Securing LNG’s leading role on the global energy stage

The use of LNG reduces GHG emissions while also nearly eliminating other tox-ins such as sulfur oxide (SO_X) and nitrogen oxide (NO_X). As such, it is the fuel of choice for diesel replacement in heavy horsepower applications. LNG is also a critical lower carbon solution that helps ensure reliable power for electricity, heating, and cooling in remote areas or during periods of peak demand, such as extreme weather events.

Given its leading role, the inspection, maintenance, and repair of LNG tanks and terminals is vital to providing energy assurance through safe, ongoing operation. Ideally this work, referred to as life cycle analysis (LCA), is completed as part of a planned lifecycle infrastructure asset management programme for a facility, but it can sometimes be triggered because of unplanned events.

A real-world example

In January 2020, a series of earthquakes rocked the island of Puerto Rico, with the most significant being an M6.4 event that originated within 13 km of the EcoEléctrica LNG import terminal and power plant. A critical energy resource, the facility supplies natural gas fuel to produce up to 40% of the island’s total power, and its natural gas combined cycle power plant is the cleanest, most reliable source of energy for the island. Seismic monitoring instrumentation on the EcoEléctrica tank indicated ground shaking caused by the M6.4 event produced accelerations on the structure that exceeded the seismic hazard developed for the original design.

As a result, EcoEléctrica was tasked with responding to multiple inquiries from US regulatory bodies including the Federal Energy Regulatory Commission (FERC), Department of Transportation Pipeline and Hazardous Materials Safety Administration (PHMSA), U.S. Coast Guard (USCG), and U.S. Geological Survey (USGS). In question were the short-term and long-term risks associated with the facility’s 160 000 m³ double-containment LNG storage tank.

For answers, EcoEléctrica turned to Matrix PDM Engineering, whose engineers possess extensive expertise in cryogenic tank design, and whose predecessor firm had designed, fabricated, and erected the EcoEléctrica tank when it was commissioned in the early 2000s.

Matrix conceptualised the problem and developed the methodology for the tank analysis considering input from engineering seismologists, EcoEléctrica, and technical staff at regulatory agencies. The Matrix engineering team conducted multiple seismic and structural analyses on the tank system; the first was a forensic analysis following the M6.4 event. This analysis consisted of a desktop study for the seismic and structural analysis of the tank for accelerations developed from the ground motion recorded during the M6.4 event. The team was able to verify that the tank system remained undamaged during the M6.4 event and was safe for continued operation. Sample results for modal response spectrum analysis of the fluid structure system can be seen on the left in Figure 2.

Additionally, regulatory agencies implemented restrictions on the liquid level inventory until EcoEléctrica could satisfy concerns for plant and public safety due to potential increases in short and long-term seismic hazards because of the M6.4 event. The Matrix team performed multiple seismic and structural analyses for seismic hazards including those developed based on current standards, as well as potential hazards developed to consider elevated return periods. Several of the evaluations considered seismic hazards beyond what is currently required by US LNG codes, including 49 CFR Part 193.

Ultimately, the LNG tank and its prestressed concrete secondary container were deemed to have performed safely during the M6.4 event. The analysis was also used to support regulatory approval for liquid level that the facility could use for continued operation.

Planned LCA

While an unplanned event resulted in the LCA performed at EcoEléctrica, given LNG’s role in achieving global energy goals, planned LCAs should be a priority with today’s owner/operators – especially with ageing facilities – to ensure their facility’s safe, continued operation and compliance with changing regulations. In the US alone, there are 107 active LNG facilities (excluding mobile, temporary, and satellite facilities), 70 of which were constructed between 1965 – 1995, according to PHMSA.

The remainder of this article presents a structured approach for performing an LCA on the cryogenic storage systems at these facilities, whether double-wall, single, or full containment.

While assessment methodologies are similar for different tank systems, each tank has its own characteristics and requires a facility-specific process. The complexity requires that all phases of an LCA be carefully planned and executed.

Phase 1: Data collection

Assessment begins with collection of information such as:

• Past tank loading and unloading cycles.
• Design calculations.
• Design and fabrication drawings.
• Construction documentation (including material certificates, material test reports, and weld procedures).
• Geotechnical reports, commissioning, operation, maintenance, repair, and modification records.

Certain facility operating information is also necessary, such as transport logs, which indicate loading/unloading information, temperature data, foundation settlement information, and historical tank vapour pressure information.

The operating history and anticipated past and future loading cycles form the basis for a fit for service (FFS) and remaining service life (RSL) assessment. Assumptions based on historical data of similar facilities and experience may be made if there are gaps in facility data or operating information. Understanding the owner’s objectives is critical to this phase, as this will significantly influence the work required in Phases 2 and 3. A longer life expectancy, for example, will require closer evaluation and potentially more repairs. Similarly, if Phase 2 analysis results in a long future life, the work in Phase 3 can be minimised.

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