As part of the Ormen Lange Northern Field Development, 36 large diameter steel cylinders were used as counter-acts to ensure pipeline stability during pipe-lay along route curves. The Ormen Lange field is situated 120 km off the mid-Norwegian coast of the Norwegian continental shelf at the base of the Storegga landslide. Due to the Storegga landslide, the soil conditions vary significantly with a stratigraphy characterized by very soft Holocene clay overlying very stiff clay till. Locally, the clay till outcrops at seabed level. The water depth is in excess of 800 m and the seabed topography is highly irregular. This makes the development of the Ormen Lange field very challenging. Ormen Lange came on stream in 2007, and the operator is A/S Norske Shell. Ormen Lange is the second biggest offshore gas field and the deepest subsea production facility in Europe. The gas is exported through one of the world's longest pipelines, and the gas-export in peak production equivalents 20% of the UK's gas demand.
The chaotic seabed topography and variable stratigraphy presented several unusual geotechnical design challenges. Stability charts were constructed based on a parametric study considering slope gradient, counter-act inclination and penetration of the skirt plate. With use of these design charts, decisions were made offshore to relocate, install additional counter-acts or increase the pre-load the counter-acts with use of a steel clump weight. The counter-acts were designed with an internal lifting mechanism, which allowed for safe installation in the harsh environment. Furthermore, optimization of counter-act locations removed the requirement for a costly subsea rock installation campaign. The methodology proved to be robust and this paper presents lessons learned for design, installation and retrieval of counter-acts in harsh environments on a soft seabed.