Torque peak reduction and overload monitoring of induction motors in offshore drilling operations

Current drivetrain design procedures for electric actuation systems operating in offshore conditions typically consider two major requirements: to provide sufficient steady-state and maximum motor torques. As a result of this, no information regarding the transient state (e.g. during motor acceleration) is utilized when selecting drivetrain components. This leads to potentially dangerous situations when motors undergo saturation in these regions due to too high dynamic loads. A common reason for this (apart from lack of information about transient state when designing a drivetrain) is applying trapezoidal reference motion profiles that cause discontinuities in system acceleration and infinite values of jerk. To overcome this drawback, the current paper presents an application of a trigonometric scurve trajectory to lower values of torque peaks of motors operating in offshore drilling equipment. In addition, we show a method to monitor time spent by motors in overload regions. These two combined contributions allow to avoid unexpected motors saturation and verify if the components selected for a given electric actuation system will provide for a smooth machine operation. Practical significance of the presented work is demonstrated on a gripper arm mechanism of a full-scale offshore pipe handling machine. The results show not only a serious reduction in motor torque peak values when applying the trigonometric motion profiles (up to two times lower values), but also adaptation of the overall motor torque shapes to avoid too high overloads during the steady-state operation.