Thirteen buried GRE firewater subsystems were modelled in CAESAR II and assessed to ISO 14692 for hydrotest, operating and sustained load cases. On implementation of the recommendations, static pipe stresses are acceptable and flange loads are moderate.
The buried glass-reinforced epoxy (GRE) piping in the firewater system upgrade at a main degassing station is type EST 20, with a winding angle of 55 degrees. The axial elastic modulus is 12000 MPa and the hoop elastic modulus is 20500 MPa. The system consists of thirteen subsystems carrying water at a design pressure of 20 barg and a maximum design temperature of 52.0 °C. The installation temperature is 21.0 °C.
At the EST 20 GRE flanges there is no allowance at design conditions for the accommodation of external loads. The flanges have to be protected from operating loads on the hydrants and from loads applied by connected steel piping. The isometrics normally indicate the burial depth. Where they did not, it was estimated from nearby hydrant locations with specified burial depths or from the general arrangement drawings. In most cases the cover depth was approximately 800 mm, measured from the top of the pipe. In some cases it was less than 800 mm. The system design was in a preliminary state, and the assessment includes assumptions and suggestions.
CAESAR II Modelling of Buried GRE Piping to ISO 14692
The thirteen subsystems were modelled and analysed in CAESAR II, a pipe stress analysis program. Each subsystem is named after the lowest pipe number it contains. The pipe material properties and the allowable stresses are based on material data provided by the pipe manufacturer. The properties used are a shear modulus of 11500 MPa, Poisson ratios of 0.38 for hoop load and 0.65 for axial load, a thermal expansion coefficient of 20 x 10⁻⁶ mm/mm/°C and a specific gravity of 1.85. The structural wall thickness is 2.4 mm for ND 150 and 4.1 mm for ND 250. The stress assessment was performed in accordance with the allowable load case dependent stress envelope. The GRE pipe stress was assessed in accordance with ISO 14692.
Three static load cases were considered, where W is the weight of the water-filled pipe, HP the test pressure, T1 the maximum design temperature and P1 the design pressure. The hydrotest case (W+HP) applies 30 barg at 21 °C. The operating case (W+T1+P1) applies 20 barg at 52 °C. The sustained case (W+P1) applies 20 barg at 21 °C. The pipe content is water with a density of 1000 kg/m³.
Generally, node numbers below 10000 denote GRE piping in the scope of the assessment, and node numbers above 10000 denote steel piping and GRE piping outside the scope. Steel piping and some existing GRE piping outside the scope were modelled from a general overview drawing as boundary to the GRE piping. Piping that is not visible on the isometrics but is connected to the existing GRE firewater lines was included for overlap (boundary condition) purposes, based on the general arrangement drawing. Where the connecting piping was not known, two situations were analysed. In the first, an anchor at the connection gave the restraint loads. In the second, the connection was unrestrained, which gave the displacements of the free end. The stress assessment and the measures for improvement were based on the worst case.
Flange Rating, Restraint, Wall Thickness and Cover Depth
According to a memo from the flange manufacturer, the qualified pressure is two times the rated pressure. The combined flange loads from internal pressure and external loading were checked against the rated pressure, corrected by the applicable load dependent part factor.
A rigid connection from the steel hydrant piping to the concrete floor is needed so that operating loads on the hydrants do not overload the GRE flanges. To limit the moment loads transferred to the GRE by manipulation of the hydrants, the top flange of the valve connecting the steel hydrant part to the GRE piping is restrained in all directions except the vertical direction. This is implemented with a clamp with brackets that connect the flange to the anchor foundation. The hydrant connection piping is anchored at grade level, and at the hydrants and monitors the piping is assumed to be encased in and fixed to a concrete floor. Where GRE piping connects to steel piping, the GRE flange has to be protected from loads applied by the steel piping. This may be done with restraints in the steel piping, shown on the isometrics. In some subsystems, axial stops were introduced as a boundary condition, positioned to prevent high loads and moments on the GRE flanges.
To bring the stresses within allowable limits, adjustments to the standard arrangement are to be made and are indicated on the stress isometrics. Where the pipe wall thickness has been increased, the effective thickness TE is shown on the isometrics, and the model plots show the applied TE by colour for each subsystem. Anchors that have to be applied and pipe length changes are also indicated. A cover depth of at least 800 mm was applied in all models. The report strongly advises this depth to assure buried stability and prevent pipe upheaval. Sufficient cover depth is also required to provide the support the pipe needs, in the form of friction and side support, which limits the pipe stresses.
GRE Pipe Stress and Flange Load Results
It may be concluded from the computer results that, upon implementation of the recommendations, static pipe stresses are acceptable and flange loads are moderate, the latter due to the measures taken. The report is issued as preliminary.
Cover Depth Check and Displacement Limit at Existing Piping
The cover depth of the buried piping has to be at least 800 mm in all subsystems. If the actual cover depth differs substantially from the values indicated on the isometrics, the stresses have to be checked. The existing piping shall not impose displacements at the connection with the new GRE piping in excess of the free-end displacements established with the connection unrestrained.
Frequently Asked Questions
What allowance do EST 20 GRE flanges have for external loads at design conditions?
None. At design conditions the EST 20 GRE flanges have no allowance for the accommodation of external loads. A memo from the flange manufacturer states that the qualified pressure is two times the rated pressure. The combined flange loads from internal pressure and external loading were checked against the rated pressure, corrected by the applicable load dependent part factor.
What cover depth applies to the buried GRE piping in this analysis?
A cover depth of at least 800 mm was applied in all models and has to be assured in all subsystems. The report strongly advises this depth to assure buried stability and prevent pipe upheaval. Sufficient cover also provides friction and side support, which limits the pipe stresses. If the actual cover depth differs substantially from the values indicated on the isometrics, the stresses have to be checked.
How was unknown connecting piping handled in the CAESAR II models?
Two situations were analysed. With an anchor at the connection, the restraint loads were obtained. With the connection unrestrained, the displacements of the free end were obtained. The stress assessment and the measures for improvement were based on the worst case. The existing piping shall not impose displacements at the connection with the new GRE piping in excess of the free-end displacements established with the connection unrestrained.