Active heave compensation

[Brian Cooper]

Hi
I have a application where I need to do active heave compensation on a ship.
The equipment used to do the control are listed below.
2 x hydraulic motors
1 x winch
1 x MRU ( To measure heave)
1 x Siemens PLC to do active heave compensation.

The system will function as follow.

The winch will pay out its load to a certain depth without heave compensation. When the depth has been reached heave compensation will be switched on to stabilise the load hanging from the winch. the load will then be lowered slowly to the seabed.

The PLC has to control the hydraulic winch by paying out and in as to keep the load still in the water while also lowering the load at the same time.

Can anybody help me with how to do the control of the heave compensation?

Regards,

 

[Peter Nachtwey]

This is not a simple application. There is a lot of motion code you will have to write. Your version of active heave control seems relatively simple though. What is the second hydraulic motor for? I see the need for only one that controls the line feed.

I wouldn't do this in a PLC. You would be starting from scratch. Get a good motion controller or even write the code from scratch on a PC compatible PC104 platform with a real time kernel.

You will have to superimpose the payout with the roll and heave motion.
You will need to calculate the roll and heave and compute what affect that will have on the motion. This will require some trigonometry.
You will have to integrate accelerations to velocities and position. I am assuming your MRU uses accelerometers.
The ability to compute rates of change will greatly help in keeping the load still.

 

[Brian Cooper]

the winch will be working in two areas. In the first area the winch will only be used with light weights up to about 15 tons and for this only one hydraulic pack will be used. the second area the winch will be lifting heavier loads up to 30 tons and for this part of the application both hydraulic units will be used.

I can read the following from the MRU from all axes.
Linear acceleration
relative dynamic heave position
velocity of linear motion
angular acceleration and velocity

I contacted the MRU manufacturer and according to them the MRU uses all three axis and utilizes the roll and pitch measurements in the unit to find the vertical direction.
Also I found that PLC are used in these type of applications.

 

[shooter]

So you want to stabilise a load hanging overboard.
You will have to measure the movement of the hook, or end of the arm, or the movement of the ship and calculate the movement of the load, via some more sensors like the position of the arm, the position of the cranehouse etc.

for a start:
mount your MRU on the load, easiest way and for testing fast, now control the winch by this MRU, the winch should be varying speed when you move the crane. (heaving)

a better solution is:
mount the MRU on the crane arm, you can calculate the movement of the end of the arm, thus you know the speed of the cable due to seamovement.
And now you can control the position of the load by the hydraulic winch.
Not done often as the cable will need to move fast up/down.
mostly they use a separate system, where all of the crane is moved(it is hanging in compasslike ring system), by hydraulics, or the kursk it had a bunch of hydraulic cylinders moving up/down with movement of the sea, the winch was separate on deck, due to the very long cables.
here the MRU (several) were mounted on the ship and even the torsion of the ship was measured.

When doing proper you will have to compensate also other movements sideways, an expertsystem (fuzzytype) is best solution for this but will involve a lot of study, so first try it on a model sitting on the table.
you only need a simple electric motor as winchsimulation the rest by hand.


this control should be on always to avoid strong forces in the cable while travelling under water.

 

[Peter Nachtwey]

If I were only concerned about forces I would use some sort of tension sensor and use a derivative gain on the rate of change in the tension. The cable and the load will increase slowly as the load is lowered but monitoring the rate of change in tension will provide a pretty good indicator if the load is being pulled up due to a swell or suddenly lighter due to a trough in the waves.

If a MRU is used it is easier to put the MRU at the tip of the crane. This way there isn't a need to rotate vectors in 3D.

I don't agree with shooter's fuzzy expert system suggestion. If your feed back devices are good you should be able to do the math and calculated exactly how your crane is moving and how to compensate. I do agree that it would be best to model your system so you have code pretty close to running by the time the active heave system is built.