Stress analysis of the midship transverse frame in an offshore production ship is performed. Four different models are applied for linear finite element stress analysis, namely, a conventional beam model as typically applied for tanker hull structures, a hybrid beammembrane model with a superelement technique, a two-dimensional shell model considering half of the frame, and a most accurate threedimensional shell model considering a quarter of the midship tank with two frames. Two critical load cases are applied, that IS, full draught, cargo hold empty and ballast empty; and minimum draught, cargo hold full and ballast empty. Due to complex geometry of the transverse frame, the conventional beam model predicts maximum stress levels which are excessively large and beyond acceptance. The hybrid beam-membrane model results in improved stress predictions which become comparable With the results obtained by the shell models. The 3-D shell model is regarded as the most accurate, but the difference between the 2-D and 3-D models is small, and the 2-D model is numerically much more efficient.
Ships are attractive as production facilities for offshore hydrocarbons during early production and reservoir testing. They are commonly designed as oceangoing merchant vessels such as tankers. However, production ships, with a typical profile as shown in Fig. 1, are normally operated in designated regions, have zero speed and are held head on waves. These and other factors make both still water and wave response different from those of tankers. Furthermore the structural characteristics of production ships, such as transverse frames, may differ from those of conventional tankers. Consequently, it is necessary to investigate the applicability of conventional strength analysis methods for production ships. A typical approach to the stress analysis of transverse frames of tankers is to model the frame by simple beam elements as described in DnV Classification Notes No. 31.3 (1988)