SPECTRAL FATIGUE ANALYSIS OF LIQUEFIED NATURAL GAS CARRIER STRUCTURAL DETAIL

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SPECTRAL FATIGUE ANALYSIS OF LIQUEFIED NATURAL GAS CARRIER STRUCTURAL DETAIL N Vladmr and I Senjanovc, Unversty of Zagreb, Croata S Malenca and C Ouled Houssene, Bureau Vertas, France BK Cho and HI

1 SPECTRAL FATIGUE ANALYSIS OF LIQUEFIED NATURAL GAS CARRIER STRUCTURAL DETAIL N Vladmr and I Senjanovc, Unversty of Zagreb, Croata S Malenca and C Ouled Houssene, Bureau Vertas, France BK Cho and HI Im, Hyunda Heavy Industres, South Korea SUMMARY The spectral fatgue analyss of a structural detal of Lquefed Natural Gas (LNG) carrer s presented. The analyss was performed by means of general hydro-structure tool HOMER (BV), where 3D FEM model for the structure and 3D potental flow code for flud modellng, respectvely, s appled. Mode superposton method s used to calculate shp hydroelastc response n waves. Numercal procedure for the fatgue assessment based on the so called top-down procedure s descrbed n detals and appled to determne stress concentratons n fne mesh FE model of a selected structural detal. Based on the calculated stress RAOs and takng nto account proper S/N curve, the fatgue lfe of certan structural detal s obtaned for a selected operatonal profle. 1. INTRODUCTION Natural gas s consdered as a cleaner fuel compared to MDO or HFO. In lne wth ncreasng envronmental care, there s a rsng demand for natural gas worldwde and nternatonal natural gas markets are contnuously growng. One of the most mportant parts of LNG transportaton system are LNG shps, whch can be found n servce from A typcal LNG carrer s doublehull vessel wth four to sx tanks located along the centrelne, and there are nowadays several contanment systems n use, that can be classfed nto self-supportng ones (also referred to as Moss type) and membrane type ones. Both alternatves are desgned, constructed and equpped wth sophstcated systems for carryng LNG over long dstances at storage temperatures around -162 C [1]. Recently, the membrane tank system has been adopted wdely due maxmzng shp load capacty. More detals on LNGC cargo contanment systems can be found n [1]. IHS Farplay (IHSF) database [2] ncludes data on all shps operatng worldwde, and here the DWT of shps delvered from 1999 to 2015 (nclusve) s presented n Fgure 1, whch shows us that largest LNG shps have been bult about 10 years ago (several shps wth DWT smaller than t are omtted from the representaton). However, t s more nterestng to look at the number of delvered shps and number of orders by the end of 2019 (as stated n August 2015), Fgure 2. Although there s some slght trend to buld larger unts, t seems that market of LNG shps s rather unstable, whch s a consequence of economc crses. There are dfferent ssues assocated wth the desgn (cargo contanment system, hydrodynamc aspects, structural aspects, propulson ssues...) and operaton (LNG transfer systems, partal fllng ssues, problem of bol-off ) of LNG shps makng them rather complex objects, and ths paper s orented to the structural one,.e. how to assess fatgue lfe of a shp structural detal wthn so-called drect calculaton approach. Fgure 1 DWT of LNG shps delvered from 1999 to 2015 Fgure 2 LNG shp fleet and order book The appled procedure s elaborated n detals, as well as the used general hydro-structure tool HOMER [3,4]. More nformaton on the mentoned software s gven wthn the paper, but t s to be noted that t can be appled to any knd of shps and offshore structures n the analyss of both quas-statc and dynamc structural responses, caused by lnear, weakly nonlnear or mpulsve nonlnear hydrodynamc loadng, respectvely. Although hydroelastc effects are not expected to be of

prmary mportance n ths case (due to relatvely hgh hull grder rgdty and low operatonal speed), computaton of stress RAOs needed for fatgue damage computaton s done by hydroelastc model (lnear sprngng

