HN039 PIPELINE DERWENLAS TO ABERDYFI Review of Options for Future Operation of HN039
Wales & West Utilities
Report No.: 10165347-1, Rev. 1 Document No.: 10165347-01 Date: 29/11/2019
Project name: HN039 Pipeline Derwenlas to Aberdyfi DNV GL Oil & Gas Report title: Review of Options for Future Operation of HN039 Gas Technology Customer: Wales & West Utilities, Holywell Park Wales & West House Ashby Road Spooner Close Loughborough Celtic Springs LE11 3GR Coedkernew Tel: +44 1509 282000 Newport NP10 8FZ Customer contact: Grant Rogers Date of issue: 29/11/2019 Project No.: 10165347 Organization unit: Gas Technology Report No.: 10165347-1, Rev. 1 Document No.: 10165347-01 Applicable contract(s) governing the provision of this Report: Wales & West Framework Objective: To determine if pipeline HN039 requires to be replaced or can remain in service with routine care & maintenance. Prepared by: Verified by: Approved by:
Stewart Myles Andrew Ramage Michael Pritchard Prncipal Consultant Senior Consultant Vice President
Michael Gardiner Principal Consultant
Adam Farrance Senior Engineer
Joanna Lucas Senior Geotechnical Engineer
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Rev. No. Date Reason for Issue Prepared by Verified by Approved by
1 2019-11-29 First issue S Myles A Ramage M Pritchard
M Gardiner
A Farrance
J Lucas
DNV GL – Report No. 10165347-1, Rev. 1 – www.dnvgl.com
Table of contents
1 EXECUTIVE SUMMARY ...... 1 1.1 Conclusions 1 1.2 Recommendations 2
2 INTRODUCTION ...... 3 2.1 Purpose of this Report 3 2.2 Problem Statement 3 2.3 Scope of Review 6
3 PREVIOUS REPORTS ...... 7
4 CURRENT MAINTENANCE ACTIVITIES...... 8
5 CONSEQUENCES OF FAILURE ...... 9 5.1 Operational & Supply Consequences 9 5.2 Legal Penalties 10 5.3 Reputational Damage 10
6 GEOTECHNICAL REVIEW ...... 11 6.1 Pipeline Route & Environmental Setting 11 6.2 Geological Setting 12 6.3 Hydrological Setting 13 6.4 HN039 Geotechnical Issues 14 6.5 Geohazard Summary 14
7 OPTIONS CONSIDERED ...... 16 7.1 Option Baseline: ‘Business as Usual’ 16 7.2 Option 1: Replace 1964-Vintage Pipeline 16 7.3 Option 2: Downrate to 7 Bar 16 7.4 Option 3: Replace Entire Pipeline 17 7.5 Option 4: Proactive Cut-Outs 17 7.6 Retained Options 17
8 COST ESTIMATES FOR RETAINED OPTIONS ...... 19 8.1 Baseline 19 8.2 Option 1 19 8.3 Conclusion 19
9 REFERENCES ...... 20
APPENDIX A ACRONYMS & ABBREVIATIONS ...... 21
APPENDIX B STRIP MAPS ...... 22
APPENDIX C FIGURES ...... 34
APPENDIX D TABLES ...... 42
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1 EXECUTIVE SUMMARY Wales & West Utilities (WWU) have a non-piggable pipeline (HN039) in the West of Wales. This pipeline was built in 1964 between Derwenlas and Tywyn. It was extended in 1979 to the town of Aberdyfi. This pipeline now supplies some 370 Non-Domestic and 13,500 domestic customers.
The original section of pipeline was built when the standards for high-pressure pipelines were not as well developed and material technologies were not as robust as more modern systems, therefore, aspects of its design and build do not meet any of the standards that now apply, or any previously recognised standards.
The original section of this pipeline has a history of features. Corrosion has been reported, and there have also been a number of leaks that have required replacement of short sections of the pipeline. This pipeline is one of only a few onshore pipelines in the UK to have leaks associated with it where third party interference was not the cause. (The other pipelines have in most cases been replaced or where viable been repaired where this has satisfied a fitness for purpose assessment.)
WWU commissioned DNV GL to review the history of the pipeline, including undertaking a review of work previously done by GL Noble Denton and Pipeline Integrity Engineers as well as a number of internal activities undertaken by Wales & West Utilities.
1.1 Conclusions We have briefly reviewed previous reports and freely-available information to understand credible hazards, known faults and potential outcomes for loss of supply.
There are geohazards along the route of the HN039 pipeline which warrant further consideration and could be expected to lead to pipeline exposures in the future. The age, thin wall, and nature of the pipeline’s joints suggest it will be less resilient to ground movement and changes in stress conditions than a pipeline which conforms to any edition of TD/1. Reactive interventions for geohazards are likely to be required in the future, which may involve new fittings and/or excavation of the pipeline which can cause additional stress on nearby sections of the pipeline.
Stress Corrosion Cracking (SCC) is a credible threat to this pipeline due to its historical use. This is a result of the pipeline previously transporting manufactured gas, although for more modern construction techniques this should be a dormant condition. For this pipeline, the construction techniques, choice of materials, and the likely changing ground conditions mean that it is still to be treated as a credible threat mechanism. As such this requires additional mitigation methods to be employed. Previous research has concluded that SCC can initially manifest as a pin hole leak. Although not confirmed as SCC, this pipeline has suffered from pin hole leaks. SCC often manifests at low points in the geometry of the pipeline which can coincide with features such as river crossings, which are themselves prone to ground movement. Analysis of the data provided for this report indicates there may have been failures at locations where you might expect both SCC and ground movement. It may, therefore, be construed that SCC was the cause of these failures.
Reduction in pipeline pressure, especially down to atmospheric, can impose significant additional stresses on a pipeline especially where it is subject to loading. Should a significant failure occur on the pipeline, this may then cause additional defects to propagate with the potential to fail shortly after re- commissioning.
