Very Long Step-out Subsea Umbilical Fast-Track Design Approach based on Zohr Experience

Abstract

Design of deepwater Subsea Control & Umbilical Systems is a challenging process subject. Challenges are emerging from subsea flow assurance ever demanding requirements as well as control and data transmission implications through long step-out. Zohr project Accelerated Start-up Phase FEED design adopted a Centre Control Platform (CCP) to accommodate the chemical injection, power & control Topsides facilities feeding and controlling subsea equipment at different drill centres through umbilical network. Subsea control is based on enabling multiplexed electrohydraulic Subsea Production Control System (SPCS) with Fiber Optic (FO) communication. Control of each drill centre is independent based on segregated power and data transmission scheme. The development adopted tight schedule due to the significance to country economics. Chemical Injection and control with related data transmission through very long step-out umbilical has demonstrated to be a complex job in terms of assuring reliable connection of the CCP with subsea equipment located about 160km far away from the CCP. This complexity is merited to tight coupling between SPCS, umbilical system and installation engineering. Also, the heavy impact of failure downtime attributed to production loss along with increasing cost of intervention has significant footprint on every design aspect. The current paper highlights a Fast-Track parallel design approach for very long step-out subsea development based on Zohr project achievements. With the tight schedule and massive amount of material involved in umbilical manufacturing (i.e. 2.2millions meter cables, 2million meter of tubes, and 2.7 million meter of fillers), any change after umbilical purchase order issuance will have significant impact on project execution and will probably put the project schedule into major risk. The traditional relay-based design scheme is replaced with an approach minimising the dependency of Umbilical Design on SPCS and Installation engineering. The criticalities include impact of power distribution/sparing scheme on electrical cables configuration and design of Umbilical Termination Assembly. Also, the work covers FO link budget design challenges, need for midway repeater and related impact on connection design between main umbilical sections. The proposed approach is supported with conservative deployment scheme to eliminate installation risks. Finally, the paper will conclude with a summary for key aspects to be taken into consideration during FEED in case of very-long step-out projects. Very-long step-out subsea field development projects being limited worldwide, the work will be valuable reference for similar future projects as being handling technicality from project management perspective.