Explosives Safety: Safety Strategies for Operating Electroexplosive Devices in a Radio-Frequency Environment

Abstract

Transmitters operating in the radio-frequency (RF) spectrum create the potential for accidental detonation of electroexplosive devices (EEDs). This risk is magnified with the widespread use of EEDs in oil- and gas-well perforating services, the potential for an EED to be ballistically connected to other explosives, and the proliferation of radio transmitters. To prevent accidental detonations while using an EED in such an environment, one or more safety strategies are necessary. At least three different strategies—radio silence, site risk analysis, and RF-immune EED—have been used over the past 50 years. All three, when properly implemented, are successful in improving safety during EED use, but each strategy has the potential for disaster when not fully implemented.

This paper reviews the RF environment and the potential for accidental detonation of an EED caused by RF energy. It describes the problems and risks associated with the strategies of radio silence, site risk analysis, and RF-immune EEDs, and explains how the key elements of each strategy address these issues.

Introduction

Detonating the explosives used in an oil- and gas-well perforating gun is routinely started by detonating an EED, commonly referred to as a blasting cap or detonator. Through the application of electrical energy, the EED is detonated, and this detonation is used to initiate the other explosives in the perforating gun. Uncontrolled sources of electrical energy, such as radio-frequency transmissions, have the potential to cause an accidental detonation in most EEDs used in oil- and gas-well perforating guns.

The user of an EED must control RF energy if an accidental detonation is to be prevented. Companies providing perforating services typically use one of three safety strategies to prevent accidental detonations: RF silence, site risk analysis and the use of RF-immune EEDs. RF silence and site risk analysis rely on procedural controls to be effective. The use of an RF-immune EED relies on the design of the EED, or engineering controls, for effectiveness. Each strategy can provide effective control of RF energy and prevent accidental detonations if properly implemented, but provide only limited protection if not implemented properly.

EEDs

The majority of perforating guns used today have three basic explosive components: an EED, detonating cord and shaped charges. Electroexplosive devices are designed to be initiated by electrical energy and generate enough energy to detonate the detonating cord and shaped charges, which are ballistically connected to the EED.

There are different types of EEDs, and these can be organized by the power required to detonate them and whether they use a primary high explosive such as lead azide. Lowpower EEDs can detonate with power levels as low as 40 mW. High-power, intrinsically safe EEDs require unique power sources with voltages ranging from several hundred to several thousand volts to detonate. Expanding on the EED categories (Categories 1 through 4) found in Reference 1, I categorize EED as follows:

• Category 1. A low-power EED using bridge wire and a primary high explosive. These EED do not have additional resistance added for safety.

• Category 2. A low-power EED using bridge wire and a primary high explosive but with additional resistance (approximately 50 ohm) added to increase safety.

• Category 3. Either 1) a low-power EED using bridge wire and a primary high explosive but with an electronic circuit used to increase safety, or 2) an EED without a primary high explosive requiring a few watts to detonate (e.g., a semiconductor bridge).