Redesign of NEMO-Type Spherical Acrylic Submersible for Manned Operation to 3000 ft (914 m) Ocean Depth

Abstract

A detailed nonlinear three-dimensional (3-D) finite element analysis (FEA) was performed in Phase I on a NEMO (naval experimental manned observatory) type HBOI (Harbor Branch Oceanographic Institution) spherical acrylic submersible in order to overcome the crack generation problem at its interfaces. An FEA has been carried out to develop a new design guideline, by reducing the higher stresses and relative displacements at the interfaces, which are believed to be two of the main reasons for crack generation at these areas. Variation of the stiffness between dissimilar materials at the interface, lower nylon gasket thickness, overdesigned aluminum hatch and bottom penetrator plug, attachment of the retainer ring with hatch ring, close placement of the rubber padding to the interface between the acrylic sphere, and the retainer ring are found to be the various causes for higher stresses within the acrylic sphere at the nylon gasket/acrylic interface. Based on the new design guidelines, in Phase II, a new spherical acrylic submersible was fabricated and tested. This design resulted in significant improvements by reducing the peak stresses and relative displacements at the gasket/acrylic interface areas. This improvement in design is expected to significantly extend the crack-free cyclic fatigue life of the acrylic submersible at 3000 ft (914 m) ocean depth.