The concept of proof testing engineering structures has its origins in antiquity. The pre-commissioning hydrostatic test (also known as the pre-service pressure test) has been an important part of the process of commissioning a newly constructed pipeline for over 50 years, since its beginnings in the 1950s in the USA. The purpose of the hydrotest is several-fold: to prove the leak tightness of the pipeline system at a pressure above the design pressure, as a strength (proof) test to identify (fail) defects and sub-standard pipe, and to prove a safety margin above the pipeline design pressure. Historical data, from PARLOC (Pipeline and Riser Loss of Containment), the OPS (Office of Pipeline Safety) 30 day Incident Reports, and the published literature on the number and causes of pre-commissioning hydrotest failures has been reviewed. The historical data covers onshore gas transmission pipelines in the USA and the UK, and gas and liquid pipelines in the North Sea. The data covers the period from 1952 to 2005, although there are significant gaps in the data (e.g. the OPS data for the USA does not report test failures after 1984). In this paper, the historical data is summarised over this period, by year, in terms of the number of failures per km, and trends in the frequency and type of failures are identified. Comparison of USA and UK experience, or onshore and offshore experience, is contentious because of the influences of different design codes, and local custom and practice. The USA and UK pipeline design code requirements for the hydrotest are summarised in the paper, and it is shown that some of the trends in the failure data may be explained by the differences between the codes. Failures during the hydrotest are rare, but occasionally they do occur. The general consensus is that failures during the precommissioning hydrostatic test are now less common, and that failures due to defective line pipe (rather than due to leaking fittings) are rare. The historical data supports this consensus, but it also highlights that it is largely based on anecdotal evidence rather than data and analysis, because information on test failures is not now routinely gathered and published. The results of the historical review demonstrate that understanding the causes and reasons for hydrotest failures is important for learning from past mistakes, and also for identifying those cases where it may be possible to dispense with a pre-commissioning hydrotest. Reliable historical data on hydrotest failures is necessary to quantify trends over time, and to understand the causes of failures. The pipeline industry as a whole is not coherently recording this data. It should be.
Failures during the pre-commissioning hydrostatic test of a newly constructed pipeline are rare, but occasionally they do occur. Structural reliability techniques can be used to estimate the probability of failure during the precommissioning hydrotest, and to investigate the sensitivity of the probability of failure to the test pressure. This paper describes a study of the probability of failure during the hydrotest, based on data for the BBL Pipeline. The BBL Pipeline is a 36 in. outside diameter, approximately 235 km long pipeline designed to export natural gas from the Netherlands to the UK. The definition of failure is limited to failure of the line pipe due to internal pressure loading. Failure of fittings (e.g. flanges, valves, etc.) is not considered. With this definition of failure, three different scenarios are considered: 1. Failure of defect-free pipe. 2. Failure of pipe containing a ‘workmanship’ defect (i.e. a defect in the pipe body or a weld that is acceptable to the relevant specifications or standards). 3. Failure of pipe containing a defect not acceptable to workmanship levels (e.g. a crack, or a dent on a weld). Defects larger than workmanship defects encompass defects that would not fail at the design pressure, but would fail at higher pressures, and ‘gross’ defects that would fail at very low pressures. It is difficult to estimate the probability of failure of such defects because it is highly dependent on the probability of these defects being present in the pipeline. Conversely, it is relatively straightforward to obtain a reasonable upper bound estimate of the probability of failure due to defect-free pipe or a workmanship defect. Consequently, in this study, only the probability of failure due to (1) or (2) has been calculated using structural reliability techniques. Inferences about (3) are drawn from the results of (1) and (2), and from historical data on hydrotest failures. It is shown that there is a hydrotest level below which the probability of failure is predicted to be zero. For defect-free pipe this is at least equal to the level of the mill test. In general, this hydrotest level depends upon factors such as: the ratio of the axial stress to the hoop stress in the mill test and the hydrotest, and the size of defects in the pipeline.
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