The effects of fast passing craft on moored vessels

Date 2013

Author Pinkster, J.A. a n d J.A. K e u n i n g Address Delft U n i v e r s i t y o f T e c h n o l o g y

Ship H y d r o m e c h a n i c s a n d S t r u c t u r e s L a b o r a t o r y

Mekelweg 2, 2628 CD D e l f t

TUDelft

Delft University of Technology

The effects of f a s t passing craft on moored v e s s e l s .

by

J.A. Pinkster and J . A . Keuning

Report No. 1 9 0 0 - P 2013

Proceedings of the 12'" International Conference on Fast Sea Transportation, FAST2013, Amsterdam, The Netherlands.

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T H E E F F E C T S O F F A S T PASSING C R A F T ON M O O R E D V E S S E L S

J.A. Pinkster

Pinkster Marine Hydrodynamics BV Rotterdam, Ttie Netlierlands

SUMMARY

A recently developed analysis method to determine the behaviour of a moored ship due to wash waves created by a passing fast vessel is applied to wash waves generated by an existing fast inland passenger catamaran and to a concept design of a triple-hull passenger vessel. The analysis method used to determine the motions of a barge can be based on a measured or computed longitudinal wave-cut close to the passing fast vessel. The method is briefly described and some previously obtained results regarding the predicted wave propagation and wave loads on a captive barge in a towing tank are shown.

The analysis method is subsequently applied to determine the transient motions, accelerations and transverse drift forces of two stationary rectangular barges in open water due to the two fast passing vessels. 1. INTRODUCTION

Fast ships create water motions which appear in the form of waves. Relative to the passing ship itself, the wave pattem is stationary but for moored ships the waves represent a transient , dynamic disturbance which may cause unexpected motions hampering loading/discharging operations. In extreme cases this can lead to personal injury or breakage of mooring lines. With the advent of more , larger and faster vessels on inland waterways passing ship effects are on the increase with corresponding increases in the material damage, loss of time and personal injuries. A recent survey carried out by the Netherlands ministry of transport and waterways among the skippers of inland vessels revealed that some 40 % of the damage sustained to mooring equipment were due to passing ship effects. The same study has resulted in a re-assessment of the design loads of mooring equipment of such vessels to reduce the number of serious incidents.

Many of the incidents recorded in the past have focussed on the wave-induced motions of the moored vessel as the cause of personal injury and damage to mooring systems. In the Netherlands a comprehensive study of the effect of passing vessels on the subjective assessment of the

J.A. Keuning Delft University of Technology

Delft, The Netheriands

ensuing motions of a moored inland waterway vessel by a number of professional inland waterway skippers on board the vessel was carried out. See Bos et al, 2007,Ref. [1] and Luth et al, 2009, Ref.[2].

2. E F F E C T S OF WASH WAVES OF FAST CRAFT We base the development of the prediction method for the disturbance due to wash waves on the input due to a so-called 'wave-cut' of a wash wake pattem taken a short distance from the centreline of the passing fast vessel. Short distance in this context means , close to the vessel but outside the region of strong non-linearities (steep or breaking waves) so that linear wave theory may be applied to the waves progressing away from the vessel beyond the wave cut. Based on this relatively short wave record we propagate the wash waves to the surrounding port geometry and moored vessels using linear potential wave theory. We have chosen linear potential flow to propagate the wash waves partly based on the findings of Whittaker et al (2001), Ref. [3]. with respect to ftill scale measurements of the wake wash of fast ferries. He concluded from analysis of the measurements that the propagation of the wash waves conformed well with predictions based on hnear dispersion.

2.1 THE COMPUTATIONAL METHOD

The computational procedure has been treated in detail Ref. [7]. Here we give a short summary of the main aspects of the method.

