This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18613, "An Alternative Approach to Wellhead Flowline-Pressure Protection," by Angela E. Summers and Bryan Zachary, SIS-Tech Solutions, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May.
For many years, pipe-specification standards have required that piping downstream of the wellhead be adequate to sustain a full wellhead shut-in. This inherently safe design practice ensured that the flowline pipe was specified with a maximum allowable working pressure (MAWP) equal to or greater than the maximum pressure expected to be produced by the well. This practice has proved to provide adequate protection in thousands of wellhead installations throughout the world. Inherently safe practice has been challenged when retrofitting electrical submersible pumps (ESPs). In such cases, the maximum discharge pressure under block-in conditions is greater than the MAWP of existing flowline pipe. A safe alternative to replacing the pipe is the use of a high-integrity protective system (HIPS) designed and managed as a safety-instrumented system (SIS) with a Safety Integrity Level (SIL) of 3.
When the price of crude oil rose above USD 50/bbl, oil-producing companies pursued efforts to increase or renew production in existing oil wells. One solution, especially in ultradeep (4,000-ft water depth) subsea applications, is to boost well pressure. A major challenge in developing these deepwater solutions is the MAWP limitation of the downstream flowline pipe. An HIPS can close block valves placed in front of the low-specification pipe, protecting the lower-rated downstream piping.
The introduction of high-pressure ESPs in many offshore fields has increased the maximum shut-in pressure beyond the MAWP of the downstream flowline pipe. The design of the instrumented-system response is complicated by the need to respond quickly. Upon shut-in, the ESP can produce pressure sufficient to challenge the mechanical integrity of the flowline very quickly. The pressure rise is related to the compressibility of the material, but the pressure rise generally is rapid, resulting in a very-short process safety time (e.g., in liquid service, the time may be less than 5 seconds).
In typical onshore process-industry applications, an overpressure hazard such as that encountered by the ESP-well flowline would be addressed by a pressure-relief device (PRD) and a disposal system, such as a flare or scrubber to safely process the material relieved during an overpressure event. When PRDs are installed in conventional flowline designs, they typically are sized to mitigate only thermal expansion of the material when the flowline is blocked-in. For HIPS flowline designs, the PRD sizing also should consider possible block-valve leakage.
A safe and reliable alternative to replacing the existing wellhead flowlines relies on the use of an HIPS to detect unacceptable operating pressure and initiate closure of block valves. An HIPS is a special class of SIS.
