4.3 Calculation results for changing the δTh

　

 As shown in section 4.2, by including a measurement error in the estimated value, the thruster power fluctuation becomes large. Therefore, it is necessary to investigate the influence of δTh on the position keeping control of the FPSO. The numerical calculations were then made for the standard deviation SD = 0.7 , and δTh = 1, 3, 8% of the maximum thruster power. The calculation results are shown in Fig.13 〜 Fig.15.

　

 In the condition of δTh = 1 %, Fig.13 shows that the horizontal displacement of the FPSO exceeds 25(m), which is the limit of controllable horizontal displacement. It is thought that since the thruster output is small, it is impossible to output the thrust based on the estimated value acquired from the Kalman filter. Particularly, from the time history of the heading angle, it is found that the FPSO rotates slowly in the direction of the target angle.

　

 When δTh is 3% of the maximum thruster power, it is possible to achieve the control objective. However, compared with a no measurement error situation, the horizontal displacement of the FPSO increases. Also, fluctuation of the angular velocity γ and the thruster power (τB,τS) occurs.

　

 From the comparison of Fig.10 and Fig.15, it is found that when the value of the δTh increases, the difference of the control performance by δTh is clear. Especially, the horizontal displacement of the FPSO decreases greatly. However, there is a tendency for the fluctuation of the thruster power to increase.