Development of a Real-Time Riser Fatigue Monitoring System

Abstract

Abstract As risers are deployed into deeper water, they are subject to increasingly severe environmental loading due to ocean currents and surface weather. Analytical models of these risers often predict premature failure due to the required safety factors and conservative modeling assumptions. As the design boundaries are extended, field measurements become necessary to assess the accuracy and safety margins associated with these models. Additionally, such measurements are expected to play a prominent role as quantitative structural integrity management programs are formalized and mandated. We have developed a Riser Fatigue Monitoring System (RFMS) to provide field measurements of drilling riser stress and fatigue in real-time. To our knowledge, real-time fatigue monitoring of an entire drilling riser has not been accomplished previously. The RFMS calculates drilling riser stress states using measurements from accelerometers and angular rate sensors inside Subsea Vibration Data Logger (SVDL) modules installed at strategic locations along the riser length. This method is preferred to resistive or fiber optic strain gauges due to the measurement quality and reliability concerns associated with installing such devices subsea. The SVDL units are connected via fiber optic subsea cabling to a central data acquisition system, located topside. Data from each SVDL is displayed as it is acquired, and processed with a sophisticated online computer algorithm to synthesize stress estimates along the entire riser length using a database of riser dynamic modes. The estimates are then processed chronologically via rainflow counting, recording fatigue damage accumulated during the deployment. The fatigue estimates are updated at 15-minute intervals, thereby providing actionable information to the drilling crew in real-time. This enables the crew to make informed and timely responses to damaging events such as loop currents, wave action, or improper tensioner settings. Additionally, the accumulated fatigue damage estimates may be tracked on a per-joint basis as the riser joints are rotated among different well sites, thereby aiding decisions regarding the frequency of joint inspection intervals. Significantly, since the only required online inputs are the dynamic riser response, top tension (TT), and mud weight (MW), fatigue estimates may be calculated without knowledge of the impinging currents or other forcing events (although the fatigue response may be correlated to these data, if available). We deployed the system at two well sites off the coast of Japan at 1,180- and 1,939-meter water depths. It successfully collected and processed data from August to November 2012, recording a number of riser excitation events (and attending fatigue damage) due to operations, weather, and vortex-induced vibration (VIV) while connected to and disconnected from the wellhead.