Summary
In gas applications with high-pressure drops and/or high-velocity gas flow, acoustic-induced vibration (AIV) occurs when very high sound levels are propagated in piping. If the sound levels are high enough, they will excite the circumferential vibration modes of the pipe. This allows the pipe to vibrate in and out as the sound wave moves through the piping. Most often this occurs downstream of pressure-reducing valves or safety-relief valves, but AIV can occur at any location where the pressure drop or mass flow rate is sufficiently large. The level of pipe wall vibration is so high that it can induce fatigue cracking of the pipe and can potentially cause catastrophic pipe failure. The West Delta Deep Marine (WDDM) concession onshore of the slug catcher M9001 had a recent failure in the flare header as result of accumulating AIV effect. As a consequence of this failure Enppi/Burullus and Foster Wheeler have undertaken an engineering assessment study to investigate the risk of AIV for the plant and determine the corrective and engineering solutions to protect the plant from any failure that may occur as a result of AIV. This paper will present a case study as reference for gas-production facilities to consider the AIV application in regard to the design phase, and what might have prevented fatigue of pipes during the operation phase.
Introduction
The Burullus Gas Company (Burullus) has developed the Scarab/ Saffron, Simian/Sienna and Sapphire, and Sequoia and Serpent fields within the WDDM concession and plans to develop further fields there as well. The concession is located offshore Egypt in the eastern Mediterranean.
The WDDM development consists of a number of subsea wells, all of which have differing compositions. The product of these fields feeds three separate subsea tieback pipelines which deliver the three-phase fluid (gas, gas condensate, and water) to the onshore facilities for processing by three separate slug catchers (M-9000, 9001, and 9002).
During a controlled blowdown of the WDDM gas plant, fatigue failure has occurred on the flare header of a 20-in.×16-in. unreinforced branch of the Slug Catcher 9001, as shown in Fig. 1.
A failure of the flare header is downstream of the blowdown valve, and associated restriction orifices during a blowdown are initiated in response to a uninterruptable-power-supply failure.