The devastating storms last summer had a terrible impact on some small hydroelectric power plants, particularly in the Upper Styria region.

The damage ran into tens of millions of euros. What was particularly striking from the assessment that followed was that those penstocks which had been constructed using restrained cast-iron pipes withstood even the strongest forces and remained intact. Non-restrained systems, by contrast, were broken into their individual components in some sections by the forces of nature and had to be reconstructed at great cost. In a situation like this, the issue of insurance does of course play a major role. The insurance industry already offers specific solutions for the hydropower sector – and will react to the knowledge gained from the storm damage in Upper Styria and take appropriate action.

Storm Petra will live long in the memories of the residents of the Sölk Valley in Upper Styria in particular. In the night from 5 to 6 August last year, up to 100 millimetres of rain per square metre fell in the area, causing the water level to rise to a high water mark HQ100 that is only expected once a century. The effects were devastating. Hundreds of mudslides were triggered, and some slopes slipped right down to the bedrock. The fact that no people and no livestock came to any harm really was bordering on a minor miracle. Nevertheless, the physical damage to property was vast. In an initial assessment of the damage, state governor Hermann Schützenhöfer estimated the total cost to be more than 100 million euros.

Massive damage in the Sölk Valley
The main victims of this disastrous bad weather also included the operators of the small hydroelectric power plants in the region. “In the Sölk Valley, all the power plants on the Liezen and on the Murtal side were damaged in the storms. With two exceptions,” says Rudi Stelzl from the traditional Tyrolean pipe manufacturer TRM, who makes reference to two plants that remained intact: “The first was a small hydroelectric power plant on the Liezen side and I am also aware of another one on the Murtal side.” What makes these two small power plants different from the others is that they are both designed with a penstock made from ductile cast iron (GGG for short) and with a connection that is exclusively restrained against longitudinal forces. Even in places where entire slopes broke away, the restrained pipeline remained – albeit free-standing and uncovered – but it was still preserved fully intact. The difference was quite striking and was particularly evident at the Schöder power plant in Upper Styria, where on a “mixed” pipeline the pipe fractured in the exact place where the plug-in sleeve connection merged into the restrained one.

Restrained connection holds
This is also described particularly vividly by DI Peter Neumann, the planning engineer who was given the task of restoring the two power plants Schöder 1 and Schöder 2. “At the Schöder 1 power plant, there were two sections where up to ten pipes in a row had broken out of the pipeline. They were later found in the river bed and could no longer be used. They had evidently been properly pulled apart by the force of the torrent. But there were also sections of the pipeline that were originally laid underground that were exposed, and here the pipeline was still completely intact,” recalls Peter Neumann. The section of the penstock for the Schöder 1 power plant which runs underground through the village also displayed no damage whatsoever. In addition, he mentions the pipe bridge which also withstood the abutment shifting. Here too one thing was abundantly clear: the sections of the pipeline which were coupled together with restrained connections survived the storm unscathed. The others did not.

Challenging repairs
The big clear-up and assessment of the damage was followed by the repair work, which would prove to be especially challenging, particularly on the broken penstocks. Design engineer Peter Neumann comments: “In some cases, it was really very difficult to get the rocks and mud which had entered the pipeline out again. The fundamental approach that we adopted was to start from the water catchment and flush down in sections from top to bottom. But to do this it was first necessary to free the sand trap completely of gravel and fine matter so that there was no further ingress on flushing. By contrast, in the area upstream of the power house of the Schöder 2 power plant we flushed out the mud from the bottom upwards over a length of around 100 metres so that no more sediment would be conveyed towards the machinery. This approach ultimately proved successful.” During the process of relaying the sections that were affected, an additional manhole was integrated. “Immediately before the power house of the Schöder 1 power plant there is a low point where a lot of material settled. A manhole with an integrated flush line has now been installed here,” says the planner, who mentions the fact that the restored sections have a restrained connection – such as the patented pipe connection that is restrained against longitudinal forces from the company TRM. “As you have seen with these power plants, it makes complete sense to lay pipelines with restrained pipe connections, particularly in Alpine regions. One alternative to this would be to lay the penstock much deeper. However, one drawback of this is that it entails higher costs – and sometimes the geological conditions do not make this economically feasible.” As an additional safeguard, the bank revetments made from large armour stones were laid in a concrete bed. The power plants are now ideally equipped to cope with the next storm, which hopefully will be a long time coming.

