The motion of a deep-sea remotely operated vehicle system Part 1: Motion observations

Rapid and high-resolution motion and tension measurements were made of a ca ged deep-sea remotely operated vehicle (ROV) system. Simultaneous measureme nts were made of all six components of motion at the cage and ship A-frame and of the tension in the tether at the ship. Data were collected for cage depths of 0-1765 m. The most significant forcing was in the wave-frequency band (0.1-0.25 Hz) and accounted for over 90% of the variance of vertical a cceleration. The vertical acceleration of the cage lagged the acceleration of the A-frame by up to 1.9 s and its variance was larger by up to a factor 2.2. For moderate displacements of the A-frame ( less than or equal to 2 m ), the system is only weakly non-linear because the harmonics (3rd and 5th) of the vertical acceleration of the cage account for less than 2% of the t otal variance. The system is essentially one-dimensional because only the v ertical motion of the cage and the vertical motion of the A-frame were cohe rent, while horizontal motions of the cage were weak and incoherent with an y component of motion of the A-frame. The natural frequency of the system i s 0.22 Hz at 1730 m, and we estimate that it is within the waveband for dep ths between 1450 m and the full operating depth of 5000 m. Large vertical excursions of the A-frame produce momentary slack in the tet her near the cage. Retensioning results in snap loads with vertical acceler ations of 0.5 gravity. Large rates of change of tension and vertical accele ration first occur at the cage during its downward motion and propagate to the surface with the characteristic speed (3870 m s(-1)) of tensile waves f or the tether. Six echoes are clearly detectable at both ends of the tether , and their pattern is extremely repeatable in different snap loads. Due to misalignment of the tether termination with the centres of mass and buoyan cy, the cage pitches by up 14 degrees during a snap. The resulting small ra dius of curvature poses the greatest stress on the tether. (C) 1999 Elsevie r Science Ltd. All rights reserved.