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Structural Design and Response in Collision and Grounding
Alan Brown, Member, Virginia Tech, Kirsi Tikka, Member, Webb Institute, John C. Daidola,
Fellow, AMSEC LLC, Marie Lützen, Student Member, Technical University of Denmark, Ick-
Hung Choe, Student, Pusan National University
Abstract
The results summarized in this paper represent the work of SNAME Ad Hoc Panel #6 convened under
the SNAME Technical and Research Program. This is a summary and overview paper. Topics
discussed will be addressed individually and in more detail in later publications. The 2nd
International Conference on Collision and Grounding of Ships, to be held in Copenhagen, July 1-3,
2001, will also present and discuss many of the results of this panel and other related research. The
paper discusses four primary areas of panel work: collision and grounding models, data, accident
scenarios and design applications. A probabilistic framework for assessing the crashworthiness of
ships is presented. Results obtained from various grounding and collision models are compared to
validating cases and to each other. Data necessary for proper model validation and probabilistic
accident scenario development are identified. Deformable striking-ship bow models and their
application are described. Potential design applications and alternatives for improving
crashworthiness are discussed.
1 INTRODUCTION displacement, striking ship bow characteristics,
impact point, bottom, rock characteristics, rock
On May 14, 1998, SNAME Ad Hoc Panel #6, Structural height above baseline, sea state, ice mass, etc.
Design and Response in Collision and Grounding, was • Define standard criteria and methodologies for
applying structural performance in grounding and
formed under the SNAME T&R Steering Committee with
collision including oil outflow, carriage of nuclear
support from the Hull Structure Committee, Ship Design
waste, damaged stability, ultimate strength and
Committee and Marine Safety and Environmental
serviceability. Provide a basis for performance
Protection Panel (O-44). The objectives of the panel are: specifications in these areas.
• Investigate, compare and assess tools for predicting • As tools, standard scenarios, methodologies and
grounding and collision damage (including collision performance criteria are evaluated and defined, apply
with ice). Emphasize simplified methods, which can them to a matrix of typical and innovative structural
be readily applied to the design and rapid analysis of designs and concepts. Identify promising concepts
new structural concepts, including applications using for future study.
probabilistic scenarios. Finite element and other
This paper is a summary report of work performed by
more rigorous methods may be considered and used
the panel. It is the first of the deliverables specified in the
to validate simplified methods. Include comparisons
to available data. panel’s charter. Additional papers and reports will
follow. The panel has three working groups studying: 1)
• Propose a format for a collision and grounding
database. The database may include actual, tools for predicting damage in grounding and collision; 2)
experimental and simulated data as long as it is collision and grounding scenarios; and 3) innovative
properly identified. Gather initial data. design concepts. Funded research is centered at Virginia
• Define standard scenarios and conditions for Tech and Webb Institute.
grounding and collision analyses. This may include The work of the panel, as presented in this paper,
probabilistic and/or deterministic descriptions of ship consists of four essential elements:
speed, trim, collision angle, striking ship • Benchmarking of grounding models
Other Members of the Ad Hoc Panel contributing to this paper are:
Donghui Chen Jeom Paik Bo Simonsen Surya Vakkalanka Ge Wang Rong Huang Rick van Hemmen
John Sajdak Arne Stenseng Brett Fox Ryszard Kaczmarek Peter Gooding
• Benchmarking and development of collision fb2
LONGITUDINAL EXTENT FOR BOTTOM DAMAGE
models 5.0
•
4.5
Definition of grounding and collision scenarios
4.0
including striking bow geometry and bow
Probability Density
Tanker Data - 63 Cases
Piecewise Linear Fit
stiffness. 3.0
• Innovative design concepts 2.0
1.0 0.5
2 MOTIVATION 0.5
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
The serious consequences of ship grounding and collision Damage Length/Ship Length
necessitate the development of regulations and
requirements for the subdivision and structural design of Figure 1 - Damage Probability Density Function [4]
ships to reduce damage and environmental pollution, and
improve safety.
The International Maritime Organization (IMO) is A major shortcoming in IMO’s current oil
responsible for regulating the design of oil tankers and outflow and damage stability calculation
other ships to provide for ship safety and environmental methodologies is that they do not consider the effect of
protection. Their ongoing transition to probabilistic structural design or crash-worthiness on damage
performance-based standards requires the ability to extent [5,6]. The primary reason for this exclusion is
predict the environmental performance and safety of that no definitive theory or data exists to define this
specific ship designs. This is a difficult problem relationship.
requiring the application of fundamental engineering
principles and risk analysis. Other deficiencies include:
IMO first introduced probabilistic standards in
damage stability regulations for passenger ships [1] and • Damage pdfs consider only damage significant
later for cargo ships [2]. IMO’s first attempt to apply a enough to breach the outer hull. This penalizes
probabilistic methodology to tankers was in response to structures able to resist rupture.
the US Oil Pollution Act of 1990 (OPA 90). In OPA 90 • Damage extents are treated as independent random
the US required that all oil tankers entering US waters variables when they are actually dependent variables,
must have double hulls. IMO responded to this unilateral and ideally should be described using joint pdfs.
action by requiring double hulls or their equivalent. • Damage pdfs are normalized with respect to ship
Equivalency is determined based on probabilistic oil length, breadth and depth when damage may depend
outflow calculations specified in the "Interim Guidelines to a large extent on local structural features and
for the Approval of Alternative Methods of Design and scantlings vice global ship dimensions.
Construction of Oil Tankers Under Regulation 13F(5) of
Annex I of MARPOL 73/78” [3], hereunder referred to as It is logical and essential that crashworthiness is
the Interim Guidelines. considered in oil outflow and damage stability
All of these regulations use probability density calculations. An analytical method is required to
functions (pdfs) to describe the location, extent and define the relationship between structural design and
penetration of side and bottom damage. These pdfs are damage extent in collision and grounding. This is a
derived from limited historical damage statistics [4], and primary objective of the work of this panel.
applied identically to all ships without consideration of A method that considers crashworthiness must be
their structural design. Figure 1 is the IMO probability sensitive to at least the basic parameters defining unique
density function used to define the longitudinal extent of structural designs while maintaining sufficient generality
bottom damage from grounding in oil outflow and simplicity to be applied by working engineers in a
calculations. The histograms represent the statistical data regulatory context for a ship in worldwide operation. It
collected by the classification societies and the linear plot should not require detailed finite element analysis or be
represents IMO's piece-wise linear fit of the data. Other limited to a single accident scenario.
IMO pdfs are constructed in a similar manner. An additional advantage of performance-based
models is the potential for their application to
different and innovative designs. This is a second
primary objective of this panel.
2
3 PROBABILISTIC FRAMEWORK A damage pdf generated in an early application of
this approach is shown in Figure 3 [6]. This pdf was
Figure 2 illustrates the process proposed to predict the generated for a MARPOL single hull tanker typical of the
struck ships represented in the data used to develop the
probabilistic extent of damage in collision as a function of
current MARPOL damage pdfs.
ship structural design [7]. A similar process is proposed
for grounding.
4 GROUNDING MODELS
Struck ship design variables:
Type (SH,DH,IOTD,DS,DB,DS) 4.1 Background and Plan
LBP, B, D
Speed & displacement
Subdivision
Structural design
The Exxon Valdez accident in 1989 and the regulatory
events following it lead to active research on structural
behavior in grounding. The earlier studies had been
Monte Carlo
Extent of mostly empirical, the best-known being a study by Card
Damage
Probability given collision Simulation Specific Calculation
[8] who surveyed 30 grounding incidents to determine the
Point puncture, raking puncture, collision
penetrating collision scenario's joint pdf for
effectiveness of a double bottom in reducing pollution.
Pdf's:
Striking ship speed
longitudinal, vertical and
transverse extent of
More recent studies have included large numerical
Striking ship displacement
Striking ship draft & bow height
damage: simulations, small and large scale experiments, and the
Striking ship bow shape and stiffness Pdf parametrics for extent
Collision striking location & angle of damage as a function
Regression
analysis
development of simplified methods. Many of these
of struck ship design
studies were supported by the project “Protection of Oil
Spills from Crude Oil Tankers” carried out by the
Figure 2 - Process to Predict Probabilistic Damage [7] Japanese Association for the Structural Improvement of
The process begins with a set of probability density Shipbuilding Industry (ASIS). In the United States, the
functions (pdfs) defining possible grounding or collision Carderock Division of the Naval Surface Warfare Center
scenarios. Using these pdfs, a specific scenario is (NSWCCD) conducted grounding experiments, and the
selected in a Monte Carlo simulation, and combined with MIT-Joint Industry Project on Tanker Safety developed
a specific ship structural design to predict damage. This software to analyze structural damage in grounding.
