Recent Advances in Structural Design of Ice-Strengthened Vessels

Transcription

1 ABS TECHNICAL PAPERS 006 Recent Advances in Structural Design of Ice-Strengthened Vessels Ge (George) Wang American Bureau of Shipping, Corporate Technology Houston, TX, USA Shewen Liu American Bureau of Shipping, Corporate Technology Houston, TX, USA Kaj Riska ILS OY Consulting Naval Architects and Marine Engineers Helsinki, Finland Originally presented at the 006 SNAME ICETECH conference held in Banff, Canada, July 16-19, 006. Reprinted with permission of the Society of Naval Architects and Marine Engineers (SNAME). Material originally appearing in SNAME publications cannot be reprinted without written permission from the Society. 601 Pavonia Ave., Jersey City, NJ This paper highlights the latest ABS research in the analysis and design of ice strengthened tanker structures, which supports the development work by the Finnish Maritime Administration (FMA). In search for a rational basis for design, FMA has introduced the equivalency concept and accepted application of nonlinear FEM as an alternative to applying prescriptive requirements. The recent development of ice strengthening guidelines for large commercial ships is characterized by active studies supporting this FMA initiative. ABS has developed formalized procedures for designing alternative ice-strengthened side structures, applying a nonlinear FEM approach. These procedures specify the extreme ice load definition, structural modeling, nonlinear material modeling, and acceptance criteria. This paper focuses on the analysis of the side stringer. The Finnish Swedish Ice Class Rules (FSICR) criteria for the side stringer has been under debate particularly after the new criteria for the side shell and the side longitudinal have been formalized by FMA (005) Guidelines supported by ABS (005) Guidance Notes. The paper describes a series of linear and nonlinear FEM analyses conducted to investigate the behavior of side stringers. The stringer thickness and ice loads were varied and the impact on the buckling and ultimate strength of the side stringer was studied. A design procedure is proposed using nonlinear a FEM and ultimate strength formulation with assumed extreme ice loads, a selected corrosion addition and acceptance criteria. The paper discusses the interaction of structural components in the given structural designs, and potential improvements of the proposed methods. KEY WORDS: ice-strengthened hull design, Finnish- Swedish Ice Class Rules, direct calculation approach, buckling, ultimate strength, side stringer INTRODUCTION Oil exports from Russian Port of Primorsk are steadily growing, and set to reach 65m tonnes in 006 from 17.7m tonnes in 003. This is the driving force behind huge rise in the orderbook for large, high-specification ice class ships. According to Clarkson Research, by 008, 10% of the tanker fleet will have some kind of ice class as against 3% in 199. Since 1999, 15% of the total investment in tanker buildings has been on new ice class ships (Corkhill 006). The increasing demand for ice-strengthened tankers and the need for construction friendly structural designs have been driving the regulatory agencies to rationalize ice-strengthening design standards. At the same time the available design codes are challenged by lack of experience and limited data associated with longitudinally framed ships with breadth much wider than the vessels normally sailing in the Baltic Sea. Direct calculation approaches have been regarded as pivotal in addressing the uncertainties involved in operation, ice loads and structural responses. More and more direct calculations are being introduced into design standards, and some guidelines have been developed to support analyses and evaluations of alternative designs. In response to the industry needs and the development of direct calculation approaches, the Finnish Maritime Administration (FMA) issued Guidelines (FMA 003a, 004) to incorporate a new ice load definition for large longitudinally-framed vessels, and to allow direct calculation approaches to be used for designing side structures under ice load. Following feedback from the industry to these Guidelines, FMA issued new Guidelines (FMA 005) to redefine the design procedures for large longitudinally framed vessels, including the design ice load and acceptance criteria. Recent Advances in Structural Design in Ice Strengthened Vessels 151

