Appendix A 

 SURFACE VESSELS 



Specifications and costs of available and/or desirable surface vessels 

 for heavy-lift systems are discussed in this appendix. The three general 

 classifications of surface vessels, their assets and liabilities, and any additional 

 points of interest are given consideration. 



A portion of the appendix is devoted to a specific but important 

 topic related to operations employing surface vessels; the problem of 

 maintaining position on the surface during an operation. The possibilities 

 of using a conventional anchoring system or a dynamic positioning system 

 are discussed. 



SURFACE VESSEL TYPES 



There is a large number of trade-offs to be considered before the 

 more desirable types of surface vessels can be chosen. It is convenient to 

 define three general types of floating vessels: surface, semisubmersible, and 

 submersible. 



1. Surface Type. A surface-type vessel is characterized by having 

 most of the hull near the water surface. Any ship could be considered a 

 "surface-type" vessel. These vessels are designed so the center of gravity is 

 below the metacenter. The stability is dependent upon the amount of mass 

 initially above the water line and its location with respect to the center of 

 roll. Vessels of this type, namely ships, have short beam dimensions when 

 compared to their length, making them susceptible to roll. 



2. Semisubmersible Type. Semisubmersible vessels have reduced roll 

 and pitch which result from increasing the natural periods of these ship 

 motions. The latter are achieved by placing large masses significant distances 

 from the center of roll. Much of the added mass of the hull is below the 

 water surface, although some of the hull is still above the water surface. 



3. Submersible Type. The hull of a submersible vessel is completely 

 below the water surface. To provide stability, the structure must usually be 

 ballasted; if it were not, the center of gravity for most geometric shapes 

 would be above the center of buoyancy and a very unstable vessel would 

 result. These vessels have very deep drafts when in the operational mode. 



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