12.7.1 Vessel selection
The transport vessel should be selected based on its capacity to suit operations of the transport, including loadout and offload, and to demonstrate that the vessel meets deadweight, deck space, strength and stability requirements for the marine operation.
12.7.2 Stability
For stability requirements, see Clause 9.
12.7.3 Under-keel clearance
The minimum under-keel clearance for offshore transportation should be 5 m after considering the effects of roll and pitch, heave, towline pull, wind heeling, tolerance on bathymetry, differences in water density, squat effects and deflections of the vessel.
12.7.4 Special considerations
Special considerations should be given to transit in limited areas with smaller under-keel clearances subject to weather-restricted conditions, tidal windows and accuracy of the survey data.
12.7.5 Sea fastening
12.7.5.1 For sea fastening of objects for dry tow on a barge or aboard a heavy transport vessel, the following considerations apply.
The provisions of 12.2.5 apply to the design of the sea fastening.
Sea fastening should be designed with details that are robust with regard to fatigue; for critical transports, a fatigue assessment should be made and the calculated fatigue lives of the sea fastening and their connections shall be at least five times the anticipated transit time.
For possible slam loads and their effects on overhanging parts of the cargo, such as capped pile sleeves, appropriate slam calculations should be prepared and documented.
If the cargo becomes submerged when the vessel heels or rolls, the weight of the cargo is reduced by an amount equal to the buoyancy of the submerged cargo. This reduces the friction forces between the object and the vessel and the sea fastenings should be designed to account for this loss of resistance.
A detailed analysis of heavy transport vessel or barge and its cargo, taking into account the cargo overhang, arrangement of cribbing and sea fastening, should be performed to enable appropriate local and global strength checking.
For cargos supported on the deck by cribbing or dunnage, and in the absence of more detailed analysis, the coefficients of friction in Table 13 may be used for the combination of cargo weights, cargo overhangs and arrangements of cribbing and sea fastenings listed therein. The computed frictional force on the cribbing can be deducted from the computed loading when determining the forces to be carried by the cribbing and sea fastenings. The sea fastening design strength shall be greater than the minimum force derived from the data in Table 13, which presents the maximum coefficients of friction and the minimum force, expressed as a percentage of cargo weight.
12.7.5.2 When using the information in Table 13, the following apply.
Friction forces shall be computed using the reaction normal to the deck between the vessel and the cargo.
The cargo should be supported by wood dunnage or cribbing; friction is not allowed for steel-to-steel interfaces.
The overhang is the distance from the side of the vessel to the extreme outer edge of the cargo.
For wood cribbing less than 600 mm high and with a width no less than 300 mm, the full friction force may be assumed to act in any direction relative to the cribbing.
For cribbing heights between 600 mm and 900 mm high and with a width no less than 300 mm, the calculated percentage friction force assumed to act in a direction at right angles to the line of the cribbing should not exceed the calculated factor, ffr, expressed as a percentage, as given by Equation (7):
f −H
= c
fr
900
3 (7)
where Hc is the height of the cribbing above deck, expressed in millimetres.
For wood cribbing over 900 mm high, or with a width less than 300 mm, no friction force shall be assumed to act in a direction at right angles to the line of the cribbing.
For cribbing with a height greater than its width, movement in the direction orthogonal to the length of the cribbing shall be prevented by steelwork designed to carry the full friction force in that direction.
When the cribbing arrangement is predominantly in a single direction, the calculated friction force orthogonal to the predominant cribbing direction should be reduced accordingly unless the sea fastenings acting in the orthogonal direction are fitted with an interface that is compliant with the cargo and allows the development of the cribbing friction as the actions on the sea fastenings develop. Such interfaces can be comprised of an elastomeric bearing pad.
The value of the minimum sea fastening force listed in Table 13 is the minimum value, expressed as a percentage of cargo weight, for which the sea fastening should be designed in the event that the computed sea fastening force is less than this value.
For very short duration moves in sheltered water, such as turning a barge back alongside the quay after a loadout, friction forces may be considered. It should be demonstrated that all parts of the load path, including the potential sliding surfaces, are capable of withstanding these forces. For additional information, see 11.12 and 11.13.
Table 13 — Allowance of friction in sea fastening design [36]
Overhang m
Maximum coefficient of friction Cargo weight
Wc kN
< 1 000 1 000 to
< 10 000 10 000 to
< 50 000 50 000 to
< 100 000 100 000 to
< 200 000 200 000 to
< 400 000 ≥ 400 000
None 0 0.10 0.20 0.20 0.20 0.20 0.20
< 15 0 0 0.10 0.20 0.20 0.20 0.20
15 to < 25 0 0 0 0.10 0.20 0.20 0.20
25 to < 35 0 0 0 0 0.10 0.20 0.20
35 to < 45 0 0 0 0 0 0.10 0.10
≥45 0 0 0 0 0 0 0
Direction Minimum sea fastening force
% Wc
Transverse — 10 10 10 10 a 5
Longitudinal — 5 5 5 b c 1.5
a ≥ 15 − Wc/40 000.
b ≥ 7.5 − Wc/40 000.
c ≥ 3.5 − Wc/200 000.
12.7.6 Navigation systems
Requirements for navigation systems are given in 12.6.4.