Submarine Power Cables 1

[bigamp]

A proposed development will include a number of small offshore production platforms. Each platform load is 415V 10kW!! The distance to each is approximately 12km. We plan to provide power to each at 11kV, via submarine power cable, with each platform to include a suitable transformer to 415V. Does this sound OK? What are the things to watch for with submarine power cables? Would we be better off providing power at a lower voltage (3.3kV is possibly available)? What sort of limits are there on circuit lengths, charging current etc?

Regards, "bigamp"

 

[electricuwe]

To my mind there are atwo different approaches:

Search for a the cable with the lowest available cross section for 3,3 kV and 11 kV. Calculate voltage drop and input current using the formulas for transmission lines. With this result you will see if one of the two solutions is more suitable (if any). It might be useful to operate the cable not at its rated voltage but with a lower voltage! So you should also check this alternatives.

Another option would be to built a "small scale HVDC" system with about 3 kV of DC. Converters in that voltage and power range might be available as auxillary power converters for DC-railway applications.

 

[jbartos]

Suggestion: Visit

http://www.pirelli.com/en_42/cables_systems/submarine/power_

systems/index_power_systems.jhtml;jsessionid=MD2USJFEDW415FY
KH5DCFEY?s1=4200140&s2=4200143&s3=5200004

http://www.pirelli.com/en_42/cables_systems/submarine/power_

systems/references/italy_sps.jhtml

for more info
11kV will safe copper noticeably since the voltage drop will have to be taken into consideration at the length of 12km.
If the load happens to vary widely on your platform, you will need transformers with on-load tap changers.

 

[jwerthman]

Wouldn't it be much simpler and more cost-effective to install a small local generator at each platform. If reliability is an issue, then multiple units could be installed, probably for less investment than submarine cable and all the other equipment. I'm speaking somewhat out of school since I have no experience with off-shore platforms, but I would certainly evaluate it. For 10 kW, some type of photovoltaic and/or wind generation might even have some application. The wind regime in many of the coastal areas is quite good for wind power.

 

[Etrix]

bigamp,

I looked at subsea cables ages ago while doing a feasibility study for an interconnector and looked into an existing project with a 105km long cable carrying 40MW of power. The cost of installing the cables alone (rule of thumb - GBP 1 mil per 10km) is enough to put most ppl off! Of course this really depends on the condition of the seabed etc etc. I tend to agree with JW that having a small genset may be more cost effective, even if you implement an N+1 scheme.

Photovoltaics cease to be economical where continuous loads are more than 1.5kW (bp Solar literature), and are also very much dependent on the 'peak hours of light' (e.g. 5 hours for Africa, 4 hours for Far East and India and so on and so forth).

The largest wind generators currently used in Europe average about 1MW or so, and are not too cost effective, besides having a power output that is difficult to control accurately if that is an issue. Very climate/weather dependent - obviously.

Hope that sheds light on the issue if not to complicate things further.

Etrix

 

[electricuwe]

I do not aggree to jbartos:

using 11 kV for such a low power will lead to excessive losses due to charging currents. Unfortunately I did't find the data necessary for the calcultion on the pirelli page. Load current will be smaller than 2 A for 3,3 kV !

 

[jbartos]

Comment:
1. The submarine cable will probably have to be custom made. Considering the length of the cable(s), it will be feasible to have it custom made.
2. The cable charging current and leakages are dependent on the cable structure. Theoretically, 3.3kV cable could have the higher charging current than 11kV cable, depending how it is designed.
3. 11kV cable will save copper.
4. 11kV cable can be oversized to achieve small voltage drop and to avoid voltage regulators compensating voltage due to loading.
5. If the cost and amount of cable copper or size is not an object, the supply voltage could be of 415V and cable 600V voltage level (carrying about 13.9A/lineconductor), for example.

 

[electricuwe]

To jbartos,

1. Of course, but I guess for that application that's not a problem. Bigamp needs several of that links.
2. Charging current does depend on nominal voltage (thickness of insulation and type of dielectric), but the most important factor is operating voltage. My be I would use an 11 kV cable if its easily available, but I wouldn't operate it at its nominal voltage.
3) It will not save copper for that distance because of charging current (if operated at 11 kV). And if charging current wouldn't be an issue it would be impossible to find someone willing to manufacture an 11kV submarine power cable with 3*0,..mm² cross section.
4) That should be also possible for a lower voltage
5) That would need an awfull amount of copper

With some calculation you will find that the optimum voltage level for that application is somewhere between 415 V and 11 kV

 

[jbartos]

Comment: Had it been my project and investment, I would shop around to find the best deal with the manufacturer. Of course, the loses can add to the operation and maintenance cost. Therefore, it would be good to have any losses minimal to save money in long run.

