Improved Signal-On Power Technique
Authors: Blankenship H.C. , Marosko R.J. 
Published: October 2008

Communications with the electronics modules required for production control at subsea well sites have generally used separate power and signal conductors in the umbilical. This paper describes a signal-on-power technique that can be used to operate single or multiple subsea electronics modules at offset distances of 30 miles using a single, 2-conductor umbilical to carry both signal and power at higher power levels than has previously been achieved.

INTRODUCTION

Earlier subsea production control applications have used signal on power in a limited way. One of the earliest was the 1985 Eniepsa Casablanca 11 development offshore Spain. The project employed a data acquisition system built by Hughes offshore that combined a data signal (Bell 202) and a DC power signal on a twisted pair. This system communicated well data over an offset distance of 7.4 km (4.6 miles). Since the tree control was powered by piloted hydraulics, no control functions were supported through the electrical umbilical.

In 1986, the ONGC Bombay High project offshore India employed a data acquisition system supplied by the OCS Group of Vetco Gray. It combined a 7–10 KHz data signal on 60 Hz power over a coaxial cable. The offset distance was less than 10 km (2 miles). Four subsea electronics modules (one per well) were supported by each umbilical. Tree production functions were controlled by direct hydraulics, again leaving control functions out of the electrical umbilical.

For the Troll Oseberg Gas Injection (TOGl) development in the North Sea in 1989, Vetco Gray combined an 1800 Hz PSK data signal and 60 Hz power on a quad cable. The system operated subsea control functions and data readbacks over an offset distance of 50 km (30 miles). In this system, the subsea electronics module supported both data acquisition and production control functions over a single pair of wires.

DEVELOPMENT AND TEST PROGRAM

The present program extends earlier signal-on-power techniques in several ways:It increases the number of subsea electronics modules that can be supported by each umbilical.It increases system power capability.It operates both the control functions and the data acquisition systems over a single pair of wires.

The concept of signal-on-power employs frequency division multiplexing of power and signal components. Fundamentally, power at one frequency (nominally, 60 Hz) and signal at another (1700 Hz) are transmitted simultaneously on the same pair of wires. The W frequencies are separated by frequency allocation. Circuits in the surface equipment that combine the power and signal frequencies are designated as "combiners". The subsea electronics module (SEM) must accept the combined power and signal envelope and separate one component from the other. Circuits in the SEM that separate power and signal are designated as "splitters". Within each SEM, a splitter demodulates the multiplexed power/signal, permitting normal operation of each SEM.

Since this system must communicate from the surface equipment to the subsea equipment and vice versa, the communications sections of the combiner and splitter must be bidirectional. See Figure 1.