Paul Zing 



McClure [ 1965] described a platform which was designed for the 

 MOHOLE deep sea drilling project. This platform was to consist of 

 two submerged main horizontal pontoons 3 5 feet in diameter, and 390 

 feet long, with a centerline separation of 215 feet. Three vertical 

 caissons extended from each horizontal pontoon through the free 

 water surface to support the nnain working deck. This platform was 

 intended to operate in a water depth of 14,000 feet, and was to be dy- 

 namically positioned by means of trainable propulsion units controlled 

 through a central computer system. The foregoing types of platforms 

 are referred to as "column stabilized, " which implies that pitch and 

 roll static stability are obtained primarily from the waterplane mo- 

 ment of inertia of the surface piercing vertical column. 



A third type of platform for which a tubular space frame con- 

 figuration has been proposed is the tension leg platform, an example 

 of which is shown by Macy [ 1969] and described by Paulling and 

 Horton [ 1970] . This is a moored stable platform for which the 

 buoyancy exceeds the platform weight, and the net equilibrating 

 vertical force is supplied by vertical tension mooring cables secured 

 by deadweight or drilled-in anchors. As a final example of tubular 

 stable platform structure, we mention the spar-type platforms, the 

 prime example of which is FLIP, described by Fisher and Spiess 

 [ 1963] . The platform consists of a single cylindrical member of 

 tapering cross section arranged to float vertically, with a small 

 portion of its length projecting above the surface of the sea. 



All of these platforms share a common characteristic in that 

 their configuration consists of a space frame assemblage of relatively 

 long, slender, cylindrical members, with the addition in some cases 

 of small buoyancy chambers or pontoons. All share a common ob- 

 jective of producing a working platform having minimum wave- 

 induced motions, even under relatively severe sea conditions, i.e. , 

 a platform which is "transparent" to the waves. The positioning 

 methods used differ greatly in each case, ranging from essentially 

 no positioning, in the case of FLIP, dynamic positioning with no 

 physical connection to the sea bottom. In the case of the MOHOLE 

 platform, to various types of anchoring systems exemplified by the 

 tension leg and the column stabilized platforms. The sea environ- 

 ment and resultant platform responses are similar In each case. 

 I.e. , all are Intended to operate In relatively deep water under 

 severe environmental conditions, with platform motions which are 

 small compared to the overall dimensions of the platform and to the 

 length of waves involved. Our objective here Is to describe a pro- 

 cedure for analyzing the forces and motions which can be applied 

 equally to all of these platforms If suitable account Is taken of the 

 type of anchoring or positioning restraint Involved. 



Such an analysis of wave- Induced forces and motions forms 

 an essential part of the process of designing a platform to perform 

 a specific mission. At least three such functions are envisioned. 

 First, for a platform of given geometry, a range of sea conditions 

 may be Investigated to determine what limitations may be Imposed 



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