Development of analytical models for the software was
process is repeated for thousands of scenarios and a range
supported by experimental studies. Active research effort
of structural designs until sufficient data is generated to
on grounding analysis has been carried out also in
build a set of parametric equations relating probabilistic
Europe, mainly in Denmark and the Netherlands.
damage extent to structural design. These parametric The Specialist Panel V.4 of the 1997 International
equations can then be used in oil outflow or damage Ship and Offshore Structures Congress (ISSC 1997) [9]
stability calculations. reviewed the state-of-the-art of the research and
Bottom Damage Longitudinal Extent PDF concluded that nonlinear finite element analysis coupled
12 with calculation of ship motions had reached a level at
which a fairly accurate prediction of the structural
10 response was possible. However, the report noted that
DAMAGE Generated Points since the method is time consuming and requires a high-
8 MARPOL Standard level of expertise, it is not suitable for the design or
regulatory environment. The report also concluded that
the simplified methods that existed at the time required
6 further validation.
4.5 This study concentrates on evaluation of the existing
4 simplified methods, which have been further developed
since the ISSC 1997 report. The objective is to assess
2
their suitability for design and regulatory work.
0.5
0 0.5 4.2 Models Included in the Current Study
The methods evaluated in this study include DAMAGE, a
-2
0 0.2 0.4 0.6 0.8 1 software tool developed by the MIT-Joint Industry Project
Length of Damage / Ship Length on Tanker Safety, and a simple analytical method
Figure 3 – Bottom Damage pdf [6] developed by Dr. Wang at the University of Tokyo
(method by Wang). Both methods calculate grounding
3
force and extent using closed-form solutions, but they event, the contact force between ship and ground induces
differ in the detail in which the structure is defined and its heave, roll and pitch motion on the ship, and eventually
behavior is analyzed. The methods also differ in their causes the ship to stop.
treatment of ship motions. The program DAMAGE was The primary mechanisms of energy dissipation are:
selected for further testing and analysis, because it is
applicable for a wider range of grounding scenarios. 1. A change in potential energy of the ship and the
surrounding water as the ship is lifted by the
ground reaction.
4.2.1 DAMAGE [10]
2. Friction between the ground and hull.
The computer program DAMAGE was developed at MIT 3. Deformation and fracture of the hull.
under the Joint MIT-Industry Program on Tanker Safety.
Since the lifting of the ship off the rock generally causes a
This project, lead by Professor Tomasz Wierzbicki, was
reduction in crushing and tearing forces it is important to
initiated in 1991, and in addition to the program
include the coupling between the global ship motions
DAMAGE the project has produced more than 70
(external dynamics) and the local damage process
technical reports about prediction of grounding and (internal mechanics). Detailed descriptions of the model
collision damage. The program DAMAGE Version 5.0 for the external dynamics can be found in References [11]
can be used to predict structural damage in the following and [12].
accident scenarios: For the model to be applicable to optimization for
• Ship grounding on a conical rock with a structural crashworthiness it is important that it captures
rounded tip (rigid rock, deformable bottom) the effect of :
• Ship-ship collision (deformable side, • Material strength and ductility
deformable bow) • Dimensions and arrangement of major structural
Compared to previous models for prediction of grounding components such as bulkheads, decks, girders
and collision damage, a major advantage of DAMAGE is
and large stiffeners.
that the theoretical models are hidden behind a modern
The theoretical model is based on a set of super-element
graphical user interface (GUI) [10]. The program has
solutions, i.e. closed form, analytical solutions for the
been developed with the objective of making crash
energy absorption of rather large structural components
analysis of ship structures feasible for engineers that do
undergoing large deformations. All structural components
not have any particular experience in the field of
are assumed to follow the same overall mode of
crashworthiness. Figure 4 shows the input screen for deformation around the rock. This way it is possible to
defining the “ship-ground interaction”, the relative take into account the structural resistance of all typical
position and velocity of ship and ground, coefficient of
members in a ship bottom. The fracture criterion is based
friction, etc.
on analytical solutions for plastic response of a
membrane. Figure 5 shows a window from an animation
of the ship motion over the rock.
A description of the procedure including detailed
derivations of the super-element solutions can be found in
References [13] and [14].
Figure 4 - Input screen for defining the “Ship-ground
Interaction” in the program DAMAGE [10]
Initially, the ship is assumed to be on a straight-line
course with a known velocity and trim. The rock is at
some distance to port or starboard of the ship’s centerline, Figure 5. DAMAGE simulation of ship motion over a
and at some height above the ship’s baseline. During the rock (only selected structural elements are shown).
4
4.2.2 Method by Wang [15] type of obstruction (pinnacle only), and the structural
model (the structure is modeled for the cargo block only).
Dr. Wang developed a method to predict structural
resistance in raking type grounding in his doctoral work 4.3.1 Validation of DAMAGE
under Professor Hideomi Ohtsubo’s supervision. The
method was further developed and published in 1997 Simonsen presented verification of the theory behind
[15]. DAMAGE models by comparing calculated results with
US NAVY 1/5-scale grounding experiments, with large-
scale grounding experiments carried out in the
Netherlands, and with an actual grounding of a VLCC
[14,16].
Based on the limited number of validation cases
DAMAGE is found to predict the damage extent well. In
the case of a VLCC grounding, the difference between the
calculated damage length of 177 meters and the observed
damage length of approximately 180 meters is only 1.7%.
The result is sensitive to the transverse location of the
rock relative to the ship’s centerline as is illustrated in
Figure 7. As the rock moves away from the centerline the
effect of ship motions increase and the predicted damage
length increases.
DAMAGE predictions for average rock penetration
and grounding force are also excellent. Predictions for
Figure 6 - Four Failure Modes in the Method by Wang minimum and maximum values are not as good. Figures
8-10 illustrate a comparison of DAMAGE results with a
The method includes closed-form solutions for four large-scale grounding test carried out in the Netherlands
failure modes: stretching (beam mode), denting, tearing by ASIS. Large differences in the penetration at the
and concertina tearing illustrated in Figure 6. initial stages of grounding are probably due to
The rock is modeled as a wedge, and the ship’s simplifications in the global motion calculations.
bottom structure is modeled with periodic structural 5
members where the period corresponds to transverse e=0
Rock Penetration (m)
frame spacing. Only horizontal ship motions are
4
considered in the calculation.
The grounding damage is calculated by combining e=5
the failure modes to model periodic resistance of the 3
structure. The resistance of the transverse structure is
predicted with a beam model. The denting failure mode is e=10
assumed for the plate immediately behind the transverse 2
-50 0 50 100 150
structure. As the wedge advances in the plating and a
Longitudinal Position (m)
crack develops at the tip of the wedge, the rupture is
modeled by tearing failure mode. Concertina tearing is
used to model the accordion like behavior of the plating, Figure 7 - Rock Penetration vs. Rock Eccentricity
which can lead to cracks at other locations than the tip of 1.4
the wedge. The entire calculation can be carried out by 1.2 DAMAGE Measured
hand or with a simple spreadsheet program, but the
Vertical Penetration (m)
1
analyst must decide how the failure modes are combined
to achieve the final damage. 0.8
0.6
4.3 Application
0.4
DAMAGE was selected for further testing and analysis, 0.2
because of its applicability to a wider range of grounding
0
scenarios. Although the method by Wang is elegant in its 0 0.5 1 1.5 2 2.5 3
simplicity, its application in the current formulation is Time (sec)
limited to raking type damage only. The major
limitations of the current version of DAMAGE are the Figure 8 - Vertical Penetration (ASIS Test 2)
5
3 D A M AG E M easured
DAMAGE Measured 2
2.5
Horizontal Force (MN)
2 1.5
Vertical Force (MN)
1.5
1
1
0.5
0.5
0 0
0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3
Time (sec) Tim e (sec)
Figure 9 - Comparison of Horizontal Forces (ASIS Test Figure 10 - Comparison of Vertical Forces (ASIS Test 2)
2)
Figure 11 - Sketch of Four Grounding Scenarios
4.3.2 Sensitivity Analysis extent, rather the objective was to investigate the
sensitivity of the DAMAGE model to the parameters
The sensitivity of DAMAGE results to changes in defining the grounding event.
