Comparison of structural behaviour of wet deck panels made of different materials under slamming loads

Comparison of structural behavior of wet deck panels made

of different materials under slamming loads

Katsaounis,

Georgios M.’,

ABSTUACT

i%e paper presents aninvestigation of the behavior of wet deck panels @a SES under slam”ng loads. The aarn.t”nation wws pwftmned by the execution of slannu”ng tests, using fidl scale models of the inq-wcted structure qf the wet &ck of the ship. Two difenmt stnufuml &signs of the SES wre aann”ned. l% model corresponding to the first &sign w made by ahuniw”um alloy, while the second had a composite structure made up by Cm skins atui balsa core. 71w expen-ments aimed to &termine the response @the wet deck under dynamic pressure load and to compare the behavior of the dl@immt structural solutions. Representative time histories @the induced pressures and stmins are presented. i%e maximum recorded pressures are related to the drop height, the impact energy and the momentum of the panel prior to the slam. lle compmison of the two structumi solutions, the @ect of air cushioning on the induced pressures, the relationship between induced pressures and inalmed strains and the useof drop tests results in the estimatkm ofskmuning &sign pressures are also discussed.

INTRODUCTION

The object of the paper is the investigation of the behaviour of wet deck panels of large SES vessels,

under slamming Ids. The work was performed

within the project MATSTRUTSES - Advanced

materials and design pmwkre for large size SES

structures, which was tinancd by DGXII of the

European Commission. The work presented herein

mainly focuses on drop tests, conducted on full scale

models of the wetdeek, shown encireled in figure 1.

The goals of the tests were:

> The comparison of the behaviour of different

structural solutions of the wet deck

> The detetmimtion of the reqxmse of the wet deck

under dynamic pressure loads

E The generation of adequate and emugh time

histories of the response of the wet dee~ which

could be used for the evaluation of numerical simulation procedures of the wet deck slamming. Tests were carried out on models corresponding to two

structural solutions, and they were both full-scale

models of the simulated wet deck panels.

A numerical analysis was ihrther performed in

order to obtain a relationship between the pressures and strains, developed during the water impact.

THE MODELS

The first model tested - thereafter aluminium

model - which is shown in figure 2 (Samuelides et al,

]ResearchAssociate,Dqmtrmmtof Naval Arebitedure and MarineEngineuing NationalTeeltnkalUniversityofAthensjtet(301) 7721425, fkX (301) 7721412, e-mail: kats@Mab.@ua.gr

2AssistamPmfeaw, Dqartm@ of NavalArcbktore andMarineEngimering,NationalTechniealUniversityof Athens, tel (301) 7721421,

fax (301) 7721412, e-mail: mssrouel@kslab.ntua.gr

CA%CARGOCONMTKM Tmnxs CARGOCOND17I(M

— ~

-.

_._%l. .- . –.-.. _.+. –.–. _ - .. ..K.-...

Figure 1: SES under investigation

199’7)was fiibricated by Chantiers de 1’ Atlantique and

it is made of aluminium. The bottom plating is 12mm

thick and the stiffeners attached to it have the same

spacing and characteristics as the longitudinal

stiffeners of the panel of the aluminium solution of the

target vessel. Further, tbe spacing of the I-beams,

which are normal to the stiffeners, equals to the

spacing of the web frames of the wet- deck.

The second model (Katsaounis et al, 1998)

-thereafter sandwich model - was constructed by the

Stiffeners * \_\ “7 “4 ‘1

I

●

Figure2:Ahmdnmnmodel and location of sensors

+ 1974--4s, , ~r (>8 4 ,)38 A?$ _A A 4 !ss 6s 7s )2s v z 01 () 2 (33 v - 9s Ss B

F@IW 3: CFRP balsaconstruction and sensors

Institute for Design Methods and Construction of the

Eidngenoessische Technische Hochschule

Zuerich-ETH (see figore 3). It is a sandwich construction having a 14cm thick balsa core and skins comprising 21 CFRP layers each. The model was fixed to an orthogonal steel frame made of I beams.

