Design Analysis for Grounding Experiments

(i,

Design. analysis for grounding

experiments

Pinkster,, J.A., P.M. Lemmen and

A.W. Vredeveldt

Report No. 1061-P

1996

Deift University of Technology and

ThO Centre for Mechanical Engineering

International Conference on Designs

and Grounding Protection of Ships,

San Francisco, California, August 22-23, 1996

'i"LJ Deift

Faculty 'of Mechanical llngmeering andiMarine Technology Ship I{ydromechanica Laboratoty

International Conference on

Designs and Methodologies for

Collision and Grounding Protection of Ships

SNAME

PROCEEDINGS

THE SOCIETY OF

NAVAL ARCHITECTS & MARINE ENGINEERS

San Francisco, California

August 22-23, 1996

SNAJ

THE SOCIETY OF

CONTENTS

Keynote Speech

IMO's Role in Improving Ship Safety and Mitigating Environmental Damage ¡li

Martin.B&kenhâuer(Chainnan, 1MO BLG Sub-Committee)

Prediction of Oil Outflow basedon Energy Considerations

i - i

Manolis Sainuelides (National Technical University of Athens)

Residual Strength Assessment of Ships After Collision and Grounding 2 - i JeomKee Paik, Soo H Yang (PusanNationalUniversity)&

Anil K. Thayamballi (American Bureau of Shipping, New York)

Energy Dissipationof Plastic Hinges Under Dynamic Loads 3 - i Eike Lehmann (Germanisher Lloyd, Hamburg) &

Xing Yu (Technical University - Hamburg-Harburg)

Application of Structural Collision Analysis Procedures to High-Speed Craft John Daidola & Ernesto Pet (M Rosenblatt & Son, Inc.)

ValidatiOn of Minorsky's Ship Collision Model and Use of the Model to

Estimate the Probabilityof Damaging a Radioactive Material Transportion Cask duringa Ship Collision

Philip C. Reardon (PCRT) & Jeremy L Sprung (SandiaNational Laboratories)

KComparison of Methods for Evluatiñg Structure During Ship Collisions

Doug Ammerman (Sandia National Laboratories) & John Daidola (M. Rosenblatt & Son, Inc.)

Numerical Simulation of Actual Collision & Grounding Accidents Takao Kuroiwa (Mitsubishi Heavy Industries, Ltd.)

Strength Analysisof a New Double Hull Structure for VLCC ¡n Collision Atsushi Sano, Osamú Muragishi, Takao Yoshikawa, Aldo Murakami, & Tatsuya Motoi (Kawasaki Heavy Industries, Ltd.)

Comparative Study on Collision Resistance of Side Structure

Où Kitamura (Mitsubishi Heavy Industries, Ltd.)

Strength of Ships during Collision and Grounding

Hideomi Ohtsubo (University of Tokyo)&

Ge Wang (American Bureau of Shipping, Yokohama)

il. Development of a Computational Model for Predicting Damage to Tankers

Patrick Little & Daniel Pippenger (US Coast Guard) Bo Cerup Simonsen (Technical University of Denmark)

12. Search for a Predictive Modelof Structural Damage in Ship Collisions: from a Case Study toSome Proposals for a New Approach

Mario Maestro & Alberto Marino (University of Trieste)

SNAMEISNAJ Conference I August 22 - 23, 1998

6-i

7-i

8-i

9-1

lo - i

12-1

SNAMEISNAJ Conference il August22 - 23, 1996

Observations on Conventional andAdvanced Double Hull Grounding Experiéments 13 - 1 James L. Rodd (Carderock Div., Naval Surface Warfare Center-ONR)

Design Analysis forGrounding Experiments 14- 1

Paul P.M. Lemmen, Alex W. Vredeveldt (TNO Centrefor Mechanical Engineering) & Jo A. Prinkster (Deift University of Technology)

Tanker Environmental' Risk - Putting the Pieces Together 15-1 Alan Brou & Michael Amrozowicz (Massachsetts.lnstitute of Technology)

A DesignPerformance Standard for Oil Tankers 16 - I

Paul Cojeen, PatrickLittle, Daniel Dippenger & Jaideep Sirkar (US Coast Guard)

Presentations and Panel Discussions (no written documents)

Dutch-Japanese Large Scale Model Test Program

DAMAGE (DAMage &ssessment ln'roundingvents) Computer Program

Monique Sinmao (ICF Kaiser International, Inc.)

