AC Plies the high seas

DN Staff

March 25, 1996

7 Min Read
AC Plies the high seas

Singapore, Malaysia--The challenge was as formidable as an angry sea: Design a ship-board system that will gently lay almost three miles of oil and natural-gas transfer pipe onto the ocean's floor--every hour, around the clock. Such a system must not only withstand more than 660 tons of tension as the 5-ft, steel-and-concrete pipline sinks to depths reaching 3,000 ft, it must counter the rolling, pitching, and pounding of an often hostile environment.

These were the specs issued by Allseas of Holland when converting its M/V Solitaire--a 985-ft, former bulk carrier--into the world's largest pipelay vessel. Meeting them required the integration of three critical systems: The biggest and most powerful caterpillar-type, linear pipe tensioners ever built; an emergency Abandonment and Recovery System, designed so the ship can ride out a sudden squall or driving gale; and an on-board, pipe-assembly plant that rivals the most efficient of shore-based operations.

It is the combination of ac motors, adjustable-speed drives, and ac gearmotors that optimize these three systems. Run through a distributed control system with universal operator interface, the ac network surpasses more conventional dc and/or hydraulic systems from both a size and efficiency standpoint. Credit for the design and integration of all power, drive, and control units belongs to subcontractor Reliance Electric, based in Cleveland, OH.

Ship-board assembly. Pipe comes on board in 40-ft straight lengths. Delivered from supply ships or barges, and stacked below decks in the holds, the steel sections are pre-coated with four inches of concrete. This heavy sleeve anchors the pipe, which would otherwise pop to the surface when empty.

Also below decks, in a double-joint factory equipped with multiple pipe conveyors, welders convert the 40-ft lengths into 80-ft lengths of pipe. The longer segments allow non-stop production of continuous pipeline--a process that takes place downstream at the ship's "firing line." Designed and constructed by CRC Evans, of Tulsa, OK, the on-board assembly plant features an unprecedented degree of automation.

For example, Reliance ac gearmotors and GP2000 inverters, ruggedized for severe vibration and a corrosive salt-water environment, provide adjustable-speed control of the angled-roller assembly conveyors that move pipe from one station to the next. At each weld station, additional drives lift and rotate the pipe sections in conjunction with the welder.

Exposed steel flanges are butt-welded, X-ray inspected, and sealed with a tar/asphalt compound, before transfer to the ship's firing line. Here, welders work "on-the-fly." Their job: join each 80-ft segment to the end of the pipeline prior to passing through the ship's giant tensioners into the sea, utilizing vector-controlled drives to feed the final assemblies in place.

The caterpillar-type tensioners, built by WesTech Gear Corp., Lynwood, CA, apply tension to the continuous pipe as it drops off the ship's stern. Without this controlled rate of deceleration, gravitational pull on the huge concrete mass could upset, and even sink the vessel. The tension also ensures that the system lays the pipe down comfortably on the ocean bed, protecting its integrity.

Each tensioner comprises an upper and lower track. Two 300-hp motors per track add up to 1,200 hp per tensioner--largest ever constructed. Insurer Lloyds of London, moreover, requires three tensioners, for a total of 3,600 hp of adjustable speed drives. This figure, they calculate, enables the ship to lay the largest pipe at the fastest rate, and yet maintain an acceptable safety factor.

Why ac? During operation, the upper and lower caterpillar tracks, each controlled by the two 300-hp motors, push the pipe through their respective tensioners. At the same time, 28 hydraulic cylinders per track apply the squeeze force for the tensions required to hold the pipe in position. DC or hydraulic motors have traditionally been used to power these tracks, but present several drawbacks to ship-board application.

For starters, "There simply isn't enough room on the ship for twelve 300-hp hydraulic motors and all the associated plumbing," explains Reliance Project Engineer Tom Renner. DC motors, on the other hand, must be totally enclosed because the sea air will quickly corrode the commutator brushes. The air/water heat exchanger needed to cool such units, he says, would also result in a massive assembly.

Renner adds that "DC can't stand full torque at stall," and describes a situation where the pipe must be held still to make a valve connection. "Holding tension and torque with a dc machine," he says, "heats up the commutator bar, which begins to expand. If the bar expands beyond the commutator's normal concentric arrangement, it will act like a file, destroying the brush, and eventually the motor."

The common solution-- mechanical brakes--doesn't fix the situation. "Can you imagine 3,600 hp worth of brakes in addition to the already oversized motors?" asks Renner. WesTech Gear, consequently, uses Reliance DutyMaster ac induction motors to supply primary power to each tensioner; twelve Reliance Invertrona regenerative, vector-controlled inverters provide tensioner control.

WesTech Program Manager Kurt Schantz says his company selected these motors and drives because they have proven themselves in heavy-duty mining applications where space is a premium. And while the water-jacketed motors take up little space, they also match the ship's existing power system. Schantz can tick off several other advantages, too:

* A power factor of 96% vs 88% for the best dc drives.
* Lower maintenance costs.
* Regenerated power to other on-board systems, for maximum efficiency.
* Integrated, distributed control.

Central control. Reliance Electric's AutoMax Distributed Control System, combined with the company's SIGMA Operator Interface System, coordinates operation of the pipe assembly plant and the tensioners to ensure continuous pipe- assembly rates. It also compensates for the rolling action of the ship. As the stern and bow alternately rise and fall, the pipe must be lowered into the ocean as if the ship were constantly level.

The overall system, therefore, must account for a wide range of variables, such as total ship displacement, required pipe weight, tensions and lay speeds, as well as the effect of ocean swells and wave frequency on the ship and pipe. AutoMax DCS accommodates these varying inputs and requirements through its multi-tasking operating system, multi-processing hardware, and multi-language programming capabilities.

Reliance Electric's SIGMA Universal Operator Interface and Universal Process Monitor System function as the pipelay operation's supervisory-level architecture. Located on the ship's bridge, they integrate data acquisition, supervisory control and information management into one comprehensive package. Custom screens indicate distributed power drive status, maintenance and troubleshooting tools, process overview, trending, and event history.

The SIGMA systems run on an IBM PC-compatible; screens are produced using Microsoft(R) Visual Basic(R). A customized real-time data acquisition system, called SIGMA Kernel, links the complete package to the AutoMax Drive System via a standard ISA card. To prevent interference with navigational sonar and radar, every drive, console, and cabinet features fiber optic connections.

Safety at sea. Central command via AutoMax/SIGMA controls the emergency Abandonment and Recovery System as well. Also developed by WesTech Gear, the system serves as the ship's first line of defense; when seas are rough, it drops the capped pipline to the ocean floor on a heavy steel cable marked by a buoy--ridding the ship of its potentially deadly tether. After the storm, the ship retrieves the pipeline using a double-drum cable winch designed by Bodewes, of Holland.

Operation is similar to any block-and-tackle approach, where multiple wraps around a couple of capstans generates a tremendous amount of pull. AC control, however, adds electrical efficiency to the cable winch's inherent mechanical advantage. Driven by eight 400-hp, water-cooled vector drives and motors, the winch--like the tensioners--regenerates extra energy that goes directly into the ship's power grid. This occurs during abandonment, when gravity overcomes the motors and turns them into alternators.

Once the Abandonment and Recovery System reels in its cable to bring the pipe end back through the tensioners, the pipelay operation can resume. Designed to operate year round in any climate--from the frigid Arctic to the steamy tropics--the world's largest pipelay ship is presently under construction in Singapore. Testing in the North Sea is scheduled for early 1997.

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