the impact of green water on fpso design · 2 the impact of green water on fpso design otc 7698...

OTe 7698

The Impact of Green Water on FPSO DesignB Buchner, Maritime Research lnst Netherlands

Copyright 1995 Offshore Technology Conference

This paper was presented at the 27th Annual OTe in Houston Texas USA 1--4May 1995

This paper was selected for pre~entation by the QTC Program Committee following review of information contained in an abstract submitted by the aUlhor{s). Contents 01the paper,as pres~nted, have not been reviewed by the Offshore, Technology Conference and are subject to correction by the authcnsj. The material. as presented, does not necessarily reflectany POSition ~fthe Off~hore Technoloqy Conference or Its officers. Penmssicn to copy is restricted to an abstract of not more than 300 words Illustrationsmay not be copied The abstractshould contain conspicuous acknowledgment of where and by whom the paper is presented

ABSTRACT

For weather-vaning turret moored FPSO's in survivalconditions the bow is always exposed to the wave action.When the waves come above the freeboard, this canresult in considerable amounts of green water on thedeck. This can cause damage to the sensitive equipmentat the bow, like the fluid swivel, piping, turret structureand chemical stores. Taking also into account thetendency in several new designs to place theaccommodation superstructure forward of the turret at thebow, the impact of green water on FPSO design willbecome more important in the future

Based on a series of model tests this paper presents anorientation into the various aspects which playa role inthe green water loading on FPSO units. First theoccurring phenomena are described Second the effect ofthe environmental conditions, such as the wave height,wave period and current velocity, are highlighted Finallyseveral design aspects are considered The influence ofthe bow shape, the position of the equipment on the deckand the shape of breakwaters will be discussed

INTRODUCTION

For ships and naval vessels, green water damage tosuperstructures, deck plating, hatches and topsidesequipment is a common occurrence However, methodsfor the prediction of green water or deck wetness arelimited in number Based on model tests with a frigate,the author showed in [1] that these methods aresometimes based on assumptions which can be

References and nomenclature at the end of the paper

45

questioned It was concluded that the development orimprovement of these types of prediction methods isindispensable for the evaluation of the operability andsafety of ships in rough seas

For turret moored FPSO's the same conclusion is validHowever, contrary to ships or naval vessels, permanentlymoored FPSO's cannot change their heading or speedwhen they come into survival conditions They shouldtherefore be able to deai with all the loads related tothese extreme conditions In the past the investigation ofsystem safety was generally limited to the mooring loads,the low-frequency motion behaviour and related topics.However, in recent years the impact loading due toslamming and green water has become a more importantfactor in the design of FPSO's Due to the fact that thebow of this type of weather-vaning system is alwaysexposed to the wave action, considerable green waterloading can occur to the sensitive equipment at the bow.The fluid swivel, piping, turret structure and chemicalstores can therefore be damaged. Taking into account thetendency in several new FPSO designs to place theaccommodation superstructure forward of the turret at thebow, the impact of green water on FPSO design willbecome more important in the future

The main objective of the present study was therefore toinvestigate the phenomena, problems and trends in greenwater occurrence and loading This will be done based onan extensive model test series with a typicai FPSO Thepresent paper presents an orientation into the problem. Ina later stage, more detailed analysis [2] andcomputational aspects will be highlighted

2 THE IMPACT OF GREEN WATER ON FPSO DESIGN OTC 7698

MODEL AND INSTRUMENTATION

Modelling

For the model tests a typical 160,000 tDWT FPSO wasused at scale 1:60 It was tested at its fully loaded draftof 1752 m without trim It had a freeboard of 8.88 m Noforecastle was present and no camber was applied at thedeck At this stage no bulwark was placed around thebow to avoid added complication to the flow. Although theeffect of a bulwark needs further investigation in thefuture, a first comparison between the present test resultsand tests in the past showed that a bulwark increases thefreeboard, but does not change the flow onto the deckdramatically. A drawing of the instrumented foredeck ofthe ship is shown in Figure 1. The main dimensions,weight data and stability parameters are given in Table 1.

Figure 1 - Instrumented foredeck of the FPSO

The original tanker had an almost vertical bow withoutmuch flare (see the body plan in Figure 2) To investigatethe effect of flare on the relative motions and green water,an interchangeable bow part was made with a significantflare above the waterline This is also shown in Figure 2by the dashed line.

. '------,'-

Figure 2 - Body plan with and without flare

The flare has also effect on the deck shape, as is shownby the dashed line in Figure 1 The results presented in

46

this paper refer to the bow with extra flare, if not statedotherwise.

Lenoth 260.34 m

Beam 47.10 m

Draft (even keel) 17.52 m

Freeboard 8.88 m

Depth 26.40 m

Displacement weight 1830S3 t

COG above base 14.22 m

COG forward of midship 6.72 m

Longitudinal radius of gyration 65.1 m

Pitch period 11.0 s

Heave period 11.3 s

Table 1 - Main dimensions of the 160,000 tDWT FPSO

To simulate a turret protection house or superstructure,an instrumented vertical wall was designed for the presentstudy. It had a height of 20 m and a width of 15 metresIt is shown in Figure 3

Instrumentation and measurements

The present model was instrumented with a number ofdifferent transducers Referring to Figures 1 and 3, thetransducers will be described below.