2 ABS TECHNICAL PAPERS 006 American Bureau of Shipping (ABS) has been developing design-oriented procedures for evaluating ice-strengthening of side structures. These procedures formalize the steps for the extreme ice load determination, structural modeling, nonlinear material behavior formulation, and for establishing the acceptable permanent deformation. A nonlinear finite element analysis is a critical part of this approach. A complete set of procedures has been developed for structures including the side shell, side longitudinal, side stringers and web plates in wing ballast tanks (ABS 004, 005). Incorporating all structural members in the FEM modeling has the advantage of capturing the interaction between the members, which leads to more balanced designs. ABS research work (Wang et al 005, 006) provides support to FMA for the development of new guidelines for design criteria. The paper highlights the latest ABS developments on ice strengthened hull designs, which provide the first formalized procedures for direct calculations to evaluate ice strengthening of side structures. LATEST DEVELOPMENT OF DESIGN CRITERIA In search for a rational basis for design, FMA has introduced the equivalency concept and accepted the application of nonlinear FEM as an alternative to applying prescriptive requirements (FMA 003a, 003b, 004). These initiatives have been welcomed by the industry. The Tentative Guidelines, officially released in 003 (FMA 003a) and slightly revised in 004 (FMA 004), were extensively reviewed and studied by the industry. The feedback on the application of these Guidelines have led FMA to revise the guidelines with alternative design requirements replaced by prescriptive requirements (FMA 005). Recently, FMA and SMA have initiated a project to revise Chapter 4 Hull Structural Design and Chapter 5 Rudder and Steering Arrangements of the FSICR. See Table 1. Note that FMA (003a, b) and FMA (005) are official releases, but FMA (004) is not. In support of the initiatives by FMA, ABS has developed formalized procedures for the design of alternative icestrengthened side structures, applying the nonlinear FEM approach (ABS 004, 005). These procedures specify the extreme ice load definition, structural modeling, nonlinear material modeling, and acceptance criteria including acceptable permanent deformations. The latest ABS development has focused on the side stringer thickness requirement. The side stringers in the double sides are termed deck strips in FSICR ( 4.5.3). However, FSICR do not have requirements applicable to side stringers in longitudinally framed double sides. Neither was this side stringer criteria discussed in the recent FMA guidelines (FMA 003a, 003b, 004, 005). Nonlinear FEM approach by ABS was used to investigate the stringer s buckling strength. Parametric studies were conducted to find the trend between the stringer thickness and the ice loads. The FMA Tentative Guidelines (FMA 003a) as well as the present FSICR ( 4.1) allow alternative design methods but do not specify the necessary procedure for such an evaluation. With an objective to supplement FMA (003a) ABS started to develop a procedure for conducting nonlinear FEM for designing ice-strengthened side structures in late 003. The development resulted in publishing the ABS Guidance Notes on Nonlinear Finite Element Analysis of Side Structures Subject to Ice Loads (ABS 004). The development continued in 004, and the ABS procedure was refined and expanded to cover the criteria for side shell, powering requirements and propeller strength. Samsung Heavy Industry joined ABS in developing the side shell strengthening procedure (Wang et al. 005a). In April 005, ABS published the Guidance Notes on Ice Class (ABS 005), which formalize these three direct calculation approaches. The developed ABS procedure (for hull) provides a consistent method using state-of-the-art technology. Detailed technical background can be found in Wang et al (005, 006). The procedure is based on the experience gained in the application of ice class rules (FMA 00, IACS 001), the knowledge on ice loads (FMA 003b, Riska 005, Kendrick 005) and data on ice damages (Kujala 1994). The procedure suits well the design needs and has been applied to structural designs of some ice-strengthened tankers. Having addressed all the pieces in the direct calculation procedure, these ABS Guidance Notes (ABS 005) provide designers with a practical engineering procedure, and therefore, supplements the development of FMA Guidelines. See also Table 1. Table 1. Recent FMA/SMA and ABS development of criteria for ice strengthened designs for large commercial tankers Year FMA and SMA ABS Tentative guidelines (FMA 003a): accept alternative side frame designs based on equivalency concept Guidelines (FMA 004): refined FMA (003a) Guidelines (FMA 005): use prescriptive requirements for side longitudinal design Further development on Chapter 4 Hull Structure Design and Chapter 5 Rudder and Steering Arrangement Internal studies GN (ABS 004): specify a procedure for designing side longitudinals using nonlinear FEM GN (ABS 005): specify a procedure for designing both side longitudinal and side shell using nonlinear FEM Additional design guidance for side stringers and webs CHALLENGES IN ICE LOADS DEFINITION Two main factors affect the scantlings of the ice-strengthened side structure. The first is the ice loads imposed on the hull structure; the second is the allowable response of the hull structure to the ice load, e.g., elastic or plastic response, or damage. Once the ice load is properly defined, the structural response can be determined either with a linear analysis, or with more advanced methods such as the nonlinear FEM analysis. The proper definition of the stochastic ice load is critical for a balanced design of the hull structure. 15 Recent Advances in Structural Design in Ice Strengthened Vessels