grounding parameters was studied and presented in [17]. As can be expected the results are very sensitive to
The base ship used in the sensitivity analysis was a the ground characteristics and to the parameters defining
237,000 DWT single-hull VLCC and the grounding case the ship-ground interaction (rock elevation, rock shape
studied corresponded to the VLCC validation case. and transverse location, ship velocity and displacement,
In the sensitivity analysis only one parameter was friction coefficient and trim angle).
changed at the time and the other parameters were kept at The parameters affecting the global ship motions
their initial values. The objective of this analysis was not (longitudinal center of floatation and the longitudinal and
to study the effect of the structural design on the damage the transverse metacentric height) had little effect on
6
damage results. Surprisingly, the tank spacing as well as The steel weight (longitudinal structure only) was
the characteristics of both the transverse bulkhead and the calculated for each design, and the effectiveness of a
longitudinal bulkhead had very little effect on damage design modification was measured in terms of the
results. The results were also not very sensitive to reduction in inner bottom rupture as a function of the steel
material characteristics. weight. Changes in the transverse structure caused by the
Damage results were sensitive to the thickness of the modifications were not taken into account.
outer bottom. Reducing the spacing and increasing the Some conclusions based on the selected scenarios
scantlings of longitudinal girders and transverse floors include:
also affected damage results, but not as effectively. The • In the low rock elevation case, which represents
same applied to longitudinal stiffeners. raking type grounding, none of the designs had inner
bottom rupture. At service speed the outer bottom
ruptured throughout the entire length of the ship. In
4.3.3 Structural Modifications
this type of scenario, the structural modifications
DAMAGE was found to provide a good tool for have little impact on the damage extent and no effect
comparative studies based on the validation cases and the on the oil outflow assuming that the vessel survives
sensitivity analysis. The next step in the study was to the raking damage.
investigate the effect of structural modifications on the
damage extent in selected grounding scenarios. The study • In the sharp rock tip, high-rock elevation case the
was limited to eight grounding scenarios, and the rock penetrated through the entire cargo block in all
structural changes were limited to changes in scantlings designs at service speed. Even at port speed all
of a conventional double-bottom structure. The analysis designs had serious damage. However, increased
was intended to provide insight into the effects of the outer bottom or inner bottom thickness, two
modifications and scenarios to help design a probabilistic additional girders, or reduced spacing of transverse
analysis, which will be the next step in the study. floors did reduce outer bottom and inner bottom
The base ship used in the analysis was a 150,000 rupture. This was the only grounding scenario where
DWT double-hull tanker. The grounding scenarios were increasing double bottom height had very little effect
selected to represent high, medium and low rock on the inner bottom rupture.
elevations relative to the ship’s baseline. One of the • In high and medium rock elevation cases most of the
scenarios had a sharp rock tip (30 degree semi-apex design modifications improved structural
angle), whereas the shape of the rock was kept constant in performance. Reducing the double bottom height
the other scenarios (45 degree semi-apex angle). Two resulted in the worst performance. The design with
velocities were analyzed: 7 knots to represent port speed two additional longitudinal girders performed best in
and 14 knots to represent service speed of the tanker. The reducing inner bottom rupture, but it also had the
rock shapes and elevations are illustrated in Figure 11. heaviest steel weight among the studied designs. If
More details on the base ship and the grounding scenarios the steel weight was taken into account, increasing
can be found in [17]. the thickness of the outer bottom was found to be
Eight structural modifications, all designed according most effective in reducing the inner bottom rupture.
to the requirements of ABS SafeHull (97/98), where Increasing both the inner bottom and outer bottom
investigated. The scantlings met the minimum thickness reduced the inner bottom rupture more but
requirements except for the dimension under at the cost of a higher steel weight.
investigation. The modifications included:
1. Increase in the outer bottom plate thickness. 4.4 Ongoing and Future Work
2. Increase in the inner bottom plate thickness. The work discussed above was carried out to provide a
3. Increase in both outer bottom and inner bottom basis for a probabilistic analysis, which will take into
plate thickness. account a range of possible grounding scenarios.
However, to add credibility to the results further
4. Additional longitudinal girder. validation is needed. Data needs for validation work will
5. Two additional longitudinal girders. be discussed later in the paper. Further review of the
results from the presented validation cases is ongoing.
6. Reduced spacing of transverse floors. The analysis on the effect of structural modifications
7. Decrease in double bottom height. discussed above measured the performance in terms of
the damage extent without taking into account the
8. Increase in double bottom height. subdivision of the vessel. Therefore the observed changes
in the damage extent may have no effect on the resulting
oil outflow. To determine the effect of the structural
7
modifications on oil outflow the DAMAGE calculations • Low energy raking puncture
should be coupled with outflow analysis. • Penetrating Collision – right angle and oblique
The next step in the study is to carry out a sufficient to penetrate outer hull with significant
probabilistic analysis of damage in grounding scenarios damage extending in at least 2 directions
defined by probability density functions of the input (penetration, horizontal, vertical)
parameters. This analysis will be coupled with outflow Work thus far is focused on models for penetrating
calculations. The objective of the study will be to provide collision.
a tool for comparative studies between vessel designs. Each collision model must consider two very
different aspects of the collision event:
• External problem – includes the global
5 COLLISION MODELS characteristics of the striking and struck ships and
their motion before, during and after collision;
5.1 Background and Plan
• Internal problem - includes the internal deformation
In 1979, the Ship Structure Committee (SSC) conducted a mechanics and structural response of the struck ship
review of collision research and design methodologies and the striking ship bow during collision.
[18]. They concluded that the most promising simplified The four models studied by the panel take different
collision analysis alternative was to extend Minorsky’s approaches to solving and coupling these two problems.
original analysis of high-energy collisions by including Although not a formal validation, comparison and
consideration of shell membrane energy absorption. A agreement between these models provides useful insight
simple and fast model is important in probabilistic into their performance and increases confidence in the
analysis because thousands of different scenarios must be validity of their results.
run to develop statistically significant results.
5.2.1 Simplified Collision Model (SIMCOL) [22]
A more recent review of the literature and of the
applicability of available methods for predicting structural
SIMCOL uses a time-domain simultaneous solution
performance in collision and grounding was made in the
of external dynamics and internal deformation mechanics
1997 International Ship and Offshore Structures Congress
similar to that originally proposed by Hutchison [23].
(ISSC 97) by Specialist Panel V.4 [9]. Their report states:
Figure 12 shows the SIMCOL simulation process. The
“Knowledge of behavior on a global level only (i.e., total
external sub-model uses a three degree-of-freedom
energy characteristics like the pioneering Minorsky
system for ship dynamics shown in Figure 13. The
formula) is not sufficient. The designer needs detailed
internal sub-model determines reacting forces from side
knowledge on the component behavior (bulkheads,
and bulkhead structures using detailed mechanisms
girders, plating, etc.) in order to optimize the design for
adapted from a 1981 Rosenblatt study [24,25]. It
accident loads.”
determines the energy absorbed by the crushing and
The approach taken in this study is to progressively
tearing of decks, bottoms and stringers using the
increase the complexity of SIMCOL (Simplified Collision
Minorsky correlation [26] as modified by Reardon and
Model), the Ad Hoc Panel’s baseline collision model,
Sprung [27].
until results with sufficient accuracy and sensitivity to
design characteristics are obtained. In order to assess the At time step i
model’s consistency and sensitivity, the model is applied
in a series of collision scenarios with a range of tanker Based on current velocities, calculate the
next positions and orientation angles of the
sizes and designs. SIMCOL results are compared to ships, and the relative motion at impact
results obtained using three other collision models:
Calculate the change of impact location
DAMAGE [10,19], ALPS/SCOL [20,21], and a Technical along the struck ship and the increment of
penetration during the time step
University of Denmark (DTU) model. The DTU model
and ALPS/SCOL were also further developed during this Calculate the average reaction forces during
the time step by internal mechanisms
study. Determining the ideal trade-off between simplicity
and sufficiency is a primary concern of this process. Calculate the average accelerations of both
ships, the velocities for the next time step,
and the lost kinetic energy based on external
ship dynamics
5.2 Models Included in the Current Study
Go to the next time step: No Yes Calculate maximum
Meet stopping
Three types of collision were identified for investigation i = i+1 criteria ?
penetration and damage
length
by the panel:
• Low energy puncture at a point Figure 12 - SIMCOL Simulation Process
8
y
beginning of time step n
Striking Ship
P4,n+1, P5,n+1 P1,n P1,n+1
G2 ξ P4,n, P5,n side shell
′
φn P2,n
φ
Striking Ship
θ2 α
P2,n+1
end of time step n
G1 P3,n
damaged area during
θ1 Struck Ship P3,n+1
time step n
Struck Ship
l
η
Note: The positive direction of angle is always
counterclockwise.
x
Figure 14. Sweeping Segment Method
Figure 13. SIMCOL External Ship Dynamics
Web frames acting as a vertical beam
V.U. Minorsky conducted the first and best known of distort in bending, shear or compression
the empirical collision studies based on actual data [26].
