Industrial Automation Networks Inc.
Every offshore facility has a riser, but unfortunately not all facilities have real-time monitoring systems to confirm the integrity of that riser. The importance of riser monitoring is demonstrated by the existence of two industry standards: DNV-OS-F201 "Dynamic Risers" and API RP 16Q "Recommended Practice for Operational Inspection of Drilling Risers." In addition to providing much of the theory on which riser monitoring systems should be based, as with all standards, these documents tend to reflect the best practices offered by multiple experts in this area of work.
The common challenges for offshore drilling and production that need to be managed via riser monitoring and associated management systems include: riser curvature, fatigue loading damage, and high tensions at the BOP and base. The causes of these challenges are a result of the environment in which offshore facilities operate. It is impossible to avoid the impact of wave action, tides, and temperature gradients, especially in deepwater installations as well as the pressures of the production system itself. All of these variables are classified into three types of loads on the system:
Pressure loads - external hydrostatic pressure and internal fluid pressure, both of which can easily be measured and are normally measured for process reasons.
Functional loads - which include the weight and buoyancy of riser, tubing, coatings, marine growth, anodes, buoyancy modules, contents, and attachments as well as the weight of the internal fluids. Again, these are loads that by and large are under the operator's control or can be managed with proper maintenance.
Environmental loads - predominantly waves including internal waves and other effects due to differences in water density and floater motions induced by wind, waves, and current.
Calculations from API16Q provide guidance and recommendations on the maximum top tension ranges recommended for drilling risers with corresponding mud weights. However, riser analysis is required to define acceptable tension ranges for in-service loading conditions to manage flexjoint angles, component capacities, and riser fatigue damage. During drilling operations, the mean flexjoint angles should be limited to 1-2° so this does not leave much margin for error.
This is reinforced by Clause B (301) of the DNV specification that states: "The riser's internal and external operating condition should be monitored to reveal whether design conditions have been exceeded. This monitoring should include the recording of riser response and tension (if relevant) as well as the composition, pressure, and temperature of the riser contents. Wall thickness measurements by internal means, e.g. pigs and by external means at selected reference points should be considered."
Real-time monitoring systems provide sequence data, pressure, depth, and stress levels on equipment during the entire life cycle to increase system reliability. These systems can help users predict vortex-induced vibration (VIV), where the drilling riser vibrates perpendicular to the dominant current direction. This phenomenon is responsible for the majority of the fatigue damage in deepwater drilling risers. Because of the uncertainties involved in VIV prediction, it has the potential to be very dangerous. These uncertainties come from various sources: the variation in magnitude and direction of deepwater long-term currents; complex multi-modal characteristics of VIV in the deepwater environment; non-scalability of tank test results that are used to determine potential VIV impact; and uncertainties in the design input parameters which require calibration based on measurements in the field. Installing real-time systems and then comparing the actual results against those predicted by the models will not only improve the reliability of the system but also the veracity of the model as well.
Much of the technology to provide the required life cycle data is available today, but integrating the measurements into a system is the challenge. For example, RFID tagging is frequently used to identify each asset through its life cycle and is being done today by manufacturers as part of their required traceability. So, this technology exists.
Fixed components of the system and production platforms can be connected to the data collection and analysis system installed in the "top works" via wire or fiber. Of course, for non-fixed components such as the riser, during drilling acoustic (water equivalent of wireless) technology can be used to transmit data from subsea to platform for processing. Acoustic transmission can also be used with fixed systems.
Acoustic systems like wireless need to be low power, so they need to be designed to be able to run off batteries. However, the batteries could be charged by waves or the temperature gradient of the water for longer life. One of the simpler instruments capable of providing a significant number of the measurements needed for riser monitoring are based on a strain gauge as part of a Wheatstone bridge, which is common low energy circuit.
Be sure to keep the individual sensors and network simple because it is in a harsh environment that is difficult to access for maintenance. This means put as much intelligence "up top" in software where it can be accessed by local experts who recognize and correlate patterns. Also, if the real-time riser data acquisition system is working properly, the platform control and data capture/archiving system can also be analyzed in real time remotely by experts and quality assurance teams. An additional benefit of using a distributed sensor network is that if one sensor fails, the majority of the system is still functional; and to a certain point, the model and other data points can "fill in" the missing information. Avoid typical engineering of overdesign as represented by bloatware in computers.
Because the industry is now putting the intelligence in a computer with "unlimited" processing capability, one thing that must be avoided is the idea of developing the "perfect" standalone system, since the trend today is to distribute intelligence. The Internet of Things (IoT) is all about gathering lots of data and then making sense of it in a central location. Improve the operator interface so that the data is presented in a meaningful way. This will allow users to get a better understanding of how a riser system is performing as a part of an integrated offshore platform, and it will also increase the overall reliability and safety as well.
Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional, Automation Hall of Fame member, and a recognized authority on process analyzer sample systems, Foundation Fieldbus, and industrial communications technologies. Verhappen provides consulting services in the areas of field level industrial communications, process analytics, and hydrocarbon facility automation. Feedback is always welcome via e-mail email@example.com.