ABSTRACT

In 2005, a complete liquids gathering system was installed at a natural gas field to remove liquids from the wells. These liquids, which primarily consist of water, are separated at the wells and sent to three Central Delivery Points (CDPs) via individual pipelines. The liquids gathered at the CDPs are further separated and then pumped via variable speed triplex positive displacement pumps to a stabilizer plant several miles downstream. The original, pre-existing, liquids gathering system utilized a 4-inch composite flexible pipe from the wells to the CDPs and from the CDPs to the stabilizer plant. Failures began to occur near the Northern most CDP where the liquids were being pumped over a hill. As the number of failures increased in 2007, the decision was made to replace the composite pipeline with an 8-inch steel pipeline with a pull through poly liner. A transient analysis was requested to determine if water hammer was a factor in the composite pipeline failures and to perform a transient analysis of the new, existing, 8-inch steel pipeline for potential problems. A case study of the modeling and simulation effort related transient analysis of the liquids gathering system including modeling the triplex pumps is presented. Each phase of the effort is discussed including the field study and data collection, model development, simulation, and analysis. In addition, the findings are presented as well as a discussion of several problems and discrepancies that arose during the analysis effort and how those issues were resolved.

INTRODUCTION

An oil and gas company (O&GC) is producing numerous natural gas wells in the United States. The wells produce sweet gas along with liquid byproducts associated with the gas production. These byproducts, in the form of water and condensates, are separated at the well and sent along separate pipelines to Central Delivery Points (CDPs). At the CDPs, the liquids are separated, some of the overhead gas is flashed off, and the remaining fluid is pumped to a stabilizer plant. Well pressures vary, but are typically between 300 to 350 psig and the discharge of liquids from CDP pumps is on the order of 500 psig. A flexible composite 4-inch pipeline which carried the liquids from CDP pumps to the stabilizer plant began to fail in late 2005 continuing into 2007. Initially, many of the failures were attributed to incorrect installation or other environmental issues. As the number of failures increased, O&GC began to look for potential causes of the pipeline failures. An investigation into several of the failures provided no clear determination as to the cause. In addition, flexible composite pipe is also used to carry liquids from the wells to the CDPs and, to date, has not suffered failures like those seen on the discharge lines. The 4-inch flexible pipe was replaced in 2007 by an 8-inch steel pipeline with a pull-through poly liner and no further failures have occurred.

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