Application of New Common Structural Rules on Aframax Tankers

Transcription

1 TSCF 2016 Shipbuiders Meeting Appication of New Common Structura Rues on Aframax Tankers CAI Shijian 1 SHENG Lixian 2 SHAN Penghao 1 SUN Yu 2 LIUYinhua 1 LIU Kun 2 1: Marine Design & Research Institute of China (MARIC), Shanghai, China <mcb@maric.com.cn> 2: Shanghai Waigaoqiao Shipbuiding Co. Ltd., Shanghai, China <rd@chinasws.com> Abstract Research institutes, shipyards and design offices in China are deveoping New Eco-friendy Tankers which are aso to compy with New (Harmonized) Common Structura Rues (CSR) for Buk Carriers and Oi Tankers. Athough New CSR have been reviewed by the Industry, it is found that there are sti some issues to be discussed for rea ship design, especiay for some new designs. This paper wi give a detaied introduction to one new Aframax tanker designed to meet the requirement of New CSR rues. Not ony the prescriptive cacuation but direct strength anaysis for the whoe cargo hods, incuding yieding, bucking and fatigue, are carried out. The impact of New CSR wi be discussed as we as some technica issues, such as the shear force adjustment outside the cargo region, modeing and bucking evauation of manhoe region. How to optimize the structura arrangement and scantings wi aso be studied briefy at the same time. 1 Introduction The New (Harmonized) Common Structura Rues for Buk Carriers and Oi Tankers [1] entered into force on the 1st Juy 2015 and supersede CSR-OT [2] and CSR-BC [3]. During the deveopment and maintenance period for New CSR, not ony IACS members but aso the industry carried out ots of consequence assessments to determine the effect of appying New CSR, incuding some ramification studies [4]. But it is to be pointed out that in the consequence assessment or ramification study of New CSR from China, the buk carriers and oi tankers are mosty od CSR vesses, and no structura rearrangement, no stee materia grade change, no optimization and no basic design parameters change. As we a know, a good ship design shoud be within the cost boundaries specified by Owners, within the safety requirement by Rues and Reguations (e.g. New CSR) and in compiance with other needs of society and the maritime industry (e.g. Environmenta protection and Occupationa heath). Some needs are correative or potentiay contradictory, e.g. New CSR and EEDI index. How to baance such needs and design eco-friendy or green ships with New CSR wi give more chaenges for designers and shipyards. The paper wi firsty give a brief introduction of new design trends for oi tankers and the corresponding impact to meet the requirements of New CSR. By prescriptive cacuation and direct strength anaysis, incuding fatigue evauation, one new eco-friendy Aframax design in accordance with New CSR wi be presented. At the same time, some technica issues in New CSR wi be studied or discussed. It shoud be noted that there are some additiona requirements for the new ship compared to the parent ship, i.e., ice cass (LR 1A), ower rate of higher strength stee, etc. Some pates and stiffeners of side she aong the whoe ength of the new ship have been strengthened due to ice cass. This effect is more obvious for the fore ship region than the aft ship region. The percentage of higher strength stee is about 55% for the new ship, but 68% for the parent ship. Some resuts may be not typica for this type of ship. 2 New design trends for oi tankers Page 1 of 12

