PSI - Issue 5

Rui F. Martins et al. / Procedia Structural Integrity 5 (2017) 640–646 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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exhaust system (Martins et al. 2010). These values were used to determine the nodal temperature distribution in the exhaust systems under analysis. Inertial loads were introduced in the model in order to simulate the accelerations/decelerations to which the exhaust system is submitted to by the vessel operation at sea. Additionally, an acceleration of 1G was defined in the negative sense of Y axis, in order to simulate the acceleration of gravity applied to the exhaust system and hence the own weight effect, as well as the definition of six forces, each with a value of 2000 N, to simulate the weight of the three silencers existing in the exhaust system. The exhaust systems have wire rope or elastomer shock mounts on the right and left bottom sides of the lower support structural profiles, as well as on the right and left sides of the upper supporting structures of the exhaust systems; those were specifically applied to protect the structures from shocks, decelerations and vibrations and to sustain compression, roll and shear loads applied. The mass flow rate of the exhaust gases is generated by the functioning of the gas turbines; hence, the total fluid’s mass flow rate was considered to be equal to 68 kg/s (Fig. 1a) and 67 kg/s (Fig. 1c), and these values were used to calculate the pressure distribution along the axial direction of the exhaust systems using the Bernoulli equation. The pressure was also determined using a Pitot probe placed at the outside cross section of exhaust system presented in Fig. 1a (Martins et al. 2010). A detailed description of the numerical models previously presented can be found in Soares (2011). As referred earlier, the original exhaust system’s design (Fig. 1a) highly restrained the thermal expansion of the plates at the critical regions of the structure; this was mainly due to the different thicknesses of the plates used to build the exhaust system and due to the high thermal inertia of the support rings welded at the lower and upper regions of the structure. In order to enhance the thermal behaviour of the exhaust system, some modifications were proposed, namely: • The use of a high-strength ferritic marine stainless steel (445M2) characterized by higher thermal conductivity values and lower thermal expansion coefficients than those observed for an AISI 316L austenitic stainless steel (Table 4); • The use of plates and shells with similar thicknesses at the critical regions of the structure; • The use of non-closed contour structures to support the exhaust system at its lower and upper regions. 3. Results

Some of the numerical results obtained are presented in Fig. 3. a b