INTERNATIONAL MARITIME ORGANIZATION
E
IMO
SUB-COMMITTEE ON STABILITY AND
LOAD LINES AND ON FISHING VESSELS
SAFETY
47th session
Agenda item 13
SLF 47/INF.7
11 June 2004
ENGLISH ONLY
REVIEW OF THE 2000 HSC CODE AND AMENDMENTS TO THE DSC CODE
AND THE 1994 CODE
Research into raking damage of high-speed craft
Submitted by the United Kingdom
SUMMARY
Executive summary:
An extensive research programme has been conducted into the
probable extent of raking damage for HSC built of aluminium alloy,
mild steel, higher tensile steel or FRP sandwich. This document
provides a summary of this work, in support of proposals to amend
the Code.
Action to be taken:
Paragraph 6
Related documents:
DE 40/8/4, DE 40/INF.2 and SLF 47/13
Introduction
1
As a result of a number of significant grounding incidents occurring to high-speed craft,
during the development of the 2000 HSC Code consideration was given to the potential for
extensive bottom raking damage.
2
The proposal in documents DE 40/8/4 and DE 40/INF.2 that the length of raking damage
should be related to the displacement, speed, hull material and hull scantlings was accepted in
principle, but the specific proposal made at that time was considered not to be sufficiently
substantiated, and simpler proposals were adopted at that time.
3
An extensive research programme has now been conducted by Cerup-Simonsen
Maritime AS in association with the Danish Technical University into the probable extent of
raking damage for HSC built of aluminium alloy, mild steel, higher tensile steel or
FRP sandwich, as summarised in the annex.
4
Based on this research, proposals to amend the 2000 HSC Code have been made in
document SLF 47/13.
For reasons of economy, this document is printed in a limited number. Delegates are
kindly asked to bring their copies to meetings and not to request additional copies.
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SLF 47/INF.7
-2-
5
For a more detailed description of the work conducted, administrations are invited to
contact MCA Policy Manager for High Speed Craft via [email protected] for an
electronic copy of the 35 page Summary Report.
Action requested of the Sub-Committee
6
The Sub-Committee is invited to note the information provided in conjunction with
document SLF 47/13.
***
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SLF 47/INF.7
ANNEX
SUMMARY OF RESEARCH INTO RAKING DAMAGE OF HIGH-SPEED CRAFT
Objective
The objective of the project is to develop and propose a deterministic rule for the required extent
of raking damage in the damage stability requirements of the IMO Code of Safety for
High-Speed Craft. In particular, it is the objective to determine a rule that rationally takes into
account the main parameters governing the extent of damage such as vessel speed, displacement,
structural dimensions, and building material.
Overview
The approach taken to this problem has been:
.1
The statistics of grounding damage have been examined. The statistics of
grounding accidents with HSC include a few high-profile accidents and several
minor accidents but the database is too scarce for a reliable statistical analysis.
Instead, a recently assembled database for the grounding of conventional ships has
been considered. It is assumed that the non-dimensional probability distributions
for impact speed, location, width and height of damage also apply to HSC. The
task is then to determine how the probability distribution for the length of damage
varies with vessel speed, displacement, structural dimensions, and building
material.
.2
A simplified and accurate prediction method for length of grounding damage has
been developed and validated by comparison to a large-scale grounding
experiment and a real-life accident with a VLCC. The method was developed by
fitting an analytical formula to a large series of Finite Element Method (FEM)
simulations of grounding impact with 12 different ships, and varying indentations
and rock geometries. To produce reliable FEM simulations a comprehensive
experimental program was carried out to determine material parameters, in
particular how to model the fracture of mild steel, aluminium, and high tensile
steel. The FEM simulation procedure was validated by comparison to a large
scale grounding experiment.
.3
A Monte Carlo simulation procedure has been established for development of
damage statistics for grounding accidents. The procedure takes into consideration
the kinetic energy of the ship and the crushing behavior of the bottom by using the
developed deterministic damage prediction method. The procedure was first
calibrated to accurately produce the damage statistics for conventional ships.
Then, by using the same procedure, damage statistics for HSC have been
developed. Finally, a simple analytical formula has been fitted to the damage
statistics to express the rule damage length as a function of the ship kinetic
energy, the raking resistance of the bottom, the width and height of damage in the
rule and the probability of survival.
