With a newfound abundance of natural gas across North America, a massive shift is taking place as companies contemplate conversion of liquefied natural gas (LNG) import terminals into import/export terminals or export alone. Likewise, owners of peak shaving facilities are contemplating facility upgrades to facilitate larger throughputs. Before doing so, however, these companies should first perform a Fitness for Service (FFS) and Remaining Service Life (RSL) assessment to ensure maximum benefit from any financial investment made.
This assessment requires in-depth analyses of the complex systems that make up a facility. As such, the assessment should be made by specialists who have practical experience in designing, constructing and commissioning facilities; possess a background in nondestructive examination methods and techniques; have performed internal and external inspections; and are skilled in FFS and RLS assessment. The assessment is performed system-by-system within the facility and requires both desktop analyses and on-site inspection. Input from client engineering and operations personnel is also critical to achieving a successful outcome.
A key component at these facilities are the cryogenic storage systems that consist of large, double-wall, single or full containment tanks used for the storage of products such as LNG. Analyzing the future service life of these storage tanks requires in-depth review of past and future tank loading and unloading cycles, design calculations, design and fabrication drawings, construction documentation (including material certificates, material test reports and weld procedures) and geotechnical reports. Assumptions, based on historic data of similar facilities and experience, may be made if there are gaps in facility data or operating information.
A typical assessment is iterative, with various methods used to perform engineering studies and evaluate the tanks. The analyses result in recommended inspections and potential modifications to upgrade and increase the service life of the tank. The operating history and anticipated past and future loading cycles form the basis for the FFS and RSL assessment.
The assessment should include:
- Engineering studies focused on the highly stressed, fatigue sensitive components of the liquid containing inner tank. These components include circumferential weld of the shell to the annular plate, shell penetrations, local shell stiffeners, portion of the shell at stiffener ring attachment and circumferential lap weld of the bottom plate to the annular plate.
- Finite element analyses to identify critical areas that are susceptible to fatigue and generate stresses and stress ranges used in the assessments. Fatigue analyses of the fatigue-sensitive regions are per-formed using fatigue crack growth models and damage accumulation mechanisms. Fatigue studies estimate the design life consumed and help determine if adequate life is available to sustain future demand.
Once the assessment is complete, inspection requirements can be developed and potential upgrades defined with emphasis on the components with limiting fatigue value in order to increase the design life of the tank. The LNG tank system, consisting of the inner and outer tanks and appurtenances, should be inspected based on the developed inspection requirements, industry norms and API inspection standards.
The inspection data validates the engineering analyses assumptions or requires them to be modified. Information from final analyses coupled with inspection data facilitates the development of repairs and upgrades that enable the tank to meet future facility requirements.
Although the methodologies of assessments are very similar for different tank systems, each tank is unique in its own characteristics and requires a facility-specific process to be developed.
Richard Insull is director of business development for External Engineering Services at Matrix PDM Engineering, a subsidiary of Matrix Service Co. Matrix PDM Engineering provides comprehensive engineering solutions for energy, power and industrial facilities across North America.
For more information, call Richard Insull at (724) 934-7023 or email him at rinsull@matrixpdm.com.
