Abstract

Flexible riser systems are critical for the use of floating production systems in harsh shallow water environments. The use of flexible risers in such environments is driven by large vessel offsets, large vessel motions, and harsh shallow water environmental conditions. Where turret disconnectability is required in ice-infested waters, the riser system must be able to accommodate not only the normal FPSO motions, but also the disconnection and freefall of the spider buoy from the FPSO, and the spider buoy motions in the disconnected position. The Terra Nova riser system is the first large capacity disconnectable riser system in the world and the systems successful design and installation have demonstrated the ability to utilize flexible pipe technology for this type of application.

Introduction

The Terra Nova field development is located in 94 meters of water off the East Coast of Newfoundland, Canada in the Grand Banks Offshore area. The region has a harsh environment with intense storms occurring frequently in winter and the presence of pack ice and icebergs.

Terra Nova is an FPSO development. The Terra Nova FPSO has an overall length of 292.2 meters (277.0 meters between perpendiculars) with a nominal crude oil storage capacity of 960,000 barrels. The first phase of field development utilizes four remote drill centers connected to 34.4 km of static flexible flowlines and umbilicals. Eleven dynamic risers and four dynamic umbilicals provide the link between the static pipelines and the FPSO. Considering future development, the entire riser system accommodates for 14 dynamic risers and 5 dynamic umbilicals.

The selection of a floating production design for Terra Nova, the seasonal presence of ice, and the possibility of an iceberg encroaching on the field introduced basic functional requirements for the field's development. These main requirements were the design/fabrication of an ice strengthened vessel and a facility capable of disconnecting from its risers and mooring chains;thus enabling the vessel to move off location to avoid a potential collision with an iceberg. Pursuant to these functional requirements and the overall design requirements, this paper details the challenges imposed on the global design of the Terra Nova riser system and its associated equipment. Furthermore, a synopsis of the riser installation in year 2000 is provided along with lessons learned from the design and installation of the Terra Novariser system.

Riser Configuration Selection

Three types of riser configurations were considered for the Terra Nova riser system. The first of these configurations was the Pliant Wave® (Fig. 1). The riser's wave configuration is shaped via a distributed buoyancy module section. Following this section the riser is anchored to the seabed via a tether that is connected to a swivel clamp on the riser and a gravity base on the seabed. The riser then descends to the seabed and ultimately is terminated at the flowline connection.

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