Long-Offset Subsea Production Control Systems, Which Facilitate "Subsea-to-Beach" Field Developments
Authors: Douglas N. , Hutchings V. 
Published: December 2007

The Snøhvit project in the Barents Sea is a milestone in offshore oil and gas development, with a record breaking offset and all subsea facilities tied-back to shore, without any surface-piercing structures such as platform or floating production facilities, with a consequent dramatic reduction in environmental impact.

Clearly the use of a subsea development with remote control from shore is a cost-effective solution that contributes to ensuring safe shipping routes and in line with Norwegian practice.

The key elements of technology required to implement such a new development with a subsea Production Controls System operating over a distance of 200km are:- Long-distance optical communications without the use of repeaters3-phase 3kV electric power transmission over 200+km from shoreSubsea data communication distribution "routers"Local data communication using "high-speed" communication superimposed on power linesUse of a "piggy-back" comms-on-power signalling system for back-up communication from shore

Details of the field development will be described.

From a controls perspective, the most significant aspect is, of course, the provision of high control availability over an extremely long distance. Long-range communication without repeaters and relatively high data requirements dictate that a fibre-optic link is used as the principal means of communication from the subsea location to shore. The long range also led to adoption of a high voltage, 3-phase power transmission system operating at 3 kV.

The challenges of long-range, electrical power supply are addressed, such as the copper content of the umbilical being minimised, and the need to produce a subsea transformer of a size such that it can be conveniently packaged in standard retrievable subsea modules. A back-up communication, low bit-rate data channel is superimposed on the 3kV, 3-phase link. A subsea power and communication distribution module and an intelligent communication router, to de-code and manage the various data communication paths are described.

The control system design for the Snøhvit project has addressed and overcome some difficult technical challenges and represents a significant advance for long offset control.

A number of technologies have been combined to provide the necessary safe and reliable, long offset control system. These are illustrated in the figure.

Fibre-optic communications modems have been developed together with a 3-phase, 3kV power delivery system with back-up communication superimposed on the power lines. In addition, a local high-speed, communication- on-power system is used to provide communication and power to the subsea wells and a communication router implements the communication switching and prioritisation. We believe that these technologies provide a significant stepping-stone for other subsea-to-beach projects in the future, as well as coincidentally providing virtual data channels from surface to subsea, which provide the potential for incorporating the IWIS standards emerging in the industry.