ABSTRACT EXPERIMENTAL METHOD AND MODEL

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1 Proceedings of The Twelfth () Interntionl Offshore nd Polr Engineering Conference Kitkyushu, Jpn, My 31, Copyright by The Interntionl Society of Offshore nd Polr Engineers ISBN (Set); ISSN (Set) Drift Force nd Drift Motion of the Bow Prt of Broken Ship Kunihiro Hoshino, Shoichi Hr, Kenji Ymkw, Kzuhiro Yukw Ntionl Mritime Reserch Institute -3-l Shinkw Mitk Tokyo 11- JAPAN ABSTRACT Russin tnker nmed Nkhodk snk in the Jpn Se Jnury in The leked hevy oil ws wshed up to the cost nd considerble dmge ws brought bout. The bow prt of Nkhodk ws seprted nd drifted for bout 5 dys in the Jpn Se. In this pper, experimentl result of drift force nd drift motion in wves concerning bow prt of tnker model like Nkhodk is described. The mesurement of the wve drift force ws mde using the model supported by the wek spring. The mesurement system giving the rection force, which mtches with the stedy drift force ws dpted. The torque motor ws used in the mesurement of electromechnicl method. The wve drift motion of model ship ws mesured by utomtic trcking system using the TV cmer. Quite interesting results were obtined by this experiment. There exists multiple ship stble sttus during drifting. The ship drifts in the rectngulr nd opposite direction with the wve coming direction in some cses. As the tnk experiments on wve drift force nd drift motion of the very bnorml shped ship like Nkhodk hve not been so fr crried out nywhere, those dt re considered to be quite vluble. KEY WORDS: Drift Force; Drift Motion, Broken Ship, Nkhodk INTRODUCTION In the cse of the wreck ccident of the Russin tnker nmed Nkhodk Jnury in 1997, the ship snk due to wves in the rough wether nd the bow prt seprted from Nkhodk drifted for severl dys in the Jpn Se In this ccident, the dmge ws expnded becuse the bow drifted for some dys nd ws wshed up to the cost. It my be becuse tht the informtion ws insufficient on the site nd the dt necessry for the prediction of the drift course of the bow prt could not be obtined promptly. It could be cuse of the expnsion of dmges. If the drift course is predicted, it cn be modified by force. Thus the industril fcilities such s power plnts, the min fctories, the fishery fcilities nd the sensitive se re such s the ntionl prks cn be protected from the dmge. It lso becomes possible to prepre for the wshing up in dvnce. On the other hnd, it is necessry to develop the system for supporting the prompt decision on the site concerning the slvge of the disbled ship. The optimum towing support system is being developed t the Ntionl Mritime Reserch Institute (NMRI) s the equipment to support for slvging the disbled ship. The drift motion simultion of the broken ship is going to be included in this system. EXPERIMENTAL METHOD AND MODEL Model is the section of VLCC type cut off between S.S.7 nd S.S. used (bow broken ship). Fig.1 shows the ppernce of the model nd Tble 1 shows the principl prticulrs of the model. The experiment of wve drifting force ws crried out using the mesuring system of wve force (Ueno et l., 1) developed t NMRI. The system mesures the constnt component of surge force, (Xn) swy force (YD) nd yw moment (ND), llowing degree of freedom. Fig. shows the schemtic ide of mesuring equipment of wve drifting force. The system controls 3 independent constnt torque motors which cn pply vrible constnt force continuously in the horizontl motion (swy, surge, yw). Thus the system mkes esy nd ccurte mesurement possible similr to the typicl wve drifting force mesuring method (Ando, 197; Tomong, nd Htnk, 19), in which the restoring force without effect of the motion with wve period is pplied on the model by the soft spring nd the constnt drifting force is blnced by djusting the weight hnging through pulley. For mesuring wve drifting motion, the movement of trgets psted on the front nd rer of the model ship were recorded with TV cmer nd the motion chse of model ship using the uto trcking system of the crrige in Ocen Engineering bsin. The drifting motion of model ship ws nlyzed from the position of the crrige nd obtined imge dt. Fig.3 shows the outline of mesuring wve drifting motion, The yw motion ws nlyzed from only imge dt, llowing the crrige not to chse with the control gin. Fig. shows the time history of mesuring error between the position of the crrige nd tht of model ship nlyzed from imge dt obtined by TV cmer. The true coordintes of the model ship cn be obtined by modifying the position of the crrige using dt in Fig.. 55

