Contents lists available at ScienceDirect Applied Ocean Research journal homepage: www.elsevier.com/locate/apor Selection of subsea distribution systems Sirous F. Yasseri a, ⁎ , Hamid Bahai b a Research Fellow, Department of Mechanical & Aerospace Engineering, Brunel University London, Howell Building, Uxbridge, UB8 3PH, United Kingdom b Professor of Mechanical Engineering, Department of Mechanical & Aerospace Engineering, Brunel University London, Howell Building, Uxbridge, UB8 3PH, United Kingdom ARTICLE INFO Keywords: Availability & Reliability Subsea Control System Architecture Selection Design Structure Matrix (DSM) Systems engineering ABSTRACT Equipment repair and intervention in subsea oil and gas fields are expensive, mainly due to vessel mobilization time, retrieval, repair, and replacement costs. The loss of revenue due to downtime could also be significant and the producer could face penalties in not meeting contractual commitments. These costs are part of the life cycle cost, which must be considered at the design stage. Estimating the reliability and availability of subsea systems at an early stage of design is important in assuring the quality of the system architecture, which leads to a more reliable choice of configuration and equipment. Availability analyses should be undertaken very early in the development process, while the operational concept is still under review, and the choice of components is still to be finalized. Postponing this assessment could prove to be too costly to improve the availability and depend- ability. This paper presents a reliability assessment using a systems engineering framework by combining the system's requirements and reliability requirements. A Design Structure Matrix (DSM) is employed to map the system and visualize the inter-relationships (dependencies) between components/subsystems. The DSM is then augmented with reliability data, including intervention times, to determine the overall system availability. It is also ex- plained how to use the system's DSM to aid integration and interface management decisions. A case study is presented to demonstrate this procedure. 1. Introduction Controlling deepwater Subsea Production Systems (SPS) requires hydraulic fluids for actuators, communication lines, power cables, and a variety of chemicals e.g. MEG to prevent waxing, hydrate prevention, corrosion inhibitor, etc. These control lines are normally small in dia- meter, and for ease of installation, they are bundled together inside a protective sheath. These bundles are known as umbilicals. The major function of an umbilical is to provide a link between the control equipment on the topsides of the host (a platform, or a floater), and the production equipment. Umbilicals can be up to 200 km long, but they are flexible enough to be installed by reeling. Because umbilicals are required for multiple tasks high reliability and dependability are de- manded of them. Umbilicals, together with their Termination Assemblies (UTAs), Subsea Control Modules (SCMs), and Subsea Distribution Units (SDUs) are collectively known as the Distribution System. The umbilical makeup, i.e. the number of power cables and steel (or flexible) tubings for chemicals and hydraulic fluids depends on the field demand and sparing policy. Figure 1 shows the major equip- ment of a subsea control system. It is also important to consider the hookup requirements for Remotely Operated Vehicles (ROVs). In deep-water applications, the deployment time and hook-up costs for an ROV will have a major im- pact on the optimization of the subsea field architecture. Commonly Subsea Control Modules (SCMs) are located on the X-tree during run- ning but can be retrieved with or without the tree. From the SDU, the cable and jumpers distribute out to the SCMs. The feasibility of the final arrangement is governed by voltage drop limitations and input voltages required at the SCMs. Reliability is a cornerstone of the offshore petroleum industry (ISO 2015, ISO 2008). Repair and intervention operations at a subsea oil and gas field are costly due to expenditure needed for vessel mo- bilization times, retrieval of the failed unit, and replacement. Loss of production (revenue) and intervention costs are an integral part of the life cycle cost of an SPS. The primary reliability measure for a main- tained system is its availability (OREDA 2009). In other words, the availability of a system is its ability to deliver the required function and performance (OREDA 2009) for the required duration. Subsea avail- ability depends on equipment reliability, retrievability, maintainability, as well as the performance of the maintenance crew. Reliability and https://doi.org/10.1016/j.apor.2020.102399 Received 10 June 2019; Received in revised form 6 August 2020; Accepted 5 October 2020 ⁎ Corresponding author E-mail addresses: Sirous.Yasseri@Brunel.ac.uk (S.F. Yasseri), Hamid.Bahai@Brunel.ac.uk (H. Bahai). Applied Ocean Research 105 (2020) 102399 0141-1187/ © 2020 Elsevier Ltd. All rights reserved. T