In the event of a serious defect resulting in the pipeline having to be shut down, the regulatory authorities may impose sanctions on WWU which would prevent WWU from re-introducing gas into this pipeline until certain assurances are made as to whether it is safe to do so. This could prove very
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challenging for those being tasked with making this decision, as the normal method of hydrotesting would not be possible in this case.
There are 539 vulnerable customers in the towns of Tywyn and Aberdyfi, as well as there being a hospital supplied from this pipeline. Despite significant processes and procedures being in place to ensure that these members of the public are suitably cared for, there is the potential for some to be badly affected should gas supplies fail in an un-planned manner.
The financial analysis undertaken by WWU concludes that if the pipeline was approved for replacement in GD2, then the cost of replacement versus the ongoing increased costs of maintaining and repairing the existing pipeline will show a cost benefit in 2035.
Based on the known history of the work that has been undertaken on this pipeline, and the works undertaken by WWU since 2005, it is concluded that there is both a safety and technical justification for the replacement of the pipeline. The key weakness in the model used for financial justification is that the unknown costs associated with potential future leaks cannot be clearly defined. An increase in the number of leaks, which is a credible scenario, and the potential for a complete loss of supply to the towns of Tywyn and Aberdyfi should be anticipated.
The available evidence supports that replacing the pipeline during GD2 is a cost-effective solution, as the condition of the existing pipeline is susceptible to damage and the maintenance costs will continue to increase. The pipeline will still require to be replaced in the near future, with significant additional costs incurred if the pipeline has to be replaced under emergency conditions.
1.2 Recommendations WWU should undertake the following work to support justification of pipeline replacement in GD2:
WWU should undertake an additional financial analysis of retaining the pipeline but building in a scenario whereby there is a pipeline failure that results in loss of supply in the towns of Tywyn and Aberdyfi, but where the pipeline can be re-commissioned as quickly as possible. This analysis can then determine a new financial breakeven point should such a scenario occur.
Should the complete replacement of the pipeline not be approved, the WWU should undertake the following works to ensure that the investment in maintaining this pipeline provides a suitable return on the investment both in terms of safety and technical compliance.
• Undertake a full assessment on any future joints that are removed to specifically look for signs of internal stress corrosion cracking (ISCC).
• Undertake a risk review of the geohazards and maintenance frequencies to determine if the current regime remains suitable for this pipeline. This should also determine if any additional works give a greater assurance that the increased cost will detect or prevent additional failures before they actually occur.
• Review the pipeline crossing at Nant Caer to verify if the plant in question is an invasive plant and make arrangements to manage any future inspections or repair works of this and nearby crossings accordingly.
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2 INTRODUCTION Wales & West Utilities’ (WWU) HN039 is a non-piggable 18.6 km high-pressure (HP) carbon steel pipeline from Derwenlas to Aberdyfi. It has a nominal diameter of 100 mm and declared MOP of 24.1 bar.
HN039 is the sole supply to Tywyn and Aberdyfi, serving 13,500 domestic and 370 commercial customers.
Figure 1 shows the pipeline route and Appendix B contains strip maps.
2.1 Purpose of this Report WWU is actively managing ongoing integrity issues associated with pipeline HN039 through both proactive and reactive maintenance. However, maintenance costs are high and increasing.
WWU commissioned DNV GL to prepare this report as a technical review of the case for replacing HN039 with a new-build pipeline, to be used in considering options for cost-effectively ensuring public safety and security of supply.
2.2 Problem Statement 2.2.1 Construction History HN039 was built from Derwenlas to Tywyn in 1964, with the extension from Tywyn to Aberdyfi constructed in 1979.
Line pipe jointing on the original section used the ‘spigot and socket’ method but the Aberdyfi extension was conventionally girth-welded. Spigot and socket welded joints are not a conventional jointing technique for high-pressure pipelines, but are likely to have been used at the time of construction as being the best option available.
There are no available photographs of the joints used on this pipeline, but the photograph above shows a spigot and socket joint on a water pipe, to show the general configuration of such a joint. The joints within HN039 are not as well manufactured, and do not have a sealing ring, but are fillet welded on the outer joint.
In 1964 there were no published pipeline codes used within the UK but the IGE had prepared initial recommendations for the Gas Council in 1956 /1/. The Gas Council used those recommendations for the preparation of their Pipelining Specification /2/, first issued in 1962. These pipeline specifications recommended the use of API 5L or API 5Lx, which specified appropriate material properties. However, production and QA procedures at the time allowed pipes to leave the pipe mill with inconsistent material properties. These recommendations also indicate that butt welding should be the jointing method, but
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for reasons that cannot be obtained, the decision was made to manufacture joints using a spigot and socket technique.
The Gas Council document, which gives guidance on appropriate depths of cover, was initially published in 1962. This indicates that new pipelines will be constructed with a cover depth of 3ft, but reduced depth of cover is permitted within the specification.
The section of line between Tywyn and Aberdyfi was built to IGE/TD/1 Edition 2 (TD1).
The pressure reduction station (PRS) at Tywyn was reconstructed in 1982.
Considering the content of IGE Communication 6741 /3/ and subsequent revisions, the following aspects of HN039’s construction would now be regarded as being sub-standard:
Material toughness properties.
Toughness is the ability of a material to absorb energy and plastically deform without fracturing. This property within a steel pipeline provides protection from the effects of third-party interference. The original section of this pipeline is recorded as being built with API 5L Grade B line pipe. At the time of construction, quality control procedures were not as robust as the present day, and the material properties cannot be verified as meeting appropriate toughness requirements but very likely to be below what is now accepted. Additionally, it cannot be verified as to how the socket joints were formed, but it is likely that this process has reduced the toughness properties further. In other pipelines within WWU with this joint technique, cracking has been found at the back of the socket. These joints have had to be cut out and replaced, so that they never reached a similar time in service as those in HN039.
It can therefore be assumed that this pipeline’s material toughness does not provide what would now be regarded as minimum acceptable resistance to the failure of any damage.
