Hoop stress procedures

When constmction is complete, the pipeline must be tested for leaks and strength before being put into service industry code specifies the test procedures. Water is the test fluid of choice for natural gas pipelines, and hydrostatic testing is often carried out beyond the yield strength in order to reHeve secondary stresses added during constmction or to ensure that all defects are found. Industry code limits on the hoop stress control the test pressures, which are also limited by location classification based on population. Hoop stress is calculated from the formula, S = PD/2t, where S is the hoop stress in kPa (psig) P is the internal pressure in kPa (psig), and D and T are the outside pipe diameter and nominal wall thickness, respectively, in mm (in.). 

The analysis did not consider the effects of temperature loading. The general effect of operational temperature conditions Is to add compressive vertical and hoop stresses to the Inside of the barrel and tensile vertical and hoop stresses to the outside. It Is considered that the Inclusion of planes of weakness would not significantly affect the stress distribution due to temperature loading. It should be noted that small tensile stresses occur close to the liner during PCRV cool down procedures. However, It Is expected that these would be accommodated within the thin layer of reinforced In-sltu concrete Immediately behind the liner referred to In Section 7.5. 

Particular attention must be given to transfer pipework where seam-welded pipe runs are used. In such cases, experience from the American power industry steam reheat line catastrophic failures should be borne in mind. The critical factors identified are weld geometry, weld metal and flux compositions and pipe ovality. Generally, older CRU units utilise seamless pipe in this service but some instances are known, particularly in larger capacity units, where seam-welded pipe is in service. Where necessary, this factor can be taken into account in the assessment procedure described below. Seam welds are subject to the full pressure hoop stress, as well as to many of the system loads. Failure of such a weld is likely to lead to a full-scale rupture, rather than a leak. Accordingly, they must be treated as critical items for assessment, and replacement, rather than repair, should be considered the ultimate outcome. 

In simplified terms, the PPl procedure, which is performed by the materials manufacturers themselves, involves recording how much time it takes stressed pipe samples to rupture at a standardized temperature of 73 °F. The stresses used in the tests are recorded as hoop stress, which is tensile stress in the wall of the... 

If a thermoplastic composite pipe is pressurized rmder laboratory conditions (with the pipe ends freely moving) the stress/strain in the two principal, axial (ej and hoop ( y), directions exhibit a behavior schematically illustrated in Figure 1. Three aspects of this response, namely (1) the initial stiffness of the pipe associated with the slopes and Sy (2) nonlinearity in the cr/e response and (3) the ultimate failure of the pipe corresponding to the burst pressure, are rationalized by means of micro- and macromechanical modelling. These types of modelling of the short term structural response are the initial steps in the overall design procedure. 

---