Non-linear hydroelastic response analysis in irregular head waves, for a large bulk carrier structure, and fatigue based preliminary ship service life prediction

Abstract

The objective of this master thesis is to provide the specific knowledge to determine the steady state and transitory ship hull hydroelastic dynamic response (oscillations and vibrations) induced by irregular head waves and coupled with cumulative damage factor fatigue analysis to predict the preliminary design ship life.

The studied ship model is a large double hull bulk carrier, with length between perpendiculars 279.00 m. The significant loading cases considered in this work are: full cargo and ballast. The numerical analyses are divided in three-interlinked parts:

- Part 1 - Global-local Strength Analysis in Equivalent Quasi-static Head Waves - there are considered two types of analysis models, the equivalent ship 1D-girder and 3D-FEM hull model extended over the whole ship length. In order to obtain the floating and trim equilibrium condition in vertical plane, the iterative numerical procedures, analogues to the method developed in the MARSTRUCT EU-FP6-Project, Work package Methods and Tools for Strength Assessment by Domnisoru & Stoicescu 2005, 2007, are applied. The 3D-FEM hull model has been developed by NASTRAN NX for FEMAP ver. 10.2 program (Femap 2010). Taking into account the thesis objective, form this analysis the 3D/1D stress correlation factors are obtained for selected structural details.

- Part 2 - Non-linear Hydroelastic Response Analysis in Head Irregular Waves - there are analysed the wave induced dynamic response, including: the linear and non-linear oscillation components, taking into account the bottom and side slamming phenomena, and also the vibration components, on the first and higher order elastic modes, the springing phenomenon, linear and nonlinear steady state dynamic response, due to the ship structure-wave resonance, and the whipping phenomenon, slamming induced transitory dynamic response. The analyses have been carried on with DYN program codes package developed at Galati Naval Architecture Faculty (Domnisoru et al., 2009, 2011), for head waves first order ITTC power density spectrum, with the significant wave height h1/3 = 0-12 m and second order wave interference components (wave model Longuet-Higgins).

- Part 3 - Fatigue Analysis and the Preliminary Ship Service Life Prediction - the analysis is based on the fatigue strengths assessment of the ship hull structure, using the wave induced short term significant stresses. In order to evaluate the ship fatigue strength criterion, according to Germanischer Lloyd Register 2011, was applied the cumulative damage ratio D method, based on Palmgren-Miner method and steel standard design stress-cycles S-N curves. The stress values are selected from the hydroelastic analysis (part 2 analysis) multiplied by the 3D/1D correlations factors (part 1 analysis). The fatigue analysis was carried out considering the dynamic response with or without higher frequency dynamic response (vibrations), besides the low frequency dynamic response (oscillations), involving two different long-term wave significant height histograms North Atlantic and World Wide Trade. For the two loading cases, full cargo and ballast, it is considered a simplified travel scenario with the same occurrence probability (D = 0.5Dfull + 0.5Dballast), for a reference time of R = 20 years.

The numerical results obtained from the first part (ship global-local strength analyses), based on 3DFEM and 1D girder models, are the stress 3D/1D correlation factors for the deck structural details with maximum stress values. The numerical results from the second part outlined the extreme wave loads, from slamming-whipping and springing hydroelastic dynamic responses. The third part is focused on the influences of the hydroelastic responses on the fatigue strength assessment and how the ship preliminary service life prediction can be obtained more realistic for large elastic ships.