2 EXPERIMENTAL RESULTS AND CONSIDERATION Constnt wve drifting force of the bow broken ship The mesurement of wve drifting force ws crried out restricting yw moti& by the strong spring in order to mesure the constnt component with constnt wve incident ngle x between wve nd the bow direction. The wve incident ngle x ws vried from to 1 degree every 5 degree. The experiment ws crried out in regulr wves. Fig.5 shows the definition of the coordinte system of constnt wve drifting force. Fig. nd 7 show the constnt wve drifting force in reltion with wvelength divided by ship length (hn) in the longitudinl wves nd the cpsized condition (x = nd 1 ). The solid lines in both figures re the men line of experimentl results insted of theoreticl clcultion vlue. The figures were omitted becuse the trnsverse force FYD to the wve coming direction ws smll due to the smll symmetry in the longitudinl tis of fhe ship. Judging from those figures, on both the following wve (x = ) nd the hed wve 1 = 1 ) condition, Fxn increses s the wvelength becomes shorter. When h/l is less thn., Fxn is the constnt vlue of bout 1. in the cse of x =. However, Fxn continues incresing in the cse of x = 1 within the experiment wvelength rnge. Especilly when h/l is smller thn., it increses drsticlly. The wve drifting force in the cse of hed wves is lrger thn tht in following wves. Fig. shows the constnt wve drifting force when the direction of the hull is t right ngles ginst the wve coming direction. In the cse of x = 9, not only fhe drifting force of the wve coming direction FYD but Fxo lso becomes significnt. It is becuse the symmetry of the hull shpe long the longitudinl direction is strong. The direction of the trnsverse drifting force Fvn reverses when hn is ner 1.9. When 3JL is smller thn 1.9, the ship drifts down leeside. However, when h/l is bigger thn 1.9, the ship drifts up wetherside. The force pplied on wetherside becomes the biggest t h/l = The direction of the longitudinl drifting force Fxn lso reverses when h/l is ner 1.9. The constnt wve drifting force towrd the stem direction is generted when the wvelength is short, nd it is generted towrd the bow direction when the wvelength is long. The force towrd the bow direction becomes the biggest t ner h/l =.3. Those results show tht the bow broken ship could drift not towrd the wve coming direction but its reverse direction depending on the blnced condition. It lso could drift t right ngles ginst the wve coming direction. It cn be supposed tht the restriction of the yw motion should hve n effect on the vrition of the constnt component of yw moment ND. As for ND, yw moment mking the bow direction turn to the wve coming direction increses s h/l becomes smller. Therefore, even if the initil condition of ship sttus is x = 9, it seems tht the bow drifts turning towrd the wve coming direction when h/l is smll. Fig.9 shows n exmple of the constnt wve drifting force cused by the vrition of the wve height. The solid line in Fig.9 indictes n pproximte curve of the qudrtic function clculted by the lest squre method. The ship sttus is degree trim on the cpsized condition nd the wve comes towrd the trnsverse direction. It cn be understood tht the wve drifting force is considered to be proportionl to the squre of the wve height, lthough this condition is extremely specil for the ordinry ships. Fig.1 nd 11 show the vrition of the constnt wve drifting force in the longitudinl wves nd on the upright condition (x = nd 1 ). Judging from those figures, in the following wves, Fxn increses s the wvelength decreses when ML is smller thn, but it decreses when VL is lrger thn. As shown in the figures on the cpsized condition, FXD increses s the wvelength decreses in the hed wves nd the drifting force in the cse of x = 1 is bigger fhn tht in x =. When X/L is smller thn.5 both in the cse of x = nd x = 1 Fw increses drsticlly on the cpsize condition (Fig. nd 7) rther thn on the upright condition s the wvelength decreses. The difference between two conditions is firly big. Fig.1 shows the vrition of the constnt wve drifting force when the hull