Circumstances at Nord Stream may be unusual, but damage to submarine pipelines is almost commonplace. Corrosion from salt water can cause leaks, and commercial shipping accidents are common.
Nord Stream’s pipes also lie beneath some of the world’s busiest shipping routes. That’s why there are sophisticated protective measures and repair techniques for pipelines, and plans for dealing with leaks and accidents are already formulated during the construction of the pipelines.
The massive anchors of large container ships or tankers in particular can damage the pipes – as happened in 2008 with the Kvitebjørn pipeline in the North Sea off the Norwegian coast. Objects falling from ships, such as containers, and even sinking ships themselves can also hit the pipeline.
Erosion and landslides under water also pose a potential hazard. That is why the pipes are protected using different methods depending on the circumstances and risk. You can surround them with large rocks that fend off anchors, cover them with concrete mats, or bury them entirely in the seabed.
However, such measures would have been very expensive for the more than 1,000 kilometers of Nord Stream lines. The pipelines are laid on the seabed or, where the seabed is not stable or flat enough, on a bed of gravel. Only near the shore are they laid under the seabed.
The pipelines are protected by their 2.7 to 4.1 centimeter thick steel walls and a concrete jacket up to eleven centimeters thick, which also serves as additional weighting. Without the concrete, the pipeline would simply be too light and would float.
In fact, the possibility of an explosion near the pipelines was taken into account in the planning. There are still countless bombs from the Second World War in the Baltic Sea – especially in the area around Bornholm.
Experts therefore cleared a strip 50 meters wide along the pipeline; According to the risk assessment by Nord Stream, however, it cannot be ruled out that currents will transport ammunition near the route.
According to the operator, the pipes would withstand an explosion of two tons of explosives at a distance of 12 meters from the pipeline without leaking.
It is difficult to estimate the extent of the current damage to the pipeline. It is true that the state of the pipeline can be precisely calculated with the help of computer analyses, for example with the finite element method. To do this, however, you have to know exactly what forces the material was exposed to.
So far, the evidence, such as the size of the gas leak, indicates that the massive pipes are badly damaged or even completely severed.
The extent of the destruction will determine how the pipeline needs to be repaired. Most pipelines constructed today have a specially developed “repair strategy” that describes the procedure in the event of damage to the pipeline.
At Nord Stream, according to the operators, this strategy includes five different scenarios of varying severity – including a complete rupture of the pipeline.
If large explosive charges have actually damaged the pipeline, as security authorities suspect according to media reports, a longer segment of the pipeline will probably have to be renewed. There are various techniques to repair such damage.
This repair, known as “tie-in,” can be performed in a number of ways. In some cases, the new, undamaged section of pipe is inserted above the water surface.
For example, when an anchor completely severed one line of the Transmediterranean Pipeline System in 2008 and severely damaged a second.
The pipeline was about 70 meters deep, similar to the Nord Stream section near Bornholm. Due to the comparatively low water depth, the damaged ends of the pipeline were lifted above the water surface with special ships. A new segment was then fitted and the connections welded at both ends.
This is how a team from a special ship assembled the individual segments of Nord Stream 2 in 2019. However, it is also possible to connect the ends of pipeline sections directly under water – a technique that was used on Nord Stream 1.
The ends of the pipeline are guided into a special hyperbaric chamber and welded together, a process known as “hyperbaric tie-in”. This technology has already been tested at Nord Stream – for example in 2011 to connect the individual sections of the first pipeline, each laid by a different special ship.
Somewhat less complicated are other ways of tightly coupling the separate ends together without welding them. The pipe ends can be connected using special flanges, just like with ordinary pipes. Such components are commercially available and can be installed either by divers or with the help of unmanned underwater vehicles.
When repairing the Kvitebjørn pipeline at a water depth of 210 meters, the Norwegian state oil company Statoil cut out an approximately 25 meter long section around the damaged area and attached the new segment using special sleeve connections. These are pushed over the pipe ends and then close the connection tightly with hydraulics.
When the pipeline itself is repaired, the water that has penetrated has to be pumped out of the line, the inside dried with a stream of air and the corrosion protection renewed if necessary.
At Nord Stream, this consists of a plastic coating and so-called sacrificial anodes made of zinc and aluminum, which prevent electrochemical corrosion.
Which technology will actually be used to repair Nord Stream depends on various factors. Last but not least, the extent of the damage is decisive, but the type of device available also plays an important role.
For example, a key factor in repairing the Transmediterranean Pipeline System above the waterline was having a suitable specialist ship in the region.
How quickly special ships, equipment and specialists are available determines how long it takes to repair the pipelines. After all, Nord Stream says it has access to a pool of repair equipment that several pipeline companies jointly operate.
The extent of the damage, the water depth and the conditions at the site of the damage also determine how long it will take before the pipelines can be operational again.
The repairs for the Kvitebjørn pipeline took five months and the Mediterranean pipeline was operational again nine months after the accident. At Nord Stream it is likely to take even longer due to the unusual circumstances.
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The original of this article “How the pipeline can be repaired” comes from Spektrum.de.
