Structural optimisation for ice-strengthened vessels : R.A. Pedersen D.A. Molnes L.S. Stokkeland
2014
Consequently, it is of utmost importance for the
vessels operating in the conditions defined above,
that they can withstand the load resulting from the
ice-structure interaction. The latter may however
be in conflict with the economical feasibility of the
structural design. Previous studies have indicated
that a significant increase in structural weight can1 INTRODUCTIONThe growing interest in arctic sea-based transportation has increased in recent years due to the vast
amount of expected natural resources to be found
therein. In the context of sea-based transportation
the arctic environment may be defined as a sea area
with cold climate and at least partial ice-coverage
for a period of time. Thus, areas within the arctic
circle may be considered as such, but as a sole criterion this does not suffice since areas within the
Barents Sea are ice free on a year round basis while
the Baltic or the Caspian Sea is ice covered during
the winter period. Besides the general presence of
ice, which increases the resistance of the vessel and
induces local loads to the structure, the cold climate represents another challenge for the vessel’s
structure and systems. In other words, the knowledge of current and future ice and metocean condition at the site in question is limited. Prediction
models are associated with high uncertainties andoccur as a result of higher ice classes. In a study
of the structural integrity of cargo containment
systems in LNG carriers (Kwon et al., 2008), an
increase of about 4-6% was found when changing
scantling compliance from Baltic Class Ice 1A to
IACS Polar Class 7. This is a significant increase in
weight, especially since the classes are considered
to have equal performance. An increase of this
degree will for a merchant vessel result in a proportional reduction in payload capacity. This poses
a challenge in a conceptual engineering phase, as
equivalency between classifications does not necessarily translate to similar structural mass and
therefore cost. In an attempt to find an approach
to this complex problem, a method was suggested
in a report for the Krylov Shipbuilding Research
Institute (Appolonov et al. 2007). It suggests a
system of determining classification equivalency,
by comparing class requirements for frame cross
sectional area, with one spacing plate flange width,
in the ice belt. In the report it is noted, that the
problem of estimating ice strengthening structure weight is especially important for ships of
new types that do not have close analogies, such
as large Arctic tankers and LNG carrier. Another
approach comparing class equivalency was performed by the Helsinki University of technology
and Lloyd’s Register (Bridges et al, 2005) by comparing the principal scantlings between the Russian
Register Rules and the IACS unified requirements,
for a selected case study in the Russian Varandey
region. Similar to the Appolonov et al. (2007) only
the ice-strengthened regions are considered and
thereby the influence on the possible changes in
local and global scantlings outside these regions
have been neglected.
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