The primary station keeping objective during drilling operations is to keep the riser angle relative to the BOP within tolerance to ensure safe operation and minimise wear at the riser flex joint. As a minimum, the riser and BOP inclinations need to be measured and the differential riser/BOP angle calculated to make sure they remain within tolerance. The inclinations measured subsea are transmitted up to the rig using high speed digital acoustic telemetry, where they are received by the rigs acoustic transceiver, decoded, and then displayed on the bridge or drill floor. The rig’s position and bearing relative to the BOP is also calculated and the information displayed to the dynamic positioning operator or barge engineer. They use the data to steer the rig and return the riser flex joint back to vertical.
Extend operating envelope and maintenance intervals
Whilst measured data is used at the flex joint, mathematical models are used on the rest of the riser to predict the effects of currents, stresses and strains on riser lifespan and rig operating windows. The existing models are recognised as conservative, sometimes resulting in unnecessary downtime due to reduced operating windows and the expense of riser over-inspection and maintenance. Instrumenting the riser to provide measured, rather than modelled, data can result in more realistic operating windows and maintenance schedules; however wired sensors can be impractical, especially when a retro-fitted solution is needed. Therefore, wireless sensors connected to an acoustic network offer the only practical solution. Risers can be instrumented at intervals and connected wirelessly to the surface to measure, for example, inclination, accelerations, stress, strain and water current. Near real-time displays of riser measurements taken either on demand or automatically at predefined intervals can aid decision making and extend a rigs operating envelope in marginal conditions. Data analysis to assess wear and tear can be used to determine the most appropriate inspection regime.