EXPERIMENTAL PROCEDURE

The testswere carried out with the co-operation of

the Ship and Marine Hydrodynamics Laboratory and the Laboratory of Harbour Works both located at tie

Zografou C&PUS of the National Technical

University of Athens. The model tested was lefi to fall

freely from a predetermined drop height in a water

tank, equipped with wave generators. The parameters that varied during the tests were the initial drop height,

the mass of the model and the surface of the water. The variation of the input parameters and the number of the tests conducted are presented in Table 1.

In general the pressures induced during the impact and the strains developed within the model, depend both on the impact speed and the mass of the model.

To investigate the effect of the mass two

configurations of both the ahuninium and the sandwich

models we~ tested: the light configuration and the

heavy configuration. The light configuration included

the maids and the minimum extra weights, which

were necessary for tbe tests. These were the

suspension mechanism, the added freeboard in order to prevent the model from emerging in the water, the covers of the upper side of the models and the cables. For the construction of the heavy configuration I beam frames were added along the edges of the model. The weight of the light aluminum model was equal to 311 Kg and that of the heavy configuration 698 Kg. The

on the pads and accelerations of the structure. The

sensing instrume nts installed on each model were:

> 6 Piezoresistive pressure sensors (type KYOWA

PS-1OKB)

> 2 Piezoelectric pressure sensors (type KISTLER

7261)

> 1 accelerometer (type Bruel & Kjaer 4370)

> 8 strain gauges (type KYOWA

KFG-5-120-Cl-23L1M2R) for the aluminium model

> 9 strain gauge rosettes (type KYOWA

KFGIO-120-D16) for the sandwich model

Figures 2 and 3 show the location of the strain and

pressure gauges mounted on the aluminium and

sandwich models respectively. The strain and pressme gauges are closely spaced in the aluminum model. The

numbering of the location of the strain gauges is

followd by an “s” in figure 3.

The above instruments were cxmnected via

appropriate cables to following amplitle~ whose

TABLE 1: Number of drops - range of input parameters

Light configuration Heavy configuration

Model Water Alumimun 311 kg Aluminum 698 kg

surface Sandwich475 kg Sandwich 867 kg sum

Drop heigl ,t in meters

0.50 1.04 1.39 1.70 2.00 2.50 0.50 1.04 1.39 2.00

Aluminum Flat 4 4 6 5 5 6 13 11 12 11 77

Disturbed 5 5 6 - 5 - 11 - - - 32

Sandwich Flat 10 11 11 - 16 9 8 8 8 12 93

Sum 19 20 23 5 26 15 32 19 20 23 202

correspding weights of the sandwich model were

475 and 867 Kg.

In order to examine the effect of the geome@ of

the water snrfkce on the response, the ahnninium

model came to impact with an undisturbed and a

disturbed water surface. The disturbed surface was

gemated by sinusoidal waves having a length of 1.5

m and a height of 7 cm. The generation of waves aimed as already mentioned in the investigation of the effect of the disturbed water surface on the wet model response and not in the simulation of the impact of the wet deck on waves included in a specific sea state.

MEASURING AND ACQUISITION SET UP

The acquisition set up includes instruments

capable to record slamming pressures, strains induced

output were monitored by digital acquisition devices,

installed inside three persomd computers. The

measuring and acquisition equipment was placed on a moving platform located near the drop area.

