Line length navigation system

Abstract

The invention is a guidance system used in laying undersea pipe or cable from a pipe-laying vessel when radio transmissions are unreliable. The system processes parameters derived from sensors normally aboard a pipe-laying vessel and sensors cooperating with active winches applying tension to the several anchor lines. The vessel's pitch, roll, and heading angles are supplied from gyrocompass and vertical gyroscopic instruments. Anchor line elevation and azimuth angles with respect to deck coordinates are yielded by conventional angle pick offs associated with fairlead devices, as are the line axial tension and the length of anchor line paid out. Initial conditions, including the initial position of the vessel, are easily obtained in the usual manner. From certain of these parameters, the invention derives intermediate anchor line parameters for use in computing fairlead motion. The intermediate data is used to compute compensated fairlead motion data by first computing apparent fairlead motion from anchor line tension changes. This version of fairlead motion contains undesired terms due to apparent fairlead motion caused by anchor line length changes and also due to actual fairlead motion caused by vessel attitude changes. Therefore, second and third computations are made of the contributions of the latter effects. This is done so that apparent fairlead motion due to anchor line length change and actual fairlead motion due to vessel attitude change may be subtracted from the measure of apparent fairlead motion as calculated from anchor line tension change, yielding the desired compensated fairlead motion value free of significant disturbing components. An estimated value of vessel position change is then obtained by a least squares estimation involving these fairlead motion terms. Correction of vessel position then follows by cooperative operation of the winch and thruster assemblies.

Dependencies are inferred from the claim text.