Success through planning and risk mitigation: Part two

Operational integrity of LNG facilities relies on the foundation set in codes and standards that dictate engineering designs and material specifications for constructing storage tanks and related equipment. These guidelines serve as a crucial layer of protection, ensuring facilities maintain safe containment of LNG, thus allowing companies to mitigate risks, safeguard personnel, and maximise reliable operations.

In the final part of this two-part series, this article will outline key early inputs and their impact on the selection of the optimal tank configuration, as well as considering risk detection and mitigation measures which help provide additional safeguards.

Key early inputs and their impact

Key inputs from early planning and risk assessment have a direct impact on selecting the optimal configuration, sizing, layout, and level of integrity for complex storage structures, such as LNG tanks. These inputs often help drive the decision on selection of the LNG storage concept and configuration to make sure the necessary safeguards are built into the design of the tank.

Soil conditions

Early geotechnical investigations play a crucial role in LNG tank projects by providing essential soil properties and performance characteristics. These investigations establish the foundation type required for the tank and help determine seismic design criteria for structures on the foundation, such as the storage tank and tank top-side platforms and piping. By assessing soil conditions and seismic hazards, geotechnical investigations enable engineers to design foundations that can withstand anticipated loads and seismic forces, thus contributing to the structural integrity and safety of the entire LNG facility. Additionally, these investigations inform construction planning and cost estimation, mitigating risks associated with foundation failure and seismic events, and ultimately supporting the successful execution of the project.

Lack of sufficient soil information can result in critical conditions being over-looked. Voids, soft soils, or buried debris can delay the construction schedule.

It is important that a thorough soil investigation guideline is prepared by a storage tank EPC contractor before performing any investigation. The investigation guideline will help identify the number of soil borings and their distribution to adequately cover the footprint of the LNG tank.

The input from the geotechnical report helps define the type of foundation selected for the LNG tank based not just on loading, but also on settlement criteria defined from the tank design. The foundation selected for the LNG tank not only serves to support the inner and outer tanks, but also assists in preventing detrimental effects of freezing of the soil underneath. Ring wall and slab on grade type foundations typically require foundation heating systems to prevent the soil freezing, while an elevated pile cap has the necessary air gap to avoid a risk of soil freezing.

Site seismicity

The geotechnical investigation for the tank site also needs to address the probability of earthquakes and the magnitude of the ground motions from seismic activity and the influence of the soil. The seismicity study defines the ground motion hazard for the project location, and is determined by analysing many variables, including but not limited to: regional seismicity, subsurface soil conditions, and proximity to known seismic faults. Additionally, the study provides information to quantify the soil liquefaction potential of the site, or the ability of the soil to shift and consolidate due to seismic activities.

Storage tanks are designed for two levels of seismic events: the operating basis earthquake (OBE) and the safe shutdown earthquake (SSE). The OBE is the maximum earthquake for which the structure sustains no permanent damage, and restart and safe operation can resume after the earthquake. The SSE is the maximum earthquake for which the structures may sustain some permanent damage, but there is no loss of overall structural integrity and containment of contents.

The seismic loading on the LNG tank has a direct impact on the dimensions of the LNG tank (proportioning diameter and height), liquid sloshing height, and need for anchorage.

In locations with high seismic, it can be beneficial to add base isolation to the tank system in the form of friction pendulum isolators. Use of seismic isolation often results in an economical tank design with a reduced footprint, while providing the most reliable mechanism for accommodating the large seismic displacements that occur during an earthquake. The implementation of a base isolation system allows a significant reduction of the earthquake forces on the tank by approximately 80%.

In summary, early geotechnical investigations, guided by comprehensive site-specific soil investigations guidelines, are indispensable for de-risking LNG storage EPC projects, as they provide critical data to inform foundation design, mitigate seismic risks, and ensure the technical integrity and safety of the facility

You can read an extract of part one, from the April issue of LNG Industry here.

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Read the article online at: https://www.lngindustry.com/special-reports/06062024/success-through-planning-and-risk-mitigation-part-two/