RESULTS

The induced pressure and strain histories as well as

the rigid body acceleration of the model were

memnred and recorded at various locations. F@ures 4 to 7 present the measurements of pressures and strain histories during four tests, whose parameters are rdso included in the figures. Results in terms of the spatial

average of the induced pressures versus the drop

height, impact energy and the momentum of the

impacted panel prior to the slam are shown in figures 8 to 10. Further the induced strains at some locations plotted versus the spatial average of the maximum

I

I

Light configumtion Heavy configuration

Water 311kg/475kg 698 kg 1867 kg

Model surface

Drop heig % in meters

0.50 1.04 1.39 1.70 2.00 2.50 0.50 1.04 1.39 2.00

Flat 0.36- 0.68- 0.80- 0.93- 1.04- 1.43- 0.87- 1.36- 1.49-

2.04-Alumi- 0.60 0.97 3.18 2.14 2.30 5.57 1.44 2.21 3.56 3.72

nnm Dis- o.17- 0.36- 0.33- 0.46-

0.41-Turbed 4.32 4.43 6.21 6.10 4.19

Sandwich Flat o.55- 0.98- 1.18- 1.69- 1.97- 1.12- 2.06- 2.56-

3.24-0.97 1.79 2.24 3.25 3.28 1.78 3.43 3.69 5.30

values of pressures are shown in figure 11. These

figures also include the liis corresponding to the

uniform presmre -strain curves obtaked by astatic

finite element analysis of the tested panels described below.

Tables 2a and 2b show the range of the measured pressures and stains respectively for all tests.

RELATIONSHIP OF PRESSURE VSSTRAIN

A numerical analysis of the structural response of the models under uniform pressure was pertlmned. The goal of the analysis was

a. To examine whether the induced strains are of the same order of magnitude when compared to the strains developed if the spatial average of the local maximum values of the induced pressures is applied statically on the models, and

b. To predict the induced stresses during the tests

in locations other that the locations of the strain

gauges. The accuracy of such a prediction will heavily depend on the resolt of a.

Aluminium Model

The analysis was performed using the FWe

element code MSC/NASTRAN. The bottom plate of

the drop model was modeled by 4,500 shell elements. The bottom stiffeners were modeled by 240 beam elements. The frames, surrounding the bottom plate,

were not modeled, a consequeme of the imposed

boundary conditions, as explained below. In total

4,636 nodes, having six degrees of freedom each, were used.

The FEM model was supported at the perimeter by the application of boundary conditions, in order to

restrict the displacements and rotations of the

corresponding nodes so as to simulate the connection of the test model to the rest of the actual wet deck

Mass 6e8kg, Iie4gtO.sm,DMwtedwdeJ

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bdor-1 kulbn-2 Iw#cwKl

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F-4: Impact of aluminum model on disturbed water surface.

Pressure histories

Mms8698kg,lil@to.an,Diah9bi* 300

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54 1o11 1% ZO050 100 150 200YI Im 150 Zoo

IO@br=? klmUW—8 lc.@ul-9

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IOcdkn-1 kwauDn=2 bcdiaP3

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Figure 5 Impact of aluminum model on disturbed water surfiwe.

Ma55xw~,Iml@xlml 6,0 4,5 . . . . ... i----,.. 50 100 150 Zuoso 100 w 20050 Im 1s4 200 ~1 bc81kQ.2 Iwm@l—3 llnm(M6ec)

Fiaure 6 Immct. of sandwich model on Rat water surf’e. Pressure historb

Mass 867kgr, Height 2.On

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Drop Height in m

Tigure 8: Spatial average of peak pressures versus drop height

5 4 3 2 1 0 o 1 2 3 4 5 -6 Momentum of model in kN-s

Figure 9: Spatial average of peak pressures versus momentum prior to impact

5 4 3 2 1 0 o 3 6 9 12 15 18 Impact energy in kJ

Figure 10 Spatial average of peak pmasures versus impact energy

structure. The fixed boundary conditions approximate also the stiff web fkames surrounding the bottom panel and the stiffimers.

The FEM model was then submitted to a uniform

static pmsure, applied over the bottom surface and

having a magnitude of om bar. A hear static analysis

was performed. The von Mises stress pattern,

computed by this analysis is shown in figure 13. The results show that the plating between the stiffeners is

deformed like an isotropic plate, subjected to a

uniform pressure and having clamped edges along the lines of intersection with the bottom stifkmers and the web frames. Examining the bottom plate of the model, the maximum stresses are observed at the boundaries of each panel with the longitudinal.

