technical article | Pipe lining
2,00 1,80 1,60 1,40 1,20 1,00 0,80 0,60 0,40 0,20 0,00
0
50,0 45,0 40,0 35,0 30,0 25,0 20,0 15,0 10,0 5,0 0,0
0 30 45 time (days)
Figure 3: The elastic modulus and the ultimate strength follow the swelling trends, the slight decreases observed in the fi rst months are due to the swelling effect. Then the properties remain stable over time. Also the mechanical performance trends confi rm that Solef PVDF is not infl uenced by H2S.
Source: Solvay Specialty Polymers
The permeation of gaseous species dissolved in the transported fl uids through the permeable wall of the polymer liner to the annular space between the polymer liner and internal surface of the host steel pipe; A depressurisation event where the lowering of pressure in the pipeline causes any annular gas to expand to a volume that is suffi cient to collapse the liner. Collapse models have been developed in order to
provide a conservative design basis. The most widely used equations were developed by Frost et al[4]
. Modulus
is the key parameter to determine the requisite thickness of liner relative to the size of the steel bore (SDR ratio). As the modulus of a polymer reduces with increasing temperature, key considerations are the maximum design temperature and the infl uence of any swell of the liner. Even a modest degree of swell reduces the modulus, and hence the critical pressure the liner can withstand without increasing the wall thickness of liner. Polymers can be selected for a wide variety of
applications primarily because, unlike carbon steels, they are inert to most chemical species. However, polymers are derived from hydrocarbons causing them to swell and soften in the presence of hydrocarbon species that are small enough to migrate into the
32 PIPELINE COATING | November 2012 30 45 time (days) Sweet Sour Sweet Sour 180 370
spaces in the loose amorphous chains surrounding the crystalline regions of their structure. Swell is less pronounced in denser, more crystalline polymers. Swell does not imply chemical attack on the polymer and is generally harmless, apart from an associated reduction in mechanical properties such as modulus, which necessitates the use of a thicker liner (Figure 3). When using the Frost equation it is important to recognise that the output is a critical pressure given in bar, which is factored into the numbers in the formula together with an assumption for Poisson’s ratio. A common misconception when designing with polymers is that modulus decays with time due to creep. How- ever, this is not the case. The more relevant design condition for a compressive fi tting polymer liner inside a steel pipe is stress relaxation.
180 370
The maximum design temperature would prohibit PVDF being used above 130°C. The design basis is conservative because the assumption is that all transported gases will eventually permeate into the annulus. The reason more realistic permeation models are infrequently used is that their inherent complexity, with the movement of different species at different rates, makes adequate modeling diffi cult. Although PVDF is much less permeable than PE, the rate of permeation is immaterial as permeation will eventually take place through all pipeline polymers.
The Swagelining technique accounts for the size of the annular space. It aims to achieve a tight compres- sive fi t everywhere on the steel bore including the maximum tolerance, essentially leaving only a micro- annulus associated with the size of the surface roughness features of the steel. With such a limited micro-annulus, a hypothetical gas pressure of double the critical pressure determined for the appropriate polymer would have insuffi cient volume to cause collapse of the liner.
The equations used by Frost et al to determine whether a liner is tight fi tting may be better defi ned as close fi tting liners. There is nothing in the equation to account for the additional residual pressure. This effect should mean that the tighter the compressive fi t of the liner the greater the critical collapse resistance becomes, as confi rmed by Wang et al[5]. This additional pressure also needs to be
overcome by any annular gas pressure acting to collapse the liner and it is postulated that this contributes to the inherent collapse resistance of the liner. Calculations of the residual pressure induced by a
recently installed PE100 liner for a water injection pipeline suggest that, after accounting for stress relaxation, the long term residual pressure contributed by that particular liner is of the order of 5 bar. When all these factors are considered, the critical
Stress Break [MPa]
Elasto Modulus [MPa]
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