Jetting 'sword' system adapts to different soil conditions

July 1, 2002
The continued expansion of economic development is increasing the need for direct connections across barriers, esp-ecially water bodies.

The continued expansion of economic development is increasing the need for direct connections across barriers, esp-ecially water bodies. These include comm- unication cables and fiber, power trans-mission cables, water pipelines, and pipelines to carry hydrocarbons and refined products.

Also, more subsea facilities are being installed with control lines and pipelines that connect to existing stationary platforms or to shore. All of these applications require the lines to be buried for safety and security.

Nordic Offshore is a specialist in seabed trenching. The company has developed a flexible and adaptable jetting system to cut trenches as deep as 3 m, which will accommodate most seabed pipelines.

The jetting system is contained in a remotely operated trenching system (ROTS), which uses a set of two, side-by side, adjustable "swords" lined with jets. The swords are connected to the underside of the frame, which also holds the pumps, steering impellers, and controls. Three different jet settings can be used, depending on soil conditions.

Along the length of each sword, the jets have different tasks, and up to five different water pressures can be set along each sword. Jets in the top third of each sword fluidize the mud at the seabed; the middle jets cut the seabed, while the lowest jets flush soil backward out of the cut and away from the trench. For pipelines, soil material must be fluidized for 20 m to 30 m behind the trencher.

The swords can be adjusted in three dimensions to set the proper cutting depth and width while the top jets can be turned off to get the increased hydraulic cutting action needed in the seabed.

The ROTS launch and recovery system can be operated by one person and launched in conditions up to Sea State 7. Lifting the trencher using the umbilical is also possible. The trencher itself weighs 14 tons, while the full system weighs 125 tons. Impellers on the trencher move it through the water and limit the skid weight on the seabed when necessary. A separate power system is used to power the trencher. This preserves ship power and allows power to be varied as needed to make the trenching cut.

Oseberg, Troll, Veslefrikk

Last year, Nordic Offshore completed the burial of the last 45 km of 200 km of fiber optic system connecting Troll, Oseberg, and Veslefrikk platforms in the North Sea. Due to the hard soils, the original contractors failed to complete the task. Nordic Offshore stepped in to finish the job and buried the cable to 1.2 m.

In May 2002, Nordic Offshore buried a 10-in. pipeline to 1 m (top-of-pipe) for a distance of 4.3 km. It took only 24 hours to complete the burial.

One current project involves laying and burying a 96-fiber optic cable interconnection between Helsinki and St. Petersburg for the Baltic Cable System. The Baltic Cable System is a ring of cable connecting Stockholm, Sweden; Helsinki, Finland; Kotka, Finland; St. Petersburg; Russia; the Baltic states to Kaliningrad; then to Poland, and back to Sweden. Nordic Offshore is burying the 45-mm, double armored, cable to a depth of 2 m in up to 100 m water depths.

The trencher uses two, side-by side, adjustable jetting "swords" to cut a trench up to 1 m wide and 3 m deep.
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For more information, contact: Jarle Aksenes or Gösta Nilsson, Nordic Offshore. Tel: +46 522 81 379, fax: +46 522 151 60 (Sweden) or +47 51 888 130 (Norway), email: [email protected] or [email protected].