The maximum stresses for the bottom stiffeners were observed at the connections with the transverse

frames. When brackets are inserted, between

longitudinal and transverses, the values become 20%

to 42% lower depding on the effkctive length of the

bracket - it has been taken equal to 100 mm to 200

mm respectively. In the absence of brackets the

maximum compressive stress corresponding to one bar

slamming pxessure, was fd to be equal to 91 MPa.

The pressure-strain relation for a representative

point of the bottom plate, resulted horn the FEM analysis is presented in figure 11, together with the experimentally obtained values.

Sandwich model

The analysis was performed using the finite

element code SOLVIA, which provides the element library necessary for the modeling of the composite panel. The sandwich panel of the model was divided into 80 composite shell elements using 43 layers for each one of them. The panel was simply supported along the perimeter by the application of boundary

conditions restricting the displacements of the

corresponding nodes. It was found by preliminary

analyses that this modelii simulates sufficiently the

installation of the test panel inside the surrournling I-frame (figure 3). In total 775 nodes having 6 degrees of freedom each were used.

A uniform static pressure of one bar was applied to the panel. The maximum displacement is found at

the centre of the panel. The stresses along the

orthotropic axes SAA and SBB, in the skin material (CFRP) take a maximum in the outer layers, at the centre of the panel. The shear stress takes a maximum in the vicinity of the corners of the model. The graph of the shear stress SAB for the balsa core layer is shown in figure 14.

The calculated values of the maximum stresses

correspd to a loading of the skins well bellow the

smength liits of the CFRP material (see Table 6).

The strains corresponding to the measured

Iocations during the experiments were depicted from

results of the finite element analysis. These values

were used to establii the pressure-stmin relations

based on both experimental and numerical data,

presented in figure 11.

The maximum normal stress imluced in the balsa

core material was equal to 0.97 MPa. The shear

stresses take maximum values either near the comers of the panel or inside a thin region at the boundaries of the panel. The maximum shear stress observed was equal to 0.4 MPa. The maximum stresses of the balsa material are of the same order of magnitude as the strength limits of the material according to Wallat

(1997).

2M0

A ALWIMIM 698 I@

lam _—+ ALUM!WM$11W_FEM

12M

400

0

Izm

Ma+

2d6

prw

~m 11: Pressure versus strain

measurements and numerical results

COMPARISON OF THE TWO STRUCTURAL

SOLUTIONS

The two stmctmal solutions are compared

following two alternative procedures: the first is based on the c-&nparison of the- induced pressnres, whereas

TABLE 3 Relationship between pmmure and drop heighti Ap = FH H“

r

(w

Almninium model Sandwich model

Apmr] =2.31 (M~]/1000)” H[m~ Ap~r] =2.51 (M[kg]/1000~ H[m~ n=O.79 n=O.85 311 0,92 0,93 475 1,28 1,33 698 1,74 1,85 867 2,06 2,22

TABLE 4 Relationship IMtween pressmw and impact veloeity : Ap = Fv V’

Mass Fv

(k)

Aluminium model Sandwich model

Ap~] =0.22(M&g]/1000~ V[nis]m Aphr] =0.20(M~]/1000)” V[rn/s]m

n= O.79m=l.58 n=O.85 m=l.70

311 0,09 0,07

475 0,12 0,11

698 0,17 0,15

867 0,20 0,18

the second on the comparison of the ratios of the

differeme of the maximum allowable stresses minus

the induced stresses over the maximum allowable

stresses for each material. In both cases it was

attempted to compare the results corresponding to

identical parameters.

