Photo: NOV Flexibles

Recipe for deep ocean oil pipelines

torsdag 21 jun 18
af Morten Andersen


Nicolas von Solms
DTU Kemiteknik
45 25 28 67


Flexibles has around 750 employees in Denmark.  About half of them work on the production of oil and gas pipes in Kalundborg, while the other half work in research and development and administration in Brøndby.

The company is part of the US National Oilwell Varco Group, one of the world’s largest suppliers of oil and gas exploration equipment. The name ‘Flexibles’ refers to the fact that the company manufactures flexible pipes. 

The laboratories of National Oilwell Varco subject steel and advanced plastics to mistreatment to ensure that the finished pipes are up to the task of transporting oil and gas at ocean depths of 2.5 km

Beads of plastic are melted and extruded into an elongated form, which is marinated in diesel and boiled for 12 months.

This recipe would hardly win a television bake off, but is an example of the mistreatment materials are subjected to at National Oilwell Varco Denmark. The company, also known simply as ‘Flexibles’, is one of the top three suppliers of flexible oil and gas pipes in the world.

“As the more accessible oil fields are drained, there are an increasing number of projects at greater water depths. This means extreme pressure and temperature conditions. It is a constant challenge for us to ensure that we can meet the increasing demands of our customers,” explains Jacob Sonne, Principal Engineer at Flexibles.

The company has production facilities in Denmark and Brazil, while research, development, and administration are based in Denmark, south of Copenhagen. As part of its efforts to keep at the leading edge, the company began collaboration with DTU’s Centre for Energy Resources Engineering (CERE) nearly 20 years ago. At the centre, Associate Professor Nicolas von Solms and his team investigate how CO2 and other gases transported in the pipes penetrate the pipes’ plastic layer at high pressure and temperature.

Flexibles’ oil pipes contain a polymer that is extruded on the pipes at extremely high temperatures. Photo: NOV Flexibles


From steel pipes to flexible pipelines

Modern oil and gas pipelines have a sandwich structure. Layers of steel provide strength. Layers of insulating material ensure the desired temperatures are maintained, especially to prevent the formation of ‘gas hydrates’, which look like ice and can block the pipes. Finally, layers of polymer seal the pipe, without compromising its flexibility. Hence the nickname Flexibles.

Since the mid-1980s, flexible pipes have gradually replaced steel pipes for oil and gas extraction. Flexible pipes are much more practical because they are easy to install on the seabed.

"CERE’s results have opened up a whole new way of interpreting data in our field. "
Jacob Sonne, Principal Engineer, Flexibles

The increasing number of oil and gas extraction projects at great water depths is one of the reasons why flexible pipes now dominate. It is no simple task to perform service at a depth of 2.5 km, so it is essential to have pipes that can adapt to the inevitable changes that occur in the terrain over the years.

The pipes are usually designed to last at least 20 years. But flexible pipes are not without challenges. For example, there is no polymer that completely seals out gas ingress at extremely high pressure. This is why CERE researchers conduct experiments using equipment that mimics the conditions the materials will be subjected to on the seabed. The equipment comes from DTU Chemical Engineering.

The experiments focus on the types of polymers used for the inner layers of the pipes. The focus is also on CO2 ingress.

“There is always a certain amount of CO2 present when extracting oil and gas. We also see many projects where customers want to increase the sweep efficiency by pumping CO2 into the underground to force out the oil and gas. The challenge we are currently facing is to build pipes that can handle transporting CO2 into the underground,” explains Jacob Sonne.

Like pumping air into a stone

The task is difficult because the CO2 has to be carried under extremely high pressure. Otherwise the gas will not be able to penetrate the underground and displace the oil and gas.

“Because of the high pressure that naturally exists at great water depths, recovery using CO2 is much like pumping air into a stone. You can no doubt imagine how much pressure that requires,” says Jacob Sonne.

If too much CO2 penetrates into the pipes’ polymer layer, it can cause two problems. Firstly, the polymer can swell and become soft. Secondly, if the pressure is lowered too quickly, blisters may form in a gas-filled polymer. This will destroy the polymer’s barrier properties.

It is technically possible to prevent problems by using a special polymer, but it is more expensive. And since a typical offshore oil and gas project requires many kilometres of pipes, it can quickly cost millions of dollars to change the polymer type. This price difference can decide whether it is Flexibles or a competitor that is awarded the project.

Flexibles therefore goes to great lengths to find the right balance between price and material properties for various polymers. Unfortunately, we cannot wait 20 years to see how well a new pipe performs. We have to speed up the pace and conduct accelerated tests, where materials and finished pipes are exposed to even greater strain than they will experience on the job. For example, this might mean boiling pipes in diesel for a year.

Then there are the experiments at DTU, where the key polymers used for the inner pipe layers are tested for gas ingress. These tests are conducted at pressures of up to 650 bar and temperatures of up to 125 °C.

“These are tests that we would have to do anyway, because this is a requirement in the quality standard we have to meet. But the collaboration has given us a better understanding of the processes,” says Jacob Sonne.

Flexibles is one of the world’s largest producers of oil pipes. One of Flexibles’ factories, at the harbour in Kalundborg, accounts for about half of the company’s pipe production. Illustration: NOV Flexibles


Major Brazilian customer

Flexibles has now changed its models in collaboration with CERE.

“It’s a big job when you have to redo your models. But there was no way around it, and we can be pleased that our calculation predictions now match what is actually measured in the trials. This is very motivating and will hopefully improve our future competitiveness,” notes Jacob Sonne.

There are currently two regions in the world where oil and gas are being extracted at great ocean depths. One is off the west coast of Africa, and the other is off the east coast of Brazil. Extraction from the Brazilian region is handled by Petrobras, the national energy company, which is an important customer for Flexibles.

“Petrobras has its own pipeline research department. We have a good dialogue with them. They have experience from the field. If they see something that looks strange, we want to be able to explain it. To that end, it’s important that we have the right laboratory tests and models,” says Jacob Sonne.

From 700 to 2,500 metres’ depth

Flexibles can now supply pipes certified to a depth of 2,500 metres. The previous certification was for a depth of 700 metres. The company also aims to be able to supply pipes suitable for even greater depths. It is hard to say when these will be in demand, according to Jacob Sonne.

“Many different factors will play a role, and especially what happens to the oil price. But at some point there will be demand, and of course we cannot wait until that day to get started.”

Chemical engineering equations are important in this regard:

“CERE’s results have opened up a whole new way of interpreting data in our field. They have not led to us using new polymer materials, but we have become confident about using our current materials across a broader pressure and temperature range than before,” says Jacob Sonne.

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