Ormen Lange Pipelines Installation and Seabed Preparation
Authors: Eklund Trond , Høgmoen Kåre , Paulsen Gunnar 
Published: October 2008

Innovative pipeline engineering and installation together with heavy seabed preparation work has been a key factor in the success of the Ormen Lange project. Along its 120 km route to shore in two 30" pipelines, the gas travels through a highly irregular seabed with depressions and large scattered slide blocks, passes across large fishing areas with trawling, and areas with thousands of large boulders, climbs steep hills with 30° slopes, resting on a sea floor varying from the softest clay to hard moraine, and snake their way through narrow subsea valleys and passages close to shore.

Through the implementation of new technology and challenging design codes, as well as development of new engineering practices, the pipeline design was elevated to a new level. The consequence of route selection is major and its feasibility has been highly dependent on the significant use of seabed preparation work. Considering the use of new and assorted technology together with large volumes and associated costs, developing, choosing and optimizing between the different seabed preparation methods have shown to be of great importance.

Seabed intervention work for pipelines has been performed to a large extent as route preparation and post-installation work to accommodate required route corridor width, control of free-span, expansion, stress and stability, as well as to protect the lines. This has been done by means of trenching, ploughing, excavation, jetting and rock installation.

The installation work has challenged existing limits on size and installation tolerances in combination with depth. Never before has a pipeline of this size been installed at such depth. The installation tolerances have been reduced to a minimum at the same time as the lines are passing through irregular terrain with curved routing and dramatic consequences to pipeline integrity if failing to meet the installation requirements.

This paper describes the methods and equipment used, the planning, engineering and execution of the work and achievements.

Introduction

The Ormen Lange Field is located 120 km offshore Mid-Norway. The multiphase gas is transported from the field to the terminal through two 30" pipelines. The field is controlled from shore through two umbilicals and two MEG lines. The second pair of MEG line and umbilical serves as a redundancy for potential future failure. Considering the long distance, trawler intensity, seabed conditions, complexity of repair and consequences for production, this redundancy is vital to the system's overall required robustness. Figure 1 shows the pipeline layout for Ormen Lange. Summarized, the pipeline and umbilical system from the field to shore include:2 off 30" multiphase production lines2 off 6" MEG Lines2 off umbilicals1 off infield MEG line1 off infield umbilical lineIn-line tees for template tie-insHot-tap tees for future tie-ins

Fig. 1 Pipeline and umbilical layout (available in full paper)

Pipelines engineering.

Design aspects.

Pipeline installation on an extremely uneven seabed may create major free spans that require excessive amounts of seabed preparation work in order to control pipe stresses and provide a sufficient fatigue lifetime.