Learnings

Outcomes

The overall outcomes met the scope and objectives of the project, and provided detailed information on oxygen concentration limits.

This study highlighted that there is a range of current oxygen limits around Europe, with many countries considering specifications to enable a wider range of gas into their networks. European gas quality standards are undergoing a review and the oxygen concentration limit is one factor that it being considered. The proposed European Standard (EN 16726) will probably result in a two-tier oxygen limit approach. This will propose a 1 mol% limit for networks where there are no “sensitive” users, but a much lower limit if there are “sensitive” users. The classification of “sensitive” users includes underground storage and feedstock assets, and may also include interconnectors if the limits for the different countries differ.

A main driver for this review of oxygen limits is to expand options for biomethane injection. Biomethane producers face significant challenges in meeting low oxygen content from their overall production process, and although injection into distribution networks is widespread, the high pressure oxygen limit acts as a barrier to injection into the transmission network. The impact of 1 mol% oxygen on physical and chemical parameters is small and will often result in changes that are within the normal range of factors as the composition of natural gas changes with changes to the hydrocarbon content and the amount of inert gas that is present.

The discussion on physical and chemical parameters is extended to combustion parameters, and these are also within an acceptable operating variability, for oxygen concentrations up to 1 mol%.

Corrosion and pipeline integrity was considered and this highlighted that the wall loss rates (decreasing the metal pipe wall thickness) are within acceptable ranges, and as long as the gas meets the water dew point limit then there should be no negative impact on pipelines, especially as most are epoxy lined and exposure to native steel surfaces is limited. The elevated oxygen content is not thought to impact on seals and elastomer reliability also.

The impact of 1 mol% oxygen on end use and overall gas utilisation activities was considered, and although there may be small changes to the overall operation of some installations (gas turbine power plant or energy intensive industries), it is not expected that the increased oxygen limit would have any significant negative impact. One aspect that was raised was the potential impact on emissions, but here too it is suggested that impacts will be small and within manageable, usual variability (unless the plant is operating right at the top of an existing emission limit).

The work undertaken also looked into network operation. If the flow of high oxygen content gas is significantly lower than the main pipeline flow then co-mingling at the injection point should ensure rapid dilution and blending out to form a mixed stream with a very low oxygen content. This mixing should be complete within about 20D (a distance equivalent to 20 pipe diameters) downstream of the injection point. 

This project also considered “no flow” conditions where the high oxygen content gas could “accumulate” around the injection point and form a slug of unmixed gas. In order to understand how this slug of high oxygen content gas would mix as the overall pipe flow increased transient Computational Fluid Dynamics (CFD) studies were performed to understand the impact of the mixing as a function of time and distance.  The results from the CFD work identified that the “slug” could be dispersed and “blended out” within a few hundred metres providing evidence that it would be unlikely for any particular end user to receive gas that contains 1 mol% oxygen, unless they are adjacent to the entry point. In addition, it is recognised that in many cases the actual concentration of oxygen is likely to be lower than 1 mol% and this will reduce the impacts overall.

A small number of connections to the NTS have been identified as being particularly sensitive to higher concentrations of oxygen in natural gas. These sites include underground gas storage facilities and sites using natural gas as chemical feedstock. Although the current GS(M)R specification allows for 0.2 mol% oxygen, the level in the NTS is typically around 10 – 100 ppm. The main category for sensitive users is underground storage, and it is known that oxygen content can impact on their operations and the ability to deliver the stored gas to meet the entry specifications. Chemical processes that use natural gas as a feedstock may also be classed as sensitive users, and the Interconnectors may be classified in the same manner, if the oxygen limits differ in the interconnected countries.

In conclusion, although the majority of end users, pipelines, metering points and offtakes should be able to function safely and effectively with higher oxygen content gas, there remains the potential impact on underground storage which may not be able to accept gas with oxygen content higher than the current limits. This potentially indicates that there a two-tier limit approach is the best approach, which is in line with current European gas specifications. If there are no sensitive users near the injection point then a limit of 1 mol% is reasonable recognising that additional risks are low and that the high oxygen content gas will mix and “blend out” to a much lower concentration. If a sensitive user is nearby then the risk of oxygen reaching the end user is high and the concentration of the oxygen at that point would need to be risk assessed.

Value tracking                   Data Point                         Data Point Definition

Maturity                             TRL 8-9 achieved                Information provided to demonstrate established view on oxygen

Opportunity                       Achieved                            Approach on oxygen limits discussed and appraised

Deployment costs              N/A (site specific)               N/A

Innovation cost                  £78,137                              Review completed, meeting scope and outcomes discussed

Financial Saving                 N/A                                     No direct financial saving but has potential for future instrumentation

                                                                                       requirements

Safety                                 Achieved                            Safe oxygen limits investigated and form part of outcomes.

Environment                      Achieved                            Gas quality specifications including oxygen are a key aspect of network operation

and potential environmental impacts

Compliance                        Achieved                            Oxygen limits with GSMR legislation and also for Network Entry Agreement

 specifications. The outcomes of this project support compliance requirements.

Skills & Competencies       In part                                Information provided to support knowledge of importance of oxygen limits and

 the impact of oxygen concentration on high pressure pipeline networks.

Future proof                      In part                                Oxygen limits are discussed widely in Europe and worldwide. Current information

is valid for future utilisation.

The net benefits delivered so far are associated with the provision of information relating to the impact of oxygen on high pressure transmission system pipelines. 

Lessons Learnt

Additional stakeholder engagement with regard to the end user impact of oxygen concentration would have been beneficial. Although information was obtained from several end users, additional data would have widened the experience and knowledge base.