Comparison of induced presmms

The analysis of the pressure results, revealed that

the induced pressures may be presented as a

transcendental tition of the ewrgy of the impacted

model prior to the impact, i.e. Ap=CE’,

where Ap is the induced pressure over the atmospheric

pressure, E=mW/2 is the kinetic energy of the

impacted model prior to impact, m is the mass of the model, V is the speed of the model prior to the impact

and C and n are constants. On the basis of the

maximum values of the local pressures as defined from

610 arul 668 dme histories of 77 drops for the

alurninium model and 93 drops for the samiwich

model respectively, it has been fotnxl that the

constants C and n are as follows for both models, and

for Apinbarand EinkJ:

Ahnuinium model C: 0.38 m 0.79

Sandwich modd C: 0.36 m 0.85

The relationships which were established between

Ap and E may be easily transfimned in relationships

between Ap and drop height H or impact velocity V, which however depend on the mass of the model M.

Tables 3 and 4 pment the relationships between Ap

and H and Ap and V for both models. The

relationships have been established for the following values of the mass for both models: 311 kg, 475 kg, 698 kg and 875kg.

The relationships between pressure arrl drop

height are described in Table 3 and are plotted in figure 12. Lines corresponding to all four values of the mass are plotted in the figme, but it must be stressed that the four lines representing the behaviour of the aluminium model are based on the results of models having a mass of 311 kg and 698 kg and those four, representing the behaviour of the sandwich model, are based on the results of models having a mass of 475

kg and 867 kg. Figure 12 also includes the

experimental results. The comparison of the defined

lines versus the experimental results lead to the

cordusion that for equal mass of models, the induced

pressures during slamming expected in an aluminium

model should be lower than the corresponding

pressures induced on a sandwich model. This

observation, which is attributed to the elasticity of the models, is apparent for the case of the heavier range of masses tested. However the effect of the elasticity is 43

not so strong to consider it as dominating the phemnnenon.

Comparison of the induced dresses

On the basis of the measured strains and the finite element anslysis described above, it is possible to deduce the strain and stress patterns induced on both

models, during the impacts. The results of the

numerical analysis comespond to the response of the models under uniform static loading. Tables 5 and 6 show the values of stresses obtained from the FE

analysis, in critical @ts and for a uniform applied

pressure Ap of 1 bar. For both models, the values of strains obtained from the FE analysis correlate well with the values of the measured strains at the locations of the strain gauges when the applied spatial average of the induced pressure is also 1 bar. This agreement is also valid for the range of pressures, in which the

response of the pads is limar.

Using the above results, it is possible to compare the structural response of the two different structural solutions under slamming, by comparing the “safety

margin” of the induced stresses under the same

pressure impulse. The “safety margin” is calculated as

the ratio of the difference of the strength of the

material from the induced stress over the value of the material’s strength. It is clear that such a ratio may be calculated for any point (see Tables 5 and 6) within the model, so that the values compared are the minimum values for each model.

As presented in Tables 5 ad 6, the most critical

point for the aluminum model was fti to be the

connection of the stiffeners to the tmnsverse frames

and, for the sandwich model, the balsa core. The

corresponding safety margins were 44% and 68%

respectively. Other critical points with comparatively

low safety margin are the intersections of the

almninum plating with the longitudinal stiffeners.

The comparison presented in this section is based

on models of different material having equal

dimensions, i.e. length and breadth. These models

correspd to the same panel of the wet-deck.

However, it is pointed out that the aluminum model

corresponds to a panel of the wet deck between

longitudinal and tmnsverses, whereas the sandwich

model does not correspond to any supported pad,

which in geneml has larger dimensions. Thus the

response during slamming is expected to yield higher values of stresses in this case.

