AC power station aids tanker maneuvers in Haltenbanken

Oil offload operations should begin shortly on Heidrun. Heidrun's three shuttle tankers have been specially designed to cope with the field's production requirements. Due to the low rate of oil output, the offloading vessels need to be able to maneuver into position close to the TLP and remain there for long periods in the choppy waters of the Haltenbanken.

ABB package ensures stable offloading of crude,
even in five-meter significant waves

Heidrun's three shuttle tankers have been specially designed to cope with the field's production requirements. Due to the low rate of oil output, the offloading vessels need to be able to maneuver into position close to the TLP and remain there for long periods in the choppy waters of the Haltenbanken.

Another requirement was the ability to discharge cargo quickly at the onshore terminal using electrically-driven pumps. The consequent need for adequate and flexible electrical power supply led to the choice of a diesel electric AC power station concept powering DP thrusters, as well as cargo pumps, and providing main propulsion via a cycloconverter drive. ABB Industri in Norway and Finland has provided much of this equipment.

Knutsen, which commissioned two of the field's newbuild tankers, had previously stuck with slow-speed diesel engines and controllable pitch propellers. But it recognized that using four medium-speed diesel engines would suit Heidrun's power requirements better, and that the finite control characteristics of the ABB AC cycloconverter drive would eliminate the need for a controllable pitch propeller in the main drive train, as well as improving dynamic performance.

Model tests at MARIN in The Netherlands established the optimum propeller and thruster configuration for DP-aided station keeping in significant waves up to 5 metres. The solution was five 1,700kW thrusters located at the bow and stern, operating with an 8m dia. fixed pitch propeller driving across a Becker high lift rudder.

The thrusters also had to cope with submerged turret loading in 6m waves and 20m/s winds. An output ratio of 0.15kN/kW was selected to give maximum thrust of 1,275kN. Adding in the main propeller needs pushed the total power requirement to 22,000kW.

Taking into account redundancy requirements, this has been achieved using four medium-speed 6.6MW Sulzer diesel engines, each of which drives an ABB generator rated at 6.275MW and outputting 6.6kV and 60Hz using a flexible coupling. This power is accessed by the main ABB propulsion electric motor (19MW at 98rpm) and the five thrusters.

Hull considerations

The two new Knutsen tankers, built by Astilleros Espanoles, are a modified version of an earlier hull form developed by the Spanish shipyard. Changes for Heidrun included lengthening the parallel midbody, adding a third bow thruster and a second stern thruster.

Initial trials at MARIN suggested that the three bow thruster tunnels were hampering efforts to increase power needed to meet the contact transit speed. This led to tests of two different bow line configurations, and an improved variation.

It was assumed for these trials that maximum power available from the ABB drive motor was 17.1MW at 90rpm, but discussions between Knutsen and ABB suggested the increase to 19MW at 98rpm was feasible. The decision was based on the motor's over-speed margin of 11% being able to run up to 103rpm.

Redundancy is also provided by the motor's double armature arrangement: two independent motor windings mounted in tandem on the same common shaft. If one fails, the other can supply 9.5MW and low enough rpm (76) to meet propulsion needs.

Another plus of the power station concept is the redundancy in power supply provided by the four generators, ensuring safe operation of the vessel should any major component fail. Also for any single operation such as traveling, DP or unloading, only the necessary number of generators will be assigned, which should cut fuel consumption and operating costs.

Most of the time, the vessels are expected to operate in DP mode for loading purposes. Even then, power requirements will be affected by prevailing weather. During unloading, the same power supply will be used to drive the four cargo pumps: this means exhaust emissions are likely to be reduced.

ABB has also supplied the propulsion motors and generators for the other Heidrun tanker, built by Samsung in Korea. Now the company is winning work packages also for floating production systems.

Statoil's Norne Field oil production and storage vessel will feature five ABB SAMI Megastar PWM thruster drives each rated at 2.8MW. And for Smedvig's speculative newbuild the SPU 380, under construction at FELS in Singapore, ABB is delivering all the electrical, communication and positioning systems.

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