Ormen Lange Gas Field: Immediate Settlement of Offshore Rock Supports

Abstract

Located approximately 120 km offshore, Ormen Lange, with an estimated 400×109 m3 of natural gas, is the second-largest gas discovery on the Norwegian shelf. The water depth is up to 850 m, making Ormen Lange the first deepwater project on the Norwegian continental shelf. The development of Ormen Lange is under shared operatorship between Norsk Hydro and Shell. Ormen Lange’s untreated well stream will be transported to shore in two 120 km long, 30 in. diameter pipelines to a processing plant at Nyhamna, Norway. From there, gas will be exported via a 42 in., 1200 km subsea pipeline (Langeled) to Easington at the east coast of the UK. The pipelines have to pass over the Storegga slide edge, which rises 200–300 m toward the continental shelf in very steep slopes, which are also encountered in the nearshore Björnsundet area. The uneven and steep seabed conditions require the use of approximately 2.8×106 tons of rock to support and stabilize the pipelines. The sea bottom conditions on the Norwegian continental shelf are characterized by many outcrops as well as very soft clay deposits. The immediate settlement of the rock supports during installation forms a significant amount of the total required rock volume. In this paper a procedure is presented on how to assess these immediate settlements recognizing four contributing components all being discussed separately. The calculation results are compared with a back analysis, performed during the execution of the Ormen Lange rockworks, proving the suitability of the calculation method.