Ultra Deepwater Mooring & SCR Solution for Disconnectable FPSO's
Authors: Banon Hugh H. , Lavagna Philippe , Connaulte Xavier Serge 
Published: April 2012

The industry has many leases in the western Gulf of Mexico (GoM) where there isnot extensive infrastructure and the Floating Production, Storage andOffloading (FPSO) concept can be economically competitive. As part of conceptscreening and appraisal of new deepwater fields, BP retained Single BuoyMoorings (SBM) to evaluate various disconnectable FPSO options with focus onthe vessel turret, the disconnectable system, and potential riser solutions. The study assumed the development would comprise of 12 subsea wells in 6,200ft. of water tied back to four subsea manifolds. The flowlines would becomposed of two loops connecting to the FPSO facility via four risers. Thesmall amount of produced gas would be exported via a pipeline and export of theproduced oil would be via shuttle tankers. The possible need for high pressure(and high volume) water injection was also assumed.

A hybrid riser system is a feasible but expensive riser solution for adisconnectable FPSO in the Gulf of Mexico. The emphasis in the conceptevaluation was on cost effective alternatives to hybrid risers, i.e., evaluating turret and mooring systems which would make the steel catenary riser(SCR) or its variations feasible.

This paper describes the main results and conclusions of the evaluation andsizing of an External Turret system of the MoorSpar type. The system comprisesa spar buoy to which the FPSO is connected via an articulated yoke system hencedecoupling the FPSO heave/pitch motions from the SCR friendly spar buoy. Thistype of external turret allows the steel risers and umbilicals to be in simplecatenary configuration.

The concept evaluation included a HAZID performed by DnV and a third partyreview performed by Atkins. The following issues were also addressed:technology readiness, complexity and safety of operations, uptime, CAPEX andOPEX.

Introduction

For the many leases in the western GoM where there is little infrastructure, the FPSO concept can be economically competitive. There are several reasons whyan operator may opt to select an FPSO concept in the GoM:The FPSO offers the lowest " regret cost" in case of subsurface surprisesbecause it can be easily moved to a new location. This is especially importantfor fields that have high subsurface uncertainty.The FPSO may offer a lower CAPEX due to the use of shuttle tankers (whichcan be leased) instead of a new oil export pipeline. This is especiallyimportant if a prospect is not close to existing pipeline infrastructure.The FPSO is a versatile concept because it offers a large deck space andhence can accommodate future expansions of process facilities. This isimportant if the operator would like to keep the initial investment low whilekeeping the option of future expansions.A disconnectable FPSO mitigates the risk of hurricane damage because thevessel would sail away from the storm hence avoiding the hurricane inducedenvironment. The recent experience in the GoM has demonstrated that hurricanespose a relatively significant risk to permanently moored deepwaterfacilities.