During the design phase of a sea jetty for an oil terminal, Dynaflow Research Group (DRG) conducted a comprehensive stress analysis for the glass reinforced epoxy (GRE) firewater system. This system, lined with fire insulation that influences its strength properties, is mounted on several independent trestles approximately 10 meters above sea level. The primary challenge was accommodating the significant thermal expansion of the 1.5 km pipeline, especially under the influence of seismic and wave loads, which cause substantial displacements.
Analysis
Before performing the stress analysis, the effective properties of the pipeline had to be determined. While the properties of the GRE material were well known, those of the fire insulation were not. In collaboration with the pipe supplier, DRG adopted a conservative approach that accounted for the axial stiffness, Poisson effect, and weight of the insulation lining, but excluded its strength. This method accurately described the system’s stiffening due to the fire insulation but was conservative because it did not include the additional strength provided by the insulation layer.
The jetty structure comprised 33 trestles that supported several pipelines. Midway along the jetty, a firefighting platform housing all water pumps was present. The firewater system was designed for both marine and terrestrial fire extinguishing. The structure rested on tall poles embedded in the seabed, resulting in relative flexibility. Several trestles were connected in groups, and the movement between these groups had to be incorporated into the analysis to accurately determine the pipe’s displacement relative to each trestle.
Given the jetty’s location in a seismically active area, the analysis had to anticipate the following challenges:
- Significant displacements of the jetty under seismic loads
- High seismic acceleration affecting the piping
- Large displacements of the jetty under wave loads
Results
To minimize stresses within the system, several expansion loops were designed along with effective supports. The accompanying figure illustrates how one of these loops functions to reduce material stresses if the firefighting platform moves relative to other trestles. The analysis reported material stresses, support loads, and axial and lateral displacements. Additionally, restraint loads were reported according to an axial and longitudinal coordinate system to facilitate client understanding.
By addressing these factors, the analysis ensured that the GRE firewater system could withstand the combined effects of thermal expansion, seismic activity, and wave loading, thereby enhancing the safety and reliability of the sea jetty’s firefighting infrastructure.
