Enhanced IM of subsea control systems

Claire Zhao and Dharmik Vadel

May 1, 2015

Claire Zhao and Dharmik Vadel, of Clarus Subsea Integrity, discuss condition monitoring as a proactive approach to minimizing downtime.

Control system failure is one of the top contributors toward unplanned shutdowns in offshore subsea production systems. Current practice includes a reactive approach to failure management including equipment replacement or a retrofit solution, which can be costly. Condition monitoring of control equipment using existing operational data can provide leading indicators of equipment degradation and potential failure. This approach enhances visibility of equipment health, estimates time to failure and increases production uptime via planned equipment replacement. Three key performance indicators are discussed below including hydraulic pump performance, subsea control valve utilization and digital communications efficiency to demonstrate the benefits of condition monitoring.

Hydraulic fluid mass balance. Photo from Clarus Subsea Integrity. 

The hydraulic system, from the hydraulic fluid pumps topside to the directional control valves (DCVs) subsea, forms the cardiovascular system of a production system. The pumps are the heart of the hydraulic system and are the source of the fluid supply. Frequency and duration of pump operation to maintain required pressure are indicators of system performance. An operational norm can be established by trending pump runtimes and cycles over a period of time. An increasing trend of pump usage translates to a growing demand for hydraulic fluid downstream. With no significant changes to system configuration, such as startup of additional wells, a leakage in the system can be a logical conclusion requiring further investigation.

The DCVs and their companion actuators are on the opposite end of the hydraulic system. Similar to muscles in the body requiring blood to function, the DCVs are the consumers of hydraulic fluid. Wear on the DCVs is known to cause hydraulic fluid leakage over time. As there are no remediation methods available, they are operated until leakage rates reach critical levels and the hydraulic system can no longer provide sufficient pressure to actuate valves. The entire subsea control module (SCM) is then replaced. By trending the accumulated DCV operation cycles over time, the wear out date can be estimated allowing advanced planning for replacement hence avoiding unplanned shutdown. DCV operations can also be used in fluid leakage analysis.

Combining pump performance with DCV utilization, it is possible to begin identifying imbalances between the hydraulic fluid demand and supply. Each DCV controlled actuator is designed to consume a fixed volume of hydraulic fluid during each operation cycle. The pump runtime can be converted to hydraulic fluid volume output. The imbalance is thus determined by comparing the total volume output by the pumps to the total volume consumed by the valves. Such imbalance may indicate a leakage in the system. If additional flow information can be made available in the hydraulic system, such as on the hydraulic distribution systems, the location of leakage may be narrowed by performing localized mass balance within distribution system branches.

If the hydraulic system is symbolic of the cardiovascular system, then the digital communication system represents the production nervous system. As systems age, response time to commands typically increases, commonly referred to as “sluggish control response.” This is often misdiagnosed as a software issue, whereas in reality it is due to decreasing efficiency in the electrical/optical components. Success and failure to execute a command from topside are recorded as success and error messages. Communication efficiency is determined by dividing the number of successful messages by total messages recorded. Moreover, the degradation rate can be used to predict noncompliance with the emergency shutdown response window based on data trends.

These monitoring algorithms serve as a step improvement in proactive management of control system health and aim to increase uptime by predicting failure and planning mitigation. A condition monitoring approach also improves the understanding of operational performance and practices and serves as a valuable input in future design of new or retrofit systems. 

Claire Zhao
is a senior engineer at Clarus Subsea Integrity Inc., with four years of experience in risk based integrity management, non-destructive evaluation techniques and riser engineering. Her project experience includes working with major operators in the Gulf of Mexico providing integrity management support and solutions. She holds an M.S. in structural analysis of monuments and historical constructions and a BS in civil engineering.

Dharmik Vadel
is a vice president at Clarus Subsea Integrity Inc. He co-manages technical and commercial areas of Clarus, a recent spin off from 2H Offshore. In his 10 years of service, Dharmik has managed multi-platform SURF integrity management projects and focused on delivering integrity engineering solutions. Vadel holds an MS in environmental engineering from Oklahoma State University and a bachelor’s degree in civil engineering from REC Calicut India.