ABSTRACT

Modelling algorithms are described for mechanical rock-cutting wheels for subsea rock trenching. The models are used in design and for optimising system operation. Performance prediction algorithms are based on current theoretical, experimental and field data on standard conical picks. Inputs include maximum wheel torque, wheel speed and trencher advance rate. Also considered are the effects of pick spacing and estimated rock unconfined compressive strength (UCS). Benefits of such modelling include new wheel design parameters, improved trenching system designs and more effective operation.

INTRODUCTION

The growing field of offshore exploration and production necessitates an in depth review of the tools and equipment used. Moreover, the cost of offshore time requires optimisation of processes done from ships, especially at depths. Subsea trenching is an example of such an area for study - in this case, optimisation of the design and operation of a rock-cutting wheel. The prediction of the performance is based on previous studies and on field performance data. This combined with the mechanical constraints of the trencher wheel can dictate optimal rotational and translational speeds along with predicted performance output.

This paper presents the basic physics required to predict the performance of a mechanical rock cutter in a subsea application. The objective is to predict the forces, torques and production rate given a rock type and the mechanical design of the machine, including a cutter geometry, pick layout, power supply and drive system.

This paper focuses on mechanical trenching, but the principles can be extended to rock excavation (e.g. subsea mining) as well.

Overall Cutting Model

A basic power flow/causality diagram for the mechanical cutting and drive system is shown in Figure 1. The figure represents an electric motor driving a variable swash-plate hydraulic pump, which drives a hydraulic motor that in turn drives the rock wheel through a gearbox.

The diagram assumes that the electric motor runs at a nominal rpm which, through the swash plate setting (fluid displacement per revolution), determines the fluid flow through the hydraulic motor, the resulting motor rotational speed, the speed of the output shaft of the gearbox and the pick speed. The forces on the picks result from two effects: the penetration of the picks into the rock(which dominates), and the drag force of the picks through the slurry.

The depth of penetration of the picks into the rock is determined by the ratio of the advance rate of the cutter to the rotational speed of the cutter and the number of sets of picks on the wheel:

penetration =(trencher advance rate)/((pick-sets/rev) *(rev/sec))

The net pick force then translates back into torques, pump pressure, and electric motor torque through the same set of relationships that were used on the rotational speed/flow path in Figure 1.

Note that, without careful design, the drive system will be prone to stalling. From Figure 1, it can be seen that, as the cutting forces rise, the hydraulic pressure increases, which will result in decreased pump output flow, which causes lower cutter speeds.

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