All of the above mentioned types of surface vessels have been built. 

 The submersible units are more stable and, therefore, have fewer restrictions 

 on the limits for safe operation. The surface-type vessels are more common, 

 of course, and have proven to be very successful in offshore oil well drilling. 

 For simplification, the surface-type vessel will be called a "ship" and the 

 semisubmersible and submersible will be called "platforms" in the following 

 discussion. 



A surface vessel meeting the requirements for the heavy-lift operation 

 could be designed to be any one of the three types described. Each has its 

 own advantages and weaknesses. For the project under consideration, it is 

 important to study the following factors. 



Availability. There are many ships suitable and available for 

 conversion to heavy-lift operation. There are very few, if any, platforms 

 suitable for heavy-lift operations. The latter would undoubtedly have to be 

 custom-built. 



Ease of Construction. There is much more experience in building 

 ships than platforms. The result is that it is easier to design and build an 

 acceptable ship; it is also easier to estimate the total cost. The limited 

 experience in designing and constructing platforms has made this type of 

 vessel relatively expensive to build. Shipyards are not designed for 

 constructing platforms, nor are many of the personnel familiar with the 

 construction procedures. In addition, special facilities are needed for 

 alterations or repair of platforms; this is particularly true if drydocking is 

 required. 



Mobility. Ships are more mobile than any platform yet built. Many 

 platforms have no means of propulsion and must be towed by a tugboat 

 between work sites. Up to the present, there has been no need for a platform 

 which has its own propulsive power, the only requirement being that it is 

 safely towed. Towing speeds are on the order of 6 to 8 knots. 



Platforms can be designed to accommodate a propulsion system. 

 A large amount of power is required to move a platform since the hull 

 configuration is not the best for movement through the water. It can be 

 safely assumed that unless a tremendous (almost unrealistic) amount of 

 power is provided, a platform is considerably slower than a ship. 



91 



