DN Staff

August 19, 2002

12 Min Read
Electric propulsion turns the tide

New Orleans-based Tidewater Marine operates more than 550 ships that support energy companies drilling offshore for oil and gas. Its vessels sustain the materials transport necessary to keep drilling platforms operating, and perform vital towing and anchor-handling activities.

Borrowing from electric propulsion concepts pioneered by the U.S. Navy and more recently used on modern cruise and deep water drilling ships, this platfor-supply vessel uses on-board diesel-engine generators to supply electricity for powering electric motor driven propulsion screws. WHile not the firs tsuch vessel to do so, its completely integrated electrical automation system, redundantt fiber-optic communications, systems diagnostics, and easy-to-use touch screen controls make it one of the most sophisticated in its class.

Reliability is king in the world of platform supply, towing, and anchor handling vessels because the high cost of drill rig downtime leads to quick replacement of failing support ships. Consequently, staying competitive means having more reliable vessels than the competition, according to Tidewater VP of Technical Services E.J. Hebert.

With its fleet aging, the company embarked on a $300 million program in 2000 to build 12 new diesel-electric powered ships that, instead of driving the screws directly with diesel engines, use on-board diesel-engine generators to supply electricity for powering electric motors. While not the first vessels of this type to do so, Tidewater claims its completely integrated electrical automation system, redundant fiber optic communications, easy-to-use system diagnostics, and advanced touch screen controls make its vessels some of the easiest to operate, maintain, and troubleshoot.

Electric muscle: six dc motors drive two main propulsion 360- degrees, z-drive units producing more than 6,000 hp. Two other 1,100-hp dc motors independently drive the bow thruster units. One is a drop-down, 360 degrees, azmuthing, z-drive unit. The other is a reversible tunnel thruster unit. In all, there are 10 Siemens SCR drives controlling the propulsion and thruster units.

The program's first completed Platform Supply Vessel (PSV), the Miss Jane Tide, set sail from Houma, LA in March under contract with an oil company drilling in Trinidad. Six other ships in the program, like the Miss Jane Tide, are general purpose 260-ft PSVs. Tanks below deck store fuel, drilling fluid, cement, or mud, while casing, drill pipe, tubing, or other materials are secured on deck. Five additional ships, classified as 280-ft Anchor Handling Towing Supply vessels, can position and moor drilling rigs, and come equipped with a 600-metric-ton line-pull dc electric winch for hoisting massive iron anchors. All 12 vessels will have a completely integrated electrical automation system designed by Houston-based systems integrator EPD International Ltd. that uses major components from Siemens Energy & Automation (Alpharetta, GA).

As the electrical systems integrator, EPD supplies the main propulsion generator controls that allow ship operators to start, stop, and load share power between the generators; distribution switchgear comprising feed breakers for all the loads; the propulsion and thruster water-cooled dc motors that drive the screws; SCR drives that control the speed of the screw motors; ac drives for bulk unloading; all 600V and 480V motor control centers that house the motor starters governing auxiliary equipment such as fans, blowers, and cooling systems; control drop-in consoles for the pilot house and engine control room (ECR); a redundant PLC system that monitors most on-board functions and delivers data to the touchscreen controls; the power management system that prevents blackouts by controlling load distribution between generators and power limits the variable speed drives; and the high power phase shifting transformers and reactors that greatly reduce electrical harmonics.

"We use and apply standard industrial products to the marine market," explains EPD Manager John Norwood IV. "We buy the power bridge, field supply, and microprocessor controls from Siemens, then package them with our own dc drive specifically designed for the marine industry."

Anchors away? Tidewater chose diesel-electric propulsion for all 12 of the DP-2 [Dynamic Positioning Class 2] vessels as part of its plan to stay ahead of the competition in terms of cargo capacity, reliability, and safety, according to Hebert. DP vessels and drill ships don't require cables and anchors to hold a position over a drill site. Instead, their main propulsion and thrusters are tied into a GPS (Global Positioning System) that automatically controls the ships' position by applying thrust as needed.

These DP systems can be coordinated so that the drill ship and its support vessels maintain a 5- to 10-ft diameter circle around a particular drilling position, according to Norwood. Consequently, DP vessels are suitable for operation in deep waters (up to 12,000 ft to date) where attaching anchors to the ocean floor becomes increasingly difficult.

There are three classes of DP. DP-1 vessels are the simplest class, without system redundancy. Such a system is generally used on smaller, less sophisticated support vessels with lower day rates than DP-2 and DP-3 vessels. DP-3, in contrast is 100% redundant, and is commonly used by very large, sophisticated deep water drill ships. "DP-3 means two of everything," Norwood explains. "Two engine rooms, two control rooms, so even if you flood a compartment completely, you still have 100% power." DP-2 lies between. "It doesn't have two of everything, but it has a lot of redundancy," says Norwood. "So if a main propulsion shaft motor fails, only a percentage of the power is lost, not all of it."

Fiber optic nerve: For EMI immunity, which eliminates power and control cable segregation, engineers use fiber optics wherever there is a Profibus run of more than 100 ft. This approach simplifies wiring and saves money and space because it eliminates the expense of secondary cable trays. Profibus Optical Link Modules located in all the motor control centers (MCCs) and drives connect bridge consules. The system comprises more than 2,000 I/O points with a redundant fiber-optic ring that encircles the ship. If one cable is cut or fails for some reason, the secondary ring automatically takes over communications of control signals, drive settings, motor parameters, fault-code and generator control data, and real-time status monitoring of motor starters, switches and other components.