US Navy ONR'ModèlTests'of Single and Double Hull Tanker Groundings

Incorporatiùg Structural Research into Regulations: Presentationsand PanelDiscussion from the Major Classification Societies

Don Liu (AmericanBureau'of Shupping), Elke Lehmann (Germanischer Lloyd) & Jacob Polderman (Lloyd's Register)

Design analysis for grounding experiments.

Paul P.M. Lemme&, Alex W. Vredeveldt' and Jo A. Pinkster2

ABSTRA CT

in / 995 a series of six grounding experiments has been carried OUI with a 600 Tonne inland water way tanker. .41 the how of the t'esse! test sections could befitted, _{which were run into an artilcial rock. The design of the}

support structures for the tesi sections and for the rock requïred the_{prediction of grounding forces and ship}

nzoti(ms. The crash behaviour of the test sections was assessed by applying explicit finite element calculations. Hvdrodvnan,ic forces were intes1igated by running 3-D diffraction calculations. This paper deals with the design approach for the g ounding experiments and gives some details. on the calculation methods used. Predicted loads and penetrations are compared with measured_values.

INTRODUCTION

Throughout history an interest has existed forthe

assessment of the crashworthiness of ship structures Initially this interest originated from _{war fairing needs.}

Rut since the late fifties non military motives_{became the}

main drive. Especially when the feasibility of nuclear

powered ship was investigated crash worthiness _became a paramount issue. During the sixties the preventions of

outflow of hazardous cargo became of major _{interest to}

naval architects. _{Initially the safety of operating gas} tankers was the main factor but soon the hazards_{of oil}

pollution became equally important. Another _{reason for}

interest lies in the assessment of damage stability of ships. 1MO regulations in this area rely on a assumed extent of damage based on inadequate damage Statistics.

In recent years explicit finite element _{methods have}

become available on cheap computers. This has_opened

the way to damage prediction calculations taking into

account the actual lay out and scantlings of the structure.

With this possibility the need has risen for _{validation by}

means of controlled crash tests. Two major problems exist with these crash tests:

Largely scaling down the size of the structure cannot be done in any reliable way.

2 _{Speed of deformation grossly influences the}

materialj behaviour.

Full scale crash testing. would, be the. most obvious solution. Unfortunately such testing is _{very expensive.}

However a balance between costs of testing and

usefulness of the measured results could be found _by testing at a moderately reduced scale. With _{respect to}

VLCC's the test scale which could be obtained was 1:4. In case of coastal and inland shipping the scale is 1:1.

mo Ccnu for Mechanical Engineeflng' Delit_{Ufliveralty of Technology}

TEST SET UP

In order to enable a large scale grounding test, a

scrap bound inland waterway tanker and a submersible pontoon were taken. After conversion at the bow, the

tanker could support testsections of about 7000x5000x800 mm.

The pontoon was fitted with a rock súpport ttructure. This structure supportedanartificiaI rock. The test section would be run into the rock. Figure 1 shows the general lay out.of the test set up.

Figure 1 Test set up.

The test section is supported between two catamaran hulls.