Insurance companies are already making allowances
How small-scale power plants ought to be equipped for future bad weather events in order to prevent any damage from occurring in the most effective way possible has long since also become an issue for the insurance industry. After all, the total cost of the damage from the small-scale hydropower sector in the Sölk Valley alone ran into tens of millions of euros; precise figures are not yet available. For an acknowledged expert in this field, Anton Alt, managing director at the insurance broker Alt & Walch in Voitsberg, this represents an extremely worrying development: “Natural disasters such as those in the Sölk Valley in the summer of 2017 appear to be increasing both in frequency and in intensity. An HQ100 should by definition only occur once every 100 years, but we have had such an event two or three times in recent years.” It seems to him highly plausible and very likely that this will have repercussions for the insurance industry. He points out that with this in mind the premiums of the insurers that are linked to the reinsurers will probably increase. “I expect that power plant operators will have to reckon with higher premiums in the future,” thinks Anton Alt, but in the same breath he concedes: “From my perspective it is very much conceivable that individual adjustments can be made in respect of insurance risk management.” In this case, what that means is quite simply that the level of premium might depend on the quality of the design of a power plant project. In other words, power plant operators with a penstock that is restrained against longitudinal forces would enjoy an advantage when it comes to insurance.

Robust insurance indispensable
However, the insurance specialist is still cautious and restrained in the statements he makes. After all, some of the damage still needs to be assessed, analysed and evaluated as part of the risk management process. But there is one thing that he is very clear about: “It is vital for power plant operators to have professional insurance. Particularly with the most recent damage we have seen that one or two have been caught out here because they did not have any insurance or had insurance cover that was entirely inadequate. This really should not happen.” An important factor is that operators should not opt for insurers that do not have any experience in this area. According to Anton Alt, this is also a reason why his company, which is known as a specialist insurer for industry and commerce, has focused in great depth on the complex issue of hydropower. “It is essential for an operator to have an expert discussion with a specialist from this sector. For example, we have devised a special product for the sector which is based on the experiences we have gained since 2010. Over this period of time, we have been working very closely with planners and operators and have also drawn up a number of risk analyses. An expert discussion with a specialist that can implement the visions of a hydropower plant operator in a way that meets their needs should really be mandatory,” says Anton Alt.
 
Extreme events overcome
The experiences from the storm events and the damage caused in the Sölk Valley in Upper Styria will live long in the memories of not just the small-scale hydropower plant operators that were affected. They will also long be an issue for the industry, which it seems in all likelihood will have to get used to such disastrous storms occurring more frequently. One very important aspect for preventing the worst from happening has also become abundantly clear: TRM pipelines made from ductile cast iron with connections that are restrained against longitudinal forces demonstrate their resilience even under the most extreme loading and guarantee that the plant installation will remain intact even after such events. This point should always be borne in mind with new projects in particular.

Restraint or restrained VRS®-T connection:
To be precise, the restrained connections are plug-in sleeve connections based on positive locking which are restrained against longitudinal forces. The weld bead on the spigot end and the retaining chamber provide the basis for this. The actual form fit is achieved between the weld bead and the retaining chamber through the insertion of locking segments. This creates a mechanical transfer of force between the spigot end and the coupling of the next pipe or next moulded pipe fitting. Depending on the nominal diameter, the locking is done using two to 14 bars, which are generally very easy to attach. They are inserted via a sleeve window and spread around the circumference of the pipe. It is possible to insert a clamping ring on cut pipes. Pipes with a VRS®-T connection are available in unit lengths of 5 and 6 m. The primary advantage of this connection is that it can absorb very high permitted operating pressures and enormous tensile forces. For example, the system restrained against longitudinal forces with DN900 pipes can tolerate tensile loading of up to 1,845 kN. Depending on the nominal width, they still permit certain deflections.