Apparatus and methods of monitoring mooring lines of an offshore drilling
rig, with the apparatus including a communication system in communication
with a global positioning system, providing position coordinates of the
apparatus at a first point on the line, and also includes an inertial
navigation system supported by the frame, providing position coordinates
of the apparatus at a second point on the line, by double integration
with respect to time of inertial accelerations measured by the device as
it is moved from the first point to the second point. Correction at the
second point is provided by obtaining at the second point, depth,
position relative to magnetic north, and deviation from vertical.