3 ABS TECHNICAL PAPERS 006 Normally, interactions between the hull and ice are modeled using deterministic laws of mechanics. It is generally known that such interaction modeling involves many uncertainties associated with random variation of the initial and boundary condition of the load events between hull and ice, variations in ice conditions and the chaotic nature of the breaking pattern, etc. Many factors affect the ice load, including the ice condition (first-year or multi-year ice), ice mechanical parameters (e.g., compressive and bending strengths and Young s modulus), ice cover parameters (e.g., concentration, thickness, ridge density, size of ice floe after broken by icebreakers in brash ice channel), vessel size and speed, hull form, frame spacing and many others. The stochastic characteristics of ice load require that the design ice load is based on actual measurements at different conditions. FMA definition of the rule ice load is based on l measurements data (Kujala 1994, Hänninen 004). In the present FSICR, the ice load is defined as a patch load on the side structure with certain load length and load height, where the load length relates to frame spacing and the load height depends on the ice thickness. There are several challenges relative to the FSICR ice load definition for larger commercial vessels in the Baltic Sea. All the ice loads measurements (based on which the FSICR defined the design ice loads) were collected on smaller, transversely framed vessels. How well these data can be used for larger longitudinally framed vessels is not very clear yet. Another factor is that the measured maximum ice load usually is about 3 times larger than the design ice load defined in the FSICR (FMA 003b, Muhonen 199, Hänninen 004). In most modern hull design standards hull structures are designed to sustain the maximum loads during the vessel s service life. In icestrengthened structural design however, some plastic deformation is allowed. This level of allowable plastic deformation has been established to allow damage that does not endanger the vessel or the environment, but does not lead to excessively heavy structures. Therefore, there is a need to properly define the extreme ice loads that correspond to the maximum loads in the vessel s design life, normally assumed to be 0 or 5 years. The FSICR design ice loads are not the extreme ice loads; they are more frequently encountered (see also, FMA 003a, 003b, 004). These FSICR design ice loads are encountered roughly once a year on ships in regular traffic to northern Baltic. To meet the challenges associated with the larger frame spacing and longitudinally framed vessels, FMA and SMA (003a, 004) Guidelines re-define the ice load on side structures in those parts of the rules where some reserve for the extreme loads was provided in the structure. One such item is the requirement of brackets on longitudinal frames. The industry can use these ice loads for designing larger ice-strengthened and longitudinally framed vessels. But many uncertainties on ice loads still need to be more clearly defined by more measurements and analysis on these kinds of vessels. Especially the proper use of plastic design requires knowledge about the statistics of ice loads. DESIGN OF THE SIDE LONGITUDINAL prescriptive FSICR criteria, characterized by frame spacing up to 450 mm and brackets on longitudinal frames. The ABS (004, 005) Guidance Notes have formalized a procedure that uses nonlinear FEM to evaluate the permanent deformation of side longitudinals. Figure 1 shows a typical permanently deformed side structure after ice loads are removed. To simplify the design procedure, based on previous industry studies, the new FMA Guidelines (FMA, 005) use a prescriptive formula to define the strength requirement for the side longitudinals. The frame section modulus requirement is increased by 0% for frames when there are no brackets connecting longitudinals and webs (FMA 005). As shown in Table, this 0% increase is comparable to the results of direct calculations based on the FMA (003a, 004) Guidelines. In Table, the column FSICR refers to FSICR rule based design; column FMA prescriptive refer to a design with large frame spacing (about 800 mm) and brackets; column FMA