Transportation of CO2 – a ‘special’ issue
CLIMATE CHANGE has been attributed to greenhouse gases with carbon dioxide (CO2) being the major contributor. 60-70% of CO2 emissions worldwide originate from the burning of fossil fuels. Government authorities and power companies in the UK, along with oil- and gasfield operators, are proposing to capture CO2 from their power plants and either store it in depleted reservoirs or saline aquifers, or use it for enhanced oil recovery (EOR) in depleted oil- and gasfields. The capture of ‘anthropogenic’ CO2 (broadly-speaking, CO2 that has been produced as a result of human activity) will mitigate against global warming and possibly reduce the impact of climate change.
The United States has over 30 years of experience in transporting CO2, mainly from naturally-occurring CO2 sources, and mostly for the purpose of EOR. Both the source and the function of the CO2 pipelines in the USA have dictated a relatively-pure CO2 stream. In comparison, the UK’s proposed carbon capture and storage (CCS) projects will be focusing on anthropogenic sources from major polluters such as fossil-fuel power plants, and the CO2 transport infrastructure will involve both on- and offshore pipelines. The fossil-fuel power plants will produce CO2 with varying combinations of impurities depending on the capture technology used. CO2 pipelines have never been designed for some of the impurities released from these power plants. Other key differences between the transport of CO2 in the US and the UK relate to design codes and legislation, pipeline routes, offshore transportation and also storage.
The presence of impurities has a great impact on the physical properties of the transported CO2 that consequently affects pipeline design, compressor power, recompression distance, and pipeline capacity, and could also have implications for the prevention of fracture propagation. The effects could be either negative or positive: for example, the addition of some impurities tends to reduce compressor power while others increase the power required. These effects have direct implications for both the technical and economic feasibility of developing a CO2 transport infrastructure on- and offshore.
In this issue of the Journal of Pipeline Engineering, we are pleased to publish four important papers on the subject of CO2 transportation, all of which were first presented at the recent conference held in Amsterdam on the use of the existing pipeline infrastructure for the transportation of CO2 and H2, organized by our sister publication Global Pipeline Monthly.
Patricia Seevam and co-authors form the University of Newcastle discusses the ‘engineering gap analysis’ surrounding the issue of sequestration of CO2 in the UK. Their paper looks at the key differences between the CO2 transport scenarios in the US and the potential transport infrastructure in the UK, and provides an understanding of these differences and the implications for designing a CO2-compliant pipeline. The paper focuses on factors which include recompression distance, flow assurance, and phase equilibrium, and the authors present results of some initial hydraulic modelling work done using proprietary software.
Continuing this general theme, Aude Fradet from Gaz de France, and her co-authors from GdF, IFP, and Saipem, provide a technical and economic assessment of CO2 transportation for CCS purposes. In a CCS project, transportation will obviously be necessary between the capture plant and the storage site, and this paper look at the state-of-the-art of CO2 pipeline transportation, including existing experience, CO2 thermodynamics, corrosion, hydrates, impurities, and safety aspects. Two case studies – based on conventional coal-fired power plants but using different processes – are outlined from an economic viewpoint, and are compared. The study was done first for a 100-km distance between source and storage site, and was then extended to consider greater distances. In addition, an overview of transportation by ship is given, where the thermodynamic properties of CO2 will probably imply pressurized and refrigerated capacities. Finally, some ideas are given for what could be a future French network.
The subject of Dr Andrew Cosham and Robert Eiber’s paper is the fracture control of CO2 pipelines. As the authors point out, the transportation of CO2 by long-distance transmission pipeline is nothing new, and there are examples of CO2 pipelines in USA, Europe and North Africa. However, the required infrastructure for CCS brings with it issues that are different from the transportation of natural gas or oil.
Fracture control is concerned with designing a pipeline with a high tolerance to defects introduced during manufacture, construction, or service; and preventing, or minimizing the length of, long running fractures. ‘Captured’, or anthropogenic, CO2 may contain different types or proportions of impurities from ‘reservoir’ CO2, and this may have implications for the initiation of defects, such as corrosion. The decompression characteristics of CO2 mean that CO2 pipelines may be more susceptible to long running fractures than hydrocarbon gas pipelines. Many existing CO2 pipelines have mechanical crack arrestors installed at regular intervals along their length. Fitting crack arrestors is expensive. Advances in steelmaking practices have meant that the toughness of linepipe steel has significantly increased over the years. It is therefore informative to look at the issues associated with achieving fracture control in CO2 pipelines, review previous work, and consider the implications of developments in the understanding of how fracture control can be achieved.
The steps necessary to develop a fracture-control plan for a CO2 pipeline are discussed. The differences between the properties of hydrocarbon gas and CO2 that make CO2 pipelines more susceptible to running fractures are explained, and the additional complexity of the problem is outlined. Simplifications that allow conservative estimates of the toughness required to achieve fracture control are presented. Areas where there gaps in existing knowledge are highlighted, including the accuracy of gas-decompression models, and the effect of impurities. It is shown that fracture control can, in principle, be achieved in CO2 pipelines constructed using modern linepipe steel without the need for mechanical crack arrestors.
