EXPLANATIONS AND INTERPRETATION OF METHODS TO ASSESS THE EFFICIENCY OF
SECURING ARRANGEMENTS
1 The acceleration figures given in table 2, in combination with the
correction factors, represent peak values on a 25-day voyage. This does not imply that
peak values in x, y and z directions occur simultaneously with the
same probability. It can be generally assumed that peak values in the transverse
direction will appear in combination with less than 60% of the peak values in
longitudinal and vertical directions.
2 Peak values in longitudinal and vertical directions may be associated more
closely because they have the common source of pitching and heaving.
3 The advanced calculation method uses the "worst case approach". That is
expressed clearly by the transverse acceleration figures, which increase to forward and
aft in the ship and thereby show the influence of transverse components of simultaneous
vertical accelerations. Consequently, there is no need to consider vertical
accelerations separately in the balances of transverse forces and moments. These
simultaneously acting vertical accelerations create an apparent increase of weight of
the item and thus increase the effect of the friction in the balance of forces and the
moment of stableness in the balance of moments. For this reason there is no reduction of
the force m · g normal to the deck due to the presence of an angle of heel.
4 The situation is different for the longitudinal sliding balance. The
worst case would be a peak value of the longitudinal force Fx
accompanied by an extreme reduction of weight through the vertical force
Fz.
5 The friction coefficients shown in the tables of this annex are generally
lower than the ones given in other publications, such as the CTU
Code. The reason for this can be seen in various influences which may appear
in practical shipping, such as: vibration of the ship, moisture, grease, oil, dust and
other residues.
6 There are certain stowage materials available which are said to increase
friction considerably. Extended experience with these materials may bring additional
coefficients into practical use.
7 The principal way of calculating forces within the securing elements of a
complex securing arrangement should necessarily include the consideration of:
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.1 load-elongation behaviour (elasticity);
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.2 geometrical arrangement (angles, length); and
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.3 pre-tension, of each individual securing element.
8 This approach would require a large volume of information and a complex,
iterative calculation. The results would still be doubtful due to uncertain parameters.
9 Therefore, the simplified approach was chosen with the assumption that
the elements take an even load of CS (calculated strength) which is reduced
against the MSL (maximum securing load) by the safety factor.
10 When employing the advanced calculation method, the way of collecting
data should be followed as shown in the calculated example. It is acceptable to estimate
securing angles, to take average angles for a set of lashings and similarly to arrive at
reasonable figures of the levers a, b and c for the balance of
moments.
11 It should be borne in mind that this annex contains a number of
assumptions based on approximations. Even though safety factors are also incorporated,
there is no clear-cut borderline between safety and non-safety. If in doubt, the
arrangement should be improved.