2 prmary mportance n ths case (due to relatvely hgh hull grder rgdty and low operatonal speed), computaton of stress RAOs needed for fatgue damage computaton s done by hydroelastc model (lnear sprngng ncluded) takng nto account also the effect of nternal lqud n the cargo tanks, just to llustrate applcablty of the software wthn the complete procedure. 2. DESCRIPTION OF THE ASSESSMENT PROCEDURE Lnear hydroelastc analyss performed here s based on the mode superposton method [5]. Wthn the modal approach, total dsplacement of a shp s expressed through a seres of modal dsplacements: N Hx, t th x, (1) 1 where xt, pont, h x s modal dsplacement (mode shape), t H represents total dsplacement of one s modal ampltude, and N represents the total number of modes [4]. Generally, the procedure s very smlar to rgd body analyss descrbed n [6] except that the number of degrees of freedom s extended from 6 to 6 plus a certan number of elastc modes. The used modal approach mples the defnton of supplementary radaton potentals wth the followng body boundary condton: Rj j hn, (2) n where n s unt normal vector. After solvng the dfferent boundary value problems for the potentals, the correspondng forces are calculated and the equaton of moton s wrtten 2 ( ) ( ) m A B b k C F DI, (3) where m s the modal structural mass, b s the structural dampng, k s the structural stffness, A s the hydrodynamc added mass, B s the hydrodynamc dampng, C s the hydrostatc restorng stffness, whle F DI s the modal hydrodynamc exctaton vector. Once the modal ampltudes have been calculated the total stresses can be obtaned, at least theoretcally, by summng the ndvdual modal contrbutons and one can formally wrte, [6]: x x Σ, N, (4) 1 where Σ x, s the total stress and x s the spatal dstrbuton of modal stresses. In order to practcally take nto account hydroelastc effects on the structural response, dynamc computatonal scheme s appled, startng wth modal analyss n dry condton, Fgure 3, [7]. Once the dry modes are obtaned, the modal dsplacements are transferred from the structural model to the hydrodynamc one, and correspondng hydrodynamc problem s formulated. After that, fully coupled dynamc equaton s solved, gvng the modal ampltudes. Fgure 3 Dynamc analyss computatonal scheme [7] In order to cover all types of hydro-structural nteractons nherent shps and offshore structures descrbed n [4], the numercal software HOMER s developed n Bureau Vertas Research Department for the drect transfer of the seakeepng loads from the general seakeepng code to a structural FE model, Fgure 4, [3,4]. HOMER modules presented n Fgure 4 are ntended to be used as follows: HMFEM to compute mass and nerta propertes of FE model. Run modal analyss, HMSWB to analyse stll water load case and perform balancng, HMHST for runnng hydrodynamc pressures computatons usng the seakeepng code, HMMCN - solves mechancal problem, HMFEA to run FE analyss on load cases, HMRAO - to create RAOs and HMTIME to perform tme-doman computatons. Fgure 4 Flowchart of HOMER software applcaton [3] Three man deas ntroduced through HOMER software to obtan the perfect equlbrum of the structural model are the followng, [4]: 1. Recalculaton of the pressure at the structural ponts (nstead of nterpolaton). 2. Separate transfer of the dfferent pressure components, and calculaton of the dfferent hydrodynamc coeffcents by ntegraton over the structural FE mesh. 3. Soluton of the moton equaton usng the above calculated hydrodynamc coeffcents and nerta

propertes of the FE model. Ths pont ensures the perfect equlbrum of the FE load case because of calculaton of all the coeffcents of the moton the FEM model.

Fatgue assessment of LNG shp structural detal s performed accordng to the flowchart presented n Fgure 5.

applyng the coarse mesh dsplacements at the boundares of the local fne mesh later [10].

3 propertes of the FE model. Ths pont ensures the perfect equlbrum of the FE load case because of calculaton of all the coeffcents of the moton the FEM model. Wthn the nvestgaton presented n ths paper, HOMER s used wth Hydrostar [8] as the hydrodynamc solver, and NASTRAN [9] as the structural solver. Fatgue assessment of LNG shp structural detal s performed accordng to the flowchart presented n Fgure 5. For the fatgue lfe/damage calculaton, very local stress concentratons n some partcular structural detals are needed, and generally they can be calculated by refnng the global coarse mesh or usng the so called top-down approach. The former approach seems to be mpractcal leadng to excessve number of fnte elements, and therefore here, the latter one s used, whch mples solvng the global coarse mesh FEM problem at frst, and applyng the coarse mesh dsplacements at the boundares of the local fne mesh later [10]. In ths way the fne mesh FEM calculatons are performed n a next step wth the load cases defned by the prescrbed dsplacements from the coarse mesh and by the local pressures and nerta of the fne mesh. The above procedure should be performed for each operatng condton, defned by loadng, wave frequency and headng, and for both real and magnary part of the loadng, resultng n the RAOs of the stresses n each partcular structural detal. A specal care s gven to the separaton of the quasstatc and dynamc parts of the response to ensure a proper convergence of the results. The quas-statc part of the response s calculated usng the so called quas-statc method as descrbed n [7], and dynamc part of the response s calculated by summng up the dynamc contrbuton of each mode. Fnally, t s to be noted that the procedure for the fatgue assessment s the same rrespectve on calculaton of stress RAOs n quas-statc or hydroelastc manner. 3. SHIP DATA, NUMERICAL MODELS Applcaton of HOMER software to fatgue assessment of shp structural detal s llustrated on 175 KM3 LNG vessel, wth tank general arrangement shown n Fgure 6. The man shp partculars are the followng: Length between perpendculars: L pp = m Breadth: B = 46.0 m Depth: H = 26.0 m Draught: T = 11.6 m Dsplacement, full load: Δ f = t Dsplacement, ballast: Δ b = t Fgure 6 General arrangement of shp cargo tanks Global FE model of the consdered shp, havng elements s used for the calculaton, Fgure 7. Actually, much coarser mesh can be used for ths purpose, but one of the ams of ths nvestgaton was software testng at very demandng fnte element models. Fgure 7 FE model of the analysed shp Besde global FE model, fne mesh model of a structural detal s requred, to be ncluded n the calculaton wthn the top-down scheme, Fgure 8. Fgure 5 Fatgue analyss flowchart [11] After obtanng transfer functons of stresses, the spectral analyss s performed and based on the selected S/N curve and wave scatter dagram, fatgue lfe/damage s calculated [11]. Fgure 8 Fne mesh FE model wth selected boundary nodes for top-down procedure

