What are Hydrostatic Release Units?

What are Hydrostatic Release Units?

Lifeboats and liferafts are essential for saving lives in emergencies. This article explores a fascinating aspect of the subject: Hydrostatic Release Units (HRUs).

As we know, lifeboats and liferafts differ in terms of availability, characteristics, and their use in saving lives at sea. Lifeboats are usually large and can accommodate a large number of passengers in the event of a ship sinking. Liferafts, on the other hand, are smaller, mostly inflatable, and portable, and can accommodate fewer passengers.

One of the primary advantages of liferafts is their ease and rapid release in emergencies. This makes them a reliable means of lifesaving, as required by the International Convention for the Safety of Life at Sea ( SOLAS ) for any seagoing vessel.

The speed with which liferafts can be released on board makes them an essential part of lifesaving appliances (LSAs), which are indispensable equipment for ships. Currently, all types of lifesaving appliances must be released with minimal human input, i.e., to minimize human intervention in times of crisis.

This means that this important equipment must be immediately ready and available when the situation requires it.

For practical purposes, automation has been widely used to meet this need. A mechanism must be provided to enable lifesaving equipment to be activated quickly in situations where manual intervention is tedious and time-consuming.

The launch mechanism is the most critical part of the lifesaving equipment, ensuring that they are safely launched when needed. Life rafts rely on a device called a hydrostatic launch unit ( HRU ).

Parts of an HRU

The HRU is a safe launch unit that automatically launches a life raft into the water in an emergency, such as a sinking ship. The device is informally known as a "hydrostatic lifesaver".

The principle of the HRU is very simple. It is similar to a hydrostatic switch, which activates the release mechanism that retracts the life raft, suspending it so that it can be freely thrown into the water and save lives.

How does a hydrostatic release unit work?

If the ship sinks, the increasing water pressure activates the hydrostatic release unit switch. The hydrostatic release device is connected to the life raft's stowage device, disconnects the lanyard or carabiner, and releases the life raft into the water.

In simple terms, the release device of a life raft is similar to the oxygen mask on an airplane: a drop in cabin pressure causes the mask to drop over the seat, allowing the passengers or crew to breathe.

The trigger is air pressure, specifically hydrostatic or water pressure.

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The hydrostatic release device is typically located at a specific position on the hull, usually within a sealed enclosure. When the ship begins to sink, rapid changes in draft or waterline (which manifest as changes in hydrostatic pressure) trigger the release device.

Now the question is: at what depth of water does such a device work?

The design and ease of use vary from ship to ship. Still, for practical purposes, hydrostatic lifts are designed to operate at draft changes ranging from 1.5 meters to 5 meters below the ship's maximum design draft.

The hydrostatic lift is well protected within its enclosure and will remain intact even in extreme situations such as fire.

The sequence of operation of the hydrostatic lift can be briefly explained as follows:

Life raft rumble and activation unit (HRU)

When the ship sinks or submerges beyond a certain depth, the life raft's umbilical activation switch is designed to detect the change in hydrostatic pressure caused by water in contact with its membrane. This activates the umbilical activation unit (HRU).

This pressure is transmitted mechanically (and sometimes electronically) to the life raft storage unit, where the rigging, safety belts, and lanyards are disconnected.

Life rafts equipped with HRU.

Life rafts are usually launched after automatically inflating in the inflatable raft compartment.

This device saves time, effort, and crew deployment. In these cases, the crew usually rescues and assists other people on board, usually those who are missing or incapacitated.

HRU designs must meet specific mandatory standards and are usually tested as part of the safety checklist before sailing. In practice, ships must replace them in a maximum of 4 to 5 years.