alternative refer to a design with large frame spacing without brackets; and column FMA (005) refer to design requirements specified by the latest Guidelines. Ice class Table. Side longitudinal of a Suezmax tanker (scantlings in mm) FSICR 350x x17 FMA (003a, 004) prescriptive 400x1+ 100x18 FMA (003a, 004) alternative 400x x18 FMA (005) IA cm cm 3-00x11+ 90x15 100% 117% 10% 50x9+ 90x15 80x1+ 90x15 IC - 530cm 3 685cm % 19% 10% DESIGN OF THE SIDE SHELL The FMA (003a, 004) Guidelines use the same prescriptive formula to calculate the side shell thickness as in the original FSICR, except that the ice load was raised by adjusting the parameter Ca which is related to load length. The Guidelines do not change the side shell design principle. More sophisticated methods can, however, be used. The ABS (005) GN use nonlinear FEM to establish the required side shell thickness, allowing permanent deformation up to % of the frame spacing under extreme ice load (3 times of FSICR design ice load). In addition, thickness of the side shell is not allowed to be more than 10% below the FMA (003a, 004) Guideline requirement. Table 3. Side shell thickness of a Suezmax tanker (mm) Ice class FMA (003a, 004) FMA (005) ABS (005) IA 9.5 HT HT HT36 IC 0.5 HT HT HT The FMA (003a, 004) Guidelines define criteria for the larger frame spacing without brackets to connect longitudinal stiffeners and web frame. The criteria are based on the concept of equivalency: The large frame spacing design without brackets under the extreme loads should have permanent deformations equivalent to a design that fully complies with Recent Advances in Structural Design in Ice Strengthened Vessels 153

4 ABS TECHNICAL PAPERS 006 Suezmax tanker. The side stringer web thickness required by the FMA (005) Guidelines is smaller than typical design of non-ice-strengthened side stringers. It clearly shows that specific design guidance for side stringers needs to be added, and additional engineering analyses are needed. Table 4. Side stringer thickness of a Suezmax tanker (mm) Figure 1. Remaining strain in side shell and side longitudinal after ice loads are removed from acting on shell plating (Wang et al. 005) In the FMA (005) Guidelines, the requirement of icestrengthened shell plate thickness uses the same formula as the FMA (003a, 004) Guidelines. However, the Ca factors, once increased in the FMA (003a, 004) Guidelines, are brought back to their values in FSICR (FMA 00). This leads to about a 7% thickness reduction compared to the FMA (003a, 004) Guidelines. This result is compatible with the direct calculations carried out on a IA class Suezmax tanker based on the ABS Guidance Notes on Ice Class (ABS 005, Wang et al 005), shown in Table 3. Therefore, this FMA (005) prescriptive rule requirement, revised from the previous FMA (003a, 004) Guidelines, approximately reflects a more refined approach that takes into account extreme ice loads, interaction among different structural members, and acceptable permanent deformations. The trend differences in ice classes revealed by nonlinear analyses suggest that more analysis is needed to clarify the relationship between the different classes. DESIGN OF SIDE STRINGERS Main supporting members such as side stringers and transverse webs in wing ballast tanks and double sides provide support to side shell and side longitudinals, and transfer ice loads to main structural members. These structural members should not buckle when subject to local intense ice loads. This consideration is clearly stated in the original FSICR (FMA 00) and the recent FMA (003a, 004, 005) Guidelines. Ice class IA IC FMA (003a, 004) 15.0 HT MILD 10.0 HT MILD FMA (005) 9.0 HT MILD 9.0 HT MILD Non icestrengthened design MILD MILD Several methods can be used to assess the strength of the side stringer under the ice load. As usually, the strength of the side stringer can be evaluated using buckling and ultimate strength formulations in modern hull design standards. Generally based on the theory of simply supported plate panels, these formulations neglect the interaction between the side stringer and other structures (such as the stiffeners on the side stringer, side shell and transverse webs). A better alternative is to model a wider range of structures using FEM techniques, such as those defined in the ABS (004, 005) GN. The structural response to the ice load can be more realistically captured, and buckling and the ultimate strength can be more precisely calculated. Major commercial FEM software calculates the eigenvalue, which corresponds to the elastic buckling strength, and the ultimate strength, which corresponds to the maximum load-carrying capacity of the structure taking into account both geometric and material nonlinearity. Extending the application of ABS (004, 005) GN) to the side stringer, the side structures can be modeled, as shown in