A Light %mdwioh -475 Kg Q Heavy Sandwich -867 Kg

A Light Aluminium -311Kg ● Heavy AI.mini.m -698 Kg

— Equation based on the Sandwi.h Mcdel — Equs&icm based O. the A1.mirmmnModcl

0.0 0.5 1.0 1.5 2.0 2.5

Drop Height in m

Figure 12 Average of peak pressures versus drop height

TABLE 5 Snfety margins for the ahmdnium model

I

I

Yield Stress Panel urder 1 bar

Mated slam. Pressure. Safety marginl

MPa MPa

Aluminium pklte (t = 12mIn) 36 77%

124..200

Stiffeners (without brackets) 91 44%

Stiffeners (with bracket 100mm) 73 55%

Stiffeners (With bracket 200mm) 53 67%

1Based on an averme vield strerwth of 162 MPa

TABLE 6 Safety margins for the sandwich model Max stress

(FEM)

Strength Panel under 1 bar Safety margin

Material slam. Pressure

MPa

MPa

CFRP skin - Tensile 693 12 98%

CFRP skin - Compressive -611 -12 98%

Balsa core - Tensile 13 0,97 93%

Balsa core - Compressive -13 -0,97 93%

Balsa core - Shear 2,98’ 0,40 87%

*the value was furnished by the manufacturer of the pane 1 -ETH. DISCUSSION

Ahmdnium Model

The superposition on one hand of the lirwar

pressure - strain relationship obtained by FE analysis for 8 location on the surt%ce of the model, and on the other hand of the pressure - strain points Obtaimd from the measured pressures and strains, reveal that the induced strains obtained if the average of the measured pressures is applied uniformly and statically on the panel are of the same order of magnitude. This occurs in the case of slamming on flat water surface, whereby the pressures act for relatively long time with

respect to the natural periods of the impacted pad. A

discrepancy of the afbrementiomxi results, which

concerns measuring locations 4 and 6, is attributed to the interference with the installation of the two large diameter pressure sensors near to these locations. It is therefore concluded that the damming of tit plates on undisturbed water surface produced during the tests may be considered as “quasi-static”.

The maximum stresses induced on the model were

observed at the conmdon of the longitudiml

stiffeners to the transverse frames. The level of the stresses at this point results in a small safety margin. However 20% to 42% lower stresses may result if brackets with 100 mm to 200 mm effective length are inserted. A further critical point having high stresses is the boundary of tbe plating, along the line of the intersection with the longitudinaks.

Sandwich uwdel

The superposition on one hand of the linear

pressure - strain relationship obtained by FE analysis for 14 location on the surface of the model and for strains in two directions, and on the other hand of the

pressure - strain points obtained from the

memurements, reveal that the induced pressures

during slamming could be simulated as uniform static pressure. This occurs in the case of slamming on flat water surface whereby the pressures act for relatively short time with respect to the natural period of the impacted panel.

For impact velocities up to 4(2 x 9.81 m/s2 x 2 m)

= 6.3 nds the induced avemged pressures range

between 0.62 bar and 4.5 bar, which is well above the pressures obtained by the extrapolation of the current regulations to the SES under consideration (Gran et al, 1994). However for slow application of this pressure, i.e. quasi-static response of the panel, the stresses

which develop in the skins are well below theh

strength limits. This remark is based on both the measured strains and the strains obtained from the FE calculation. The lowest margin of saibty is expected for the balsa core, whose strength may be even lower than that presented in Table 6.

Air cushioning effect

A strong air cushioning eftkct is detected when the surface of the water was flat. Alr compression under

the model resulted in relatively smooth pressure

histories, which is clearly indicated when the presswe histories of figure 4 (impact on a wave valley) are

compmed to those of figure 6. The prestme of the air

cushion was also investigated through the examination of photographs taken mseconds before the impact of the models.

During slams on calm water surface, the

development of the air entrapment below the tilling

model caused the pressure histories to have

frequencies well below the first natural frequency of the structure. In these cases the spatial average of the maximum pressures, applied statically, yields strains, which are of the same order of magnitude as those

measured. The calculation of the strains is based either on pure bending theory of isotropic plate, or on finite element calculations.

Presswe impldae

During impacts on calm water surface the shapes of the recorded pressure histories were similar to those presented in figure 6. Although the peak values of the

pressures were varied, comspodng to tbe drop

height, the overall shape of the history was the same,

independently of the specific model or mass

considered. The pressure reaches a maximum during

anintervalof 8- 10msec from the start of the impact.