Wall thickness.
The non-TD1 section was constructed using 4.8 mm wall thickness pipeline. This is less than would be required for a modern design. As well as increasing the amount of stress within the pipeline and increasing the likelihood of failure caused by third-party interference, welding on to live pipelines <5mm thick presents additional complexities and risk, and requires specially- approved welders. In certain circumstances, some companies even prohibit welding onto thin wall live pipelines due to the risk of burn-through.
Jointing techniques.
The non-TD1 compliant section used a basic form of spigot and socket arrangement, with a fillet weld being used to prevent leakage. This increases the amount of residual stress within the joint and is not an approved jointing technique for any pressure tiers — especially not for a major accident hazard pipeline.
Reduced Depth of Cover.
Reduced cover increases the likelihood of third-party interference.
1 The first edition of the standard later known as IGE/TD/1 and now IGEM/TD/1.
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The non-TD1 compliant section was built with a nominal minimum cover of 0.9 m (3ft), whereas the current revision of TD1 requires a minimum of 1.1m. Based on the findings from the previous GL report, it was recorded that some sections had as little as 0.5m cover. This reduced depth of cover is likely to have been in place since construction as opposed to a reduction in cover during service. However, natural erosion may have added to the reduced depth of cover in some locations. There are no records available to validate the initial depth of cover where not in compliance with the Gas Council requirements.
As can been seen from the above, the materials and construction techniques used for the original section of HN039 increase the consequence of a third-party impact. The reduced depth of cover increases the probability of third-party damage. 2.2.2 Operating History The Derwenlas to Tywyn section was commissioned before conversion to natural gas; therefore, it will have previously transported town’s gas, which is known to have contributed to formation of Internal Stress Corrosion Cracking (ISCC) in certain circumstances. Although no internal examination has been undertaken on this pipeline, the presence of ISCC can be expected. 2.2.3 Failure History HN039 is one of only a few HP pipelines in the UKOPA database with any record of gas escape, other than losses associated with third party interference.
Table 1 lists all FR1 fault reports for the HN039 pipeline.
All leaks were repaired by stoppling to replace a short section of pipeline unless the leak was close to a major river crossing, in which case the complete crossing was replaced.
Table 1 shows that:
All recorded faults were located on the original 13Km section of the pipeline, laid in 1964.
There have been no reported failures for the 5Km section of pipeline between Tywyn and Aberdyfi.
Cracks and pin-hole leaks are the key failure modes.
It cannot be confirmed whether these failures were caused by ISCC because the removed sections were not examined for this failure mechanism. However, having reviewed the location of the defects, it is noted that the defects are located at low points, and concentrated in 3 areas. As shown in Section 6, these failures are also concentrated in areas prone to flooding. This suggests a direct relationship between the failures and higher stresses associated with environmental loading.
It is recommended that WWU should look to undertake a full assessment on any future joints that are removed to specifically look for signs of SCC and to also determine if there are any additional failure scenarios that may be indicated from a full assessment of these joints.
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2.3 Scope of Review The area covered by this report is shown in Figure 1.
Following discussions with key WWU staff and an initial document review, it was found that:
The section of HN039 between the tie-in near Tywyn PRS and the PRS at Aberdyfi, which was built to a recognized code, does not suffer from the same issues as the original Derwenlas to Tywyn section.
All sections between the connection of HN039 with pipeline HN040 and the tie-in on the North side of the River Dyfi were replaced following previous failures of the pipeline.
Therefore, unless there were to be a clear benefit in a completely new route, pipeline replacement should run from the tie-in on the North side of the River Derwent and the connection at Tywyn PRS.
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3 PREVIOUS REPORTS DNV GL has reviewed the reports from two previous investigations of this pipeline:
DNV GL report2 11432 /4/, and
Pipeline Integrity Engineers (PIE) report R261 /5/.
We note that:
The recommendations of both reports remain valid.
o In addition, the geotechnical review in Section 6 provides more evidence of a relationship between failures of HN039 and flood zones.
Reference /5/ points out that ISCC may appear as pin hole leaks on the pipeline. This does not appear to have been considered by WWU.
Given that risk factors for ISCC are present, and that several failures have been recorded as pin hole leaks, we conclude that:
o ISCC is likely to be present, and
o The additional stresses imposed on the pipeline through ground movement (for example as a result of “predictable” flooding and more extreme weather events (climate change), may cause any existing cracks to propagate, potentially fail as a leak and this should be anticipated.
2 Published as a GL Noble Denton report
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4 CURRENT MAINTENANCE ACTIVITIES WWU are maintaining HN039 in accordance with T/PM/MAINT/5, applying standard risk methodologies to determine the frequency of examination. Table 2 lists the activities and frequencies being used.
In addition to the minimum requirements of T/PM/MAINT/5, WWU are undertaking leakage surveys to locate failure points whilst they are small leaks, and prior to becoming unmanageable defects.
This maintenance and inspection regime is the most effective practicable regime at the present time. However, there is an increasing likelihood of failure because of the ageing nature of the asset, its operating history, and known deficiencies of design and construction3. Therefore, DNV GL recommends that:
There should be a detailed risk review to determine the specific operating risk of this pipeline, taking account of its history.
Results of this review should be used to demonstrably align inspection and maintenance (I&M) frequencies to the current specific operating risk. This exercise must balance the increased costs of undertaking more activities against their potential for preventing further failures from occurring, so that risk will be made ALARP4. It is likely that there will be situations where the risk due to the potential failures cannot be accurately predicted so that, no matter what additional activities are undertaken, the additional Opex would provide no benefit to WWU stakeholders.
3 As previously noted, these deficiencies are ‘products of their time’ for a pipeline of this vintage. There is no suggestion of negligent behaviour by WWU or its predecessors. 4 As Low As Reasonably Practicable.
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5 CONSEQUENCES OF FAILURE There has not been a major incident on HN039 but the likelihood of failure of this 1964 pipeline should not be overlooked. The age and history of the pipeline, together with the threats highlighted in this report, mean that future risks from failures are credible and their likelihood can be expected to increase.
