More research is required on the impact of high loading rates on
bulk carrier safety, according to leading classification societies.
Dr Jan Jankowski Board president of the Polish Register of
Shipping told DCI that studies of bulk carrier losses had revealed
a shortfall in research on the impact of loading on structural
safety. He called on classification societies and owner groups to
work together to bridge the information gap.
Extensive research by PRS has seen the society produce a
series of loading impact hypotheses and assumed theoretical
models but they required validation, he said. “A full-scale
experiment on an existing ship of the double bottom structure
response to the impact load caused by cargo falling and hitting
the bottom of the hold should be carried out,” he told DCI.
“The magnitude of the project — the theoretical models and
experiments — are complex endeavours that require the
co-operation of owners and classification societies.”
Research by PRS based on studies of historical casualties
dating back almost 30 years identified a number of reasons for
bulk carrier casualties. The need to adapt vessels for highdensity
cargo led to the concept of bulk carriers with very
rigid bilge and wing tanks with flexible sides between them
which in certain combination of wave and cargo loading
conditions had led to side failures.
PRS research, he explained, had revealed that the failure of
sides and then progressive collapse of bulkheads due to the
sloshing in flooded holds had been significant factors in a
number of bulk carrier losses.
Other factors contributing to the losses were: the collapse of
the first hatch cover due to green seas impact and the
progressive collapse of the bulkheads; the loss of ultimate
strength of the hull girders due to the certain combination of
wave and cargo loading; and the failure of Capesize bulk carrier
structures due to high loading rates.
The impact of loading at rates up to 30,000tph (tonnes per
hour) when the distance from the loader to inner bottom can be 40 metres remains an area of concern for Dr Jankowski and PRS and this is where future research should focus. “From that height the impact of three tonnes of cargo per one cubic metre affects the structure of Capesize carriers,” he added. “The failure of Capesize bulk carrier structures due to the high loading rate originates in still port waters. Then wave loads generated at sea cause further failure of the structure and the sinking of the ship.” The failure of designers to apply traditional forecastle on some of the biggest bulk carriers should also be rectified. The forecastle protects the ship forepart against the impact of green seas on the closing of openings and hatch covers. While any ship’s response to waves and the events during vessel operations are random, the modern approach to the development of safety standards relies on risk analysis. PRS has developed a fault tree of various sequential and parallel combinations of faults — events — that result in the sinking of the ship and the society has also developed mathematical models enabling simulations of particular events of the fault tree. “Such an approach enables us to convert the results of simulations into the probability distributions, to calculate the likelihood of structural failure, and to develop safety standards, comprising criteria, for designers,” said Jankowski. “The risk model describes the logical interrelations of the key events that lead to ships sinking. But
this is not a model of all possible causes of structural failure but
only those faults that contribute to an undesired event. It is a
qualitative model which can be evaluated quantitatively.”
PRS’ model for testing the impact of loading rates on bulk
carrier safety “assumes reasoning from general to specific, from
risk model comprising various scenarios to the criterion for
each event.”
He added: “This approach intends to harmonize ship
structures with wave loads and operational loads.”
He said that while maintenance was important to safe bulk
ship operation, the most important facet was design. “The term
‘quality of ship safety assurance’ frequently appears in various
situations, mainly with reference to the maintenance of ship
structure,” he added. “Maintenance is obviously important.
However, the core factor of quality ship safety assurance lies
with ship construction safety criteria and resultant safety
standards.
“Shortcomings in ship safety standards for ensuring safety at
sea may result in ships being launched with inherent flaws. This
generates problems for ship operations which are then usually
assigned to poor ship maintenance.”
He insisted that the assumptions, theories and benchmarking
of safety standards for ship construction should be available
publicly for verification. “Such an approach could give grounds
for constructive class competition in developing rules to obtain
more rational rule criteria based on the first principles.”
DNV is one classification society that has picked up the
baton on the impact of loading rates on both design and supply
chain efficiency.
Michael Aasland, DNV segment director for bulk carriers, said
the society was now conducting a surprisingly large number of
Specification Reviews to help owners ensure the design is as safe
as possible during the pre-contract phase of placing newbuilding
orders with yards. “The Specification Review includes
comparing the design with other designs using our Pre-Contract
Tool, and also EEDI calculations,” he said.
“We are also doing an increasing number of projects for
cargo owners. Here we, and the cargo owners, benefit from the
holistic view DNV has — looking at the complete logistical
chain, including the cargo operations.”
Safe and efficient loading are critical to DNV’s supply chain
analysis. One project has seen the society conduct full scale
measurements including strain gauging as well as advanced Finite
Element Modelling. “The results are positive so far, but need to
be analysed further before we can go further into the results,”
he said, adding that the loading process was key to the analysis.
“Safety and efficiency are very closely linked through the design
of the vessel. Ships that are not designed with focus on
efficiency and fitness for purpose may be operated in a manner
which is unsafe.
“A cargo owner will typically have other priorities when
optimizing the vessel, and DNV can help the owners prioritize.
This is particularly valuable for some cargo owners who have
not been very active on the ship owning side for some years.
“With regard to cargo operations the design must consider
both loading rate and number of pours as well as the vessel’s
longitudinal strength, double bottom strength, ballast capacity
and stripping system capacity. All these parameters are interrelated,
and must be viewed together.
“With regard to discharge we must consider the structural
design, to minimize e.g. trapped cargo — which can be a safety
issue during cargo discharge.”