Features: - Tanker Operator

INDUSTRY - FOCUS ON GERMANY
Predetermined breaking point. In a collision, the malleable inner hull of the tanker will bulge inwards without causing an oil spill.
(Graphics – Lindenau)
On any ship you will find the cage's
equivalent. It is formed by longitudinal
stringers and vertical frames. Stringers and
frames make up the skeleton that supports the
outer and inner hulls of the ship and which are
spaced about 2 m apart. The critical question
is how to enable the inner shell to separate
from this supporting cage, thereby enlarging
this space.
As far as the longitudinal stringers are
concerned, the solution is straightforward
enough- the stringers are welded only to the
outer hull and have no contract with the inner
hull. A more complicated matter is the case of
the vertical frames. A series of holes, known
as perforated pre-determined breaking points,
are made in the sections closest to the inner
shell. In a collision, the frames are intended to
rupture at these locations, thus releasing the
inner shell. The larger number of breaking
frames, the better.
This design concept was first developed by
Lindenau partner and former managing
director Guenter Stehn, who estimated that
frames may break over a length of 20 m along
a ship's body, allowing the austenitic inner
shell to bulge inwards by several metres
without rupturing. To ensure that this works,
tanks may not be fully loaded and must be
fitted with burst plates designed to break on
impact. The cargo then flows into a
neighbouring tank or ballast tank.
What made Stehn's approach appealing was
its simplicity. The vessel's inner shell deforms
without causing an oil spill. But how far can
this be taken before the ship's structure
becomes unstable? As it is, sailing in rough
seas is a punishing experience for a tanker,
exposing it to incessant assaults from different
load combinations, a constant interplay of
compressive and tensile stresses.
This relentless strain is particularly
aggressive around cut outs and holes where
cracks may form. Perforated pre-determined
breaking points are not exempt from this
threat. The critical question is, how should the
perforations be designed so the frames only
fail in a collision as intended while
safeguarding the structural integrity of the
vessel during normal operations?
In his dissertation completed under the
supervision of TUHH's Professor Eike
Lehmann, Tautz analysed the hidden pitfalls
of calculation methods that simulate the
planned failure of pre-determined breaking
points. The ductile yield of austenitic steel,
while known, does not help in determining the
highly important parameter of time of failure
when calculating collision simulations.
Knowing the time of failure is critical. If a
tank shell separation from the frames is too
late, the striking vessel might penetrate it.
No matter how many calculations are
performed, there is no substitute for physical
measurements and practical tests. To
accommodate this, Lindenau has requested
research funding from the German Federal
Ministry of Economic Affairs. The research
project aims at clarifying important aspects of
operational and collision safety.
Operational safety measurements have
already begun on a ship section built
according to the new design concept and
installed on the 44,700 dwt product tanker
Seychelles Patriot.
Seychelles Patriot. One side
section was built with integrated
predetermined breaking points.
(Photo – Lindenau)
August/September 2008 � TANKEROperator 25