Figure. The ice loads are idealized as line loads that act directly on the location of the side stringer. The load length is the web frame spacing, shown in Figure 3. Details of boundary conditions, modeling extents, materials, etc. are the same as defined in ABS (005) GN. FSICR (FMA 00) specify that minimum web thickness of ice frames is to be half of the shell plating thickness and and no less than 9 mm. The same principle was followed in the FMA (003a, 004) Guidelines, but was revised in the FMA (005) Guidelines according to the following text: the web thickness of transverse and longitudinal frames need not exceed one half of the shell plating thickness as required for frame spacing of 0.45 m assuming the yield stress of the plate not more than that used for the frame. It appears that these current prescriptive requirements do not, however, apply a buckling formulation. Neither the original FSICR nor the latest FMA guidelines provide a reasonable buckling guidance for determining the required side stringer thickness as the requirement stated above applies to ice frames i.e. profiles. Table 4 lists the side stringer thickness requirements, if the web thickness requirement in the FMA (003a, 004, 005) Guidelines is used and the comparison with that of a typical non-ice-strengthened design for a Figure. Typical FEM model for side stringer strength analysis 154 Recent Advances in Structural Design in Ice Strengthened Vessels

5 ABS TECHNICAL PAPERS 006 Dimensionless Strength (Relative to Midship IA class) mm - 0.5mm side shell 11mm - 9.5mm side shell 13mm -9.5mm side shell 13mm - 0.5mm side shell 15mm - 9.5mm side shell 15mm - 4mm side shell Maximum Displacement, D (mm) Figure 5. Progressive collapse of side stringers: both side shell thickness and side stringer thickness are varied Figure 3. Line load applied on side stringer Interaction between the Side Shell and the Side Stringer FSICR requires the web frame thickness and thus logically also the side stringer thickness to be half of the thickness of the side shell plating. It is interesting to find the impact of side shell thickness on the strength of the side stringer. Figure 4 shows the eigenvalues for three different models with different side shell thicknesses as a function of desired side stringer thickness. The side shell thicknesses of the three models are 0.5mm (IC midship), 9.5mm (IA midship) and 4mm (IA forebody). Figure 4 shows that the eigenvalue curves for these three models closely collapse together. It appears that side shell thickness has very small influence on the strength of the side stringer, e.g., for side stringer thickness of 14mm, the side stringer strength of the IC class at midship (with side shell thickness of 0.5mm) is only about 5% different from that of the IA class at forebody region with side shell of 4mm. The side shell is exposed to lateral pressure from ice, while the side stringer is subject to in-plane loads. The side shell and the side stringer have a weak interaction. This is further confirmed by the ultimate strength analysis using nonlinear FEM for the same models as shown in Figure 5. In Figure 5, the load-deflection curves of the post-buckling analysis for one side stringer thickness design (the first number in the legend) collapse together closely, with less than 3% different strength for different models. This conclusion might lead to simplifying the FEM modeling process for the side stringer buckling and ultimate strength analysis. Eigenvalue (normalized to midship IA class ice load) mm side shell 4mm sides shell 0.5mm side shell t (mm) Ultimate Strength of the Side Stringer Figure 6 shows the progressive behavior of the side stringer of a Suzemax tanker. Loads in this figure are normalized with respect to the FSICR design ice loads. The deflections are the maximum in the side stringer. The maximum points on the load-deflection curves correspond to the point when the side stringer reaches its ultimate strength. Contour plots for stress distribution on the side stringer are shown in Figure 7 for the thickness t=1mm. Each contour plot corresponds to three different stages: before, at and after the ultimate strength. It appears that side stringer behaves differently from the side shell and the side longitudinal when subjected to ice loads. The Figure 6, indicatess the ultimate strength a side stringer when subjected to ice loads mm 1m m 15m m Maximum Displacement, D ( mm) Figure 6. Progressive collapse of side stringer of a Suezmax tanker: Loads are normalized with respect to FSICR design ice pressure. Deflections are the maximum in the side stringer. The side stringer retains some level of initial imperfections as a result of manufacturing. It is assumed that the initial imperfections follow the shape of the first buckling mode of the side stringer, and resembles the so-called hungry horse shape. The maximum initial imperfection w is assumed to be of an average level, and is determined based on statistics of Smith et al (1988): w = 0.1 β t, (1) Figure 4. Eigenvalues (elastic buckling) of side stringers: both side stringer thickness and side shell thickness are varied Recent Advances in Structural Design in Ice Strengthened Vessels 155

