Rules and Intepretation
According to ABS Guide for Dynamic Positioning Systems Section 5 Chapter 9, Subchapter 9.1
"A simple device is to be provided in the DP control station for the selection of the thruster control modes (i.e., manual thruster control, Manual Position Control and DP control). The device is to be designed so that it is always possible to select manual thruster controls after any single fault in the DP control mode. Thrusters within the DP control system may also be individually de-selected from DP control to manual thruster control for service and vessel specific operations."
A simple device here means that the device should be as electrically simple as possible and without needing complex integrated IO cards to function. Complexity increases the number of potential failure points and as such increases the likelihood of an operational-crippling failure occurring. The device can be a simple switch or a series of push buttons. The operability of the device shall not be dependent on the health of the DP system; if a catastrophic failure occur in the DP system such as a major blackout of one control cabinet the device must still work to allow the operator to switch between modes.
At least three modes are required, namely “manual thruster control”, “manual position control”, and “DP control” or more commonly known in the offshore industry as “Manual”, “IJS”, and “DP” mode respectively.
Although not mentioned in this rule, it is explicitly stated in the guideline that the selected mode shall be indicated clearly. On a side note, manual thruster controls and independent joystick system should be placed somewhere near the DP operator station. This is to make sure the DP operator have easy access to all available control modes. It goes without saying that the mode selector should also be nearby the DP operator. Means against inadvertent operation of the switch shall also be set in place.