Preventing LNG Stratification and Roll-Over Events

by , | Jul 3, 2024 | Industrial Software, Operator Performance | 0 comments

The AspenTech Optimize 24 conference was held in April in Houston, Texas. It is the premier global event for AspenTech customers, including users, operational leaders, and executives within asset-intensive industries looking to elevate their business performance, improve their resiliency, and meet their sustainability targets.

Here’s a recap of an excellent presentation, Prevent LNG Tank Stratification and Rollover Events through Operator Training, by Emerson’s Vicky Athanasiou.

Vicky discussed LNG weathering, stratification and rollover events, and strategies for incident prevention. Weathering is the change in LNG composition due to the evaporation of more volatile components over time while in storage. This phenomenon is caused by heat leak-in, typically causing a boil-off of 0.05% of the tank’s contents daily.

Stratification is when the LNG liquid phase separates into different layers of different densities: inadequate mixing, different LNG feeds, and high nitrogen content cause this condition.

Roll-over is a reversal of stratified layers and rapid mixing, causing a release of vapor that is multiple times the normal boil-off rate. Roll-over events can cause damage to the plant equipment and disrupt operations for extended periods, resulting in revenue losses of $5-USD 10 million/day.

Typical process trends for monitoring stratification and roll-over events include density differences more significant than 1 kg/cubic meter at different tank levels and subsequent density convergence between layers, indicating a potential upcoming roll-over event.

From a temperature measurement standpoint, temperature differences of 0.5 degC between the lighter LNG layer and denser LNG layer indicate stratification. This temperature difference is responsible for rapid evaporation conditions when density differences collapse. The boil-off rate reduces below expected levels during stratification and increases dramatically, up to 20x and more, during roll-over conditions. It returns to normal after the roll-over condition ends.

Some control strategies to address these conditions include top filling denser LNG than stored in the tank and bottom filling less dense LNG. Upon detection of stratified conditions, use pump recycling from the bottom to the top of the tank. LNG can also be mixed by transferring between storage tanks from the bottom of one tank to the top of the second tank. This process can also be performed between the storage tank and LNG export carrier.

A digital twin simulation is helpful in all phases of an LNG project—concept definition, process design, integrated control & safety system development, factory acceptance testing, commissioning & startup, operations support & maintenance, and plant optimization.

From a personnel training perspective, training simulators help LNG producers:

  • Retain Experienced Operator Knowledge
  • Accelerate Operator Training – 2 Years of knowledge in 6 months
  • Develop and Validate Operating Procedures
  • Familiarize Operators with the Process Dynamics and Controls
  • Train Operators on Normal Operation and Handling Process Upsets
  • Hold Unbiased Training program for Startup and Refresher training
  • Keep Track of Training Needs & Progress for Each Operator
  • Certify Operators

These operator training systems (OTS) help these producers ensure safe operations and greater uptime by increasing operator competency. Vicky shared a case study of an LNG terminal with process model and controls developed in Aspen HYSYS Dynamics and training features in Aspen Operator Training. Process models were developed for weathering, stratification, and roll-over conditions.

The model configured the heat transfer characteristics from ambient conditions for the LNG tanks to predict the weathering effect and normal boil-off rate accurately. The HYSYS model assumes ideal mixing in separators and needs special modeling to represent the two LNG layers. Special modeling was also needed to represent the rapid release of boil-off gas.

For the OTS, the instructor could trigger rollover conditions. Using various training scenarios based on density and temperature measurements, the operators learned to quickly spot these impending conditions and how best to respond to mitigate these conditions. These operator training systems help retain the knowledge of experienced staff and prepare operations to handle normal and abnormal situations for the safe and reliable operation of LNG terminals.

Visit the Aspen HYSYS Dynamics and Aspen Operator Training pages for more information on how these software applications can drive performance improvements in your operations.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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