is t right ngles with the wve coming direction x = 9 on the upright condition. The trnsverse drifting force Fvo could not be mesured becuse of the trouble of the force mesuring equipment. Different from the cse of x = nd x = 1, the difference of drifting force between cpsize nd upright condition cnnot be seen in the cse of x = 9. It cn be supposed tht it my be becuse the surge in the longitudinl wves is different gretly on the ship conditions, lthough the swy in the trnsverse wves is not very different between on the cpsized nd upright condition. The detil exmintion is going to be mde with the result of numericl clcultion of drifting force nd motion. Drifting motion of the bow broken ship The mesurement of the drifting motion ws crried out s the stndrd condition of the initil wve incident ngle x = O, on the sme wve conditions s mesuring the experiment of the wve drifting force. The experiment ws crried out both in regulr wves nd irregulr wves. As for the irregulr wves, the white noise generted by the rndom noise genertor through bnd pss filter (centrl frequencies re.,.9, 1.O nd 1.3Hz) ws used s the wve genertion signl without using the prticulr spectrum. Fig.13 shows the definition of the coordinte system of drifting motion. Fig.1 shows the component of drifting speed long the wve coming direction Ux on the cpsize condition with the trim by the bow of degree nd the initil wve incident ngle of x = 1. The model ship is restrined before pssing of the wves nd is relesed slowly. When h/l is between 1. nd.3, the model rottes 1 from the initil wve incident ngle nd drifts with its bow towrd leeside stbly. When h/l is less thn 1., it drifts trnsversely ginst wves nd turns slightly towrd wve coming direction. When VL is more thn x = 1, the drift direction becomes unstble nd the drifting speed becomes smll. Fig.15 shows the reltion between the bow direction nd the drifting speed Ux with the initil wve incident ngle x in the cse of VJL = 1.. The initil wve incident ngle x ws set keeping the initil wve incident ngle x constnt first nd relesing it fter wves pssed. The bow direction in the figure indictes the stble ship sttus during drift nd the drifting speed is n verge of the speed long the coming direction Ux in the stble drift. In the cse of L& = 1. there re 3 steps of chnge concerning the drifting speed within the rnge of given initil x In the cse of x =, the drifting speed is much lrger thn tht in other blnced conditions becuse of gret drifting force nd smll drifting resistnce. As is shown in Fig.1, the ship drifts keeping x 1 when the initil direction x is 1 degree. Therefore there re stble drift sttuses. Fig.1 shows the wve height effect on the wve drifting speed t 1JL = 1.. The figure shows tht the drifting force increses linerly s the wve height increses. As the wve drifting force is proportionl to the squre of wve height nd drifting resistnce is proportionl to the squre of wve speed s is shown in Fig.9, this result is considered to be resonble (Tnizw et l., 1). Fig.17 shows the drifting speed long the wve coming direction Ux on the upright condition t the constnt wve height of cm. Fig.1 shows the reltion between the initil hed ngle nd the drifting speed on the upright condition with stble ship sttus. Fig.17 shows tht the ship turns 1 degree nd drifts on the following condition insted of drifting with n initil ship sttus even when h/l is greter thn. on the contrry to the cse of cpsized condition. As the ship drifts with its hed towrd the wve coming direction, the drift resistnce becomes 557

3 smll nd the drift speed is two times s high s tht on the cpsized condition. When h/l is smller thn., the ship becomes stble directing its hed slightly lrger thn 9 degree ginst the wve coming direction. When UL becomes lrger thn 3., the ship sttus chnges with time nd the drift speed becomes extremely slow. It cn be found from Fig.1 tht some blnced points exist concerning the stble sttus during drift in wves like the cse of cpsized condition. There re 3 conditions of stble sttus during drift becuse the ship on the hed wve condition (x = 1 ) is not stble. Fig. 19 nd Fig. show the typicl exmples of the drifting trjectory on the cpsized condition. Fig.19 shows n exmple in the cse tht UL is smller thn 1.. The ship in the initil hed wve condition immeditely rottes 9 degree due to