A Dry Tree FPDSO Unit for Brazilian Waters
Authors: Poldervaart L. , Pollack J. 
Published: October 2008

In the last decade Brazilian offshore oil fields have become one of the most interesting areas containing prospects in water depths ranging from 500 - 3000 m. Many of these field developments require a stand-alone production and storage facility with tanker export. An FPSO is the natural solution and should ideally be installed during the drilling phase to obtain early recovery. To further improve the FPSO concept a dry production tree deck has been included in the moon pool of a spread moored FPDSO. This concept presents to the industry a cost effective solution where drilling, production and work over activities can be carried out from one single floater. The paper describes this new concept with its unique feature that the rigid production risers are suspended from, and tensioned by, a Tension Leg Deck (TLD) located above the water level in the middle of the FPDSO moon pool. The BOP stack and the surface completed production trees are located on the TLD and therefore easy accessible. The tension or uplift force of the deck is provided by weights. The weights are located in dedicated hull compartments where their vertical motions are guided by hinged lever arms. These lever arms are located transverse in the longitudinal direction of the FPDSO vessel. The FPDSO-TLD facility has the flexibility to support all required drilling and production equipment for a wide range of field development scenarios. The FPDSO-TLD concept presented in this paper was studied for a Brazilian deepwater field development scenario where an FPSO and a nearby Dry Completion Unit is the base case development scenario. The FPDSO-TLD concept has been model tested.

Introduction

The current drive to produce oil in deeper and deeper waters has pushed oil companies to review their field development strategies. Deepwater fields generally differ from their shallow water counterparts in some or all the following aspects:Absence of or limitations in existing infrastructures,Reservoir size,Reservoir horizontal extent and thickness of oil pockets,Oil properties and well maintenance,Faster return on large capital expenditure.

In the new and promising deep water regions (West of Africa and Brazil) infrastructures are scarcely available on the seabed to export the oil and gas production. This has paved the way for floating production systems which have flourished in recent years in various forms (FPSOs, TLPs etc..).

The size of the proven reserves in these deepwater fields calls for large storage facilities that can only be provided by ship-shaped floating production systems (new-built barge or converted tanker).

The new deepwater fields often differ from shallow water fields by their geology. While the latter consisted of horizontally localized oil pockets with a substantially thick oil layer, the deepwater fields come in shallow and thin oil pockets scattered over a large area. This new situation requires wellheads to be placed at large distances from each other.

Optimum production rates and oil recovery often requires a comprehensive work-over and well maintenance program.