Deepwater Natural Gas Pipeline Hydrate Blockage Caused by a Seawater Leak Test
Authors: Hatton Gregory J. , Pulici Massimo , Curti Gianbattista , Mansueto Massimiliano , Kruka Veet R. 
Published: October 2008

Natural gas transmission pipelines are being laid at water depths in excess of 2,000 meters (6,562 ft). At these water depths, small pipewall penetrations can provide a path for water to enter the pipeline, form hydrates, and block gas flow. This paper discusses a test in which water was "leaked" into a 200-bar (2901-psi) "pipeline" and the resulting hydrate blockage was safely and quickly cleared by pressure reduction on one side of the blockage. The formed blockage contained gas pockets and the measured blockage permeability was low " indicating that reducing the pressure throughout a line, including blockage-trapped gas pockets, can take a long time.

Executive Summary

A small penetration of a deepwater gas transmission pipeline can lead to water ingress (due to high external pressure) and hydrate blockage of the pipeline with large differential pressure across the blockage and partial flooding of the line.

In the test discussed in this paper, water was leaked into a flowing natural gas stream at deepwater pipeline pressure and temperature. This resulted in formation of a hydrate blockage system consisting of at least two blockages that entrapped one or more high-pressure gas pockets. In a system with entrapped gas pockets, using traditional "two-sided" pressure reduction (without calculations and evaluations) to clear a blockage can be unsafe. After appropriate calculations and evaluations, "one-sided" pressure reduction was used to clear the formed hydrate blockage safely, quickly, and effectively.

Based on the test results, the estimated time for clearing a similar blockage in a large (300-km long, 24-inch) deepwater line, while maintaining pressure uniformity across the system, is over a year - resulting in over a year of lost gas delivery.

With "one-sided pressure reduction," the blockage cleared safely with a 138-bar (2,000-psi) pressure difference across the test section. The pressure difference across the test section dropped to a few bar within 4 seconds of blockage freeing. The clearing time for a similar blockage in a large (300-km long, 24-inch) deepwater line with "one-sided pressure reduction" (including the time required to drop the line pressure on one side of the blockage) is much less than the clearing time with "two-sided pressure reduction."

Introduction

Natural gas transmission pipelines are being laid along the floor of deepwater seas. Some of these lines run for hundreds of kilometers at water depths in excess of 2,000 meters (6,562 ft). The deepwater environment is typically cool (2°C to 10°C), and the deepwater pipeline external pressure is in excess of 200 bar (2901 psi). In this deepwater environment, the pipeline external pressure may exceed the internal gas-stream pressure. Small pipeline penetrations - for example, those resulting from corrosion - can result in water leaks into the pipeline gas stream. Water in a natural gas stream at these conditions may form hydrates.

Hydrates.

Hydrates are ice-like solids with guest molecules occupying cages in a crystalline structure formed by water molecules.