His method relates the energy dissipated in a collision
event to the volume of damaged structure. Actual
collisions in which ship speeds, collision angle, and Strike at web Strike between
frame web frame
extents of damage are known were used to empirically
determine a linear constant. This constant relates damage
volume to energy dissipation. In the original analysis the
Analyze each shell Analyze each shell
collision is assumed to be totally inelastic, and motion is
separately separately with
limited to a single degree of freedom. Under these consistent with nodes consistent
assumptions, a closed form solution for damaged volume web deformation. with web
can be obtained. With additional degrees of freedom, a deformation.
time-stepped solution must be used.
Crake and Brown developed SIMCOL Version 0.0 as
part of the work of SNAME Ad Hoc Panel #3 [6,28].
Based on further research, test runs and the need to make
the model sensitive to a broader range of design and
scenario variables, improvements were progressively
made by Chen and Brown at Virginia Tech [22]. A Figure 15. Web Deformation in SIMCOL 2.0 [24,25]
sweeping segment method, shown in Figure 14, is added
to the model in SIMCOL Version 1.0 to improve the 5.2.2 DAMAGE 4.0 [19]
calculation of damage volume and the direction of
The DAMAGE 4.0 collision module solves the external
damage forces. Models from the Rosenblatt study [24,25]
problem uncoupled from the internal problem, and applies
are applied in Version 1.1 assuming rigid web frames. In the calculated absorbed energy to plastic deformation of
Version 2.0, the lateral deformation of web frames is the struck ship. Structural components, motions, masses
included as shown in Figure 15. In Version 2.1, the etc. are described in ship coordinate systems local to each
vertical extent of the striking ship bow is considered. ship and in one global coordinate system. Degrees of
Table 1 describes the evolution of SIMCOL over the freedom in DAMAGE include striking ship surge and
course of the study thus far. struck ship sway and yaw.
Design data required for the striking ship includes The following assumptions are applied in DAMAGE
bow half-entrance angle, bow height, length, beam, draft 4.0:
and displacement. • Both ships are perpendicular before and during
Scenario data required includes striking and struck impact, i.e. only right angle collisions are considered.
ship velocity, collision angle, and longitudinal location of • The forward motion of the struck ship is assumed to
impact in the struck ship. be zero.
9
Table 1. SIMCOL Evolution The model for the internal mechanics is based on the
direct contact deformation of super-elements. The super-
Version 0.1 1.0 1.1 2.0 2.1
elements used to model the side in DAMAGE are:
Simulation Simulation in time domain
External Model Three degrees of freedom
• Shell and inner side plating (laterally loaded
(Hutchison and Crake) plastic membrane)
Horizontal Minorsky mechanism as re-validated by Reardon and Sprung • Deck and girder crushing
Members
Crake’s Sweeping segment method to calculate damaged area
• Beam loaded by a concentrated load
model and resulting forces and moments • X-, L- and T-form intersections crushed in the
Vertical Jones and Van Mater McDermott / Rosenblatt Study methods axial direction
Members
w/o rupture Crake’s Van Does not Considers Striking
of plate model Mater’s consider deforma- bow with The bow geometry is defined by eight parameters.
Internal Model
(Jones) extension deforma- tion of limited Figure 16 shows an example the bow geometry.
of Jones tion of webs, depth
webs, friction
friction force and
force and the force to
5.2.3 ALPS/SCOL
the force to propagate
propagate yielding
yielding zone ALPS/SCOL is a coarse-mesh 3-D non-linear finite
zone element code using super-elements based on the Idealized
Vertical Neglected Minorsky method for Structural Unit Method (ISUM) [20,21]. The geometry of
Members calculating absorbed the striking and the struck ships is described in a global
w/ ruptured energy due to
plate longitudinal motion (three-dimensional) rectangular coordinate system. The
stress in an ISUM unit is described in a local element
coordinate system. ALPS/SCOL considers sway and yaw
In DAMAGE 4.0, the striking ship bow is assumed to
of the struck ship with the following assumptions:
be rigid. DAMAGE Version 5.0 (Released June 2000)
• The added masses of the striking and the struck ships
includes crushing of the bow.
are calculated based on ships of similar type and size
Based on conservation of linear momentum, angular
momentum and energy, the velocities after the impact are using a linear strip theory-based computer program.
calculated as well as the loss of kinetic energy, which is • The striking ship is assumed to be rigid.
available for the structural deformations. • The analysis of the external and the internal
In order to determine the deformation of the bow and dynamics is undertaken separately.
the side, the striking ship is moved into the struck ship in • The longitudinal velocity of the struck ship is not
small increments. In each increment, the total resistance considered.
forces from crushing of the bow and penetration into the • Since ALPS/SCOL is based on a simplified 3-D
side are compared. The actual crushing/penetration nonlinear finite element approach, damage in the
increment takes place in the ship with lowest resistance. three directions (i.e., including penetration, vertical
DAMAGE 4.0 considers the material and structural and horizontal damage) are considered.
scantlings of all major structural components for the side • The geometry of the striking ship bow shape is
structure. described by gap/contact elements. The bow is
assumed to be rigid.
• One cargo hold of the struck ship is taken as the
extent of the analysis.
1 .8 0 E + 0 1
• ISUM stiffened panel units are used to model the
1 .6 0 E + 0 1
struck vessel structure.
1 .4 0 E + 0 1 The geometry of the struck ship is described using about
1 .2 0 E + 0 1 600 rectangular or triangular ISUM units. If the
1 .0 0 E + 0 1 deformation of the struck ship is symmetric, the total
8 .0 0 E + 0 0
6 .0 0 E + 0 0
degrees of freedom in the numerical model are reduced by
4 .0 0 E + 0 0 half. Each node has 3 degrees of freedom.
1 2 .0 0 E + 0 0 Figure 17 shows damage calculated in an
0 .0 0 E + 0 0
5 ALPS/SCOL simulation.
S15
Design data required for the striking ship includes a
S13
9
S11
S9
13 detailed bow geometry description, length, beam, depth,
S7
S5
draft and displacement.
S3
S1
Design data for the struck ship includes, length,
beam, depth, draft and displacement, transverse bulkhead
Figure 16. DAMAGE Bow Geometry
10
location, COG, and detailed structural design and
scantlings. Ship-Ship collision
Scenario data required includes striking ship velocity 60 Deg.
and longitudinal location of impact in the struck ship. 1 90 Deg.
120 Deg.
Energy ratio
0,8
150 Deg.
0,6
0,4
0,2
0
-0,5 -0,3 -0,1 0,1 0,3 0,5
Collision location (d/L)
Figure 18 - Energy ratio defined as the ratio between
energy released for crushing and the total kinetic energy
of the two ships before the collision as function of the
collision location, d, for collision angles equal 60, 90, 120
and 150 degrees. The two tankers are identical and have
the same speed prior to the collision [29].
