The Use of Optical Fiber in Safety-System Design
Authors: Kugler B.A. 
Published: December 2007

This paper summarizes the design, installation, and performance of an optical-fiber safety system installed for testing purposes on a production platform in the Gulf of Mexico. This system was installed on a production platform in the Gulf of Mexico. This system was installed on a chemelectric treater to monitor five end devices and to determine the feasibility of using fiber optics rather than pneumatics or an electrical system in a harsh environment. Included with a performance summary is a brief history of optical fiber and the potential for these types of safety systems in oilfield applications.

Introduction

Efficient, reliable, and economical safety systems are an intrinsic part of production operations on offshore platforms. These systems part of production operations on offshore platforms. These systems prevent pollution, avert damage to expensive equipment, and are prevent pollution, avert damage to expensive equipment, and are directly responsible for the safety of field personnel. The majority of safety systems in use are pneumatic, but an increasing number of electrical configurations are being used. Both have proved to be efficient and reliable. The purpose of using an optical fiber rather than a conventional safety system was to take advantage of the unique physical characteristics of the optical-fiber cable and to determine whether future installations could be reliable, more efficient, and less expensive. This would be determined at a minimum cost with the greatest degree of safety. There were two primary reasons optical fiber was used as a test medium. First, the size and flexibility of the cable offered a significant reduction in installation time and cost vs. bending 3/8-in. [0.953-cm] stainless-steel pneumatic tubing. Second, when compared with electrical wiring, this system provided a completely safe transmission medium in a potentially hazardous environment. The testing and objectives were accomplished by installing the optical fiber on a single vessel and by providing 100% backup with a pneumatic safety system. To determine reliability, a group of "first-out" indicator lights were used to determine whether the optical fiber or pneumatic system enacted the shut-in.

History

Fiber optics was first introduced in 1966 when the use of glass fiber as a viable transmission medium was discovered. Today, the total world investment in fiber optics is estimated to be on the order of $300 million per year (corresponding to more than 62,000 miles [100,000 km] of installed fiber) with an annual growth rate of about 40%. This growths can be directly attributed to the unique physical characteristics of the fiber vs. conventional transmission media. These include the ability to transfer more information, freedom from electromagnetic interference, and the inherent safety features of being nonelectric and nonsparking. Today, optical fiber is used primarily in the telecommunication and military industries. Because of optical fiber's advantages, however, increasing numbers of fiber-optic control systems are being introduced into the process-control business. Optical fibers are being manufactured in a variety of materials at different costs that depend on the type of service required. Lengths of transmission, size limitations, and the environment in which cables are installed are important criteria that determine the type of cable used in various applications. Glass, plastic, and plastic-clad silica (PCS) fibers are the three primary source materials used in fiber manufacturing. Glass is generally the most efficient material used for small-diameter fibers and long transmission requirements, while plastic is generally more rugged and economical in less-demanding applications where transmission distances are shorter. The PCS core fibers perform somewhere intermediate to glass and plastic. Light is transmitted through the fiber in a zig-zagging fashion down the length of the core by total internal reflection off the cladding material. The light propagation and analysis of the actual rays within a given material are beyond the scope of this paper and are not discussed here. Attenuation in a fiber is the loss of power occurring because light pulses lose some of their photons during transit, reducing their amplitude. Attenuation can range from 1 dB/3,280 ft [1 db/km] for premium small-core glass fibers to more than 2,000 dB/3,280 ft [2000 dB/km] for large-core plastic fibers. The attenuation within fibers results from two causes: absorption and scattering. Absorption occurs when impurities in the glass absorb light energy, fuming photons to heat. Scattering results from imperfections in the basic photons to heat. Scattering results from imperfections in the basic fiber structure when unintentional variations in density and fiber geometry occur during fiber manufacturing and cabling. The angle of incidence of rays striking such variations at the core-to-cladding interface changes enough that some rays are refracted onto new paths not subject to total internal reflection, resulting in inefficient paths not subject to total internal reflection, resulting in inefficient light transmission.

Test Configuration

The fiber-optic safety system was installed 40 miles [64 km] off the Louisiana coast on the Ship Shoal 182 "E" platform and was designed to monitor five safety devices on a 10,000-B/D [1600-m3/d] chemelectric treater. These included a pressure safety high (PSH), pressure safety low (PSL), level safety high (LSH), and two level safety lows (LSL's). This system was designed as is shown in Fig. 1 and operated as follows: A 24-V power source was supplied to five transceiver devices within an explosion-proof box and mounted 6 ft [1.8 m] from the platform master panel. Each transceiver in turn generated a light signal, which was directed into the optical fiber and transmitted about 150 ft [45.7 m] to each safety end device. After passing through a pneumatic pressure-switch transducer located on passing through a pneumatic pressure-switch transducer located on each end device, the light beam returned to the transceiver, completing a loop. When any upset occurred, pressure was bled off at the end device as in any pneumatic system and the pneumatic/optical transducer would activate, closing a gate and blocking the flow of light detected by the transceiver. The transceiver in turn sent an electrical signal to a first-out indicator (blinking light) and controller, which de-energized a solenoid valve and pneumatically tripped the master panel to give a platform shut-in. panel to give a platform shut-in. Although testing a 100% optical system rather than using pneumatic/optical transducers was preferred, proven optical end devices pneumatic/optical transducers was preferred, proven optical end devices were not found at the time of installation.

JPT

P. 906