The fourth paper is from Hans Bratfos and colleagues At Det Norske Veritas in Norway. The paper presents a review of the challenges faced to pipeline transport of dense CO2. One of the initiatives in this direction is the Norwegian government’s decision to capture CO2 at the Kårstø gas power plant for permanent storage in a subsea aquifer in the North Sea. As the authors point out, both the capture and the storage issues have received considerable attention in the debate of how to realize CCS, while the transportation has been regarded as fairly trivial. There are, however, several important technical issues to be resolved related to pipeline transport of dense CO2, and some of these are highlighted in DNV’s report to the Norwegian Water Resources and Energy Directorate (NVE) as input to their feasibility study of the Kårstø CCS concept. Such issues are limitation of gas humidity to avoid corrosion of C-Mn steel pipes, severity of accidental entrance of wet gas, toughness requirements to ensure fracture arrest properties under a release of dense CO2, corrosivity of reproduced CO2 from enhanced oil recovery (EOR) reservoirs, and procedures for requalification of oil and gas pipelines for transport of dense CO2.
Spillage summary – getting better
CONCAWE – the European oil companies’ association for the environment, health, and safety in refining and distribution – has recently published its annual report on the Performance of European cross-country oil pipelines , which is also available as a pdf from the Concawe site at www.concawe.org. As in past years, this important statistical summary of reported spillages covers the most recent year from which data are available (2005), but this report also a looks back over the 35 years since the organization first started collecting data from its members.
Concawe annually collects data on oil pipelines in Western Europe with particular regard to spillages of 1 cum or more, the clean-up carried out, and the environmental consequences, and the results have been published in annual reports since 1971. Of great significance is the fact that the data are non-attributable, so although the statistics they provide are trustworthy and informative, no details are given of companies or individuals who may have been involved in any particular incident.
From its early beginnings, when there were only a few members of the group, approximately 70 companies and other bodies operating oil and products pipelines in Europe are now providing statistics for the annual report. These organizations operate over 250 different service pipelines which, at the end of 2005, had a combined length of 34,826km, slightly less than that for 2004 although, Concawe points out, the difference is mainly due to corrections to the reported data. The volume transported in 2005 was 789 million cum of crude oil and refined products, which is 7% less than in 2004; total traffic volume in 2005 amounted to 127 x 109 cum-km, 10.6% less than in 2004.
There were 11 reported oil spillages from pipelines during 2005, and there were no associated fires, fatalities, or injuries. The gross volume of these was 511 cum, equivalent to 0.65 parts per million (ppm) of the total volume transported; 407 cum (80% of the total spillage) was recovered or safely disposed of, and the net loss into the environment therefore amounted to 105 cum, or 0.13ppm. Of the spillages, five resulted from mechanical failure, two from operational causes, two from corrosion, and the last two resulting from third-party activities.
The report also provides comparative data for the five-year period between 2001 and 2005, and for all reported incidents since 1971. In terms of numbers of spillages, the 2005 performance (11) was slightly better than average: the long-term average is 12.5 per year, and it was 11.2 per year for 2001 to 2005. The system length is, of course, much longer now than in earlier years (the reported length in 1971 was around a third of that of the present day, at 12,800km), and this means that the spillage frequency of 0.32 spillages per 1000km/yr was the same as the average over the last five years, but less than the long-term average of 0.52 per 1000km/yr. The performance was also good in terms of volume spilled: the gross spillage volume per 1000 km of pipeline was 14.7 cum, compared to the (considerably-higher) long-term average of 89 cum per 1000km.
The report concludes with an analysis of intelligent pig inspection activity, both in 2005 and since the technique was first used. In 2005, 109 inspections were reported using a variety of intelligent pigs, covering over 6000km of pipeline, the greatest length of reported inspections in any year.
Pipelines constitute one of the main means of oil transport in Europe and are correctly considered to be one of the safest. Whereas major and sometimes repeated accidents with large media exposure have occurred with road, rail, and sea transportation, nothing similar has happened with oil pipelines. Almost inevitably though, with such a massive undertaking operating for 35 years, a handful of incidents has occurred that have resulted in a small number of fatal injuries and fires, although none in 2005.
The system is ageing. Whereas in 1971, 70% of it was 10 years old or less, by 2005 only 7% was 10 years old or less, and 35% was over 40 years old. However, this so far does not appear to have led to any increase in spillages.
Most pipeline spillages are very small, and just over 5% of the spillages gave rise to 50% of the gross volume spilled. Pipelines carrying hot oils such as fuel oil have, in the past, suffered very severely from external corrosion due to design and construction problems, and many have been either shut-down or switched to cold service. The great majority of pipelines now carry unheated petroleum products and crude oil.
The two most important causes of spillages have been third-party incidents and mechanical failure, with corrosion well back in third place, and operational and natural hazards making minor contributions. Third-party accident frequency has been significantly reduced progressively over the years. However, after having made great progress reducing mechanical failure frequencies during the first 20 years that the reports were issued, by the mid-1990s it appeared that something of an upward trend could be setting in.