FPDSO with Near Surface Disconnect Drilling System
Authors: Bye Morten , Wanvik Leiv , Karp Terje 
Published: October 2008

The paper describes an FPDSO (Floating Production Drilling Storage and Offloading unit) equipped with a drilling plant and a near surface disconnectable drilling riser ("free-standing riser"). Drilling operation takes place through a dedicated moonpool, which is located just aft of the turret area. This allows a 100% clean split between the drilling and station keeping functions with minimum interfaces between two systems, which is a novelty for concepts based on turret moored drilling vessels.

The relatively small internal turret makes use of taut leg mooring system and provides a number of slots for import/export risers as well as a swivel/drag chain transfer system. The mooring legs are configured in clusters, typically 120° apart. The vessel may change its translatory position by adjusting top chain sections using winches at the turret, while simple weathervaning or thruster systems perform heading control. The subsea wells are located in the "down wind" sector of the mooring leg clusters, whereas the import/export risers are laid out in the remaining sectors which are normally observed "upwind".

The paper describes the operational procedures for the drilling (or well work over) from the vessel. Drilling/work over may take place under conditions when weathervaning is required; featuring the novelty that the drilling/work over riser may stay connected as long as the weathervaning takes place within approximately a 180° sector (+/-90° from prevailing heading). The weathervaning, naturally, has to take place about the axis, which runs through the well/centre and drilling riser. This means that the vessel has to change heading (weathervane) at the same time as it performs a translatory adjustment of its turret position. This is made possible through stepwise, predetermined manipulations of the chain section and some thruster control, if necessary. If weather forecast suggests heading beyond the 180° sector, a disconnection of the drilling riser system is planned for (upper portion only - the rest remains free-standing in the water), and executed if necessary. After disconnect of the riser the vessel may rotate around the turret in normal FPSO manner and a full n × 360° weathervaning capability is achieved. Drilling has in this situation been suspended, however, kill and choke lines and BOP controls may still be connected as these may be routed through the turret/swivel arrangement and connected to the top of the submerged free-standing drilling riser. This allows for continuos mud circulation and well control while disconnected.

The paper also outlines the subsea wells and seabed arrangement, riser configuration, turret swivel solution, the mooring leg configuration, the drilling plant and the overall FPDSO layout. The potential of the proposed FPDSO design carried out by Kvaerner will be briefly discussed in the paper, like the main features of the FPDSO unit, the status of the technology, and the areas which may need further studies.

Introduction

As the industry is increasingly expanding exploration activities in deepwater blocks, demand for competitive solutions based on stand-alone field development emerges. Besides being subjected to deepwater challenges these prospects are often characterised by great distances to existing infrastructure.