7.1 Assumption of external forces
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Statutory Documents - IMO Publications and Documents - International Codes - CSS Code - Code of Safe Practice for Cargo Stowage and Securing1 – Resolution A.714(17) - Annex 13 - Methods to assess the efficiency of securing arrangements for non-standardized cargo - 7 Advanced calculation method - 7.1 Assumption of external forces

7.1 Assumption of external forces

 7.1.1 External forces to a cargo item in longitudinal, transverse and vertical directions should be obtained using the formula:

  • F(x,y,z) = m · a(x,y,z) + Fw(x,y) + Fs(x,y)

where

  • F(x,y,z) = longitudinal, transverse and vertical forces

    m = mass of the item

    a(x,y,z) = longitudinal, transverse and vertical accelerations (see table 2 below)

    Fw(x,y) = longitudinal and transverse forces by wind pressure

    Fs(x,y) = longitudinal and transverse forces by sea sloshing.

The basic acceleration data are presented in table 2.

Table 2 – Basic acceleration data

Remarks:

The given transverse acceleration figures include components of gravity, pitch and heave parallel to the deck. The given vertical acceleration figures do not include the static weight component.

 7.1.2 The basic acceleration data are to be considered as valid under the following operational conditions:footnote

  • .1 operation in unrestricted area;

  • .2 operation during the whole year;

  • .3 length of ship is 100 m;

  • .4 service speed is 15 knots; and

  • .5 B/GM ≥ 13 (B = moulded breadth of ship, GM = metacentric height).

 7.1.3 For operation in a restricted area, reduction factors for accelerations may be considered, taking into account the season of the year, the accuracy of the weather forecast affecting the wave heights during the intended voyage and the duration of the voyage. Restricted area means any sea area in which the weather can be forecast for the entire sea voyage or shelter can be found during the voyage.

 7.1.4 Reduction factors, fR, may be applied to significant wave heightsfootnote, Hs, not exceeding 12 m for the design of securing arrangements in any of the following cases:

  • .1 The required securing arrangement is calculated for the maximum expected 20-year significant wave height in a particular restricted area and the cargo is always secured according to the designed arrangement when operating in that area.

  • .2 The maximum significant wave height that a particular securing arrangement can withstand is calculated and the vessel is limited to operating only in significant wave heights up to the maximum calculated. Procedures for ensuring that any operational limitation is not exceeded should be developed and followed and documented in the ship's approved Cargo Securing Manual.

  • .3 Required securing arrangements are designed for different significant wave heights and the securing arrangement is selected according to the maximum expected wave height for each voyage for which an accurate weather forecast is available. Thus, the duration of the voyage should not exceed 72 hours or a duration as accepted by the Administration.

 7.1.5 The basic acceleration data in table 2 may be multiplied by the following reduction factor:

fR = 1 – (Hs – 13)2 / 240, where Hs is:

  • .1 the maximum expected 20-year significant wave height in the area according to ocean wave statistics; or

  • .2 the maximum predicted significant wave height on which the operational limitations are based; or

  • .3 for voyages not exceeding 72 hours the maximum predicted significant wave height according to weather forecasts.

 7.1.6 When weather-dependent lashing is applied, operational procedures for the following activities should be developed, followed and documented in the ship's approved Cargo Securing Manual, or otherwise included in the ship's safety management system:

  • .1 decision on the level of cargo securing based on the length of the voyage and the weather forecast;

  • .2 communication to all concerned parties of the decided level of cargo securing for the intended voyage;

  • .3 execution and supervision of appropriate cargo securing efforts in accordance with the Cargo Securing Manual; and

  • .4 monitoring of environmental conditions and ship motions to ensure that the applied level of cargo securing is not exceeded.

 7.1.7 For ships of a length other than 100 m and a service speed other than 15 knots, the acceleration figures should be multiplied by a correction factor given in table 3.

Table 3 – Correction factors for length and service speed

Length (m) 50 60 70 80 90 100 120 140 160 180 200
Speed (kn)
9 1.20 1.09 1.00 0.92 0.85 0.79 0.70 0.63 0.57 0.53 0.49
12 1.34 1.22 1.12 1.03 0.96 0.90 0.79 0.72 0.65 0.60 0.56
15 1.49 1.36 1.24 1.15 1.07 1.00 0.89 0.80 0.73 0.68 0.63
18 1.64 1.49 1.37 1.27 1.18 1.10 0.98 0.89 0.82 0.76 0.71
21 1.78 1.62 1.49 1.38 1.29 1.21 1.08 0.98 0.90 0.83 0.78
24 1.93 1.76 1.62 1.50 1.40 1.31 1.17 1.07 0.98 0.91 0.85

 7.1.8 For length/speed combinations not directly tabulated, the following formula may be used to obtain the correction factor with v = speed in knots and L = length between perpendiculars in metres:

  • correction factor = (0.345·v /⁄)+(58.62·L-1034.5)L2⁄

This formula should not be used for ship lengths less than 50 m or more than 300 m.

In addition, for ships with B/GM less than 13, the transverse acceleration figures should be multiplied by the correction factor given in table 4.

Table 4 – Correction factors for B/GM

B/GM 3 4 5 6 7 8 9 10 11 12 13 or above
on deck, high 2.64 2.28 1.98 1.74 1.56 1.40 1.27 1.19 1.11 1.05 1.00
on deck, low 2.18 1.93 1.72 1.55 1.42 1.30 1.21 1.14 1.09 1.04 1.00
'tween deck 1.62 1.51 1.41 1.33 1.26 1.19 1.14 1.09 1.06 1.03 1.00
lower hold 1.24 1.23 1.20 1.18 1.15 1.12 1.09 1.06 1.04 1.02 1.00

 7.1.9 The following should be observed:

  • .1 In the case of marked roll resonance with amplitudes above ±30°, the given figures of transverse acceleration may be exceeded. Effective measures should be taken to avoid this condition.

  • .2 In the case of heading into the seas at high speed with marked slamming impacts, the given figures of longitudinal and vertical acceleration may be exceeded. An appropriate reduction of speed should be considered.

  • .3 In the case of running before large stern or quartering seas with a stability which does not amply exceed the accepted minimum requirements, large roll amplitudes must be expected with transverse accelerations greater than the figures given. An appropriate change of heading should be considered.

  • .4 Forces by wind and sea to cargo items above the weather deck should be accounted for by a simple approach:

    • .1 force by wind pressure = 1 kN per m2

      .2 force by sea sloshing = 1 kN per m2

  • .5 The wind force may be reduced by the same principles as the accelerations, i.e. multiplying it with a reduction factor, fR, based on the expected significant wave height.

  • .6 Sloshing by sea can induce forces much greater than the figure given above. This figure should be considered as remaining unavoidable after adequate measures to prevent overcoming seas.

  • .7 Sea sloshing forces need only be applied to a height of deck cargo up to 2 m above the weather deck or hatch top.

  • .8 For voyages in a restricted area and with forecast wave heights for which no sea sloshing is expected, sea sloshing forces may be neglected.

Parent topic: 7 Advanced calculation method
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