At an oil refinery plant in Saudi Arabia, a fiberglass-reinforced plastic (FRP) pipe fire water system has been installed, predominantly buried underground. Since its installation, the system has experienced several failures. DRG was tasked with conducting a comprehensive pipe stress analysis to assess the system’s integrity. This analysis focused on critical sections identified either due to past failures or potential vulnerabilities based on DRG’s extensive experience with fiberglass piping systems.
Analysis
Using the pipe stress analysis software CAESAR II, DRG computed stresses on selected sections of the fire water system. The analysis incorporated data on pipe properties, layout, design conditions (pressure and temperature), coupling joints, thrust block specifications, and above-ground equipment such as fire hydrants and monitors. Various scenarios were evaluated, including the impact of thrust block settlements and soil conditions on the system’s performance. Particularly, the adequacy of thrust block sizing was scrutinized, as inadequate sizing can lead to significant displacements and potential piping failures in underground installations.
Reports
Key findings from the analysis include:
- Under nominal conditions (normal temperature and pressure without thrust block settlements), underground FRP piping is expected to perform adequately. Non-tensile resistant coupling joints effectively accommodate any expansion.
- Above-ground GRP piping exhibits high stresses at specific points where the FRP pipes are restrained vertically.
- Poor soil conditions causing significant thrust block settlements, particularly for smaller diameter pipes, result in elevated stresses.
- Thrust blocks were found to have minimal safety margins. However, given the system’s operational status and completion of critical load scenarios like hydrotests, the current thrust block sizing combined with soil properties was deemed sufficient.
Recommendations
Based on the analysis, the following actions and modifications are proposed:
- Implement polystyrene wrapping around pipes entering or leaving thrust blocks to mitigate potential settlements, focusing on sections identified as susceptible based on the analysis.
- Replace U-bolts restraining vertical motion of above-ground piping with supports featuring guide and rest support functions only, optimizing the system’s structural stability and longevity.
By addressing these recommendations, operators can enhance the reliability and performance of the fiberglass fire water system, reducing the risk of future failures and optimizing overall operational efficiency.
