Geotechnical engineering specialist GEO is into the second year of its Ormen Lange seabed study. On behalf of Statoil, the project's development operator, the Lyngby-based institute has been collecting and analyzing soil samples along the route of the Langeled pipeline, which extends from the Norwegian Sea south to Sleipner, before veering southwest to Bacton on England's east coast.
According to Jens Brink Clausen, GEO's head of marine operations, "Statoil and Norsk Hydro are examining soil strength parameters, as these vary widely. The seabed also features extensive undulation – we are investigating the upper 10 m. The Ormen Lange partners want to establish whether the layers follow the contours of the seabed, or if erosion is responsible." These factors need to be clarified before pipelay can get under way next year.
The Geoceptor CPT/vibrocorer rig.
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Last fall, GEO undertook 160 combined cone penetration tests (CPTs) and vibrocoring tasks over two periods lasting three months in total, deployed from the Geoconsult vessel Geobay/Geofjord. For this program, over the planned stretch in the Norwegian Sea, it employed its Geoceptor CPT/vibrocore rig. This rig, in service since 2000, is 2.9 m tall, with a weight in water of up to 12.1 tons, depending on instrumentation.
Features include dual hydraulic clamps that provide 100 kN, or 6 tons of thrust for CPTs, with a rate of penetration of 20 mm/sec, and a wheel drive, dual-vibrator system for vibrocoring, which operates in a low frequency range of 20-35 Hz, or 100-130 Hz high frequency.
"The wheel drive gives us improved control of the speed of penetration or vibration," Clausen says. "As the vibrocorer disturbs the soil, we get better accuracy. The weight of the vibrocorer, being so heavy, means that it doesn't vibrate in the early stages when we push it down. In that way, we do not disturb the soil."
This year, GEO has been tracking the main trunkline section between Sleipner and the UK, working from Stolt Offshore's newly built geophysical survey vessel Seaway Petrel. GEO is also applying its new ROV-mounted, mini-CPT toolkit, which can undertake CPTs to a depth of up to 3 m, with a maximum thrust of 15 kN (depending on the ROV's reaction force). This is stationed at the front of the ROV on a small mounting plate, Clausen explains, and weighs 200 kg.
"It can also be applied for density measurements of backfill on laid subsea pipes. Some companies that have used it have been surprised to find that their pipe wasn't buried as deep as they had expected. With this system we can perform 70-100 tests per day, according to the client's needs. In addition, we can examine soil cross-sections to confirm the width of the trench, or to attain a depth profile."
Also last year, GEO was contracted by Statoil to take soil samples for the Snøhvit project in the Barents Sea, both at the site of the subsea templates and along the route of the gas pipeline to Hammerfest.
"These are very variable soil conditions, with sand and clay." An earlier job involved use of the GEO mini-CPT for another subsea-shore field development, BG's Scarrab/Saffron, under sub-contract to Stolt Offshore.
Another GEO specialty is spudcan soil penetration analysis for jackup legs. Mærsk is a long-established customer. Using conventional analytical techniques, or large deformation finite element modeling, GEO can evaluate potential risks for jackup failures, caused by punch-through, sudden penetration, cavities, or squeezing of the thin soft clay layer. FEA software employed at its geotechnical laboratory in Lyngby includes Abaqus and Elfen. The same facility also provides services in rock mechanics for exploration purposes.
CPTs and vibrocoring are done mainly in water depths of 400-500 m, but GEO is looking to invest in new cables to take its instruments down to 3,000 m for offshore analyses. "Fugro currently dominates the deepwater geotechnical market," Clausen says, "but oil companies are looking to promote more competition. To do this, we need more regular access to vessels, as Fugro does. We are therefore interested in teaming up with a vessel operator to bid for multiple projects for the same operator."
