Checking the subsea system pulse

OE Staff
Thursday, March 4, 2010

While inspections are important in understanding the current status of subsea systems, limited information can be received about the internal health of critical subsea systems such as electrical power, hydraulic control and chemical injection. That, say Himanshu Maheshwari and Jim McMahan of 2H Offshore, is where condition monitoring plays a pivotal role in an effective integrity management program.

An integrity management engineer can learn a lot by understanding the parallel between a human body and a subsea system.

Compare the heart with the hydraulic system. The heart is a pumping motor. It pumps the blood all over the body through a network of pipes. The blood travels along 100,000km of blood pipes, which is equal to about 2.5 times around the equator. The heart pumps 13,640 litres of blood and beats 100,000 times each day.

An external visual inspection cannot provide a status of the heart’s health except in the case of a severe cardiac condition. However, even a simple pulse measurement indicating beats per min, rhythm and strength can indicate a heart condition. Additional tests can be done to measure any blockage in the artery or valve malfunction.

Similarly, an external visual inspection of a subsea hydraulic system cannot provide the status of its health unless there is, for example, a visible leak in the system. However, a mass balance calculation of the amount of hydraulic fluid consumed by the system during operation can adequately indicate the hydraulic system’s condition. Additional tests can be done using valve signature information, recharge time, flow rate and reservoir levels to refine the health check to identify specific anomalies within the hydraulic supply and distribution system of typical subsea production systems.

Prevention vs cure
Prevention is better than cure. A periodic check to track the heart’s health is better than being in cardiac arrest and losing partial or complete functionality of the organ.

Likewise, a carefully defined plan of key performance indicators (KPIs) as part of the integrity management plan of a subsea system can serve as an early warning system. The value of using KPIs is often highlighted when they are applied to intervene before an anomaly develops into an unplanned shutdown, or even damage to the environment or injury to personnel. Even one day of lost production for a major asset can be significant. Given the time delay in ordering and installing specialist replacement subsea equipment, lost revenue can easily run into millions of dollars, in addition to the cost of replacement components and their installation costs.

To inspect or monitor
In the implementation of risk-based integrity management processes, one of the key observations has been that situations are rarely binary. You seldom have a choice between two options, but instead, a range of choices and possible solutions present themselves in most instances. Good decisions are about balance, and looking for long-term systemic solutions along with the quick fix of an immediate problem.

Setting up inspection and monitoring in isolation of each other is not the most useful way to look at risk mitigation strategies for subsea assets. Inspection and condition monitoring go together, as an effective approach to mitigating offshore risks over the long term of the facility infrastructure.

Inspections provide a snapshot of the visible structural health of the system. An ROV video of a deepwater subsea system can show signs of potential wear, impact damage, external corrosion signs, ROV valves status, potential leaks, and vibrations of subsea structure or other major structural damage. In order to maximize the returns from a subsea inspection, it is important to have trained personnel to carry out inspection work. Training and experience are required in order to reliably assess the visual significance and translate what can be viewed into an accurate estimate of equipment condition.

Benchmarking the condition of the subsea equipment following installation and tracking its status over time can provide a history of the equipment for external structural damages. Unfortunately, limited information can be obtained in this manner to assess the internal integrity of the subsea system, and visibility due to marine growth and inaccessibility can also play a role in how reliable of an inspection can be carried out.

Threats relating to the internal health of subsea systems can include a ground fault in the electrical system due to water ingress, poor communications integrity, blockages or small leaks in the hydraulic or chemical injection systems, and critical control valve failure, all of which can go undetected by inspections and can lead to subsea system downtime.

Types of monitoring systems can include condition monitoring or system specific response monitoring. Condition monitoring targets the essential parameters that are required to ensure the safe operation and verify the condition of the system, while the system specific response monitoring targets the critical areas which typically are only relevant to the specific operation being conducted at a specific point on time.

Condition monitoring examples include the pressure, temperature, flow rate, environment and vessel motions. Depending on the subsea development operational philosophy, sand erosion probes, corrosion coupons and regular fluid sampling may be required to monitor internal corrosion. Environmental loading such as current, wave and wind impart loading on the vessel and the resulting vessel motions affect the subsea systems that are connected to the vessel.

An example of system-specific response monitoring can be geared towards capturing motion, strain or curvature at/near fatigue critical locations based on the system design. Other examples of typical system-specific response monitoring include hydraulic reservoir level alarms, valve command failure alarms, data readback communication outages, low hydraulic pressure alerts, and computer malfunctions.