For this purpose the bow of the vessel and _{the forward} cargo tank were replaced by the catamaran hulls. At the

front of these hulls a portal beam was fitted for additional stiffness. Sections were connected to the support structure by instrumented stud boults. The artificial rock is similarly connected to the rock support structure.

o

DESIGN _PHILOSOPHY

The main objective of the grounding experiments has been to obtain experimental data on a large/full scale

test. _{This objective could only be met by causing}

sufficient, damage to the test section. On the other hand to much damage. i.e. damage to the supporting structures,

would disable following tests. 'The main difference with

normal design 'is that 'in ihecase of full scale testing there exists no' "safe side of the boundary'. Therefore applying some sort of safety factor is not possible

To cope with this situation a two step approachwas used:

I. _{Design the test set up as if all available input} data is exact and reliable (e.g. yield behaviour of steel is known including strain rate effects).

2. _{Consider the consequences of faulty input data.}

Three main areas of intrest could. be identified;

I. _{Crash behaviour of tesi sections,}

Planar ship motions 'prior to crashing.

Rigidity of section support and rock support.

Crash behaviour was of importance because of the design of load cells and the choice of the data recording method

The planar ship motion was of intrest because of the

accuracy of targeting.

The rigidity of supports had to be considered 'in order'to

make sure a realistic grounding was simulated. The latter

could be tackled by applying conventional analysis

methods. The main consequences of this analysis were a

rather heavy portal beam at the forward ends of the

catamaran hulls. Moreover is was decided to apply a

submerged pontoon as rock support rather than a single

pole driven into the river bed. In this_{paper no further}

attention

_{will be given to the rigidity}

_{issue. Crash}

behaviour and planar motions are discussed in the

following sections

CRASH BEHAVIOUR TEST SECTIONS

To determine forces in the stud boults of sectionand rock, numerical simulatiöns were performed using the explicit'

finite element program DYNA3D. A 'test in which the rock strikes the double 'bottom and the tank top was simulated. At the time of the design analysis this was

considered to be the heaviest collision. Before describing

the simulations an outline will be given of the solution scheme for integration in time as used in DYNA3D.

The explicit integration scheme

The integration scheme in DYNA3D expresses the

discretized equations of motion at time s, at which

displacements are known as follows

with M the diagonal (lumped) mass matrix' at time _{n' ,}

the stiffness matrix,f,,, the column of external _{nodal forces}

and ; the column of nodal degrees of freedom. _Since

displacements .znare known, accelerationsat time i simply

follow from

d2xIdl2=M,' i,flnXnJ

or

d' x, / dr2 = iii,, fin]

with f the column of internal nodal forces at time i,,.

Velocities and displacements at times resp. are

obtained from

dXn.i,7/ di = dXn.j./ dz+d2X,,/ dr'st,,

X,,.,j = X,, +dx,,,,7 / di ¿tjj7

with Lii,, the time step size at time i,,.

The advantage of this method is that only thelumped mass

matrix appears in the denominator and that there is no need

to invert the stiffness matrix. The obtained, set of equations is an uncoupled set of algebraic equations. As_{accelerations} areexpressed in known quantities, equilibrium iterationsare

not required.

A disadvantage of the method is that the requirementof stability puts .an upper limit on the allowable timestep ist,,

&,, 2 I

with _{the maximum.frequency of thesystem. The stable} time step size 'is equal to the time it takes for _{the fastest}

propagating wave to travel from one nodal point across an element to the closest other nodal point.

Use of this _{integration scheme is recommended for}

problems were highly dynamic or strongly nonlinear

phenomena occur. In those cases the time step size has to

be small anyway in order to' accurately _{trace the event.}

Collisions between ships and grounding events are typical

examples of events which can be simulated _{using the}

explicit finite element method.

CrashSimulations

For the investigation it was not _{necessary to include all}

relative degrees of freedom between rock and section. The

section was restricted from moving by _{restraining nodal}

displacements at 'the fixtures. Only nodal degrees of freedom along the connecting rods were restrained. The

rock was pushed into the section with _{constant velocity.} Computed results were corrected afterwards for inertia

forces of the section. The latter _{were found to be}

comparatively low. The rock strikes the _{section along its}

centre line.

