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Thrusters improve dynamic positioning on OSVs

Mon 20 Jun 2016

Thrusters improve dynamic positioning on OSVs
When engaged, the DP System simultaneously and continuously controls the bow thrusters

Alabama has the largest concentration of builders of metal fabricated vessels under 90m in the US. One of those companies, Master Boat Builders Inc of Bayou La Batre, has established its niche by building highly sophisticated offshore supply vessels to serve the oil industry.

Master Boat has cultivated its oil rig supply boat business, in large part, by being one of the first builders to transition to thrusters powered by electric motors and variable speed drives (VSDs). The electronic thrusters provide numerous benefits over their hydraulic predecessors, and are highly compatible with Dynamic Positioning (DP) systems that automatically maintain a vessel’s position while it is serving an offshore oil platform.

Master Boat has a long standing relationship with Gulf Coast Air & Hydraulics Inc of Mobile, a full line distributor and fabricator specialising in air, hydraulic and electric products and services for the heavy industrial and marine markets.

Gulf Coast’s early work with the offshore boats included supplying mechanically operated bow and stern thruster systems driven with hydraulic motors. The hydraulic systems were loud, relatively complex and inefficient, and a potential environmental liability if a pipe or hose broke at sea. As alternative technologies began to evolve and become more available, boat owners began to demand a change. Hence in the early years of the 21st century Master Boat decided to transition to electric thrusters and variable frequency drives.

In addition to the environmental safety benefits, electronic thruster systems are far safer to operate and service than hydraulic systems, creating less risk for crew members and allowing boat owners to carry less insurance. They are also easier to install, cheaper to operate, and more efficient.

The biggest benefit of the electric thrusters, however, is how smoothly they interface and respond to the sophisticated Dynamic Positioning (DP) systems that are required by the American Bureau of Shipping (ABS), and the oil companies that contract the boats to supply their offshore oil rigs.

A DP system is an automatic operational mode that triangulates between GPS satellites to maintain a vessel‘s position within a 1m to 3m radius. It is typically used while a ship is in very close proximity to an oil rig while loading and offloading materials and supplies. When engaged, the DP System simultaneously and continuously controls the bow thrusters, main propulsion system and rudders to hold the ship in place, no matter the wind, current or wave action. The orchestra of control is essential in the supply process to keep a boat from hitting the floating oil rig, and potentially moving it off its mark, causing major damage while drilling in three to five mile sea depths.

From a macro perspective there were immediate DP system benefits realised by moving from hydraulic to electric thrusters. In general, the interface of an electric motor and VFD is more efficient, the system reacts far quicker and requires less maintenance.

The electric system also eliminates the complex interface between the mechanical thrusters and the DP software required with the hydraulic system; including an electronic board that converted signals to a processor that drove the hydraulic motor coupled to the shaft of the thruster. This intricate set of moving parts limited the performance of the DP system and was prone to frequent breakdown.

Baldor motors and ABB drives have been integral components of Master Boat thrusters since the early stages of the transition, because both components are uniquely able to withstand the harsh marine environment and the tremendous demands of the DP system. In 2011, just after ABB acquired Baldor, the switch was made to the ABB ACS800 VFD and the Baldor RPM AC motor, providing additional system performance and longevity.

When the vessel is on station or in a positioning mode, depending on wave height, current and wind, there can be quite a demand put on the thrusters depending on the position of the boat. The ability of the ABB ACS800 and the Baldor RPM AC motor to execute very quick and rapid accelerations and decelerations simplifies the positioning process.

“In the DP application, the VFD will typically be ramping from 1,000 rpm in one direction to 1,000 rpm in the other direction in less than 10 seconds - a full reversal in less than 10 seconds. You can’t do that consistently with a standard induction motor and a standard scaler type VFD, they just can’t take the abuse,” says Mike Mitchell, the ABB application manager who has worked with Master Boat on the electrical systems from the beginning.

The direct torque control (DTC) technology in the ABB drive, and the low inertia aspects of the AC motor, provide excellent control throughout the entire speed range, from 2 rpm to 1,800 rpm. They allow highly efficient acceleration, deceleration, and full reversals, reducing generator load and increasing the overall performance of the system.

The workboats will effectively hold in 10 kts of wind at the beam. In other words, it is possible to have the wind coming from the beam into the side of the vessel and it will hold. If the wind or current was coming from the bow, it could probably hold in hurricane force winds.

Bow and stern thrusters are also used for docking the boats and manoeuvring in tight spaces like rivers and canals where boats may be passing within three feet of each other. The demand on the drives is not as severe as in the DP system, but precise performance is still required. The thrusters essentially allow the boat to move sideways to negotiate tight spaces without the need for a tug or push boat.

Master Boat's new class of 220 foot vessels contain the fourth iteration of drive enabled design at 800 hp. This move increased the width of the drives beyond the tight space allotments of the boat, where every inch of space is a premium. Working together Master Boat, Gulf Coast and ABB created a custom design that reduced 30 inches of width on each drive cabinet, saving five feet of space with the two cabinets mounted side by side.

ABB provides a complete system including the drive and the motor, and then works with Gulf Coast to integrate them into the master DP control system, from initial engineering through to commissioning and start-up.

At of the close of 2015 there were at least 60 Master Boat built vessels floating around the Gulf of Mexico with Baldor/ABB drive systems powering the thrusters, the vast majority of them integrated by Gulf Coast Air & Hydraulics.

Gulf Coast sees challenges and opportunities ahead, and understands that the boats, along with their electronic motor and drive packages, will need to evolve.

“Our big challenge moving forward is that the boats are going to get bigger and the thrusters are going to get bigger. The drives will need to get bigger in power, and we will need to further custom-size them so they fit within the limited space in the boat,” predicts Chuck Moorehead, Gulf Coast president and co-founder.