712
by wear and tear and the line can be reeled paying out
irrespective of the top or bottom of the drums;
probably due to acquisition and maintenance costs
“… few, if any, winch manufacturers offer them as an
alternative to band brakes” [10].
A split-type drum comprises a tension section that
holds one layer of the mooring line and a storage
section that the remainder of the rope is wound onto.
They were designed to cope with two disadvantages
that undivided drums have:
− To prevent the spooling problem encountered with
undivided drum winches due to lines biting into
each one when under load (undivided drums have
several layers on one drum, whereas split drums
have one layer on the tension section), and;
− As on undivided drums all the line required for
mooring the ship is stored on a single drum, the
layers of the unused line increase the diameter of
the drum effectively, thus decreasing the BHC
whereas, as split drums operate with only one
layer of the mooring line on the tension section,
theoretically they can maintain a constant BHC.
Each drum has its respective band brake that lets it
hold mooring lines fast on brakes with the winch
disengaged, after being previously heaved tight
against the rubber fenders with the winch engaged
until achieving the desired pretension to use friction
in order to reduce the fore and aft movement of the
vessel [2], [10].
Brakes have two circular steel bands connected by
an articulated joint pin. At the opposite end, the top
half is anchored to a pivot point fixed to the deck or
bed plate and the bottom half (the free or floating
end) is connected to the brake control lever (Fig. 2).
Brass screws or copper rivets attach steel bands to
high-grade non-asbestos marine brake linings. Due to
the wear and tear in ordinary use of brake linings,
when their thickness has reached a certain level they
must be replaced as per manufacturer´s operations
manual (it usually features a wear indicator to this
end), in order to maintain effectiveness of the winch
brake by suitable friction between the linings and the
rim of the winch drum. The brake linings rely on the
coefficient of friction against the brake drum to hold
the force on a line. How to tight or release the band
brakes depends on their application type, either by
means of a simple conventional brake handle (or hand
wheel) manual screw or in conjunction with a spring-
applied by manual setting and release or by hydraulic
release.
As a static device, the prime aim of the band brake
is to secure the winch drum and the mooring line
spooled and layered on it, holding the line tight when
moored.
When the load on the mooring line becomes
excessive (overloaded) beyond a predetermined level,
the brake has an added safety function of rendering to
allow the line to shed its load by releasing the tension
in a controlled manner [14]. This serves to prevent a
mooring line from breaking and snapping back.
To manage the safety function of the brake
effectively, a winch brake test for the winch operator
guide should be carried out in order to hold the line
correctly with the winch disengaged. However, the
load at which the brake renders is also dependent on
its actual condition.
At the ship design stage [10], the MBL of the lines
is determined taking into account factors such as the
ship’s size and hull form, the number of mooring lines
to be deployed, mooring restraint requirements, the
maximum windage and lateral underwater area to
cope with wind and tidal conditions respectively.
Regarding the windage and underwater area, Oil
Companies International Marine Forum -OCIMF-
uses standard environmental criteria to this end [10].
Based on the MBL, the mooring winch general
requirements and parameters being dealt with in ISO
3730:2012 [6] and ISO 7825:2017 [7] are determined.
Regarding the winch brake, their BHC will not be less
than 80% of mooring line MBL [6] in the first layer
although, through a brake winch test, it should be
operationally set to hold 60% of the MBL on the first
layer to permit slippage before the line breaks [11].
The mooring set-up on-board should be such that
the winch brake should be the weakest link in case of
overload followed by the primary line, the synthetic
tail if any (MBL for mooring tails should be 25%
higher than that of the primary mooring line to which
they are attached [10]); and lastly the fixed structures
such as the winch foundation and the fairleads. This
set-up is the standard and its intention is to minimize
the consequences in case of overload.
The drum load of the winch (also called hauling,
hoisting or rated load), i.e. the pull that the mooring
line can develop at the rated speed on the first layer,
should be within 22% to 33% of the MBL of the line
[6] assuring adequate force to heave in against
environmental forces, but being low enough to
prevent the line from overstressing in the stalled
condition i.e., the maximum rope tension, in
kilonewtons -kN-, is measured at the drum exit when
the drum ceases to rotate in the haul direction, the
prime mover being set for maximum torque and the
rope being wound on the drum in a single layer. The
rendering load (maximum rope tension measured at
the drum exit when the drum just starts to rotate in
the opposite direction to the applied driving torque)
will not be more than 50% MBL of the line [6];
therefore, the BHC is always greater than the
rendering load. Thus, when occasionally
unanticipated changes of load generated by extreme
winds, waves or tide cause the brakes to render and
the vessel to be at risk of moving off the berth, it is not
possible to engage the winch and haul the lines after
releasing the brakes in an attempt to put the ship
alongside again [11]. Instead, good seamanship in
those cases dictates to maintain engines ready for
manoeuvring, to require tug assistance, to ballast the
ship down to reduce the total forces acting on the ship
as the wind gradient is greater than the under keel
clearance effect [11] and to disconnect hoses or any
other fixed cargo handling system on-board from the
harbour.
As most modern large ships are equipped with
dedicated mooring winches with split drums fitted
out with band brakes, in this paper we analyse how to
manage the safety function of the band brake to
render before breaking the line and their reliability
depending on the type of application. To that end, the