After the second girder the crushing_{process repeats itself.}

Therefore the analysis was stopped _{after failure of the}

second girder which is'equivalent to a penetration of 2.5 rn In the regions were large deformations are expected a fine

mesh has been applied with element lengths of typically

65 mm. Further away coarser elementsare used. deformation. The Cowper-Symonds relation is often used'_{to express the dynamic yield stress ad as function of the}

strain rate t

c3d=a,II +(tID)"l

with D and p material constants, and a1 the static yield

stress. This expression was derived for the 0.2% yield

stress. This relation shows a steep increase of the yield stress at small strain rates (< 5 - IO Ils). At higher rates an

asymptotic behaviour is found.

Apart from the increase of the yield stress, material tests

also indicate a reduction of the tangential stiffness at

increasing deformation rates.

In the computations strain rate effects should be included.

However, reference [5] describes a detailedanalysisof drop

tests using DYNA3D. There it is found that computations overestimate the stiffness when strain rate dependency is

included. This may bedue to the element formulation used

in DYNA3D which employs a reduced integration. The elements suffer from spurious zero energy modes. These 'are restrained by artificial springs or dampers. Rélated coefficients are based on elastic constants of the material.

Furthermore deformations related to the zero energy modes

are not included in the equivalent strain. As a result the elements respond too stiff in caseof plastic deformations. Thiseffect cani at least partly, becompensated by reducing the yield stress.

At contact areas between rock and section a friction

coefficient M of 0.3 is assumed. In order to investigate the

influence of this coefficient additional analyses without

friction were performed.

All simulationswerecarried out with a onstant penetration

speed. To reduce CPU time this was set at 10 m/s which is higher than the actual groundiñg speed. The consequences

of this were investigated by an analysis at a penetration speed of 5 m/s.

Results

Figures 3 and 4,give typical result for the resultant forces acting on the artificial rock. Figure 5 shows deformations and cracks in the shell plating for one of the calculations. Tables Ito Ill summarize some relevant simulation results.

Maximum forces in the stud boults between catamaran and section are given in table 3.1. The section was connected to

the catamaran by eighteen transducers: four acting in

vertical direction, four in longitudinal direction and the

remaining ten in transverse direction. Data in columns five through seven of table 31 relate to maxima in timeover aH

transducers acting in direction specified in top of these columns. The last two columns give maxima of the total forces acting on the rock. Note that the rock strikes the section along its centre line. Therefore there is no fòrce resultant in transverse direction, which can also be seen

from figures 3 and 4.

SNAME/SNAJ Conference

_14-3

_{August 22-23, 1996}

Figure 2 _{Finite element mesh of test section and} rock

The test section is manufactured of 5 mm mild steel

(St32). The material was modelled as an elasto-plastic

material with linear hardening. The Young'smodule is 2.1

lO' N/mm2, the Poisson's ratio 0.3. the yield stress 235

N/mm2, and the tangential stiffness 445 N/mm2. Material

failure is included by setting stresses in an integration

point to zero as soon as the effective plastic strain c

exceeds an ultimate value £. Input for the ultimate strain is strongly related to the element size. This is due'to the

stress concentrations in front of cracks and the fact that the distance between

a crack

tip and the closest

integration point varies with element size. A relationship between the ultimate strain and the element lengthcan be

derived from fracture mechanics approaches. Crack

growth will be plastic. In that case the stress concentration

at a crack tip will vary linear with I

_{/ r, with r the}

distance to the crack tip (I]. With this relation the failure strain for arbitrary element sizes can be calculated when the failure strain for a given length is known. Based _on findings from posi test analyses on full scale collision tests with inland waterway vessels [3] the failure strain at

an element length of 65 mm was found to be 0.2. To

further investigate the influence

of this

parameter, simulations were also performed for a'failúre strain of 0.3.

lt should be noted that the above method reduces the

effect of mesh dependency but does not eliminated it. If, for instance, a crack grows between two opposite_corners

ratherthan two opposite edges some deviations will _occur In recent years advanced methods have been developed to

cater for mesh dependency. These include strain rate

dependency or higher order derivatives of the strain in the Constitutive equations

_{[2]. A disadvantage of these}

methods is the need for highly refined meshes 'in critical

regions.