Vertical and horizontal wave resistance probes were usedfor the measurement of relative motions (R1-R7),velocities of the flow onto the deck (RV1·RV3), waterheights on the deck (Hi-HiD) and water velocities overthe deck (HV1, HV2) .. The application of this type of probehorizontally is new. After time differentiation it gives agood measurement of the important velocity of thewaterfront over the deck

For the forces (F2-F4) a panel is used connected to a stiffaxial force transducer The results of these measurementsare presented as integrated pressures over the total areaof the panel (5725 rrr') For one panel, F1, a spring isincorporated between the panel and the transducer tostudy the effect of the structural flexibility on the impactsThe results of this analysis will be presented at a laterstage. In the present paper the results of the stifftransducers will be used

The pressure transducers Pi and P2 are strain gaugehigh-frequency local pressure pick-ups, based on siliciumPi is located in the centre of the flexible force panel F1.

The force and pressure channels were recorded at a

OTC 7698 B BUCHNER 3

sampling rate of 2100 Hz, whereas all other signals wererecorded at 150 Hz. All measured values were scaled tofull scale values according to Froude scaling

However, the effect of current was investigated by towingthe model with the carriage through the SeakeepingBasin

In the present paper the integrated pressures over theforce panels (F2-F4) were used as design values, insteadof the very local pressures from the pressure pick ups(P1, P2). This was done from a structural point of view.The real structure should be able to deal with theintegrated pressure over a plate field Although locallyhigher pressures can occur, the integrated pressures willdetermine the structural response to the impact

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The tests were carried out in regular and Irregular wavesThe regular waves had a period of 12.9 s or 11.2 s, whichresuited in a deep water wave length equal to the shiplength (AIL = 1.0) and three-quarters of the ship length(AIL = 0.75) respectively. The first harmonic wave heightswere equal to 17.18 m (129 s) and 17.3 m (112 s) Toinvestigate the sensitivity to variations in wave height, thewave of 112 s was also repeated with a height of 15.76m and 14.64 m In current regular waves with a period of11 2 s were calibrated for a current velocity 2.0 mls

in irregular waves the tests were carried out in aJONSWAP spectrum with a significant wave height of13.5 m and peak period of 12.9 s. It had a peakamplification factor y equal to 3..3. Tests were carried outwith and without a current speed of 2..0 mls for a periodof 2 hours full scale.

The water depth in the Seakeeping Basin amounted to150 metres full scale

OBSERVED PHENOMENA

Figure 3 - Schematic deck structure

TEST SET-UP AND PROGRAM

Tests of moored FPSO's generally show the worst greenwater at the bow In the collinear wind, wave and currentconditions. Therefore it was decided to limit the presentinvestigation to head waves.

To have a constant phase relation between the incomingwaves and the ship behaviour for the comparison withfuture numerical simulations, it was decided to restrict thesurge motions of the vessei for these theoretical tests.This was done by attaching the model to the basincarriage via a vertical cylinder with roller bearings. Thiscylinder was connected to the model at the centre ofgravity. For real design tests the FPSO is alwaysconnected to the mooring system or to an equivalenthorizontal soft spring system, to have the correct lowfrequency behaviour during the test If the last techniqueis applied, it would be better to simulate also the verticalstiffness and weight of the mooring system, to include itseffect on the pitch and heave motions, which is estimatedto be approximately 4% [3].

Wind does not have influence on the solid green water,but only on the light spray. Therefore the wind was notsimulated during the present experiments

Most of the tests were carried out without current

47

To study the trends in green water loading, it is importantto understand the occurring phenomena in this complexprocess .. In [1] it was shown that a number of present dayprediction methods are based on assumptions of theoccurring phenomena which can be questioned. Thefollowing examples can be mentioned:

the water height on the deck is equal to theexceedance of the freeboard by the relativewave motions beside the bow [4]the pressure of the water on the deck is equal to thestatic water pressure [4], or equal to the staticpressure corrected for the vertical acceleration of thedeck [5]the water falls on the deck as a breaking wave [6]the veiocity of the water on the deck is equal to theorbital velocity or phase veiocity of the incoming wavelinear ship motion theory can be used to determinegreen water occurrence

In [2] these aspects will be discussed in more detail,taking into account the results of the present model testsand the findings in [1]. In [1] it was shown for instancethat the pressure on the deck is not only due to the staticpressure corrected for the vertical acceleration of thedeck Also the rate of increase of the water height on thedeck and the vertical velocity of the deck play animportant role


It was shown that the total pressure on the deck can beexpressed as:

Response Amplitude Operator H(m):

p = p(g + aW)h + p(~)wat at

(1)H(m) = r,(m)

~,(m)(3)

