Designing Retrieval System for Subsea Sensors Accounts for Increased Numbers, Complexity
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 22323, ’Developing a Retrieval System for Subsea Sensors and Transmitters,’ by Clive Packman and Nils Braaten, Roxar Flow Measurement, prepared for the 2011 Offshore Technology Conference Brasil, Rio de Janeiro, 4-6 October. The paper has not been peer reviewed.
Operators face increased complexity in their subsea operations, with up to 1,500 subsea sensors and transmitters installed each year. Whereas the retrieval of topside sensors has proved to be successful, delivering significant cost savings, operators face maintenance difficulties and costly interventions in upgrading and replacing subsea sensors. Technology has evolved to enable operators to retrieve subsea sensors, allowing them to better maintain existing instruments, replace failed ones, and install improved versions. Such developments can save customers millions of dollars by reducing the cost and time required to maintain subsea assets.
Introduction
A single production field may have 20 to 30 operating wells, with more than 40 production trees and manifolds. Each of these assets can have anywhere from five to 15 monitoring sensors mounted.
How can we ensure reliability of monitoring devices? First, use high-quality sensors that offer a low risk of failure and accurate data. Second, improve the redundancy of sensors and transmitters. This can be expensive, however, requiring pipe spools or retrievable modules and isolation valves. Third, maintain the sensors, which carries the high costs associated with equipment mobilization (replacing a subsea component can cost up to USD 1 million), sourcing vessels capable of recovering a large subsea module, the shutdown of production systems, hydrocarbon displacement and evacuation, and the restart of production systems after sensor change out.
It was against this backdrop that a subsea-sensor-retrieval system was developed that is controlled by a remotely operated vehicle (ROV), with the goal of optimizing subsea equipment, facilitating replacement and cost-effective maintenance, and reducing the cost and time required to maintain subsea assets.
The Subsea-Sensor-Retrieval System
Possible applications of the new system and benefits for operators can be separated into three key areas—increasing instrument availability and reliability, reducing life-cycle costs and increasing performance, and ensuring optimum safety and environmental protection.
Increasing Instrument Availability and Reliability. The new system allows operators to replace a measuring point during routine ROV operations without production shutdown and extends the operating lifetime of subsea sensors.
Reducing Life-Cycle Costs and Increasing Performance. The retrieval system reduces life-cycle costs through a design in which the majority of the cost and technology is concentrated in the ROV tool.
Improved Safety and Environmental Protection. The system allows for the replacement of individual sensors, significantly lowering the environmental risk involved with retrieving larger structures.