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Bulk Carrier Safety
FSA Study
As part of the effort
to facilitate IMO’s work on measures to improve the safety
of bulk carriers, a progress report of the formal safety assessment
of single deck bulk carriers was presented by the U.K., coordinators
of this collaborative international effort.
At this stage, the
objective is to identify, from the initial list of wide ranging
hazards (150 hazards associated with over 800 initiating causes),
those high priority hazards based on risk ranking methodology.
Concurrent with this
effort, Japan presented its interim HAZID results carried out on
a Cape size bulk carrier of single side shell construction. Hazard
causes and effects were provided for four principle categories of
accidents: structural failure of a cargo hold, fore end of ship
and aft end of ship and water ingress through openings. Upon completion
in 2001, the study will be presented to IMO.
IACS presented the
results of its HAZID study on the watertight integrity of the fore
end of single side skin bulk carriers. Of the 51 hazards identified,
10 hazards were determined to pose an unacceptable level of risk
according to failure severity and frequency/probability of occurrence
criteria. Included in the 10 hazards were chemical damage due to
corrosive cargoes; mechanical damage either due to poor maintenance,
lack of controlled loading or failure to operate within allowable
structural limits and excessive slamming loads during ballast voyages.
The work by IACS and
Japan has been submitted to the U.K. Coordinator for consideration
in their effort. However, due to the late submission of the two
further HAZID lists, the risk ranking could not be completed by
the U.K. for this session of MSC.
It
was therefore agreed that work progressed during the interim should
be reported to MSC 73 (27 November – 6 December 2000). WP.9
Seakeeping Tests
The Committee was presented
with the model test results of the 1998 UK/EC Assessors’ Report
on the MV Derbyshire casualty. The report aims to determine the
sensitivity of green sea loads on hatch covers and deck wetness
in storm and hurricane sea states as a function of bow height and
forward reserve buoyancy.
Interim conclusions
presented were based on bulk carriers with B-60% reduced freeboards
and a standardized hatch coaming height of 2m. Subject to completion
of this ongoing work, tentative conclusions included:
- The forward hatch of Cape size bulk
carriers operating in severe storm conditions, as shown below
for a significant wave height (HS) in excess of 12.78m,
is more likely to experience loads in excess of those prescribed
in IACS UR S21, relative to Handy and Panamax sizes;
No.1 Hold – Maximum Measured Load vs Bow Height
(HS >> 12.78++m)
- Increasing bow height can reduce
fore deck wetness and green sea loads, but, as shown above, not
to the extent needed to avoid excessive pressures.
- Relative forward speed was determined
to be the most influential factor in terms of green water on deck.
As shown below, actual peak loads can exceed IACS requirements.
No.1 Hold – Maximum Measured Load vs Forward Speed
(HS = 12.78m)
- The Load Line Convention design
hatch cover loads should, for hatches aft of the foremost hatch,
be updated to incorporate a more appropriate margin to more directly
account for buckling and load effects and actual coaming height;
- Hatch cover loads from severe storm
beam seas do not appear significant in terms of an equivalent
water head. However, there may be substantial impact loads on
the coamings due to the horizontal velocity of the wave crest;
Several questions
concerning the possible conservatism of the model test results
were raised, particularly by IACS. IACS and the UK agreed to
collaborate, with the view to resolve the following issues:
- Proper simulation of hurricane wave
patterns and steepness;
- Use of extreme events and identification
of corresponding probability of occurrence;
- The extent to which loads from relatively
large hurricane waves can be generalized and applied to smaller
bulk carriers (in a comparative manner) recognizing that each
ship has its unique critical wave length and period and that smaller
ships will likely experience maximum loads in more moderate environments
than assumed in these tests.
When comparing
the equivalent water heads between the model tests and IACS
Unified Requirements, it was noted that due account should be
taken that the nominal pressures used in the IACS UR are established
solely to obtain the hatch cover scantlings and that safety
factors to ensure the structural integrity of hatch covers in
extreme weather conditions are applied.
Ultimately,
the MSC decided to task its SubCommittee on stability to further
evaluate these test results and the questions raised. Although
there was agreement in principle that hatch covers should be
designed for maximum peak loading that can be expected to be
encountered during their service life, Delegates also noted
that caution should be exercised when considering amendments
to long standing mandatory regulation based solely on one particular
research project. WP.9
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