2 TSCF 2016 Shipbuiders Meeting With the increasing demand for eco-friendy or green ship designs, especiay with a ow EEDI index vaue some new design trends for oi tankers resut in some effect or change to the scantings and structura design. Typicay these may incude: Leadge-Bow design which coud reduce the added wave resistance in both aden condition and the baast condition but woud increase the Rue ength and have a itte effect on the minimum requirement and hu girder oads. Thin hu ines design tends with decreased bock coefficient, especiay in the forward area, aftmost cargo area and machinery space, which wi on one hand increase the sti water bending moment in hogging condition (e.g. Suezmax and Aframax), and on the other hand induce probems for hu girder bending strength and utimate strength in the region of aftmost cargo area and machinery space. The increase of diameter of propeer and the decrease of distance between the propeer tip and hu, which woud induce greater propeer exciting forces, thereby affecting hu vibration. The popuar use of energy-saving devices, e.g. pre-ducts, which woud increase potentia safety risks. The optima design of superstructure with decreased wind resistance but woud give adverse effect on vibration characteristics of the superstructure. The more stringent requirement for the arrangement of cofferdams due to environmenta protection, which wi be chaenging for the compartment arrangement intended for iquid hydrocarbons and wi increase stee weight. The mandatory requirement for noise, which woud demand stee encosing was for some workshop areas and increase stee weight due to the minimum thickness requirement for genera pates. The above design trends wi be more chaenging for structura design, especiay when meeting the requirement of new CSR. 3 Prescriptive cacuation for Aframax tanker 3.1 Impact on scantings Different requirements between CSR-OT and New CSR, such as hu girder strength, minimum thickness, oca yied strength, bucking strength and fatigue strength, are considered and anayzed in the prescriptive cacuation stage. The modes of parent ship are used for this anaysis. It is found that there is no impact on scantings for hu girder yieding strength, hu girder utimate strength and hu girder residua strength. The impacts of New CSR on scantings are shown as foows: Fig.1 Impact on scantings of ongitudina structures within midship cargo hod region Page 2 of 12

3 TSCF 2016 Shipbuiders Meeting a) foremost cargo hod region b) aftmost cargo hod region Fig.2 Impact on scantings of ongitudina structures within foremost and aftmost cargo hod region 3.2 Fatigue strength It is found from fatigue assessments in New CSR that the scantings of some structures wi increase, such as ongitudina stiffeners on outer bottom cose to side she and on side she cose to waterine. But for some other structures, such as ongitudina stiffeners on the doube bottom, main deck and some ongitudina stiffeners on side she, the scantings wi decreased due to fatigue. Fatigue requirement has itte infuence on the size of stiffener. According to the cacuation, the number of stiffener end brackets on the doube bottom coud be decreased which wi therefore reduce workoad during construction procedure. It shoud be noted that the thickness of main deck may not be determined by the fatigue of deck ongitudinas, but by the fatigue of other detais on deck, such as pipes, supports and equipment weded on main deck, or deck openings. Because the stress concentration factors of detais mentioned above may be arger than that of deck ongitudina connections. 4 Direct strength anaysis for Aframax tanker 4.1 Cargo hod anaysis Besides New CSR, there are some other requirements, e.g. ice cass (LR 1A) and ower percentage of higher strength stee required for the target new ship. Severa modes are considered to evauate different requirements. Midship cargo hod region The anaysis of impacts of New CSR on scantings is carried out with the mode used for CSR-OT. The resuts in Tabe 1 coud refect the differences between New CSR and CSR-OT. Tabe 1 Impact of New CSR on yieding strength of midship cargo hod region Face pate of deck transverse: +60%~90% Knucke and toe: fine mesh Horizonta stringer: end of shear span +2mm Page 3 of 12

4 TSCF 2016 Shipbuiders Meeting Tabe 2 Impact of New CSR on bucking strength of midship cargo hod region Foor: +1mm; others: add stiffener Centerine BHD: add stiffener; change materia End of shear span of transverse stringer: +2mm; Others: add stiffener Transverse BHD: +0.5~2mm (port, upper) The resuts shown above are based on the mode of the parent ship. For the new ship, fewer reinforcements need to be appied after appying ice cass and ow rate of higher strength stee. The weight comparison between new ship and parent ship is shown as foows: Tabe 3 Structure weights of different modes (mid cargo hod with one transverse BHD) Mode Structure weight (t) Rate of higher strength stee Parent ship (CSR-OT, no ice cass) % Parent ship (New CSR, no ice cass) % New ship (New CSR, no ice cass) % New ship (New CSR, ice cass) % Foremost cargo hod region The FEA (finite eement anaysis) in CSR-OT has not been performed for the foremost cargo hod region of the parent ship. Transverse frame system is used for the side of foremost cargo hod region of the new ship. The fore ship region has been strengthened due to ice cass requirement and ower rate of higher strength stee. FEA is carried out for the new ship mode, and it is found that few areas need to be reinforced for New CSR. Aftmost cargo hod region The FEA in CSR-OT has not been performed for the aftmost cargo hod region of the parent ship. The anaysis of impacts of New CSR on scantings is carried out with the mode of the new ship (ower percentage of higher strength stee) with and without ice cass respectivey. Foowing resuts are based on the mode without ice cass. Tabe 4 Impact of New CSR on yieding strength of aftmost cargo hod region Centerine BHD: +2~8mm Horizonta stringer: simiar to midship Page 4 of 12