6 ABS TECHNICAL PAPERS 006 Where, β = slenderness ratio of plate panel = (s/t) (σ y /E) 0.5, s = spacing of stringer stiffeners, t = thickness of stringer, σ y = yield stress, E = Young s modulus. A common way of idealizing the structure is to assume that the side stringer can be treated as a single plate panel with simple support boundaries. Buckling and the ultimate strength of a plate panel subject to in-place compression on its longer edges have been investigated extensively. Closed form equations are available for calculating the buckling and ultimate strength. The following equations are extracted from ABS Steel Vessel Rules Part 5 with a simplified format. Predictions using other formulae should give comparable results. ( π E /( 1 ( 1 ) ( t / s) ) σ, () σ buck = σ Ei = k i ν 0 σ y (3) l β s s ult σ y C 1 = x l Where, σ buck = buckling strength of plate panel, σ ult = ultimate strength of plate panel, σ Ei = elastic buckling stress, for the considered case, ( 1 ( s / l) ) k i = +, l = frame spacing, 1.0 C x = / β 1/ β ν = Poisson s ratio. for β > 1 for β < 1 A more realistic modeling is to include a wider range of structures, such as shown in Figure, and to calculate the structural strength using FEM, which captures the interaction among the side shell, inner skin, side longitudinals, side stringer and transverse webs. V1 L C G1 Y Z X Output Set: Case 11 Time 0.4 Deformed(5.80): Total Translation Contour: Plate Top VonMises Stress V1 L C G1 Y Z X Output Set: Case 7 Time Deformed(17.34): Total Translation Contour: Plate Top VonMises Stress V1 L C G1 Y Z X Output Set: Case 4 Time Deformed(36.64): Total Translation Contour: Plate Top VonMises Stress Figure 7. Stress distribution on side stringer for three stages: before, at and after the ultimate strength Table 5 summarizes the prediction of buckling and the ultimate strength using simple formulae and linear and nonlinear FEM. The analyzed structure is in the midship area of a Suezmax tanker with IA ice class. The basic parameters for the vessel are listed below: Side stringer:.45m in depth; Web frame spacing 4.8 m; FSICR ice loads of IA class: N/mm ; Line loads: N/mm; Stiffeners on side stringer: 150x x11; Stiffener spacing on side stringer: 80mm. Figure 8 shows the side stringer strength as the function of thicknesses using different methods listed in Table 5. It appears that buckling strengths calculated using eigenvalue approach (linear FEM) are substantially higher than that predicted by buckling formulae. This can be attributed to the fact that the FEM analysis accounts for the supporting effects of the adjacent structures, and these effects are ignored in the buckling formula. The k i factor in Eq. () is applicable to simple 156 Recent Advances in Structural Design in Ice Strengthened Vessels

7 ABS TECHNICAL PAPERS 006 supported plate panels. Table 5. Ratio of predicted buckling and ultimate strength over FSICR ice loads for varying side stringer thicknesses of a Suezmax 1A tanker (predictions using linear and nonlinear FEM and design formulae) Stringer Thickness (mm) Dimensionless Strength Eigen value Buckling formula Nonlinea r FEM Ultimate strength formula Nonlinear FEM eigenvalue Ultimate Strength f l Side Stringer Thickness, t (mm) Figure 8. Buckling and ultimate strength of side stringer predicted using different approaches. Also see Table 5 The ultimate strength predicted by the nonlinear FEM also differs from the results of the ultimate strength formula. The differences are generally smaller than in buckling. The main source of the difference is again the boundary conditions, or interactions among the side shell, inner skin, stringer stiffeners and the side stringer. This will be discussed in more detail in the next section. While the difference exists, it still demonstrates the potential for designing the side stringer applying an ultimate strength formula instead of using the more time-consuming nonlinear FEM. Rationalizing Side Stringer Designs The investigations have led to a refined method for designing side stringers. The side stringer thickness can be designed to have adequate reserve against extreme ice loads. This gives a better design basis because the current FSICR and FMA guidelines are not in the format of buckling formulation and the strength of side stringer appears not to depend on side shell thickness as in FSICR. There are very high uncertainties associated with ice load definitions. As discussed earlier, the extreme ice loads measured in Baltic Sea usually are more than two times