Effects of hull deformations on ship displacement

Transcription

1 Effects of hull eforations on ship isplaceent K. Ziha Faculty of Mechanical Engineering an Naval Architecture, Zagreb, Croatia S. Goles Broarski Institut, Zagreb, Croatia A. Raica & S. Maksiovic Broosplit Shipyar Copany, Split, Croatia ABSTRACT: The paper presents first an alternative raft survey proceure for assessent of isplaceent of longituinally efore ship hull. Next, the paper consiers all potential positions of outer shell local eforations uner static loas on the ierse ship hull. The theories of isotropic an orthotropic plate bening are applie in orer to assess local eforations. Consequently, the total change in the locally efore unerwater hull volue is assesse by eploying nuerical integration over the entire ierse hull. The proceure is illustrate on a bulk-carrier recently built in Croatia. The results are presente graphically in orer to provie insight to all ajor hull volue changes ue to local eforations for ifferent loa cases in ships service, pertinent to specific copartents, substructures an levels of eforation. Finally, soe general suggestions for ships isplaceent corrections ue to local eforations are given. INTRODUCTION The ships isplaceent plays iportant role in the valiation of ships operational efficiency. Nevertheless, the eterination of isplaceent is not always accurate with respect to the environental conitions an conitions of the ship s hull. Nuerous relate conitions on-boar are practically iperceptible an ieasurable. Soe uncertainties coe fro raft reaings, ifferent global, local, theral, longituinal or transverse, as well as peranent or teporary eforations of an ierse ship hull. Aitionally, a hull is subjecte to fouling, corrosion, aages, reparations an aging. Moreover, the conitions of survey soeties o not provie precise observations ue to waves, ship otions or heavy access to raft arks on boar. Norally in the en, accurate hull local an global eforations cannot even be consiere in practical eterination of the isplaceent on-boar uring raft an eaweight surveys in the ships service, particularly in rough weather conitions. Hence, it is nearly ipossible to eterine the ships accurate isplaceent, an any etho that siplifies the proceure or iproves the accuracy shoul be highly appreciate. GLOBAL HULL DEFORMATIONS In traitional practice of a raft survey, the rafts are observe on raft arks or easure by evices place aft, forwar an on ship s portsie an starboar close to aiships. The eans of rafts are recalculate to positions in virtual coincience with respective perpeniculars an iship section, usually esignate as observe rafts aft, aiships an forwar, as T A, T M, T F., Figure. A coonly aopte etho of arine surveys for assessent of the effects of sall global hull longituinal eflection on the ship isplaceent is to perfor stanar hyrostatic calculations supposing parabolic eflection shape with an equivalent value of raft aiships, enote as "quarter ean raft" efine as: T T + T + TF = () w/ A 6 M M A ore general case enote as the CroMark stanar raft T CM, is when instea of a constant /, a variable raft correction factor C is introuce: T CM TA + TM(/ C ) + TF = () / C The iensionless raft correction factor C ue to eflection can be erive by only basic waterplane characteristics, such as the oent to change tri one unit MT, tons per unit of iersion TP, water plane coefficient C wp, longituinal water plane oent of inertia I L an water plane area A wl. The correction factor can be easily calculate fro available ship s hyrostatic particulars Ziha (00), as shown: C I / A MT L = = ) L wl pp ( Lwl /) TP ( Lwl /) ( CWP ()