The first term represents the static pressure corrected forthe vertical acceleration of the deck, whereas the secondterm includes the effect of the rate of change of waterheight on the deck,

To study whether this linear assumption holds true inthese extreme conditions, the linear ship motions andrelative motions were calculated with the programDIFFRAC and compared with the results of the presentmodel tests

The present paper focuses more on the design aspectsof green water loading on FPSO's, However, to have agood understanding of these aspects, it is necessary todescribe the phenomena briefly

In Figure 4 the measured and calculated RAO's for theheave, pitch and relative motions in the irregular wavesare shown,

RESPONSE OF z.'COG

Generally a green water occurrence can be split up in thefollowing sequence of events:

-- ORIO'NAlBOW---,.-__ • BOW WITH FlARE

-----'--- BOWwrn-< FLAAE.CURRENTo CALCULATEO, wrrnOlJT CURRENT• CALCULATED, WrT>ICURRENT

RESPONSE OF PITCH

RESPONSE OF RELMOTION

0

:c l.

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, \ \.

\ \

~~"

~0

0 e

0

'"'\

\" H\, \

\\oU: .o••

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00 os 10FREQUENCY OF WAVE ENC01.JNTER'N IWl1S

0.5 \.0FREQUENCY OF WAVIi ENCOV..reRIN MnIS

00 U 1~meOUENCY OF WAve ENGOumERIN ""DIS

Figure 4 - Measured and calculated RAG's for heave,pitch and relative motions

(2)

1, The reiative wave motions come above the freeboard2, The water flows onto the deck3 A shallow water wave flows over the deck4, The water hits a structure

The reiative motions can be calculated by subtractinq thelocal vertical motion from the local absolute wave motionsaccording to:

The local absolute wave motions are due to theundisturbed incoming waves, combined with the reflectingand radiating waves due to the presence and motions ofthe ship respectively, In head waves the local absolutevessel motions are due to the pitch and heave motions ofthe vessel From equation 2 it will be clear that thelargest relative motions occur when the local wavemotions and the local absolute motions are large and outof phase, This occurs generally with wave frequenciesjust above the peak in pitch response

Relative motions are nowadays generally calculatedbased upon linear diffraction analysis In this theory it isassumed that the relation between the relative motionamplitude and the incoming wave amplitude can for eachfrequency be expressed in the frequency domain as a

The relative motions around the bow of a vessel aregenerally seen as an input to the green water problem In[1] and [2] it is shown that also the horizontal and verticalrelative velocities play an important role

Ship motions and relative motions

These four phases will be described below,

48

OTC 769S B BUCHNER 5

Another way to determine whether ship motions andrelative motions can be considered as linear, is byexamination of the distribution of their peaks and troughsin an irregular seastate

Applying the assumptions of a linear relative motionresponse to Gaussian distributed waves, Ochl [4]developed the following expression for the probability ofexceedance P of a certain value R:

ORIGINAL BOWBOW WITH FLARE

--, ._ aOWWITH FLARE: CURREONT

(4)

A INM

FITCH IN DEG

Based upon the results in Figures 4 and 5 it seemsjustified to conclude that the considerable differencesbetween the measured and calculated RAO's for the pitchmotions and relative motions and the curved nature of theRayleigh plots points towards considerable non-linearitiesIn [2J the reasons for these non-Iinearities will bediscussed in more detail. Attention will be paid to theeffect of the bow shape above the still waterline and otheraspects related to the finite wave height assumptions inlinear theory, such as the behaviour of the relative wavesabove the deck edge. In the present paper we haverestricted ourselves to the observation that the validity ofthe use of linear assumptions in the input to the greenwater problem is limited.

The flow of the water onto the deck can also not be seenas a breaking wave problem, which could be the ideaafter a first look at the water behaviour

Flow onto the deck

The resulting probability of exceedance can be presentedas a straight line on Rayleigh distribution paper. In Figure5 the measured distributions of the crests are shown,together with the theoretical straight lines based upon themeasured and calculated significant values in the appliedseastate

Due to its complexity, the flow of the water onto the deckis an important aspect in the study of green waterproblems, During the present model tests and the testsreported in [1] it was for instance found that there is nodirect relation between the orbital and phase velocities ofthe undisturbed waves and the flow of the water onto thedeck These aspects, combined with the effect of wavereflection and radiation, can only be seen as minorbuilding blocks in the construction of this complex flowproblem

To study the flow of the water visually, it was decided toplace a thin plate at the centreline of the ship at the frontof the bow In this way it was not disturbing the wave andship motions On the plate a reference frame with blocksof 5.0 metres width and height was drawn for analysis.Recordings were made with a vessel fixed video camera,In this way it was possible to analyze the vessel fixedrelative wave contour as function of time.