wves. Further, it drifts stbly in wves with its hed towrd bow direction insted of wve incident direction, fter the ship turns towrd wve coming direction. The drifting resistnce becomes lrge becuse it drifts with its hed in the trnsverse wves. However, the drifting force is gret nd the drifting speed is high, becuse the wvelength is short. Fig. shows tht the ship drifts keeping the initil condition. It drifts long the wve coming direction without chnging its sttus. The drifting resistnce is considered to be firly lrge becuse the ship directs its broken section towrd the drifting direction. Fig.1 shows tht the experimentl results of the drifting trjectory on the upright condition t h/l of 1.9. Different from Fig. on the cpsized condition, the ship immeditely rottes 1 degree due to wves. It drifts in zigzg with its bow towrd the wve coming direction. The drifting resistnce is smll nd the drifting speed is high becuse the ship drifts with its hed leeside. Fig. nd Fig.3 show exmples of specil drifting trjectories when h/l is lrge. Fig. shows the cse where the ship moves long right ngle ginst the wve coming direction. Fig.3 shows the cse where the ship moves towrd wve coming direction. As ws lredy mentioned in the considertion of the experimentl results of wve drifting force, those phenomen cn be nlogized from the direction of mesured force. It cn be supposed tht those phenomen ws cused by the hydrodynmic force generted by the motion of the bnorml broken shped ship in wves insted of the norml wve drifting force. Fig. shows n exmple of much more specil drifting trjectory with zigzg drift in wves. As is shown in Fig.5, the ship followed lmost the sme course with zigzg drift lthough the experiments were crried out severl times on the sme condition. As is seen in the figure, this phenomenon is regrded s the oscilltion phenomenon with limited mplitude. The stble oscilltion system is constructed by the reltion between drift sttus nd pplied hydrodynmic force. Nmely it seems tht the stble feedbck system is composed mong the motion due to the wve drifting force, drift speed nd drift resistnce, nd lso between the yw moment due to both wves nd drift resistnce nd drift sttus. It is necessry to mke detiled exmintion such s the mesurement of the drift resistnce, the drift sttus nd the motion in order to investigte this phenomenon. The mesurement of the drift trjectory ws crried out in irregulr wves of which centrl frequency corresponds to the wvelength cusing the zigzg phenomenon in order to ensure the zigzg phenomenon in Fig.. Fig. shows the spectrum of the generted irregulr wves. Fig.7 shows the comprison of the trjectory between the wvelengths corresponding to the centrl frequency of irregulr wves of.5 nd?jl of.5 in irregulr wves. The drift trjectory in irregulr wves shown in the figure by the thick solid line indictes tht the ship keeps drifting with its hed direction towrd the wve coming direction insted of zigzg drift in regulr wves indicted by the thin solid line. It seems tht the disturbnce in irregulr wves mkes the zigzg phenomenon like Fig. difficult to occur. Including the exmintion on the existence of multiple stble sttuses nd drifting course in irregulr wves shown in Fig.15 nd Fig.1, the detil exmintion nd nlysis will be needed. CONCLUSIONS The optimum towing support system is being developed t the Ntionl Mritime Reserch Institute s the equipment to support slvging the disbled ship. The drift motion simultion of the broken ship is going to be included in this system. In this pper, the experimentl results of the drift force nd the drift motion of disbled ship in wves re described. As the tnk experiment on wve drift force nd drift motion of quite bnorml shped ship such s the bow broken ship hve been so fr crried out nowhere, quite vluble dt could be obtined. The summry of the experimentl results is s follows. 