5.1 Operational & Supply Consequences 5.1.1 Loss of Supply HN039 is the single feed supply to the towns of Tywyn and Aberdyfi, with a combined customer base of 13,500 domestic and 370 commercial customers. If a major leak were to occur on this pipeline, whereby the whole pipeline had to be shut down, then supply would be lost to all these customers. WWU estimates that it would take:
Approximately 4 days to ensure that all customers affected were suitably isolated from the network before any gas could be re-introduced to the network. This is based on the data within the WWU major supply loss calculator.
Between 1 and 5 weeks, depending on the severity of the incident, to make a suitable repair on the pipeline. Again, this is based on the data within the WWU major supply loss calculator. However, it should be noted that should a significant failure occur on this pipeline, then it may not be possible to re-introduce gas into the pipeline until it has been proven to be safe to do so. It may therefore be necessary to rebuild the pipeline as an emergency exercise to re-connect gas supplies to the customers in Tywyn and Aberdyfi. 5.1.2 Emergency Response WWU have a robust Emergency Response system that is in line with good industry practice. This includes ensuring that trained staff are available for immediate emergency management and providing for prompt availability of contractors and specialist equipment. 5.1.3 Societal Risks Even with suitable emergency planning provision there is always a threat to vulnerable customers from such an incident, especially in the winter months. Any delay in getting suitable support to these customers can result in hospitalization or worse5. At present, WWU have identified that there are 539 vulnerable customers between the towns of Tywyn and Aberdyfi. This figure does not include vulnerable people who are in the local hospital. 5.1.4 Economic Impact Compensation would be payable to domestic customers at standard rates for the duration of a loss of supply.
Industrial and commercial customers could suffer significant financial damage as a result of their gas supplies being removed for any significant time. Depending on the terms of their contracts, these customers might well seek to recover losses from WWU.
5 It is assumed that societal consequences such as these will be analysed by WWU as part of the CBA for interventions on HN039.
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5.2 Legal Penalties It can be reasonably assumed that WWU would be subject to substantial financial penalties from the HSE if there were to be another leak from HN039. Corporate and possibly individual prosecutions for offences up to and including manslaughter, if appropriate, would also be highly likely, especially in view of the pipeline’s history of leaks.
5.3 Reputational Damage Reputational damage from a leak and loss of supply on this scale would be extremely significant to a conscientious network operator like WWU. Such an event could be expected to be heavily reported in the national press and would be likely to be noted in international trade publications. There would be sharp regulatory and press focus on the suitability of continuing to operate a pipeline of this design well beyond its original design life.
Additionally, the town of Tywyn has a hospital. Although classed as a dual fuel customer, recent events with National Grid have shown that although they are not directly responsible for the failure of the back up system when a network goes down, the negative publicity for the loss of fuel to such an establishment will initially sit with WWU.
As noted above, it is also highly likely that there would be some form of prosecution for offences related to the failure to manage pipeline integrity effectively. Since prosecutions necessarily take place some time after the events to which they refer, this would serve to ‘re-ignite’ public interest in and scrutiny of WWU’s safety culture and give a second blow to the company’s reputation.
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6 GEOTECHNICAL REVIEW The environmental setting and geohazards present along the pipeline route are critical to understanding the risk posed to the pipeline by ground movement and third-party activities.
Ground movement may be small scale and/or slow, through to catastrophic events such as a landslide. It can be responsible for exposing the pipeline, ovalization, development of leaks at joints or rupture of the pipeline.
Third-party activities (such as, in this case, farming, forestry, highways, and construction) can additionally load the pipeline causing circumferential and/or longitudinal bending which may exceed the pipeline’s tolerance and cause permanent damage, leakage or rupture.
6.1 Pipeline Route & Environmental Setting Reference /4/ has an extended section describing the pipeline route and broad descriptions of the environmental setting. The following points summarize environmental and geotechnical issues taken from /4/:
The original pipeline route was laid from the valve bridle now identified as Quay Ward adjacent to the railway line just off the A487(T) road outside the village of Derwenlas. This is approximately 3 km south west of Machynlleth. Because of the topography of the area the pipeline continues westwards along the Dyfi Valley, generally following the (Lon Las Cymru) Class B road towards Tywyn.
Previous IGE/TD/1 Reports have not identified any high-density road, rail or watercourse crossings. However, risk assessments have been required because of the below [current] standard wall thickness of pipe used.
Records show that there was a diversion on the River Dyfi crossing and further work at the Afon Pennal crossing (in 1988 and 1992, respectively) indicating riverbank or bed erosion. This was clearly visible at the River Dyfi during the pipeline walk in August 2011.
The topography of the route starts off fairly flat from the River Dyfi and through the Dyfi Valley, but rises quite steeply up over the Happy Valley area and through several densely-wooded areas (Coed-y-Gelli and Coed Cefn-Coch) before dropping down and running westwards in and out of the mountain roads and fields towards Tywyn.
There are several steep slopes all along the route and, although there are no other major watercourse crossings, the pipeline does cross several ditches; some of these are quite wide. At the time of the route walk in August 2011 many of the fields were extremely wet with standing water covering many areas.
There are two areas showing possible signs of early-stage subsidence along the road around Llechwedd Melyn and Craig Pant-yr-Owen, which is adjacent to an old lead mine and requires further investigation.
Farther west from this location, the pipeline is exposed with ongoing loss of cover where the pipeline has been laid up a steep slope behind the property known as Pen-y-Bont. Flood water running off higher fields is resulting in washout and soil erosion around tree roots in the vicinity of and parallel to the pipeline.
There are many other areas on the original construction section (Derwenlas to Tywyn) where the depth of cover was found to below acceptable levels. It should be noted that a few areas were
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severely overgrown which restricted full access and suitable vantage points were achieved from the field boundaries, roads, lanes and footpaths.
