Ultimate elastic wall stress envelopes for (GRE) pipes

Abstract

In this thesis the procedures for qualifying fiberglass pipes are discussed in relation to industry needs. The ultimate elastic wall stress (UEWS) test appears to provide an efficient means of rating pressure pipes, and indeed vessels, for cases where weepage failure occurs resulting from the accumulation of matrix cracks. The principle behind the UEWS test is to identify from stress-strain response, a stress level below which damage growth is either negligible or at least sufficiently low to avoid failure within the design life. An experimental investigation into the UEWS of ±55⁰ filament wound composite pipes has been conducted under various load and environmental conditions. The pipes were subjected to biaxial loading, which was achieved by a combination of hoop and axial stress loading. Loads were applied as groups of 10 one-minute pressure cycles, recording the hoop or/and axial strain. These pressure groups were gradually increased until the UEWS had been determined. Various ratios of hoop to axial stress were applied, ranging from pure axial to pure hoop loading at room temperature, 65⁰C and 95⁰C. These ratios were investigated by applying different pressures in both the main and small chambers built inside the pipe, and therefore it was unnecessary to add external load to the pipe wall. Tests were stopped at the first observed leakage through the pipe wall. The UEWS test appears to provide an attractive alternative to the currently used procedure laid down in ISO 14692. This involves an expensive series of long term constant pressure tests described in ASTM 2992, running for over 10,000 hours. It is shown here that the UEWS test reflects the cyclic fatigue behaviour of fiberglass pipe, but further work on the relationship between cyclic and static behaviour is needed. The test results are presented in the form of UEWS and failure envelopes showing the effects of testing at elevated temperature, and the degradation in pipe properties as the test progressed. Acoustic emission examinations were also conducted to detect damage development in the GRE pipes during the tests, allowing its compatibility with the UEWS test to be assessed. Four different types of failure mode were observed according to the loading conditions. It has been shown that a Miner‟s Law approach is effective in modelling damage due to combined static and cyclic effects, and that damage can be directly related to matrix crack growth. This approach could form the basis of future procedures for describing the lifetime behaviour of GRE pipes under any required combination of static, fatigue, hydrostatic and non-hydrostatic (multi-axial) loading.