Abstract Pipeline networks worldwide, which can be measured Pipeline networks worldwide, which can be measured in hundreds of thousands of kilometres, represent major capital assets with enormous revenue-earning capacity and as such often figure significantly in a country's economy. Of particular economic and strategic importance are under sea pipelines which provide the critical link between production provide the critical link between production platform and shore terminal sometimes over long platform and shore terminal sometimes over long distances. Such lines often take supplies from a number of sources via gathering lines and this makes their long term value, as old fields are depleted and new ones discovered, extremely high. In general these lines have been operated for many years during which time the pipeline industry generally has experienced changes in design, construction and maintenance methods some of which reflect the fact that high pressure pipelines are subject to deterioration to some degree or other. The rate at which this occurs depends on numerous factors not least being the chemical composition of the product carried, which may determine any internal corrosive action, and the degree to which an external protective coating and cathodic protection can be proof against the environment protection can be proof against the environment including third party interference. Such factors combined with the relative inaccessibility of submarine pipelines means that cost effective maintenance methods are an important factor in their operation. More particularly such methods must be capable at any time of quantitatively describing the structural integrity of the pipeline. This can be achieved only by pressure pipeline. This can be achieved only by pressure testing or some form of non-destructive evaluation. Pressure testing will continue to be essential to Pressure testing will continue to be essential to the pipeline industry as an effective means of commissioning new and modified installations. Its value lies in the fact that it will reveal, sometimes in a spectacular manner, all defects that are significant at the particular test level. It is not however a very cheap or convenient method of subsequent periodic testing being somewhat disruptive in terms of product supply unless additional pipeline capacity and/or storage can be used to maintain supply for the duration of the test. Clearly such an approach represents a significant capital investment especially in the case of main off-shore supply lines and is unlikely to be economically justified. Neither is it a straightforward test, requiring strict attention to time-consuming stages of filling with water, testing and drying out. In addition the location and repair of small leaks occurring during test is difficult.