To limit the workload of this thesis and ensuring that sufficient time has been available to work with the main objective of the thesis, some simplifications and assumptions have been made. The main objective and most central part of the problem faced in this thesis is to develop a mode supervisor that merges nor-mal and emergency control modes into an autonomous control system with risk-mitigating capabilities.
The following limitations and assumptions have been made during the thesis work:
• Necessary state estimates are assumed to be available from an observer. The implementation of the observer is not considered, neither is measurement noise.
• The movement of the vessel is restricted to the 3 horizontal DOFs surge, sway, and yaw, with a control design model as given in Section 2.1. The parameters of this model are assumed known.
• Thruster dynamics are not considered in the control problem.
• Obstacles are assumed to be stationary.
• COLREGs are not considered in the implementation of obstacle avoidance.
• Environmental forces are not included.
4.4 Limitations and assumptions
• The case study considered is milliAmpere, with an operational area crossing Kanalen in Trond-heim. Both are presented in Section 2.11.
• Situational awareness and sensors fusion is not considered, but all necessary measurements are assumed available. This includes the position of obstacles.
• For simplicity, only the position of the center of the vessel is considered regarding obstacle avoid-ance.
• Here, all MRC modes are executed until they are finished, without reevaluating during the maneu-ver.
Chapter 5
Mode supervisor
In this chapter, the algorithm behind the mode supervisor will be explained thoroughly. The supervisor should solve the problem formulated in Section 4.1. First, a mapping of the modes used is presented.
After this, the logic behind mode switching in nominal control is presented, before the same is done for switching to MRC control modes. Lastly, some alternative methods that are not pursued closer in this thesis are presented.
5.1 Mapping of modes
As previously stated, the system proposed in this thesis will consider six modes, where three are NCMs and three are MRCs. All modes are not available or suitable at all times. Table 5.1 summarizes the relationship between the seven modes. The rows represent the current mode, while the columns represent the mode to be switched to. The reasoning behind the table will follow.
Firstly, the switching between NCMs (represented by the first three columns of the first three rows) is treated. The nominal switching is dependent on the distance to the two docks. When undocking, crossing should be activated when the distance to the origin dock is over a threshold, and the distance to the target dock is over a threshold. If the target dock is closer, then the switching should go directly into docking mode. When crossing is active, docking mode should be activated when the distance to the target dock is low enough. In docking mode, undocking can be activated if a new trip is started after the docking is completer. Also, crossing mode can be activated if the destination is changed while docking. Details for the nominal switching are given in Section 5.2.
Secondly, the switching from NCM to MRC (represented by the three leftmost columns of the first three rows) is treated. In docking and undocking phases, the only MRC available is crash stop and should be used for whatever reason an MRC is activated. This is because an evasive maneuver is considered too abrupt and uncontrolled when operating in a restricted area close to shore. Also, the speed is low, so the stop length will be short when a crash stop is performed. In crossing mode, all three MRCs are available.
Then, the system must decide which mode is best suited for the situation. The details for this are given in Section 5.3.
Thirdly, the switching from MRC back to NCM (the last three rows of the first three columns). When an MRC-mode is finished, the vessel is keeping its position in DP-mode. To go back to nominal operation,
Table 5.1:Switching table for modes
From
To Undocking Crossing Docking Crash stop Ev. maneu-ver SB
Crash stop Navigational conflict/ collision is avoided -Ev. port Collision
-it must be ensured that -it is safe to resume operation and that the s-ituation is resolved. For now, -it is assumed that this is done by a human operator at a control center. Therefore, no functionality for resuming operation is implemented. It is safe to assume that the first autonomous vessel will have continuous contact with an operator at shore. In the future, autonomous systems might be able to assess if the operation should be resumed, but this is not considered here.
Lastly, the switching from MRC to MRC is represented by the lower right corner of the table. As seen, this is left blank. In this thesis, it is assumed that once in an MRC, this mode should be completed without switching to another mode. In a real implementation of a system like this, it might be necessary to assess the risk during the execution of an MRC-mode, to ensure that the selected mode is indeed the one with minimum risk. This can for example be relevant if the situation changes a lot during the execution or if faults occur during the maneuver. But based on the assumptions in Section 4.4, this is not necessary for this thesis.