An operational modal response method for application to the structure health and integrity of pipelines is investigated. The modal response characteristics of externally supported pipe structures are quantified through flow Reynolds number (Red) variation. Pipe flow turbulence and the resulting hydrodynamic pressure fluctuations on the interior pipe wall provide the structural forcing mechanism, and signals from wall-mounted accelerometers provide the system response. During experiments, the Reynolds number is varied from 51,000 to 154,000. Over this Reynolds number range, the pipe flow turbulence was found sufficient enough to excite the structure at frequencies up to 400 Hz. Modal response characteristics obtained through Reynolds number variation were found to be in agreement with results from impact hammer modal testing. The in-situ modal response method developed was applied to two different structural health monitoring investigations, one involving loss-of-material and the other involving loss-of-fluid. The loss-of-material scenario simulated the process of external pipe wall corrosion, and the developed method was able to detect material loss as small as 1.4%. The loss-of-fluid scenario simulated a small leak. Despite the low operating pressure of 0.024 MPa, the methodology was able to detect fluid loss as low as 0.1% of the bulk flow rate. The developed method has the potential to offer in-situ continuous pipeline health monitoring that relies on the continuous changes (flow rate, product viscosity, product density) that are inherent to an operational pipeline system.