Z··COG INM

CRESTS

TEST NO MAA, , 4473 3.0574

, 4476 3.1252

, , , ,,

'",,'\\-. '\'\\,~

M&aSUTed-:Calculaled

CRESTS

TEST NO M"4473 16.3114

~4476 19.3872

, 4477 24.8839, ,, -,

'" \ -,

~, "

~\-,

-, Measured,

\§\,withoot ~are

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Calculated \<wilhftare

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o CRESTS

rEST NO M"4473 4.9636

4476 4.f1707

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Figure 5 - Rayleigh dislribution piots for the crests of theheave, pitch and relative motions with the original bowand with the bow with flare, For the relative motions elsothe distribution with current is shown

49


Figure 6 shows the results for a regular wave of 11.2 sand a height of 173 m. Lines of the wave contours aregiven with steps of 0.25 seconds full scale time.

AJB ~~~/

~

./-:-r.r(/

-

~

the dam. At that moment the dam is removed and thewater flows into the empty region.. This is shownschematically is Figure 7

Iz

I I10

IIz r--U

<I, '<,~ce'I 9 0

Iy

y=o

Figure 7 - Theoretical dam breaking problem (from [7J)

The flow onto the deck is in this way proportional to theroot-pressure height of the water before the dam breaks:

Figure 6 .. Flow of the water onto the deck in the regularwave of 11.2 s. Wave contours are given evelJl 025 s

(5)

The following steps can be distinguished:

t = 0.0 sThe pitching angle is at its maximum and an almostvertical wall of water is present in front of the bow. Thehorizontal velocity of this wall of water is almost zero.

t = 050 sThe vertical wall of water translates onto the deck andstarts to be considerably curved This gives theimpression that it starts to break

t = 075 sHowever, due to the high quasi-static pressure at decklevel the water close to the deck starts to accelerate andprevents actual breaking

t=1..25-1 ..50sA high velocity jet shoots over the deck

During this process short (non-linear) reflecting andradiating waves slowly propagate away from the bow ontop of the incoming long waves, see points A and B

As was reported by other authors [7, 8J, the bestresemblance between the flow of green water onto thedeck and another phenomenon is the theoretical dambreaking problem The dam breaking problem isdiscussed extensively by Stoker based on the method ofcharacteristics [9J. In this problem it is assumed that attime t = 0 there is a vertical wall of water on one side of

50

It is possible to include a horizontal velocity in the waterbefore breaking and to have water on the other side ofthe dam [8J

Although the dam breaking does represent the majoritems in the flow onto the deck, it should be noted that thepossibilities for its application are limited. This is due tothe shallow water assumptions in the dam breakingproblem, which cannot handle the found curvature in thewater flow for instance

Behaviour of the water on deck

As soon as the water is on the deck, it behaves like ashallow water wave (bore) under the action of gravity,pitch angle of the deck, vertical acceleration and verticalvelocity For the prediction of this flow, Dillingham [81 andMizoguchi [10] presented promising results using Glimm'smethod These methods should be improved in the futuretaking into account the pressure term in equation 2 andthe full ship motions according to the approach ofPantazopoulos [11J.

At the most forward part of the bow the water has avelocity in the longitudinal direction. To the sides of thebow a transverse component towards the middle of thedeck plays a role. The combined flow results in a highwater 'tongue' which flows with a high velocity aft alongthe middle of the deck

In Figure 8 an example is given for wave period of 11 2

OTC 7698 BBUCHNER 7

s (above) and 12.9 s (below) It can be seen that fordifferent wave periods different patterns occur

Figure 8 - Contour of the water front over the deck withtime steps of 025 s (above T=112 s, below T=129 s)

The high velocity flow along the middle of the deck resultsin a concentrated loading in the middle of the forwardmost structure on the deck

Impact dynamics

When we observe the impact of the shallow water boreon a vertical structure, we immediately see that this is avery complex phenomenon.

'bourrage' ,

As mentioned before, no complete numericai impactmodels for this problem were found in literature It shouldbe noted that the development of this type of model willbe very difficult, because all types of very local effects willplay an important role

The entrapment of air and the other local flow effectsresult for instance in a considerable scatter in the peakloads, even in regular waves. To quantify this variation,we defined a non-dimensional variation parameter E as:

(6)

(J is the standard deviation in the measured peaks in

regular waves and the x represents the mean of themeasured peaks. For the water height on the deck, E is inthe order of 003 For the pressure panels, however,values for E of 0.. 10 are found This variation should betaken into account when the results of green water testsare evaluated for design purposes.