More capacity. "In addition to efficiency gains from cleaner, smoother, and quieter operation, the electrical propulsion system frees up cargo space below deck to accommodate larger holding tanks.

Since cargo space is aft on platform supply vessels, positioning engine rooms near the screws trades off cargo space for shorter drive shafts on direct-drive diesel ships. However, this rear-engine configuration means long exhaust runs forward to the stacks near the pilothouse that take up additional space. Moving the engine room forward with direct-diesel drives simplifies exhaust routing, but requires long drive shafts linking engines to the propellers, which also consumes cargo space.

Because diesel-electric propulsion uses smaller power and control cables to link the generators and motors, the design eliminates the larger drive shafts required with direct-diesel systems. Consequently, Tidewater's new vessels can have the engine room forward for easier exhaust ventilation straight up to the stacks, without requiring long drive shafts running down the port and starboard sides. Result, the Miss Jane Tide has a cargo capacity of over 4,000-tons total dead weight, 20% more than similar vessels, according to Hebert.

Four 1,825 kW generators connected to a main 600V switchboard with Siemens RL breakers power the Miss Jane Tide. The generators have a 0.7 power factor and tie to a main bus via electrically operated, draw-out, 3,200A frame circuit breakers. "This power factor rating lets us get the maximum amount of power out of the generators for the dc variable speed system," adds Norwood. As a result, the ship has a top speed of 15 knots.

Norwood says the completely integrated electrical automation system is the lifeline of the vessel-tying together propulsion, dynamic positioning, and vessel management systems. Although these three systems all work independently, they depend on each other to operate. For example, the thruster vendor's throttle and steering controls not only have to tie into the electrical and automation systems for power and control signals, but also must respond to the DP and vessel management systems.

No overlap or omission. "We had to make sure that everything worked together, we had to eliminate duplications, and we had make sure that nothing was left out," says Norwood. Accordingly, EPD scheduled periodic integration meetings throughout development with other system suppliers, including the engine generator, thruster, vessel management system, and dynamic positioning vendors.

"At these meetings we generated the interface requirements for what each system needed from the other vendors to operate," Norwood explains. In fact, EPD spent several months early in development working with Tidewater and a Norwegian ship designer and other international suppliers writing the specifications for the electrical and automation systems aboard the vessels. EPD also coordinated specifications from vendors located near the respective shipyards that supply the PSV vessels' 480V switchboards, shore-power connections, emergency switchboards, and lighting panels.

Automation-wise, the biggest issue was pinning down a single PLC protocol to make communication fast and easy between all the vendors' systems, according to Norwood. "Using a common PLC system throughout the vessel allows the vessel management system to communicate efficiently with the DP system and the electrical automation system. It also helps the customer minimize the number of different spare parts kept on hand," Norwood explains.

Harmonic reduction. Variable speed drives are electrically characterized as non-linear loads that source significant amounts of harmonic current, which when injected into a power system cause unwanted voltage distortions. Consequently, EPD had to deal with electrical system harmonics. These electrical disturbances, if left unchecked, can shorten the life of ac motors and lighting ballasts, blow fuses, trip circuit breakers, and disrupt operation of navigation and computer control systems. "There are two design approaches when it comes to harmonics. Design and build so that you don't produce them, or produce them and then clean them up with filters," Norwood explains. "We chose to not produce them in the first place."

EPD's competence in variable speed drives and designing harmonic filters complements president and founder John Janick's experience working with harmonics on the F-16 fighter after he graduated from collage. By using phase-shifting transformers and multiple motors on each shaft the company designed a system that virtually eliminates all harmonic problems commonly associated with drive systems. To create a pseudo 18-pulse drive, EPD engineers use three 6-pulse drives to control the three motors driving each main propulsion shaft. "By phase-shifting each drive 20 degrees apart, we get 18-pulse harmonic cancellation," says Norwood. In contrast, a single motor drives each bow thruster, so true 12-pulse drives, phase shifted 30 degrees apart, control each motor.

Since its launch, the Miss Jane Tide has not had any problems with the EPD supplied equipment, according to Hebert, adding that the transition to an electrically powered fleet is paying off. After all, the bottom line in the oil and gas industry is how much a company is paid on a day-rate basis. When a supply vessel is unable to make a delivery or is disabled, the costs can be staggering. Now, Tidewater is confident of retaining a reputation for reliability, which translates into repeat business.

What does it take to automate 12 diesel-electric vessels?




250 hp Siemens 6SE70 ac variable speed drives


600V generation main propulsion switchboards, Siemens RL


4,160V main propulsion generator switchboards, Siemens GMI


600V winch distribution switchboards, Siemens MCCB


2,250 hp main propulsion dc drives, Siemens 6RA24


2,250 hp water-cooled main propulsion dc motors


1,100 hp 12 pulse propulsion dc thruster drives, 6RA24


1,100 hp water-cooled vertically mounted dc thruster motors


1,100 hp main propulsion dc drives, 6RA24


1,100 hp water-cooled main propulsion dc motors


1,100 hp winch dc drives, 6RA24


1,100 hp air-cooled winch dc motors.


4,160V - 600V high power phase shifting transformers


Small ship service transformers


600V phase shifting transformers and reactors


Vertical sections of 480V & 600V MCCs, Siemens Model 89


Fire fighting assignment cubicles


Siemens PC670 touch screen Industrial PCs


I/O points


Siemens SIMATIC(TM)S7-400 PLCs


Siemens SIMATIC(TM) S7-318 PLCs

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