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Slamming of large high-speed catamarans in irregular seas

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French, BJ (2012) Slamming of large high-speed catamarans in irregular seas. PhD thesis, University of Tasmania.

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Abstract

Current design methods are limited in their ability to predict long term loading statistics
relating to wave loads and fatigue from prolonged cyclic loading. These methods either neglect
slam loads entirely or they are included as post-processed or simplified two-dimensional methods.
The work presented in this thesis introduces a combined theoretical-empirical approach
to determining long term load trends in realistic sea conditions during the initial stages of the
design spiral. This method builds on a previously developed non-linear time-domain seakeeping
strip theory method, for high-speed multi-hull vessels, using scale model testing in irregular seas
as a basis for an empirical slam module.
Towing tank experiments, using an extensively instrumented 2.5m hydroelastic scale model
wave-piercing catamaran representative of the 112m class Incat design, were used to develop a
database of slam events in a range of realistic (but idealised) irregular sea conditions. A total
of 2,103 slam events were identified over 22 test conditions during the scale model experiments.
Large slams generally occurred in the conditions where motions were largest; however significant
scatter was present with extreme events observed to be up to four times the median for most
conditions.
Occurrence rates were found to be a function of encountered wave frequency and significant
wave height. If the encountered wave frequency coincides with the motion resonance, slam
rates increased. Increasing the significant wave height also increased slam occurrences. A wave
height dependent slam threshold was identified by extrapolating occurrence rate trends with
decreasing significant wave heights. Pressure measurements also revealed that the cross-deck
structure was exposed to large local pressures at each measurement station, suggesting that ship
designers should ensure the structure can withstand large local loads along the entire length of
the bow.
The non-linear time-domain seakeeping program was extended to simulate motions and
loads in irregular seas and a method for constructing idealised wave spectra was developed as
an input to the seakeeping code. The extended code was verified by conducting a series of
program tests and then validated by comparing computational ship motions with results from
the scale model experiments in the absence of slamming.
A new module for predicting slam loads, based on a statistical analysis of scale model
tests was then developed and integrated into the extended time-domain seakeeping method,
allowing for the slam events to be determined on-line in the time-domain. Slams are identified
by defining a location dependant immersion threshold criterion based on the geometry of the
hull form combined with a stochastically determined variation originating from experimental
observations.
In the event of a slam, the maximum load and slam duration are determined by empirical
methods stemming from regression analyses on experimental data. Vessel forward speed and
relative vertical velocity at the centre bow truncation are used to predict the maximum slam
load. Slams loads are ramped up and down over a number of time steps. Forward speed
and immersion at the centre bow truncation are key explanatory variables in the duration
calculation. The slam load is then applied over a number of time steps according to its duration
at the location where it was first triggered.
A case study is finally undertaken demonstrating the application of this method. A highspeed
ferry service route was selected and real wave data used to determine expected wave
environments. The computer simulation was run for a range of sea conditions and slam events
are identified. Slam event statistics are then extrapolated to produce expectations for long
term (20+ years) loading expectations showing how this method could be a valuable tool when
considering the long term loading implications of a vessel for a particular route.

Item Type: Thesis (PhD)
Keywords: slamming, catamaran, high speed craft, irregular waves
Copyright Information:

Copyright 2012 the Author

Date Deposited: 14 Jan 2013 00:28
Last Modified: 24 May 2016 22:43
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