Viewing data
A large amount of sensor data exists in the subsea field that is extensively used for making production decisions. At the same time, opportunities exist to employ algorithms in conjunction with selected specific response sensor data to yield system health information. For example, production operators refer to sensor readings to maintain production at a desired level. Flowing pressure downstream and upstream of an adjustable tree-mounted flow choke indicates the rate of production from a well. That same data combined with the choke position sensor reading can indicate the condition of the choke trim and give preliminary notice of incipient choke insert replacement.

Subsea communication efficiency monitoring provides another opportunity to gain early warning of an impending anomaly when combined with a software algorithm. Most subsea production control systems are programmed to monitor successful communications between surface and subsea and are set to annunciate a notice to personnel when the communications error rate reaches a preset level. While this monitoring method serves well to indicate a communications anomaly which has already occurred, the annunciation alarm level is set to prevent nuisance trips and only indicate when the integrity of communications is severely compromised.

For improvement, an algorithm can be implemented which closely trends the same communication error information, compares the current trend to historical trends previously collected and logically determines if an increasing error indication could be a leading indicator of incipient communication channel failure. Obviously, an early warning of this type of anomaly allows time to prepare a cost-effective mitigation response before actually losing all communication with the subsea equipment and thereby forcing a production shut-in.

Alert levels
Suitable alert levels in the form of KPIs need to be assigned to each monitored parameter. KPIs are established to determine when these parameters have reached a critical or near-critical level. They can even be automated for continuous monitoring to notify the appropriate personnel if a measurement exceeds the preset alert level.

The key performance indicators can be either leading or lagging. A leading KPI provides a pre-warning for an imminent failure of the subsea system. An example of leading KPI can be corrosion inhibitor injection rates. In case the actual injection rate deviates from the required injection rates for the asset, it will indicate an imminent potential corrosion of the subsea system during the life of the field. By taking steps in advance to address these gaps, integrity and risk to the assets can be managed proactively.

A lagging KPI provides a warning following an incident. An example of lagging KPI can be excessive hydraulic fluid loss in the subsea control system loop. In case the hydraulic fluid loss is higher than the alert levels for a subsea system based on a valve actuation pattern, it will indicate a leak in the hydraulic fluid system. If available, spare lines can be used to isolate the potential leak in the fluid circulation. An early warning of the incident can help mitigate the potential severity of the consequence and losses.

Design for monitorability
Over the past two decades, design for manufacturability became a widely accepted practice among product designers. It facilitated products to be designed in such a way that they are easy to manufacture.

The safety, criticality and economy of a subsea system demands designers’ attention to provide provision for long-term condition monitoring. Most subsea production systems are designed for adequate system-specific response monitoring, but very few, if any, have been designed to extract the most in terms of system condition monitoring.

Integrity management of subsea infrastructure utilizing both underwater inspection and condition monitoring is an emerging methodology. Looking ahead, system designers can dramatically improve the industry’s ability to perform condition monitoring by planning for and providing in the system designs the necessary instrumentation to permit better condition monitoring.

A few additional, basic subsea measurements would enable more precise fault detection and source identification than is possible with most systems today. For example, more accurate hydraulic flow rate measurement from the hydraulic power unit would improve the fluid consumption algorithms used to detect leakage. Additional pressure sensors combined with strategic isolation valves within the subsea hydraulic distribution termination assemblies would permit isolation of hydraulic anomalies associated with blockage or fluid leaks in complex subsea system architecture.

No subsea system can be expected to operate unattended over its useful lifetime; therefore, ongoing maintenance is to be anticipated and supported by a thorough integrity management plan that includes both inspection and condition monitoring of all the subsea equipment. Condition monitoring of subsea risers and flowlines is well established among most operators, but the concept has a way to go toward being applied to the remaining subsea equipment making up an underwater production facility. Fortunately, it is relatively easy during the design phase to add a few additional instrumentation points into the system to facilitate significant improvements in our ability to identify and locate system anomalies.

Integrity management of subsea systems goes well beyond simply inspection or inspection management, although these are key requirements. Inspection activities alone provide a fraction of the information required in order to give an informed opinion in integrity assurance. A carefully developed condition monitoring plan must be developed and implemented to manage integrity and extend the life of a subsea system. OE

Categories: Equipment Maintenance

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