Section 3 Emergency release system requirements

3.1.1 The in-board end of the towline is to be attached to the winch drum with a weak link or similar arrangement that is designed to release the towline at low load.

3.1.2 All towing winches are to be fitted with an emergency release system.

3.1.3 Where required for testing the winch, adequately sized strong points are to be provided on deck.

3.2.1 The emergency release system is to operate across the full range of towline loads, fleet angles and ship heel angles under all normal and reasonably foreseeable abnormal conditions (including, but not limited to vessel electrical failure, variable towline load (for example due to high strain rate loadings i.e. ‘snap loads’), etc.).

3.2.2 The emergency release system shall be capable of operating with towline loads up to at least 100 per cent of the maximum design load.

3.2.3 The emergency release system is to function as quickly as is reasonably practicable and within a maximum of three seconds after activation.

3.2.4 The emergency release system is to allow the winch drum to rotate and the towline to pay out in a controlled manner such that, when the emergency release system is activated, there is sufficient resistance to rotation to avoid uncontrolled unwinding of the towline from the drum. Spinning (free, uncontrolled rotation) of the winch drum is to be avoided, as this could cause the towline to get jammed and disable the release function of the winch.

3.2.5 Once the emergency release is activated, the towline load required to rotate the winch drum is to be no greater than:

1. the lesser of five tonnes or five per cent of the maximum design load when two layers of towline are on the drum; or
2. 15 per cent of the maximum design load where it is demonstrated that this resistance to rotation does not exceed 25 per cent of the force that will result in listing sufficient for the immersion of the lowest unprotected opening. Unprotected openings include openings fitted with closing appliances that must remain open for operational reasons.

3.2.6 Emergency release of the towline is to be possible in the event of a blackout. For this purpose, where additional sources of energy are required, such sources are to comply with Pt 7, Ch 16, 3.2 Emergency release system operation 3.2.7.

3.2.7 The sources of energy required by Pt 7, Ch 16, 3.2 Emergency release system operation 3.2.6 are to be sufficient to achieve the most onerous of the following conditions (as applicable):

1. Sufficient for at least three attempts to release the towline (i.e. three activations of the emergency release system). Where the system provides energy for more than one winch, it is to be sufficient for three activations of the most demanding winch connected to it.
2. Where the winch design is such that the drum release mechanism requires continuous application of power (e.g. where the brake is applied by spring tension and released using hydraulic or pneumatic power), sufficient power is to be provided to operate the emergency release system (e.g. hold the brake open and allow release of the towline) in the event of a blackout situation for a minimum of five minutes. This may be reduced to the time required for the full length of the towline to feed off the winch drum at the load specified in Pt 7, Ch 16, 3.2 Emergency release system operation 3.2.5 if this is less than five minutes.

3.3.1 Emergency release operation must be possible from the bridge and from the winch control station on deck. The winch control station on deck is to be in a safe location. A position in close proximity to the winch is not regarded as a ‘safe location’, unless it is documented that the position is, at least, protected against towline break or winch failure.

3.3.2 The emergency release control is to be located close to an emergency stop button for winch operation, if provided, and shall be clearly identifiable, clearly visible, easily accessible and positioned to allow safe operability.

3.3.3 The emergency release function is to take priority over any emergency stop function. Activation of the winch emergency stop from any location is not to inhibit operation of the emergency release system from any location.

3.3.4 Emergency release system control buttons are to require positive action to cancel. The positive action may be made at a different control position from the one where the emergency release was activated. It must always be possible to cancel the emergency release from the bridge regardless of the activation location and without manual intervention on the working deck.

3.3.5 Controls for emergency use are to be protected against accidental activation.

3.3.6 Indications are to be provided on the bridge for all power supply busbar voltage and frequency and/or hydraulic/pneumatic pressure levels related to the normal operation of the emergency release system. Alarms are to activate automatically if any level falls outside of the limits within which the emergency release system is fully operational.

3.3.7 Wherever practicable, control of the emergency release system is to be provided by a hard-wired system, fully independent of programmable electronic systems.

3.3.8 Computer-based systems that operate or may affect the control of emergency release systems are to meet the requirements of Pt 6, Ch 1, 2.13 Programmable electronic systems - Additional requirements for essential services and safety critical systems.