2. Speed reduction in head seas as accepted operational parameter

[German version] 
The present Annex 13 requires to assume the full nominal service speed of the vessel for assessing external forces to the cargo. There are situations however, where the sensitivity of certain cargo units or the restricted feasibility of securing measures limit the endurable forces. Therefore it seams advisable to include a deliberate speed reduction, exclusively in head seas, into the operational parameters. That means in clear words that speed reduction would become a legal measure of cargo securing which should be specified and approved in the ship's Cargo Securing Manual, as has been practiced by certain flag states already. In general, this measure may be limited to distinguished cargoes like timber on deck or pipes on deck or to unusual and sensitive cargo items.
The problem of securing against longitudinal sliding and tipping arises predominantly in stowage positions at the forward and aft parts of the vessel and increases with the vertical level of stowage. The mathematical model of ship behaviour in the Annex 13 assigns a gentle random influence of the speed of the vessel on the magnitude of forces. Lower speed means a certain reduction of all forces. In the case of longitudinal securing a twofold relief is obtained by speed reduction, namely the direct drop of longitudinal forces and the drop in temporary loss of weight. This provides a better friction or a better tilting stability than with high speed.
Another possible revision of the Annex 13 regards the worst case approach within the longitudinal sliding and tipping balances. This is addressed in chapter 5 below and may justify a modification of the assumed weight reduction by 80% of Fz, concurrent with the maximum longitudinal force Fx.
Experience has indeed shown that certain cargoes, where longitudinal securing is difficult, like timber or pipes on deck, do not move longitudinally in headon sea, if the vessel's speed is reduced accordingly, a fact which cannot be reflected by the present Annex 13 calculation model. The following example demonstrates the modelled effect of a speed reduction and of the modified weight reduction.
Example: The demand of longitudinal securing of a distinguished cargo unit is checked with two different ship's speeds according to the algorithm of the Annex 13.
Ship: Lpp = 160 m. Cargo: m = 100 t, stowed on deck low at 0.8 L, cargo dimensions l x b x h = 10 x 8 x 4 m.
Service speed v = 18 kn
Fx = 2.9 · 0.82 · 100 + 48 = 286 kN (48 kN = impact forces from wind and spray)
Fz = 7.6 · 0.82 · 100 = 623 kN
Longitudinal securing demand = Fx  μ · (m · g  Fz)
= 286  0.3 · (100 · 9.81  623) = 179 kN
with 80% vertical force Fz:
Longitudinal securing demand = Fx  μ · (m · g  0.8 · Fz)
= 286  0.3 · (100 · 9.81  498) = 141 kN
Reduced speed v = 12 kn
Fx = 2.9 · 0.65 · 100 + 48 = 237 kN
Fz = 7.6 · 0.65 · 100 = 494 kN
Longitudinal securing demand = Fx  μ · (m · g  Fz)
= 237  0.3 · (100 · 9.81  494) = 91 kN
with 80% vertical force Fz:
Longitudinal securing demand = Fx  μ · (m · g  0.8 · Fz)
= 237  0.3 · (100 · 9.81  0.8 · 494) = 61 kN
The longitudinal securing demand drops to about 34% by a speed reduction of 1/3 in this case and with the modified weight reduction. But there is no simple rule about this relation because of the various influences from stowage position and impact forces to deck cargo. Each case must be investigated individually. An amendment to the Annex 13 in paragraph 7.1 under the Table 3 might read:
Table 3 should be entered with the full service speed in principle. However, for cargoes with a particular sensitivity against vertical accelerations in head seas or for which longitudinal securing creates substantial difficulties, a deliberate reduction of speed in head seas may become part of the longitudinal securing concept. Such cargoes and the appropriate parameters and evidence procedures of speed reduction should be specified in the ship's Cargo Securing Manual.
