As it had
been established by the before explained analysis of the visor structure that
the Atlantic lock was exposed to less load than the side locks as long as
the hinges were intact and that the force direction would be aft, it was suggested
to the technical group of the JAIC - Börje Stenström - Tuomo Karppinen - Klaus
Rahka - to have mock-ups built and tested to destruction by an institute of
their choice, also by the Finnish VTT where both Karppinen and Rahka were
employed. This was, however, not possible as this institute did not have sufficient
pulling power, agreement was reached to have the break- tests carried out
by the Institute of Naval Architecture (Institut für Schiffbau) in Hamburg
and four mock-ups were built according to the suggestions of Stenström/Rahka
which were identical with exception of the welding seams between the bushing
starboard lug and the housing/centre- and port-lugs.
These
were:
(1) welding seam thickness 3 mm (as found on the lug remains recovered from the wreck);
(2) bushing and housing only spot-welded to the lugs;
(3) no welding seams at all;
(4) welding seams 3 mm but visor lug strengthened above dimensions according to drawings.
The tests were carried
out in the presence of members of the technical group of the JAIC as well
as B.V., the University of the Armed Forces, Hamburg and this 'Group of Experts'.
The lugs were
made of normal steel and the bolt, bushing and housing of high tensile steel.
The results were as follows:
Test (1) : The big visor lug broke at a load of 204.3 ts. The bolt was slightly deformed, the lugs and welding seams remained unaffected.
Test (2) : The starboard and centre lugs broke at a load of 142.5 ts., the port lug was deformed towards port but did not break the visor lug and the bolt remained unaffected.
Test (3) : The starboard and the centre lugs broke at a load of 99.2 ts., the port lug in its forward part was slightly deformed towards port but did not break, the visor lug and the bolt remained unaffected.
Test (4) : The starboard and the centre lugs broke at a load of 204 ts., the port lug did not break at the test because the pulling was discontinued after the breakage of the starboard and centre lugs; it has to be assumed, however, that it would have broken if the pulling had been continued as it had been in practice. The bolt was slightly deformed, the visor lug apparently remained unaffected.
The complete Report is attached as Enclosure 34.3.431.
34.4
Determination of the Steel Quality of the Atlantic Lock Mock-ups
by the Institute for Failure Analysis and Failure Prevention, Hamburg
The systematical fracture tests with Atlantic lock mock-ups by the Institut für Schiffbau, Hamburg have been discussed in Subchapter 34.3 and the respective report is attached as Enclosure 34.3.431. These tests were performed upon recommendation and in the presence of members of the JAIC and the resulting report - Enclosure 34.3.431 - was sent to them soon after the tests in 1996. Although the report clearly states that the lugs were made of normal (mild) steel and that the bottom lock assembly broke at 2.04 MN load the JAIC has stated on page 189 of their Final Report among other things :
»Two contributing lugs would then have given the complete bottom lock assembly a holding capacity of about 1.5 MN including a small contribution from the starboard bracket. The Commission considers this to be a realistic maximum value.
The Commission is aware of a series of tests carried out in 1996 at the Technical University of Hamburg on behalf of the yard with full scale mock-ups of the bottom lock assembly made of high-tensile strength steel. In these tests, characterised by different extent of welding between the attachment lugs and the locking bolt housing, failures occurred between 1.0 and 2.0 MN. A test that incorporated intermittent welds resulted in failure at 1.42 MN.«
In order to prove that the lugs were not made of high tensile steel as stated by the JAIC the Institute for Failure Analysis and Failure Prevention, Hamburg carried out tensile tests of the mock-up remains and the result was: The lugs of the Atlantic lock mock-ups were made of
normal shipbuilding steel, e.g. according to GL.
The letter with test results and findings is attached as Enclosure 34.4.432.
34.5
Bow Impact of Ro-Ro Vessels by Det Norske Veritas
This paper - given to the Society of Naval Architects of Japan by Det norske Veritas (DnV) - concentrates on design load aspects for the bow area and presents results of recent work carried out by DnV with the aim of evaluating the long term bow impact loads that a Ro-Ro vessel may be subjected to during its operational life. The reason why it has been incorporated into this report is the obvious fact that the results are partly absolutely contrary to the findings of the JAIC. The paper is attached as Enclosure 34.5.433 and the conclusions are as follows:
»Bow impact of five different Ro-Ro vessels is studied numerically and empirically. Impact is considered both from an ultimate and a fatigue point of view. For the ultimate load case, the loads are found to be strongly a function of the wave height and the forward vessel speed. this illustrates the importance of speed reductions in rough seas. The study also shows that the highest waves do not necessarily induce the highest impact loads on the structure. The bow impact loads from the DnV Classification Rules are intended to cover all types of designs and design conditions.
A fatigue investigation of the bow door support system for one typical vessel has been carried out. A long term distribution of the bow forces has been established based upon a gradual speed reduction when wave height exceeds significant wave height = 5 meter. The results of the analysis show a fatigue life exceeding 66 years when using the North Atlantic wave climate as basis and constant operation in head seas. This corresponds to 400 years fatigue life for operation in all headings and on this basis it is concluded that fatigue does not represent a problem provided the vessel is operated with good seamanship. These conclusions are based upon the assumption that the bow doors are designed for ultimate loads according to the DnV 1994 Rules and that NV36 strength steel or lower is applied. The welded details are according to normal ship practice and with an intact corrosion protection.«
