forces due to lifting 

 the capsule off the deck 



padeye —J 



padeye 



retaining bolts 



service module 

 (power pack and winch) 



Another approach that 

 was investigated and subsequently 

 discarded was to attach the cap- 

 sule to the service module by 

 bolting on the metal penetrator 

 plate where all of the feedthroughs 

 were located (Figure 6). For this 

 purpose, the penetrator plate would 

 have to be located at the bottom of 

 the capsule so that it could resist 

 best the upward force generated by 

 the buoyancy of the capsule when 

 attached to the negatively buoyant 

 service module. Such an attach- 

 ' ment would possess some good 

 points: (1 ) rigidity, (2) equal 

 resistance to upward and down- 

 ward forces, (3) ease of design and 

 fabrication, and (4) unobstructed 

 visibility. It has, however, two 

 serious disadvantages: (1) it would 

 generate considerable tensile 

 stresses in the acrylic plastic joints 

 when the NEMO system is lifted 

 out of water by means of a line attached to the top hatch insert ring, and (2) it 

 would generate flexure stresses in the acrylic plastic capsule when underwater 

 currents impinge upon the capsule rigidly attached to the service module, 

 thus creating a moment loading on the capsule. Of these two disadvantages, 

 the first is the more serious. 



Analysis of the shortcomings of the rigid attachment to the metal 

 penetrator plate provided an approach to the problem that eliminated all the 

 bad features of the previous design approach and retained all the good ones. 

 In this design approach, the capsule was attached by bolting the metal pene- 

 trator plate to the service module, but the tensile stresses across the bonded 

 acrylic plastic joints generated by hoisting operations were eliminated by the 

 incorporation of pretensioned tie rods connecting the hatch insert ring to the 

 penetrator plate located at the opposite pole of the capsule (Figure 7). With 

 such an arrangement, the upward force applied to the hatch insert ring is 

 transmitted to the service module through the tie rods rather than through 

 the acrylic plastic hull. The incorporation of pretensioned tie rods introduces 

 an additional compressive loading on the acrylic plastic hull when it is on deck, 

 floating on the surface, or shallowly submerged. Because the tie rods are under 

 tension, the acrylic plastic hull is placed under diametrical compression. If the 



Figure 6. Bolt-on restraint concept 

 for attaching the acrylic 

 plastic capsule to the 

 NEMO service module. 



16 