1) It ws found through the mesuring experiment of drift motion in wves of the bow broken ship tht the drifting trjectory followed multiple courses depending on the incident wve ngle. ) The drifting trjectory of the bow broken ship follows right ngle ginst wves, the wve coming direction nd zigzg course very differently. Even if the drifting trjectory is complex, the drifting course is repetble provided tht the wve height, the wvelength, the initil sttus of the model ship re the sme. 3) The drift motion cn be estimted by the direction nd its mgnitude of pplied hydrodynmic force obtined by the mesuring drift speed is proportionl with the wve height. ) The wve drifting force is proportionl with the squre of the wve height nd the drift speed is proportionl with the wve height. As for the experimentl results of the wve drifting motion in regulr wves the broken tnker model, it ws found tht the prediction of the drifting course is difficult due to the existence of multiple stble drift sttus nd drifting course. However, it ws ssured by the experiment tht the drifting force nd the drifting motion in irregulr wves like rel ocen could be expressed in simpler mnner. The detil experiment nd nlysis of the drifting motion in irregulr wves is minly going to be mde minly. The possibility to predict the drifting course of the bnorml shped ship is lso going to be pursued. REFERENCES Ando, S (197). On the Drifting Force for the Floting Body in Regulr s, Trns. of West-Jpn Society of Nvl Architects, No.5, pp 5-5. Tnizw, K, Minmi M, nd Inoue, Y (1). On the drifting speed of floting bodies in, Journl of the Society of Nvl Architect of Jpn, No.59, pp 33-. Tomong, Y, nd Htnk, K (19). Mesurement of the Drifting Force nd Moment on Floting Type Offshore Structure in s, Trns. of West-Jpn Society ofnvl Architects, No.59, pp 33. Ueno, M, Nimur M, Miyzki H, Nonk S, nd Hrguchi T (1). Model Experiment on Stedy Forces nd Moment Acting on Ship t Rest, Journl of the Knsi Society of Nvl Architect, Jpn, No.35, pp

4 Tble 1 Prticulrs of model Full-scle Model ship - I \ I ^^^ I ^,. LPPtml 3lJU Y.U L(m).3. B(m) 5..5 D(m) Trget Imge A Control PC [-Positioni EX.EY.EY ; & i PID control i ~~~------~ I 1 Speed commnd : wwyr 1 Crrige controller 1 Drive of crrige i Fig.3 Mesuring system of wve drifting motion Time(sec.) 5 Fig. 1 Model for experiment O.OSy Y I Time(sc.) Fig. Time history of mesuring error Constntxque motor 9 Surge Fig. Mesuring system of wve drifting force Fig.5 Coordinte system 559

5 N3 ul. Q 1. s : Y :: u. x =Odeg. 11 Y,p:ig, C$ p FXD cpsize stem trim Gdeg. Hw+O.Om IIL Fig. Drifting force (x=o Cpsize). N3.1 %P :. E? L , nx=sodeg. cpsize stem trim Gdeg. Hw+Cl.Om AIL Fig. b) Drifting force (x=9 Cpsize).- N3 Q A. 9 g -1. z" cpsize stem trim Gdeg. Hw+O.Om ; -1.?! % -. P cpsize stem trim Gdeg. Hw+O.Om AIL -1.5 AIL Fig. c) Drifting force (x=9 Cpsize) Fig.7 Drifting force (~1~ Cpsize) - x lt - >. - AIL=l.il I fnhl) b 3. x=godeg. -. AIL Fig. ) Drifting force (HO Cpsize) ( C: I km) Fig.9 height effect on wve drifting force (x =9 Cpsize) 5

6 . x =Odeg. N3 ti J ol Q 1. - FxD is H 5 upright HwSO.m. N3 Q -.5 "_I z F c -1. p z stem trim deg x = 9deg. Hwe.m ND AIL -I B- AIL Fig. 1 Drifting force (x= Upright) Fig. 1 b) Drifting force (r9 Upright). 31 upright bow trim deg. Hw%O.Om x=1odeg. -. IL FXD Fig.1 1 Drifting force (x=1 Upright) A : Drift direction : Directionl ngle + Fig. 13 Coordinte system.. wve + N3.1 Q p". 3 : Y $ g Hw%O.Om A/L -.1 : SO..1 : cpsize stem trim We. X e initil condition HwiO.m Fig. 1 ) Drifting force (r9 Upright) Fig.1 Drifting speed (Cpsize) 51

7 B E E 3..1 wve wve + wve I wve ble direction + I L---&-, c--m go x Weg.) l.15.1 z I Fig. 15 Reltion between initil bow direction nd drifting speed (Cpsize) wve Cd-) stble direction e Fig.1 height effect on wve drifting speed 1 C$ b : 1 w$e :::%ngde. t 1 I E * Hw:O.Om Strt 1 Initil conditio Fig. 19 Exmple of drifting trjectory (Cpsize AL=1.l 1) 1 - E o- * cpsize A,'L:.33 Hw:O.OOm Strt I Initil condition I -1 I Fig. Exmple of drifting trjectory (Cpsize k&=.33) AIL Fig. 17 Drifting speed (Upright) 1 Strt. ii I E.1 3; 3 upright Hw+O.Wm stem trim deg. /L= i x Fig. 1 Reltion between initil bow direction nd drif?ing speed (Upright) z - * - - Initil condition wright bow trim deg l/l:i.9 tlw:o.o7m x (In) Fig.1 Exmple of drifting trjectory (Upright IL=1.9) 5

8 1 II 1 cpsize 1 f9 ; 7 I/L= Fig. Exmple of drifting trjectory (Specil exmple - 1) t-t g - -1 bow trim deg Fig,5 Reproducibility of zigzg type drifting trjectory z I > 9 7 cpsize _ A/L:53 _ Hw:O.OIZm initil condition ' I Fig.3 Exmple of drifting trjectory (Specil exmple - ).. I c/y. A&=.5 1 f(hz) 3 Fig. Irregulr wve spectrum 1 1 Ii- T x (ml / irregulr Fig. Exmple of drifting trjectory (Zigzg type) Fig.7 Comprison of drifting trjectory in wves 53