The model for the internal mechanics is based on a
set of super-elements, where each element represents a
structural component. The calculation method is based on
Figure 17. Damage from ALPS/SCOL Simulation the principle that the area of the struck vessel affected by
the collision is restricted to the area touched by the
5.2.4 DTU Model striking vessel. The super-elements are:
• Lateral plate deflection and rupture. Large
The Technical University of Denmark (DTU) model deflections are assumed; this implies that the bending
also solves the external problem uncoupled from the resistance can be neglected
internal problem, and applies the calculated absorbed • Crushing of structure intersection elements (X- or T-
energy to plastic deformation of the struck ship. elements)
Solution of the external dynamics is accomplished • In-plane crushing and tearing of plates
based on an analytical method developed by Pedersen and • Beam deflection and rupture
Zhang [29]. This method estimates the fraction of the The design data for the struck vessel includes length,
kinetic energy that is available for deformation of the ship beam depth, draft, displacement, COG and detailed
structure. The energy loss for dissipation by structural structural design and scantlings.
deformation is expressed in closed-form expressions. The The bow of the striking vessel is assumed to be rigid.
procedure is based on a rigid body mechanism, where it is The basic data for describing the striking ship bow are
assumed that there is negligible strain energy for stem angle, breadth and bow height. The horizontal shape
deformation outside the contact region, and that the of the deck and the bottom are assumed to be parabolic. If
contact region is local and small. This implies that the the striking vessel is equipped with a bulb, this is assumed
collision can be considered instantaneous as each body is to have the form of an elliptic parabola.
assumed to exert an impulsive force on the other at the Scenario data required includes striking and struck
point of contact. The model includes friction between the ship velocity, collision angle and longitudinal location of
impacting surfaces so those situations with glancing impact at the struck vessel.
blows can be identified. Both ships have three degrees of
freedom: surge, sway and yaw. The interaction between
the ships and the surrounding water is approximated by 5.3 Application
simple added mass coefficients, which are assumed to
remain constant during the collision. In order to assess the models’ consistency and sensitivity,
The loss in kinetic energy by the method is they are tested in a series of collision scenarios with a
determined in two directions, perpendicular and parallel range of struck tanker sizes and designs. The Baseline
to the side of the struck ship. Both right and oblique angle Tanker design is a 150000 dwt double-hull tanker. It was
collisions are considered and both vessels may have developed to be consistent with the dimensions of the
velocity before the collision. 150000 dwt reference tanker in the IMO Interim
Guidelines. HECSALV and SafeHull were used to
11
develop the details of the design, and to insure that the
arrangement satisfies IMO regulations and the structural Deadweight, tonnes 150,000
design satisfies ABS classification requirements. A 60000
dwt double hull tanker and a 283000 dwt double tanker Length L, m 264.00
were also developed in accordance with the Interim Breadth B, m 48.00
Guidelines [3]. Results for the baseline 150000 dwt
double-hull tanker are presented in this paper. Depth D, m 24.00
5.3.1 Struck Ship Draft T, m 16.80
Double Bottom Ht hDB, m 2.32
The 150000 dwt Baseline Tanker design shown in Figure
19 is the primary struck ship used for the initial model Double Hull Width W, m 2.00
testing. Tables 2 and 3 list principal characteristics and
structural data for this design. Displacement, tonnes 178,867
Table 4 lists the input data for each test matrix. The
first test matrix considers damage for a series of strike
locations on the web at the center of each cargo tank. This 5.3.2 Model Test Matrices
represents a large global variation in strike location. The
second test matrix considers damage for a series of strike Three scenario test matrices were used in the initial study.
locations on either side of the web at the center of the All matrices use the Baseline Tanker as the struck ship
midship cargo tank. This represents a relatively small and a 150000 dwt bulk carrier as the striking ship.
local variation in location on and between webs. The Principal characteristics for the striking ship are listed in
third test matrix considers damage for a series of collision Table 4 and the resulting vertical alignment of the two
angles with a strike location on the web at the center of ships is shown in Figure 20. Since the focus of the initial
the midship cargo tank. tests is on penetration damage, zero struck ship speed is
used in all cases. Scenarios for the test matrices are
described in Table 5.
Table 3. Baseline Tanker
Ship 150,000 dwt
double hull tanker
Web Frame Spacing Ls, m 3.30
Deck 47.32
Smeared Inner Bottom 26.92
Thickness
th, mm Bottom 28.29
Stringers 3 a 15.34
Side Shell 21.92
Smeared
Thickness Inner Skin 22.94
tv, mm
Bulkhead 22.28
Figure 19 - Baseline Tanker Design [3] Web Upper 12.00
Table 2. Baseline Tanker Principal Characteristics Thickness
tw, mm Lower 18.00
Table 4. Striking Ship Principal Characteristics
12
Strike Location (m fwd MS)
150,000 dwt
Striking Ship Speed (knt)
Ship Type
Struck Ship Speed (knt)
bulk carrier
Collision Angle (deg)
Length L, m 274.00
Breadth B, m 47.00
Depth D, m 21.60
Bow Height H, m 26.00
Draft T, m 15.96
Displacement, tonnes 174,850
Matrix 1 0 3,4,5,6,7 90 -62.5,29.5,
Half Entrance Angle, α 38° 3.5,36.5,
69.5,102.5
Matrix 2 0 3,4,5,6,7 90 1.85,2.675,
5.3.3 Model Results 3.5,4.325,
5.15
Representative model results for struck ship penetration Matrix 3 0 3,4,5,6,7 45,60,75, 3.5
are shown in Figures 22-27. The figures show transverse 105,120,135
penetration into the struck ship as a function of the
particular variables in each matrix. The results show
good agreement between the models. DAMAGE
generally predicts the lowest penetration, and Matrix 1: pen(x) Vb=3 knt
ALPS/SCOL generally predicts the highest.
Figures 22 and 23 (Matrix 1) show the effect of the 4 DTU
external dynamics. More energy is absorbed in strikes SIMCOL2.11
around midship. SIMCOL shows a larger variation in 3
DAMAGE
pen [m]
penetration as a function of global strike location than the
other models, particularly at low energy. SIMCOL’s 2 ALPS/SCOL
coupled dynamics predict larger changes in the relative
motions at the strike point of the two ships as the strike 1
location moves away from midship. This results in more
0
longitudinal damage and less penetration. ALPS/SCOL
shows a similar, but lesser trend. This has the greatest -100 -50 0 50 100 150
effect on shell and web deformation, and is most evident x [m fwd MS]
in the lower energy case, Figure 22. Figure 22 – Matrix 1 Low Energy Collision
Matrix 1: pen(x) Vb=7 knt
150,000 dwt
10
Bulk Carrier DTU
8
150,000 dwt
pen [m]
Double Hull SIMCOL2.11
Tanker 6
4 DAMAGE
2 ALPS/SCOL
0
-100 -50 0 50 100 150
x [m fwd MS]
Figure 21 - Collision Strike Vertical Alignment Figure 23 – Matrix 1 High Energy Collision
Table 5 - Test Matrices
13
using FEA can be used to fill in the gaps in actual data,
Matrix 2: pen(x) Vb=3 knt
but only if data and FEA are consistent. FEA using
LSDYNA has just begun. Collaboration and comparison
4
of SIMCOL results with DAMAGE, ALPS/SCOL and
DTU will also continue.
3
DTU
pen [m]
SIMCOL2.11 Matrix 2: pen(x) Vb=7 knt
2
DAMAGE
ALPS/SCOL 10
1
8
0 DTU
6
pen [m]
1 2 3 4 5 6 SIMCOL2.11
x [m fwd MS] 4 DAMAGE
ALPS/SCOL
2
Figure 24 – Matrix 2 Low Energy Collision
0
In Figures 24 and 25 (Matrix 2) the results for 1 2 3 4 5 6
SIMCOL, DTU and ALPS/SCOL are remarkably x [m fw d MS]
consistent. In the lower energy case, Figure 24, SIMCOL
shows a significant reduction for a perfect right angle Figure 25 – Matrix 2 High Energy Collision
strike, on the web, at midship. Membrane tension is
balanced on both sides of the strike in this case, and in
combination with a direct strike to the web, this Matrix 3: pen(beta) Vb=3 knt
substantially reduces penetration in lower energy cases 4
where membrane tension dominates. ALPS/SCOL and
DTU show a similar, but less pronounced decrease. 3
Figures 26 and 27 (Matrix 3) also show the effect of
pen [m]
DTU
the external dynamics, i.e. more energy must be absorbed 2 SIMCOL2.11
in a right angle collision. SIMCOL, DTU and
ALPS/SCOL show excellent agreement. SIMCOL again ALPS/SCOL
1
shows a marked reduction at 90 degrees for the lower
energy case, Figure 26. The current version of DAMAGE
0
is only able to consider right angle collisions, so
DAMAGE is not used in Matrix 3. 40 60 80 100 120 140
beta [deg]
5.3.4 Collision Model Discussion and Conclusions
Figure 26 – Matrix 3 Low Energy Collision
Although not a validation, the results from four very
different models are remarkably similar, and this
increases confidence in their results. SIMCOL results are Matrix 3: pen(beta) Vb=7 knt
less homogeneous for different scenarios because of
coupling to external mechanics. The SIMCOL bow 10
geometry may be oversimplified, but at least for these test 8
cases, it provides consistent and sufficient results. The
pen [m]
6 DTU
advantage of this simplified geometry is its single
parameter description (half-entrance angle) which SIMCOL2.11
4
facilitates its application for probabilistic analysis. ALPS/SCOL
SIMCOL’s simple geometry and coupling with external 2
dynamics should also facilitate calculation of longitudinal
0
extent of damage that is very important in damage
stability and oil outflow analysis. 40 60 80 100 120 140
beta [deg]
5.3.5 Ongoing and Future Work
The single most critical next step is model validation Figure 27 – Matrix 3 High Energy Collision
using actual or large-scale model test data. This will be
discussed in the next section of this paper. Validation Other ongoing and future work includes:
14
• Improved modeling of longitudinal extent of The pdfs for these variables are not independent. Possible
damage including the effect of transverse webs relationships for the collision variables are illustrated in
and transverse bulkheads. Figure 28. Given the struck ship design and requirement:
• Application and comparison of models in cases
where the struck ship has forward speed. 1. Since specific struck ships trade in specific ports on
• Application of models to other designs and specific routes, it is expected that they will encounter
assessment of damage length, beam and depth a related subset or distribution of other ships (striking
scalability with struck ship principal ships) that may not be a distribution representing all
characteristics. ships in worldwide trade.
