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In-depth | DAMAGE STABILITY Estonia failure was regulatory


Naval architect Anders Fischer says Estonia was in compliance for operations in restricted waters, and not for the open sea. Tis regulatory failure led to the catastrophe 16 years ago this month.


E


stonia was designed as an “inland ferry” with vehicles expected to roll on at one end and roll off at


the other end. It was unprepared and ill-equipped for the stormy Baltic waters that it encountered on the night of 27-28 September 1994. Meeting waves head on in stormy


weather was disastrous for Estonia, locks and hinges were turned into breaking pins. The opened ramp closing like a return valve made for slow water ingress, preventing an immediate capsize, floating upside down. Up-flooding into cabin cluster on upper decks prolonged the belated capsize-process from seconds to minutes. Sixteen years have passed since


Estonia sank. The two new investigations (from the HSVA, the Hamburg Ship Model Basin, Consortium and the SSPA Consortium), presented in May 2008, confirmed the earlier report from JAIC, the Joint Accident Investigation Commission, except for a difference in time of 15 minutes, three very competent commissions have arrived at the same conclusion. They could then hardly have been


anything but right! However, the objection from Margus Kurm, the state prosecutor of Estonia, came as a real dash of cold water. The ramp was continuously under surveillance on the monitor in the machinery control room and it was never seen down. Like JAIC, the new independent consortia had staked their investigations on its being down. Not up. Now further investigation failures were


imminent because Mr Kurm insisted on his witnesses being trustworthy. Was a new dive necessary? Instead of having to accept an indefinite stalemate certainly all could be resolved from existing knowledge. At the time that Estonia was on the drawing board there were no computer


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Estonia. (From SSPA final report.)


codes in general use for calculating stress and strain as a function of both time and space as is experienced by a ship’s hull in stormy weather, with the stern the first and worst. Formulas meant for static loads are in this case of little or no relevance. Nevertheless experience and sound


thinking has over the years led to sound constructions. The keyword is ‘continuity’. Wave impacts (slamming) are transmitted into the material as stresswaves. They are sent further into the hull in a ‘soft’ way. The energy will then be consumed mainly in flexural propagating vibrations. A considerable part of this energy is in fact returned to the water by the vibrating hull. Thresholds, sharp corners and edges or anything that narrows the stressflow, i.e. constitutes a discontinuity are forbidden. On Estonia (and her kind) hinges and


locks (Gångjärn, Sidolås, Bottenlås in figure 1 below), the elements that joined the visor to the hull - were particularly evil in this sense. They were subject to stresswaves in their interior when the stem with its visor smashed into the waves.


They functioned like “breaking pins”.


Normally such an element is there in order to save the construction from overloading. In this case however, they brought disaster when Estonia for the first time encountered waves generated by a storm, having blown freely over an open stretch of water from the South Baltic to Ålands Sea. This should be compared to the maximum 20 nautical miles from nearest land, which was what she was certified for. The new investigations did not pursue


this problem, however. They were solely dedicated to the sinking process, which was strictly according to the instruction given by the Swedish government. And consequently they arrived at the same conclusion as the 10 year older investigation, with methods 10 years more modern. But, the government was driven by the general public, and many of its questions, for instance about sabotage, were left unanswered. The uncertainties regarding the sinking


process remain. With an abundance of in themselves admirable diagrams, animations, pictures and drawings stretching over hundreds of pages the said public may have become tired but


The Naval Architect September 2010


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