Modular Stacked DC Transmission and Distribution System for Ultra-deepwater Subsea Process
Authors: Lai Rixin , Chi Song , Garces Luis , Elgsaas Kristin Moe , Alford Mike , Dong Dong , Zhang Di , Masoud Haji , Gunturi Satish , Harfman Todorovic Maja , Sihler Christof , Song-Manguelle Joseph , Datta Rajib , Pappas James Marcus , Gupta Rajan , Rocke Svend Erik 
Published: May 2013

Abstract
The offshore oil and gas industry is developing subsea processing systems far away from the shore and in ultra-deepwater. These subsea systems are usually power intensive, and thus a reliable electrical transmission and distribution (T&D) system is desired. In this paper, the modular stacked direct current (MSDC) architecture is presented to meet the technical challenges of the ultra-deepwater subsea systems. Instead of using a bulky centralized high voltage direct current (HVDC) converter station, the high dc voltage is achieved by stacking a number of power converter building blocks in series on both the on-shore station and the seabed stations. The dc-link current is controlled to be constant, and the dc-link voltage will vary according to the loading condition. This architecture renders fault-tolerant capability and feasibility for field extension. The details for the system architecture, the control algorithm, the simulation and experimental results will be described. The test results and the studies show that the MSDC system is a very attractive solution for subsea applications.

Introduction
The diminishing conventional oil and gas reservoirs and the always growing energy consumption have been driving the efforts to explore the deep-sea water petroleum resources, as shown in the diagram in Fig. 1. These resources are usually in ultra-deepwater and far away from the shore, where subsea process is usually preferred for technical and/or economic reasons. Subsea processing is especially attractive for the locations with tough topside environment since the manned topside platform can be removed. However, in order to achieve subsea process, turbo machines, e.g., pumps and compressors, and other processing equipment need to be deployed on the seabed. Accordingly various electrical equipment, including variable speed drives, power supplies, motors, connectors, switch gears, etc., are required to drive the machinery loads subsea. The overall power requirement ranges from tens of kW to tens of MW. As the scale and power consumption of the subsea process continue to grow, there is a clear need for a reliable power transmission and distribution system to power the subsea equipment over long step-out.
Long step out distances and the ultra-deepwater environments have brought significant technical challenges to the electrical system. In ultra-deepwater and long step-out situation, accessibility of any equipment deployed in the subsea field is extremely limited. Subsea operation and maintenance is dependent on availability of vessels with remotely operated vehicles (ROV) and sufficient lifting/installation capacity, hence unplanned stops due to failures will cause large losses in production due to lost time. The cost of the vessel and offshore operation is also significant. Therefore, other than being designed for installation in ultra-deepwater, the electrical system must be very reliable and require minimum regular maintenance in its lifetime. The electrical system should also be as compact as possible and modularized, so that they can be installed and retrieved without heavy vessels and lifts which have very limited availability. In addition, the electrical system should be flexible and easy to be reconfigured since the power requirement, the tie-back length, and the number of loads will vary from site to site. And it should also have the flexibility for future expansion since additional field might be found in the adjacent area.
In order to meet the above mentioned challenges and requirements, the MSDC architecture has been developed for the subsea power transmission and distribution with long step-out. This paper compares different technology options for subsea transmission and distribution (T&D), presents the MSDC architecture, describes the details of the system operation, and discusses the results of the field case simulation and the lab demo.