Since impact models are not available yet, we restrictourselves to the investigation of empirical relations in theimpact dynamics .. In [1] it was shown that the square ofthe horizontal velocity of the waterfront over the deck isan important factor in the peak impact pressure. This canbe expressed as:

p = C P u~ (7)

100

JF1kPa

Based on experiments with the impact of a water jet on aplate, Suhara et al. [12] present a factor C of 14.. in aclassification note [13J, DNV uses a factor 15 for smoothcircular cylinders

Figure 9 - Typical time trace of impact pressures

We can observe the following stages in the impact of thewater on the structure:

At the moment the horizontal water hits the structureits flow direction is deflected 90 degrees, see thephotograph at the end of the paper. This results inthe peak pressure on the structure in the typical timetrace in Figure 9. In the impact literature this peakpressure is referred to as the 'giffle pressure'.A part of the water shoots up vertically along the wall,which results in a quasi-static load at the lower levels,combined with the effect of flow stagnation. Thisresults in the secondary peak in the time trace inFigure 9, which is generally referred to as the

51

During the present study it was found that not only thewater velocity is important. The water height on the deckalso plays an important role In [2J empirical relations forthe pressure as a function of the water height and watervelocity will be given

During the present tests also the effect of the stiffness onthe impact loads was investigated This, combined with astudy into the structural response to green water impactloads, is presented at a later stage

SENSITIVITY FOR ENVIRONMENTAL CONDITIONS

The previous sections showed that green water loading isa complex combination of phenomena which are stronglydependent on local effects and combined phasing.


Due to this, the green water problem is very dependenton the input parameters. In this section it will be shown inwhich way the green water phenomena is sensitive to thewave period, wave height and current

Wave period

Although the use of linear RAO's is not fully valid, theycan be used to study the sensitivity of green waterproblem on the wave period . From the relative motionRAO in Figure 4 it can be seen that the relative motionsare very dependent on the wave period

To check this dependency on the wave length, tworegular wave tests were carried out with periods of 11..2sand 12.9 s. In Table 2 the main results of these twotests, with the deck house in the aft position (30 m aft ofFP), are shown

No. 4481 No. 4482T=12,9 s T=11.2sH=1718 m H=17,30 m

Pitch (deg) 5.55 2.B9

R2 (m) 16.3 17.9

HB (m) 7.04 7.23

H5 (m) 4.92 4.B9

H3 (m) 4.47 4.93

F2 (kN/m') 103 194

Table 2 - Effecf of wave period, deck strucfure 30 m aftof forward perpendicular

The following should be drawn to the attention:

In the shorter waves the pitch motions decreasedramatically. However, the relative motions increasesignificantly This is due to the phase shift of the pitchmotions in this frequency range and the increase ofthe wave diffractionThe water height on the deck is approximately thesame in both tests, at the forward part of the deck.Further aft the water height on the deck decreases inthe longer waves, whereas in the shorter waves itremains almost constant.The pattern of the flow over the deck is dependent onthe wave period, see Figure 8 This figure shows thatfor the shorter waves the flow is more concentratedon the middle of the deck. In these waves there isalso a larger variation in the velocity of the flow overthe deck than in the longer waves.

The combination of the larger water height on the deckand the estimated highervelocity, results in impact peak

52

pressures which are much larger for the short waves (byalmost a factor 2)

Wave height

To study the effect of the wave height on the green waterloading, the wave with the shortest period was repeatedat two smaller heights (0 91Hand 0.85H)

In Table 3 an overview is given of the results of this waveheight variation. The first column shows the original waveheight results In the second and third column the resultswith the reduced wave heights are given. For these lowerwave heights the ratio between the measured item andthe value for the original wave height is given betweenbrackets

No. 4482 No. 4483 No. 4484T=11 2 s T=11,2s T=11 2 sH=17 30 m H=15,76 m H=14,64 m

(0.91) (0.B5)

Pitch (deg) 2.89 2.71 (0.94) 2.49 (0.86)

R2 (m) 17.9 16.7 (0.93) 15.6 (0.87)

HB (m) 7.23 6.38 (0.88) 5.47 (0.76)

H5 (m) 4.89 4.37 (0.B9) 3.96 (0.81)

H3 (m) 4.93 4.55 (0.92) 4.11 (0.83)

HVI (mls) 21.B 19.4 (0.89) 19.1 (O.BB)

F2 (kN/m') 194 149 (0.77) 107 (0.55)

Table 3 - Effect of wave height, deckstructure 30 m aft offorward perpendicular

The table shows that the pitch angle, the relative motions,the water heights on the deck and the velocity over thedeck are reasonably linear will the wave height

The measured peak pressure, which is a result of boththe water height and water velocity [2], is not linear withthe wave height The present results indicate that thepressures are also not dependent on the square of thewave height, but more or less to the cube of the waveheight This will be discussed In more detail in [2J.

The observed sensitivity of the green water loading tosmall differences in the wave height shows that it will bevery difficult to couple the green water loading predictionto linear ship motion predictions which do not result invery accurate predictions in these extreme conditions

Current

Due to the fact that in most of the survival conditions ofFPSO's the tidal current and storm surge plays an

OTC 7698 BBUCHNER 9

important role, the effect of current on green waterloading was investigated. Surprisingly this was found tobe rather large. It can be divided in the following twoaspects:

the effect on the FPSO motionsthe effect on the water flow around the bow

In the RAO's for pitch, heave and relative motions inFigure 4 it is shown that a 20 m/s (4 knots) currentresults in a significant increase of the FPSO motions andrelative motions

velocity on the deck increases from 218 mls to 224 m/sand the impact pressure from 194 to 221 kPa.

