Managing Flow Assurance and Operation Risks in Subsea Tie-Back System
Authors: Song Shanhong 
Published: October 2008

Abstract
Chevron's Lobito-Tomboco subsea field in Block 14, Angola was successfully commissioned and started up in 2006. This tie-back system with multiple subsea production centers was developed within budget and delivered ahead of schedule. Effective flow assurance and subsea operability managements played a significant role for these achievements.
Developing this deepwater tie-back production system consisted of various phases: feasibility studies, concept selections, front end engineering, detailed engineering, installation, and commissioning and start up. At each phase, project team had to resolve various commercial and technical challenges. One of the key technical challenges was how to design a robust production system to economically transport the production fluids from subsea wells to topsides and to be able to sufficiently mitigate all the flow assurance and operation risks. The subsea system was thus, greatly impacted by the flow assurance mitigation strategies. Well-defined flow assurance criteria and strategies helped better technology selections and helped improve project economics.
This paper covers all the major flow assurance and operability risks for Lobito-Tomboco field and how these risks were defined and managed during the design stages, and during the commissioning and startup. Covered in the paper will also include the best practices used to help better define the flow assurance mitigation strategies in the course of the project development and the various lessons learned. Actual field data and the designed parameters are compared to have a better understanding of the accuracy of the design models.

Introduction
The Lobito-Tomboco development composes of three subsea centers tied back to the Benguela Belize (BB) compliant tower (CPT). The BB compliant tower is located in approximately 1280 feet of water; the three subsea centers are located in approximately 1300 to 1800 feet of water, offshore Cabinda, Angola. Two subsea centers are located to produce the Lobito reservoir. The third subsea center is located to produce the Tomboco reservoir (Figure 1).
Each subsea center has a combined production / gas lift manifold and a water injection manifold. Each production manifold has round trip pigging capability. Each production manifold has slots for five producers and each water injection manifold has slots for four injectors. Flowlines, gas lift lines, and water injection lines from the Tomboco subsea center (Subsea Center A) and the southern Lobito subsea center (Subsea Center C) will be tied back directly to BB. Both centers will have one eight-inch and one ten-inch flowline going back to BB. The northern Lobito subsea center (Subsea Center B) will be daisy-chained to the Tomboco subsea center and additionally will have one ten-inch flowline tied directly back to BB. For these flowlines, gas lift lines, and water injection lines, conventional risers with diverless connections will be pre-installed at the BB tower. Well testing will be via the 8-inch flowlines from the subsea centers and multi-phase flow meters located on the BB tower.
A single umbilical for controls and chemical injection runs directly from each subsea center to the BB tower. Umbilicals are sized for the subsea center facilities. A total of nine " I??-tubes are pre-installed on the BB compliant tower for the installation of umbilicals.