To make the deepwater Ormen Lange project off the coast of Norway feasible and in order to save seabed intervention costs, a strong focus has been put on design procedures for long free pipeline spans.
The Ormen Lange development is Norway’s biggest ever, and involves a total investment of 66 billion NOK where 1/3 is for the pipeline transportation system. The potential gains are enormous, but so are the challenges. All the gas is to be transported through pipelines. The Ormen Lange’s well installations are located at depths of 850 metres. The seabed is extremely uneven following a huge sub-sea landslide, with stronger current flows than are usual in similar depths.
A team, led by Hydro’s Finn Gunnar Nielsen, who also is an adjunct professor at the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway, has carried out tests, calculations and research in order to develop pioneering ways of laying gas pipelines on the seabed. DNV has been involved, in developing the calculation methodology, planning the tests, updating the calculation methods and preparing the project design guidelines. Consequently DNV has been able to update its Recommended Practice for free spanning pipelines.
“An update to this Recommended Practice has been in great demand and will be very relevant for use, for example, in relation to the difficult seabed on the Shtokman field in the Barents Sea,” states Olav Fyrileiv, project manager for DNV’s pipeline verification activities towards Ormen Lange and one of the authors of the Recommended Practice.
Surprised by the challenge
The major gas finds in the Ormen Lange field gave the developers a problem that was crucial to the field’s success: “How to get the gas out of the field?”
“We saw that this was a challenge, but we didn’t see how uneven the seabed was,” says Finn Gunnar Nielsen. “It was only when we were mapping the details and carried out shallow seismic work to chart the seabed that we saw how great this challenge really was”. What was found was steep slopes, strong current flow and long free spans.
From the well installations at depths of 850 metres, the pipelines have to rise up a steep slope to the so-called Egga edge of the continental shelf. The upper part of this slope has a gradient of 30 degrees. On the pipe route selected, there would be free spans of up to 200 metres in length. Based on these findings two questions were raised: What length of spans and what kind of vibrations could be accepted? The result was a relatively large research and development project on free span vortex-induced vibrations.
Finn Gunnar Nielsen states: “One of our success criteria has been that this must be a new way of laying pipes, one which is better, safer and cheaper than before. This work has been exciting because the pipelines had to be laid. We have worked all the way from the detail engineering to create a Recommended Practice that contributes to this field being commercially exploitable.”
Finn Gunnar Nielsen further believes this project is a good example of the Norwegian way of working in an offshore context. “While many oil companies keep their knowledge secret from each other, we often share our data. We have also seen the benefit of the historical long term contribution that DNV represents when it comes to knowledge and experience.”