5 TSCF 2016 Shipbuiders Meeting Tabe 5 Impact of New CSR on bucking strength of aftmost cargo hod region Foor: +1~2mm and stiffener Centerine BHD: add stiffener; +1.5~2mm Side bottom girder: add haf frame; +0.5mm; Others: add stiffener Horizonta stringer: add stiffener; +0.5mm She: +3mm, or +1mm and stiffener Transverse BHD: +0.5~1mm (port, upper); add stiffener The impact of ice cass on scantings is about 60 ton increasing for side she of aftmost cargo hod region. The effect of ice cass on the resuts shown above is not obvious. Engine room region In New CSR, the evauation areas of aftmost cargo hod mode incude: A structura members being part of the forward transverse bukhead of the machinery space and a hu girder ongitudina structura members aft of this transverse bukhead within the extent of 15% of the aftmost cargo hod ength excuding sop tanks. The impact of New CSR on scantings of engine room region is mainy on patform and inner bottom, shown as foows. Insert pate is added for the 2 nd patform; 2mm increased for the 3 rd patform; 4mm increased for the inner bottom; stiffeners are added for others. The effect of ice cass on the resuts is not obvious. Tabe 6 Impact of New CSR on scantings of engine room region Yieding Bucking Page 5 of 12

6 TSCF 2016 Shipbuiders Meeting 4.2 Fine mesh anaysis From fine mesh anaysis, the increase of scantings in midship cargo hod region is mainy appied to opening and manhoe areas. Some areas are shown as foows: Tabe 7 Some areas to be reinforced according to fine mesh anaysis Manhoe of side vertica web near upper knucke: repace manhoe stiffener with face pate Hopper web to face pate: increase the breadth of face pate Manhoe of vertica PSM on centerine BHD near arge bracket toe: repace manhoe stiffener with face pate, and increase web thickness In New CSR, fatigue factor f f is introduced in the anaysis criteria for fine mesh anaysis. For areas that compy with the fatigue assessment criteria, the anaysis criteria coud be magnified 20% for fine mesh anaysis. This requirement has the merits of decreasing the pate thickness and materia yied strength of ower and upper hopper knuckes which have aready compied with the fatigue assessment. It shoud be paid attention that the stresses of hopper knuckes outside midship cargo hod region may be arger than that in midship cargo hod region, due to the tank oads increasing outside midship cargo hod region according to the ship motion. There may be 20% thickness increasing in foremost cargo hod region compared to midship cargo hod region. 4.3 Fatigue anaysis The fatigue requirement of hopper knuckes can be satisfied even when the scantings are reduced according to fine mesh anaysis. The fatigue resuts of hopper knuckes are shown as foows. Tabe 8 Fatigue resuts of hopper knuckes Lower knucke Upper knucke Hot spot ocation T F (year) Hot spot ocation T F (year) HS1: inner bottom 27 HS1: side stringer 300 HS2: hopper sopping pate 27.2 HS2: hopper sopping pate 25.1 HS3: hopper web 60 HS3: transverse web, beow stringer 35.7 HS4: doube bottom foor 36.9 HS4: transverse web, above stringer 37.0 HS5: side girder 115 HS5: inner hu ongitudina bukhead 25.5 HS6: scarfing bracket to the inner bottom 132 It is found that the fatigue screening of some bracket toe areas of horizonta stringer is not satisfied. The fatigue resuts and fatigue screening resuts are shown as foows. The target stringer is the midde horizonta stringer. The hot spot ocation is described in New CSR Pt1, Ch9, Sec 2, Tabe 14 and Tabe 15. Tabe 9 Fatigue resuts and fatigue screening resuts of horizonta stringer Structure Hot spot ocation Type T F (year) Hot spot 1: free edge of bracket free edge 57 Backing Hot spot 2: inner hu at bracket toe type a 49 bracket Hot spot 3: transverse bukhead at bracket toe type a 27 Hot spot 1: inner hu at bracket toe type a 40 Stringer toe Hot spot 2: toe in way of face pate termination type b 40 Page 6 of 12