 

[bigamp]

Gentlemen,

Many thanks for the constructive and helpful ideas you have put forward. I am closer to resolution. The platform loads look to have increased somewhat so cable supply at 11kV is the way to go. I have approached three cable manufacturers for information and (interestingly) so far their preferred cable type is paper insulated/lead sheathed steel wire armoured.

Power generation on the platforms is not being considered for this project because the platforms are intended to be simple and "maintenance free", just one visit per year!

Regards

 

[elecman]

Bigamp should be careful of advice from manufacturers. First of all, they don't appreciate the vibration that takes place with platforms. The cables will no doubt be installed in J-tubes up the side of the jacket. Constant vibration will work harden lead and eventually it will fail due to fatigue cracking. Secondly, nothing prevents water from entering submarine cables, even lead. Water leaches in and that wouldn't be compatible with paper insulation. I would recommend a wet design, EPR insulated 11kV cable with steel wire armour, laid on the seabed rather than buried. There are some existing examples of long submarine cables supplying small loads in the Southern North Sea and around Morecambe Bay on the Weat coast UK, and they use these principles. I would suggest that for offshore platforms losses are of secondary importance to getting the right technical solution. The distances of about 12 miles are not a problem, nor are charging currents. If the platforms are in UK waters there will be no teredos to worry about either.

 

[BJC]

Interesting thresd.
Here are some thoughs just to keep the pot boiling.
Why not use 600 or 1000 volt cable and DC power? At 10KW and 240 volts the load current is about 42 amps. It was not clear to me wheather the power was going to be 3 phase or single phase at the platform. DC to 3 phase inverters are pretty common.
A DC system will take a larger cable but the cable will be cheaper, easier to get and easier to splice.

12 Km of 4/0 cable has about 2 ohms of resistance or 80 volts of drop. There is some power loss but is the cost offset by the lower cable cost?
Can you get by with one wire? If you use an earth-sea you only need one cable. The system could also cathodically protect the stucture. The annode on shore has to be carefully considered. You can disolve a disolve a boxcar in short order.
Even AC systems need to addres stray currents and the potential for inducing corrosion they can have.

 

[bigamp]

Elecman,

Thanks for your comments, you have been most helpful and have provided answers to all of my questions. By the sounds of your posting, you have been involved in similar installations. It is really good and helpful to hear how installations similar to the one I am involved with have been done, successfully.

In particular, thank you for making me aware of the vibration issue. This alone will preclude the use of PILS type cable. At this stage, the power cable to each of the platforms is planned to be at 11kV and to be included in the "umbilical" to each platform and so would be either XLPE or EPR insulated. EPR sounds to be a better way to go.

I also concur with your comments about manufacturers, it is a good general comment applicable to all areas.

BJC,

Thank you for your comments. Supply at DC as per your suggestion would be viable for some applications I think, but not really for my one for various reasons:

(1) No wish to put in a 600V or 1000V DC system at the onshore point of supply. This would have to be a dual redundant system to satisfy maintenance and reliability requirements.

(2) There are reliability issues with inverters, you would probably need to have two of these on each platform as well.

(3) The whole of the platform will be a hazardous area. It would be necessary to attempt to house the inverters in explosion proof enclosurers on the platform and this may be a problem.

(4) For these sorts of applications, simplicity really pays and it is difficult to beat 11kV supply plus cable to transformer plus DB. Hazardous area transformers, 11kV switchgear and 400V DB's, although hellishly expensive, are all available as standard production items from a range of vendors.

Regards

 

[elecman]

Two more thoughts - (i) If you might have to pull the cable, eg to the beach from the boat, over a long distance consider using hard drawn solid conductors for more strangth and smaller cross section. (ii) if you were using dc in UK waters the British Admiralty will not allow sea return as suggested by BJC. The reason - sea returns affect magnetic compasses. (Not only does the British Admiralty no longer use magnetic compasses, but probably no sailor uses them any more!)

 

[johnwg]

For such small loads on platforms that are spread so far apart, local generation is far better than cables which are very high cost. You can have very reliable small generators (Trinity House type) that will show a much lower life cost and acceptable reliabiity doubled up with auto cut-in devices, even allowing for fuel storage/usage etc. Instrumentsa could be on PV for added reliability (keep it limited the cost of PV is high)

 

[jbartos]

Suggestion to the previous posting. I think that the requestor prefers the cables since the diesel generators have higher operating and maintenance costs.