Design Assesment of the Sawing Wire System Used in Salvage Operations

Abstract

The Smit sawing wire system was used in the salvage operation of the Baltic Ace. The sawing wire was drilled underneath the wreck and connected to two barges on opposite sides. Winches were placed on the barges to make a controlled cut through the wreck by pulling the sawing wire back and forth. The cut was made in an upward direction. While the sawing wire was pulled in by one winch, the winch on the other side kept a holding tension on the sawing wire to create a normal force in the wreck. This normal force was necessary to make the cut through abrasive wear. To get a better insight into the forces experienced in the sawing wire system, measurements were conducted during the cutting operations. The objective of this research project is to contribute to the assessment of the sawing wire system, by providing a simulation model based on theory and compare this with measurements of the salvage operation of the Baltic Ace. The measurements show a peak tension at the beginning of the sawing cycle that was not expected in calculations made prior to the operation. In friction theory, this is called the breakaway force that is needed to get the sawing wire moving. The breakaway force is caused by the difference in static and kinetic friction coefficients between the sawing wire and the wreck. The simulation model is made in Orcaflex and includes the friction at the contact points, barge movement, and the heave compensators. The friction is included by a Coulomb friction model for the kinetic friction, a tension for the breakaway force and a stick-slip damper for the intermittent motion of the wire. The heave compensator stroke is controlled by a code in python that measures the sawing wire angle and the heave motion of the barges at the fairlead position. The barge motions were included by force-RAO’s, calculated in AQWA. The wave environmental forces are simulated using a JONSWAP spectrum with the wave height and period similar to those during the sawing operation. After completion of the model, the results were compared with the measurements. The tension graph of the model has similar peak forces at the beginning of the cycle (first 10s.) and similar lower force fluctuations in the constant part (10-50 s.). These results together with the comparison in the frequency domain are used to verify the model, hereafter the model is considered correct and ready for use in future sawing calculations. The simulation model is made in Orcaflex and includes the friction at the contact points, barge movement, and the heave compensators. The friction is included by a Coulomb friction model for the kinetic friction, a tension for the breakaway force and a stick-slip damper for the intermittent motion of the wire. The heave compensator stroke is controlled by a code in python that measures the sawing wire angle and the heave motion of the barges at the fairlead position. The barge motions were included by force-RAO’s, calculated in AQWA. The wave environmental forces are simulated using a JONSWAP spectrum with the wave height and period similar to those during the sawing operation. After completion of the model, the results were compared with the measurements. The tension graph of the model has similar peak forces at the beginning of the cycle (first 10s.) and similar lower force fluctuations in the constant part (10-50 s.). These results together with the comparison in the frequency domain are used to verify the model, hereafter the model is considered correct and ready for use in future sawing calculations.