We have had a lot of marine accidents that involved failure of the hull structures.
Whether it was a crack in the midship region, or a total split-off of the hull girder, or failures due to propagation of cracks, the crux of the matter boils down to a handful of causes that are of great concern to ship designers and operators.
Mostly, crack propagation takes place due to fatigue, which is not something this article is about. This article gives an insight into the causes that lead to the failure of the hull girder from a longitudinal strength point of view.
And before we start, we have to ask ourselves a few questions. Why have ships split-off? Why have many hull structures split-off after grounding? Why are midships highly prone to such failures? Is it because of a design flaw? Or for that matter, a glitch in the operation standards that have not been maintained?
Marine Engineers and ship operators use loading manuals to maintain the distribution of deadweight load on the hull girder. But what if the key to the design of those manuals lies in a deeper context? What if we tried to understand the phenomenon from a ship designer’s point of view? When ships are designed, a Naval Architect takes into consideration each and every factor that might affect the structural response of the hull girder with respect to the required loading conditions. In order to see the inner picture, we need to look at it from designer’s point of view.
When the hull girder of a ship is designed, the designers analyse the structure as a beam. But this beam is different from those that are used by civil engineers, in as much as ships are structures that are subjected to unpredictably variable loads. Look at it this way- the buoyancy on the hull is never predictable given the fact that sea surface being characterised by waves, the buoyancy on the ship is always varying periodically along the ship’s length. Also, ships are not always in the same cargo loading conditions. Where they may ply one voyage in a fully loaded condition, in the return voyage the ship may not have cargo but be induced to ballast loading condition. So we design the ship structure keeping in mind the uncertainty of the loads, and what helps in quantifying the response is a theory that has been widely used- Euler’s Beam Bending Theory.
