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Reconstruction of Riser Profiles by an Underwater Robot Using Inertial Navigation

This paper proposes a kinematic model and an inertial localization system architecture
for a riser inspecting robot. The robot scrolls outside the catenary riser, used for underwater
petroleum exploration, and is designed to perform several nondestructive tests. It
can also be used to reconstruct the riser profile. Here, a realistic simulation model of robot
kinematics and its environment is proposed, using different sources of data: oil platform
characteristics, riser static configuration, sea currents and waves, vortex-induced vibrations,
and instrumentation model. A dynamic finite element model of the riser generates
a nominal riser profile. When the robot kinematic model virtually scrolls the simulated
riser profile, a robot kinematic pattern is calculated. This pattern feeds error models of a
strapdown inertial measurement unit (IMU) and of a depth sensor. A Kalman filter fuses
the simulated accelerometers data with simulated external measurements. Along the riser
vertical part, the estimated localization error between the simulated nominal and Kalman
filter reconstructed robot paths was about 2 m. When the robot model approaches the
seabed it assumes a more horizontal trajectory and the localization error increases significantly.