Subsea Fibre Optic Communications for Production Control and Data Acquisition.
Authors: High G. , Wright P.J. 
Published: October 2008

The trend to reduce topside facilities and the frequent use of subsea completions over extended step-out distances has increased the volume and complexity of subsea date communications beyond the capacity of conventional systems. Improved reservoir management using intelligent sensors, metering and process equipment, requiring real time monitoring and control, dictates the use of widebandcommunication. Fibre optics offers the necessary volume of data transmission, with the high noise immunity needed for data integrity and safety, for the severe EMI environments created where motors and power cables are used.

The system uses conventional optical fibre conductors with 1550 nm laser generated optical pulses as the information carrier. Data rates of 2.0 Mbs−1 are achievable, unrepeated, over distances in excess of 100 Km, with extremely low error rates. Equipment and installation costs will be comparable with current hard-wired technology, demonstrating that single mode optical communication is a technically and commercially feasible, and reliable, alternative to existing electrical systems.

Introduction

The high cost and hazardous nature of recovering hydrocarbons offshore, have led to the trend to reduce topsides facilities, and to the growth in subsea production control. Subsea control capabilities have been considerably enhanced by the recent generation of microprocessor based systems. The latest control facilities enable the provision of a subsea interface for advancedtechnology installations, These include artificial reservoir lift systems, multiphase flow measurement, down hole monitoring and intelligent sensors, as well as the conventional tree control functions. However theseadvanced technologies impose increased data processing and communication overheads in the form of closed loop motor control, local data processing multi channel communications, downloading of calibration software, and intelligent diagnostics.

This expanded functionality has created a need for increased data transfer. In particular the controls umbilical must now provide a data highway capable of supporting communications between topsides control systems and a network of Subsea Control Modules (SCM's), and their extended control functions. Traditionally this has been implemented using either a twisted pair or coaxial conductor, or by utilising the power conductors(power and signal combined). The available data transmission rates have been necessarily low, often of the order of a few hundred bits to reduce errors, maintain reliability and provide an economical solution. This transmission performance is no longer sufficient for the inevitable requirements of the offshore industry, A high bandwidth communication link is now a critical enabling technology for the future of subsea control.

The move towards placing high power electrical equipment subsea, has created a need for a communications system with greater immunity to Electro Magnetic Interference (EMI). The maximum impact of power transmission on a subsea communication system will be encountered where subsea produced fluid boost facilities such as electrically powered multiphase pumps are used. These high power, variable frequency motor drives, generate unacceptably high levels of EMI for conventional screened data conductors, Optical fibres offering inherently high noise immunity and improved attenuation characteristics over conventional conductors are suitable for such applications.