Gryphon A: The First Purpose-Built Permanently Moored Fpso In The North Sea

Abstract

ABSTRACT The Gryphon 'A' Floating Production Storage and Offloading (FPSO) unit is the first purpose built permanently moored FPSO in the North Sea. The ship is moored some 1500m from the field wellhead cluster. Each well is connected to the ship with a jlexible J70wline using a "steep S" catenary from the seabed to the turret. The vessel is moored with a 10 anchor mooring system supplemented by an active position mooring system. Five fully azimuthing thrusters provide automatic thrust assistance. The Gryphon 'A' incorporates a "turret transfer system" to transfer hydrocarbons, utilities, electrical power and instrument signals between the "fixed" turret and the "turning ship". This system is unique in that it uses a vertical drag chain to effect the transfer. The main processing plant is single train with two stages of separation followed by an electrostatic coalescer. The ship has a storage capacity of 525,(XM barrels and is designed for offloading to a tandem moored shuttle tanker for export. INTRODUCTION Among an increasing number of floating production facilities throughout the world, the Gryphon 'A' is the first purpose built, field specific Floating Production, Storage and Offloading Unit (FPSO) operating in the North Sea. This paper describes the technical aspects of the Gryphon 'A' facility including the overall field layout, the subsea configuration and riser system, the vessel's marine systems and utilities, and the topside process equipment. FIELD LAYOUT The Gryphon field development comprises a total of 14 wells: 8 producers, 4 water injectors, I aquifer supply well and 1 gas supply/injector well. Figure 1 illustrates the FPSO with a shuttle tanker connected, the riser system, the flowline corridor and the wellhead cluster. The orientation of the field, with the wellhead cluster located 1500m to the South West of the FPSO, accounts for the prevailing weather from that direction. It also reduces potential interference between tanker operations during approach and connection and any drilling or well workover operations that may be taking place. With this arrangement it is possible to continue drilling and/or working over wells while maintaining production and offloading operations from the FPSO. Figure 2 shows the combined anchor patterns of the FPSO and the drilling semi-submersible on location over the wellhead cluster. In order to avoid any anchor line crossovers or interference during reconnection of semi-submersible anchors during future workover operations, the two shorter lines for the semi-submersible are connected to anchor piles. SUBSEA Well Cluster The subsea wells are tied back by individual flexible flowlines to the FPSO. The well cluster consists of two rows of individual wells spaced 10m apart with 30m between the two rows. Figure 3 shows the Well Cluster Layout. FIowlines The flowlines and the control/gas lift umbilicals from the ship to the well cluster are arranged in a flowline corridor 60m wide. Flowlines run directly between the xmas trees and the riser bases located beneath the FPSO. Risers connect between the riser bases and the turret via riser guide tubes.