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Sea outfall project in Stockholm, Sweden

Sea Outfall project in diameter DN/ID3400mm and DN/ID1400mm for a power plant in Stockholm, Sweden

For a power plant in the capitol of Sweden a sea-outfall pipe was planned originally in concrete. The construction-site is located directly at the ferry-harbour in Stockholm.

Because of the impressive product-properties and the professional design the German Henze GmbH convinced the client to use a helical extruded polyethylene pipe with the diameter DN/ID 3400 mm. As part of the overall project also a provisional pipeline DN/ID 1400 mm was necessary for crossing a brackish water basin.

The DN/ID 3400 mm pipes are designed with a strong solid wall part and a reinforced profiled wall structure, so that the total wall thickness reached 180 mm and the outer diameter was ca. 3760 mm. The profiled wall is specially designed for filling with concrete during installation at site. The additional ballast ensures that the pipe-system can be launched and submerged in a safe and easy way. Also the DN/ID 1400 mm pipes are designed similar to a wall structure - with a strong solid wall part and a reinforced profiled wall structure.  

The complete project covers around 135 m of DN/ID 3400 mm pipe and furthermore 100 m of DN/ID 1400 mm.

The used PE 100 provides excellent physical and mechanical properties and guarantees the required life-service time of more than 100 years!

The pipes are manufactured in 5 m lengths at the pipe factory of Henze GmbH in Troisdorf, Germany. All pipes fulfil the requirements of national and international standards. Especially considered are:

  • DIN 16961      “Thermoplastic pipes and fittings with profiled wall and smooth inside”
  • DIN 16917      “Pipes and fittings made of thermoplastic materials with profiled wall and smooth pipe inside – Large pipes over DN 1200 for underground installation”

For the production of the special pipe diameter a tailor-made Krah steel mandrel is used to guarantee the required tolerances and the smooth inner-surface. All pipes are produced with light and an inspection-friendly inner surface.

Helical extrusion processPipe production / helical extrusion process

Pipe after dismantelingPipe after dismanteling

After production of the DN/ID 3400 mm pipes Henze joint the pipes homogeneously to 15 m and the DN/ID 1400 mm pipes to 12 m long pipe-elements before final quality inspection is performed and the pipes gets transported.

Pipe is leaving the factory Pipe is leaving the factory
Pipes are leaving the factory

The 15 m pipe elements are transported on trucks to Stockholm and joint there to 45 m pipe strings. The pipe-strings are equipped with flanges at the pipe ends for the underwater-jointing. Engineers of company Henze and Krah were delegated to ensure the correct joining-process at the construction-site.

For the welding and installation a special area of the harbour was reserved.

Prepared area at construction sitePrepared area at construction site

Joining Pipes by using KRAH ElectrofusionJoining Pipes by using KRAH Electrofusion

The Krah-Henze Electrofusion technology was used to guarantee a quick and safe pipe-joint without any losses in strength and stability! The Electrofusion-process for ID 3400 is designed according the Henze and Krah expertises in large diameter pipe joints. The different welding-zones are positioned in a way that a homogenous welding is ensured.

For the welding the new designed welding-machines of Krah are used. Two weldings can be performed parallel and in one step only. That saves a lot of time and money on site. All welding parameters can be entered by barcode and the full welding-process is monitored. Furthermore all welding-data gets recorded automatically

Welding to pipe-strings of 45 mWelding to pipe-strings of 45 m

Krah Electrofusion Machine E-Box 44 in useKrah Electrofusion Machine E-Box 44 in use

Later the pipes-strings are lifted into the sea, where the profiled wall could be filled with concrete and the sinking process could start in a controlled and secured way. For better controlling additional lifting-bags in different sizes (500-1000 ltr) were fixed to the pipe and activated if needed.

The pipes will be placed in their final position on the sea bed. The pipes will be jointed with the integrated flanges at the pipe ends by divers.

More impressions from the construction site:

Lifting pipes into the sea Lifting pipes into the sea

Lifting pipes into the sea

Pipes fixed at quay wallPipes fixed at quay wall

Concrete-filling processConcrete-filling process

Sinking processSinking process

Henze GmbH
Josef Kitz Str. 9
53840 Troisdorf
Germany