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Mxx Towing winch emergency release systems
(2017)
(cont)
1. Scope
1.1 This UR defines minimum safety standards for towing winch emergency release
systems provided on winches that are used on towing ships within close quarters, ports or
terminals.
1.2 This UR is not intended to cover towing winches used on ships used solely for long
distance ocean towage, anchor handling or similar offshore activities.
1.3 For the purposes of this UR, ‘emergency release system’ refers to the mechanism and
associated control arrangements that are used to release the load on the towline in a
controlled manner under both normal and dead-ship conditions.
Definitions
‘Girting’ means the capsize of a tug when in the act of towage as a result of the towline force
acting transversely to the tug (in beam direction) as a consequence of an unexpected event
(could be loss of propulsion/steering or otherwise), whereby a couple of transverse forces is
generated (towline force is opposed by thrust or hull resistance force), causing the tug to heel
and ultimately to capsize. This may also be referred to as ‘girthing’, ‘girding’ or ‘tripping’ and is
particularly relevant to conventional single screw tugs. Tractor and ASD (Azimuthing stern
drive) tugs are less likely to girt because their tow is self-aligning and the tug master is able to
produce significant thrust in all directions. It is clearly understood that towing from a point
near amidships on a conventional tug is inherently unstable and can result in situations where
the load on the towline can heel the tug over to a large and dangerous angle.
Note:
1. This UR is to be uniformly implemented by IACS Societies for vessels contracted for
construction on or after 1 July 2018.
2. The “contracted for construction” date means the date on which the contract to build the
vessel is signed between the prospective owner and the shipbuilder. For further details
regarding the date of “contract for construction”, refer to IACS Procedural Requirement
(PR) No. 29.
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(cont)
Figure 1: Forces during towing in vertical plane
‘Brake holding force’ is the maximum force that can be exerted on the winch before the brake
slips.
2. General requirements
2.1 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.
2.2 Means are to be provided to prevent the fleet angle from becoming great enough to
inhibit operation of the emergency release system.
2.3 All towing winches are to be fitted with an emergency release system.
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3. Emergency release system requirements
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(cont) 3.1 Functional requirements
3.1.1 The emergency release system shall operate under all normal and reasonably
foreseeable abnormal conditions (these may include, but are not limited to, the following:
vessel electrical failure, extreme list/trim angles, load applied at the limits of operating load,
fleet angle, variable load (for example due to heavy weather), etc.).
3.1.2 The emergency release system is to allow the winch drum to rotate and allow the
towline to pay out in a controlled manner.
3.1.3 An alternative source of energy shall be provided such that normal operation of the
emergency release system can be sustained under dead-ship conditions.
3.2 Performance requirements
3.2.1 The emergency release system is to function as quickly as is reasonably practicable
and within a maximum of three seconds after activation.
3.2.2 Arrangements shall ensure that when emergency release system is activated, there
is sufficient resistance to rotation to avoid uncontrolled unwinding of the towline from the
drum.
3.2.3 The towline load required to rotate the winch drum is to be no greater than:
(a) the lesser of five tonnes or five per cent of the maximum brake holding force when two
layers of towline are on the drum, or
(b) 15% of the brake holding strength where it is demonstrated that the resistance to
rotation does not exceed 25% of the force that will result in listing sufficient for the
immersion of the lowest unprotected opening.
3.2.4 The emergency release system shall be capable of operating at 100 per cent of the
brake holding strength.
3.2.5 The capacity of the alternative source of energy required by 3.1.3 is to be sufficient
for at least three complete emergency release system operations of the most demanding
winch connected to it.
3.2.6 Where the winch design is such that 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*, in a dead-ship 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 minimum load as specified in 3.2.3 if this is less than five minutes.
* NOTE: After the emergency release system has been activated it is considered good
practise for the brake to first completely open and then automatically tighten
slightly to ensure a controlled release of the towline.
3.3 Design requirements
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 shall be in a safe location.
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3.3.2 The emergency release control is to be located in close proximity to the emergency
Mxx stop button for winch operation and both should be clearly identifiable, clearly visible, easily
(cont) 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 shall not inhibit operation of the
emergency release system from any location.
3.3.4 Emergency release system control buttons are to be of the ‘lock-in’ type or require
positive action to cancel.
3.3.5 Controls for emergency use are to be protected against accidental use.
3.3.6 The following emergency release system alarms and indications are to be provided
on the bridge:
(a) Low fluid pressure in the control system.
(b) Low accumulator / air pressure.
(c) Low battery voltage (separate alarm and indication not required where electrical power
is supplied from the tug’s emergency batteries).
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 Programmable electronic systems that operate or may affect the control of
emergency release systems are to be considered Category III systems, see UR E22.
3.3.9 The emergency release system reset function is to be always available from the
bridge regardless of the activation location and without manual intervention on the working
deck.
3.3.10 Components critical for the safe operation of the emergency release system are to
be identified by the manufacturer.
3.3.11 The method for annual survey of the winch is to be documented and adequately
sized strong points are to be provided on deck if required.
4. Test requirements
4.1 General
4.1.1 All testing defined within Section 4 is to be witnessed by a Classification Society.
4.1.2 For each emergency release system or type thereof, the performance requirements
of Section 3.2 are to be verified either at the manufacturer’s works or as part of the
commissioning of the towing winch when it is installed on board. Where verification solely
through testing is impracticable (e.g. due to health and safety), testing may be combined with
inspection, analysis or demonstration in agreement with the Society.
4.1.3 The performance capabilities of the emergency release system are to be
documented and made available on board the ship on which the winch has been installed.
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4.2 Installation trials
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(cont) 4.2.1 The full functionality of the emergency release system is to be tested as part of the
shipboard commissioning trials to the satisfaction of the surveyor. Testing may be conducted
either during a bollard pull test or by applying the towline load against a strong point on the
deck of the tug that is certified to the appropriate load.
4.2.2 Where the performance of the winch in accordance with Section 3.2 has previously
been verified, the load applied for the installation trials is to be no lower than 30% of the rated
brake holding strength.
End of
Document
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