2 The ore accurate raft correction factor allows an alternative placeent of raft arks, Ziha (00). The positions alongsie the ship s hull, centere with respect to longituinal centre of flotation LCF, where the observe rafts are equal to the equivalent raft Ziha (00), Figure, aounts to: x =± Lwl C / () Following the sensible assuption that the position of the axial eflection is closer to the center of flotation LCF than to the eflection aiships, the equivalent raft is efine by only two CroMark raft reaings aft CM A an forwar CM F, Figure. The CroMark stanar equivalent raft CM yiels to ore accurate isplaceent assessent of a eflecte hull eploying the stanar hyrostatic ata. The usually applie "quarter ean raft" or ean of ean raft correction for hull hogging or sagging is not appropriate for oern large erchant ships with relatively high waterplane coefficients. Therefore, the CroMark proceure offers an alternative, quick, practical an ore accurate rational etho base on the stanar an peranent hyrostatic particulars for assessents of rafts an isplaceent of a sensibly eflecte an trie ship hull uring a raft an eaweight survey. x x LCF CM A CM L CM F 9 Aft perpenicular Figure. The CroMark raft arks position alongsie ship hull of / t Stinice Figure. The CroMark raft arks aft on /t «Stinice» Figure. The CroMark raft arks iships on /t «Stinice» Since the observe CroMark rafts aft an forwar are centere with respect to the LCF at positions x, the CroMark stanar equivalent raft CM for isplaceent calculations accounts for the hull longituinal eflection an virtually coincies with respective center of flotation, Figure : CM A + CM F CM = (5) Forwar perpenicular The applie CroMark raft correction factor base on the true waterplane shape iffers significantly fro the traitionally applie constant correction factor of / for ost of the oern erchant ships. Moreover, the CroMark correction factors can be easily calculate on the basis of hyrostatic particulars an attache to the ship s instruction book for routine application uring arine surveys. The siplest application of the CroMark iprove raft correction factors on traitional raft reaings aft, aiships an forwar yiels the ore accurate isplaceent of a eflecte ship uring a eaweight survey. However, both the "quarter ean raft" an the CroMark proceures can be applie only for reasonable ship s tri an hull eflection. Many erchant ships cannot have raft arks on perpeniculars an aiships ue to the hull for an aitional recalculations ue to raft arks position are neee anyhow. Since the traitional placeent of raft arks was institute when the ships were of ifferent hull for an not as long as oern erchant vessels, the alternative raft arks place closer to the aiships, usually in the area of the parallel hull sies, Figure,, ay lea to ore accurate an less troublesoe raft reaings on large ships ue to less intense otions an ue to easier, near siultaneous view by observers on boar or ashore, eliinating the nee for using a boat for inspection of raft arks. It appears feasible to assess a eflecte ships isplaceent with iprove accuracy by placing raft

3 arks an/or autoatic easureent evices nearby the loa line only in two positions alongsie a ship hull, preferably portsie an starboar, which provie ientical equivalent an observe rafts. Since the stanar equivalent raft resulting fro the averaging of raft reaings at alternative placeent of raft arks virtually coincies with respective ship s center of flotation, the CroMark survey proceure yiels to iprove isplaceent assessent of a sensibly eflecte hull, an oreover, in the sae tie, it accounts for the correction ue to the reasonable ship s tri Ziha (00). The thir raft reaing is to be introuce only when the aount of hull eflection is of a particular interest, what is selo require in ship service. In the cases when the ship s tri or the eflection of the hull are too large, the aitional CroMark raft reaings in cobination with the traitional raft arks at perpeniculars an aiships iprove the accuracy of the lengthwise integration over incline an eflecte sections using Bonjean s curves. The alternative placeent of raft arks in positions of greater hull breath in the area of the parallel i-boy facilitate the fitting of raft arks an as sense for averaging both portsie an starboar raft arks, even for ships with significant heeling angle. The loae conitions with sensible tri an sall eflections are of utost interest for eaweight survey, uch ore than the ballast conitions, an therefore the placeent of CroMark raft arks nearby the loa line, in cobination with raft arks at stern which ipacts on propeller iersion an ruer effectiveness an at bow, which affects aneuverability an slaing, appears practical for surveyors ue to easier inspection an to ship-owners an crew ue to accuracy. The purpose of the next consieration is to estiate the upper liits of the effects of local eforations on changes of ship s isplaceent using appropriate proceures base on ship s theory, an to present results on an exaple ship built in Croatia. LOCAL DEFORMATIONS The ship s hull is also subjecte to local eforations affecting the hull volue. Local eforations can be abstracte on several levels of substructures, e.g. hull section Figure, or a botto Figure 6. Since the ship s isplaceent in service is in general eterine on a still water, only the local eforations uner static loas on the ierse shell are relevant for assessent of hull volue changes. Peranent plastic eforations on ol ships outer shell ight be inclue as initial losses of isplaceent. Following the reasonable engineering assuptions generally use in sei-epirical approach to ship construction about utually supporting role of oinant hull substructures an girers, such as longituinal an transversal bulkheas with respect to botto, sies an ecks, or sies with respect to botto an ecks, or botto with respect to bulkheas an sies, or ecks with respect to bulkheas an sies, etc., the relevant local eforations of the hull stiffene panels are consiere in three levels, e.g. hull section Figure an botto panel Figure 6, as follows: I level (priary) eforations: panels between oinant substructure (e.g. botto panel between two longituinal an transversal bulkheas, or sie panels between botto, eck an bulkheas) II level (seconary) eforations: stiffene panels between oinant girers (e.g. botto an sie panels between longituinal an transversal girers of large substructures ) III level (tertiary) eforations of plates between two stiffeners or stiffener an girer (e.g. botto plating between longituinals an sie plating between fraes). Assessents of local eforations Nuerical an analytical ethos are consiere. The finite eleent etho (FEM) for this application requires a very fine -D FEM oel of the entire ship structure, or at least, a set of very fine -D FEM oels pertinent to all principal panels of the hull structure, Figure 5. Such an approach is feasible but tie consuing, particularly if the FEM oel is not available fro the construction phase. Therefore, for this application, analytical ethos base on the plate bening theory are preferre. The isotropic plate etho is foun appropriate for hull plating between stiffeners an/or girers which contribute to the thir level of local eforations. WL substrucure (section of tank) supereleent (panel of plating) acro esh icro esh I level eforations II level eforations III level eforations Figure. Presentation of transverse local eforation level Figure5. Finite eleents esh of the ships hull