higher than the FSICR design ice loads. In the direct calculation approach for the side shell, ABS specifies that extreme ice loads are three times of the FSICR design ice pressure. Following the same consideration, the extreme ice loads for the side stringer can also be defined as three times the FSICR design ice loads. This would require that the ratio of ultimate strength over ice loads be over 3.0. For the examples shown in Table 5, the side stringer needs to be 15.5 mm using the ultimate strength formula. If one selects to use nonlinear FEM, the required thickness would be 1.0 mm. As is the common practice in modern ship design, net scantlings with corrosion additions deducted are used in evaluating structural strength. If we assume a.0 mm corrosion addition, the required as-built thickness of the side stringer becomes 17.5 mm when the simplified ultimate strength formula is used, and 14.0 mm when the nonlinear FEM is used in the strength evaluation. The ultimate strength formula (Eq. 3) is based on plate panels with simple support boundaries. Therefore, it predicts strength lower than the nonlinear FEM, as shown in Figure 8 and Table 5. Figure 9 shows the required thicknesses compared with FMA (003a, 004, 005) Guidelines. The Option 1 is the required thickness using the ultimate strength formula defined in Equation (). The Option is the required thickness using the nonlinear FEM analysis. The current analyses using Option 1 require stringer thickness close to the FMA (003a, 004) Guidelines, which is much higher than the FMA (005) Guidelines. The Option provides a more rational design, which is between the requirements of the two FMA (004, 005) Guidelines. The different results of Option 1 and Option show that the nonlinear FEM analysis could lead to a lighter scantling requirement on the side stringer while sustaining the same level of the ice load. The IC class has ice loads about 43% that of IA class. The ultimate strength of typical side stringers, say 1.0 mm in gross thickness, have adequate reserve against buckling failure, and therefore is not a critical concern. It is also noted that the influence of the longitudinals on the side stringer is not on the strength of the side stringer. The relative strength of stiffeners to side stringer plays an important role on buckling of the side stringer. Figure 10 shows the different buckling failure modes for two side stringer thicknesses, 9 mm (a) and 1mm (b), with the same stiffeners on side stringer (150x11+90x11). The deflected side stringer is plotted along the centerline of the FEM model. To better show the deflection of the side stringer, the deflected shapes for both the stringer and the stiffener on it are exaggerated. It clearly shows that for a thin side stringer, a large deflection takes place close to the side shell, while in a thick side stringer buckling tends to be more global and the deflection maximizes in the center. This explains why in Figure 4 and Figure 7, the side stringer strength calculated by nonlinear FEM does not have noticeable improvement as the thickness increases when its thickness reaches a certain level. It should be more efficient to increase the stiffener strength than the thickness of the side stringer. Recent Advances in Structural Design in Ice Strengthened Vessels 157

8 ABS TECHNICAL PAPERS 006 Required Web Thickness (mm) Side Stringer Thickness of IA Suezmax Tanker FMA 004 FMA 005 Non-ice-strenthened Option 1 Option Figure 9. Required side stringer thickness with different criteria. Option 1: Ultimate strength formula; Option : Nonlinear FEM CONCLUSIONS This paper highlights the latest ABS research in the design of the ice strengthened tanker structures in support of the FMA/SMA development. In search for a rational basis for design, FMA and SMA have introduced the equivalency concept and accepted application of nonlinear FEM. These initiatives are welcomed by the industry. The experiences gained have led FMA to revise the guidelines with alternative design requirements replaced by prescriptive requirements. In support of the FMA and SMA initiatives, ABS has developed formalized procedures for designing alternative icestrengthened side structures applying the nonlinear FEM approach. These procedures formalize the extreme ice load definition, structural modeling, nonlinear material modeling, and the acceptance criteria including acceptable permanent deformations. The latest ABS development was focused on the side stringer thickness requirement. A series of linear and nonlinear FEM analyses were conducted to investigate the behavior of side stringers. The stringer thickness and ice loads were varied and the impacts on the buckling and ultimate strength of the side stringer were studied. A design procedure using nonlinear FEM and ultimate strength formula was proposed with the assumed extreme ice loads, selected