After that moment, a decompression follows and the

pressure becomes negative, reaching the minimum

value. Some decaying oscillations of the pressure

follow. The period of the first positive pressure

impulse was about 16-20 msec. The variation of the

pressure histories during impact on cahn water was found to be in good agreement with the following

formula, prOpOSedby ~uang (1966):

-L4~

p(t) = 2pwe T sin n ~,

where T =41J cair, L the half breadth of the model ad G& the speed of sound in air.

Impacts on disturbed water surface

In the cases of slams on disturbed water surface the pressme histories were substantially different from those of flat impact. The recorded maximum pressures

were either greater than the cahn impact results

(impacts on wave crests), or lower than them (impacts on wave valleys). The records demonstrate high peaks during impacts on wave crests. The interval for the pressure to reach the peak value is about 0.8 msec. The pressures measured during such impacts are about four times the pressures measured during impacts on

tit water surface. However such peaks are very

sensitive to the sampling rate used by the acquisition

devices and to the subsequent filtering analysis.

Further, the pressure peaks do not relate well to the recorded strains, as in the case of impacts on calm water surface. The strains in the case of impacts on

disturbed surface were lower than those recorded

during impacts on cahn surface, for the same level of the maximum recorded pressure.

Integration of time histories

The recorded pressme histories were integrated in time, in order to examine the impulse of the pressures transferred to the models. The first peak value of the

integral, occurring just after the impact, is related to the change of momentum of the tilling body, which when integrated over the impacted area should equal

the momentum of the model prior to the impact.

Under the assumption of a uniform application of the pressure over an “effective area” it has been found

that this “effective area” equals to 80-90 % of the

nomiml wetted am of the tilling model, which is

1.85x1.5 m.

Similar conclusions were made from the

integration of the acceleration signals. The resulted

maximum velocities were 90-95 % of the change of

velocity of the models calculated on the basis of the formula ~(2XheightX9.81). Taking into consideration,

that the experimental mea surements were ve~ ‘fist’,

with a sampling frequency at a level of 10 Id% and a very short duration, then the integration of pressure and acceleration histories provide an indication of the accuracy of the measuring and acquisition procedme, which has been followed.

CONCLUSIONS

Comparison of the behaviour of ditYerent structural

solutions of the wet deck

During impact of flat plates on undisturbed water

surface, the recorded pressws were uniformly applied

over the wetted surface of both the models. During all such impacts, the shapes of the pressure histories were

similar. The slamming pressures for the heavy

aluminium model were somewhat lower than the

corresponding values of the sandwich model. This is a

result of the different elasticity of the models.

However, the test results did not make apparent a

dominant effect of the elasticity on the induced

pressures. This is attributed to the effiwt of the air

entrapment, which produced a cushion having a much higher elasticity that the structural elasticity of the models. As far as the structural response is concerned,

the “safety margin” was higher for the sandwich

model.

However it should be noted that this finding is

based on investigation of pads made of different

materials but having equal unsupported span - length and breadth. If a panel made of a sandwich material has considerable larger unsupported dimensions than those of a panel made of steel or aluminium, which is often the case, then the bending rigidity of the former may be considerable lower of the latter and in this case it may have an influence on the induced pressures, and the corresponding stresses.

slamming design presmra for the SW Vemel

~ tests on full-scale models of the wet

deck pads constructed by difi%rent materials were

performed. The impact velocity was ranged from 3.2 to 7 rn/sec. During impacts on caIm water surfhce the recorded pressures were found to be larger than the pressure values obtained by the extrapolation of the current regulations.

During impact of tit bottom on cahu water the

measured pressures were uniformly applied over the

wetted surface of the model. The shapes of the

pressure histories, which have a rather slow variation, were smooth, due to the effbct of the air entrapment.

