To extend the life of a subsea production system, 2H engineering specialist Himanshu Maheshwari argues the inspection must be done along with condition monitoring.
All but the simplest subsea oil and gas production systems are likely to require maintenance at some point during their lives. For that maintenance to be most effective, it needs underpinning by a carefully formulated integrity management system.
Inspection will doubtless form a key element of the management system. If undertaken by well-trained, experienced personnel, inspections using remotely operated vehicles can provide valuable information about the structural health of subsea equipment. Wear, vibration damage, structural deformation, impact damage, leaks, external corrosion and the general condition of valves and other key items will all be revealed – always assuming good visibility and access around the structure, and no excessive marine growth.
Unfortunately, inspections of this kind will tell you very little about the internal health of a system. Faults in electrical systems due to water ingress, poor communications integrity, blockages or small leaks in hydraulic and chemical injection systems, and critical control valve failings will be missed. Moreover, inspection provides only a snapshot in time of the state of a system: you are essentially in the dark until the next inspection takes place.
To manage integrity properly and extend the life of a subsea production system, inspection needs to be closely coupled with condition monitoring. At least, this is the opinion of Himanshu Maheshwari, an engineering specialist with 2H Offshore.
“Subsea condition monitoring can be extremely powerful,” says Maheshwari. “This has recently been demonstrated by applications involving riser pipes of various kinds and flowlines. As yet, however, the concept has not been widely applied to subsea production equipment. There is a great opportunity here; it just needs people to be convinced of the value of the approach.”
Subsea field operators already record much of the data that can be used for condition monitoring in the normal course of their activities. It is fundamental to most production operation decisions. Temperatures, pressures and flow rates are obvious examples. Sand erosion probes, corrosion coupons and fluid sampling systems are commonplace. The recording of environmental data, such as currents, wave forces and wind conditions, is also normal, along with monitoring the motion of surface vessels, which can be important for assessing their structural impact on the subsea equipment to which they are connected.
In contrast, system-specific response monitoring, the capture of motion, strain or curvature at or near fatigue-critical locations, is less common. Monitoring of this kind has proved extremely valuable for riser pipes and could also provide useful insights into the health of some seabed equipment.
Maheshwari claims that the key to successful condition monitoring is how you handle the available data, which often involves developing suitable algorithms to provide the required insights. He points out that operators use flow pressures up- and downstream of adjustable tree-mounted flow chokes to estimate the rate of production from a well. “That same data combined with a choke position sensor reading can indicate the condition of the choke trim and provide notice of the need for a choke insert replacement,” he says.
As another example, monitoring of subsea communication efficiency can provide early warning of potential problems. Maheshwari explains: “Most subsea production control systems are programmed to monitor successful communications between the surface and subsea, and are set to indicate when the error level reaches a preset figure. This monitoring method shows a communications anomaly, but only once it has occurred.
“Further, to avoid nuisance trips, the alarm level is normally set so the system flags only when the integrity of communications is severely compromised. As an improvement it is possible, with the right algorithm, to accurately track the same communication error information, compare this with historical trends and determine if an increasing error level could be a leading indicator of incipient communication channel failure. An early warning of this type of anomaly allows time to prepare cost-effective mitigation measures before losing all communication with the subsea equipment and thereby forcing a production shut-in.”
Early warnings of all kinds of problems can also be obtained by a judicious choice of key performance indicators, defined alert levels assigned to selected parameters. The value of using key performance indicators is that they enable intervention to sort out system anomalies before unplanned shutdowns are necessary or there is the possibility of personal injury or damage to the facility or the environment.
Maheshwari is adamant that there is immense potential for operators to improve the integrity management of their subsea production systems. The breakthrough will come, he believes, when equipment designers embrace the idea and begin to build in to equipment the means to gather specific data more suited to the intended purpose. “Over the past 20 years, the concept of designing equipment with ease of manufacture in mind has become normal. System designers, however, also need to become more aware of condition monitoring needs and respond to these when there is the opportunity, before systems are built and installed.”
To illustrate this, Maheshwari describes how a mass-balance calculation of the amount of hydraulic fluid consumed by a system during operation can indicate the hydraulic system’s condition. Additional tests can be done using valve signature information, recharge time, flow rates and reservoir levels to refine the health check and to identify specific anomalies within the hydraulic supply and distribution system of typical subsea production systems.
However, 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 measurements from the hydraulic power unit would improve the fluid consumption algorithms used to detect leakage. Additional pressure sensors combined with strategic isolation valves in the subsea hydraulic distribution termination assemblies would permit isolation of hydraulic anomalies associated with blockages or fluid leaks in complex subsea system architectures.
“The inspection, repair and maintenance of subsea systems by the offshore industry have advanced considerably in recent years,” says Maheshwari, “though we believe a step change is still possible. There is much we can learn about the true condition of critical subsea equipment by astute observation of data from the heart of these
systems. Combining this approach with thorough inspection can provide a revelatory insight into the health of a subsea system and a means to ensuring a trouble-free production life.”