Flags Gasline - Design For Seabed Safety And Stability
Authors: Broussard Douglas E. , Kiltau Donald K. , Killin E.L. , Taylor Bryan G.S. 
Published: December 2007

The offshore section of the Far North Liquids and Associated Gas System (FLAGS) Gasline will carry gas and liquids at high pressure from the Brent Field to St. Fergus in Scotland. It is being installed by Shell U.K. Expro on behalf of Shell U.K. Ltd. and Esso Petroleum Co. Ltd. The 36" pipeline follows a route where water depths are predominantly more than 100 metres, and crosses a series of important fishing grounds. The environment in the northern North Sea is very different from that previously encountered in the southern and central North Sea.

For this reason and because there have been significant advances in submarine pipeline technology Shell Expro commissioned a comprehensive research programme to ensure the safety and stability of the pipeline by examining in detail the FLAGS route environment and designing the pipeline to meet local conditions. The main conclusions of this programme are that:pipeline stability will be obtained by concrete weight-coating alone in the deep water sections (over 70% of the route),the high strength concrete coating will tolerate repeated contact by the heaviest fishing gear,trenching would provide no additional protection to the pipeline in the deep water section and would increase the risk of damage to fishing gear,trenching would provide no additional protection to the pipeline in the deep water section and would increase the risk of damage to fishing gear,trenching in shallow water - needed for additional stability - will prevent contact with fishing gear because natural backfill will occur,no known trenching techniques will protect the pipeline against large anchors.

Trenching in deep water is not necessary for stability, it provides no additional mechanical protection, and furthermore, it causes an increase in the risk of damage to fishing gear.

Introduction

The Far North Liquids and Associated Gas System (FLAGS) Gasline is a joint venture of Shell U.K. Ltd. and Esso Petroleum Company Ltd. The offshore pipeline will carry gas and associated liquids from the Brent Field in the North Sea to the liquids extraction plant at St. Fergus. (See Figure 1). The high pressure gas and flow rate dictate the use of high strength, thick wall, steel pipe of 36" (915mm) diameter which will be effectively protected against corrosion and coated with reinforced concrete.

The 450 kilometre offshore route was chosen after detailed surveys of the seabed and discussions with other northern North Sea users (fishermen's associations, offshore operators, etc.). The main characteristics of the environment of the route are:80% of the route lies in water over 100 metres deep; the deepest part is in 165 metres at kilometer 355.deep trenches and pockmarks occur in some adjacent areas, although these were avoided when the exact route was selected. Otherwise there are no major seabed irregularities.the seabed in the inshore section consists mainly of hard clay and sands with some exposed rock and rock rubble. In deep water soils are mainly soft silts or clay, the soft layer varying in thickness from several centimeters to a few metres.inshore, near bottom tidal currents sweep around the coast at up to 1 1/2 metres per second and storm-driven waves also produce high bottom currents. In deep water, storm and tide effects produce negligible bottom currents.