Unconventional Gas-Dehydration-System Failure Resulting in a Gas-Hydrate Blockage

Abstract

Summary In this paper, we present findings of how liquid-hydrocarbon carry-over from production separators could form solid contaminants that later became a root cause for an unexpected failure of the triethylene glycol (TEG) gas-dehydration unit to deliver dry export gas. This incident was later followed by an unanticipated capillary-aided gas-hydrate blockage inside the gas-export line during a planned-maintenance shutdown. After a detailed operational review before and after a gas-hydrate blockage incident, all sections of the TEG-dehydration tower unit were opened and inspected for (1) any presence of solid contamination and blockages and (2) an evaluation of the presence of potential equipment damage inside the dehydration unit. A compositional analysis with a gas-chromatography/flame-ionization detector (GC/FID), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectrometry (EDS) was conducted on samples collected from the unit to understand its nature. Finally, using a customized gas-hydrate rocking-cell apparatus, gas-hydrate growth was observed under an 8°C subcooling temperature. From vessel inspection, it was found that a liquid-hydrocarbon carry-over results in the accumulation of high-chain hydrocarbon deposits (C19–C36) with some mineral scales inside the unit, even when filtration units are present. These deposits form when liquid carry-over passes through the reboiler unit, which gradually blocks the TEG-distribution spray nozzles, and reduces the efficiency of gas dehydration significantly. Later, with only 20.5% of the spray nozzles in working condition, the differential pressure (dP) across the distribution bar had exceeded its maximal operating limit, causing the distributor bar (VRGF) to split open along the weldment area. Hence, this resulted in total functional failure to continuously deliver dry gas and the formation of gas-hydrate blockage inside the gas-export pipeline. On the basis of the detailed operational review, field testing, compositional analysis, and hydrate-growth observations, the sequence of events that caused a catastrophic capillary-aided gas-hydrate blockage under a shut-in condition even in the presence of a hydrate-risk-management program is presented in this paper.