The circumstances at Nord Stream may be unusual, but that undermeal pipelines are damaged is almost everyday life. Corrosion from salt water can cause leaks, and again and again there are accidents due to commercial shipping. The pipes from Nord Stream are also among some of the most busy ship routes in the world. That is why there are plans for dealing with leaks and accidents for pipelines sophisticated protective measures, repair techniques and when building the lines.
Especially the massive anchors of large container ships or tankers 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 are also a possible danger. That is why the pipes are protected with different procedures depending on the circumstances and the risk. So you can surround them with large stones, repel the anchors, cover them with concrete mats or bury them completely in the seabed.
However, such measures would have been very expensive with Nord Stream's more than 1000 kilometers of pipelines. The pipelines lie on the seabed or, where it is not stable or even enough, on a bed of gravel. Only near the shore they are 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 an additional weighting. Without the concrete, the pipeline would simply be too light and would float up.
Explosion risk on the pipeline route
In fact, the possibility of an explosion near the pipelines has been taken into account in the planning. There are still countless bombs from the Second World War lying in the Baltic Sea to this day – especially in the area around Bornholm. Therefore, experts cleared a strip of 50 meters wide along the pipeline; however, it cannot be ruled out that currents could be transporting ammunition close to the route, according to Nord Stream's risk assessment. According to the operator, the pipes would survive an explosion of two tons of explosives at a distance of twelve meters from the pipeline without leaking.
It is difficult to assess how big the current damage to the pipeline is. With the help of computer analyzes, for example with the finite element method, you can precisely calculate the condition of the pipeline. To do this, however, you have to know exactly what forces the material was exposed. So far, the evidence, for example the size of the gas leakage, indicates that the massive pipes are very badly damaged or even cut completely.
The extent of the destruction determines how the pipeline needs to be repaired. Most of the pipelines constructed today have a specially developed "repair strategy", which describes the procedure for 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.
How to repair the pipeline
If large explosive charges have actually damaged the pipeline, as security authorities suspect according to media reports, it is likely that a longer segment of the pipes will have to be replaced in each case. There are several techniques for repairing such damage. This repair, called "tie-in", can be carried out in various ways. In some cases, the new, undamaged pipe section is inserted above the water surface. For example, in 2008, an anchor completely severed one line of the Transmediterranean Pipeline System and severely damaged a second one.
The pipeline was about 70 meters depth, similar to the section from Nord Stream near Bornholm. Because of the comparatively low water depth, the damaged ends of the pipeline with special ships over the water surface were raised. Then you fit in a new segment and welded the connections at both ends.
In this way, the team of a special ship joined the individual segments of Nord Stream 2 in 2019. However, the ends of pipeline sections can also be connected directly under water – a technique that was used for Nord Stream 1. The ends of the pipeline are fed into a special overpressure chamber and welded together, a process known as "hyperbaric tie-in". This technology has already been tested at Nord Stream – for example, in 2011, in order to connect the individual sections of the first pipeline, each laid by a different special vessel, with each other.
The repair takes months
Other ways of coupling the separated ends tightly to one another without welding them are somewhat less complex. You can connect the pipe ends with the help of 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 210 meters of water depth, the state-owned Norwegian oil company Stadil cut out a section of about 25 meters long around the damaged area and attached the new segment with the help of special sleeve compounds. These are pushed over the pipes and then close the connection with a hydraulics.
If the pipeline itself is repaired, it is necessary to pump the water that has entered from the pipe, dry the inside with a stream of air and, if necessary, renew the anti-corrosion protection. At Nord Stream, this consists of a plastic coating as well as so-called sacrificial anodes made of zinc and aluminum, which prevent electrochemical corrosion.
Which technology will actually be used in the repair of Nord Stream depends on various factors. Last but not least, the extent of the damage decides, but an important role also plays which type of device is available. A central factor when repairing the transmeditive pipeline system above the water surface was, for example, that there was a suitable special ship in the region.
How quickly special ships, device and specialists are available is determined by how long it takes to repair the pipelines. After all, Nord Stream claims to have access to a pool of repair equipment that several pipeline companies operate together. The extent of the damage, the water depth and the conditions at the location of the damage also determine how long it takes for the pipelines to be ready for operation again. With the Kvitebjørn pipeline, the repair lasted five months, the Mediterranean pipeline was ready for use nine months after the accident. At Nord Stream, it should take longer due to the unusual circumstances.