From the above points it can be seen that the pipeline crosses terrain which would warrant additional design consideration; for example, the Dyfi Estuary, other river crossings, forestry land, agricultural land, road crossings, wet or boggy ground, steep slopes, surface water run-off zones and former mine areas. 6.1.1 Invasive Plant Species On review of a previous report /4/ for this assessment, a photograph of a river crossing of Nant Caer (reproduced in Figure 2) was noted to apparently show an invasive plant species (Japanese knotweed) at the crossing. This may interfere with future access to the crossing for both visual and physical surveys and, if earthworks or repairs are required at the crossing, will be extremely costly and time consuming to remove. It is recommended that WWU verify if the plant identified at the crossing is an invasive plant species and prepare to manage future inspections and any remedial works of this and nearby crossing accordingly.
6.2 Geological Setting The shape file for the HN039 pipeline was combined with maps from the British Geological Survey (BGS) Onshore Geoindex /6/. Figure 3 shows the combined mapping with the pipeline shown in dark red. There is a complex underlying geology due to folding and faulting in the area and this is emphasized by the presence of valleys cutting into the successions. 6.2.1 Superficial Deposits Superficial deposits exist in the Dyfi and Happy valleys (as mapped in Figure 4) and are at their thickest in low lying areas. The type and locations of these deposits is summarized below:
The pipeline within the Dyfi Estuary is situated in Tidal Flat deposits (clay, silt, and sands), which graduates into Alluvium (clay, silt, sand, and gravel) around Pannal.
Fluvial deposits (shown as yellows and orange in Figure 4) are good indicators of the extent and position of past and existing river systems and flood extents. There are several areas where the route of HN039 traverses fluvial deposits, which are often suspectable to erosion.
Head deposits (poorly sorted clay, silt, sand, and angular gravel) are noted around Cwrt, Gwyddgwion Farm and Bod Talog.
Glacial deposits (Till (diamicton) and hummocky mounds) are noted around Tyddyn Gwilym to Tyddyn-Y-Briddell Farm. Till also extends from Tollgate Cottage to Tywyn.
Alluvium and Alluvial Fan deposits (gravel) cut into the glacial deposits along the Happy Valley and Alluvium deposits are typically associated with the course of the Afon Dyffryn-gwyn.
Superficial deposits are typically unconsolidated and/or comprise compressible layers and therefore are prone to movement such as settling due to loading and vibration and are relatively easily eroded, especially by flowing water. This can result in the soils supporting the pipeline being washed away and pipeline exposure. 6.2.2 Solid Geology The solid geology (bedrock) mainly consists of mudstones, slumped mudstone sequences, siltstones and sandstone (see Figure 5). Exposed and upper sections of these sequences are likely to weather to clays.
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To briefly summarise the structure there are a series of anticlines and synclines and several faults in the area. Some persistent faults strike west-east while less persistent discontinuities strike north-south. Mineralization may be associated with the faulting.
Very few historical boreholes are recorded in the BGS repository. There is a mineral site recorded close to Gwynfan for lead mining (Melyn Llyn Pair), which was researched by DNV GL using the Royal Commission on the Ancient and Historical Monuments of Wales website (reference /7/)6. The website indicated that this was a former lead and silver mine, believed to have worked until 1884. It is shown as disused on the Ordnance Survey first edition 25” mapping of 1889. The pipeline strip maps (see Appendix B) record multiple shafts, tips, sluices, conduits and kilns in the immediate area.
There is a glacial overflow channel centred around the source of the Afon Dyffryn Gwyn and Happy Valley.
The complex geological structure of the region exposes discrete sequences of rocks near surface, which may give rise to differential settlement. As the pipeline is for the most part buried in the Superficial Deposits this is not considered to be significant, however collapse of the mining areas near Gwynfan close to the pipeline route would have the potential to affect the pipeline. 6.2.3 Geological Summary Although the route of the pipeline crosses a plethora of different rock and soil units these pose limited geohazard risk to the pipeline. However, it should be expected that some differential settlement could occur as the pipeline passes over soils and weathered rocks with differing geotechnical properties.
Some ground movement should also be expected in superficial soils which are loaded or subject to vibrations.
The presence of fluvial deposits suggests the pipeline route will interact with watercourses and these areas maybe more prone to erosion from river, flood, or overland scour.
Mineralization may be associated with the faulting; this may have been mined, potentially causing subsidence due to settling waste tips or collapsing shafts.
6.3 Hydrological Setting 6.3.1 Sections of HN039 in Flood Zones The shape file for the HN039 pipeline was combined with flood data provided by Natural Resources Wales (reference /8/). Figure 6 shows the combined mapping.
Figure 6 shows that several sections of the pipeline (shown in dark red) are within Flood Zones (shown in blue). Flood zones are categorized as follows:
Flood Zone 2 is designated as the extent of a flood from rivers or sea that has up to a 0.1% (1 in 1000) chance of happening in any given year.
Flood Zone 2 is important to planning as it form the basis of Zone C for the Welsh Government Development Advice Map (DAM).
Zone 2 includes areas recorded to have been flooded in the past.
6 It is recorded in /5/ as the ‘Caethlu Mine; Aberdyfi No. 3, Felin-Llyn-Pair Lead Mine, Happy Valley, southeast of Tywyn’.
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Flood Zone 3 is designated as the combined extent of:
o A flood from rivers with up to a 1% (1 in 100) chance of happening in any given year, and
o A flood from the sea with a 0.5% (1 in 200) chance or greater of happening in any given year.
Figure 7 shows Flood Zone 2 (blue) in the locality of HN039.
Figure 8 shows Flood Zone 3 (light blue) in the locality of HN039.
The main sections of HN039 in flood zones are between Derwenlas and Pennal, and at several locations along the Happy Valley.
Based on information from Natural Resources Wales, in addition to above:
Low to medium surface water flooding risk should be expected in Pennal, Cwrt, and in the villages in Happy Valley.
