11. FOREBODY AND AFTBODY

Calibration factors are rampant in the forebody rules. In part this is because the finite element models haven't as a matter of course extended to this part of the hull. In part, it is because the forces, especially in the forebody are particularly difficult to predict. This means we should be particularly conservative.

Unfortunately, this is not the case. Class goes to some trouble to estimate peak slamming pressures and then immediately applies a bunch of calibration factors of unknown or dubious origin. In many cases, the resulting scantlings are lower than the less highly loaded structure in the midbody. The result is that forebody damage is still the most common form of structural failure.

Another big problem is structural continuity. Hellespont was amazed to find out that the yards felt no compulsion to extend the main horizontal members: the innerbottom and the double side stringers into the forebody or aft body in any reasonably continuous manner. Most were abruptly ended with minimal scarfing brackets and new flats and decks placed at whatever level suited the yard in the ends of the ships. At the aft end, longitudinal bulkheads deteriorate abruptly into a series of widely spaced pillars with no shear strength whatsoever. The yards argue that this is required by machinery arrangement issues but both the Hellespont V and U have real longitudinal bulkheads extending to the aft peak tank and the engine room works fine. Even the upper deck is abruptly ended, usually just aft of the accommodations with a big increase in vertical deflection.

Transversely, the engine room is inherently weak due both to the narrow sterns now being used and the large hole in the structure forced by the main engine. There is essentially no transverse structure between the forward and aft engine room bulkheads in the center third of the ship. Once again the yards claim that this is required by machinery arrangement. In fact, the web just aft of the main engine and just forward of the boilers and generators could be effectively filled in without affecting the engine room operation.

Class seems to have no rules that prevent this. They even allowed upper deck stiffeners to be cut at the engine room bulkhead in direct violation of the rules for primary longitudinal members. The argument seems to be these are not high stress areas so we don't have to worry about it. But the fact is that the stresses are not even analyzed. And the best way to turn an area that should not be a problem into a problem is with structure that violates basic principles of good design. The rules with respect to continuity between the cargo tank area and the ends of the ship must be massively strengthened. The side shell stringers and the inner bottom should extend from one end of the ship to the other in a continuous fashion. Ditto for the longitudinal bulkheads. And we need a real transverse bulkhead between the Main Engine and the boilers.

Another big problem in the aft body is hull deflection and its impact on shaft and engine bearing loads. Amazingly, Class doesn't even require this deflection to be calculated, despite the fact that there have been a rash of stern tube bearing failures and the repair yards are reporting very rapid weardown of VLCC shaft bearings. Needless to say shaft bearing reliability is absolutely crucial to these single screw ships. The rules should require detailed analysis of hull deflection -- both vertical and transverse -- in way of the main engine and shaft, and the results of those calculations fed to the shaft alignment analysis (see Section 13 below). The hull deflection numbers will be an automatic output of the full hull FEM discussed in Section 4 above, provided we pose a proper range of vertical and transverse loads. The transverse loads should be based on high speed turns, port and starboard.

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