7 TSCF 2016 Shipbuiders Meeting According to the fatigue anaysis, areas that do not satisfy the requirement coud be detected and some optimization methods, such as increasing the pate thickness and extend the ength of bracket toe, shoud be conducted. Foowing figure shows the optimization of bracket toe based on the fatigue anaysis. (a) before optimization (b) after optimization Fig.3 Optimization of bracket toe based on the fatigue anaysis (owest side stringer) 5 Items to be discussed 5.1 Shear force adjustment in FEA of aftmost cargo hod region According to New CSR, a hu girder ongitudina structura members aft of forward transverse bukhead of the machinery space, within the extent of 15% of the aftmost cargo hod ength excuding sop tanks, need to be evauated. The shear force at the forward transverse bukhead of the machinery space is normay very high, and the effective shear area of the cross section aft of the transverse bukhead is much ess than that of the cross section forward of the transverse bukhead. The scantings of evauation areas in the engine room region is mainy determined by hu girder strength, especiay shear strength for ongitudina bukheads. But the target position of shear force adjustment is ony at transverse forward or aft bukhead of center cargo hod. If shear force adjustment cannot be performed correcty in engine room region, the shear stress of ongitudina bukheads woud be incorrect. The comparison of target shear force distribution and the shear force distribution in mode after shear adjustment (enveop curve) is shown as foows: Fig.4 Shear force distribution of aftmost cargo hod mode Page 7 of 12

8 TSCF 2016 Shipbuiders Meeting From the figure above, it coud be found that the adjusted mode shear force curves in evauation area after the forward transverse bukhead of the machinery space exceed the target shear force curves. So the shear stress of ongitudina bukheads in engine room may be too conservative. If the aftmost cargo hod is much onger, the affective area in engine room wi be much arger. The trends of shear force distribution in engine room are much different between mode vaues and target vaues. It is suggested that the hu girder shear structura members in engine room are not to be evauated in cargo hod anaysis, or the shear force adjustment method is to be amended so that the mode shear force curves in evauation area of engine room can meet the target shear force curves. 5.2 Bucking cacuation of manhoe area According to New CSR, the modeing method of manhoes in webs of PSM (primary supporting members) in cargo hod mode is removing the appropriate eements, and reating to the corresponding screening procedure for manhoes. This is different from CSR-OT and od version of New CSR. There are sti some probems that need to be considered. What kind of openings shoud to be treated as a manhoe? How to remove the appropriate eements? Different treatments wi give different resuts. Liu [5] has studied the rationaity of manhoes modeing in FEM anaysis based on yieding evauation. The bucking evauation in the manhoe area sha aso be considered. According to the New CSR, the web pate adjacent to the opening on both sides is to be considered as individua unstiffened pate panes. For the opening with edge reinforcements, the reduction factors are shown as foows: With edge reinforcements Tabe 10 Reduction factors for opening with edge reinforcements in PSM C x, C y Separate reduction factors are to be appied for areas P1 and P2 using C x for case 1 or C y for case 2 in CSR Pt1, Ch8, Sec5, Tabe 3 with stress ratio: ψ = 1.0 Opening modeed in PSM C τ Opening not modeed in PSM Separate reduction Separate reduction factors are to be appied for areas P1 and P2 using case 15 in CSR Pt1, Ch8, Sec5, Tabe 3. factors are to be appied to areas P1 and P2 using case 15 in CSR Pt1, Ch8, Sec5, Tabe 3 with: τ av = τ av (web) h/(h h 0) Where: h : Height, in m, of the web of the primary supporting member in way of the opening. h 0 : Height, in m, of the opening measured in the depth of the web. τ av(web) : Weighted average shear stress, over the web height h of the primary supporting member. The bucking resuts for manhoe area with different modeing methods (opening modeed or not) are shown as foows (foor, manhoe with edge reinforcements, h=2300, h 0=700, 0=600, s=815): a) Opening modeed b) Opening not modeed Fig.5 Bucking resuts of manhoe area (foor) with different methods Page 8 of 12

