Troll Phase I Pipelines: Tie-ins to the Subsea Tunnel

Abstract

ABSTRACT Subsea approaches to the Norwegian coast are characterised by very rugged topography. Landfall of offshore pipelines therefore often require dedicated subsea tunnel and pipeline tie-in concepts. To land the 36" and 40" offshore pipe[intw associated with the Troll Phase I development, a 4 km long landfall tunnel was constructed terminating at a water depth of165 m with vertical shaft connections to the seabed. This paper describes the design of the Troll Phase I tie-ins of offshore to tunnel pipeline sections. These comprise two main elements, i.e. 180 Te tie-in spools - which are installed between the offshore pipelines and the piercing shafis - and pt-(fabricated 450 Te riser bundles - which are installed into the vertical tunnel piercing shafts. INTRODUCTION The Troll Phase I development (Norske Shell) comprises an offshore production platform in 303m water depth, two 65 km long 36" wet gas pipelines with multi-phase flow and a gas treatment plant located at Kollsnes, some 40 km northwest of Bergen, Norway (Figure 1). The dry gas will be exported through two 40" Zeepipe Phase 11(Statoil) pipelines to mainland Europe, 650 km to 830 km away. The seabed in the last 8 km of the pipeline route towards shore is characterized by very rugged topography, featuring bare rock outcrops and very soft sediments. Seabed routing of the pipelines over the last 4 km was not fetusible and a subsea tunnel system hats meanwhile been constructed (Figure 2). The design of the tunneltermination area, the piercing method and tie-in facilities are key elements in the Troll Phase I landfall design development. The main problem faced was how to connect to the tunnel with so many large diameter pipelines, while providing space for the tie-in work and maintaining construction and operational safety. The tie in design development was also constrained by the following factors; the routing of the fairly stiff 36 º-40º pipelines on the rugged near shore seabed, the tunnel length (construction time) and the tunnel routing (rock conditions and tunnel pipeline design). The tunnel terminates with three vertical, blasted, piercing shafts to the seabed at a water depth of 165m in an area with exposed bedrock outcrop. The offshore pipelines terminate about 150m away in a bassin of very soft clay deposits (Figure 3). The riser bundles are lightweight concrete cylinders, into which pipeline risers are embedded together with steel outfitting for installation/protection support and arrangements to seal off the tunnel from the sea (Figure 4). The tie-in spools have a complex 3D configuration to accommodate the seabed and interfacing riser. pipelines. Rock dumping will be used to improve foundation conditions for the spools and hyperbaric welding equipment and also to facilitate expansion movements. (Figure 5). TIE.IN DESIGN APPROACH It was recognized early that the tunnel tie-in design had to be viewed in a wide perspective. The tunnel and offshore pipeline routing would significantly impact on the possible tunnel piercinghie-in arrangements. On the other hand, the piercing options - vertical/horizontal/blasted/ drilled - and the tie-in methods - direct pull-in or spools and hyperbaric welding, would all affect the tunnel and pipeline routing and produce different requirements for rock conditions, seabed sediments and bathymetry.