The fne mesh model s located n the fore tank, whch s for the purpose of ths calculaton set at 70% of fllng capacty, whle other 3

It should be noted that ths loadng condton s not one of the standard ones from loadng manual and the shp s not amed to operate n ths

Example of tank ntegraton mesh that s also used as a hydro mesh s shown n Fgure 12.

structure, appled procedure also requres generaton of the so called ntegraton and hydrodynamc mesh, respectvely, Fgures 10 and 11.

13. Fgure 10 Integraton mesh Fgure 13 Detal ntegraton meshes 4.

4 The fne mesh model s located n the fore tank, whch s for the purpose of ths calculaton set at 70% of fllng capacty, whle other 3 tanks are full, Fgure 9. It should be noted that ths loadng condton s not one of the standard ones from loadng manual and the shp s not amed to operate n ths way, but t s used here n an academc way for the purpose of nvestgaton. The same meshes are requred for all cargo tanks contanng lqud. Example of tank ntegraton mesh that s also used as a hydro mesh s shown n Fgure 12. Fgure 9 Illustraton of the selected loadng condton on deformed shp structural model Besde both FE global and local models of a shp structure, appled procedure also requres generaton of the so called ntegraton and hydrodynamc mesh, respectvely, Fgures 10 and 11. The former s extracted drectly from the structural model, and then the latter one, s generated automatcally usng the exstng software routnes. Fgure 12 Tank ntegraton and (hydro) mesh Fnally, for the analysed detal, both nternal and external ntegraton meshes are needed, Fgure 13. Fgure 10 Integraton mesh Fgure 13 Detal ntegraton meshes 4. RESULTS Fgure 11 Hydro mesh Due to reason of smplcty, the analyss was performed for zero forward speed, whle wave headngs are consdered unformly dstrbuted from 0 to 350 wth step of The range of wave frequences s set from 0.0 to 2.0 rad/s wth a step equal to 0.05 rad/s. The frst computaton step s the modal analyss gvng dry natural modes and frequences, Fgure 14, whereas 8 elastc modes are retaned for hydroelastc computatons. Fgure 14 Dry natural modes and frequences of analysed LNG shp

Pror to runnng hydrodynamc computatons t s very useful to analyze stll water case results, that serve as

coordnate system, mass modellng wthn the structural model and basc calculaton setup, respectvely, etc.

showng the values accurately reflectng gven draught, Fgures 15-17.

on hull After performng hydrodynamc computatons and solvng equaton of moton, the global hydroelastc

The global results are mposed to detal FE model and FE analyss for each combnaton of shp speed, wave

Example of stress pattern n a structural detal s shown n Fgure 20, whle typcal stress RAO s presented n

