A New Approach for Finger Storage Slug Catcher Design

Abstract

ABSTRACT Two options are available for separating the gas liquid mixture at the exit of a two phase flow pipeline operating under slug flow conditions. These are a traditional vessel type separator and a finger storage type slug catcher. Use of a vessel separator is usually due to space limitations that exist, for example, on offshore platforms. Fingerstorage slug catchers are the obvious choice for long, large diameter pipes, especially those which undergo pigging. They are more cost effective and more simple to construct and operate. In the past, sizing of finger storage slug catchers were based primarily on experience andrules of thumb. Not surprisingly, most of the existing slug catchers have been oversized. Withthe recent trend of using a subsea compact finger storage slug catcher upstream of the platform riser, the need for more accurate design methods is even more crucial. This paper presents a new, innovative approach for the prediction of the required dimensions of slug catcher fingers. The approach is based on the effect of the finger pipe diameter and inclination angle on the transition boundary between slug flow and stratified flow. Predictionof the slug characteristics at the slug catcherinlet, under normal flow or pigging conditions, are incorporated. Based on the new approach, the required length and optimal downward inclination angle of the fingers can be determined. The new approach has been used to design a finger storage slug catcher for actual field conditions. The effect of operational conditions, e.g. pipeline diameter and finger inclination angle on the required slug catcher dimensions isdemonstrated. INTRODUCTION Pipelines can be operated under two phase flow conditions for several reasons. In hostile environments such as arctic and offshore fields, oil and gas are often transported in a singlepipeline to reduce the construction cost. In natural gas transportation, due to pressure andtemperature drop during flow in the pipeline, condensation causing two phase flow may occur. Depending upon the operating conditions, normal or terrain induced slug flow, may develop. Also, artificial slugs, possibly the largest ones, can be created during the removal of accumulated liquid by a pigging (sphering) operation in gas pipelines. It is a common practice to install a slug catcher to accommodate liquid slugs at the exit of a pipeline. A slug catcher can serve as both a separator and as temporary storage. There are several unconventional slug catcher types, such as a project slug catcher1, a self supporting fluid separator2, and a flexible subsea slug catcher3. However, the vessel and finger storage types of slug catchers are the most widely used in the petroleum industry. Use of traditional vessel type separators as slug catchers are mainly dictated by space limitation and relatively small slug sizes. A number of studies have been conducted to design such catchers4 5,6,7,8,9,1O. In references 4, 5, and 6 the acceleration of the slugs during their production into the catcher and resultant loadson bends, fittings and slug catcher internals have been investigated for a specific slug catcher.