8/10/2019 R106_p18-19_Live testing of fatigue predictions for USCG.pdf

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18 report 19report

Ingo Drummen

i.drummen@marin.nl

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