A Study on Development of Structural Safety Assessment System of Damaged Ships due to Marine Accidents

Transcription

1 A Study on Development of Structural Safety Assessment System of Damaged Shps due to Marne Accdents Tak-Kee, Lee, Chae Whan, Rm, Byung-Hyun Km Korea Insttute of Machnery & Materals, Daejeon, Korea K-Sup, Km Martme and Ocean Engneerng Research Insttute, KORDI, Daejeon, Korea ABSTRACT Shps are lkely to be subjected to accdental loads such as collson and groundng. Once she has damage on the hull, her ultmate strength wll be reduced. The system addressed n ths paper s to assess the safety of shp structures wth damages due to marne accdents The safety assessment s based on the ultmate longtudnal strength obtaned by usng Smth s method. In order to verfy the system, the expermental result carred by Dow usng 1/3 scaled frgate shp hull test was used. As the result, the system gves a good correlaton wth the experment wthn 8% n dfference. KEYWORDS Ultmate longtudnal strength; Collson; Groundng; Marne accdent; Smth s method; Structural safety assessment. INTRODUCTION In general, the ultmate strength can be defned as the maxmum load-carryng capacty of a structure. For the longtudnal hull grder strength, the ultmate strength could be defned as the maxmum bendng moment n the relatonshp between hull grder bendng moment and curvature. No addtonal load can be carred beyond the ultmate strength. The frst attempt to evaluate the ultmate strength of shp structure was made by Caldwell (1965). He appled Rgd Plastc Mechansm Analyss to evaluate the ultmate hull grder strength. To am more ratonal desgn, t could be qute natural to consder the ultmate strength as the strength standard nstead of bucklng strength. Recently, there are three bg movements n the marne socety, whch are Goal-Based New Shp Constructon Standards (GBS) by Internatonal Martme Organsaton (IMO), Common Structural Rules (CSR) by Internatonal Assocaton of Classfcaton Socetes (IACS) and Ultmate Lmt State (UL) assessment by Internatonal Organsaton for Standardsaton (ISO). The GBS conssts of fve ters, and CSR are closely related to GBS through Ter IV. In CSR, t s requred to evaluate the ultmate hull grder strength as well as the ultmate strength of plates and stffened plates n shp structures. Also n ISO, new standards for lmt state assessment of shp structures ncludng bucklng/ultmate strength are now comng up. Under such crcumstances, the ultmate strength assessment s now becomng more and more mportant ssue to ensure the safety of shp structures (ISSC, 2006).

2 Recently, a structural safety assessment system of damaged shps based on the ultmate strength was developed. Ths paper descrbes the calculaton process of ultmate strength based on Smth s method and the confguraton and features of the developed system. Ths system can be used n evaluatng the safety of damaged shp structures due to marne accdent lke collson and groundng. CALCULATION OF ULTIMATE STRENGTH Basc Assumptons Accordng to Hughes (1983), the most accurate and most general method for calculatng the ultmate strength s to perform ncremental fnte element analyss of the entre hull module, but the computatonal requrements are too great, or too costly, wth present day computng capablty. It s therefore necessary to develop a smplfed approach whch retans suffcent accuracy but nvolves an acceptable amount of computaton. From the consderatons on collapse behavours of hull module, the followng two smplfcatons have been ntroduced as basc assumptons n calculatng the ultmate strength n generally (Hughes, 1983). 1) Snce the transverse structure s approxmately orthogonal to the longtudnal structure and the shell and deck platng prevent sway n the longtudnal drecton, there are only two ndependent modes of overall collapse: longtudnal collapse and transverse collapse. 2) By consderng the relatve szes of the transverse frames and the longtudnal structure between these frames t s possble to ensure that longtudnal collapse would only occur between two adjacent transverse frames. Calculaton Process by Smth s Method Two basc elements are a stffened plate wth a plate and a stffener and a corner element wth adjacent two plates n the corner. When the vertcal bendng moment s domnant, the calculaton process for the ultmate strength s as follows: [Step 1] Generate basc elements of transverse secton of shp structures wth only longtudnal members. [Step 2] Defne the relatonshp between the axal average stress and average stran of each element. [Step 3] Calculate the ultmate stran ε ult (= σ ult /E) and the dstance from ntal neutral axs y for each element, where σ ult s the ultmate stress and E s Young s modulus of each element. [Step 4] Defne the ntal curvature φ 0 of the transverse secton as the curvature of element wth mnmum value. ( ε ) ( ε y) ult φ 0 = MIN, (1) y y [Step 5] Calculate stran of each element ε = φ y when φ = φ 0, and then calculate stress of each element σ by usng the relatonshp between average stress and average stran defned n Step 2. [Step 6] Determne the locaton of new neutral axs by usng the stresses of each element. [Step 7] Recalculate the dstance from the new neutral axs y, and then calculate the ultmate bendng moment Mu as follows: u = M σ A y (2) [Step 8] Increase the curvature by addng an ncremental curvature ( φ = φ 0 + φ ), and then repeat the process from Step 5 to 7 untl when there s no ncrease n Mu as the ncrease of curvature. It s assumed the ncremental curvature s 0.1 tmes of ntal one. Stress-Stran Relatonshp In ths system, the relatonshp developed by Rahman & Chowdhury (1996) has been

