GOM Export Gas Pipeline, Hydrate Plug Detection and Removal
Authors: Kashou Sam , Subramanian Sivakumar , Matthews Patrick , Thummel Lawrence , Faucheaux Eric , Subik Daniel , Qualls David , Akey Rollin , Carter Jerry 
Published: October 2008

Hydrate blockage detection, remediation and removal are successfully applied in a hydrate plug incident in the GOM. A hydrate plug occurred in mid-October 2002 in the Williams Field Services (WFS) owned and operated, 36-mile long, 10- inch gas export pipeline from ChevronTexaco's Genesis Green Canyon 205 (GC 205) platform (2,600 ft water depth) to the downstream Ship Shoal 354 platform (SS 354) platform (460 ft water depth). ChevronTexaco's Flow Assurance experts, ChevronTexaco's Genesis Asset Team, and WFS team worked together to detect, locate and remediate the hydrate plug in a safe and timely fashion while ensuring personnel safety, maintaining pipeline and riser integrity and minimizing loss of production. This paper shows how a concentrated team effort and deployment of the right tools such as transient models, hydrate formation and dissociation models, can be used to predict and alleviate a flow assurance problem, such as hydrates, which can occur in deepwater oil and gas production in a subsea system.

Introduction

The Genesis development1 was brought on stream in February 1, 1999. In mid-October 2002, following two major shutdowns due to strong hurricanes and one for WFS planned maintenance, gas flow into the dry gas export pipeline to SS 354 was obstructed and gas delivery to shore was halted. Analysis of the measured hydraulic data prior to and during the incident showed that the obstruction in the gas pipeline was due to the presence of hydrates corresponding to low spots in the pipeline. Operations personnel in New Orleans, working in conjunction with Flow Assurance personnel in Houston were able to locate the hydrate plug via hydraulic and hydrate models based on the measured pressure, temperature and gas flow rate during the incident. Hydrate formation caused by the presence of water and natural gas components at high pressures and low temperatures in a deep-water subsea pipeline can occur during shutdown or start-up of a pipeline as was the case with Genesis. Under normal flowing conditions, dehydrated gas flows through the uninsulated Genesis export gas pipeline. Although pressures in the line were high (~1,600 psig at inlet) and temperatures as low as 42 °F in the coldest sections of the pipeline, hydrate formation was not expected due to the low water content of gas from dehydration at GC 205. However, in the presence of sufficient water, pressures and temperatures of the flowing stream in the pipeline were conducive to hydrate formation. Since water content of gas entering the pipeline was routinely monitored below hydrate formation content while flowing, hydrates should not have formed in the pipeline during steady state flow.

When hydrates did form in the pipeline, many factors may have contributed to that formation such as the multiple shutdowns within a few days of each other and the added complexity in the procedures of shutting the platform down and starting it back up. During these back to back platform shut downs and start-ups, if any water had entered into the pipeline due to non-optimal dehydrator performance, the potential for hydrate formation would have been be high.

Overview

Genesis is ChevronTexaco's first deepwater oil and natural gas drilling and production facility located in 2,600 ft of water in the Gulf of Mexico, 150 miles south of New Orleans. The field is operated by ChevronTexaco with 56.67% working interest, and partner ExxonMobil with 38.38% working interest and PetroFina Delaware, Incorporated, with 4.95% working interest.