A wave cut is the transient time record of the wash wave elevation in a fixed point taken as the vessel passes close by on a straight course at constant speed. The record can be obtained based on computations , model tests or full scale measurements. This is an interesting aspect of the approach since it introduces a degree of flexibility i.e. a full scale measurement of a wash wave can be based on a single wave-rider buoy measurement for an existing vessel. I f it concerns a new design, computations may suffice i f the hull form is amenable to a numerical analysis, see, for instance Raven, Ref. [4. I f that is not feasible a short model test program with the new design can be carried out to measure the wave-cut. A example

of the wash waves generated by a fast vessel is shown in Figure 1. This wave field is typical for vessels travelling at high, super-critical speed and shows only diverging waves. At sub-critical speed transverse waves would also be present.

Figure 1: Wash wave field of a fast craft

An example of a wave-cut taken from a model test is shown in Figure 2. 0.75 [ . . , 1 0.50 I 0.25 E • 0 10 20 30 40 s

Figure 2 : A typical wave-cut from a model test. Full scale wave height and time.

The procedure to obtain the pressures and velocities induced by the wash waves on the port geometry and the moored vessels involves extending the duration of the wave-cut by padding the record with zeros to the expected duration of the simulation. This is related to the propagation time of the waves to the moored vessels and the time for the subsequent transient vessel response to reduce to near zero. Lengthening the wave record by adding zeros is in accordance with the assumption that after the vessel passes wave action ceases. As we are considering fast vessels travelling at super-critical speed, no significant transverse waves are present leaving only the diverging wave system.

Based on the wave-cut record the amplitudes and phase angles of the frequency components are determined

using Fast Fourier methods. This information is not sufficient as we still do not know the wave directions. The wave directions are found based on linear wave theory and knowledge of the water depth h and the speed U of the passing vessel. See Figure 3. This figure shows the relationship between the wave direction, the speed U of the vessel and the frequency-dependent phase speed c of the wave component.

Figure 3 : Relationship between wave direction, phase speed and ship speed.

The direction of the wave frequency component is found from:

a(co) = arccos(^^) (1)

In which the frequency-dependent phase speed c((i)) follows from:

c m = y (2) k

The wave number k follows from the solution of the dispersion relationship :

=kgtar\hkh (3)

Wave amplitude, phase angle and direction can be used as input for frequency-domain linear diffraction computations to compute wave propagation and wave forces and motions of floating bodies. In our case we use the program DELFRAC. See Pinkster, 1996,Ref [5], Pinkster and Naaijen, 2003, Ref [6]. The thus found amplitudes, phases and wave directions replace the usual uni-directional frequency-domain wave input with unit wave amplitude , zero phase angle and fixed direction. As indicated above, each wave frequency component in a given wave cut has its own unique direction, amplitude and phase angle.

After solving the frequency-domain diffraction problem for wave forces , motions and wave elevations, time-domain results for forces, motions and wave elevations are obtained by inverse Fourier transform methods. Besides these quantities, it is also possible to compute the mean and low-frequency second order wave forces acting on the moored vessel. See Pinkster and Naaijen, 2003, Ref [6].

3. WAVE F O R C E S ON A CAPTIVE BARGE. The computations were vaHdated based on model tests with a fast passenger ferry with a displacement of approximately 60 tons. The vessel was sailing at 17.5 m/s (34 kn) in 10.0 m water depth. The model tests were carried out at scale 1:25 in the Shallow Water Basin at MARIN, Wageningen. This basin measures 210 m x 15.8 m X 1 m(max). For the tests the water depth amounted to 0.40 m which corresponds to 10.0 m full scale. The basins sides are vertical, smooth and rigid thus assuring full reflection of wash waves generated by the passing vessel.

The set-up for the tests is shown in Figure 4. All dimensions are for full scale. This figure shows the fast ferry (schematic) sailing on the basin centreline and an earth-fixed array of wave probes along with the captive model of a 80 m x 10 m X 2.5 m barge . The wave-cut on which the subsequent computations are based is measured at Wave Probe 1 situated at a distance of

16.25 m off the centreline of the basin, close to the track of the passing vessel. The duration of the wave-cut measurement was restricted so that only outgoing waves would be measured. The measurement was stopped at a point when reflections from the basin side were becoming apparent. For the extrapolation , the time record was padded with zeros to 100 s. In Figure 5 the measured and computed wave elevations at Wave probe

1 are shown for the first 30 s.

