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. Introduction Gas lift is an artificial lifting method used to produce oil from wells that do not flow naturally. Gas is injected through the well annulus and into the well tubing at a down-well location (as shown in figure 1). The gas mixes with the oil in the tubing, aerating the oil and causing it to rise to the surface [1]. Gas lift valves are one-way valves that allow gas to pass from the annulus to the tubing but prevent oil from passing through to the annulus [1]. Most valves contain a pressurized bellows and an internal check valve (see figure 2). The bellows opens when the injection gas is pressurized above a threshold value, and the internal check valve prevents oil from passing through the gas lift valve [8] . Figure 1. Schematic of oil well with gas lift valve (GLV). Top of figure represents sea floor.