5 Pror to runnng hydrodynamc computatons t s very useful to analyze stll water case results, that serve as recommended checks of structural and hydrodynamc model consstency, ther relatve postons n global coordnate system, mass modellng wthn the structural model and basc calculaton setup, respectvely, etc. Just for the llustraton, here the hydrostatc pressures on shp hull, tank and structural detal are gven showng the values accurately reflectng gven draught, Fgures Fgure 18 Stll water deflectons (mm) Fgure 19 Stll water stresses (N/mm 2 ) Fgure 15 Hydrostatc pressures on hull After performng hydrodynamc computatons and solvng equaton of moton, the global hydroelastc response s obtaned. The global results are mposed to detal FE model and FE analyss for each combnaton of shp speed, wave frequency and headng, seres of stress RAOs s done. Example of stress pattern n a structural detal s shown n Fgure 20, whle typcal stress RAO s presented n Fgure 21. Fgure 16 Example of hydrostatc pressures on tank Fgure 20 Example of Von Mses stresses (N/mm 2 ) n a shp structural detal Fgure 17 Hydrostatc pressures on detal Fgures 18 and 19 show stll water deflectons and stresses, where also reasonable numercal values are obtaned. Fgure 21 Typcal stress RAO

Fatgue lves are computed for all fnte elements n the very fne mesh model, and here several ones wth the lowest fatgue lves are only dentfed, Fgure 22.

6 Fatgue lves are computed for all fnte elements n the very fne mesh model, and here several ones wth the lowest fatgue lves are only dentfed, Fgure 22. Fgure 22 Identfcaton of fnte elements wth lowest fatgue lves Fatgue lves for the above fnte elements are lsted n Table 1, and ndcate good fatgue performance of a shp. However, more detaled analyss wth realstc shp loadng condtons s requred to offer fnal conclusons on her fatgue strength, snce ths calculaton s ntentonally performed n a rather academc manner. Table 1 Fatgue lves of selected fnte elements Element ID Fatgue lfe (years) CONCLUSIONS Spectral fatgue assessment procedure by means of general hydro-structure tool 3D FEM structural model and 3D BEM hydro model, respectvely, s descrbed n detals. All necessary calculaton steps and requred numercal models are dscussed, as well as top-down procedure for the assessment of local stresses, whch are further used for the fatgue damage computaton. Snce fully coupled hydroelastc model s used, shp elastc deformatons and her motons n waves are smultaneously taken nto account. Fne mesh model of a tank structural detal of LNG carrer, exposed to both nternal and external flud pressure, was used as an applcaton case. The obtaned results ndcate that the analysed structural detal s well desgned from the vewpont of fatgue, whch s n accordance wth the fact that analysed shp s n servce for several years wth no reported damage for the consdered detal. Also, the software was found to be user frendly and very robust for applcaton to fatgue assessment of shps and offshore structures, n spte of the very large fnte model analysed n ths nvestgaton. After these fndngs, the future work wll be orented to fatgue analyss of the consdered shps wth forward speed and a set of more realstc loadng condtons. 6. ACKNOWLEDGEMENTS Ths work was supported by a Natonal Research Foundaton of Korea (NRF) grant funded by the Korean Government (MSIP) through GCRC-SOP (Grant No ) and by Unversty of Zagreb, Croata wthn the short-term research grant: Safety, Energy Effcency and Envronmental Elgblty of Lquefed Natural Gas Carrers. The frst author would also lke to acknowledge support of Bureau Vertas for hs 3- month vst to ther Research Department n Pars. The obtaned results and conclusons reflect personal opnons of the authors and do not represent offcal atttude of ther nsttutons. 7. REFERENCES 1. KIM, H.S., CHUN, M.S., LEE, J.M., KIM, M.H., A comparatve evaluaton of fatgue and fracture characterstcs of structural components of lquefed natural gas carrer nsulaton system, Journal of Pressure Vessel Technology, Vol. 135, pp. 1-9, IHS, World Regster of Shps WROS manual, SIRETA, F.X., DE LAUZON, J., SURMONT, F., HOMER User gude, Bureau Vertas, Pars, France, MALENICA, S., DERBANNE, Q., SIRETA, F.X., BIGOT, F., TIPHINE, E., DE HAUTECLOCQUE, G., CHEN, X.B., HOMER Integrated hydro-structure nteractons tool for naval and offshore applcatons, Internatonal Conference on Computers Applcaton n Shpbuldng, Busan, Korea, BISHOP, R.E.D., PRICE, G., Hydroelastcty of Shps, Cambrdge Unversty Press, MALENICA, S., SENJANOVIC, I., VLADIMIR, N., Hydro structural ssues n the desgn of ultra large contaner shps, Brodogradnja, Vol. 64, No. 3, pp , BUREAU VERITAS, Whppng and Sprngng Assessment, Rule Note NR 583 DT R00 E., Pars, France, BUREAU VERITAS, Hydrostar, User's manual, Pars, France, MSC SOFTWARE, MD Nastran 2010 Dynamc analyss user's gude, Newport Beach, Calforna, USA, 2010.

