Advanced Hull Design Intersecting Titanium Spheres
Authors: Leon George F. 
Published: October 2008

An evaluation of a bisphere hull configuration and the investigation of the use of ceramics in pressure vessels is presented. A structural model test program was initiated to experimentally evaluate the design of a titanium 6AL-4V intersecting sphere (bispherical) configuration with and with-out a light weight, high strength ceramic (AL203) reinforcing ring. Experimental result were in good agreement with analytical prediction and serve to verify the validity of present analytical techniques to predict the state of stress in the design of intersecting spheres and in the use of higher strength reinforcing rings.

Introduction

One of the major problems that confronts the designer of deep submersibles is to minimize the weight of the pressure hull. The hull with the lowest weight to buoyancy ratio for a given depth is advantageous since the remaining weight can be applied to propulsion systems, payload, life support, etc. There has been much discussion on how to achieve minimum weight in pressure hulls.

This paper describes the complete program for an advanced hull concept including the design, construction, theoretical and experimental analysis of a test model. It is based on research work conducted at the Electric Boat division of General Dynamics over the past two years.1,2

At present, the most promising configuration for deep submersibles are the single sphere, the ring-stiffened cylinder, and complexes of intersecting spheres. The latter of these, the intersecting spheres, combines the functional advantages of the cylinder and the weight advantage of the sphere. This configuration is currently being used for the hulls of the Deep Quest and the DSRV and is being proposed as a possible hull configuration for other advanced submersibles.

An intersecting sphere (bispherical) configuration made of titanium alloy 6AL-4V was selected as a deep submergence test model. This alloy is equivalent on the strength/weight ratio basis to a 200 ksi steel. Another modification of this confi5LITation was the replacement of the titanium reinforcing ring by a high strength, light weight ceramic (AL203) resulting in a 10 percent increase in payload over the original titanium model.

A structural model test program was initiated to experimentally evaluate the intersecting sphere concept with and without the non-metallic reinforcing ring. Specific objectives included: the evaluation of analytical techniques by comparison of experimental and theoretical stress levels, the determination of material behavior (creep and yield characteristics), and the evaluation of ceramic performance in the structural model.

This test program included two phases asfollows:Phase (1) Titanium Bisphere Model TestsTest Runs No. 1 and No. 2Phase (2) Titanium-Ceramic Bisphere

Model Tests

The model was designed for an operating depth of 20,000 feet or a pressure of 9000 psi and for a collapse depth of 30,000 feet or a pressure of 13,500 psi. The model was also designed with the concept of interchangeable reinforcing rings at the spheres intersection.

The model was pressure tested, creep tested and fatique tested for pressures up to 12,000 psi or a depth of 26,400 feet. Experimental results were in good agreement with analytical predictions and gave no evidence of material failure.