corrosion addition and acceptance criteria. The paper discusses trade offs in structural designs, interaction of structural components, and potential improvements to be incorporated in the new methods. ACKNOWLEDGEMENTS Figure 10. Buckling failure modes for different side stringer thicknesses with same stiffeners on stringer, (a): 9mm side stringer, (b): 1mm side stringer. Deflected stringer and stiffeners are enlarged to better show the buckling failure modes The impact of the relative strength of the stiffeners on the side stringer is more prominent in a vessel s forebody region, where ice loads become larger than amidships. It may be more desirable that buckling is local and global buckling of the side stringer between inner skin and side shell does not take place. Thus, when the side stringer needs to be thick, the stiffeners on the side stringer may also need to be strengthened to avoid global buckling. This issue needs further investigation. Further studies are recommended also on the ice loads definition, other failure modes, existence of other loads components, abrasion/corrosion additions of all structural members, and the acceptance criteria. Mr. Jorma Kamarainen at Finnish Maritime Administration has reviewed the manuscript. We appreciate very much his clarification regarding the recent FMA development. We would like to thank our ABS colleagues, Dr. Pixin Zhang, Dr. Bo Wang and Dr. Haihong Sun, Roger Basu, Ken Tamura, Phil Rynn, MS Lee, Ryu Nagayama and Han-Chang Yu, for interesting discussions, suggestions and comments. The authors appreciate the help of Jim Speed in proofreading the manuscript. ABBREVIATIONS ABS GN IACS FEM FMA SMA FSICR REFERENCES American Bureau of Shipping ABS Guidance Notes International Association of Classification Societies Finite Element Method Finnish Maritime Administration Swedish Maritime Administration Finnish-Swedish Ice Class Rules ABS, Guidance Notes on Nonlinear Finite Element Analysis of Side Structures Subject to Ice Loads. American Bureau of Shipping, 004. ABS, Guidance Notes On Ice Class. American Bureau of Shipping, 005. Corkhill, M. Ice Class Comes into Its Own, Llyods List, Jan. 4, 006 FMA, Finnish-Swedish Ice Class Rules, FMA bulletin No. 13/ Finnish Maritime Administration, Recent Advances in Structural Design in Ice Strengthened Vessels

9 ABS TECHNICAL PAPERS 006 FMA, Tentative Guidelines for application of direct calculation methods for longitudinally framed hull structure (30 June 003), Finnish Maritime Administration, 003a. FMA, Background for the tentative Guidelines for application of direct calculation methods for longitudinally framed hull structure (5 June 003). Finnish Maritime Administration & Helsinki University of Technology, 003b. FMA, Guidelines for the application of the Finnish-Swedish Ice Class Rules. Finnish Maritime Administration (This is not an official publication, and was distributed only for industry review), 004. FMA, Guideline for the application of the Finnish-Swedish Ice Class Rules (Version 1, 0 December 005). Finnish Maritime Administration, 005. Hänninen, S. The Use of Statistical Methods in Determination of Design Ice Load on Ship Hull Frame in the Baltic Sea. M.Sc. Thesis, Helsinki University of Technology, 00. IACS, Unified requirements for polar ships, PS1 polar ship structures (draft). International Association of Classification Societies, 001. Kendrick A. EER in Ice-Covered waters, PERD Workshop, October 005, Calgary, Canada, 005. Kujala, P. On the statistics of ice loads on ship hull in the Baltic, Acta Polytechnica Scandinavica, the Finnish Academy of Technology, Legland E., Diettrich D., Conachey R., Baker C., Wang, G, Operation of Arctic LNG carriers: conditions, crew and cargo, GASTECH 006, 4-7 December 006, Abu Dhabi, UAE, 006. Muhonen, A. Ice Load Measurement onboard the MS Kemira, Winter Helsinki University of Technology, Ship Lab, Report M-11, 199. Riska, K., Ice seminar to ABS Houston, October 005, Houston, TX, USA, 005. Smith, C.S., Davidson, P.C., Chapman, J.C., Dowling P.J., Strength and stiffness of ships plating under in-place compression and tension, Transaction, Royal Institute of Naval Architects, Wang G., Basu R., Chavda D., Liu S., Rationalizing the design of ice strengthened structures, International Congress of International Maritime Association of the Mediterranean (IMAM 005), Lisbon, Portugal, 005. Wang, G., Basu, R., Chavada, D., Liu, S., Lee, M.-S., Suh, Y.-S., Han, Y.-J., Rationalization of Design of Side Structure of Icestrengthened Tankers, IJOPE, Vol. 15, No.3, 005 Wang, G. & Wiernicki, C. J., Using nonlinear finite element method to design ship structures for ice loads. Marine Technology, The Society of Naval Architects and Marine Engineering, 006. Recent Advances in Structural Design in Ice Strengthened Vessels 159

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