7.2.1 Fittings
The design of the Beach Valve has incorporated the earlier recommendations regarding elimination of flanges and reduction of small bore connections, and will be fully welded and contain minimal connections or leak paths
7.2.2 External Interference
The most significant risk reduction measure which reduces the effect of external interference is the high wall thickness which provides mechanical strength to resist excavator bucket teeth and other such implements. This is accepted in all recognised stress based design codes which decreases design factor and increase wall thickness in areas deemed to be of higher risk potential or higher consequential damage. The design of the Corrib pipeline, with its design pressure, has led to a very thick pipeline when compared to other gas pipelines of a similar size installed in Ireland, the UK and continental Europe (normally 8 to 12 mm thick versus 27.1mm for Corrib).
The wall thickness of the Corrib pipeline is such that there is no additional risk reduction in areas where design factors would normally have been lower, e.g. road crossings. This is recognised in many design codes where, once the wall thickness reaches 19 mm, there is no requirement to alter the design factor at crossings or other sensitive locations. The Corrib pipeline therefore will not gain from any increased wall thickness and may actually suffer due to increased weight and consequential subsidence effects and difficulty in construction. The reduction in internal diameter to 384mm from 453mm, resulting from an increase in wall thickness when the design factor is increased from 0.72 to 0.3, may interfere with the running of intelligent inspection pigs, which is not a desirable outcome.
Increasing the depth of cover provided for a pipeline can reduce the likelihood of external interference damage by reducing the proportion of excavation activities reaching a depth which could interfere with the pipeline [Ref. 26]. A report by British Gas has analysed the effect of increasing the depth of cover by studying damage data for the UK Gas Transmission System. The method used relates the frequency of damage at any depth of cover to the pipeline length and exposure at that depth, so that comparisons can be made for various depths. The report concludes that the risk of damage is reduced by a factor of 10 by increasing the depth of cover from 1.1m to 2.2m.
British Gas also conducted a series of experiments involving a range of excavating machines and various forms of protection [Ref. 26] which concluded that some credit could be taken in
risk assessment work for protective measures such as plastic warning tapes, concrete barriers and a combination of the two.
However, as external interference is not a major risk contributor for the Corrib pipeline the beneficial effect of increased depth of cover or additional protective measures is not considered significant.
For road and ditch crossings, where the risk of excavator activity and other similar activity can be considered to be higher than for other sections of the pipeline route, the use of 150mm thick re-inforced concrete slabs, which extend a minimum of 1000mm beyond the road, track or ditch edge complete with warning tape and marker posts, have been specified on the basis of the ALARP principle. However, no credit for this has been taken in the risk assessment in line with the overall conservative approach used.
It is recommended that the use of plastic warning tapes (in the ground above the pipeline) and pipeline markers (at field boundaries) should be adopted along the entire route due to the very low cost of their installation. These measures may provide a small reduction in the risk of external interference (e.g. during peat cutting activities), and therefore be consistent with the ALARP principle.
Both of these recommendations have been accepted and implemented in the design. 7.2.3 Ground Movement
For the purposes of this assessment a significant proportion of the total pipeline failure rate has been attributed to ground movement, due to the terrain crossed by the onshore section of the pipeline. The detailed design has assessed ground movement issues and pipe settlement, and addresses means by which the risk of pipeline failure due to ground movement can be reduced in the Onshore Sealine Mechanical Design Report [Ref 18]. The bearing capacity of the soil (peat) along the areas of peat bog along pipeline route has been examined. Construction plans will involve the use of pillars or islands of stone (supported on the bed-rock below the peat) at set distances to provide support to the pipeline, or alternatively the peat will be excavated to a suitable rock or sand / gravel layer. Road crossings and points where other loads can be placed on the pipeline have been addressed in the design.
Field tests have demonstrated that the undisturbed peat strata has the load capacity to resist sinking of the pipeline, even when full of hydrotest water, with settlement limited to the initial two to three days. These tests included the use of 20" pipe, weighted to be identical to the Corrib pipeline, when full of water. For most of its life, the weight will be less than that, thus reducing the propensity for settlement. The Corrib pipeline, unlike many other gas pipelines, will always be negatively buoyant, due to its weight, even in saturated peat soils.
JPK has studied the effects of a peat slide or land slip across various lengths of the pipeline [ref 5] from 200 m to 25 m and the results of this analysis show that the pipe may move downhill with the slide, especially for the larger slides, but that it does not exceed the yield stress, nor does it buckle, hence the pipeline would not fail in service. This is primarily due to the thick wall of the pipeline.
The failure rate used in this assessment may, therefore, be over-conservative. However, as the overall risk estimates indicate a tolerable level of risk, this will not be revised.
7.2.4 Demonstration of ALARP
The Corrib pipeline has been designed in accordance with the ALARP principle in relation to the risks to the public.
The pipeline has the following safety features:
• Pipeline design pressure is equal to well shut-in pressure at start of production and therefore cannot be exceeded under any circumstances.
• Shut in pressure declines rapidly after initial 3 year operation
• Pipeline routing has ensured separation distance to occupied houses is the maximum practicable
• Pipeline is all welded and all construction welds will be examined by NDT methods
• Pipeline has been designed to a conventional, well proven design code
• Nominal wall thickness of 27.1mm
• Depth of cover, minimum of 1.2 m
• Pipe joints have been subject to automatic seam weld inspection and manufactured to modern techniques to minimise the possibility of material defects.
Risk reduction measures which have been incorporated into the design include:
• Use of warning tape and marker posts throughout the pipeline route
• Use of concrete slabs at ditch and road crossings with warning tape & marker posts
• Use of all welded connections at Beach Valve and removal of small bore valves and fittings
• Separation of gas pipeline from umbilical to avoid potential effects from maintenance activities or damage from one to another
Other measures considered
Use of lower design factor and hence thicker pipeline has been considered, especially in areas such as road crossings or where proximity to occupied buildings is less than elsewhere. However, examination of the historical data and logic behind the increase in wall thickness shows that it is designed to resist third party damage. For the Corrib pipeline, external interference resulting in a leak or rupture of the pipeline represents a small proportion of the estimated overall release frequency. The use of lower design factors and hence thicker wall pipeline cannot be justified in this instance due to the increased cost of materials, construction and additional risk from handling very heavy pipe.
A similar argument is applicable in terms of additional impact resistance such as concrete covers, sleeves and other mechanical protection to the pipeline. For these items there would be a quantifiable reduction in risk, but it is extremely low and therefore the additional cost is not reasonable compared to the reduction in risk.
Other measures to reduce the risk from third party activity, or to locate any leakage from the pipeline include enhanced inspection of the pipeline route from the air or countryside and regular maintenance activities. These activities will be carried out, but as there is no reliable data on which to quantify any reduction in risk generated from increased levels of surveillance or inspection, no credit is taken for this in the QRA.