4 Figure 6. Local eforation levels of ships botto The proceure benefits of plate bening equation solutions presente by iagras. The eflection in the ile of the plate bane uner lateral pressure p accoring to Schae s iagras in Hughes (9), using factor K (epening on η i ρ) is as follows: pb w = K (6) Ei b where η = torsion coefficient; ρ = virtual plate aspect ratio; an i b = unit oent of inertia of girer parallel with the plate shorter sie. The paper actually ae use of ore sophisticate iagras accounting also for torsional stiffness given by Marguerre an Woernie in Hughes (9), which efine the eflection aist the plate uner bening as: w ( ab) 5 p = A (7) D D x y Eix( y) where Dx( y) = =bening stiffness in irections µ x(y); an A=factor that epens on η an ξ=/ρ. The plate bening calculations for the purposes of this paper were carrie out by two coputer progras, one using igitize curves fro iagras an another by using Fourier-series expansion.. The proceure of calculation Calculations of changes of hull volues were perfore in five steps, as presente in the sequel:. For every ierse substructure of length a an with b between two ajacent transverse bulkheas uner local pressures, the isplaceent at crossings of principal girers are calculate, supposing orthotropic plate fully rigily clape at eges. Each volue V ( n ) of the I level eforations is calculate by nuerical integration, where V ( n ) = volue of -th substructure of n-th copartent.. For every stiffene panel between ajacent transvers an longituinal girer of length a an with b uner local pressures, the isplaceents in the ile of the panel are calculate, supposing orthotropic plate fully rigily clape at eges. Each volue V( n ) k, of the II level is calculate by nuerical integration, for k-th panel of -th substrucuture.. All volues are suarize: V ( n ) k, k = no. panel. V( n) k, k = V = () ( n) uk where V( nuk ) = total volue of the II level eforations of n-th copartent of -th substructure.. The eflection curve of the flat plating between stiffeners of length a an with b, rigily clape at eges is taken in a trigonoetric for as the first ter in a series of Navier s solutions: w πx πy w( x, y) = ( cos )( cos ) (9) a b The axiu eflection w in the ile of the plate is obtaine uner assuption of an isotropic plate using appropriate iagra ata. The overall volue V ( n ) j, of III level for j-th part of flat plating between stiffeners is efine by integration as: a b ab V( n ) j, = w( x, y) x y = w (0) 0 0

5 V( n ) j, 5. All volues are accuulate: j= no. ofsooth. pl. fiels ( n) j, j= V( n ) uk = V () V ) where ( n uk = total volue of III level eforations of n-th copartent of -th substructure. Steps fro to 5 were carrie out for each part of the test ship for all three levels of eforations, an the results are presente in the sequel.. Exaple of calculation The effect of local eforations on ship s isplaceent is illustrate on the bulk-carrier Don Frane Bulic (Yar no. 99 Shipyar BRODOSPLIT, Split, Croatia) with five cargo hols an engine roo place aft. The ain particulars are as follows: L oa =7.6 length overall L pp =79.7 length between perpeniculars B =0.0 with, D =5.5 epth D =0.0 / 0.0 esign raft / axiu raft =50 / 659 t corresponing isplaceents DWT=600 / 00 t corresponing eaweights. The ship was norally longituinally stiffene, except the transversely frae sies, Figure 7. Coplete calculations for all three levels of eforations of botto an sies were carrie out only for the cargo hol copartents. While in case of engine roo an fore an aft peaks, the first two levels of eforations were unattainable ue to high rigiity of these parts of the hull, an only the III level eforations are accounte for. Calculations were carrie out for rafts where the structure significantly changes its character Figure 7. Draft = 5.9 correspon to the top of the longituinally stiffene hopper tank, an botto of the transversely frae sies. Draft =.5 correspons to the top of the sies an botto of the longituinally stiffene wing tanks. Exaples of how sub-structuring is carrie out an the eforation level consierations are given in Figures, 9, 0. Calculations accoring to proceural steps to 5 gave the following results: -for = 5.9 loss of isplaceent is V= 7. -for =.5 loss of isplaceent is V=7.59 Results by levels of eforations for cargo Hol no. are presente in Figure. The results for the whole ship inicating the contribution of each copartent are given in Figure, in orer to estiate the contribution of each copartent an each eforation level, wherever that ay be of interest. with b y Fr x lenght a Figure. Double botto, Hol no., I level eforations y Fr y Fr.5 x LC Fr.5 x Figure 9. Wing tank, Hol no., II level eforations Fr Fr.77 Figure 0. Sie, Hol no. 5, III level eforations [] S 0 D substr.side 6 II LEVEL III LEVEL substr.double BOTTOM I LEVEL substr.bilge TANK TOTAL VOLUME OF DEFORMATION OF HOLD V [ ] V( D)=75. V( S)=59. Figure 7. Bulk-carrier hull substructures Figure. The volues of local eforations at Hol no.

