Example B.3.1 – Loop lashings on a 16,600 DWT ship
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Statutory Documents - IMO Publications and Documents - International Codes - 2011 TDC Code – Code of Safe Practice for Ships Carrying Timber Deck Cargoes, 2011 – Resolution A.1048(27) - Annex B – Samples of Stowage and Securing Arrangements - B.3 Example calculation – Loop lashings - Example B.3.1 – Loop lashings on a 16,600 DWT ship

Example B.3.1 – Loop lashings on a 16,600 DWT ship

Figure B.3 Midship section of 16,600 DWT ship with packages of sawn wood secured with loop lashings

 Ship particulars

Length between perpendiculars, LPP: 134 metres
Moulded breadth, BM: 22 metres
Service speed: 14.5knots
Metacentric height, GM: 0.70metres

 The deck cargo has the dimensions L x B x H = 80 x 19.7 x 2.4 metres. The total weight of the deck cargo is taken as 1,600 tons. Sliding between the layers is prevented by packages of different heights in the bottom layer.

 Dimensioning transverse acceleration

With vessel particulars as above and considering a stowage position on deck low, Annex 13 of the CSS Code gives a transverse acceleration of at = 5.3 m/s2 , using the following basic acceleration and correction factors:

at basic = 6.5 m/s2 = Basic transverse acceleration
fR1 = 0.81 = Correction factor for length and speed
fR 2 = 1.00 = Correction factor for BM/GM
at = at basic • fR1 • fR2 = 6.5 • 0.81 • 1.00 = 5.3 m/s 2

 Cargo properties

m = 1,600 ton = Mass of the section to be secured in tons, including absorbed water and possible icing
μdynamic = 0.32 = Coefficient of dynamic friction between the timber deck cargo and the ship's deck/hatch cover
H = 2.4 m = Height of deck cargo in metres
B = 19.7 m = Width of deck cargo in metres
L = 80 m = Length of the deck cargo or section to be secured in metres
PW = 192 kN = Wind pressure in kN based on 1 kN per m2 wind exposed area, see CSS Code, Annex 13
PS = 160 kN = Pressure from unavoidable sea sloshing in kN based on 1 kN per m2 exposed area, see CSS Code, Annex 13
α = 70° = Angle between the horizontal plane and the lashings in degrees
n = 36 pcs = Number of loop lashings pairs
LL = 25 m = Length of each lashing in metres
PTV = 16 kN = Pretension in the vertical part of the lashings in kN
np = 13 pcs = Number of stacks of packages abreast in each row

 Number of required loop lashings

The number and strength of the lashings are to be chosen so that the following equilibrium is satisfied:

 If the number of loop lashings pairs is 36 then the required strength in the lashings can be calculated as:

The required MSL in the lashings is calculated as:

Parent topic: B.3 Example calculation – Loop lashings

Transverse movement of cargo due to elongation in lashings

 The transverse movement of the deck cargo due to elongation of the lashings is calculated according to the formula below. If chains are used the elongation factor is set to ∊ = 0.02, and the transverse movement is calculated as:

If web lashings are used the elongation factor is set to ∊ = 0.07, and the transverse movement is calculated as:

In accordance with 6.5.16 the transverse movement of the cargo should not generate a greater heeling angle than 5 degrees. In order to comply with this requirement significantly more and/or stronger lashings than described above have to be used.

Racking strength

 To prevent the packages in the bottom layer from collapsing due to racking, the weight of the cargo stowed on top of the bottom layer should be limited so that the following equilibrium of forces is satisfied:

Units denoted with a consider cargo units above the bottom layer only.

 Thus the required racking strength can be calculated as:

 There is no requirement on the racking strength of the packages, since the calculated value is less than zero.

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