The application of a unihrn static pressme as

obtakd from drop tests on flat water surhce appears

to result in strains which correspond to those develop in the structure dudng slamming. This has been also implied by Zhu et al (1995). It should be stressed, that this conclusion is not in conflict with the findings of Kvalsvold et al (1995), as the presstue s measured during the tests reported threrein are extremely high, even up to 80 bars, and they were recorded during slamming on wave peaks, where no substantial air cushioning occurred.

Numeried simulation of wet deek shrmdng

The drop tests produced adequate ad enough

results for the validation of computer codes simulating

water impacts under various coalitions. Spatial and

temporal variations of pressures and strains and the

~AmAuv-?n !s-reb-mlls:19

time history of the acceleration have been produced in

hard copies and in electronic format. The tests to

check the validity of the experimental results were

successfid and the measurements may be looked with confidence. CETENA has already performed a Fhite

Element simulation of the tests Dambra (1997),

including the interaction of the structure, water ami air which produced promising results.

Experimental investigation of wet deck slamming

A question of great importance is relevant to the usefidness of drop tests for the direct prediction of the design pressures against wet deck slamming, as well as for the direct prediction of the panel’s structural response. This question could be put more generally also for slamming of conventional type hulls. From the

experience gained fkom the conducting of a large

number of tests and from the studies perfoxmed it is

comluded that for the direct application of the test

results to the case of a ship the following information is needed:

> The impulse which is appIied gIobally to the pad

in question, and

9 An indication on the air cushioning during

slamming of the ship in question.

This information could be extracted from towing

tank tests and from sea trials respectively. Having

these data the simulation of the experimental

simulation of the water impact of the panels using full

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F@re 13: FE analysis. Von Mises stress on the alumirdum model under 1 bar alZltic pressure

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F@re 14 FE analysis. Shear stress in the balm core of the sandw4ch model under 1 bar static

presure

wale models and subjecting them to water impacts, will produce results directly applicable to the case of a ship.

Further, drop tests d@gned in an appropriate

way, me an exeellent tool fbr understanding the

slamming phemxnenon, the comparison of diflkrent

structural solutions, for the prediction of the response

of pads under hydrodynamic loading and for the

validation of numerieal simulation tools.

ACKNOWLEDGEMENTS

The work was part of contract BRE2-CT94-4)582.

The anthers acknowledge the finamial assistance of

DGXII of the European Commission.

REFERENCES

1. CHUANG, S-L., Experiments on Flat-Bottom

Slamming, Jnl. tip Research, Vol. 10, No. 1,

1966.

2. DAMBRA, 1?., Theor./exp. Correlation slamming

pressure loads: drop testddynarnic F.E. analysis,

3.

4.

5.

6.

MATSTRUTSES report TEC/64/105/001/27,

CETENA, 1997.

GRAN, S., Local and Global Rule Design Loads,

MATSTRUTSES Report, WOR/12/002/001/04,

1994.

KATSAOUNIS, G. M., SAMUELIDES, M.,

HAYMAN, B. armiWALLAT, R., Slamming on a

fiat cfiplbalsa sandwich structure, Proc. 4th Int.

Gmf. On Sandwich Construction, Stockholm,

Sweden, June 1998.

KVALSVOLD, R., FALTINSEN, O., ad

AARSNES, J.V, Effect of Structural Elasticity on Slamming against Wetdecks of Mukihull Vessels, Pmt. of 6th Int. Symp. on the practical Design of Ships and Offshore Mobile Units (PRADS), Seoul, S. Korea, 1995.

SAMUELIDES, M. and KATSAOUNIS, G.M.,

Experimental modelling of wet-deck slamming,

Pmt. 4th Int. Conf. Fast Sea Transportation

(FAST’97), Sydney, Australia, 1997, pp. 413-422. WALLAT, R., Private communication, 1997.

ZHU, L., and FAULKNER, D., Design Pressnre

for the Wet-Deck Structnre of Twin-Hull Ships,

Proc. 3rd Int. Cmf. Fast Sea Transportation

(FAST’95), Luebeck-Tmvenmede, GelllMlly, PP.

257-268, 1995.