3 appled. They had been used the approach by Hughes (1983), whch was derved from bucklng theory of column based on the assumpton that a stffened plate could be replaced by a beam-column. For example, Fg. 1 gves the axal average stress stran curves under tensle or compressve loads. Fg. 1: Axal average stress average stran curves. (a) under tensle load (b) under compressve load Fg. 3: Input modules of plate and stffener CONFIGURATION OF DEVELOPED SYSTEM Ths system has been developed by usng Vsual C and OpenGL. Fg. 2 shows the confguraton of the system. The module for defnng stffened panels has a functon whch generates automatcally the stffened panel elements from cross secton members. Fg. 3 s the nput module of plate and stffener. For ths module, a specal purposed modeller developed by the Korean Shppng Regster was appled. After generatng the geometrc data for plates and stffeners, thckness, materal propertes and locaton of each member are set. Fg. 4: Watch bar of data for structural members Fg. 5: Generaton of stffened panel element C ro ss S ecto n D ata Inp ut o r S hp D atab ase S tffened P aneld efnton C o m p artm ent D efnto n D am aged A rea, D raft, H eelng A ng le, U ltm ate S treng th C alculato n A ctuallo ad ng Inp ut S m ulato n o f P ro g ressve C o llap se D ecson of Falure Fg. 2: System confguraton and calculaton flow In order to check the nformaton of the generated members, the numerc data for the member appears smultaneously wth the generaton of the member n the watch bar below the model vew as shown n Fg. 4. The watch bar conssts of four components: plate, stffener, element and moment-curvature. Among them, moment-curvature gves the calculaton results for the ultmate strength. Fg. 5 shows the cross secton conssted of the generated stffened panels of double hull tanker.

4 In ths process, mrrorng functon was appled to copy the members n the port sde nto starboard sde. Fg. 6 shows the process to defne the compartment and loadng condtons. Here, one of the compartments must contan the outer sde shells to defne hydrostatc pressures. Also, n order to consder the added mass due to floodng, all dry cargo holds should be defned as compartments. Fg. 7 s the user nterface to do the structural safety assessment of an objected shp. Age of the shp s needed to take account of the effect of corroson due to agng. The damaged part can be selected drectly by user on the graphc dsplay. Also, the heelng angle occurred from floodng due to damage can be chosen. After settng the damaged condton ncludng heelng angle, the pressure onto structural members by lqud cargo or ballast recalculated as n Fg. 8. The fnal step of ths process s the calculaton of ultmate strength as n the process bar n the rght bottom of the check vew. Fg. 8: Calculaton of actual pressure due to damage Fg. 9: Assessment for bottom damaged case of D/H tanker Fg. 10: Assessment for sde damaged case of D/H tanker Fg. 6: Defnton of compartment Fg. 7: Interface of structural safety check Fg. 9 and 10 are examples for the assessment for bottom/sde damaged cases of an double hull tanker. The range of damages was selected based on IGC Code 2.5, 2.7, 2.8 and IBC Code (IMO). In these fgures, the actual saggng moments were the sum of stll-water and wavenduced bendng moments from UR S11 (IACS). The vertcal and horzontal bendng moment n two damaged cases was - 16,050MN-m and 1,500MN-m respectvely. The draft was 23m, and the added water draft was 1m. From the results, t can be assessed as very dangerous f the safety margn of vertcal bendng moment s below 1.0.