X

C « n t « r l i n « t>nl(

f4

Figure 4 : Model test set-up. All values full scale. From Ref.[7]

In this part of the procedure high frequency components obtained from the Fourier analysis of the wave cut are ignored since they do not contribute significanfly to the wave forces on the barge. Leaving these components out of the inverse transform leads to small differences with the input wave cut.

The total number of Fourier components amounted to 652 between 0 r/s and 5.0 r/s

-0.2

Time (s)

Figure 5 : Measured and reconstructed wave train. From Ref.[7].

3.1 PREDICTION OF WAVE ELEVATIONS

Based on the wave directions, amplitudes and phase angles of the Fourier components of Wave Probe 1, the wave elevations were predicted for other wave probes. The results of the predictions are compared with the measured wave elevations in Figure 6 using 80 s data for the measured wave elevations i.e. until basin wall reflections are suspected.

The results shown in Figure 6 confirm that the wave elevations are well-predicted with some deviations for the shorter wave components. Noteworthy is the fact that as the distance from the passing vessel increases, the wave elevation records show longer period wave components first, followed by the shorter components. This is a manifestation of the dispersion of the waves.

-si—-40 60 T i m . (!)

Figure 6 : Measured and predicted wave elevations. From Ref.[7].

3.2 PREDICTION OF WAVE FORCES

The wave forces on the captive barge have been measured for a sufficiently long duration for the wash wave pattern to reflect back and forth between the vertical basin walls. Consequently, in order to be able to compare computations with the measurements, due regards must be taken of those wave reflections. Details are given in Ref[7]. The method involved including reflected waves from the basin side opposite to the moored barge in the wave record. Reflected waves from the basin side near the barge were accounted for by appropriate modelling of the vertical quay by means of panels.

The results for the forces and moments on the captive barge are shown in Figure 7. The measured records have a duration of about 260 s full scale while the computed records are given for 300 s.

The measured forces show some spikes , mainly at the beginning of the records.

The prediction for the roll moment is considerably below the measured values. No deflnite explanation can be given at this time. A possible reason could be the position of the roll moment axis in computations and experiments and not due to any signiflcant differences in the waves since other force modes correspond quite well, see, for instance , the sway force.

From these results it is concluded that both waves and forces on the captive barge were generally well predicted and that the method is suitable for predictions of moored vessel motion.

4. T H E MOTIONS OF F R E E - F L O A T I N G BARGES IN OPEN WATER.

4.1 THE MODELS

In the first part of this paper and overview was given of the development of the prediction method for wash wave induced forces. Validation of the method was based on the forces on the captive model of a barge and on wave measurements at different distances from the passing vessel. It was shown that linear wave theory can be used to predict wave elevation and forces on the barge over considerable distances. In this section we will apply the computational method for the wave-induced motions to two rectangular barges in open water. One of the barges is the 80.0 m long barge which was the subject of the previous sections. The second is a 40 m long barge with the same beam and draft. The water depth amounted to 10 m. The set-up is shown in Figure 8.

The main particulars of the moored barges are as follows: 40m Barge 80m Barge Length m 40.0 80.0 Beam m 10.0 10.0 Draft m 2.5 2.5 Displacement m3 1000.0 2000.0 KG m 2.5 2.5 Kxx m 3.5 3.5 Kyy m 10.0 20.0 Kzz m 10.0 20.0 Roll damping kNm.s 1627 3852 ! ' ' ' — H - Compul«J.W1*W3*W5*W7*W9*WI1 K Measuied =• 500 S. 250 10000 z 6OO0 * 0 •g -5000 S. -10000 _ 5000 ^ 2500 -2500 •5000 Time (s)

Figure 7 : Measured and predicted wave forces on the captive barge. From Ref.[7].

The barges are located with x-axis parallel to the passing vessel as shown in Figure 8. For the computations two barge offset values were used as shown in the figure.