9 TSCF 2016 Shipbuiders Meeting It coud be found that the maximum vaues of P1 and P2 between the two methods are very cose, but the resuts of adjacent panes are very different. The resuts of the two panes just near the opening (not P1 and P2) in a) are much higher than that of corresponding panes in b), because there is some stress concentration for the eements near the opening in a). It is normay considered that the method a) is conservative, and there may be a risk for panes adjacent to manhoe area with method b). More research sha be considered to cacuate the bucking with fine mesh mode. 5.3 New incined strut arrangement for bottom foor With topoogica optimization, it is found that the arrangement of incined strut for bottom foor with ong shear span may be effective. MARIC has appied such arrangement in the design of a new Aframax as foows: Fig.6 New incined strut arrangement With the direct strength cacuation, it is found that the incined strut can decrease the high stress at hopper knucke end of bottom foor (A), but at the same time, the inboard end of bottom foor (C) shoud be paid more attention, because ony the part of bottom foor between ocation A and ocation B wi be checked in the prescriptive requirement. Loading Scenario Tabe 11 Shear stress comparing for incined strut arrangement Seagoing conditions Harbour and testing conditions Loading pattern Shear Stress in cargo hod anaysis Description High stress at hopper knucke end and inboard end High stress at hopper knucke end and inboard end Shear force of griage anaysis A B C A B C Page 9 of 12

10 TSCF 2016 Shipbuiders Meeting From griage anaysis, it coud be found that the shear force at ocation C is at amost the same eve of that of ocation B. From cargo hod anaysis, it coud be found that the shear stress at ocation C is even arger that at ocation B, due to smaer thickness at ocation C than that at ocation B. The materia at ocation C is norma stee, and the average shear stress may exceed the permissibe shear stress for harbor condition (C t-prτ eh=95mpa). It is suggested that the shear strength at ocation C sha be checked in the prescriptive requirement. 5.4 Seection of fatigue hot spot ocations and hot spot type Some new fatigue concepts are introduced into New CSR, such as types of hot spots, fatigue screening, etc. There are aso some hot spot ocations given in New CSR, and the amount is much more than that of CSR-OT. But it is found that not a the hot spot ocations need to be check according to the consequence assessments. Foowing figure gives the resuts of hopper knucke of oi tankers. The hot spot ocation is described in New CSR Pt1, Ch9, Sec 2, Tabe 4. a) weded ower knucke b) weded upper knucke c) radiused ower knucke d) radiused upper knucke Fig.7 Fatigue resuts of hopper knucke of oi tankers For weded ower knucke, resuts of hot spot 1, 2 and 4 coud be beow 25 years, and resuts of other hot spots are much higher than the design ife. For each test ship, the fatigue strength of weded ower hopper knucke is determined by hot spot 1, 2 or 4. So it is suggested that ony hot spot 1, 2 and 4 need to be evauated, incuding inner bottom pate on cargo hod side, hopper soping pate on cargo hod side, and doube bottom foor inboard the side girder. Hot spot 3 (hopper web) may aso be incuded. Page 10 of 12