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ANNEX
Page 2
.4
A final proposal for a raking damage length has been proposed and tested on
existing vessels. An acceptable probability level is proposed together with a nondimensional height and width of damage. Then a simple formula results for the
length of raking damage as a function of vessel speed, displacement, structural
dimensions, and building material. When applied to existing HSC it is found that
most fast catamaran vessels should be designed for full length damage while some
of the monohull vessels can be designed for smaller damage lengths. Vessels with
a service speed close to the minimum speed required in the HSC Code can be
designed with relatively short damage lengths.
Application of damage statistics for conventional ships to HSC
To determine realistic impact scenarios, the basic assumption of the present work is that certain
observations for conventional ships also apply to HSC. It is assumed that the probability
distributions for the following quantities are the same for conventional ships and HSC:
•
The vessel speed at the moment of impact divided by the service speed (V/Vs).
•
The location along the hull at which impact commences divided by the ship length,
(Xf/L).
•
The width of the damage (Bd) divided by ∇1/3 (∇ is volume of displacement).
•
The penetration of the damage (Hd) divided by ∇1/3.
The normalization of the width and height of the damage corresponds to the method currently
used in the HSC Code. The main reason for using this normalization and not for example the
width or draught of the ship is that the formula has to apply multi-hull vessels so a simple length
measure (like the beam) may not be representative for the vessel size in those cases.
The table below gives the key figures from the statistical analysis of 930 grounding incident
records.
No of records
Average ship length
Statistical mean
25%-percentage
50%-percentage
75%-percentage
Maximum observed
V/Vs
Bd / ∇ 1 / 3
H d / ∇1 / 3
122
97m
0.64
0.40
0.70
0.89
1.0
104
157m
0.17
0.018
0.078
0.20
0.93
77
168m
0.032
0.0065
0.016
0.041
0.14
Xf/L (from
aft)
167
136m
0.84
0.77
0.91
0.98
1.0
Ld /L
168
136m
0.27
0.047
0.15
0.45
1.0
Data was also collated on the principal particulars, materials and scantlings of 7 conventional
ships representative of those on the damage database (4 general cargo vessels between 50 m and
150 m length, and 3 tankers of between 125 m and 304 m length), and of 8 monohull HSC
between 23 m and 146 m length and built in FRP, aluminium alloy and high tensile steel, and of
7 catamaran high-speed craft between 24 m and 83 m length and built in FRP and aluminium
alloy. These data were to aid the selection of testing materials and for use in the finite element
and grounding force prediction methods.
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ANNEX
Page 3
Development of raking damage prediction method
Grounding events are obviously highly stochastic events. The analysis has to be carried out with
due consideration to these uncertainties, which for example implies that prediction of damage
length means prediction of the probability distribution for the damage length. The following
briefly describes how a deterministic prediction method has been established. Subsequent
sections describe how this method has been used in a probabilistic framework to determine
distributions for the damage length.
If the damage does not ‘run out’ of the aft end of the vessel, the damage length is equal to the
stopping length of the vessel. A balance of energy gives the maximum damage length as:
Ld ,max =
0.5MV 2
FH
(Equation 1)
where M is the displacement (including added mass in surge), V is the impact speed and FH is the
average horizontal force.
Given a specific vessel and an impact speed, the main difficulty in predicting the damage length
is to determine the horizontal raking force as a function of the rock penetration size, and the
speed, the structural dimensions and the building material of the craft.
The following formula for the horizontal grounding force (FH) is the result of a comprehensive
experimental, numerical and analytical study.
FH = 127 km σo (teq)1.17 (Bd)0.83
(Equation 2)
where km is a coefficient that accounts for the material ductility (= 1.0 for mild steel, 0.70 for
higher tensile steel and 0.58 for aluminium alloy), and σ0 is a measure of the yield strength of the
material, taken as the average between the initial yield stress and the ultimate tensile stress, teq is
the equivalent thickness of plating plus an allowance for stiffening and Bd is the breadth of
damage.
The formula was derived in the following way:
.1
Tensile tests were carried out for the three identified representative materials: mild
steel, HTS and aluminium (A5083). The finite element program LS-DYNA was
used to simulate the tensile tests. Values for the critical plastic strain for crack
initiation were derived for different element sizes for each material and plate
thickness.
.2
Special fracture mechanics tests were carried out, where cracks were propagated
approximately 400 mm under fully plastic conditions through shipbuilding plates
of 5 mm and 10 mm thickness of mild steel, HTS and aluminium. Combined
in-plane bending and extension and pure extension were tested.
.3
The finite element program LS-DYNA was used to simulate the fracture
mechanics experiments with the objective to determine so-called fracture strain as
a function of the element size. By calibrating the fracture strain it was found that
the fracture experiments could be accurately simulated.