There is also a reservoir flood risk in River Dyfi for 0.3-2.0m and a slightly wider area at 0-0.3m from a dam burst from Lyln Cwm byr, Powys. 6.3.2 River Crossings in Flood Zones All the river crossings identified in /4/ are in, or adjacent to, Flood Zones 2 and 3 (see Figure 6). Most of these river crossings were also noted to be subject to flooding from surface water flow.
6.4 HN039 Geotechnical Issues Certain aspects of HN039’s geological and hydrological setting warrant further consideration of threat magnitude given the pipeline’s age, joints, material properties, and construction history.
Activities and processes relevant to this pipeline and the subsequent potential threats to integrity are summarized in Table 4.
A number (No. 15) of known or probable geotechnical issues were identified during this review and have been listed in Table 5.
Further assessment would be required to quantify the degree of risk posed by these issues.
6.5 Geohazard Summary A number of credible geohazard threats to NH039 have been identified by reviewing freely available mapping and aerial data. These include:
o Location in flood plains.
o River crossings.
o Road/Track crossings.
o Interaction with forestry operations.
o Interaction with farming activities.
o Steep slopes.
o Wet or boggy ground.
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o Proximity to former mining areas.
The pipeline is likely to be susceptible to damage from ground movement and flooding, especially when exposed to changes in loads or additional permanent loading, on account of:
o Non-standard joints (socket and spigot).
o Thin wall.
o Reduced depth of cover.
It is noted that:
o Areas of historical reduced cover in fields and slopes have been noted on previous line walks.
o There are many watercourse crossings along the route. Most of the river crossings and some longer sections of the pipeline are within Flood Zones 2 or 3.
o The pipeline has been exposed at watercourse crossings in the past. Further exposure should be expected in the future with potential for damage from rock and debris strikes, and washouts. Therefore, pipeline integrity cannot be assured at the river crossings and within the floodplain given the risk of erosion, age of HN039, its properties, and the lack of protection measures.
These threats to the pipeline’s ongoing integrity will require management. DNV GL recommends that line walks and inspections after periods high rainfall or flooding would be a reasonable minimum precaution.
However, it should be recognized that reactive interventions for geohazards will almost certainly be required in the future, which may involve new fittings and excavation of the pipeline. Such activities cause additional stress on nearby sections of the pipeline and would likely need to use stress analysis to assess the effects on socket and spigot joints and materials.
It is also worth noting that depressurization of the pipeline can lead to additional damage occurring to the pipeline where high loads are acting on the pipeline. These loads can cause additional circumferential or longitudinal stress and may exceed the capacity of the joint to withstand the movement.
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7 OPTIONS CONSIDERED DNV GL has discussed options for safely and efficiently maintaining security of supply to Tywyn and Aberdyfi with members of WWU’s Asset Integrity and Operations department7. The purpose of this was to determine which, if any, options could be discounted from further consideration.
The options covered were:
Option Baseline — ‘business as usual’.
Option 1: Replace remaining parts of the 1964 construction (approximately 13 km).
Option 2: Downrate to 7 bar.
Option 3: Replace all 18 km of HN039, including the Aberdyfi extension.
Option 4: Proactively cut out all socket and spigot joints.
Each option is described in Sections 7.1 to 7.5, with Section 7.6 summarizing those that have been retained for cost benefit analysis (CBA).
7.1 Option Baseline: ‘Business as Usual’ The ‘business as usual’ option would continue I&M according to T/PM/MAINT/5 (Table 2), making reactive repairs as necessary.
This option has been retained as the baseline case for financial analysis. However, as a prudent network operator WWU considers that this option will not sufficiently protect Tywyn and Aberdyfi from loss of supply. The trend is that HN039 is now beginning to see a rise in incidents; given the age and construction of this pipeline, there is concern that a larger-scale incident will eventually occur, causing a complete loss of supply to almost 14,000 customers in Tywyn and Aberdyfi (with consequences as discussed in Section 5).
7.2 Option 1: Replace 1964-Vintage Pipeline Following a failure8 in 2017, HN039 was replaced between its connection with pipeline HN040 and a point on the north side of the River Dyfi.
Option 1 would replace HN039 from this point to the existing connection with the entry to Tywyn PRS and the T-piece supplying the pipeline to Aberdyfi. This represents all the remaining pipeline that was constructed in 1964, with an as-laid length of approximately 13 km.
This option has been retained for financial analysis.
7.3 Option 2: Downrate to 7 Bar Under this option the MOP of HN039 would be reduced to 7 bar and the pipeline would be operated under IGEM/TD/3 (reference /9/). This would substantially reduce operating stresses and thus, potentially, the overall consequence of failure. However, any external stresses such as those from ground movement or hydrological effects would be unchanged.
7 At a meeting in Cardiff on 20 August, 2019. 8 Fault Report 9 in Table 1.
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Network analysis performed by WWU showed that the existing operating parameters are necessary to maintain minimum pressures at system extremities because of the pressure drop along the 100 mm pipeline. Therefore, this option was not retained.
7.4 Option 3: Replace Entire Pipeline This option is the same as Option 1, but also includes replacement of the 5 km Aberdyfi extension.
This option was not retained because the Aberdyfi extension was built to an approved code9, is of a newer vintage (hence will have been built using qualified welding procedures on line pipe with appropriate fracture toughness), and has no recorded damage or failure history.
7.5 Option 4: Proactive Cut-Outs This option would replace all spigot and socket joints by cutting out and replacing with girth-welded pups. This would require cut-outs every 12m, with stopple tees being required for each operation.
There are approximately 1083 spigot and socket joints on HN039. From previous experience each joint replacement would cost in the order of £80k per joint, giving a total cost of £86.64m
This option was not retained because it is deemed to be too expensive, not efficient and not in the customers interests.
7.6 Retained Options Two options have been retained for financial analysis:
1. Baseline: ‘Business as Usual’.
2. Option 1: Replace original 1964 pipeline.
Each retained option has ‘pros and cons’ that feed into determining which is the more appropriate solution.
