forces due to lifting the 

 capsule off the deck 



retaining bolts 



I * 



service module 



(power pack and winch) 



Figure 7. Tie-rod restraint concept for 

 attaching the acrylic plastic 

 capsule to the NEMO service 

 module. 



magnitude of pretension selected 

 for the tie rods is larger than the 

 forces applied to the hatch system 

 during lifting, the joints in the 

 acrylic plastic hull will never be 

 subjected to tensile stresses due 

 to lifting and will generally be 

 under compression. In this man- 

 ner, the danger of joint failure 

 will be completely eliminated, as 

 under compression the joints are 

 self-wedging without any shear 

 stresses at the joint surfaces. In 

 the design of the pretensioned 

 tie rods, the shrinkage of the cap- 

 sule under hydrostatic loading at 

 design depth had to be taken into 

 account. If designed properly, 

 the tie rods would be under the 

 needed tension while the NEMO 

 system is out of the water, while 

 at design depth the tension would 

 approach zero. In this manner 

 the diametral compression of the 

 capsule by tie rods will not be superimposed on the hydrostatic compression 

 at design depth thus eliminating the danger of triggering the elastic collapse 

 of the capsule by the localized tie-rod pretensioning forces. To achieve such 

 a self-adjusting pretensioning system, a spiral spring with extension equal to 

 diametral shrinkage of the capsule would have to be incorporated into each 

 tie rod. The total force exerted by the springs in the tie rods while on deck 

 would be equal to the buoyancy of the NEMO capsule while at the design 

 depth it would decrease to almost zero. 



The tie-rod system appeared to be an adequate solution to the 

 attachment problem so long as the presence of several tie rods in the interior 

 of the capsule was not considered objectionable. Since for some capsule sizes 

 or possible non-NEMO system applications the presence of tie rods may be 

 objectionable, additional approaches were considered that did not require 

 the presence of tie rods. One of such proposed approaches utilized an exter- 

 nal cage. To eliminate the tensile stresses associated with lifting of the NEMO 

 system out of water or when it just floats on the ocean surface with the 

 negatively buoyant power module pulling downward, an external rigid cage 

 was added to the NEMO system (Figure 8). The point of contact between 

 the NEMO capsule and the rigid cage was to be only at the polar metal plates 



17 