• Struck ship design-parameter sensitivity 2. A specific struck ship with known design
analysis. characteristics in a specific trade will also have
• Probabilistic analysis using scenario descriptions related distributions for draft, trim and speed. Note:
discussed in the next section of this paper. this speed is the speed at the moment of the collision,
• Modeling and application of a deformable and not necessarily operating speed.
striking ship bow. 3. Given a specific type and tonnage of striking ship, its
other characteristics will also be related including
displacement/mass, bow half entrance angle, bow
height, draft, bow stiffness or structural design and
6 COLLISION AND GROUNDING DATA speed. Again, this is speed at the moment of
collision, not operating speed.
Two basic types of data are required to support the goals 4. When two ships are maneuvering to avoid a collision
of this panel: (in-extremis), the resulting collision angle and strike
1. Data for developing pdfs for grounding and collision location are expected to be related.
scenarios.
2. Data for grounding and collision model validation,
particularly for double hull damage. Striking Ship
Bow HEA
Collision Angle Strike Location
6.1 COLLISION AND GROUNDING SCENARIOS 4
Striking Ship
As illustrated in Figure 2, probabilistic descriptions of Bow Height
grounding and collision scenarios are required to develop
probabilistic descriptions of grounding and collision
Struck Ship 1
damage. Design
Striking Ship Striking Ship
Type Bow Stiffness
The following data are required to define grounding
scenarios: 3
• Bottom or obstacle description
• Depth of water
Striking Ship Striking Ship
Dwt LBP, B, D
• Grounding ship displacement, trim, draft
2
and speed
Struck Ship
• Location of the obstacle relative to the ship's Speed
Striking Ship
Displacement,
centerline Mass,
Draft,Trim
A longer list is required for collision:
• Striking ship Struck Ship
Trim
• Speed Striking Ship
Speed
• Displacement
• Draft Struck Ship
• Bow height Draft
• Bow shape
Figure 28. Collision Scenario Variable Relationships
• Collision angle
• Strike location The data necessary to establish these relationships is very
• Struck ship limited. Collection and analysis of this data is ongoing.
• Speed
• Displacement
15
6.2 DATA FOR MODEL VALIDATION 7 STRIKING SHIP BOW
The second type of data required by the panel is for model 7.1 SIMCOL Bow Model Study
validation. In collision the following data is required:
• Collision Scenario All of the collision models used thus far in this study, and
• Striking ship most models used by other researchers, assume that the
• Speed striking ship bow is rigid. This is changing. Panel work
• Displacement in this area is focused on the following questions and
• Draft problems:
• Bow height 1. Is the rigid bow assumption rational? Is significant
• Type energy absorbed by the striking ship bow? Is this
• Collision angle energy variable in different collision scenarios?
• Strike location 2. If bow deformation is important, can this problem be
• Struck ship design solved uncoupled from the side damage problem?
• L,B,D,T,∆ 3. Identify a simple, but sufficient bow model for future
• Speed use in SIMCOL.
• Structural design 4. Develop a course-mesh bow model to speed up
LSDYNA modeling for SIMCOL validation.
• hDB, wDS, yLBHD, xTBHD
• web spacing, zstring, zstrut Most collision accidents that result in significant
• scantlings struck ship damage also result in significant damage to
• Damage description the striking ship bow. Minorsky’s original analysis [26]
considered absorbed energy in the striking bow. A
In grounding, the following model validation data is reanalysis of his data indicates that absorbed bow energy
required: in these cases accounted for between 8 and 53 percent of
• Grounding Scenario the total absorbed energy [30]. More recent studies by
• Geometry and elevation of the rock Reckling [31], Akita and Kitamura [32], and Valsgard and
• Location of the rock relative to the ship's Pettersen [33] indicate absorbed bow energies in the range
centerline of 42 to 55 percent of the total absorbed energy.
• Ship speed, draft, displacement and trim LSDYNA analyses conducted at Virginia Tech with
• Grounded ship design deformable bows and sides show absorbed bow energies
in the range of 22 to 44 percent [30]. We must conclude
• Hull type (single hull, double hull, mid-deck, …)
that absorbed bow energy can be significant and variable,
• L, B, D, LCG, LCF, GMT, GML
and therefore should be considered in collision analyses.
• Waterplane area
• Transverse and longitudinal bulkhead locations
• Frame spacing, spacing of longitudinal girders
• Scantlings
• Damage description
The availability of this data is extremely limited.
Accident data from USCG databases for the period of
1980-1998 was screened for sufficiency and applicability
to this problem. Useful validation cases must include all
of the data specified above (scenario, structural design,
damage description). In the case of collision, the
following additional criteria were applied:
• Striking location in struck ship away from bow or
stern.
• Collision angles of 45-135 degrees.
• Penetration greater than one meter.
Collection and evaluation of this data is also ongoing.
Table 6. Bow Model Test Matrix
16
Table 6 lists the bow and struck ship model cases Forecastle Deck, 26.0m abl.
considered in the SIMCOL bow model development.
Four rigid bow tests were run to validate SIMCOL
calculations. These are labeled R-1 through R-4 in Table Tank Top, 20.0 m abl.
6. Ten conventional FEA bow analyses were run to
validate the simpler intersection bow model results. These
Loaded
are labeled C-1 through C-10 in Table 6. Two types of
intersection model analyses were accomplished. Tests I-1 Collision Bhd.
Tl =15.96m
through I-4 use closed-form equations from Pedersen
[34], Amdahl [35], and Yang and Caldwell [36]. Tests I-
5 through I-16 use intersection elements applied in
LSDYNA simulations. Deck (not W.T.), 7.6m
Simplified LSDYNA intersection-element bow abl.
models were developed to increase the speed of
LSDYNA finite element solutions used in SIMCOL
collision model validation. In these models, only
intersections of sides, decks, longitudinal bulkheads and
girders were included in the model with longitudinal Figure 30 - 150kdwt Bulk Carrier Bow [34]
stiffener area smeared into plate thickness based on
plastic bending moment. Intersection elements are
modeled as truss elements in LSDYNA with material
properties replaced by properties derived for crushed L, T
and cruciform sections as illustrated in Figure 29 [35].
Transverse frames were modeled as normal truss
elements. Nodes were only allowed a single degree of
freedom in the striking ship longitudinal direction. The
150kdwt bulk carrier bow is shown in Figure 30 [34].
Figure 31 shows the intersection model for this bow (I-8).
Figure 32 shows the conventional fine-mesh FEM for this
bow with beam and panel elements (C-5). Figure 33
shows a comparison of the force-indentation plots for the
intersection and conventional models (I-8 and C-2). The
results compare very well. Similar results were obtained
for the other test cases. It is concluded that the bow
intersection model is sufficient for LSDYNA collision
simulations. Work is continuing to apply these results to Figure 31 - Bulk Carrier Intersection Element Model [30]
the development of a SIMCOL bow model.
Figure 29 - Intersection element Model [35]
Figure 32 - Bulk Carrier Conventional FE Bow
17
comparison of the crushing forces for respectively the
8.00E+08
bow and the side, it can be determined which vessel
7.00E+08 deforms during the considered step.
6.00E+08 Before calculation of the deformation of the two
vessels the following calculations are carried out:
5.00E+08
Force (N)
1. The Force-Penetration curve Fstruck(δA) for the struck
4.00E+08 vessel is calculated, where the striking vessel is rigid.
Intersection Model
3.00E+08 2. The Force-Penetration curve Fstriking(δB) for the
Conventional Model
striking vessel is calculated, where the struck vessel
2.00E+08 is assumed rigid.
