Gas Hydrate Formation Assessment in Deadlegs and Jumpers: A Comprehensive Review and Field Case Study Analysis

Abstract

This paper presents a comprehensive literature review of gas hydrate safety issues in subsea deadlegs. Essential unique insights, "rules of thumb", and guidelines are provided to help engineers and researchers address hydrate formation risks in deadlegs and jumpers. The review highlights the importance of several critical factors, including pipeline geometry, fluid properties, operational conditions, temperature distribution, length of the mixing zone, impact of natural convection, fluid distribution, and effectiveness of inhibitors in mitigating these risks. Practical recommendations derived from the literature, such as optimizing the length-to-diameter ratio of deadlegs and implementing effective flushing strategies, are discussed to aid in the design and management of deadlegs in subsea production systems. Additionally, a field case study is investigated for gas hydrate formation in deadlegs, focusing on a Norwegian subsea gas condensate asset. The study employs thermodynamic calculations and multiphase flow simulations to assess the risk of gas hydrates in a deadleg of a spool section. Given that the pressure and temperature in the spool section fall within the gas hydrate equilibrium conditions, the chosen method for mitigating gas hydrate formation was to fill the spool section with monoethylene glycol (MEG). The findings reveal that during production only the last 5 m of the section contains MEG that has been diluted due to condensed water. However, the risk of hydrate formation was minimal due to the higher manifold temperature. The simulation results also determine the minimum MEG flow rate required to displace the spool with MEG, as well as the associated changes in the fluid viscosity and density. These insights contribute significantly to a new and deeper understanding of gas hydrate formation risks in deadlegs in subsea gas systems, providing crucial information for effective flow assurance and pipeline integrity management.