I'm a subsea engineer with mechanic

[suraiya]

I'm a subsea engineer with mechanical background. Recently I was assigned to a project which require design of a subsea umbilical, which will comprise bundle of hydraulic and chemical tubes as well as power and signal cable. Being a mechanical guy, I really need some basic idea on the power cable part.

The power cable is actually carrying communication signal as well (signal on power). I need to select between a quad cable and 2-pair cable. My questions (at least for now!!) are:

1)what are the basic advantage and limitation (in term of load, distance, cost, lifecycle etc) between these two.

2)do i need to screen the cable or not, bearing in mind that this is signal on power cable. If indeed yes, should I screen every single cable within the bundle, or suffice to screen the bundle (quad/pair)only?

Well, obviously I'll talk to an expert at a later stage, but first round input from this forum will be really helpful.
Thank you.

 

[davidbeach]

You need to find your expert now. There are far too many unknown variables for you to get meaningful help here.

 

[waross]

Normally you are trying to prevent radiation from the power cable(s) from inducing noise into the signal cables (control and data.
Also you may want to prevent cross-talk between signal cables.
What may work in one instance may be a complete failure in another installation.
Some methods of interference abatement:
Twisted cables, power conductors twisted together and signal conductors twisted in pairs.
Twisted signal pairs in an overall shield.
Each twisted pair shielded.
Each twisted pair shielded and the power conductors shielded.
One of the above methods combined with physical separation between power and signals.
There may be extreme instances of interference where the type and/or frequency of the signals may have to be re-engineered to ensure reliable operation.
As to which level of protection you need, you need an expert now as suggested by davidbeach.

 

[slavag]

Hello.
Suraiya, please don't use my Q as some recommendation,
please ask your expert, as saied Davidbeach and Waross.
Why don't use for signals FO cable? In lot of cases it's best solution. Only conversion from RS485/422/232/TCP-IP to FO.
Regards.
Slava

 

[itsmoked]

I would totally agree with slavag!

One fiber could carry interference free and interference immune all the info you would ever need. And probably cheaper than copper.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

One of the problems here is that you can probably find a lot of decent advice on the power aspect, but as was mentioned above, there are a lot more variables thrown in when you add the PLC (Power Line Carrier) system to this. That alone is almost an art form to get it right in a normal factory installation, you need to look for a Michaelangelo to do it on a ceiling while laying on his back!

 

[sjohnr]

You should initially be discussing this with your potential subsea control system suppliers. Depending on the field size, i.e. tree count, distances, they can estimate the load on each line from which they can estimate a suitable electrical conductor size. They may also have a preference for quads over pairs and should be able to advise you on their preferred screening arrangement.

You then really need to speak with the umbilical vendors, Nexans, DUCO or Kvaerner. They can advise on what limitations there might be with the functional requirements you have specified.

It's not at all clear why you think you have to choose between quads and pairs at this stage. But to answer your specific questions:

1. There is little or no difference. There is an argument that quads are more efficient for the overall bundle, but there is also potentially a greater susceptibility to cross-talk.

2. I have worked on unscreened and screened power pairs and quads and it's possible to make all of them work. However, I would recommend that a twisted power pair be screened if it's carrying comms. Likewise a quad for comms on power should have an overall screen. An overall screen on the entire bundle is not required. Depending on the application the quads may need to be armoured.

I'm not a big fan of comms on power systems. There is typically a fairly low limit on the number of loads on any given circuit. Also, those loads have to be reasonably near to each other to balance the system. Separate comms and power gives greater flexibility for expansion and changes to the system.