Figure 8 : Set-up 80 m barge in open water For the computations the barges were assumed to be stationary but free-floating i.e. no mooring constraints. For this exercise we make use of two passing vessels i.e. the catamaran vessel used in the previous sections and a triple-hull concept ferry. The actual catamaran vessel and the model are shown in Figure 9 .The triple-hull vessel which consisted of three rigidly coupled, identical planing hull forms is shown in Figure 10.

Figure 9 : Catamaran Fast Ferry and tiie model

Figure 10 : Model of a triple-hull ferry concept The main particulars of the passing vessels are as follows:

Fast Ferry Tri Delta V I

Length.o.a m 37.0 23.4 Beam.o.a m 9.4 19.8 Draft m 1.3 0.7 Displacement m3 60.5 43.8 Length hull m 37.0 9.7 Beam hull m 1.2 4.8 Draft hull m 1.3 0.7

The triple-hull ferry concept was developed in order to investigate whether at high speed ,in planing condition, the effects of the wash waves would be different to the waves produced by the catamaran vessel, a displacement-type vessel, sailing at the same speed.

4.2 WAVE ELEVATIONS

Before proceeding to the computed motions of the barge , the records of the wave cut taken in location wave 1 , see Figure 11 are compared for both vessels and for four speeds of 12.5 m/s , 15.0 m/s , 17.5 m/s and 20.0 m/s. The critical wave speed for the 10.0 m water depth amounts to 9.9 m/s so that even the lowest speed is super-critical.

The speeds correspond to 24kn, 29kn, 34kn and 39kn resp. In terms of Froude number , for the Fast Ferry (length 37.0 m) these correspond with 0.66, 0.79, 0.92 and 1.05 resp. For the Tri Delta (length of hull 9.7 m) the Froude numbers are ; 1.28, 1.54, 1.79 and 2.05 resp. For clarity the results in Figure 11 have been separated along the time-scale by 50 s .

0.6 0,3 E 0 -0.3 -0.6 0.6 0.3 E 0 -0.3 20.0 m/s 12.5 m/s 15.0 m / s 1 7 . 5 m/s Tri Delia 60

Figure 11 : Wave records measured at Wave 1 for 4 speeds.

The figure shows that overall the waves of the catamaran Fast Ferry are lower than those of the Tri Delta concept. Whereas the Fast Ferry wave elevations tend to increase with speed, typical for displacement-type vessels, those of the Tri Delta vessel, which is made up of three planing hulls, decrease with speed, which is also to be expected. In order to compare in more detail the differences between the waves created by the vessels, we have computed the amplitude of the Fourier components of the wave trains shown in Figure 11. These are shown in Figure 12 along with the computed wave directions of the frequency components of the waves. The directions of the frequency components are the same for both vessels sailing at the same speed. The wave directions vary from approximately 30 degrees for zero frequency to 80 degrees for a frequency of 5 r/s for a speed of 12.5 m/s.

Figure 12 : Amplitude and direction of frequency components of Wave 1.

For the highest speed the wave direction varies from about 60 degrees to 85 degrees. The wave direction is defined in Figure 8.

Figure 12 shows that up to a frequency of approximately 2.0 r/s , the amplitudes of the frequency components of the waves generated by the vessels are almost the same.

These components (up to 2.0 r/s) reduce with increasing speed. Near 2.0 r/s the ampUtudes reach a minimum value. This effect may be associated with cancellation of waves from the hull layout of both vessels. The cancellation effects occur at different frequencies which is possibly a reflection of the differences in the layout of the hulls of both vessels. This will be investigated further.

Above 2.0 r/s the frequency components diverge rapidly, the amplitudes of the wave components of the Tri Delta reaching a maximum at about 2.7 r/s . This maximum value decreases with speed. At a passing speed of 12.5 m/s , the amplitudes of the wave components of the Tri Delta vessel are about 4 times larger than the corresponding frequency component of the Fast Ferry wave. This is reflected in the large differences in the waves shown in Figure 11 at this speed.

Above 2.0 r/s the amplitude of the Fast Ferry wave components increase with speed.