In the irregular waves the effect of the current was evenmore pronounced. In Figure 11 the distribution of thewater heights on the deck with and without current arecompared for position H5 The 1% exceedance value(which is a conditional probability based on the number ofgreen water events) increases from 270 m to 4.35 m

RAYLEIGH DISTRIBUTION OF CRESTS AND TROUGHS OF H5

OAlGlNAL BOWBOW Wll1i FLARE

-.-._-- BOWWITH FLARE: CURRENT

In Figure 10 it is shown that the wave length increasessignificantly with the current speed, depending on thewave period

This increase is due to the increase in wave length witha constant wave period when current is applied Thisdependency of the wave length to the current speed (U)can be expressed as:

CRESTS

TEST NO MAA

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4476 51556

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~,~ .'--- -

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~-

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'~ . ".

";0

""'0

o t0.0

(8)4 " U'

g - )g' + 4 U g OJ + 2 U OJ'A, (U,OJ)

Figure 12 - Histograms of peak pressures at F2 for theoriginai bow, modified bow and modified bow with currentbetween 0 and 600 kPa

Figure 11 - Rayleigh distribution plot of the water heightat position H5 with the original bow, the bow with flareand the bow with flare in current

The velocities over the deck, which are according toequation 5, closely reiated to the relative motions and theexceedance of the freeboard, will also be larger

I -'- -

J

II

",I- - ,. - , -

------- - - - - l-r-

-I--

II

- '- " - ,

This finally results in a dramatic effect on the green waterloads on the deck structure .. This is shown in Figure 12based on the histograms of peak pressures of F2 at theaft position

I

E 300 - .. ~.5:: Al( U)~-

jjA2(V)0

~• 200 -I

0 J 2

Vcurrent speed in m/s

Figure 10 - Effect of the current speed on the deep waterwave length for a period of 112 s (A1) and 12 9 s (A2)

Due to this increase in wave length, the wave excitingforces on the tanker increase, whereas the wave periodremains close to the pitch natural period of the FPSO(11.0 s) This combined effect, taking into account theinfluence of the current on added mass and damping,results in the increase of vessel motions.

The larger motions, combined with the effect of thecurrant speed on the flow around the bow, results in anincrease in green water on the deck and green waterloading. In regular waves with a period of 112 s thecurrent of 20 m/s results in an increase of water heighton deck at position H8 from 723 m to 786 m The water

53


DESIGN ASPECTS

Beside the effect of the environmental conditions, severaldesign aspects play an important role in the green waterloading on FPSO's. A number of them will be highlightedin this section.

Due to the fact that the water is 'captured' under theflare, the pressure which is buill up in front of the bowdue to the pitch motions of the FPSO and the wavediffraction, wili increaseThis results in a higher wave elevation in front of thebow. However, this wave is not only pushed upwards,but also forwards, away from the bow

Bow shape

Position of turret or deck house

This combined effect of the flare on the relative motionsand on the relative horizontal velocities finally determinesthe flow of the water onto the deck

The difference in the loads on the structure was found tobe relatively small, as can be seen in the histograms inFigure 12.

The present explanation is rather heuristic and needsfurther evaluation. At present the author is evaluating theuse of the non-linear time domain approach of VanDaalen [16] for this type of studies.

Figure 13 - Time traces of the relative motions in front ofthe bow (RI or R2) and the water height on the deck (H6,HB) with the bow with ileir (left) and with the original bow(right) for a period of 11.2 s

- H6=~~/R1"

. \.! ~-

Referring to the Rayleigh distributions in Figures 5 and11, it is found that the relative motions in front of the bowincrease when flare is applied, whereas the water heighton the deck decreases The ship motions of heave andpitch are hardly influenced by the bow shape

Because all available model test results are based onsharp ship types at a significant forward speed, it wasdecided to perform the present tests with the two differentbow shapes in Figure 2. The purpose of this variation wasnot to say the final word about the subject, but toinvestigate the sensitivity of green water loading ofFPSO's for bow variations. The development of numericalmethods which take into account the above water shapeof the vessel is indispensable for a future evaluation ofthe problem. Bearing in mind these remarks, the presentmodel tests show the following picture.

In Figure 13 this effect is shown in the time traces ofthese signals in regular waves of 112 s The left part ofFigure 13 shows the time traces for the bow with flareand right part shows them for the original bow. From therelative motions the freeboard is subtracted. In this figureit becomes clear that for the original bow the water heighton the deck is even higher than the exceedance of thefreeboard.

The effect of the bow shape on the occurrence of greenwater has been a point of discussion for Naval Architectsover a large number of years Some authors report adecrease in deck wetness when a significant flare isapplied [14], whereas others find an increase in deckwetness [15] with flare

The fact of higher relative motions and lower water heighton the deck can be the reason for the confusion indiscussions at this point Some authors assume that theexceedance of the freeboard is the most important factorfor green water occurrence, whereas others look to thefinal amount of water on the deck

One other aspect is the velocity of the water over thedeck. From the regular wave test results it can be seenthat the velocity of the water over the deck with the newbow is increased somewhat.