1.00E+08 If the striking vessel has a bulbous bow, the analysis of
the crushing forces is separated into a bulb analysis and
0.00E+00
0 5 10 15 20 25 30
an analysis of the top of bow above the bulb.
The force-deformation curve for the struck vessel is
Penetration (m)
determined by the procedure described in Section 5.2.4
Figure 33 - 150kdwt Bulk Carrier Striking Rigid Wall and for the striking vessel by the procedure described in
[30] Pedersen et. al [34].
A commonly used procedure for taking into account
7.2 DAMAGE Bow Model the deformation of the bow is to compare the two force-
penetration curves, Fstruck(δA) and Fstriking(δB), at each step.
DAMAGE 5.0 includes a deformable bow. The initial
This approach, however, only includes a very limited
bow geometry is the same as in Damage 4.0, illustrated in
level of interaction. In reality, the force-penetration curve
Figure 16. Bow crushing is modeled using L-, T- and X-
for the side of the struck vessel is a function of the
form super-elements in an ‘intersecting unit method’ [37].
deformation of the bow, and vice versa. This stronger
Bow and side force-deformation calculations are
interaction is taken into account by comparing the forces
completed separately (uncoupled). These calculations are
FA and FB, which is determined as:
performed assuming that the bow strikes a rigid wall and
F A = FStruck (δ A )
A'
that the side is struck by a rigid bow. The results are then Struck vessel: (1)
compared incrementally with deformation applied to the A' '
weakest component (bow or side) at each increment. Striking vessel: FB = FStriking (δ A +δ B ) (2)
This results in deformation and energy absorption in both where
components. FA force to crush the struck vessel;
FB force to crush the striking vessel;
Fstruck force from the force-penetration curve for struck
7.3 DTU Bow Model [38] vessel, where the striking vessel is rigid;
Fstriking force from the force-penetration curve for
Deformation of the Striking Vessel
striking vessel, where the struck vessel is rigid;
δA penetration into the struck vessel;
δB deformation of the striking vessel;
A’ cross-sectional area of the striking vessel taken
A’’ A’ at a distance of δA+δB from bow or bulb tip;
A’’ cross-sectional area of the striking vessel taken
at a distance of δA from bow or bulb tip;
Striking Vessel See also Figure 34.
The forces at the struck and the striking vessel FA and FB
Struck Vessel are compared
δA δB • If FA > FB
Deformation of striking vessel, δB is increased
• If FB > FA
Figure 34 - Deformation of vessels during collision. The
Deformation of struck vessel, δA is increased
A’s relate to areas not lengths
A more recent DTU model also predicts damage to The reason for correcting the resistance of the struck
struck and striking vessels in a collision event [38]. The vessel is that if the bow is deformed, the resistance is
analysis is carried out in penetration steps. Only one of approximately equal to the force at the side times the ratio
the involved vessels can be deformed in each step. By a between the areas. For a single hull vessel the correction
18
will have nearly no influence, but for a double hull vessel, present, and the overwhelming majority of single-skinned
there will be some corrections when the bow penetrates designs are longitudinally stiffened. OPA ’90 mandates
the inner side. See Figure 34. the use of double-skinned tanker designs, Figure 36 [39].
When the deformation patterns of the struck and the
striking vessel are known, the total absorbed energy can
be calculated and compared with the energy calculated by
the external dynamics. See Pedersen and Zhang [29].
8 STRUCTURAL DESIGN FOR COLLISION
AND GROUNDING
The function of a tank vessel’s structural system may be
viewed from the standpoints of normal operation and
casualty operation. In providing adequate resistance for
normal operations, the objective in structural design is to
maintain structural integrity of the hull girder, of Figure 36 - Double-Hull Tanker [39]
bulkheads, decks, etc., and of plating, stiffeners and
details. Other considerations relate to vessel size, The Marine Board of the National Research Council
complexity and heaviness of structure, producibility and has recently convened a "Committee on Evaluating
maintainability. In terms of casualty operations, the Alternative Tanker Designs". The committee is to
objective is to maintain vessel integrity and to protect establish, for the first time, a rational methodology to
cargo, or conversely to protect the environment from oil evaluate alternatives to the double hull. The US Coast
pollution in case of a casualty. In this case the primary Guard is sponsoring this Committee, and its report is
considerations should include: expected in the first quarter of 2001.
The emphasis of new and proposed requirements for
1. Resistance to fire and explosion damage and its reducing the probability of oil cargo outflow has been on
containment, the subdivision of cargo spaces. The impact of hull
2. Resistance to collision and grounding damage, structure on the reduction of outflow has had more limited
3. Containment of petroleum outflow if damage does attention. The complexity of determining the contribution
occur, and of structure to cargo protection and the unpredictability of
4. Maintenance of sufficient residual strength after structural response under the variety of potential damage
damage in order to permit salvage and rescue scenarios have no doubt contributed to this set of
operation and to minimize further damage and circumstances.
spilling of oil. The following subsections address the subject of
vessel structural design characteristics for collision,
grounding and bow impact. The intent is to consider what
has been learned from analytical studies regarding
structural design features that may suggest future
modifications and alternatives to enhance resistance to
cargo outflow.
8.1 Collisions
8.1.1 Analytical Indications – Struck Ship
The sequence of structural events during a collision varies
according to the nature of the collision and vessel design.
Figure 35 - Single-Hull Tanker [39] Figures 37 and 38 [24] provide general flow diagrams
representing the phenomena for single and double hull
Tank vessels have traditionally been designed as tankers. This information is useful in identifying where
single-skinned hulls, Figure 35 [39]. Depending on the greater resistance can be addressed.
size of the vessel, longitudinal bulkheads are often
19
Figure 37 - Macro flow diagram for side collision plastic-energy analysis of a single hull [24]
Initially, the stiffened hull plating distorts in a plastic stiffeners then buckle in the vicinity of the flanking web
bending phase, with plastic “hinges’ forming in the frames, and possibly “trip” in the vicinity of the strike.
vicinities of the strike and the web frames flanking the Subsequently, the stiffened hull unloads momentarily as
strike. During this phase, insignificant membrane tension the strike continues, but reloads in a membrane-tension
develops. For a typical tanker with longitudinal angles phase. The hull ruptures at the end of this phase, with
stiffening the hull plating, the longitudinal angle-shaped possibly the flanking web frames yielding or buckling
before the hull ruptures. In such cases, the membrane- Other sequences of phenomena are possible. A hull
tension phase includes two sub-phases: (1) there is no with longitudinal stiffeners, such as rectangular bars that
transverse movement of the web frames flanking the are not apt to buckle or trip, tend to rupture before
strike; and (2) the web frames flanking the strike move significant membrane tension has a chance to develop.
inward toward the ship’s centerline, and the damage Alternatively, with any type of hull stiffeners, very
extends into the adjacent web frame spaces. During these weak web frames can yield or buckle before rupture or
phases, the deck is also distorting in membrane tension. buckling of the longitudinal stiffeners, in which case the
However, the deck behavior is assumed not to affect the damaged length increases during the bending phase.
sequences of the options. These phenomena are unlikely, however, for typical
ships.
Figure 38 - Macro flow diagram for side collision plastic-energy analysis of a double hull [24]
21
enhance resistance to collision. These fall into two
categories:
• Conventional hull structural arrangements that are
modified to enhance energy absorption.
• Unconventional hull structural arrangements that
include structurally actuated mechanisms for energy
absorption.
As part of the research project for improved tanker
safety against collision and grounding, the Association for
Structural Improvement of the Shipbuilding Industry
(ASIS) of Japan conducted a study of varying structural
characteristics of a conventional VLCC design and
applying a more unconventional structural feature as well
[40]. The more conventional alterations included use of
high strength steel, additional shell stringers, a
longitudinal strut in the outboard cargo tank supporting
the double side, and a double side unidirectional
stiffening system. As the side-shell has shown to be a
significant contributor to energy absorption, an
unconventional new skin composed of double plate panels
Figure 39 - Energy Absorption of Structural Components internally stiffened by web frames, essentially a cored
(Standard VLCC) [40] panel skin providing additional net shell thickness, was
substituted for the conventional stiffened plate. The
results are shown in Figure 40. Other ideas of the more
conventional nature included:
• Bulkhead stools on the wing tank side to reduce hard
spots and increase damage length.
• Reduced stiffness of web frames to increase damage
length.
• Increased side-shell plating thickness, especially
from the ballast waterline up, to increase membrane
tension in the side-shell.
• Decreased spacing of side-shell longitudinals and
added intermediate vertical web frames on the side
shell to increase the integrity of the side-shell.
