COLLAPSE BUCKLING PERFORMANCE ASSESMENT OF PLASTIC LINERS FOR OIL DISTRIBUTION: LINER RESISTANCE AND LOADING CASE EVALUATION.

FEDERICO RUEDA; CAMILA QUINTANA; FABRICIO PIETRANI; PATRICIA FRONTINI

Congreso; SAM CONAMET 2022; 2022

The use of thermoplastic liners is a widespread technique to cost effectively protect steel pipelines againstinternal corrosion or to rehabilitate deteriorated ones (corroded, cracked or damaged). The most commonlyused polymer at present is polyethylene (HDPE). However, several other thermoplastics have the potentialto be used. In this regard, there are two important aspects in liner designing: liner behavior duringinstallation and collapse buckling prevention during operation.The collapse buckling induced by external pressure phenomenon takes place by the combined action of twoseparate factors: i) the permeation of oil derived gases through the liner wall for extended periods of time,and ii) the rapid decompression of pipelines that can occur during service stoppages or maintenanceand inspection shutdowns. There are two main critical aspects to be considered in terms of buckling collapseperformance. On the one hand the actual liner and its material behavior as a function of temperature,swelling and even solicitation rate and liner geometry (thickness, defects, fitting, etc.). On the other hand,the actual loading case as a consequence of a compressible fluid in the annulus between liner and host pipeis decisive.The buckling collapse resistance is a topic that has been widely discussed in the literature. Most of theearlier studies for the instability of liners were only concerned with elastic [1-3], or elastoplastic [4-5]constitutive behavior. Therefore, those studies may help in estimating the buckling pressure of thin linersthat are only expected to buckle elastically, prior to any material yielding. However it is known that materialyielding would affect the me chanical and buckling behavior of liners. One of the main shortcomingsregarding modeling HDPE liners performance is the intrinsic non linearity of polymer behavior. Thisproblem was addressed using experimentally verified numerical simulations combined with advancedviscoplastic constitutive models [7].The loading case in buckling collapse is driven by a constant mass of gas into the annular cavity between theliner and host pipe. Its effect is not trivial since it does not imply either load-controlled or displacement-controlled loading case, but the relation between volume and pressure is given by a characteristic isotherm.There is a single work that comments very briefly this concept [8] but it goes not further to analyze eitherhow to determine it or its importance in the design. The fact is a proper liner design -material selection,thickness, pressure and temperature operation ranges, etc- necessarily implies the knowing of both, linerbuckling resistance curve and characteristic isotherm. The isotherm determination is not a simple matter.The pressure can be easily measured, but the initial annular volume is very small and depends mainly onfitting level and roughness of liner and hosting pipe.In this work we proposed a laboratory test to measure the initial annular volume over a host pipe and linersection for the isotherm -loading case- determination. On the other hand, a collapse resistance curve wasobtained using an experimentally verified numerical method [7]. Combining the two, we obtained the actualoperating range of this lined pipeline.