Development of EPS FPSO and Riser System for Deepwater Gulf of Mexico
Authors: Chung Jin-Sug , Dupont Wayne , Mastrangelo Carlos Ferraz , Hartman Bryan P. 
Published: October 2008

This paper presents the functional description and conceptual design of an Early Production System FPSO and Riser system for developing isolated, deepwater fields in the Gulf of Mexico. The main contributions and challenge to this work are harsh environmental conditions including strong loop current and hurricanes, the regulatory compliances and design of a riser system in 7,000 feet of water. The EPS FPSO presented in this paper is capable of weathervaning 360 degrees using an internal turret and uses a Dynamic Positioning System (DPS) during operation and offloading. Under hurricane conditions or other emergency situations such as drive-off or drift-off, the vessel can disconnect from the riser system and sail to a safe location. The hybrid riser system which decouples motions between the FPSO and Riser system is designed for 7,000 ft water depth, 10,000 psi shut-in pressure and a 25-year design life. Regulatory requirements specific to the EPS FPSO are identified using previous DeepStar works: Environmental Impact Statement and Regulatory framework. Two presentation meetings were held with MMS and USCG to present the conceptual DWOP (Deep Water Operations Plan) of EPS FPSO and riser system to identify any show stoppers in terms of regulatory requirements. Issues discussed at these meetings are summarized in this paper. A qualitative risk assessment was performed to identify critical issues regarding safety and environment during the entire field life including workover and well intervention. The main conclusions of this work are:EPS FPSO is a technically sound, economically viable option for early production of deepwater GOM fields;there is no major show stoppers in terms of regulatory requirements; anda number of risks were identified which can be mitigated by design.

Introduction

Recently discovered deepwater fields in the GOM can produce oil and gas using an early production system (EPS) until extended well testing is completed and a long term development plan is determined. This EPS can be re-deployed to another field at the end of its task, which can be three to five years. An obvious option for vessel selection is to convert a shuttle tanker that has a DP system. In this case, the station-keeping of such EPS FPSO can be achieved through the DP system which eliminates the need for a vessel mooring system. A DP system also has advantages in that it enables the FPSO to self-propel to evacuate in advance of hurricanes as well as mob/de-mob and re-deployment from field to field with minimal need for assistance from tugs and installation vessels.

The proposed EPS FPSO is fitted with an internal turret for weathervaning capability which is an effective way to minimize environmental loads and motions, especially lateral and roll motions. The main benefit of smaller lateral motions is the relaxation of the riser system design.

The riser system is designed to be disconnectable from the FPSO for the following reasons. First, it allows the vessel to disconnect and sail to a safe location in advance of hurricane and severe storm conditions. This not only protects the vessel and crew from dangerous weather conditions, but prevents the riser system from being damaged or overstressed. Secondly, it allows the FPSO to quickly disconnect in case of emergencies such as DP system black out, drive-off or drift-off. The third reason is that the riser system can be used for the rest of the field life after the facility is replaced with a full field development platform. The disconnectable system can also benefit non-production related operations such as work-over, well interventions or hull maintenance that requires drydocking.

The riser system selected in this work is the so-called free standing hybrid riser (FSHR) system which effectively decouples the FPSO motions from the riser system, thereby significantly reducing the dynamic loads on the riser and subsea termination.