Pipelines in harsh environment
Authors: Henriksson Anders , Wilhelmsen Asbjørn , Karlsen Tore 
Published: October 2008

The selected development scenario for Ormen Lange is a subsea tie-back to a land facility. This paper describes the pipeline solution selected for the Ormen Lange gas field development in the Norwegian Sea. It elaborates on the harsh environmental challenges and difficult seabed conditions affecting the design and installation of different kind of pipelines.

Introduction

The Ormen Lange area is a prehistoric slide with varying water depths, from 250 to 1100 meters. The result of this subsea slide is an extremely uneven seabottom with local summits 60 to 80 meters high. The back wall of the slide is steep, up to 26 degrees. The environmental data is challenging. The solution involves development of design rules and methods, model tests to verify them, improved installation equipment and optimized seabed intervention, involving dredging, trenching and rockdumping. See Figure 1.

Design Premises

The initial two pipelines between the Ormen Lange field and the land terminal will carry an unprocessed wellstream containing natural gas and condensate, making them the largest planned "flowlines" to date. The processed gas will be exported through a new large pipeline, Langeled, to the market.

Intial Development to year 2007.

The initial development consists of, 2 × 30" pipelines, tee's, PLETs and pig loop for the multiphase production system. It also consists of 2 × 6" pipelines from Nyhamna to Ormen Lange, 6" crossover line between Template A and B for the monoethylene glycol (MEG) system. The control system consist of 2 × umbilical service lines from Nyhamna to Ormen Lange, and umbilical crossover service line between Template A and B. See Figure 2 and 3.

Fishing Activities.

The banks off the Møre coast are some of Norway's most important fishing areas, for both national and local fisheries. Fishing in this area is conducted with all types of equipment. During marine operations and pipe-laying, all types of fishing gear may, in principle, be affected. When installation is completed, subsea installations and pipelines may, as a main rule, only affect fishing with a bottom trawl. As a general rule, the gas export pipelines are laid directly on the sea bottom. Crossing of the pipelines should not be a problem on the fishing grounds operated by big trawlers. At the edge of the continental shelf, however, the amount of free spans may cause problems for the trawlers, with the risk of hooking the trawl boards in the free spans. Consequently, it has been decided to remove or minimize the length and number of free spans in this area.

Seabed topography and soil conditions.

The pipeline route corridor from Nyhamna to Ormen Lange passes through a near shore area, across the shelf area and into the deepwater area. See Figure 4.

In the near shore area from Nyhamna KP 0 to KP 25, outside Bjørnsundet, the water depth varies from 0 to 275 meters. The topography strongly limits the area that can be selected for pipeline routing and free spans cannot be avoided. The area between KP 7 and KP11 is the only feasible route for the large diameter lines.