At increasing speeds the differences between the amplitudes for frequencies above 2.0 r/s reduce. At 20.0 m/s, even though there is a frequency difference between the peak values of the amplitudes, generally they are of the same order which can also be seen in Figure 11. 4.3 MOTIONS OF THE BARGES

Using the procedure outlined in the previous sections, motions of the barges were computed using the wave records of the vessels for the different saüing speeds as shown in Figure 11. The computations were carried out for two values of the offset of the barges from the sailing line of the vessels. See Figure 8. The results of the motion computations are restricted to surge, heave, roll and pitch and are shown in Figure 13 through Figure 17. The results are shown for all four speeds. Motion results are shifted 100 s along the time axis for clarity.

From the figures it is clear that heave, roll and pitch motions are significantly larger for the 40 m barge than for the 80 m barge. This is not the case with the surge motions which are more or less the same for both barges except at the lowest speed where the surge due to the Tri Delta vessel is significantly larger than the motions due to the Fast Ferry at the same speed. Also apparent is the seemingly small dependence of the heave, roll and pitch motions on the passing speed , except for the difference between the results for 12.5 m/s and 15.0 m/s which show a distinct reduction with speed. Also apparent is the small difference between these barge motions caused by the Fast Ferry and the Tri Delta vessel. Generally though, the heave, roll and pitch motions caused by the Fast Ferry are slightly less than those due to Tri Delta. The effects of increased offset on these motions are more clear. The heave, roll and pitch motions are reduced and more spread out in time. This is due to the dispersion of the wave components over the increased distance leading to a more drawn-out and lower effect on the motions.

The surge motions are almost independent of the passing distance. This is related to the fact that the barges are free-floating and there is no horizontal restoring force. In that case the surge responses peak at very low frequencies and are only significant in a very narrow frequency band. Under such conditions dispersion effects over distance are significantly smaller.

0.10 0.05 E 0 .0.05 ^).10 0.1 -0.2 0.10 0.05 E 0 -0.05 -0.10 - A

Fast Ferry passing al 68.75 m 200 30O

A

Trt Delia passing at 68.75 m

Fast Ferry passing at 168.75 m

V—

Trl Delta passing al 168.75 m

200 s

Figure 13 : Surge motions 80m and 40m barges 0.10 0.05 E 0 -0.05 -0.10 0.10 0.05 E 0 -0.05 -0.10 0.10 0.05 E 0 -0.05 -0.1C 0.10 0.05 E 0 •0.05 •0.10

Fasl Ferry passing at 68.75 m

400 Tri Delia VI passing al 68.75 m

—

Fast Ferry passing at 168.75 m

— BarQ«eOOm Tri Delta passing at 168.75 m

Figure 14 : Heave motions barges.

In order to investigate the reason for the apparently small effect of the passing speed on the motions we tum our attention to the frequency components of the motion responses. For that we have chosen the results for heave, roll and pitch of both barges for the smallest offset of 68.75 m and the highest passing speed of 20.0 m/s. The passing vessel in this case was the Tri Delta vessel.

The results, shown in Figure 17, clearly show the motion differences between the two barges.

t . o | ^

Fast Ferry passing at 68.75 m 200

I n o e l t a v i passing at 68.75 m .

Hf^- ij^-

'f^'

Fast Ferry passing at 168.75 m

^

^

Tri Delta passing at 168.75 m

The results are shown in Figure 18. These results are RAOs of the vessel motions in which the frequency dependency of the wave direction is also included. These RAOs confirm that the motions of the smaller barge are indeed larger. Heave motions of the smaller barge remain high showing a slight increase above unity before reducing rapidly to zero at 2.0 r/s. The heave motions of the 80 m barge decrease rapidly from unity at 0 r/s to zero at about 1.5 r/s. This rapid decrease is partly due to the increase in cancellation effects in the wave loads since the waves are coming in at angle of about 65 degrees. This effects is considerably less for the 40 m barge. Roll and pitch motion responses of the 80 m barge are also smaller than the same motions of the 40 m barge partly for the same reason. From the above results it appears tliat despite the visible differences in the wave trains shown in Figure 11, their effects on barge motions are not reaHzed in the same measure. This is clearly due to the specific response characteristics of the barges which ' filter' the wave effects quite drastically.