This makes them unsuitable for analysing

crushing of large structures.

'3 OtO

Figure 3

00(0

00 00 IO 1.6 00

D.nIV.. dsV. (m) -.

Table J Maxima of forces up to a penetration depth of 2.5ni.

26

Contact force between section and rock for calc. 2: E,-= 0.2; y = 10 mis; p = 0.3

(F. = longitudinal force, F, vertical

force, F2 transverse force)

Figure 4 Contact forces between section and rock

for calc. 4: = 0.2; t' = 10 m/s; p = 0.0

(F0 = longitudinal force, F, vertical force, F2 transverse force)

CaIc. t, p

forces in transducers between section and catamaran forces on rock

-longitudinal transverse vertical longitudinal vertical

0.3 IO 0.3 1400. 1250 1000 4500 2000 2 0.2 lO 0.3 1200 800 800 3000 1500 3 0.3 lO 1000 1-200 1200 3500 2500 4 0.2 lO . 1100 1000 1000 2500 2000 5 0.2 5 . 1000 11100 _{1000 2500 2000} Caic. _s-(m/sJ p

forcesin transducers between section and catamaran forces on rock

longitudinal transverse vertical longitudinal vertical

I 0.3 lO 0.3 1:100 800 -800 3400 ' 1600

2 0.2 lO 0.3 640 500 560 2200 1100

3 0.3 10 . 640 800 900 2100 2100

4 0.2 lO 400 500 680' 1400 1400

5 - 0.2 5 . 400 500 _{'700 1400. 1400}

Cale, c y Im'J . p internal

energy [kNmj % dissipated by membrane deformation % dissipated , 'by bending deformation ' % dissipated by in plane shear -deformation % dissipated by out of plane shear

deformation I 0.3 10 - 0.3- 5505 -- 53 14 - 3j . 2 2 . 0:2 10 0.3 3864 . 50 18 30 2 3 03 lO . 5465 51 14 : _{33 '2} 4 0.2 10 3724 49 18 30 3 5 0.2- 5 - 3713 48 18 31 3

Table Il Average values of computed forces.

Table Il! _{In ernal energy dissipated in the section for a penetration depth of 2.5 ni.}

SNAME/SNAJ Conference

SflIP MOTIONS

The main question to be answered

was ìf

dUthig

approach of the ship, the bow of the ship would tend' transversely towards the rock ora fivm, the rock. In

orderto find an answer. to thisquestionaflowcailnf4

the Deift Tetthnical University. Tothis

end use was made of the PASSHIP

_{program. This} computer code Is based on a finite beundaty lPmMi

mhot ftis1rulatra a'veloclty

poalIal

ansiad finadag

Objecta based on donblebodypsezitjaIflow.

The 5Dverning;equasions.

14-5

August,22-23, 1996

Figure 5 _{Deformationsand cracksin shell plating} at 2.25 rn penetration.

Average values of forces in the stud bolts and the

resulting forces on the rock are given in table 3.11. It is found that computed forces for a failure strain of 0.3

(calc. I and 3) are considerably higher than those for a

failure strain of 0.2 (calc. 2 and 4). The

_{average load}

acting on the rock increases about 50 % in

both directions.

If friction between section and rock is omitted, the force in grounding direction reducesistrongly. The contact force in vertical direction increases slightly.

Table 3.111 gives data related to the energy dissipated in the section. The fifth column gives thedissipated_energy Subsequent columns give the contribütion of deformation

modes to the internal energy. It is found that

energy dissipation ismainly due to in planemembrane (50%) and

shear (30%) deformations. Results for the

_dissipated

energy are found to be insensitive

_{for the}

_friction

coefficient used.