For the Baseline option:
There is no planned Capex investment required in GD2.
Opex and Repex can be expected to increase from current levels over time because of:
o Increased need for I&M of an ageing pipeline with the weaknesses and threats highlighted in this report.
o An increasing likelihood of more failures requiring reactive repair and replacement.
There is non-trivial risk of loss of supply to 13,500 domestic and 370 commercial customers through failure of the pipeline, with the direct and indirect consequences described in Section 5.
For Option 1:
There is substantial initial Capex.
WWU could expect reduced Opex and zero Repex because:
9 Edition 2 of IGE/TD/1, which applied at the time of the extension’s construction.
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o All risk-based proactive I&M would be performed at maximum intervals.
o Reactive I&M, including repairs and replacements, should not be required for at least the design life of the new-lay pipeline.
Security of supply to Tywyn and Aberdyfi would be assured.
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8 COST ESTIMATES FOR RETAINED OPTIONS A cost analysis /10/ has been undertaken by WWU for the two retained options.
8.1 Baseline To continue operating the existing pipeline, WWU have estimated that £310k will be required for maintenance/repair of this pipeline in 2022. This cost will increase by approximately £10k per annum through the period of RIIO-2.
This estimate is based on the need to undertake routine maintenance on the pipeline, together with additional costs included to undertake additional pipeline repairs that are expected to appear.
In addition to this, WWU have estimated that a budget of c£2m is required to cover potential fines from the HSE as a result of the number of leaks being recorded on this HP gas pipeline. (Based on recent fines being issued from the HSE, this value might be seen as a conservative estimate.)
This equates to a total spend of £1.65m in RIIO-2.
A review of the build-up of these costs has been undertaken, and it is our opinion that these are in line with what has been undertaken to date, and seem realistic.
Business-as-usual spend does not cover the cost of the failure of the pipeline resulting in loss of supplies to Tywyn and Aberdyfi.
8.2 Option 1 WWU have prepared an estimate for replacing the original section of this pipeline.
The capital cost of replacing the 1964-vintage section of pipeline is estimated to be £13.29m. This equates to an average cost of £1m per Km. It is our opinion that this figure is in line with the construction of similar pipelines of this size.
WWU have built these costs into the OFGEM spreadsheet, and this concludes that the cost of replacing the pipeline would cross the breakeven point in 2035.
8.3 Conclusion The cost of replacing the original section of pipeline is reported as breaking even during 2035.
It is our opinion that these timescales conclude that replacing the pipeline now is a cost-effective solution, as with the condition of the existing pipeline is susceptible to damage and the maintenance costs will continue to increase, and will still require the pipeline to be replaced in the near future, with significant additional costs if the whole pipeline had to be replaced under emergency conditions.
It is also recommended that WWU undertake an additional financial analysis of retaining the pipeline but building in a scenario whereby there is a pipeline failure that results in a cessation in the towns of Tywyn and Aberdyfi, but whereby the pipeline can be re-commissioned as quickly as possible. This analysis can then determine a new breakeven point should such an occurrence occur.
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9 REFERENCES /1/ Recommendations Concerning the Installation of High-Pressure Pipelines, Institution of Gas Engineers, 19th March 1956 Institution of Gas Engineers
/2/ Pipelining Specification, The Gas Council, 1962
/3/ Steel pipelines and associated installations for high pressure gas transmission, Institution of Gas Engineers, Communication 674, 1965
/4/ Integrity inspection of the Derwenlas to Aberdyfi (HN039) pipeline, GL Noble Denton report 11432, September 2011
/5/ Review of pipeline integrity assessments for Wales and West Utilities, PIE report PIE/12/R0261, September 2012
/6/ British Geological Survey (BGS) GeoIndex available at: https://www.bgs.ac.uk/geoindex/home.html?src=topNav
/7/ Royal Commission – Ancient Monuments of Wales available at: https://rcahmw.gov.uk/
/8/ Flood Maps for Wales available from: https://naturalresources.wales/evidence-and- data/maps/long-term-flood-risk/?lang=en
/9/ Steel and PE pipelines for gas distribution, Institution of Gas Engineers and Managers, IGEM/TD/3 Edition 5, October 2015
/10/ HN039 Cost Benefit Analysis Spreadsheet, Ofgem developed spreadsheet completed by WWU, 2019
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APPENDIX A ACRONYMS & ABBREVIATIONS BGS British Geological Survey
CBA Cost Benefit Analysis
CIPS Close Interval Potential Survey
CP Cathodic Protection
HP High Pressure
IGE Institution of Gas Engineers
IGEM Institution of Gas Engineers and Managers
I&M Inspection and Maintenance
ISCC Internal Stress Corrosion Cracking
MOP Maximum Operating Pressure
NPV Net Present Value
PIE Pipeline Integrity Engineers Ltd.
PRS Pressure Reduction Station
UKOPA United Kingdom Onshore Pipeline [Operators’] Association
WWU Wales and West Utilities
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APPENDIX B STRIP MAPS The following strip maps outline the route of the existing pipeline, together with the proposed start and finish points of the suggested pipeline replacement, as well as the exact locations of the 9 faults recorded in Table 1.
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[Grab your reader’s attention with a great quote from the document or use this space[Grab to emphasize your reader’s a key attention point. with a To placegreat this textquote box from anywhere the document on or use the page,this just space drag to it.] emphasize a key point. To place this text box anywhere on Start of proposed the page, just drag it.] replacement.
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FR 1 No 9
FR 1 No 6
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FR 1 No 3
FR 1 No 4
FR 1 No 1
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FR 1 No 8
FR 1 No 7
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FR 1 No 5
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FR 1 No 2
Extent of Pipeline considered for replacement.
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APPENDIX C FIGURES
Figure 1: Extent of pipeline considered for replacement
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Figure 2: Possible Japanese knotweed at W0030
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Figure 3: Combined geological map for the HN039 area (pipeline route shown as dark red)
See Figures 4 and 5 for Key.
