Karlskrona Municipality installs Hallingplast SESU

In 2024, Karlskrona Municipality decided to build a new underwater pipeline using Hallingplast SESU, a jacketed and fully weighted pipe that does not require concrete weights. The decision meant that the municipality chose a unique solution over the traditional and time-consuming combination of standard pressure pipes and concrete weights.

Background

Karlskrona Municipality consists of 1,650 islands and is known for its unique archipelago environment. Karlskrona is also one of the few Swedish cities built on islands. The archipelago and the city’s architecture are part of Karlskrona’s World Heritage Site, recognised by UNESCO.

The municipality has a large number of underwater pipelines in use, both for water and sewage. The oldest ones were probably laid as early as the 1960s. Traditionally, concrete weights and clamping straps have been used for many years to submerge and anchor underwater pipelines. Karlskrona is also the only municipality in Sweden with its own full-time diving team, which to a large extent works with underwater pipelines.

About three to four times a year, one of the municipal underwater pipelines floats to the surface. This is usually due to inadequately weighted pipelines and the formation of gases, especially in sewage pipelines. As the municipality experiences large variations in the number of inhabitants during the year, with a sharp increase in guests during the summer, the grid must be dimensioned based on the highest load.

This over-dimensioning means that during the off-season it can take up to eight days for the sewage water to reach the treatment plant, which together with relatively hot water, can lead to the formation of gas – for example methane – inside the pipes. If the pipelines are weighted insufficiently, they can rise to the surface, causing a risk of accidents as boats and windsurfers may collide with the pipelines or with the concrete weights that are used.

In 2024, the project began to lay a new pressure sewage pipeline between two pumping stations at Verkö. After careful consideration, the municipality decided to install Hallingplast SESU, a jacketed and pre-weighted pipe that does not require concrete weights. The decision meant that the municipality chose a new and unique solution over the traditional and time-consuming combination of standard pressure pipes and concrete weights.

Undertext bild: Joakim Pettersson, dive leader at Karlskrona Municipality, is leading the work on a new underwater pipeline together with supervisor Per Persson.

Project: Riddardalen – Blixten TA

• Pressure sewage pipeline between two pumping stations at Verkö
• Total pipeline length approx. 600 metres
• Pipe length: 12 metres
• Number of joints: 49
• Pipe dimension Ø160 mm (media pipe excl. jacket/weighting)
• Weight grade: 60%
• Installation autumn 2024

Work on the current underwater pipeline began with 50 pipes being joined together by butt welding, which was done on land in a welding container. The pipeline was then pulled out into the water as it was finished. 

“The use of pre-weighted pipes with a protective jacket was new to us and it required adaptation of the outer welding jaws as the dimensions of pipes including the jacket is slightly larger on the section of the pipe that is not peeled on delivery,” says Per Persson, supervisor at Karlskrona Municipality. “The work went twice as fast with pre-weighted pipes, as the step of attaching concrete weights could be eliminated. This not only saves time, but also improves the working environment by avoiding heavy lifting. The increased efficiency allows us to lay more pipelines at the same time, which gives clear economic benefits.”

When the welding work was completed, the underwater pipeline was ready to be towed out to the place where it was to be submerged. Only three small boats were required for this task. A boat to tow the pipeline, a boat to guide the rear section of the pipeline, and a support boat ready to intervene if necessary. 
The landfalls were already prepared and excavators were on site during the installation to help pull the pipeline up onto land. The high-strength jacket on the Hallingplast SESU pipe protected the ends of the pipeline during winding, ensuring safe and gentle operation. The jacket also provides the pipe with extra protection during the service life of the pipeline. Towing, including unwinding at landfall and submersion of the pipeline, took less than three hours on this project. 

High precision and efficient installation

The design of Hallingplast SESU makes it possible for underwater pipelines to be laid with great precision. Only a few small boats are required to fine-tune the routing of the pipeline before it is submerged to the bottom. This is a major advantage in cases where the bottom characteristics or existing underwater pipelines need to be taken into account.
After the pipeline has been submerged to the bottom, it is buried and connected to the respective landfall. It is also flushed down about 30 cm below the bottom to further protect the pipeline. “Usually, our divers flush down the pipeline where the depth is less than three metres. In this project, however, we will flush down the entire pipeline as only a short section will lie at a depth exceeding three metres,” says dive leader Joakim Pettersson.
Hallingplast SESU gives you a smooth pipe without accessories such as concrete weights and clamping straps, which means that the entire pipeline has the same service life. Because the pipeline is close to the bottom and has no protruding parts, the risk of fishing gear or anchors getting stuck is also minimised. This provides safer and more secure pipeline ownership.

Undertext bild: When the underwater pipeline is in place and fine-tuning of the correct position is completed, the pipeline is submerged to the bottom.