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ANNEX
Page 4
.4
The finite element program LS-DYNA was set up to simulate grounding scenarios
where a ship bottom is forced to move horizontally over a conical rock.
A sensitivity study of the most uncertain parameters was conducted.
.5
The developed finite element procedure – including the calibrated fracture
modelling procedure - was validated by comparison to a large-scale test
performed by the Naval Surface Warfare Center (NSWC) in the United States.
Excellent agreement was found between experiment and FEM predictions.
.6
The developed finite element procedure was used to predict the horizontal
grounding force for twelve different vessels (four structures each with three
different materials) for lateral penetrations varying between 2% and 20% of the
design draught. Each bottom was deformed in a scenario like that of the NSWC
test, i.e. grounding on a conical rock with forward motion and a small trim angle.
This way the penetration and the damage width increase slowly in each
simulation.
.7
By use of the predicted horizontal forces for the twelve different vessels with
varying penetration and damage width, Equation 2 for FH was derived. This
simplified prediction method was calibrated by adjusting four free parameters to
minimise various measures of the error. By doing so, the average absolute error
between FEM results and the formula became 13%.
.8
Finally, the simplified formula was validated by comparison of the prediction of
the damage in two quite different accident scenarios: the large-scale test by
NSWC and grounding of a VLCC offshore from Singapore. Both validation
examples showed excellent agreement between predictions and real-life
observations. Recent further validation on a warship grounding showed an error
of 12%.
.9
To determine the raking resistance of FRP sandwich structures compared to metal
structures a comprehensive series of laboratory raking tests were carried out.
Rock widths of 75 mm and 150 mm were considered and bottom plates of 5 mm
and 10 mm thick aluminium, mild steel and HTS were tested. The conclusion was
that the raking resistance of aluminium structures and FRP sandwich structures
are comparable for similar sized craft.
In conclusion, the derived formula for FH is robust and at least as accurate as could be expected
in view of the complexity of the grounding phenomenon and the simplicity of the formula.
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ANNEX
Page 5
Grounding Damage Index
The Grounding Damage Index is a simple measure for comparing the kinetic energy and bottom
strength of different vessels. The GDI is given by:
GDI =
0.5MVs2
L FH
(Equation 3)
where L [m] is vessel length, M [kg] is vessel mass, VS [m/s] is vessel service speed and FH [N] is
the horizontal raking force calculated for Bd = 0.1 ∇1 / 3 .
Comparison with Equation 1 shows that the GDI is a relative damage length, Ld/L, calculated at
arbitrary values of the vessel speed and rock width. The absolute value of the GDI is not
particularly relevant, as it is associated with one particular speed (the full speed) and one
particular damage width (the 56%-fractile). GDI is characteristic for each vessel with regard to
the grounding damage, not only in the deterministic analysis but also determines the probability
distribution of the relative damage length.
The GDI varies between 0.34 and 1.1 for the conventional ships. The average GDI for
conventional ships is approximately 0.63. The minimum GDI for HSC is 1.2 (56m monohull
HTS) and the maximum is 5.5 (82 m aluminium catamaran).
The HSC catamarans generally show larger values of GDI than the monohull vessels. This is
partly due to the fact that they are shorter than a monohull of the same displacement (and kinetic
energy).
The HSC monohull vessels show a clear trend of increasing relative damage size with increasing
vessel size. This trend is less pronounced for catamarans.
HSC damage statistics by Monte Carlo Simulation
The purpose of the Monte Carlo simulations is to determine the probability that the horizontal
extent of damage (length and width) is less than the corresponding requirements in the rule.
First, the damage distributions are derived by use of Monte Carlo simulation. As mentioned, it is
assumed that the non-dimensional distributions for speed, location of damage front, height and
width of damage are the same for all ships but that the distribution for the length of damage will
be dependent on kinetic energy and raking resistance of the bottom. Finally, analytical
expressions for the probability of the damage being smaller than the rule requirements are fitted
to the simulation results.
It was found that that the calculated distribution for the longitudinal extent of damage is fully
characterised by the GDI. In other words, the ship speed, width, displacement, length etc. are not
necessary to determine the non-dimensional distribution for the longitudinal extent of damage.
The simulation procedure was calibrated to achieve agreement with the damage statistics for
conventional ships. It is then available to test any proposed rule requirement.
