By Karen Boman, Associate Editor
University of Houston (UH) researchers have successfully tested a physics-based model that estimates in real time the control fluid leak rate within the BOP hydraulic control circuit.
Faults within BOP functions and components lead to nonproductive time (NPT) and affect overall system availability and can even lead to catastrophic failures that pose HSE risks. One of the dominant BOP failure modes is leakage within the BOP hydraulic control circuit, according to a presentation on 1 May at the 2018 OTC.
To increase uptime and safety related to BOPs, the oil and gas industry is looking to a new approach, condition-based operational maintenance, said Mete Mutlu, previously a Research Assistant in UH’s mechanical engineering department and now Senior Subsea Engineer with Transocean, during the presentation. Mr Mutlu worked on creating the physics-based model while at the school. Being able to estimate this rate facilitates condition-based monitoring and maintenance for BOPS by enabling early detection of leaks. The ability to detect leaks early can not only reduce NPT but also protect workers and the environment from harm.
Just as drivers want to monitor the health of their cars, the oil and gas industry today seeks to continuously monitor equipment health via condition-based maintenance. “If your car shows a check engine light, you don’t step on the gas; you think about taking the car in to make sure nothing is wrong and it won’t get any worse,” Mr Mutlu said.
“You don’t want to change equipment just because a certain time limit has been reached,” he added. “But you also don’t just want to push equipment too far.”
Some level of leaks in subsea control fluid lines, connectors, seals and valves are inevitable. “If you go into the normal waterplowing of the connectives, they have a certain leakage rate. Because of the motions, the mating surfaces are going to be lubricating each of them. At some levels, this leakage are acceptable, and at some levels it’s not; but it’s good to quantify the source so you know at what point you are,” Mr Mutlu said.
“If you think that your system is leaking more than usual, maybe there’s something worse happening in the system itself or maybe the equipment is degrading,” Mr Mutlu said. Getting that number is sometimes difficult if you don’t have sensors set for it.
Existing methods for detecting fluid leakage are categorized into internal and external systems. Internal systems use or monitor measured process variables such as pressure and flow rate. While these systems are efficient at detecting large leaks and don’t require installation of additional sensors, they face challenges in detecting and isolating small leaks, which may go undetected since they are out of measurement range for most flow meters. External systems, which include acoustic methods, fluorescent methods and optical camera methods, require the installation of additional sensors on the BOP.
Today, engineers monitor overall BOP system leakage by calculating cycle time between pump starts of the surface hydraulic power unit. A rise in pumping frequency translates into a higher leakage rate, but the frequency also hinges on the surface accumulator stack size and the accumulator pre-charge pressure. As a result, a standard acceptable pump start frequency can’t be adopted for rigs in drilling fleets of various design and operations.
“If you’re lucky you might have sister rigs and you might be able to compare charge times to charge time,” Mr Mutlu said. “But it might still be a problem if you have different manufacturers for rigs in the same fleet.”
Seeking to improve real-time condition-based monitoring of small leaks, UH researchers proposed to quantify BOP control fluid leakage using specifications of the surface stack mounted accumulator, which would allow the approach to be adopted by a fleet of drilling rigs that vary in design and operations.
The UH researchers tested the physics-based approach by developing a virtual system that mimics BOP operations and simulating BOP operations under different operating conditions. Increased control fluid leak within the BOP control system is simulated by increasing the leak area in different BOP components, such as the pressure regulator, shuttle valve and directional valve. Uncertainty because of measurement errors and disturbances due to BOP operations also were incorporated into the simulation. The simulated surface accumulator pressure data is processed, and the control fluid leak rate is estimated using the physics-based model. This estimated rate was then compared with the simulated leakage rate.
Following testing and validation in the simulated environment, the model’s algorithm was applied to field data collected from five offshore drilling rigs. The researchers found that the rate of leakage within the BOP hydraulic circuit grew from under 10 gal/hr to 30 gal/hr in the first 10 weeks, demonstrating the monitoring of the leak rate in real time and effective detection of the leak rate increase. By quantifying the BOP leak with respect to each control pod, workers can choose which of the two pods to use based on their actual leak states, allowing for a condition-based operation.
For more information, please see OTC 28955, “Leakage Monitoring of Subsea Blowout Preventer Control System,” presented at the 2018 OTC, 30 April-3 May, Houston, Texas.