Quickly installed unit operates off GPS positioning data
There are times and places offshore where only a dynamically positioned vessel will do.
Conventional mooring systems can be inappropriate in shallow waters with heavy seabed infrastructure and in water depths beyond anchor limits. With activity on the rise in both heavily trafficked areas and increasing water depths, the need for vessels with DP capabilities is rising. Unfortunately, the costs to place such systems on existing equipment is often so high the converted vessel is priced out of competitive bidding.
According to Dave Holloway of Houston-based Thrustmaster of Texas, "The cost of a new system is about $550 per installed horsepower, exclusive of the computer and up to about 1,000 hp per thruster."
For typical ROV, pipe-lay, cable-lay, or seismic project with four 500-hp thrusters that would translate to about $1.4 million, including between $190,000 and $250,000 in computer costs. Since DP conversions are done for a specific project, the entire cost of the conversion must be charged against the project for which it was done. A dilemma is thus created. Without DP conversions, projects not possible with conventionally moored vessels suffer the economic consequences of delay or cancellation while waiting for a DP vessel to become available. While at the same time, DP conversion costs are sufficient to render some otherwise viable projects uneconomical.
Portable systemHolloway´s company believes it has an option - portable DP systems that can be leased for a project´s duration and removed. In the most favorable scenario, the PDPS is removed from one vessel at the end of its assignment and placed on another within the contractor´s fleet, permitting the contractor to spread lease costs over two projects and keep dayrates competitive.
Lease costs for the Thrustmaster PDPS is about 45% of purchase price for a minimum six-month term. A unit like that described above for a purchase price of about $1.4 million could be leased over six months for about $580,000, including the necessary computer equipment.
Also, unlike permanent conversions which require substantial dockside conversion time and extensive vessel structural changes, PDPSs are installed in a matter of days and without permanent vessel modifications.
The units are modular, deck-mounted, hydraulic azimuthing thrusters with separate diesel-hydraulic power units, a central DP control console, interconnecting hydraulic hoses between thrusters and the power units, and electrical control cables betweeen power units and the DP console. It´s small deck-space requirements open the DP market to all-size and type vessels and dayrates. Standard thrusters come in 250, 500, 750, and 1,000 hp and can be arranged to deliver up to 8,000 installed hp.
Portable systems use direct hydraulic drive to the propeller. The variable speed, hydrostatic drive motor is in the lower foot of the thruster, directly in line with the propeller shaft. This direct hydraulic drive eliminates the need for right angle gear transmission and drive shafts and moving parts in the shaft, which houses hydraulic lines to the motor. The lack of any moving parts in the shaft also means it can be cut to any length.
The closed-loop hydraulic system also serves to dampen propeller-induced vibrations.
Since it is a hydraulic system rather than a mechanical one, propeller speed is infinitely controlled through pump displacement control rather than incremental steps. And because hydraulic pressure is directly proportional to propeller torque, the DP computer calculates delivered thrust by comparing its electrical output signal to the pressure feedback from the hydraulic system.
Computer systemThe PDPS uses a Nautronix automatic station keeping (ASK) 4000 joystick system which can be installed in a portable control console for indoor installation. Independent manual control of heading and position can be achieved using the three-axis joystick.
A requirement of any DP installation is that the controlling computer be programmable to match the specific vessel configuration. The portable ASK system is equipped with interface software and a vessel mathematical model configured for each installation. Vessel parameters defined in the model include hull dimensions, displacement, thruster locations, and vessel center of rotation. Contained in an ASCII disk file, the parameters can be easily edited in the field when the system is moved to another vessel.
Sea trialsHolloway reported that though operational parameters were exceeded in recent sea trials, "performance results of the sea trials were satisfactory in every respect concerning the operational capabilities of the system." Original operational parameters included a five-meter watch circle in sea state four, significant mean wave height of 6.2 ft, and mean sustained wind speeds of 19 knots.
The tests were conducted with a 357-ft heavy-lift vessel to be used in a cable laying contract in the Bay of Campeche. During the trials a hold position of one to two meters was maintained for five hours, including during a simulated thruster failure.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.