Have it all

It’s common practice in the offshore and harbor tug industries but often a vessel’s power and propulsion systems are determined by just a couple of numbers: the bollard pull and the sea trial speed. Unfortunately that only takes into account a small fraction of the vessel’s operational life – most of the time you’re not pulling maximum loads or hitting top speed.

ALWAYS IN SUPPORT

This was the challenge faced by Seaways International when selecting the right equipment for the Seaways 24, an infield support vessel, spending extensive periods of time in DP or standby while serving their charter customer in the oil fields off the coast of Angola. The challenge was also that the charter requirements had changed from the original 100T bollard pull to an increased 120T without changing the already selected main MaK 9M25C engines. Our Caterpillar Marine Solutions Center came up with a solution using our MTA834 CP azimuth thrusters, together with the main engines boosted by a pair of electric motors mounted directly to the upper gears of the thrusters.

This setup is often referred to in the marine industry as a hybrid setup since the thruster has the possibility to run mechanically driven by the main engine, electrically for low load and standby by the electric motor, or driven by both together in a booster mode. Jonas Nyberg, regional sales manager for South East Asia and Oceania, is proud of how the Caterpillar team took on the task and delivered in style: “We were challenged to find a technical solution to this problem and through diligent work and some creative ideas we came up with what we finally offered to this customer."

INNOVATIVE SYSTEM, TRUSTED COMPONENTS

The new propulsion set-up outperforms diesel mechanical systems in all partial load conditions, resulting in improved fuel economy and major through-life savings. For OSV vessels spending a high amount of time in standby or DP service, the annual fuel savings can be up to 35%! “The efficiency gains are remarkable. We know OSVs with DP capability spend a considerable amount of time in standby or in various levels of DP. With this new system, they’ll be able to use the diesel electric mode and run off the smaller gensets with the propellers operating economically at a very low rpm.” Explains Jonas Granath, Manager Electrical Design at the Caterpillar Marine Solution Center. During the sea trials, the system performed well and achieved target speed and DP performance in diesel electric mode, as well as the bollard pull in the booster mode. Making this the largest diesel mechanic electric hybrid thruster installed in an OSV to date.

EASING THE FINANCIAL BURDEN

Along with the fuel savings, there’s clearly the benefit of helping operators cut down on emissions and noise pollution. What’s more, maintenance and replacement intervals are extended at these more efficient loads. And the best part, the whole system is completely integrated in the already existing MPC800A controls making it a very competitive system that pays for itself in the first years of efficient operation.

THE CAT® MARINE HYBRID THRUSTER

 

– A SYSTEM, AN APPROACH, OR BOTH?

The answer is both. The Caterpillar Marine Solution Center took a holistic approach to the challenge set by the customer. The first step of the process of coming up with the best solution was running a Cat® Select equipment benchmark together with the customer to analyze, discuss, and determine total cost of ownership for different propulsion concepts, covering fuel consumption, maintenance consumption, load response, and equipment finance cost. The different options (diesel mechanic, diesel electric, and hybrid) were compared against a detailed operational profile including the percentage of time under different working conditions– and we soon had a clear winner with the hybrid option.

POWER AND CONTROL

For example, a standard 7000 hp standby support vessel featuring conventional power-ing would require 2x MaK™ 8 M 25 C main engines at 2666kW @ 750rpm and a pair of cat C18 gensets. With the new Cat hybrid thruster system, the same vessel could instead use smaller 6 M 25 C main engines at 2000kW @ 750rpm, and two booster motors mounted on the back of the Cat MTA 730 CP azimuth thrusters powered by twin Cat C32 generator sets. The booster motors and drives are controlled directly from the Caterpillar MPC 800A control system, which also performs all mode selections, interfaces with the vessel’s PMS, and provides a single-point interface for the operator.

SPREADING THE LOAD AND SAVING FUEL

In transit operations and in diesel-mechanical mode, the smaller engines run at higher loads, consuming less fuel. In low-speed transit, either one or two gensets can power themain azimuth thruster in diesel-electric mode giving typical fuel savings of 10-15% at speeds in the 7-9 knots range, depending on hull profile. However, the biggest savings are made during standby and DP operations where the vessel would operate in diesel electric mode with the main azimuth propellers running in the most efficient variable speed mode.

WHAT'S GOOD:

- Significant fuel savings
- Longer service interval
- Lower harmful emissions
- Noise reduction
- ROI in as little as five years