Eric Gilbertson

Experiments in dynamic control of autonomous marine vehicles using acoustic modems

Marine robots are an increasingly attractive means for observing and monitoring in the ocean, but underwater acoustic communication (“acomms”) remains a major challenge, especially for real-time control. Packet loss occurs widely, bit rates are low, and there are significant delays. We consider here strategies for feedback control with acomms links in either the sensor-controller channel, or the controller-actuator channel. On the controller-actuator side we implement sparse packetized predictive control (S-PPC), which simultaneously addresses packet-loss and the data rate limit. For the sensor-controller channel we study a modified information filter (MIF) in a Linear Quadratic Gaussian (LQG) control scheme. Field experiments were carried out with both approaches, regulating crosstrack error in a robotic kayak using acomms. Outcomes with both the S-PPC and MIF LQG confirm that good performance is achievable.

Design of a Thermally-Actuated Gas Lift Safety Valve

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valve assemblies (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions, and repair and monitoring become difficult. Currently no safeguard exists in the GLV to prevent product passage in the event of check valve failure. In this paper a design and operational procedures are proposed for a thermally-actuated positive-locking safety valve to seal the GLV in the event of check valve failure. A thermal model of the well and GLV system is developed and compared to well data to verify feasibility of a thermally-actuated safety valve. A 3× scale prototype safety valve is built and tested under simulated failure scenarios and well start-up scenarios. Realistic well temperatures in the range of 20C to 70C are used. Results demonstrate valve closure in response to simulated check valve failure and valve opening during simulated well start-up.Copyright

Failure Mode and Sensitivity Analysis of Gas Lift Valves

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valves. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions and repair and monitoring become more difficult. Gas lift valves and oil well systems have been modeled but system failure modes are not well understood. In this paper a quasi-steady-state fluid-mechanical model is constructed to study failure modes and sensitivities of a gas-lifted well system including the reservoir, two-phase flow within the tubing, and gas lift valve geometry. A set of three differential algebraic equations of the system is solved to determine the system state. Gas lift valve, two-phase flow, and reservoir models are validated with well and experimental data. Sensitivity analysis is performed on the model and sensitive parameters are identified. Failure modes of the system and parameter values that lead to failure modes are identified using Monte Carlo simulation. In particular, we find that the failure mode of backflow through the gas lift valve with a leaky check valve is sensitive to small variations in several design parameters.Copyright

Sharp Phase Change in Shape Memory Alloy Thermal Actuators for Subsea Flow Control

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valves (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions and repair and monitoring become difficult. The authors have previously developed a thermally-actuated safety valve to prevent product backflow into the annulus in the event of check valve failure. The safety valve uses shape memory alloy (SMA) wires to translate a temperature change into a displacement and, based on commercially available SMA wire material properties, requires a 6°C temperature change to fully actuate. In some wells, however, check valve failure may result in less than 6°C temperature change. In this paper a new concept is developed to sharpen the austenitic phase change in SMA actuators. The concept has broad practical implications because it will allow thermally-activated devices, such as fluid control valves, to become much more precise, i.e., translating a small temperature change into a large displacement. The new concept uses the fact that SMA transition temperatures are stress dependent. By specifically controlling stress in the wire, the temperature difference required for austenitic transition can be decreased. This is achieved with a negative-differential spring — a spring that exerts a decreasing amount of force as it is displaced. The concept is tested experimentally by conductively and electrically heating SMA wires connected to a negative-differential spring. Results show a 2.9°C-5°C reductions, respectively, in the temperature difference required for austenitic transition.Copyright