of use. It is marked on the piece itself and comes with its certificate. An item with no legible marking and no certificate does not get used.
What is the most serious mistake in a lifting operation?
Positioning yourself under the load or in the path it could swing into if something gives way. It is the cause of the most serious accidents, and the only real defence is to get clear and confirm over the radio that you are out before the crane moves anything.
Published 1 September 2026 · By Arthur More, professional saturation diver
Photo: MyFWC Florida Fish and Wildlife · CC BY-ND via Openverse