Application of High-Speed Modem Technology to Offshore Communication and Control
Authors: Sills J.A. , Blankenship H.C. , Brown B.C. , Fenske R.G. 
Published: October 2008

This paper investigates the problem of applying high-speed modem technology to sub sea production systems. We begin bypresenting an overview of basic sub sea control requirements, signaling concepts, and channel characteristics. This includes a discussion of quadrate amplitude modulation (QAM) and the effects of inter symbol interference (ISI). Next we examine the specific functionality required to implement a high-speed modemfor a dispersive channel. We present a general hardware architecture based on a programmable digital signal processor (DSP) solution. A case study is presented applying modern DSP and communication technology to a multipoint communication network that is used for monitoring and controlling sub sea equipment. This presentation highlights the advantages gained by utilizing modem communication techniques such as equalization. Laboratory tests demonstrate that an order of magnitude improvement in data rates can be sustained on channels up to 60 miles in length.

Introduction

As control systems for sub sea oil production become more complex to support new applications, increased data, and larger multi-well networks, high-speed programmable communications become a necessity. In the past, dispersive channels have limited modems to data rates of 1200 bits per second (bps) or less. R.W. Lucky first reported in 1965 that improved data rates can be achieved using channel equalization techniques. Data rates have steadily increased as equalization technology continues to be refined2. In 1976, Ungerboeck3,4 found that substantial gains could further be achieved through a coding technique now known as trellis coded modulation (TCM). The dominant area of application for these advanced techniques has been to the voice grade telephone channel, because the narrow channel bandwidth (3 kHz) has permitted relatively sophisticated signal processing to be implemented in a cost-effective manner using commercial grade chip sets. In recent years, clock speeds on commercial grade DSP chips have improved to the point that data rates on phone-line channels have neared the SyQuest At the same time, industrial and military grade DSPs have improved to the point that high-speed modem techniques are now viable for industrial and military applications.

In this paper we investigate the application of high-speed modem techniques to the sub sea environment. We begin with a brief introduction to the sub sea application, and then discuss recent market and technology trends that improve the viability of using high-speed modems in the sub sea application. Next, we discuss quadrate amplitude modulation (QAM) and the effects of inter symbol interference (ISI) and how equalization mitigates this interference. A brief overview of signal processing hardware will lead to a case study highlighting the paper's concepts.

Sub sea Application

The sub sea application is characterized by a multipoint master slave network in which a central control unit (CCU) serves as the network master, and a suite of sub sea electronic modules (SEM)serve as slaves. The master-slave configuration is shown in Figure 1. In many applications, the communication signaling is performed on the same line that supplies power to the sub seaequipment.

All command and query messages originate from the CCU and are broadcast to all SEMs present on the line. Each SEM is addressable so that these messages can be directed to a particularSEM. In response to a CCU message, the addressed SEM always transmits an acknowledgment.