Last week, three different sites along the Nord Stream 1 and 2 subsea pipelines ruptured and started leaking gas. Circumstances at Nord Stream may be unusual, but damage to underwater pipelines is almost a daily occurrence. Salt water corrosion can cause leaks, and again and again accidents happen as a result of commercial shipping. Nord Stream pipelines also lie beneath some of the busiest shipping lanes in the world. This is why pipelines have sophisticated protective measures and repair techniques; plans for dealing with leaks and accidents are drawn up as soon as the pipelines are built. Engineers will follow a set playbook to begin repairing damaged sections of Nord Stream as soon as possible.
Pipelines must withstand many threats. Massive anchors from large container ships or tankers in particular can damage pipes – as has happened in 2008 with the Kvitebjørn pipeline in the North Sea off the coast of Norway. Objects falling from ships, such as containers and even sinking ships themselves, can also strike the pipeline. Similarly, erosion and landslides under water are a potential hazard. For this reason, pipes are protected by different methods, depending on the circumstances and the risk. For example, they may be surrounded by large stones to repel anchors, covered with concrete mats, or completely buried in the seabed.
However, such measures would have been very costly for Nord Stream’s more than 1,000 kilometers of pipelines. The structures rest directly on the seabed or, when it is not sufficiently stable or level, on a bed of gravel. They are only laid under the ground where they run closest to the shore. The pipes are protected by their steel walls 2.7 to 4.1 centimeters thick and a concrete casing up to 11 centimeters thick, which also serves as additional ballast; without the concrete, the pipeline would simply be too light and would float.
Even the possibility of an explosion near pipelines was taken into account in Nord Stream’s planning. Countless bombs from World War II still lie in the Baltic Sea today. The experts therefore cleared a strip 50 meters wide along the pipeline. But currents could also carry ammunition close to the course, Nord Stream Risk Assessment warns. Thus, according to the operator, the pipes were designed to resist without leaking a blast of two tons of explosives located 12 meters from the pipe.
Despite these precautions, something punctured the Nord Stream pipeline, although it is difficult to estimate the extent of the current damage. It is true that computer analyzes can be used to accurately calculate the state of the pipeline. But for this, it is necessary to know quite precisely what forces the material has been subjected to. Indications so far – for example, the size of the gas leak – suggest that the massive pipes are very badly damaged, if not completely severed. The extent of the destruction will determine how the pipeline should be repaired. Most pipelines built today have a specially designed “repair strategy” in the event of damage: at Nord Stream, this strategy includes five different scenarios more or less serious, including a complete rupture of the pipe, according to the operators.
If large explosive charges have actually damaged the pipeline, such as security officials would suspect, longer segments of pipeline will likely need to be replaced in sections. This repair, known as a “tie-in,” can be done in several ways. In some cases, the new, undamaged segment of pipe is inserted above the water surface. This was the case, for example, when an anchor completely severed a line of the trans-Mediterranean pipeline system and severely damaged it a second in 2008. The pipeline was at a depth of about 70 meters, similar to the depth of a section from Nord Stream near the Danish island of Bornholm. Due to the shallow water depth, the damaged ends of the pipeline were lifted above the surface of the water using special ships. A new segment was then installed and the joints at both ends were welded. This is also how the team of a special vessel joined the various segments of Nord Stream 2 in 2019.
However, it is also possible to join the ends of pipeline segments directly underwater – a technique that was used for Nord Stream 1. This involves bringing the ends of the pipeline into a special hyperbaric chamber and welding them together, a process known as “hyperbaric”. attachment. This technique 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 vessel.
There are also other, somewhat less complex ways to tightly couple the separate pipe ends without welding them together. They can be joined using special flanges, much like ordinary pipes. Such components are commercially available and can be installed either by divers or by remotely operated underwater vehicles. For example, when repairing the Kvitebjørn pipeline, which was under 210 meters of water, Norway’s national oil company Statoil cut a section about 25 meters long around the damaged area and then fixed the new segment using special sleeves. Once these are slid over the ends of the pipe, the hydraulics are used to seal the joint tightly.
After the pipeline itself is repaired, it is necessary to pump out the water that has entered the tube, dry the inside with a stream of air and, if necessary, renew the corrosion protection. At Nord Stream, it is a plastic coating and so-called sacrificial anodesmade of zinc and aluminum, which prevent electrochemical corrosion.
The technology that will actually be used to repair the Nord Stream depends on a variety of factors, including the extent of the damage and, most importantly, the type of equipment available. For example, a key factor in repairing the Trans-Mediterranean Pipeline System above the surface of the water was that a suitable specialist vessel was in the area. The speed with which specialized vessels, equipment and experts are available helps determine how long it takes to repair pipelines. After all, Nord Stream by his own account has access to a pool of repair equipment shared by several pipeline companies. The extent of damage, water depth and conditions at the damage site also influence the time it will take for pipelines to be operational again. In the case of the Kvitebjørn gas pipeline, repairs took five months, while the Mediterranean gas pipeline was back in operation nine months after the accident. In the case of Nord Stream, it will probably take even longer due to the unusual circumstances: much of the cause of the damage remains unknown.
This is an opinion and analytical article, and the opinions expressed by the author or authors are not necessarily those of American scientist.
This article originally appeared in Spektrum der Wissenschaft and has been reproduced with permission