UNIVERSITATEA TRANSILVANIA DIN BRA.OV Catedra Design de Produs 0i Robotic2 Simpozionul naional cu participare internaionald PRoiectarea ASIstatD de Calculator P R A S I C ' 02 Vol. II - Organe de ma0ini. Transmisii mecanice 7-8 Noiembrie BraGov, România ISBN 973-635-075-4 THE ANALYSIS OF LOCAL STRESS DISTRIBUTION DUE TO THERMAL LOADS IN THE STRENGTHEN STRUCTURE OF A PRODUCT CHEMICAL TANKER SHIP Leonard DOMNISORU, Alexandru IOAN, Dumitru DRAGOMIR University "Low Danube" Galai, The Naval Architect Department Abstract: In this paper there is presented the 3D finite element model of a strengthen frame for a product chemical tanker ship. As loading case it is considered a steady state thermal field from inner double hull to the outside ship hull, or reversed. The structural analysis is carried on with the COSMOS/M program based on finite element method. The study focused on the possibility to used the chemical tanker ship, with a supplementary isolation layer, for hot or cold cargo products. The numerical results pointed out the local concentration stress in the bilge and upper deck corner domain and the maximal temperature differences. Keywords: the finite element method, chemical tanker ship, thermal load, local strengths analysis. 1. Introduction The use of the Finite Element Method [1],[2] for ship structures has become a standard design procedure, involving a much wider ship types. The chemical tanker ship structure design, with double hull, involves special requirements in the case of cargo at high or low temperature. In this case it is important to determine the structure capabilities at thermal loads, so that a proper thermal isolation can be designed. In this study we have considered a chemical tanker ship with =55000 t, designed according the Germanischer Lloyd Rules [4], with the following main dimensions: L pp =175 m, B=32.20 m, D=18 m, d=12.20 m and ship speed v s =17 kn. The numerical analysis is carried on using the COSMOS/M (SRAC) version 2.6 [5] with license finite element program, on a Compaq Pentium III 500 MHz computer, with 256 Mb RAM and 20Gb HDD. We have selected COSMOS/M FEM program due to its facilities concerning coupled thermal and strengths analysis. 2. The 3D FEM model of the chemical tanker ship The chemical tanker ship involved in our analysis (see Figure 1, a) has a double hull structure. We take in our model a strengthen frame structure amidships. The model includes also, symmetrically over one frame distance (a v =3.4m), the outside and inside double hull plating, simple and strengthen longitudinal girders, brackets. The model is extended over the half of the ship breadth, using the symmetry diametric plane PD. The resulted FEM model has 4109 nodes and a total of 4051 elements. There were used only thick shell4t quad finite elements. The resulted mesh has a medium refinement, except the bilge zone with a finer mesh. The material is naval steel with yield stress value R eh =315 N/mm 2 ( vm_adm =292N/mm 2 ). We consider two main load cases: a) hot cargo (fig 2), a steady state thermal field from inner double hull to the outside ship hull, with
temperature differences T=+10,+20,,+80 o C. b) cold cargo, a steady state thermal field from the outside ship hull to the inner double hull, with temperature differences T=-10,-20,,-80 o C. The mechanical loads from ship local and global strengths have been not included at this analysis level. The idealization of the boundary conditions (see Figure 1,b) in displacements and rotations, applied to nodes, includes the following: a) diametric symmetry plane PD d 0, r = 0; (1) x = z b) the intersection of PD with frame plane d 0, r = 0, r = 0; (2) z = x y 110 stress admissible value is reached: = 290 95 < 292 N mm. (4) vm _ max, vm _ adm = If we consider superimposed over the thermal loads also the mechanical loads, from water hydrostatic pressure outside hull, the eigen weights of the cargo and ship structure [3], the maximum temperature differences supported by the steel structure (without isolation layer), is diminished to T=±50 o C, been necessary to design a better isolation layer (see Table 1). Table 1. Von Mises stress maximal values Load case: T=±50 o C T=±75 o C Thermal loads 194 N/mm 2 291 N/mm 2 2 c) the point at the intersection of PD with the cross section neutral axis (in frame plane) d = 0. (3) y In Figure 1, a,b there is presented the 3D FEM model of the chemical tanker ship for the strengthen frame amidships. 