What is a LARS: how a diving bell is launched and recovered

Published 5 August 2026 · 8 min read · 1,600 words

La closed bell is the capsule that carries the diving team from the pressurised life support system to the work point. The system that moves it has its own name in the industry: LARS, the English initials for launch and recovery system. It's a large, expensive and closely regulated machine, and it deserves its own explanation because almost all of the cycle's safety depends on it.

What makes up a LARS

The exact configuration varies by vessel, but the elements are always the same:

• The winch and main lift wire, which carries the weight of the bell with its occupants, its ballast and its equipment. It's sized with wide safety factors and inspected on a strict schedule.
• The guide wires: wires tensioned between the vessel and a weight on the seabed, or a carriage that clasps the bell as it passes through the wave zone. Their job is to stop the bell swinging about and striking the hull or the structure.
• The guide weight, a dead weight that keeps those guide wires taut and stabilises the whole assembly.
• The bell umbilical, wound on its own winch, which carries gas, hot water, power, communications and video from the vessel to the bell.
• Motion compensation, which absorbs the vessel's pitch and heave so the bell doesn't get jerked about.
• The secondary recovery path, independent of the main one, provided to raise the bell if the primary system is disabled.

Over the side or through the moonpool

There are two arrangements. On vessels with a moonpool, the bell goes down through an open well in the centre of the hull, which is the zone of least motion and also shields it from waves during the transit through the surface. In side-launch systems, the bell is launched through a gantry over the side. The central moonpool is distinctly better in rough seas, which is why dive support vessels built for the North Sea fit it as standard.

Motion compensation

A vessel rises and falls with the swell. A bell hanging from a wire, if nothing corrects for it, does the same thing with a lag and amplified motion. The result is jolts on the wire, on the umbilical and on the divers, and in the worst case an impact against the work structure.

Passive systems use a hydro-pneumatic accumulator that acts as a damping spring. Active systems measure the vessel's motion and command the winch to heave in or pay out in real time to keep the bell still relative to the seabed. Active compensation is more effective and also more complex, and its failure has to be covered in the procedure.

This is one of the points where the vessel's dynamic positioning and the LARS work together: if the vessel holds its position and heading precisely, the load on the launch system drops drastically.

The sequence of a dive

The full cycle, in brief, works like this:

• The divers transfer from the life support system into the bell through the transfer under pressure, without losing pressure at any point.
• The hatches are closed and checked, and the bell's own gas supply and communications are verified.
• The bell disconnects, moves to the launch point and descends along the guide wires to the working depth.
• Internal and external pressure are equalised, the lower hatch is opened and the working diver goes out, with the other crew member as standby diver, ready to go out at any moment.
• At the end of the shift, the diver is recovered, the hatch is closed, recompression is carried out as needed and the bell is hoisted back up to mate with the system.

The fact that the bell never decompresses during the cycle is what allows decompression to happen just once, at the end of the campaign, rather than after every dive.

What happens if the LARS fails

It's the question everyone asks, and the answer explains how the system is designed. The bell doesn't depend on the vessel to keep its occupants alive for an extended period. It carries on board:

• Gas propio in external bottles, independent of the umbilical supply.
• Carbon dioxide scrubbers with endurance for many hours.
• Heating or thermal insulation equipment, because hypothermia is the real enemy in a trapped bell: helium conducts heat far better than air.
• Emergency communications through the water, independent of the umbilical.
• Lastre soltable and, in many designs, the ability to ascend on its own.

Industry guidance sets a minimum of twenty-four hours of self-contained life support in the bell, and hyperbaric rescue procedures are sized for a longer period than that. On top of that, at the next level of contingency, there's the vessel's hyperbaric evacuation unit, designed to move the whole team under pressure if the vessel has to be abandoned.

A LARS failure is, therefore, a serious and anticipated emergency, with a written procedure, a standby diver, secondary recovery and periodic drills. As with almost everything done well in this industry, the response isn't a heroic trick: it's a plan someone already wrote and the team has practised. That's the same spirit reflected in the IMCA guidelines and certified through the industry certifications.

Maintenance and inspection

A LARS is a personnel lifting appliance, and that puts it in the most demanding regulatory category that exists for marine machinery. It isn't governed by the criteria for a cargo crane, but by those for a lift carrying lives, with higher safety factors and shorter inspection intervals.

The usual programme combines checks before every launch — a visual pass over wires, terminations, brakes, controls and the emergency system —, periodic inspections with non-destructive testing of load-bearing elements, wire replacement based on hours or condition, documented load tests and functional verification of the secondary recovery path. This last point is critical: an emergency system that's never tested isn't an emergency system, it's an assumption.

Operating limits

Every LARS has a defined operating window: significant wave height, period, wind speed and, on vessels with dynamic positioning, stability criteria for the positioning system. When the forecast goes outside that window, the bell isn't launched, and if it's already out, it's recovered.

That decision is usually the most common point of friction between the operation and the client, because every hour a vessel sits idle costs money. It's also the point where the chain of command shows whether it's properly built: the operating limits belong to the system, not to anyone's mood, and they aren't negotiated on the fly.

Why the LARS shapes the design of the whole vessel

The launch system isn't an accessory bolted onto an existing ship. Its weight, its working height and the loads it transmits to the hull determine the vessel's general layout: the position of the central moonpool, the reinforcing structure under the gantry, the location of the saturation system to minimise the length of the mating connection, and the deck space left available for construction work. When you compare one dive support vessel with another, a large part of the visible differences in the general arrangement come from decisions made around this machine.

Frequently asked questions

Published 24 August 2026 · By Arthur More, professional saturation diver

More from the blog

See the whole blog →