SECT. 5] DEEP-SEA ANCHORING AND MOORING 519 



and to do this in a manner that is compatible with the other forces and design 

 restraints. 



Insofar as the mechanical components of the mooring are concerned, the 

 conditions that impose force or other destructive influences on the mooring are : 



(a) steady or quasi-steady currents, 



(b) waves — particularly combers, 



(c) electrolysis, 



(d) sharks, and 



(e) miscellaneous : fouling organisms, fishermen and collisions. 



A. Currents 



The strong currents of the deep ocean are apparently restricted to the upper 

 or Eckman layer. This may be as much as 400 m thick, and currents of the 

 order of 2 to 5 knots may be common. Much below the level of the mixed layer, 

 currents of as much as 0.5 knot are rare. 



The current profile of an intended site imposes basic design requirements on 

 a mooring, and it is important to have some information on this. 



A rule of thumb in the design of moorings that has been used at Scripps is to 

 accept the normal measured surface current as extending through the mixed 

 layer undiminished. A current of 0.5 knot is assigned to the inflection point in 

 the thermocline, and a 0.2-knot current at the bottom. These currents are then 

 interpolated linearly and used for design. 



Obviously, a measured current profile is preferable, but this ordinarily is not 

 available until the first moorings have been used. 



This procedure is conservative to the degree that currents are not in the same 

 direction. Indeed submerged countercurrents cancel some of the stress in the 

 mooring. 



The calculation is non-conservative to the degree that transient currents 

 exceed the design current in the region. This is accommodated by factors of 

 safety in the design. 



B. Waves 



Waves, and particularly deep-sea-breaking waves (combers), result in the 

 most demanding requirements of moorings with surface floats. 



It is clear that if particular mooring requirements are met by subsurface 

 components alone, with no surface float, many of the problems of deep-sea 

 mooring can be eliminated or ameliorated. Such wholly submerged floats can 

 even be useful for station keeping by use of sonar. 



Undoubtedly the greatest forces that naturally affect a mooring in the deep 

 sea are those resulting from deep-sea combers striking the surface float. Although 

 no defensible deep-sea measurements have been made, it is apparent that the 

 float is struck by a cascade of water moving essentially at the wave (phase) 

 velocity. In the case of a wave of an 8-sec period, this velocity is about 41 ft/sec. 



