What is HRU & Weak Link in liferaft, purpose, operation and Requirements as per LSA Code?

HRU - Hydrostatic Release Unit

A Hydrostatic release unit (HRU) is a pressure-activated device that is provided to deploy the life raft automatically when certain conditions are met.

In case the vessel sinks, the HRU will activate and release the life raft at a depth of between 2 to 4 meters approximately but not more than 4 meters.   

Weak Link:

A weak link is one used in the float-free arrangement of life rafts that holds the painter line so that the necessary pull is attained to inflate the life raft by releasing the CO2 Cylinders.

When the tension in the painter line increases furthermore than at a certain limit the weak link breaks and release the life raft from the ship's deck to which it was connected.

Operation of HRU (Hydrostatic Release Unit):

• Life raft along with the container is attached to the ship's deck by means of HRU (i.e) it acts as a bridge between the life raft container and the ship's deck.

• When the vessel sinks and as it goes down further the water pressure increases and hence the pressure exerted on the HRU also increases, thus when the ship sinks below approximately 4 meters the HRU Comes into action.

• HRU consist of a Sharp Knife inside which is in spring tension, when the water pressure exerted on the HRU goes more than the spring force this knife is released and it cuts the strap. This strap is lashed over the container that is carrying the life raft, but still, the Painter line is connected to the weak link.

• Once the strap is cut the Container along with the liferaft is been released from the ship and by buoyancy, it moves up to the surface of the water.

• As the ship further sinks the tension on the painter line causes the life raft to inflate out of the container by releasing the CO2 Cylinder.

• The life raft has been inflated but still, the painter is connected to the weak link. As the ship sinks the tension on the painter further increases and once it attains a certain limit (2.2 + 0.4 KN ) it breaks the weak link and releases the life raft from the ship and now it is free to float in water. 

Types of HRU:

There are two types of Hydrostatic Release Units (HRU):

1. The disposable type (Normally replaced once in every 2-3 years)
2. The type with unlimited life provided HRU needs to be serviced and tested yearly.

Requirements of Weak Link  as per LSA Code:

If a weak link is used in the float-free arrangement, it shall:

• not to be broken by the force required to pull the painter from the life raft container

• if applicable, be of sufficient strength to permit the inflation of the life raft.

• break under a strain of 2.2 + 0.4 KN.

Requirements of Hydrostatic Release Unit:

If a Hydrostatic Release Unit is used in the float-free arrangements, it shall:

• be constructed of compatible materials so as to prevent malfunction of the unit. 

• Galvanizing or other forms of metallic coating on parts of the HRU shall not be accepted.

• automatically release the life raft at a depth of not more than 4 meters.

• have drains to prevent the accumulation of water in the hydrostatic chamber when the unit is in its normal position.

• be so constructed as to prevent release when the seawater washes over the unit.

• be permanently marked on its exterior with its type and serial number.

• be permanently marked on the unit or identification plate securely attached to the unit, with the date of manufacture, type and serial number and whether the unit is suitable for use with a life raft with a capacity of more than 25 persons.

• be such that each part connected to the painter system has a strength of not less than that required for the painter.

• if disposable type, then it should be marked with a means of determining its date of expiry.

Periodic Servicing of Hydrostatic Release Unit:

Hydrostatic Release units other than disposable type, shall be serviced:

• at intervals not exceeding 12 months, provided wherein any case this is impracticable, the Administration may extend this period to 17 months.

• at a servicing station that is competent to service them, maintains proper servicing facilities and uses only properly trained personnel. 

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Reference: 

1. IMO Publications & Documents - International Codes - LSA Code - Chapter IV - Survival Craft.
2. IMO Publications & Documents - International Convention - SOLAS - Chapter III - Regulation 20.
3. IMO Publications & Documents - Resolutions - Maritime Safety Committee - Resolutions.MSC.48(66).