Example B.1.1 – Top-Over 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.1 Example calculation – Top-over lashings - Example B.1.1 – Top-Over Lashings on a 16,600 DWT Ship

Example B.1.1 – Top-Over Lashings on a 16,600 DWT Ship

Figure B.1 Midship section of 16,600 DWT ship with packages of sawn wood in two layers secured with top-over lashings

 Ship particulars

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

 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 ship 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
f R1 = 0.81 = Correction factor for length and speed
fR2 = 1.00 = Correction factor for BM/GM
at = at basic•fR1•fR2 = 6.5•0.81•1.00 = 5.3 m /s2

 Cargo properties

m = 1,600 ton = Mass of the section to be secured in tons, including absorbed water and possible icing
μstatic = 0.45 = Coefficient of static 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
PTV = 16 kN = Pretension in the vertical part of the lashings in kN
α = 85° = Angle between the horizontal plane and the lashings in degrees
np = 18 pcs = Number of stacks of packages abreast in each row

 Number of required top-over lashings

For pure top-over lashing arrangements with no bottom blocking, the friction alone will have to counteract the transverse forces so that the following equilibrium of forces is satisfied:

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

 Thus the required number of top-over lashings can be calculated as:

Parent topic: B.1 Example calculation – Top-over lashings

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 to 0.33 kN/metre:

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