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Are The Lashings Adequate?

The 1994 / 1995 amendments to the code of safe practice for cargo stowage and securing, were introduced in Annex 13, which was incorporated in to the code. Annex 13 provides the methods of calculating and ensuring the adequacy of the lashing material and its strength to secure a cargo unit against possible movement during the ship motion, rolling, pitching etc.

The methods described should be applied to non-standardized cargoes, but not to containers on containerships.

Where very heavy units and those items for which exhaustive and special advice on stowage and securing is to be provided by experts, the methods described in this annex is not used. It may sometimes, so happen that a Master is asked to load a non standard cargo unit on say main deck and he may even be, expected to weld and create new lashing points to provide adequate securing support to the cargo. The methods given in Annex 13 provide useful information to do the required calculations to check whether the securing arrangements are appropriate. Computer software, providing the equivalent safety that is provided by Annex 13 is permitted.

The application of the methods described in annex 13 is supplementary to the principles of good seamanship and does not replace experience in stowage and securing practice.

The methods of calculations described: 

  1. Provide guidance for the preparation of a ships specific securing manual.
  2. Provide guidance for assessing the securing strength in case of cargo units not covered by the cargo securing manual;
  3. Assist qualified shore personnel in doing the relevant calculations.

Let us first, discuss in detail, one of the methods given in Annex 13, in respect of a cargo unit that is loaded on main deck.

Ex. A ship of length 120 m, breadth 20m, has a GM of 1.4 m and is moving at 15kn. A cargo unit of mass 62 t of dimensions, 6m x 4m x 4m is stowed at a distance of 70% of length from aft, on deck, in low position. The securing material used are as follows:
1. Wire rope: breaking strength = 125 kN, 2. Shackles, turnbuckles, deck rings: breaking strength = 180 kN Dunnage boards are used for Stowing.

Ans. The MSL or maximum securing load which is the load capacity for a securing device is found from its breaking strength using table 1. Thus, the MSL of shackles etc. = 50% of 180kN = 90 kN and MSL of wire rope is 80% of 125 kN = 100 kN.

CS or calculated strength is found by dividing MSL by safety factor (1.5). The CS of the two material therefore will be 60kN and 66.67 kN respectively. The smaller value viz 60 kN is used for our calculations.

From the diagram the securing arrangement is as follows:

Side                  n          CS                   α          f
STBD               4          60kN               400       0.96
PORT              2          60kN               40°       0.96
PORT              2          60kN               10°       1.04

Note: α is the vertical securing angle of the wire rope and f is found from table 6 using the values of α and μ. The coefficient of friction found from table 5.

External forces acting on the unit due acceleration caused by ship motions, gravity force etc.
In addition the thrust caused by wind pressure and sea sloshing is considered for longitudinal & transverse force calculations.
Disturbing Force, acting on unit in fore and aft direction =
Fx= 2.9 × 0.89 × 62 + 16 + 8 = 184 kN.
Disturbing Force acting on unit in athwartship direction =
Fy = 6.3 × 0.89 × 62 + 24 + 12 = 384 kN.
Disturbing Force acting on unit in vertical direction =
Fz = 6.2 × 0.89 × 62 = 342 kN.

Note:

  1. The first figures i.e. 2.9, 6.3 and 6.2 in above equations are longitudinal, transverse and vertical accelerations respectively as obtained from table 2 owing to the stowed location of the cargo unit viz at 70 % of L from aft at low deck position.

To hold the cargo firmly in position, it is necessary that the strength of lashing material is adequate to withstand the above forces.

Thus, the lashing material used on starboard side of the unit is appropriate if disturbing force acting on unit in athwartship direction is less than unit’s own weight plus the force, which the 4 lashing material can withstand on starboard side:

384 < (0.3 × 62 × 9.81) + (4 × 60 × 0.96)
384 < 412

This means that the lashing material used on starboard side of the unit is appropriate if disturbing force acting on unit, in athwartship direction is less than unit’s own weight plus the force, which the 4 lashing material can withstand on port side:

384 < (0.3 × 62 × 9.81) + (2 × 60 × 0.96) + (2 × 60 × 1.04)
384 < 422

Tipping moment acting on the unit should be less than the holding or restoring moment acting on it.

(384 × 1.8) < (2 × 62 × 9.81)
691 < 1216

This means the cargo unit will not tip even if no lashings are taken.

The second method given in Annex 13, about the calculations in respect of adequacy of lashing material used to secure the cargo unit that is loaded on board ship is described.

Ex. A cargo unit of mass 68t is stowed on timber (μ = 0.3) in the ‘tween deck at 0.7 l of a vessel. L=160 m, B = 2.4 m and GM = 1.5 m. Dimensions of the cargo unit are height = 2.4 m and width = 1.8 m. The top and the oblique view of the cargo is as given in the sketch below. The mean securing load (a % of breaking strength) of the wire ropes is 108, 90, 90, 108, 108, 90, 90 and 108 respectively. Calculate and state whether the securing arrangement is ok. (Speed 18kn)

Soln: Calculated strength, CS =

The external forces acting on the cargo unit in longitudinal, athwartship and vertical direction depends on the acceleration due the location, the length of vessel, speed, ‘breadth-GM’ relationship, wind and waves. Since, the cargo is loaded in tween deck, the wind and waves are not applicable.

External Force in longitudinal direction Fx = 2.0 × 1 × 0.82 × 68 = 112 kN
External Force in transverse direction Fy = 5.6 × 1 × 0.82 × 68 = 312kN
External Force in vertical direction Fz = 6.2 × 1 × 0.82 × 68 = 346 kN
The values of fx and fy are found from table 7 using the values of α and β.

Table 7.2 for μ = 0.3

To check if transverse lashings are adequate:

The sum of the frictional force due to weight and the athwartship tensional forces offered by the athwartship component of lashing ropes should be more than the transverse external forces.

Thus, (68 × 9.81 × 0.3) + (68.8 + 55.6 + 55.6 + 62.4) = 443, which is more than 312kN. This arrangement is ok for transverse arrangement on starboard side.

Also, (68 × 9.81 × 0.3) + (68.8 + 66.3 + 69 + 68.8) = 473, which is more than 312kN. This arrangement is ok for transverse arrangement on port side.

To check if longitudinal lashings are adequate:

The fore and aft shift of cargo unit will be during pitching, involving vertical acceleration, causing apparent reduction in weight. Thus, the apparent weight becoming equal to (68 × 9.81- Fz) = (68 × 9.81- 346) = 321.1kN.

To check the forward arrangement:

(0.3 × 321.1) + (46.4 + 30.2 + 18.1 + 46.4) = 237kN. This is more than 112kN. This arrangement is ok.

To check the aft arrangement:

(0.3 × 321.1) + (30.2 + 55.2 + 46.4 + 38.2) = 266kN. This is more than 112kN. This arrangement is ok.

To check for adequacy against tipping:

The tipping would be caused by the transverse force about tipping axis. In the figure it is shown as clock wise movement. Anti clock wise moment are provided by the weight and tensional forces on lashing wire ropes.

Note: If the lashing is connected as shown in the sketch, instead of measuring c, the length of the lever from the tipping axis to the lashing CS, it is conservative to assume that it is equal to the width of the cargo unit.

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