Heavylift Tip – Ships

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Seagoing Accelerations
How much motion is there on the ocean?

When we transport cargo at sea, the forces it experiences are typically the highest the cargo will ever have to withstand (extreme earthquakes aside). It is therefore crucial to the designer that these loads are well defined.

In an ideal world, we can carry out a motion’s response analysis (or even model tank testing). This will provide the engineers with the most accurate vessel motion predictions possible, from which the relevant loads can be derived. However, for these results to be applicable, the following details are required:

• Vessel hull form and load condition (including structural weights, tank loads and stowage plan)
• Sea state and weather data for the planned route (for example, wave heights and periods, applicable wind and wave spectrum)
• Accurate cargo design information (such as weights and dimensions)

If the above data has been collected accurately, the information can then be used within a suitable software program. It is important to note, however, that the results will only be valid for this specific case. This puts us in a bind; to design the cargo we need to know the vessel motion, but to correctly predict the vessel motion, we need to understand the cargo.

In order to solve this problem, several simplified/default motion calculations have been developed by the industry. These calculations use assumed limits and empirical formulae to estimate a conservative set of vessel accelerations. Vessel particulars and operational constraints limit the applicability of each method, but the results do vary, with some producing
more conservative results than others.

Significant savings in steel weight can be obtained, but this would not matter if your cargo fails en route to the client. Deciding which route to take can be a complicated process, and therefore, it is always best to speak with a qualified professional that understands the ins and outs of each approach.

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