McClure and Hove 



direction of the negative y axis increasing in a step function from zero to 33 

 knots, 



4. Current — Current may also be specified in terms of direction and ve- 

 locity and may be either a steady value or ramp function approximated by small 

 increments. In the first example, we have chosen zero current. In the second 

 example current is 3 knots in the same direction as the wind. 



5. Initial Conditions — Initial conditions for the problem may be specified in 

 terms of platform position, velocity, and heading. In this way a previous prob- 

 lem can be utilized as input for a succeeding problem. In the two examples, we 

 have chosen to start with the vessel at rest. In the first case the initial head- 

 ing is in the x axis direction and a 90-degree change of heading is ordered. In 

 the second case the heading is in the y axis direction and is to be maintained. 



The first example is a simple case of a step increase in wind. The vessel 

 is assumed to be holding position on-station and the wind arises from abeam. 

 At the start of the problem a command is given to change the heading of the 

 vessel by 90 degrees so that it will head into the wind. The vessel, therefore, 

 starts with zero velocity and with a 33 -knot wind from abeam. As the problem 

 proceeds, the platform is turned into the direction of the wind and is moved back 

 toward the hole after being forced off by the wind. The equilibrium point is a 

 function of wind force versus thrust. The integrating circuit gradually shifts 

 the x-y coordinate of the equilibrium point from an off -hole position to a posi- 

 tion over the hole. 



Figure 10 is an x-y plot of the maneuvering path of the platform as it re- 

 gains position. The heavy line is the path of the center of the vessel. The short 

 arrows along the line indicate the heading of the platform at each moment of 

 time. The number at each arrow is the time in minutes from the start of the 

 problem. This is a plausible situation which the vessel might encounter in the 

 event of a sudden squall. Often the exact wind direction is not known until the 

 wind actually strikes. 



The second example involves both wind and current. The initial conditions 

 for the platform are again zero velocity and zero position error. In this case 

 the vessel is assumed to be heading at 90 degrees, that is, in the direction of 

 the y axis. The wind is 33 knots and the current 3 knots, both in the direction 

 of the negative y axis. Since the vessel is headed into the wind and current no 

 heading change is called for. It is assumed that the integrator circuit has been 

 operating and that the thrust is nearly in equilibrium over the hole. It will be 

 noted in Fig. 11 that the vessel initially drifts only a short distance from the 

 hole, since the integrator has previously displaced the 50-ft-radius dead band 

 away from the hole. The integrator circuit is working and gradually displaces 

 the equilibrium point so that the vessel returns slowly to the hole. 



These two examples are not sufficient to demonstrate satisfactory maneu- 

 vering performance of the platform. A great many cases have been investigated 

 with variations in thrust programming, integrator circuit, environmental condi- 

 tions, and other variables. It is concluded that the platform can maintain sta- 

 tion in the design conditions under automatic control. It can, of course, main- 

 tain equilibrium at stronger wind with presence of less current and vice versa. 



426 



