Flexible pipes for large volume gas export and gas injection have been installed on several platforms offshore Europe. In some of these applications high frequency vibrations have been reported. Onset of the vibrations is related to flow rate. The vibration frequency is typically above 150Hz and structures with stresses above the fatigue limit will rapidly fail in fatigue. This paper covers the following: • Results from a measurement program performed at the Norsk Hydro Troll C platform. • Theoretical studies related to gas flow in flexible pipes. • Actions taken to control the flow induced vibrations at Troll C.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractInnovative 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.