6 S 0 D 6 [] HOLD NO. HOLD NO. HOLD NO HOLD NO. HOLD NO. 5 MACHINERY V [ ] V( D)=5.59 V( S )=6.9 TOTAL VOLUME OF DEFORMATIONS At the en, the total isplaceent with appenages an with effect of local eforations is obtaine as: = ρ = C C = C () ap ap sea ρsea ap l ρsea ap ap The concluing reark in this consieration is that the coefficient of appenages C ap can be correcte for local eforations C l as follows: C = C ap C l = = =.00 ap C ap Figure. The total volue of local eforations for all hols. Correction of isplaceent Norally in practice of arine surveys the theoretical isplaceent is correcte by using of a coefficient of appenages as shown: = ρ sea C ap () where = volue of isplaceent on easure raft ; an ρ sea = ensity of the seawater. The coefficient of appenages C ap accounts for thickness of ship s plating, volues of bilge keel, screw, ruer, ruer horn or posts, struts, bossings an other appenages. Norally the coefficient of appenages C ap epens on raft of ship, but in practice of isplaceent survey a constant coefficient of appenages C ap is usually assue. In orer to siplify the isplaceent survey the effect of appenages is inclue in a joint isplaceent correction factor iplying the water ensity aounting to ρ sea C ap =0 kg/. In the sequel, the aount of the coefficient of appenages C ap will be reconstructe by using the results of the forerly presente calculations. The hull volue on raft s =0.0 erive fro appropriate hyrostatic ata aounts to =56.. The calculate volue accounting for appenages aounts to ap = 59. Consequently, the volue of appenages aounts to ap - =.7 Therefore, the coefficient of appenages C ap is obtaine as follows: C ap ap 59 = = = The total loss of volue ue to all three levels of local eforations on raft s is calculate earlier an aounts to V ef = 5.6. Consequently, the volues are ecrease to aounts = 570 an,, ap = 5. Finally, the coefficient of local eforations C l in that case can be calculate as: C l ap 5 = = = ap.5 Rearks on results Calculations in the paper were perfore in case of a bulk-carrier built in Shipyar Broosplit, with 5 cargo hols, 5 ain substructures of botto an sie (I level), transverse frae panels an 6 longituinal stiffene panels (II level) an 0 flat plates between stiffeners an girers (III level). The total ship s isplaceent is in the presente exaple expectely reuce relatively by a very sall aount of 0.0% as a result of local eforation. However, the iinution of volue with respect to all appenages ue to volue losses of local eforations in this exaple is relatively high, aounting to about % of the volue of appenages. CONCLUSION The CroMark raft arks were at first fitte on /t Stinice with perission of Aerican Bureau of Shipping. The captain s reports about the practical usage of the CroMark proceure on boar are encouraging. The proceure for estiation of the effect of local eforations of ierse shell on the isplace volue of ship s hull, base on theory of isotropic an orthotropic plate bening was not intene to be a routine one. The priary ai was to provie soe experience in assessent of the orer of agnitue of these effects. Therefore, the application of ore sophisticate but tie consuing FEM proceures for the proble elaborate in this paper oes not appear rational. In conclusion of these consierations, the usually applie coefficient of appenages in service conitions ight be slightly reuce in orer to account for the local eforations of the ship s outer ierse shell. LITERATURE Hughes, O. F. 9. Ship Structural Design, SNAME, NJ. Ziha, K. 00. Patent HR P0007A, Zagreb, Croatia. Ziha, K. 00. Displaceent of a Deflecte Ship Hull, SNAME Marine Technology. 9(): 5-6.