Wednesday, September 6, 2000
Bedplate cracking in VLCC diesels
Wärtsilä NSD Corporation and HSD Engine Co Ltd, on behalf of Hanjung (KHIC) have issued a response to newspaper reports on cracking in the bedplates of some Sulzer low speed diesels.
Korean reports have suggested that Shell Oil and other operators have experienced considerable problems with Korean built VLCC's equipped with Sulzers built by Korean licensees. They indicate that owners have been considering lawsuits. Some reports say that it could take 70-90 days for the necessary repairs to be affected.
Wärtsilä NSD, however, says the reports "have tended to be rather alarmist, exaggerating the extent and severity of the problem."
Wärtsilä NSDsays that there are 41 RTA84T and RTA84T-B engines in service, all in VLCCs. The first began operation in 1994, and the
longest running engines have individually accumulated up to more than 40,000 running hours.Of these, 11 engines have cracks in one or more of their bearing girders.
The bearing girders are arranged transversely in the engine bedplate and carry the main bearings in which the crankshaft runs. The cracks are all in the circumferential direction beneath the bearing housings.
The cracks were detected in six RTA84T and five RTA84T-B engines, with the first cases being reported in November 1999. Ten of the affected engines were manufactured under license by Hanjung (KHIC) and one by Hyundai Heavy Industries Ltd. Several of the other engines
of these types in service, including engines delivered by the two above-mentioned licensees, were carefully checked by the engine
builders and Wärtsilä NSD but no cracks were found.
No unscheduled engine stops have occurred, and all affected vessels remain in service. However, three ships are sailing at a reduced speed, purely on the recommendation of Wärtsilä NSD as a precautionary measure.
Investigations by Wärtsilä NSD, licensees and the shipbuilders concerned are continuing, in close collaboration with the classification societies and
independent technical experts. So far, no definite conclusion can be made from these investigations.
Actual stress measurements, carried out on engines on test beds as well as on board ships, are much in line with the calculated design
values. Safety factors have been verified as being according to the DIN standard, taking into account the material used and the usual
deficiencies in cast steel.
Metallurgical analyses cannot be performed while the engines are still in operation, as these require cutting out samples from the cracked girders.
Wärtsilä NSD and the licensees concerned have developed appropriate repair procedures with instructions and tools. Two remedies have been
developed, involving the weld repair of cracks and replacement of bearing girders, both being made in situ. However, a complete change
of bedplate appears to be preferred by most of the shipowners concerned. The repairs can start as early as the ships and new bedplates become available. The time required for repair is expected to be about two months, and will in most cases be combined with normal drydocking of the ships concerned.
Meanwhile, maintains Wärtsilä-NSD, the engines are safe to operate as most of the existing cracks are stable and not propagating, or in a few cases only propagating very slowly. The existing cracks are carefully monitored and unaffected girders are regularly checked for cracks.
The first three RTA84T engines built had bearing girders with a web thickness of 60 mm. None of these engines has developed cracks in the
bedplate girders. Because of the moderate stress level measured on the first engine produced, the girder design was modified for production reasons to a "type b" design having a web thickness of 50 mm . Both the calculated and the measured stresses and strains in this "type b"
girder were all checked again by calculations and by measurements on board ship and on the test bed, and safety margins were verified to
be within normal practice and Wärtsilä NSD's expectations.
For future new engines, in order to restore confidence in view of the on-going newbuilding projects, a new bearing girder design has been
introduced. This modified girder design has a thickness of 60 mm to increase the tolerance for material deficiencies, and an extra
manufacturing specification regarding casting risers. In addition, it has been agreed with licensees to exert a stricter quality assurance
procedure and to adopt an improved quality and test specification. These measures will together give a 100 per cent guarantee that all
potentially critical material deficiencies will be detected and remedied.
Wärtsilä NSD says that while "it is clear that all requested answers are not yet available but it is also a fact that there is no technical need to take immediate unplanned action to replace bedplates or to replace cracked girders on engines. Such engines can clearly remain in service as normal."
It is also a firm opinion of Wärtsilä NSD that Sulzer RTA84T and RTA84T-B engines, which do not have cracks, even if fitted with "type b" girders, can continue to operate without any other action than visual inspection of the girders at prescribed intervals.
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