Abstract

The Matterhorn field, operated by TOTAL, is located in the Gulf of Mexico, on block 243 of the Mississippi Canyon. It produces oil and gas from seven wells in a water depth of approximately 2800 ft.

This paper reviews a hydrate blockage incident which occurred on the Gas Export Pipeline (GEP) of the Matterhorn platform. A comprehensive flow assurance study (fluids characterization, steady state and transient flow simulations) was conducted in order to understand the causes of the incident and to develop prevention, mitigation and remediation strategies. The study described herein gives a methodology to be used in the design and operation of gas pipelines, principally in the deep water subsea environment, from a hydrate management perspective.

The GEP was operated in the hydrate-stable and two-phase region, but still within industry standards in terms of dryness (dewpoint). The study shows that a temporary deviation from the normal operating conditions lead to a gradual water build-up, resulting in hydrates formation and eventually causing a total obstruction. The transient simulations performed with OLGA® were able to predict the condensate and water accumulation rate and holdups along the GEP.

The incident described and the subsequent flow assurance study highlight the potential risk of hydrates formation in " dry?? gas pipelines in deep water subsea environment. In subsea gas export lines, the current dryness (dewpoint) contractual requirements agreed upon by the operators are believed to provide safe operating conditions with regard to hydrates. The present work demonstrates that hydrates formation risks remain even when operating within the industry requirements. The remediation strategy adopted after the hydrate blockage, and the prevention measures applied since, provide useful guidelines for the management of hydrates in gas pipelines.

Introduction

With oil and gas developments moving to ever deeper subsea environments, hydrates-related issues become a growing concern for the industry. Hydrates blockages, beyond the safety risk they represent, result in time-consuming and costly mitigation and remediation operations. Most hydrates experiences found in the literature, and the corresponding prevention and remediation strategies, concern multiphase pipelines. This paper relates a hydrates blockage incident which occurred in January 2007 on a deepwater dry gas pipeline. A detailed review of the events and subsequent remediation actions is given, along with an analysis, including a PVT study and transient multiphase simulations.

The lessons learned from this experience are presented here and are currently being used as a basis for improving operation guidelines and hydrates prevention strategies for dry gas deepwater pipelines.

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