• Increased number of horizontal stringers in the wing
tank to increase side skin integrity and provide
additional material for membrane tension.
Although the practicability and acceptance from a
regulatory point of view of very unconventional
approaches may be seriously in doubt, it is nevertheless
possible to postulate alternatives, and in fact the greatest
Figure 40 - Energy Absorption Capacity [40] step in the level of possible energy absorption is likely
with an unconventional approaches:
The contribution of various structural components for
a ULCC with a more conventional double side structure is
• Controlled pressure fluid chamber in the wing tanks
shown in Figure 39 [40].
wherein the venting of fluid under pressure to other
spaces when compressed during collision results in
8.1.2 Application energy dissipation.
• Introduction of material in wing ballast-tanks that
With the knowledge acquired from understanding the
does not reduce ballast capacity significantly, but
process identified by the analytical evaluations, it is
increases energy absorption, like ultra-large open-cell
possible to postulate hull structural features that may
foam. Permeability and inspection are possible Model testing and computer finite element simulation
problems with this approach. of a stranding for a 280000 dwt tanker double bottom
• Truss structure separating wing tank skins. Such an resulted in the lateral load versus indentation of up to 2m
arrangement can enhance the integrity of the inner shown in Figure 42. The peak point, point 1, denotes the
hull longer as the truss deforms while supporting the buckling of the center longitudinal girder, and point 2, the
outer skin and more slowly loading the inner skin. fracture of the outer bottom plating. Point 3 indicates
• Pre-planned failure or “crumple zones” as have been fracture of the center girder-floor connection. It is
developed by the automobile industry for the more interesting to note that the response shown in Figure 42 is
critical and predictable areas of damage. similar to that predicted for collision, with the principal
• A foam release system actuated in the event of an difference being the nature of the obtrusive loading,
impending collision within the expected area of which may be a line load for a collision and a point load
contact to increase stiffness and energy absorption. for a grounding.
Raking bottom damage is of significantly different
8.2 Bow Impact character than the grounding damage heretofore
discussed. In raking damage, the obstruction ruptures the
8.2.1 Analytical Indications bottom structure and then continues destructive shearing,
The significance of the energy absorption in bow damage tearing and buckling as the vessel continues to move in its
and the effects of the bow interacting with the struck direction of motion, Figure 43. The length of bottom
ship’s side structure make it a critical element in raking is based on an energy balance concept where the
considering collision results. Calculations assuming rigid kinetic energy of the vessel is entirely absorbed by the
bows with vertical stems and no bulbs have simplified destruction of bottom structure.
analyses and fostered understanding, but do not depict the
current and likely future nature of bows. Ship resistance,
powering and seakeeping requirements are likely to
continue to foster the existence of bows with raked stems
and bulbs into the future.
Analytical evaluations do point a way to considering
bows. If they are deformable, they will account for a
sizeable portion of the total energy absorption in a
collision. If they can be made to deform in a manner that
does not cause premature failure of important struck ship
energy absorbing structure, such as the side shell, then the
same attributes of a rigid vertical stem bow will be
achieved at the struck vessel.
8.3 Application
The bows of ships incorporate a significant amount of
stiff horizontal fore and aft reinforcing structural elements
that enhance the rigidity of the structure. Alternatively,
arrangements with transverse stiffening coupled with
appropriate spacing can lead to significant folding
deformation, which can effectively reduce or eliminate
the protrusions of the upper bow and bulb as well as their Figure 41 - Model of the grounding process [41,42]
structural hard-spots. Recent studies [44] have confirmed An energy analysis for grounding which takes into
that lower crushing pressure of a bulbous bow is account energy due to the tearing that occurs during
preferable to avoid the early rupture of the struck tracking-type collisions presumes that in such cases the
sideshell and the crushing strength should not exceed that damage will be long and shallow, involving plate tearing,
of the side structure. but relatively little volume distortion of the inner
structure. However, Figure 42 indicates that if there is a
8.4 Grounding vertical component to the striking force, after fracture of
8.4.1 Analytical Indications – Grounding the outer plating at point 2, there is an increase in stiffness
leading up to point 3 and beyond as more girders and
The potential phases of grounding are depicted in Figure floors come into contact with the obstruction. Figure 44
41. In grounding and raking, the vessel has forward also indicates a retention of stiffness.
speed. In stranding it does not.
23
8.4.2 Application
In resisting both grounding and raking damage, it is clear
that the bottom shell and the internal grid of the
transverse floors, longitudinal girders and bulkheads is
critical.
In the case of raking damage, the longitudinal
stiffness of floors and their ability to absorb energy in
distortion are more critical than their support of the shell.
As in this case the distorting force is applied in the
weakest direction of the floor, the situation is further
complicated. Clearly, the more floors and the smaller the
spans between their longitudinally resistive structure, the
better. On the other hand, this arrangement will serve to
Figure 42 - Load versus lateral indentation for 280000 limit the lateral deflection and therefore, the energy
dwt tanker double bottom [42] absorption in a stranding. Other ideas have included:
• Increase the scantlings of the lower one-third part of
the hull.
• Slope the inner bottom, higher at the collision
bulkhead and lowering to the regulatory requirement
at the end of the first cargo tank.
Figure 43 - Bottom raking due to grounding • Preclude web frame and bulkhead damage
propagation through the inner bottom by introducing
a discontinuity in the rigidity between the two. A
bulkhead stool would be an example.
9 CONCLUSIONS
The almost two-year effort of the panel has achieved or
partially achieved many of the original objectives stated
in the introduction to this paper:
• Simplified grounding models have been investigated,
compared and assessed. DAMAGE 4.0 is shown to
be an excellent tool for comparative analysis of
grounding damage, and based on limited validation,
provides good results for cases with similar idealized
rock geometry. More validation is required.
Limitations include the ability to consider only
conventional ship geometries and an idealized
pinnacle geometry. Grounding on other rock
geometries, reefs, etc., cannot be analyzed using
DAMAGE, but should be considered in the future as
an important part of a complete probabilistic analysis.
Only a preliminary attempt at probabilistic analysis
using DAMAGE was completed.
• Four simplified collision models have been
investigated, compared and assessed. The models
provide similar penetration results for the cases
considered. Although some limited validation has
been accomplished for specific mechanisms used in
the models, more validation is required. The
prediction of longitudinal extent of damage,
particularly at transverse bulkheads, remains largely
Figure 44 - Raking force versus penetration length by unexplored. This is very important for oil outflow
experiments [41] and damage stability calculations. This prediction
24
requires consideration of various collision angles and can proceed in parallel with the remaining model
struck ship speed. Consideration of striking bow development and validation:
deformation is a relatively new addition to the • Probabilistic analysis
collision models. Further application and validation • Sensitivity analysis and the development of
of bow models and their coupling to struck ship parametric equations relating damage extent to
models is required. Most of the researchers are using structural design variables in conventional designs
finite element analysis as part of simplified model • Modification of the grounding and collision models
validation. Only a preliminary attempt at to consider unconventional designs
probabilistic analysis using SIMCOL was completed.
Additional analysis is in progress. 11 CONTINUING WORK OF THE PANEL
• Data for model validation and scenario definition is
very limited. A proposed data requirement is
Individual sections of this paper have discussed
specified in this paper. The search for data to fill the
possibilities for future work. This panel has been very
requirement goes on with the support of the USCG,
active and very productive, but in many ways has just
SSC and SNAME. Funding and significant IMO
scratched the surface. Individual papers will be written in
and class society support may ultimately be
most all areas covered by this summary paper.
necessary to fully satisfy this data requirement.
Subsequent work will include: completion of sufficient
• Some progress has been made in the definition of
grounding and collision models, validation of the models,
scenarios and conditions for grounding and collision
specification and acceptance of standard accident
analyses, but limited data is also hampering this
scenarios, and ultimately application of models and
effort. Data describing the worldwide population of
scenarios to improved structural design in collision and
ships that might be involved in collisions is available,
grounding. A two-year continuation of the panel will be
but since specific struck ships trade in specific ports
proposed to the T&R Committee. This continuation will
on specific routes, it is expected that they will
include a new charter with new objectives and
encounter a related subset or distribution of other
deliverables. The Ad Hoc framework has proven very
ships (striking ships). Sufficient data has not been
effective for meaningful collaboration and effort. The
obtained to quantify or assess this hypothesis. More
panel chairman thanks all those who have contributed.
data is also required for collision angle, strike
location, grounding and collision ship speeds and
grounding bottom description.
• A number of possibilities for design improvement are
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