Safety Restraint System for High-Pressure Oilfield Piping

Abstract Conveying energized fluid through piping presents a significant hazard with the potential for explosion in the event of a component failure. A method is presented for a simple restraint system that limits the movement of piping to a contained area. Basic practical design equations and considerations useful for piping constraints are presented. Introduction: The safe constraint of piping used to convey highly energized fluid is essential for safety, but has proved to be a difficult task to accomplish. This has been especially true for temporary well service applications where piping must be light enough to assemble and remove quickly, and does not usually have the benefit of permanent anchors at the worksite. Energized fluids, such as pressurized nitrogen or C02, have high explosive potential due to their stored energy. While pressurized liquids present their own kind of danger, a compressed gas has enormous potential for damage if freed rapidly. If a piping assembly fails, the initial forces released are those of the static pressure. Compounding the problem are the acceleration forces created as the stored medium continues to evacuate the piping. Both forces continue as long as any pressurized fluid remains in the system. The acceleration forces are responsible for most of the danger by causing the piping to move violently, placing personnel and equipment at risk. Testing has shown that constraints with limited elasticity, such as wire rope, perform poorly in high pressure situations. The lack of a constraint's ability to dissipate the discharge energy is a reflection of not only its strength, but also its toughness. The solution to this problem is a flowline restraint system (FSR), developed by us, that manages to contain the damaged piping within a limited zone of danger. The FSR system consists of members of tough synthetic loops that are strung the length of the piping and held in place by smaller loops attached at key points along the piping. This assembly is terminated at each end by anchoring to a substantial member such as the wellhead or a pumping truck for land based operations. Suitable structural tie-downs can be substituted for the truck and wellhead in offshore operations. Supplementary restraining loops are normally provided to further minimize flowline movement. Theory and definitions: The rapid discharge of energized fluid, such as pressurized nitrogen, will occur if a portion of the conduit piping fails. J.L. Stromberg and J.B. Surjaatmadja1 found that the forces applied to damaging the piping are proportional to the energy of the discharging fluid. They deduced that the steady state force of ejecting gas supplied by an infinite reservoir can be described by the following equation. (Mathematical equation available in full paper) The first set of terms on the right of the above equation represents the forces generated by the fluid acceleration out of the pipe. The second represents the static pressure force generated over the cross sectional area of the pipe at the exit.