I am doing a sub sea metering skid system. This is my first time.
Can anybody throw light on what is meant by water depth and free surface elevation. According to client data, i have water depth as 120mts. Would my free surface elevation also be 120mts. I have to apply wave at 0, 90, 45 and 135 angles to the piping system.

Read this thread.

Well i did go through that thread. How do i specify LAT and HAT in the program. I did understand that my free surface elevation and water depth would be approx the same as the system is on the sea bed. Wave loads are very very less. I am not sure whether i am going in the right direction

You can't directly specify LAT and HAT. On the first WAV specification set your water depth to the LAT value. On the second WAV specification set the water depth to the HAT. In both cases the global coordinates and free surface elevation must correspond - which is why I said things would be easier if your first node was topside and you coded down.

i have been given a value of MSWL-LAT as .865. Also water depth of the field as 120m LAT. How do i interpret these values.

It looks like your minimum water depth is 120m. For "MSWL-LAT" I would interpret as "mean sea water level - lowest astronomical tide = 0.865m". However, you should check with whoever gave you this data to be sure.

Yeah you said that right. It means my free surface elevation or the mean sea water level is to be 105mts. So if i set the free surface elevation to be 105mts, what i understand from you is that i have to set my global cordinates to be at 105mts in the Y-direction.

The "free surface" elevation is relative to the node point in your model that is at zero elevation. It is setup this way so that you don't have to go back and change your input.

So if the first node in your model is at the bottom, then you really should set its coordinates to (0., -105., 0.) and the free surface would be at 0.0. This is by far the most logical way to approach this data - especially if you're going to apply wind also.

thanks a lot for this useful info. One last query was about the direction cosines. Do i apply the wind and current profiles as like wind profiles.

All three (wind, wave, current) act independently, so they each have their own set of direction cosines. They may be aligned, they may not be.

Current is best applied using the "Power Law" selection. For wind, if you were here in the U.S. I'd suggest ASCE#7, but in Norway you may want to define either a velocity vs elevation table or a pressure vs elevation table. Just remember that when you add wind to a model, wind starts at an elevation of zero and goes up. You have to position your model (via global coordinates) accordingly.