Project Mohole Drilling Platform 



Station -Keeping Analysis 



Behavior of the platform acted upon by wind, current, and propulsive forces 

 was simulated mathematically with the aid of a digital computer program. 

 Written in Fortran IV the program has sufficient flexibility that any foreseeable 

 combination of wind and current can be imposed including time variations of ve- 

 locity and direction. Resistance, propulsion, and mass acceleration terms are 

 added vectorially to obtain equilibrium in the horizontal plane. In addition, any 

 desired propulsive thrust control formulation can be inserted. Movement of the 

 platform is calculated at frequent intervals affording a close approximation to 

 dynamic equilibrium. The equations and treatment of platform parameters are 

 stated in Appendix C. 



Platform parameters such as mass moments of inertia, resistance, side 

 force, and yaw moment are built into the program based on the best available 

 information from model test analysis. Some flexibility is available for varying 

 thrust programming and environmental forces from run to run. Two examples 

 requiring the following definitive information are presented to illustrate the ca- 

 pability of the positioning maneuvers program: 



1. Translational Thrust — Thrust is calculated as a linear function of dis- 

 tance from the hole. It has been found desirable to specify a dead band within 

 which no thrust is applied in order to minimize "hunting." In the example this 

 is specified as a 50-ft radius from the hole. At this distance any desired 

 amount of thrust may be called for, increasing linearly to a maximum at some 

 specified distance. For the example we have chosen, thrust equals 1070 D minus 

 37,500 pounds where D is the distance from the hole. This results in 16,000 

 pounds thrust at 50 ft and 390,000 pounds at 400 ft which is approximately the 

 maximum available thrust. Positioning unit thrust is built into the program and 

 depends upon orientation and velocity. Main propeller thrust is a variable and 

 this example is limited to a maximum of 240,000 pounds. 



Another variable in the thrust program is the instruction for the integrator 

 circuit. This device notes the distance off the hole and biases the range infor- 

 mation so that the platform can be moved toward the hole while maintaining the 

 thrust required for equilibrium with the environment. For the examples, we 

 have chosen a 60-second sampling interval and a correction of 1 percent of the 

 distance off hole. 



2. Turning Thrust — In order to correct errors in heading, power is sup- 

 plied to the positioning units in amounts and directions needed to apply torque to 

 the platform. A dead band of 3 degrees is specified, beyond which thrust is ap- 

 plied varying as the sine of the heading error angle up to a maximum moment at 

 30 degrees. The maximum thrust for turning moment is specified in the pro- 

 gram as 40,000 pounds ahead on one side and astern on the other which results 

 in a maximum torque of 40,000 times the 215 ft transverse spacing of the units 

 or 8,600,000 ft-pounds. 



3. Wind — Wind may be specified at any desired velocity and direction, 

 either as a constant value or as a ramp function approximated by a series of 

 small step increases. For the examples, we have chosen wind blowing in the 



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