Figure 15 : Roll motions barges 12.5 m/s

i^v

Fast Ferry passing al 68.75 m 200

Tri Delta passing at 68.75 m 4 ^ 0 0.4 0.2 0 -0.2 -0.4

Fast Ferry passing at 168 J 5 m

Tri Delta VI passing at 168.75 m

Figure 16 : Pitch motion barges

Comparing these results with the frequency components of the corresponding wave for this vessel given in Figure 17 shows that while the waves contain significant contributions above 2.0 r/s, the motions do not. In order to clarify the response characteristics we have divided the frequency components of the motions of Figure 17 by the corresponding frequency components of the wave given in Figure 12 for the Tri Delta at 20 m/s. Although the wave record was taken at Wave 1 (see Figure 6 ) , the amplitudes of the frequency components are fuUy valid at the location of the barge i.e. there is no energy loss of the waves.

Figure 17 : Amplitude of frequency components of motions. 1.25 1.00 E 0.75 Ï 0.50 0.25 Heave Trl Delta VI passing at 20 Im , 68.75 m 0

1

i :

Figure 18 : Motion RAOs of the barges 4.4 ACCELERATIONS OF POINT A ON THE BARGES

An item of interest with respect to wash waves of fast vessels are the accelerations generated on board of

stationary vessels. For our cases we compute the total acceleration in point A shown in Figure 8. This point was shown to display the largest acceleration values.

The total acceleration is the square root of the sum of the squares of x,y and z acceleration components.

The computations were again carried out for all four passing speeds, both passing vessels, both barges and both offsets.

The results are shown in Figure 19. As before, results for each speed have be shifted by 100 s along the time scale for clarity. 0.5 0.4 "m 0.3 E 0.2 0.1 0 0.5 0.4 > 0.3 Ï 0.2 0.1 0 0.5 0.4 0.3 E 0.2 0.1 0 12.50 nVs 15.0 m/s

i

k

k a

£

~

M

Figure 19 : Total acceleration in point A. From the results it is seen that the motion accelerations are largest for the 40 m barge. The trend in the acceleration level for the smaller barge due to both passing vessels is to reduce somewhat with higher speed at the 68.75 m offset. This trend is reversed for the larger barge. At the 168.75 m offset the peak acceleration of the smaller barge due to the Tri Delta increases up to and including the speed of 17.5 m/s but then reduces slightly for the 20.0 m/s speed. For the larger barge the peak accelerations at this offset due to the Tri Delta are relatively independent of speed.

The accelerations due to the passing Tri Delta vessel show relatively high , high frequency components in the latter phases of the accelerations. This is especially noticeable at the largest offset. This effect is related to the (small) high frequency content of the motions of the barges when this vessel passes. See Figure 17. In this figure we see that even for frequencies above 2.0 r/s there is some motion. Accelerations are proportional to the square of the motion frequency which tends to magnify effects from these frequencies resulting in the high frequency tail of the accelerations.

With increasing speed the peak accelerations of the larger barge caused by the Fast Ferry gradually increase

for both offsets. The accelerations caused by this vessel also show a high frequency tail which is greater at the larger offset.

4.5 TRANSVERSE DRIFF FORCES ON THE MOORED BARGES

For moored vessels, especially when moored to a quay, the drift forces acting on such vessels due to wash waves may be of interest. We restrict our results to the transverse forces. These forces tend to push the moored vessel towards the quay, away from the passing vessel and onto the fenders.

In our case we view the transient wave drift forces acting on both barges for the 68.75 m offset. The computations of the drift forces are based on the so-called near-field method or pressure integration method , see Ref [5]. The results of the computations of the transverse drift forces acting on the barges is shown in Figure 20.

15 10

1

200

Jen.

