The following is an excerpt.

A lucky side effect of the polymeric coatings on steel pipelines is that they also work as bumpers at great water depths. PhD candidate Ole Vestrum has found a new way to model how they react when hit by mooring lines, falling anchors or fishing trawls. 

PhD candidate Ole Vestrum in front of test rig at SIMLab
PhD Candidate Ole Vestrum at SFI CASA will defend his thesis on 26 March. Title: «Impact on porous polymer coated pipelines». (All photos: Sølvi W. Normannsen)

A lucky side effect of the polymeric coatings on steel pipelines is that they also work as bumpers at great water depths. PhD candidate Ole Vestrum has found a new way to model how they react when hit by mooring lines, falling anchors or fishing trawls.


The primary task of the polymeric coating is to insulate and protect steel pipelines from corrosion. But sometimes, some things come with unexpected benefits.
Polymer coated pipeline in test rig at SIMLabLike the very interesting bumper-effect of the polymer coating.
«When pipelines are subjected to sudden impact, we see clearly that the porous polymeric coating absorbs the energy very effectively. Thus, the coating contributes considerably to protect the pipelines», says Ph.D. candidate Ole Vestrum. Despite these exciting qualities, the positive contribution of the coating is often neglected in pipeline design evaluations. Why so?
Simply because the polymer coating has some extremely sophisticated features that make it too hard to assess. Until now.


4 years ago, Vestrum embarked on the task to document their effectiveness as energy absorbers. He has aimed to establish a new method that describes how the coating contributes to the behaviour of the pipeline when they are subjected to sudden impact.
Such pipes are often installed at great depths. Oil and natural gas flow through the pipelines for thousands of miles every day, all year round. The polymer coating protects and insulates, keeps the oil smoothly flowing through the ice-cold waters.
So, what kind of potential risks of damage could be done to the pipes, deep down in the dark sea? The answer is: several. Like mooring lines and anchors falling down. Or fishing trawls ploughing the seafloor. Pull-overs /twisting of the pipes, hooking, buckling or vibrations.

Ole Vestrum´s primary focus has been loading caused by maritime activities. Commercial fisheries and the petroleum industry often operate in areas of common interest. Subsea installations also tend to attract fish, and the industry is not allowed to hinder the fishing activity.


«The consequences of sudden impacts may range from damage to the external coatings to displacement of the pipeline or even fracturing of the inner steel hull which may result in possible leaks», Vestrum explains.
For the industry, lowering the risk by bypassing regions of high fishing activity, comes with a substantial cost. The longer the pipelines, the higher the economic expenses.
Ole Vestrum´s work represents a continuation of the work of former PhD Martin Kristoffersen, who worked on uncoated steel pipelines. Part of this project was Equinor´s planning of a new pipeline route at Åsgard A. The optimal way represented a risk of damage from a mooring line failure. Impact tests in the SIMLab test facilities documented that the coating also contributes to pipeline protection. The outcome was that Equinor could use the optimal route and do with a 5 km shorter pipeline. The company saved more than 150 MNOK on this project.

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When incidents occur at great water depths, it is often difficult to get a clear view of precisely what happened. Full-scale component experiments can replicate different scenarios. However, this costs time, resources and a lot of money. Numerical models present a cost-effective alternative to physical tests. But, including the mechanical contribution made by coatings in such models comes with this one, great challenge: The extremely complex features of the porous polymer.
According to Vestrum, polymers are more vivid, and sensitive to loading rate and variation of temperatures than, for instance, steel or aluminium. For the industry, one way around these challenges has been simply to omit the coating in design evaluations – leaving only the steel section of the pipelines to be assessed. He says that much of the secrets of the material´s complex behaviour lie within the pore structures.
«It looks like swiss cheese, full of holes and voids. In fact, the porosity across the coating could vary from zero to 30 per cent. In other words: You cannot treat a polymeric coated component as a single-material component», the PhD candidate underlines.


A quick look into the technicalities confirms that Vestrum´s work is not exactly a walk in the park. He started out with an extensive series of quasi-static indentation and dynamic impact tests in the kicking machine. Full-scale pipeline samples were tested in cooperation with Equinor.
Then he scanned several specimens with X-rays (photo above & right), which is an excellent way to study pore structures. We are talking small. Micro-scale. Meaning sizes down to one-millionth of a metre. As each of the specimen processes like 8 billion data points, it takes some hairy calculations.
With the help from postdoc Lars Edvard Dæhli in SIMLab´s Toppforsk project FractAl, Vestrum was able to describe the polymer coatings behaviour on the microstructure level. Underway, he also ran several minor, physical tests to verify his assumptions. «The results showed a surprisingly strong correlation between the porosity and the behaviour of the material», Ole Vestrum states.


All these steps eventually form the models that can simulate the behaviour of materials and components in full-scale. For instance, how a coated pipeline reacts when hit by a falling anchor or a fishing trawler.


According to Ole Vestrum, the new modelling approach will serve as a potent tool for the simulation of realistic scenarios. It makes it possible to build a model of pipelines without having to do large-scale and costly mechanical tests.

From full-scale quasi-static and impact indentation tests of pipelines, to X-Ray tomography of polymer samples. The 3D images are converted into finite element models to study how the pore structure affects the mechanical behaviour.

«Numerical models are a flexible and cost-effective alternative to experiments. They allow for interaction events under a wide set of conditions to be simulated», Vestrum says.
Besides, the beauty of this generic modelling approach is that it will, hopefully, work for other materials and properties too. For instance, to simulate the thermal insulation, which is the primary purpose of the coating. Equinor is also looking into the possibilities of using the models on different types of coating. During spring 2020, two MSc students work with impact on polymer coated pipelines, supervised by Vestrum, Martin Kristoffersen and Tore Børvik.


According to his supervisor, Professor Tore Børvik, the PhD candidate´s work is both innovative and exceptional. «Ole Vestrum has really adopted the multi-scale modelling approach of SFI CASA. He has gone deep into the microstructure of porous polymers and utilized existing technology within a new framework. His work contributes both to scientific theory and to our field of expertise. And, at the same time, his work can save the industry and society for large sums of money».
The next step will be to test the new method on concrete.


Ole Vestrum says that he, during the past four years, was determined to keep the thread running and to be consistent with CASA´s multi-scale research approach. He compares his PhD work with a long journey, with many rivers to cross.
«As you start, and underway too, those rivers can be out of sight. But I have always kept the main goal in mind. At the end of the day, we should be able to simulate coated pipeline response, with the experiments as the ground for comparison. We have been taking some risks along the way. And to be honest, from time to time we have been operating far outside of the comfort zone».


26 March he will defend his thesis titled «Impact on porous polymer coated pipelines». Due to the coronavirus situation, the opponents (Professor Carlos Guedes Soares, Universidade de Lisboa, Portugal) and Senior researcher Frode Grytten, (SINTEF Materials and Chemistry) take part via Skype. Only Vestrum himself, administrator Bjørn Helge Skallerud, internal back-up opponent Professor Arild Holm Clausen and the candidate’s supervisors Professors Tore Børvik and Magnus Langseth will be physically present.