D.P. (more detail)

Now that we have a basic idea of what is involved in DP, lets have a look at how a machine does it.

The analogy of running back to the phone and slowing down before you arrive is quite important, if every time the ship moved off station the thrusters went to full power until it was back on station, the ship would be charging around all over the ocean with absolutely no control. To get around this we use what is called a PID control loop. (I'm afraid there are lots of cryptic abbreviations coming)

The PID loop or proportional, integral, differential (derivative) control is used in many everyday items such as a central heating system.

taking them in turn.

force is applied at a level proportional to the size of the error

force is applied at a steady rate to maintain a steady state

force is applied at a rate proportional to the rate of change ( differential or derivative) of the error.

PID loop applied to a central heating system .

when the house is cold and the thermostat is set to warm,  the heating fires up at full blast, as it nears the desired temperature the system reduces the amount of heat applied to the water in the boiler until it reaches the set temperature (proportional), then it applies just enough heat to maintain the water at the temperature required to meet the thermostat setting (integral). If a door is opened and the temperature starts to drop, the system kicks in quickly to compensate for the sudden drop (differential) and then resumes  what is required to maintain the set temperature. The levels are constantly being changed depending on the environment and measured temperature.

Applying this to a ship.

When a ship is first put in to DP control it should be as near stationary as possible, as the ship moves off the set point forces are applied due to the proportional controller (how far it is from the set position) and the differential controller (how fast the ship is moving), after a period of time during which the ship has oscillated around the set position and average or integral force has been established required to keep the ship in position, counteracting the effect of current, waves etc (notice I have not mentioned wind)

Should some external force cause the ship to move a short way off location quickly, the differential controller will demand large amounts of thrust but the proportional controller will reduce that thrust as the ship is not far from location (remember the 2 meters from the telephone analogy). If the ship continues to move away from the set position at a high rate, the proportional controller will reduce its effect on the differential controller and more and more thrust will be applied. Eventually, the vessel will stop moving away from the set position and start to move back, now the proportional controller is demanding the force and the differential (which likes moving slowly) is moderating that force so as to move back to set position in a controlled way .By the time the ship arrives back at the set point the velocity should be zero. During all this of course the integral or average force required may have been changed due to changing environmental forces (that may have caused the original move away from location).

Should the original move away from set point take place slowly, then it will be the proportional element of the controller that is the prime cause for the ship to move back to location.

The way I look at these elements is that the integral is the average force, the proportional is what tries to keep us in one position and the differential is what tries to keep us at zero velocity, it's over-simplified but it works for me because I'm simple too.

So that's the PID loop taken care of then, what else is there in a DP system?

Basic outline of DP system components

Reference systems are covered on other pages. So all we have left are thrusters, TAL (more abbreviations), user input (MMI's),  and how we deal with wind.

MMI / User Input

The operator commands (i.e. position and heading required) are controlled from a computer console (now called a man machine interface (MMI) apparently). This more often than not is a PC running a version of Windows and the system builders proprietary software. Some systems do run on DOS with a Windows based graphical interface. Older systems have a may proprietary software interface.

From the console the operator can set the commanded position and heading, select and deselect the various sensors and thrusters and adjust system settings.

Wind

Wind is difficult to compensate for with a PID loop because it requires a position error to be measured before any change in force command is issued. Wind changes speed and direction quickly so the PID loop is not an efficient control in this instance.

When the DP system is designed, a mathematical model of the vessel is constructed and the forces required to counteract various wind speeds and directions are calculated, this data is then stored within the DP system software. When wind speed is measured by the anemometer this wind speed and direction is converted directly to a required thruster force and direction and applied forward of the PID loop, hence the wind sensors are known as a feed forward system.

Since we have a mathematical model of the vessel, it should be easy to see now that the average force required to maintain position (i.e. the integral component of the PID loop) can be resolved to give an estimate of direction and speed of water flow against the vessel. Water flow is better known as current, and although the estimation from the model is affected by numerous inaccuracies ,it is used as a guide to current direction and speed.

These days to distinguish this estimation from true current, it is often referred to as DP current or environment.

TAL (thruster allocation logic)

All DP vessels have at least 2 thrusters most have many more, the Sedco 709 has 8.

There needs to be a way of converting the force vector required by the PID  controller and wind feed forward to commands for the thrusters, there is no point in one thruster trying to do all the work with the others having a holiday.

So we have another segment to the DP system called the thruster allocation logic, this is a segment of computer code that "knows" the output of each thruster, its location and what command is required to obtain a certain force in a certain direction, it splits the total required force vector into components and distributes individual instructions to each thruster control unit that combine to make a resultant equivalent to the force vector commanded by the PID controller and wind feed forward.