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Figure 4: Geological map — Superficial deposits only
(pipeline route shown as dark red)
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Figure 5: Geological map — bedrock only (pipeline route shown in dark red)
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Figure 6: Flood map showing combined Zones 2 (royal blue) and 3 (light blue), pipeline route shown in dark red.
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Figure 7: Flood map showing Zone 2 (royal blue), pipeline route shown in dark red.
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Figure 8: Flood map showing Zone 3 (light blue), pipeline route shown in dark red.
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APPENDIX D TABLES
Table 1: FR1 fault reports
Fault Reports Location Year Outcome
1 813 SN 660 995 – West of Bryn Llwyn 1980 Coating Damage
2 847 SN 606 995 – Near Erwporth Chalet Park 1980 Leak
3 1192 SH 674 000 – Bryn Llwyn 1983 Crack / Leak
4 1193 SH 672 000 – Bryn Llwyn 1983 Crack / Leak
5 1300 SN 607993 – Pen-Y-Boal 1983 General Corrosion
6 1719 SN 708 994 – South of Pennal 1987 Stretched Bolts / Leak
7 SN 628 986 - Happy Valley 2011 Pin Hole Leak
8 SN 628 986 - Happy Valley 2015 Leak
9 SN 708 996 - River Pennal 2017 Pin Hole Leak
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Table 2: Inspection, maintenance, and other assurance frequencies
Maintenance Activity Frequency
Aerial Surveillance 2 Weekly
CP Monitoring Functional Check Monthly
CP Monitoring Interim Examination Annual
Critical Valve Maintenance Annual
Owner/Tenant Liaison Visit Annual
Sleeve Monitoring Annual
Non-Critical Valve Maintenance 2 Yearly
Above Ground Pipeline Survey 2 Yearly
River Crossing Survey 3 Yearly
TD1 Affirmation Review 4 Yearly
CIPS Scrub Clearance 5 Yearly
CIPS Survey 5 Yearly
CP Major Examination 5 Yearly
OLI4 5 Yearly
Route Walk 5 Yearly
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Table 3: HN039 river crossings
Crossing Minimum DoC (m) Within a Flood Watercourse Easting Northing Ref. (from 2015 survey) Zone10? HN039 / 0040 2.0m in 2001 Yes River Dyfi 270462 299560 (W0020) (survey planned 2016) (Zones 2 & 3) HN039 / 0060 Yes Nant Caer 270236 299793 2.51 (W0030) (Zones 2 & 3) HN039 / 0090 Yes Afon Pennal 270029 300014 0.62 (W0040) (Zones 2 & 3) Yes (or immediately HN039 / 0235 Afon Duffryn Gwyn 265185 299372 2.60 adjacent to) (W0100) (Zones 2 & 3) Yes (or immediately HN039 / 0245 Afon Duffryn Gwyn 264824 299232 2.10 adjacent to) (W0110) (Zones 2 & 3) Yes (or immediately HN039 / 0255 Afon Duffryn Gwyn 263397 298478 0.21 adjacent to) (W0130) (Zones 2 & 3) Yes (or immediately HN039 / 0275 Afon Duffryn Gwyn 262366 298523 0.35 adjacent to) (W0140) (Zones 2 & 3) HN039 / 0335 Yes Afon Duffryn Gwyn 260906 299341 0 (exposed in river) (W0190) (Zones 2 & 3) HN039 / 0490 Yes Afon Duffryn Gwyn 259039 299244 1.53 (W0240) (Zones 2 & 3)
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Table 4: Credible geohazard threats to HN039
Source of Threat to HN039 Activity / Process Threat Pipeline Agriculture Ploughing, movement of Loading from plant, striking of pipeline, heavy plant, soil erosion rutting (reducing cover), reduced depth of cover, unplanned crossings. Forestry, other Businesses and Tracks and crossings Unplanned crossings Housing Unstable slopes after deforestation Forestry and Vegetation growth Tree / plant root Root damage Reduced access to pipeline Subsidence, settlement, and Natural wastage of soils Unsupported or highly stressed sections differential settlement supporting the pipeline of pipeline River / Flood Erosion / scour Washing away of soils Exposure of the pipeline, possible supporting the pipeline by secondary threats from buoyancy, items flowing water striking the pipeline and unplanned stress Flood defences Bunds or embankment over Unplanned loading of the pipeline the pipeline Steep slopes Soil creep, overland flow, Highly stressed sections of pipeline debris flows
1 Interrogation of GIS data would be required to determine exact position in relation to flood zone
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Table 5: Known or suspected geotechnical issues
Issue Source
Ground movement along the road around Llechwedd Melyn. TD/1 Report
Ground movement / subsidence along the road around Craig TD/1 Report Pant-yr-Owen which is adjacent to old lead mines.
Pipeline exposures behind the property known as Pen-y-Bont TD/1 Report
Reduced depth of cover noted in several areas TD/1 Report
Abandoned pipeline exposure at the Afon Pennal / River Dyfi TD/1 Report crossing area.
River Crossing exposure: Afon Duffryn Gwyn (W0190) TD/1 Report
Reduced depth of cover at river crossing: Afon Pennal TD/1 Report
Reduced depth of cover at river crossing: Afon Duffryn Gwyn TD/1 Report (W0130) Reduced depth of cover at river crossing: Afon Duffryn Gwyn TD/1 Report (W0140) Reduced depth of cover at river crossing: Afon Duffryn Gwyn TD/1 Report (W0240)
Ground settlement near Eschuan Hall TD/1 Report
Review of Natural Resources Wales Multiple river crossings in Flood Zones Flood Maps for this report Uncontrolled / unplanned crossings and ground movement due Review of OS maps to forest management at Coed-y-Gelli for this report Susceptibility to ground movement / subsidence from old mine Review of OS maps workings and waste tips close to Gwynfan at Felin-Llyn Pair Lead for this report Mine Review of TD/1 Possible Japanese Knotweed at W0030 Report for this report
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