At present it seems that the presence of flare has thefollowing two effects:

The position of the turret and its protective structure isgoverned by the mooring and structural aspects of theFPSO system. When we look to the green water loadingon this type of structure, the position is a given fact, sothe structure should be able to deal with the applicableloads For accommodation structures the situation issomewhat different since, based upon green waterconsiderations, the longitudinal position can be changed

To investigate the effect of the position of the structure,tests were carried out with the front of the structure atthree positions: 10, 20 and 30 metres aft of the forwardperpendicular. In Table 4 the results are shown from thisseries of regular wave tests with a wave period of 112 s.Attention should be paid to the pressure distribution over

54

OTC 7698 B. BUCHNER 11

For the present study two different breakwater designswere used to highlight the above mentioned designaspects.

deflector it is important to keep their purpose in mind:breaking or deflecting the water which flows with a certainheight at high velocity over the deck, to minimize theimpact on the critical structure

The first structure is the traditional V-shaped breakwaterin the left part of Figure 14. A major drawback of such astructure is the fact that the water starts to run upvertically as soon as the waterfront hits the breakwaterThis fills the complete area in front of the breakwater,which results in the rest of the water fiowing over thebreakwater

5,,00

This results in the first requirement that the breakwater ishigher than the water height on the deck Secondly it isrequired that the structure deflects or breaks the waterefficiently, so that the amount of water which finallyreaches the critical structure will be minimized in amountand velocity. Finally the structure should be strongenough to deal with the dynamic load due to the waterimpact.

No 4482 No. 4492 No 4500aft position mean position forward(-300 m FP) (-200m FP) position

(-10.0 m FP)

H9 (m) 10.3 12.7 14.8

H10 (m) 5.46 6.48 6.17

F2 (kNlm') 194 200 235

F3 (kN/m') 26.8 23.1 44.2

F4 (kN/m') 8.51 6.93 14.5

the height of fhe structure (F2, F3, F4) and the waterheight in front of the structure (H9 and H10)

The highest pressure is found at the most forwardposition of the structure. For the two positions furtheraft the pressures remain almost constantThe water height in front of the structure after theimpact (H9) increases when the distance to the foreperpendicular becomes smaller For the two aftpositions it was observed that the sheet of waterwhich flows upwards is rather thin It falis forwardagain at some height. At the most forward positionthe water at the higher level is more solid.For the two aft positions the pressures at the higherlevels (F3 and F4) are small. For the most forwardposition the pressures remain larger This is closelyrelated to the amount of water at these levels.

Table 4 - Effect of position of deck structure, wave period11,2 s, wave height 17.3 m

Based upon the results in this table the foliowing remarkscan be made:

Not only the amount of water plays a role in this trendAlso the velocity of the waterfront over the deck isimportant. As can be seen in Figure 8 the velocityreaches for this wave period its maximum 10 metresbehind the forward perpendicular. After that it decreasesto a constant level. This was confirmed by themeasurements of the velocity over the deck.

Breakwater efficiency

If it not is necessary to keep an installation completelydry, it is possible to use a wave deflector or breakwaterin front of sensitive equipment. Such a structure can alsobe used as a first barrier for the lighter protective =:£~.......structure or the superstructure In this section the "./requirements for breakwaters wili be discussed and the ~~test results for two different breakwater alternatives will bepresented

EnEFor the design of this type of breakwater or wave

Figure 14 - Traditional breakwater (ieft) and vane typebreakwater (right) with Indication of typical flow

55


This run up causes a water ramp which reduces theeffective height of the breakwater.. During the present testthe overflow of water refiected of the structure andresulted in a secondary peak load on the breakwater inthe forward direction Another point is the peak load onthe breakwater itself at the moment of the impact, whichis also the result of the complete blocking of the waterflowTo prevent run up and peak load problems, an alternativebreakwater design can be considered For the presentstudy we used a vane type breakwater, which is designedby Pierre A Beynet [17], see the right part of Figure 14.This structure consisted of 18 vertical vanes at two levels.A horizontal plate was placed between the two levels andat the top of the structure to minimize the vertical flow ofthe water. The idea behind it is twofold:

to minimize the longitudinal velocity of the water bydeflecting it towards the side of the FPSOto prevent the run up and to minimize the peak loadon the structure itself by preventing a completeblocking on the water flow

The model tests showed that this principle worked. Hardlyany run up was observed and very little water reached thestructure itself through the open structure

harsh environmentsThe green water occurrence and loading cannot bepredicted with present day prediction methods basedon linear theory. This is due to the complex and nonlinear phenomena which have been described in thepaper, Relative motions around the bow, the flowonto the deck, the behaviour of the water on the deckand the impact dynamics have been considered inthis analysis.The green water occurrence and loading is stronglydependent on the wave period, wave height andcurrent velocityThe bow shape has effect on the green waterproblem For the present vessel is was found thatwith an increased flare the relative motions increased,whereas the water height on the deck decreased. Thefinal differences in loads on the structure were small.The position of a structure with respect to the forwardperpendicular has a minor effect on the green waterloading.The efficiency of a protecting breakwater is largelydependent on its ability to prevent run up in front of it.A new vane type breakwater has been tested whichhas good performance and experiences a minimumimpact force on itself

