Tool power is supplied by battery, a downhole alternator or both. Batteries allow tool operation without mud flow. However their energy is limited. This means that the operating time is limited, and the sensor power output is limited. While not normally a problem on directional - only services, with the addition of formation evaluation sensors, the problem becomes more obvious. In addition, batteries have limitations in temperature. Alternators solve the energy limit problem but introduce some of their own. Mud pumps must be above a 'drop-out" minimum rate for them to work, the turbines necessary to drive them can clog, and they limit the flow rate range in which an individual tool can operate.
Alternator tools must be tailored for the pumping system in use on the rig. Turbine stages are configured for the expected mud flow rates. Expected flow rates are important information to set up the job with alternator tools. The drilling engineer should be sure the MWD vendor has this information well before the job is to begin. The alternator tools have an internal over-voltage protection device which stops the tools should the alternator output exceed its limit.
Was this article helpful?