400 600 800 20 S 10 0 Tii Delta A 168.75 m . ; Barge 40.0 m — Barge 80.0 m 200 400 s 600 800

Figure 20 : Transverse drift forces

The trend of the peak value of the drift forces is quite different for the passing vessels. For the Fast Ferry model the peak value increases with speed. For the Tri Delta model it decreases. This effect is related to the characteristics of the waves of both vessels , mainly for frequencies higher than 2.0 r/s . See Figure 12. At high frequencies the waves are almost fully reflected by the sides of the barges. This leads to high force values. For the Fast Ferry , the high frequency content increases with speed resulting in higher drift forces. For Tri Delta, the high frequency wave amplitudes decrease with speed leading to a corresponding decrease in the transverse drift force peaks.

Taking into account the scale is Figure 20 we see that the transverse drift force peaks are much higher for the Tri Delta vessel. This reflects the higher ampUtudes shown in Figure 12 for this vessel above 2.0 r/s.

5. CONCLUSIONS

In this paper we have demonstrated the use of a method to analyse the effects of wash waves of a fast vessel sailing at 17.5 m/s (34 kn) on the wave loads on a captive 80 m barge in restricted water. The method is based on using a measured or computed wave-cut taken close to the track of the passing vessel. The input from the wave cut measured during model tests is combined with a standard frequency-domain Hnear 3-d diffraction code through the application of Fourier transforms to connect time- and frequency-domain results. Output of the computations are the transient wave loads on the captive barge.

Comparisons of computed and measured wave elevations at different locations have shown that linear wave theory can predict wave propagation quite accurately. Some evidence of non-linear effects in the waves are seen in the slightly different dispersion of shorter, steeper wave frequency components.

The computed wave forces, except the roll moment on the captive barge are shown to be well predicted by the method followed. The good correlation between the measured and computed wave forces and moments is maintained long after the fast vessel has passed the location of the barge confirming the validity of linear wave theory for predicting wave propagation over longer distances.

The free-floating , transient motions in open water of the 80 m barge and a shorter , 40 m barge were computed based on the computed wave forces , added mass and damping using measure wave cut data from two passing vessels. The total accelerations of a corner point of the barges were subsequent computed as were the transverse drift forces on the barges.

Results of the computations gave insight in the differences in the motions of the barges caused by the different hull forms of the passing vessel. One of the vessels being a displacement type, low-wash catamaran and the other a triple-hull vessel consisting of three rigidly connected identical planing hull forms.

As one would expect, the motions of the smaller 40 m barge were shown to be larger than those of a larger 80 m barge.

Contrary to intuition however, the motions of the moored barges did not increase with speed of the passing vessel.

For the Tri Delta vessel, consisting of 3 planing hulls, this is , perhaps , not unexpected. At higher speeds the hulls rise out of the water due to the increase in dynamic lift and less waves are produced.

For the Fast Ferry the case is clearly more complicated since it is essentially a displacement type vessel. The design of such vessels is based on what may be termed ' low-wash ' principles. The results found this study indicate that this seems to be a successful approach.

6. R E F E R E N C E S

[1] BOS, I.E., van der HOUT, I.E. and KEUNING, J.A.:" Golfhinder: een relatie tussen bewegingen en hinder op afgemeerde schepen", TNO report TNO_DV 2007 C461, December 2007 [2] LUTH, H.,BOS, J.E., KEUNING, J.A. and van

der HOUT , I . : " The Relationship between Motions of Moored Ships due to Wake Wash of Passing Vessels and die Hindrance thereof. International Conference on Innovation in High Speed Marine Vessels, Fremantle, Australia, 2009

[3] WHITTAKER, T.J.T, DOYLE, R. and ELS ABER, B. "An Experimental Investigation of the Physical Characteristics of Fast Ferry Wash" 2"'' International EuroConference on High-Performance Marine Vehicles HIPER'Ol, 2001.

[4] RAVEN, H . C ' A Solution Method for the Non linear Ship Wave Resistance Problem" PhD Thesis, Delft University of Technology, Delft,

1996.

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