Thru-tubing well interventions with coiled tubing, hydraulic workover (HWO), and wireline units are becoming more sophisticated due to the increase in rig-up complexity (i.e TLPs, Spars, etc.), higher wellhead pressures, and complicated well designs. These field conditions result in taller and more complex intervention stacks, which are more susceptible to bending excitation forces from crane, production riser, and platform movements. This environment dictated the need for a system to help verify the safety margin of the job design and to enable real-time monitoring of riser/lubricator stress levels during critical operations. The ζ (Zeta) safety system was developed with joint funding from BP and CTES to meet this need. It is comprised of two basic components, the Zeta gauge and the Zeta model. The ζ gauge uses fiber-optic strain gauge technology to monitor the force, pressure, temperature and bending moments acting on a section of lubricator or riser. These measured values are then used to calculate the stresses at the gauge for monitoring purposes during the job. The ζ model uses a dynamic finite-element analysis to calculate the stresses throughout a lubricator/riser stack, either in a job-planning mode based upon expected forces and movements, or based upon real time inputs from the ζ gauge. This presentation will describe this safety system and review results of recent field applications.