3. Numerical results The numerical results pointed out that at the case with only steady state thermal loads, the distribution and the maximal values of the equivalent von Mises stress is the same for positive or negative temperature differences T. In Figure 2 there is presented the temperature distribution, obtained from a steady state thermal analysis, for T=+75 o C. In Figure 3, a,b there are presented the equivalent von Mises stress distribution for T=+75 o C (same as for T=-75 o C), obtained from a static strengths analysis with temperatures as loads. It can be observed that the maximal von Mises stress values are obtained in the bilge frame zone and in the upper deck corner domain. In Figure 4 there is presented the vertical displacement d y at T=+75 o C (same for T=-75 o C). It can be noticed that the maximal vertical displacements are obtained at the intersection of the diametric plane with the deck panel. In Figure 5, a there is presented the diagram of maximal equivalent von Mises stress values vm _ max, function of temperature differences T. It can be noticed that for T=±75 o C, in the case with only thermal loads, the equivalent von Mises Thermal and mechanical loads 291 N/mm 2 414 N/mm 2 In Figure 5, b there is presented the diagram of maximal vertical values d y, function of temperature differences T. It can be noticed that at T=±75 o C the maximal vertical displacement satisfies the deformation admissible condition: d y _ max = 17. 3mm < L pp 500 = 350mm. (5) 4. Conclusions From the present study we can derive out the following main conclusions: a) The maximal temperature differences insideoutside hull supported by the steel structure of the chemical tanker ship is T=±75 o C, in the case with only thermal loads. b) In the case with thermal and mechanical loads, the maximal temperature differences (without isolation layers) is reduced to T=±50 o C. c) In order to increase the thermal capabilities of the chemical tanker ship steel structure, it must be taken into account a supplementary strengthens of the frame bilge and upper corner deck domains, involving bigger thickness of the frame and local strengthen elements. d) The 3D finite element structure analysis made possible to determine the chemical tanker steel structure thermal loads capabilities and to obtain the design data for a proper isolation layer between hot/cold cargo and the ship double hull. e) For a higher precision in the ship structure design it is necessary to involve the 3D FEM structure analysis for a wider ship types.
111 Fig.1, a The 3D FEM model of a chemical tanker ship Fig.1, b The 3D FEM model of a chemical tanker ship (boundary conditions and thermal loads)
112 Fig.2 The distribution of the temperature in the chemical tanker frame (T=+75 o C) Fig.3, a The equivalent von Mises stress distribution (T=±75 o C)
113 Fig.3, b The equivalent von Mises stress distribution-frame view (T=±75 o C) Fig.4 The vertical displacement y (T=±75 o C)
114 350 300 vm [N/mm 2 ] 250 200 150 100 50 0 0 20 40 60 80 100 T [ o C] Fig.5, a The diagram of maximal equivalent von Mises stresses values dy [mm] 20 18 16 14 12 10 8 6 4 2 0 0 20 40 60 80 100 T [ o C] FigF Fig.5, b The diagram of maximal vertical displacements values References 1. Bathe, K.J. Finite Elementen Methoden. Springer Verlag, Berlin, 1990. 2. Domnisoru, L. Metoda Elementului Finit in Constructii Navale. Editura Tehnica, Bucuresti, 2001. 3. DomniGoru, L. The Analysis of Global and Local Stress Distribution induced from Cargo and Water Hydrostatic Pressure Loads coupled with Thermal Loads in the Structure of a Chemical Tanker Ship. Black Sea Conference, Varna, 2002. 4. x x x Schiffbau Vorschriften. Germanischer Lloyd, Hamburg, 2002. 5. x x x Cosmos/M 2.6 User Information. SRAC Company, Los Angeles, 2000.