ACKNOWLEDGEMENTS

NOMENCLATURE

Dr. Mirek Kaminsky of Nevesbu in the Netherlands isacknowledged for his advice on structural aspects in thepresent project Jonathan Lambert and Gert vanBallegoyen are thanked for their special analysis duringthis study

In Figure 15 a comparison between the time traces of thehorizontal loads on the two designs is shown for a waveheight of 14.27 m and a period of 11.2 s The comparisonmakes clear that the peak loads on the traditionalbreakwater are much higher than on the vane typebreakwater (4160 kN instead of 2470 kN) The finalpressures on the structure (F2) were a factor two higher(600 kPa instead of 33.8 kPa) for the traditionalbreakwater. This indicates that the vane type breakwateris a good alternative to a traditional breakwater.

---- FXVANE- FXBREAKW

kN

Figure 15 - Time traces of the global force on thetredltionet and vane type breakwater

CONCLUSIONS

Based upon the results presented in this paper thefollowing conclusions seem justified:

Green water loading is becoming an important factorin the design of FPSO systems, which are exposed to

56

Phwxt/;"z"rx t

OlH(Ol)PauUgE

CPu,A(U, ro)

pressure in kPa (kN/m')water height on the deck in mvertical velocity of the deck (ship bound) in mlslongitudinal position in mtime in swave elevation as function of t and x in mabsolute motion as function of t and x in mreiative motion as function of t and x in mwave frequencyResponse Amplitude Operator (RAO)probability of exceedancestandard deviationwater velocity in mlscurrent speed in mlsgravity (9.81 m/s')non-dimensional variation parameterimpact coefficientdensity of seawater in tim'jet velocity normal to a surface in mlsdeep water wave length as function of currentspeed and frequency

OTC 7698 B BUCHNER 13

REFERENCES

[1] Buchner, B, "On The Effect Of Green Water ImpactsOn Ship Safety (A Pilot Study)", Nav'94 Conference,Rome, October 1994[2] Buchner, B., "On the Impact of Green Water Loadingon Ship and Offshore Unit Design", To be presented atthe PRADS '95 conference in Seoul, Korea, September1995[3] Aalbers, A B, Janse, SAW. and de Boom, W.C , "DPAssisted and Passive Mooring for FPSO's, OTC paperno 7722, May 1995[4] Ochi, MK, "Extreme Behaviour of a Ship in RoughSeas ..Slamming and Shipping of Green Water", AnnualMeeting SNAME, November 1964[5] Hansen, H J, "Uber die Vorhersage vonDecksbelastungen durch Grunas Wasser", Schiff & Hafen24,1972.[6] Takezawa, S, Kobayashi, K. and Sawada, K, "OnDeck Wetness and Impulsive Water Pressure Acting onthe Deck in Head Seas (in Japanese)", Journal of ZosenKlokai, SNAJ, Vol 141,1977[7] Gada, K., Miyamoto, T and Yamamoto, Y., "A Studyof Shipping Water Pressure on Deck by Two-DimensionalShip Model Test", JSNA Japan, Vol. 140, 1976[8] Dillingham, J , "Motions Studies of a Vessel with Wateron Deck", Marine Technology, Vol 18, No.. 1, January1981

[9] Stoker, J J, "Water Waves, The Mathematical TheoryWith Applications", Interscience Publishers, 1957.[10] Mizoguchi, S "Analysis of Shipping Water with theExperiments and the Numerical Calculations", JSNAJapan, Volume 163, June 1988[11] Pantazopoulos, M.S, "Three Dimensional Sloshingof Water on Decks", Marine Technology, Vol 25, No 4,Oct 1988[12] Suhara, T, Hiyama, Hand Koga, Y, "Shockpressure due to Impact of Water Jet and Response ofElastic Plates", Trans. of the West-Japan Society of NavalArchitects, No. 46, 1973[13] Det Norske Veritas (DNV), Classification note No30 5, Environmental Conditions and Environmental Loads,March 1991[14] O'Dea, J.F. and Walden, DA, "The Effect of BowShape and Non-linearities on the Prediction of LargeAmplitude Motions and Deck Wetness", 15th Symposiumon Naval Hydrodynamics, 1984.[15] Lloyd, AJ.R M, Salsich, JO and Zseleczky, JJ,"The Effect of Bow Shape on Deck Wetness in HeadSeas", RINA, 1985[16] Van Daalen, EF.G.,"Numerical and TheoreticalStudies of Water Waves and Floating Bodies", PhD-thesisUniversity of Twente, 1993[17] Beynet, PA, Private communications, March 1994

Typical impact on the deck structure in the aft position, 30 m from the forward perpendicular

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