Case Study 5 Under-Inhibition by Methanol in a Gas Line

2 Case Study 5 Under-Inhibition by Methanol in a Gas Line 

Flow assurance engineers for a major energy company (Mehta et al., 2003) indicate that for a two year periods, one of their offshore gas flowlines operated well inside the hydrate formation region. The problem arose from increased water production (to 1000 BPD) over the field life, with limited methanol delivery. Their approach was to inject as much methanol as possible, in the knowledge that they were underinhibiting the system. Due to under-inhibition, there was a gradual increase in the pressure drop (A P) in the line over a period of about 2 weeks, indicating a hydrate build-up on the walls. 

This strategy was successful in extending the field life by almost 2 years. This is one key example of the risk management philosophy, enabled by operating experience on the platform. The hydrate plug prevention technique in this case study is time-dependent and should be contrasted with thermodynamic (time-independent) inhibition methods in Case Study 4 of Canyon Express and Ormen Lange. 

While accurate thermodynamic predictions enable avoidance via use of thermodynamic inhibitors such as methanol or glycol, hydrates risk management is enabled by experience in the form of experiments, both in the field and in the laboratory. This is because, as indicated in Chapter 3, there is no comprehensive, predictive hydrate kinetic theory that can be accurately invoked at high hydrate 

The following sections present three examples of kinetic phenomena (1) kinetic inhibitors, (2) antiagglomerants (AAs), and (3) hydrate plug remediation. These kinetic phenomena were determined by field and laboratory observations. They also point to the need for a comprehensive kinetics theory, from which hydrate nucleation and growth can be predicted for industrial utility. 

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