11 TSCF 2016 Shipbuiders Meeting For the other three joints, the amount of resuts is not enough to give a concusion. Foowing concusions are ony for reference: for weded upper knucke, the resuts of hot spot 1 (side stringer on baast tank side) are much higher than 25 years, and may be omitted in fatigue assessment; for radiused ower knucke, the resuts of hot spot 6 (side girder) are much higher than 25 years, and may be omitted in fatigue assessment. Radiused upper knucke is not required in fatigue assessment if designed in accordance with detai design standard. 5.5 Arrangement of hopper web in foremost cargo hod region In the foremost cargo hod region, the strength requirement of the face pate of hopper web frame is hard to satisfy. The sectiona area of the curved face pate needs to be reduced in the mode according to New CSR (Pt1, Ch3, Sec 7, 1.3.3), and the stress concentration is very obvious in the curved part. The curved part of face pate is FB300X30. The hopper web arrangement is optimized, so the high stress concentration of face pate can be avoided. a) od design b) new design Fig.8 Optimization of hopper web in foremost cargo hod region Another way to sove the probem is to increase the radius of the curve part and reduce the height of the web pate, but it is not very effective for this narrow space and stress concentration sti exists. So it is better to use arrangement b) to avoid high stress eve of face pate. It is simiar for aftmost cargo hod region if the shape of hopper tank is simiar. 6 Concusion Some new design trends reated to demand of eco-friendy or green ship design have occurred for oi tankers, such as Leadge-Bow design, thin hu ines design, increase of diameter of propeer, energysaving devices, noise requirement, etc. These wi provide additiona chaenges to the structura design, especiay when meeting the requirement of new CSR. Prescriptive cacuation and direct strength anaysis have been appied to a new designed Aframax tanker, which have additiona requirements (ice cass (LR 1A), ower rate of higher strength stee) compared to the parent ship. The fore ship region has been strengthened due to ice cass requirement and ower rate of higher strength stee, and it is found that few areas need to be reinforced for New CSR for foremost cargo hod region. Severa items need to be noted: It is suggested that the hu girder shear structura members in engine room are not to be evauated in Page 11 of 12

12 TSCF 2016 Shipbuiders Meeting cargo hod anaysis, or the shear force adjustment method is to be amended so that the mode shear force curves in evauation area of engine room can meet the target shear force curves. Between different modeing methods (remove eement or not), bucking resuts are very cose for manhoe area, but very different for adjacent panes. For the new incined strut arrangement, it is suggested that the shear strength of bottom foor at inboard end sha be checked in the prescriptive requirement and the shear stress may be derived from cargo hod anaysis. Some hot spot ocations of hopper knucke coud be omitted in the fatigue anaysis. Large opening may be not necessary for hopper web when the hopper tank is not wide enough. Acknowedgements The author woud ike to thank Wu Jiameng from MARIC for the support in writing this paper. Thanks to corresponding coeagues from MARIC and Shanghai Waigaoqiao shipyard who have participated in the project. Thanks aso to LR for the software and technique support. References [1] IACS, Common Structura Rues for Buk Carriers and Oi tankers, 1 Jan [2] IACS, Common Structura Rues for Doube Hu Oi Tankers, Juy [3] IACS, Common Structura Rues for Buk Carriers, Juy [4] Wu Jiameng et a, Ramification study on IACS Harmonized Common Structura Rues: Impact on Structura Design and Scantings. Transactions Society of Nava Architects and Marine Engineers. Vo. 22, pp: [5] Liu Yinhua, Wu Jiameng. Rationaity of manhoes modeing in FEM anaysis. Ship & Boat, Vo.26, pp: Page 12 of 12