The Spiral Stack Turret, an Enabling Technology for High Volume, High Pressure Field Developments with FPSOs
Authors: Espinasse Philippe , Biaggi Jean Pascal , Lehning Virginie , Ottolini Patrick 
Published: October 2011

This paper presents a new concept that has been developed for turret moored FPSOs where swivel stacks are a design bottleneck should it be for production rates, pressures or temperatures. The swivel stack flow paths are replaced by the same technology used for dynamic risers, i.e. flexible pipe. The ratings are thus at least equivalent to those of the risers themselves.

Metal to metal flanged connections enable safe fluid containment without the need for maintenance and each riser can be directly accessed from the process facilities for testing and pigging.

This development is applicable to different types of offshore projects:–FLNGs where large volumes of gas are required to feed the process facilities, combined with the high pressure of subsea gas fields–FPSOs supporting a large number of wells where direct access to individual wells is preferred–FDPSOs due to the design characteristics enabling the installation of a drill or work-over rig over the center well of the turret

The SST is designed to remain maintenance free for the duration of field life. Commingling of produced fluids in the confined environment of the turret can be avoided allowing production to be optimized by routing wells into different separators in the process facilities depending on their performance. The SST is also designed to handle umbilicals with high throughput as well as the power cables that are now required for subsea processing. The system has been designed to include safety features enabling vessel abandonment in adverse conditions such as hurricanes allowing its application to all regions of the world. It is also adapted to common shipyard fabrication through its simplified structural steel works.

The SST uses the same flexible pipe based principle as the Drag Chain systems developed during the 90's in the North Sea, but which were not optimized for the flexible pipe. The SST design overcomes these drawbacks by allowing the flexible pipe to work in a much more favourable environment, in fact much more lenient than the risers underneath the FPSO which are of similar construction and designed for at least 20 years without maintenance.

Introduction

The development of the Spiral Stack Turret (SST) has been targeted to respond to the requirements of demanding projects under development such as the challenging new concepts, such as floating LNG plants which require high throughputs of high pressure gas as well as for large production FPSOs, to be installed in the coming years.

Turret systems provide the ability to weathervane the FPSO in any weather conditions to minimize environmental loads on the FPSO and thus reduce the number of mooring legs and / or reduce the wire / chain size. This represents a significant costs saving especially for deepwater applications. Motions on the processing facilities and riser systems are very significantly reduced and operability for tandem or side by side offloading operations greatly enhanced and simplified.