Electricity

Introduction

Electricity ia the most common medium to transport energy from the source to users and was the first type of transmission that we reviewed. Transmission of electricity from an offshore installation would require undersea cables. This is a well developed technology and hundreds of undersea connections have already been made in the world (England/France +/- 100 kV submarine d.c. link was inaugurated in 1961).

Description

An undersea or submarine cable is used to link two parts of land separated by a river, a lake or a sea channel (coast to coast connection) or to link an offshore platform to the shore or two offshore installations together.
All electric cables are essentially made with a low resistance conductor to carry the current and insulation to isolate the conductors, from each other and from their surroundings.Generally, as the voltage increases, the construction becomes much more complex.

The life time of submarine is expected to have a minimum of 25 years and probably 40 years however an undersea cable can be subject to failure and repair is very expensive costing between Ł0.5M to Ł1M.

Economics

It is not easy to get cost figures from cable manufacturers, not only due to the commercial confidentiality but also because each subsea cable installation is specific.

Cost of submarine cables
The cost of submarine cables is much higher than overhead cables and can be estimated between Ł50 and Ł125 per metre.
For example, the cost of the 26 km cable of 33 kV ac between the North Sea platform Beatrice and the shore was around Ł100 per metre.
Cost of installation of submarine cables:
The final cost of laying cable highly depends of the location (nature of sea-bed and depth) and is at present between Ł2 million to Ł5 million per cable kilometre.
We can apply the same economical criterion than for the grid-connection of an oil rig or a gas platform which estimates that a distance longer than 50 km is unlikely to be viable.

Criteria of feasibility

If the electrical prodution can justify the high cost of the connection to the shore, other criteria have to be looked at:

Due to the risk of anchor damage, the cable route must not pass through a busy shipping lane or anchorage,
Due to the risk of dredging up by trawl nets or gear, the cable route must not pass through popular fishing grounds,
For maintenance purposes, the cable route must be through reasonably shelterd waters,
The sea bed must be suitable for burial of the cable as a means of protection. Drifting sands, solid rock or strong tidal currents would have to be avoided.

HVDC or AC

Three phase AC transmission is feasible for moderate distances (around 100 km but depending of the site) and power levels. The feasibility of AC transmission is limited by the reactive power generated in high voltage AC cables .HVDC transmission is feasible for longer distances and large amounts of power therefore for lower levels of power and distances, the cost of the converter stations needed in each end of the transmission will make this option too costly. The converter station also consumes large amounts of reactive power that have to be produced at the platform.
HVDC Light is a new and promising technology for transmission of power to and from offshore installations. The amount of power and acceptable distance to shore is higher than for AC transmissions. The converter stations at each end is more compact, and there is no need for support of reactive power at each end of the transmission. The HVDC Light technology consists of voltage source converters located at the terminal stations. The connection to shore is through two extruded DC cables.

Environmental aspect

During the installation of a submarine cable there will be changes in the sea-bed can affect the environment and marine life.

D.C cables can induce magnetic compass errors, especially when cables are laid on a North-South axis.

Conclusion

The electric transmission from an offshore installation to the shore is very expensive and it is more economical to transmit a high amount of power.
This type of transmission would also require electrical devices on the platforms and the coastline to meet the technical obligations needed to supply the grid. These devices would add another extra cost.

Long distance transmission would need HVDC technology and would require a large area on the platform to site the converter.

Taking account all these points, we decided that electric transmission could not be applied to all situations of re-using offshore platform as renewable energy centre and we looked at other possibilities of transmission from the platform or storage on the platform.