7 10. SIRETA, F.X., DERBANNE, Q., BIGOT, F., MALENICA, S., BAUDIN, E., Hydroelastc response of a shp structural detal to seakeepng loads usng a top-down scheme, 31 st Internatonal Conference on Ocean, Offshore and Arctc Engneerng - OMAE, ASME, Brazl, IM, H.I., VLADIMIR, N., MALENICA, S., RYU, H.R., CHO, D.S., Fatgue analyss of HHI SkyBench TM TEU ultra large contaner shp wth sprngng effect ncluded, 7 th Internatonal Conference on Hydroelastcty n Marne Technology, Splt, Croata, AUTHORS BIOGRAPHY Nkola Vladmr s Assstant Professor at Faculty of Mechancal Engneerng and Naval Archtecture, Unversty of Zagreb (UNIZAG), Croata. He obtaned MSc degree n Naval Archtecture n 2007 at UNIZAG, and PhD n the feld of Hydroealstcty of shp structures n 2011 at the same Unversty. Hs research nterests nclude vbraton of shp structures (hydroelastc sprngng & whppng, engne & propeller nduced, etc.), vbraton phenomena n shp propulson system (man engne, shaftng, propeller ), fatgue, specalzed software development and applcaton as well as all other dynamc loadng and response ssues nherent to shps and offshore unts. He publshed over 100 papers n journals and specalzed conference proceedngs. Member of Dynamc Response Commttee of ISSC. Ivo Senjanovc, Professor Emertus of Naval Archtecture at the Unversty of Zagreb, Faculty of Mechancal Engneerng and Naval Archtecture, Zagreb, Croata. Teachng several courses on strength and vbraton of shps, offshore structures and submarnes. Investgatons felds are shp stablty, launchng, shp strength and vbraton, shell theory, desgn of pressure vessels, non-lnear dynamcs, shp hydroelastcty, numercal methods, numercal smulatons, etc. Books: Theory of Plates and Shells, Fnte Element Method n Shp Structure Analyss, Shp Vbratons. Publshed over 300 papers n scentfc journals and conference proceedngs. ISSC member actng n Techncal Commttees on shp strength, vbraton, load and response durng 10 threeyear perods. Fellow of the Croatan Academy of Scences and Arts. both n scentfc journals and at the specalzed conferences. Dr. Malenca s Deputy Drector of Bureau Vertas Research Department, leadng several natonal and nternatonal research projects. Hs man felds of research concern the hydro-structural nteractons due to the sea waves (lnear, nonlnear, frequency doman, tme doman, shps and offshore structures ). Actual topcs concern the hydroelastcty ssues for shps (sprngng, whppng, slammng, sloshng, green water ). He s a member of edtoral board of several respectable journals, Dynamc Response Commttee member of ISSC and correspondng member of Croatan Academy of Scences and Arts. Charaf Ouled Houssene receved MSc degree n Hydrodynamcs & Ocean engneerng n 2014 from Ecole Centrale de Nantes (ECN), France. Snce then, he has been workng n Bureau Vertas Marne & Offshore Dvson, France, as a hydro-structure research engneer. He has been n charge of developng smulaton tools and performng advanced studes of shps and offshore unt hydroelastc response. Hs current research nterests nclude manly flud-structure nteracton under dfferent approaches: rgd, elastc, lnear, nonlnear, etc., as well as other hydrodynamc loads such as Morson forces and lnear sloshng. Byung-K Cho, PhD, holds a poston of head researcher n the Structure Research Department, Hyunda Heavy Industres. He s responsble for structural assessments of shps and MODU structures. Member of Fatgue and Fracture Commttee of ISSC. Hong-Il Im currently holds a team leader poston as a senor engneer n basc hull desgn department of Hyunda Heavy Industres. Specalzed n evaluaton of hull grder vbraton ncludng offshore, basc hull desgn of contaner carrers, tankers, offshore structures and specal purpose vessels. Sme Malenca graduated n Naval Archtecture at the Faculty of Mechancal Engneerng and Naval Archtecture, Unversty of Zagreb n He obtaned hs PhD degree n 1994, at the Unversty Pars VI, on the problem of nonlnear water wave dffracton. He has publshed more than 100 papers

Fatigue analysis of HHI SkyBench TEU ultra large container ship with springing effect included

7 th International Conference on HYDROELASTICITY IN MARINE TECHNOLOGY Split, Croatia, September 16 th 19 th 2015 Fatigue analysis of HHI SkyBench 19000 TEU ultra large container ship with springing effect