Introduction

Given the high day rates of deepwater drilling and workover rigs, the overall drilling and well maintenance costs for subsea wells are a significant part of the total costs for deepwater field developments.

Recent drilling and completion technologies have resulted in improved reliability, higher production levels and an increase in recovery for individual wells.This has allowed for an optimization of the well system design and total well count in deepwater areas. A cost-effective access to the well and regular well maintenance during the field life are considered key issues for optimal reservoir management, which allows for accelerated production, an increase in Ultimate Recoverable Reserves (URR)and significant savings in operating costs during the field life (e.g. optimal design of process facilities).

A Floating Production and Workover Unit (FPWU), allowing for simultaneous production and workover operations from a large monohull floater, without the need for an additional well intervention vessel has been developed (Figure 1).

The FPWU combines the low CAPEX and schedule advantages of a wet tree solution (e.g. FPSO)with the OPEX advantages of a dry tree solution (e.g. spar or TLP).

Concept Overview

Based on a large monohull, the Floating Production and Workover Unit (FPW U) has been designed [Ref. 1] for applications in the Gulf of Mexico (GoM). The FPWU is kept on station by means of a fully passive station keeping system, consisting of an internal turret system.

The FPWU provides a direct vertical access to a number of subsea wells clustered directly under the vessel/turret, allowing for an optimal reservoir management over the field life. A deepwater riser system connects the main (subsea)well system, to the turret system of the surface facility. The produced fluids are transferred via a swivel to the process facilities located on the main deck of the freely weathervaning vessel.

The vessel has no crude oil storage capacity and produced oil and gas is directly exported via export pipelines, avoiding offshore crude oil storage and offloading operations.

The FPWU design is based on three existing technologies:

  1. Floating production system based on a monohull floater: FPSO and FSO systems have proven to be technically sound and economically attractive solutions for a range of field developments worldwide.

  2. Large-diameter turret design: Based on a bogie system, a large-diameter turret has been used for the Schiehallion FPSO [Ref.2], which has been in operation since 1998 offshore W est Shetland.

  3. Workover operations from floating offshore structures: Workover operations and equipment arrangement on the FPWU is similar as per drillships and drilling & production semis.

From the FPWU, light and medium well interventions can be performed, based on rigless well intervention techniques [Ref. 3]including wireline and Coiled Tubing (CT)applications. A dedicated well intervention riser can be deployed through the large-diameter turret. On the turret structure and the vessel main deck sufficient space is available to install the workover equipment.

The large-diameter turret enables segregating the two main functional areas on top of the turret structure:

  • On one side of the turret, well streams are manifolded and linked to the centrally located swivel, which ensures the fluid transfer from the turret structure to the main process facilities on the vessel main deck.

  • The other half of the large-diameter turret structure provides space to install workover equipment (packages) as per requirements.

For safety reasons, both areas are clearly separated by a wall.

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