Main principles of lashing & securing of non-standard cargoes on deck
Needless to say that lashing & securing is crucial in any transports. You can one day 'put a blind eye' on something, whereas lashing - is right what has to be kept paramount and cannot be neglected under any reason whatsoever.
Wrong or insufficient lashing will entail long-lasting reclamations in the best case, otherwise significant harm to people and environment.
Planning of lashing and securing requires certain skills and experience. Practically in transportation of non-standard cargoes rarely it is possible to come across at least 2 identical lashing plans for same cargo. Indeed, lashing & securing is based on common understanding of technology and lashing means, and only later there are explicit rules and methods (adopted by IMO) of checking the quality of such lashing and securing arrangements.
Therefore, the common practice for planning of lashing operations on board implies at least two stages:
To design preliminary lashing plan based on own experience an skills. For example, at this stage it can be supposed (based on experience), that a steel tank off 20 tons might be secured on weather deck with 4 span-sets MSL 10 t - two per each side applied at 45' to horizontal pane.
To check the preliminary plan if it satisfies all the criterias of safety. For this purpose complex methods of CSS IMO code are widely used. The common principal of evaluation is about a balance of forces (affecting cargo against total maximum safe loads of lashing and securing means in reverse direction). In addition, forces affecting cargo (caused by wind, waves, longitudal and vertical sway) are being modeled in relation to navigation environment including characteristics of vessel and cargo, as well as location of cargo on board. Balance of forces is calculated in longitudal, transverse and vertical directions each being checked to satisfy the maximum safe loads of lashing and securing equipment.
Providing that the preliminary plan is insufficient under at least one criteria, the plan is being revised (securing means are either added or changed, an applied angle is adjusted). However there are situations where lashing and securing means are over used - to such an extent when it would be reasonable and logically efficient to use less complex equipment maintaining the same level of safety. After the preliminary plan has been checked, it can be accepted as operational.
At the time of loading, the Master may encounter to reinforce lashing in some certain places or completely redesign. E.g., the vessel may decline lashing by span-sets of heavy units due to necessity of adjusting stretching components during the voyage. Therefore, it is strongly recommended to mutually agree lashing & securing plans in some advance and settle any possible issues before commencement of operations.