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ANNEX
Page 6
The probability (P) for the damage being smaller than the rule damage was determined for each
of 500 combinations of GDI and rule damages, using 20,000 grounding events in the Monte
Carlo simulation procedure. For each GDI and each accident scenario it was counted which of
the combinations of rule length and width would embrace the damage.
The probability (P) for the damage being smaller than the rule damage is finally given by:
H 
P = 1.67 1dR/ 3 
∇ 
0.30
 BdR 
 1/ 3 
∇ 
0.40
 LdR 


 L 
0.69
 1 


 GDI 
0.48
(Equation 4)
The formula should be used within the limits: 0 < H dR < 0.14∇1 / 3 and 0 < BdR < 1.0∇1 / 3
The coefficient and the exponents in this formula were determined by minimizing the maximum
deviation between the formula and the result of the Monte Carlo simulations.
Proposal for damage length requirement
Applying Equation 4 to the current rule (HdR = 0.04∇1/3, BdR = 0.1∇1/3 and LdR = 0.55L for
Category A craft) gives: P = 0.15 for GDI = 1.2, P = 0.10 for GDI = 3 and P = 0.07 for GDI = 6.
By increasing the rule requirements for height and width of damage to the maximum values
observed, i.e. HdR = 0.14∇1/3 and BdR =1.0 ∇1/3 these probabilities increase to 0.55, 0.36 and 0.26
respectively. These figures still do not offer a very satisfactory probability of survival.
Rearranging Equation 4, the rule damage length can be expressed in terms of the probability P,
the width and depth of damage in the rule and the GDI:
LdR
0.48 GDI 0.70 P 1.45
=
L
(H dR / ∇1 / 3 )0.43 (BdR / ∇1 / 3 )0.58
(Equation 5)
Equation 5 can be used to determine the raking damage length in the rules by setting the
survivability probability, P, and the width and height of damage. It is noted that the required
relative damage length is not proportional to the GDI. If it is assumed that the height and width
of damage in the rule is set to the maximum observed values, i.e. HdR = 0.14∇1/3 and
BdR =1.0 ∇1/3 then Equation 5 simplifies to:
 MVS2
LdR
= 1.12GDI 0.7 ⋅ P 1.45 = 0.69
L
 L ⋅ FH
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



0.7
⋅ P 1.45
(Equation 6)
SLF 47/INF.7
ANNEX
Page 7
The figure below shows the relation between the required damage length and the GDI at different
levels of survivability, P = R. Note that the GDI for HSC was found to be between 1.2 and 5.5
and that the HARDER project found the survivability to be above 0.5 for conventional
passenger ships.
1
P=0.8
P=0.7
P=0.6
P=0.5
P=0.4
0,9
0,8
Ld(Rule)/L
0,7
P=0.3
0,6
0,5
0,4
P=0.2
0,3
0,2
0,1
0
0
1
2
3
4
5
6
7
GDI
The proposed changes to chapter 2 given in SLF 47/13 are based on a 60% probability that
damage will not exceed the requirement.
Application of proposed requirement to existing HSC
The effect of this proposed rule on twelve existing HSC designs is shown in the table below, both
for actual speed, and if the speed just complied with the minimum required for application of the
Code.
Craft
Type
Hull
Material
Length
(m)
Mono
Mono
Mono
Mono
Mono
Mono
AL
AL
AL
HTS
HTS
HTS
23
56
95
56
94
146
Actual
Speed
(m/s)
12.86
15.95
15.43
15.95
18.52
19.03
Cat
Cat
Cat
Cat
Cat
Cat
AL
AL
AL
AL
AL
24
33
40
53
78
83
16.48
17.21
17.80
18.87
20.93
21.28
AL
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GDI
LdR/L
1.51
1.83
2.56
1.20
2.50
2.93
0.71
0.82
1.03
0.61
1.01
1.13
3.46
2.66
3.44
3.12
3.22
5.53
1.27
1.06
1.27
1.18
1.21
1.77
Min HSC GDI_min
speed
(min HSC
(m/s)
speed)
7.60
0.53
9.57
0.66
12.67
1.73
9.57
0.43
12.11
1.07
14.69
1.75
7.93
7.83
8.95
9.29
11.52
12.16
0.80
0.55
0.87
0.76
0.97
1.81
LdR/L_min
(min HSC
speed)
0.34
0.40
0.78
0.30
0.56
0.79
0.46
0.35
0.48
0.44
0.52
0.81
SLF 47/INF.7
ANNEX
Page 8
Administrations are invited to contact MCA Policy Manager for High Speed Craft via
[email protected] for an electronic copy of the 35 page Summary Report.
__________
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