5 COMPARISON WITH TEST RESULT A few expermental studes on the ultmate longtudnal strength by usng a large scale model have been done (Lee et al., 2008). In recent, the authors carred out the experments by usng box grder models wth sde/bottom damage (Lee et al., 2008; Rm et al., 2008). These expermental studes have provded a very meanngful data for the verfcaton of related assessment tools. Among them, Dow (1991) performed the ultmate strength test usng a 1/3 scaled model of frgate naval shp under saggng condton. The total length ncludng test jg reached 18 meters. The dmenson of cross-secton was 4.0m (breadth) 2.8m (depth). Fg. 11 gves the cross secton of test model by Dow. In ths study, ths test was used to verfy the accuracy of the developed system. Fg. 12 shows the model and result by the developed system. Here, No. 2 deck and center grder were modelled by plate elements because of ther dmensons. As the result, the moment curvature curve lke Fg. 13 was obtaned. In the graph, the sold lne s the result from the present system, and dotted lne s the one from ALPS/HULL (2006) wth ntal deformaton of 10% of the thckness and resdual stress of 5% of yeld stress. Ths program was used n order to compare the ultmate strength of the same model under hoggng condton whch s not carry out by Dow. The calculated ultmate vertcal bendng moment s bgger by 4% than the test result and smaller than that of ALPS/HULL by 1% n saggng condton. Meanwhle, n hoggng condton the ultmate strength was bgger by 8% than that of ALPS/HULL. These dfferences mght be nvolved the ntal deformaton and weldng resdual stress. The results by present system were not to take nto account the two effects. Therefore, the present system could be thought as t gves a relatvely good correlaton wth test result. Fg. 11: Half md-shp secton of 1/3 frgate test model Fg. 12: Assessment result by usng the developed system (M_actural_saggng = -10MNm) Vertcal Bendng Moment (Nmm) 1.5E E E E E E E+010 Experment by Dow ALPS/HULL (w/t=0.1, s rc /s Y =0.05) Present System -6E-006-4E-006-2E E-006 4E-006 Curvature (1/mm) Fg. 13: Relatonshp between vertcal bendng and curvature on Dow s test model

6 CONCLUSIONS In ths paper, the structural safety assessment system developed to assess the ntegrty of damaged shp structure due to marne accdents lke collson and groundng s descrbed. The present system has a specal purposed modeller for the modellng of shp structure. Ths modeller enables users to do easer and faster modellng of structure. The accuracy of present system was compared wth the test result by usng large model and specal purposed commercal program. As the result, the present system gves a relatvely good correlaton. ACKNOWLEDMENTS Ths work was supported by the Mnstry of Commerce, Industry and Energy of Korea through the project, Development of technologes on shp structural safety assessment and nose/ vbraton reducton. The supports are gratefully acknowledged. REFERENCES ALPS/HULL, A Computer Program for Progressve Collapse Analyss of Shp Hulls (Verson ), Pusan Natonal Unversty, Korea, Caldwell, J.B., Ultmate Longtudnal Strength, Trans. RINA, 1965, Vol. 107, pp Dow, R.S., Testng and analyss of 1/3-scale welded steel frgate model, Proceedngs of the Internatonal Conference on Advances n Marne Structure, Dunfermlne, Scotland, pp Hughes, O.F., Shp Structural Desgn, John Wley & Sons, Inc. 1983, Chapter 17, pp IACS (Internatonal Assocaton of Classfcaton Socetes), UR (Unfed Requrement) S.11 IMO (Internatonal Martme Organzaton), Internatonal Code for the Constructon and Equpment of Shps Carryng Lquefed Gases n Bulk (IGC Code), IMO, Internatonal Code for the Constructon and Equpment of Shps Carryng Dangerous Chemcals n Bulk (IBC Code), ISSC, Proceedngs of the 16th Internatonal Shp and Offshore Structures Congress, Eds. Freze, P.A. and Sheno, R.A., Southampton, U.K., 2006, Vol. 1, Commttee III.1. Lee, T.K., Rm, C.W., Han, D.S., Km, B.H., Lee, J.M. and Km, K.S., A study of ultmate collapse strength n saggng of shp structures wth sde collson damage, Proc. of ISOPE 2008 (to be prnted). Rahman, M.K. and Chowdhury, M., Estmaton of ultmate longtudnal bendng moment of shps and box grders, Journal of Shp Research, Vol. 40, Sept. 1996, pp Rm, C.W., Lee, T.K., Han, D.S., Km, B.H., Lee, J.M. and Km, K.S., The effect of bottom damage sze of stranded shp on ultmate strength, Proc. of OMAE 2008 (to be prnted).