From tables I to 111 it follows that results _{for different} grounding speeds agree well. Effects of local inertia do

not affect the results for the considered grounding speeds.

with: _N number of independent moving or flxed vessels,

1

potential due to vessel jwhich may be flxed or

moving at a constant speed y along a straight line.

In general the potcntials have:to satisfy the equation of continuitT

62. 62.

_62.

6X

aXoX

with: X, i-1,2,3'the earth boundcoordinates

At the sea floor (X3 -h) and at the mean waterline (X3 O) vestiraI velocity is zcro

60 -6X3

Onthewetted surface of pontoon and ship thó normalvelocuy mu be equal: to nonnal velocity of considered surface:

ân'

with: velocity at point of the hull of vessel j,

normal velocity of the hull of vessel j, positive

'into the fluidi

A panel method is used to find a solution for the poteadel. AttheLcentre f cach panel: aRANKINEsource is dnfbwd

lfl:sUCh:a way that aliconstinints are sathfied

When the potential function is 'own,pressures can be calcelated by applying Bernouillj!s law. Integration over the wetted surfuce yields hydidynamlc forces.

Figure 6 shows the panel distribution for ship and pontoon

Figure 7. shows the calculated Yaw moment during the approach manoeuvre for 0:0 and 0.5 m offset between

centrelines of ship and pontoon.

Yaw moment

Figure 8 Predicted and measured forces in longitudinal direction.

CONCLUSION

The available calculation tools proved to be

adequate for the design of the grounding experiments.

Input data for the calculations must be based on

previous test results. Especially failure strain in relation to the element size strongly affects the calculation results.

Because of the

satisfactory predictions it is recommended that crash analysis tools are used in the early design stage.

ACKNOWLEDGEMENT

TNO .entrefôr Mechanical Engineering expresses

its gratitude towards the Japanese Association for the

Improvement of the

Shipbuilding Industry for their

permission to use some of their results iñ this paperi

Discussions with Mitsubishi Heavy IndUstries, the

Technical University Hamburg Harburg and Royal ScheIde

proved of great value for successful completion of the

experiments.

REFERENCES

Edwards, H.L., and Wanhill, R.J.H., "Fracture

Mechanics", Delftse Uitgevers Maatschappy, 1984.

Sluys, L.J., "Wave Ptopagation, Localisation

and Dispersion in Softening Solids', Dissertation,

Deift University of Technology, DeIft, 1992.

Lemmen, P.P.M., "Application of thç Explicit

Finite

Element

Method

in

Ship

Collision

Analyses", TNO-Report 93-CMC.R1 153, 1993.

[4]; NeWman LN., "Algorithms for the Free-surface

Green

Function",

Journal

of

Engineering

Mathematics, 1985.

[5]

Langseth, M., "Impact Loading of Plates A

Comparison between Numerical Simulations and

Experimental Resúlts", Proceedings of the Sixth

International Symposium on the Interaction of Non

Nuclear Munitions with Structures, 1993.

14 - 6 AugUst 22 - 23, 1996

0.0 0.6 1.0 1.5 2.0 2.5

Figure 6 _{Panel distribution for ship and pontoon.} p000iration d.pm Im)-.

Figure 7 Calculated yaw moment.

It can be seen that an offset of 0.5 in to PS results into a

yaw moment counteracting the offset. The bow will tend towards the rock.

cOMPARISON WITH MEASURED RESULTS

A comparison has been made between measured and calculated results.

Figure 8 shows the contact force in longitudinal direction versus penetration depth for test 2. Calculated results refer to the case where friction is included and failure strain is set at 0.2. As òan be seen predicted results are larger than

the actual values. This can be; explained by the fact that the vertical motion of the section was restrained in the

analyses.

Unfortunately measured ship motions data could not be used for comparison with the calculated hydrodynamic

forces during approach. The numerical values of the

motions are orders of magnitude lower than the values

during the grounding. Thus these values disappeared into general noise levels.

However no problems

were

encountered with targeting.