r106_p18-19_live testing of fatigue predictions for uscg.pdf
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8/10/2019 R106_p18-19_Live testing of fatigue predictions for USCG.pdf
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18 report 19report
Ingo Drummen
redicting a hulls fatigue lifetime
involves the prediction of hull
loading in a seaway. This is then
compared to the structural capacity. This type
of prediction requires input from a multitude
of disciplines, therefore in July 2007, the
USCG decided to initiate the VALID Joint
Industry Project, which is also known as the
Fatigue Lifetime Assessment Project (FLAP).
In order to ensure the best possible results
MARIN was keen to involve other stake-
holders and eventually the American Bureau
of Shipping, BEA Systems, Bureau Veritas,
Damen, Defence R&D Canada, DGA hydro-
dynamics, Lloyds Register, Northrop Grumman
Ship Systems and the Office of Naval Research
came on board.
The main goals of the project are to forecast
the structural maintenance needs of USCG
Cutters and to further improve the under-
standing of wave loading leading to fatigue
damage, and ultimately, to increase the con-
fidence level in predicting this phenomenon.
Dedicated trials The first step in the
project was the instrumentation of the ship
and during 2007/2008 the USCG Cutter
BERTHOLF was heavily instrumented, which
included long base strain gauges, unidirec-
tional strain gauges, accelerometers, fatigue
damage sensors and a wave radar. In total
about 150 sensors were eventually installed.
Dedicated trials were then conducted in
August 2009 and several tests were carried
out. Trials in waves were particularly
important with a total of 80 executed.
Trial conditions included several speeds and
relative headings in waves with a signifi-
cant height ranging between 1 m and 3 m.
These conditions are very suitable for the
validation of numerical tools and model
tests used for the purpose of fatigue lifetime
assessment.
Flexible model tests In 2010, model
tests were carried out in three phases with
a flexible model with a scale o f 25, resulting
in a length of 4.7 m. In order to make the
model flexible, it was cut into six segments
and each of these was connected to the
backbone. The properties of this backbone
were tuned in such a way that the two and
three node horizontal and vertical global
flexural vibration modes match those of the
BERTHOLF. These had been determined
during the dedicated trials. More than 300
runs in regular waves were performed in
different headings, speeds and wave
heights. In irregular waves about 60 tests
were done consisting of several runs in
significant wave heights of between 3 m
and 9 m, again with different headings and
speeds. Furthermore, 10 tests in multi-
directional, irregular waves were used to
compare to the trial results.
Finite element models Bureau Veritas
built three finite element models in 2009
and 2010; the coarse mesh model and the
first and second level refined model. The
coarse mesh model has around 70,000
nodes and 140,000 elements. And as part
of the first level refined model, the number
of elements in the amidships area was
increased by a factor of four. The first level
refined model comprised around 120,000
nodes and 200,000 elements. For the
second level refined mesh model a further
refinement was done in the vicinity of a
number of fatigue sensitive locations.
Eventually, this model consisted of about
190,000 nodes and 270,000 elements.
230 days analysed In a bid to evaluate
fatigue life prediction methodologies and
to forecast structural maintenance needs,
a 48-month monitoring campaign is being
performed. The monitoring campaign
got underway in early 2009 and over
the course of 2010 and 2011 analysis of
the data has been fully automated. At a
sample rate of 200Hz, 600Gb of data is
collected each month and data disks are
sent to MARIN. After processing, this
results in a deployment report, which
includes information about the e ncountered
environmental conditions, the adopted
operational conditions and the cumulative
damage per strain gauge amongst other
things. Up until now, three deployments
have been carried out and 230 days have
been analysed.
Live testing of fatigue
predictions for USCGOne of the core services ofMARIN is to carry out
measurement campaigns on
board ships at sea and to
analyse the collected data
for the development and
validati on of des ign too ls.
Currently, MARIN is carrying
out an extensive project
for the United States Coast
Guard (USCG), which aims
to increase confidence in
fatigue lifetime prediction.
P
Six segmented test model