2015_2 Offshore performance permutations master

Offshore performance permutations

Engine offerings for the new Wärtsilä AHTS (Anchor Handling Tug Supply) vessels ensure economy and adaptability.

Text: NICO HÖGLUND Photo:

The main design criteria for the propulsion system of the new WSD 46-Series from Wärtsilä Ship Design were: high but flexible power for versatile vessel operation combined with market-leading fuel consumption.

One enabler of these characteristics is the Wärtsilä two-speed reduction gear for use with medium-speed engines. Another point to the very source of fuel efficiency – engine power.

According to their 150, 180 and the 220 ton bollard pull (tbp) capacities, the WSD 46-Series anchor handlers feature mechanical/electric hybrid propulsion and onboard electrical power systems based on 8-cylinder Wärtsilä 31 in V-configuration, 6-cylinder version of Wärtsilä 32 and Wärtsilä 26 diesel engines. The engines burn distillate fuel and are arranged in pairs of main and auxiliary engines with equal cylinder counts. Via motor/generators on the PTI/PTO shafts of the two-speed gears, the generator sets can provide propulsion power boost, or power can be generated from the main engines when the anchor handlers are steaming between destinations. In the boost mode, the vessels attain, on a hybrid mechanical-electric basis, the same flexibility as a “father-and-son” mechanical twin main engine propulsion system.

The idea was to design propulsion systems for the WSD 46-Series that are both modular and scalable.

Taking scalable first, this means that whichever of the three nominal bollard pull ratings the vessel has, its major arrangement remains unchanged, instead the main and auxiliary engine configurations are changed. And, in terms of modularity, the system is modular in the sense that it allows several different machinery concepts, so that it could also be readily executed as either fully electric or fully mechanical drive concepts.

Illustrating the adaptability of the basic concept, the 220 tbp AHTS vessel is offered with two 8-cylinder Wärtsilä 31 state-of-the-art engines, recently also acknowledged by Guinness World Records as being the world’s most efficient 4-stroke diesel engine. On the intermediate 180 tbp size, there is a choice of 6-cylinder Wärtsilä 32 engines or 9-cylinder Wärtsilä 26 main engines. In both cases, the auxiliary engines remain the same, i.e., 2 x 8-cylinder in-line Wärtsilä 26 on the 180 tbp and 2 x 6-cylinder in-line Wärtsilä 32 on the 220 tbp version.

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Typical hybrid mechanical-electric engine configurations and engine permutations on the WSD 46-Series anchor handler range.
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Fuel consumption graph of the Wärtsilä 31 engine in steaming (transit) mode.

Bollard pull power

For the 180 tpb size, there are two variants which are capable of meeting the nominal bollard pull with a varying number of cylinders, according to the needs of the anchor handler’s operating profile.

This benefit of close adaptability to a given load profile is primarily enabled by Wärtsilä’s “tug rating” concept.

 Market research shows that the maximum bollard pull mode of an AHTS vessel is around 5% of the operating profile and sometimes much less. Thus, for those customers requiring lower operating times at maximum bollard pull, we have developed the tug rating. It states that the twin main engines on the WSD 46-Series vessels can be operated at up to 10% above their normal maximum rated output for 30 minutes out of every 12 hours. On the other hand the “tug rating” can also be used as a compliment to reach an additional max-plus bollard pull for a shorter period of time, which can give the operator further peace of mind knowing there is additional back-up power available.

A schematic of the 180 btp AHTS in its max 180 ton bollard pull mode illustrates the benefit of the tug rating option. With both the 9-cylinder in-line Wärtsilä 26 engines at 108% load, they are boosted via the PTO motor/generator on the 2-speed gear with input of around 1.9 MW to each CP propeller – i.e., with gen-sets running at 90% load while also feeding other onboard consumers.

Looking at the benefit of the tug rating option, maximum rated bollard pull is also available but, as per the tug rating specification, for a considerably shorter time. Therefore, the owner saves fuel by using an engine with fewer cylinders that can easily cope with the anchor handler’s other operating modes and saves on maintenance since, as is well known, servicing on medium-speed engines is primarily determined by the number of cylinders to be maintained.

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Tug rating operating field of the WSD 46-series AHTS vessels.
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The 180 tbp AHTS variant with 9-cylinder in-line Wärtsilä 26 main engines in maximum bollard pull mode.

Fuel efficiency

The tug rating benefit for the smaller engines reflects the fact that operating modes other than max bollard pull represent 95% of the typical operating modes of the WSD 46-Series anchor handlers. Accordingly, the engines of the vessels, and their sophisticated control systems, have been chosen and tuned to achieve optimised fuel economy in every operating mode. Assisted by the two 2-speed gear concept, this is primarily achieved by automatically maximising the time the engines consume in their most economical operating range.

Transit mode

Anchor handlers consume most of their fuel in transit between harbours and offshore installations, meaning that favourable fuel efficiency with the engine at its optimal loading has the biggest impact on operating costs.

The graph of fuel consumption versus load for the Wärtsilä 31 engine shows that it is more fuel efficient than its 32 cm bore competitors under all load conditions.Building on this advantage, the propulsion control systems also include a sophisticated load sharing system designed to keep the engines in operation, at any given time, within their optimum operating ranges for fuel economy and low emissions.

The effect of maximising the engines’ time spent in economical, load operating ranges becomes clear when looking at the time spent in dynamic positioning when engines are running at part load. The Wärtsilä 32 engine’s fuel consumption versus engine load is best in 32 cm bore class also considering part load range where anchor handlers spend most of their operating time and therefore significant fuel savings can be achieved.

Dynamic positioning

The third major AHTS mode is dynamic positioning. This involves using the propulsion system to hold the vessel on a precise station, sometimes in strong winds, waves and currents, so that anchor handling and other functions can be executed close to a drilling rig or production platform.

For this function, the WSD 46-Series AHTS vessels use fore and aft tunnel thrusters powered from the generating sets. During dynamic positioning, which can be accompanied by loads from other electrical consumers such as the winches, it is important that the engines have good response to load changes since load on the thrusters varies considerably from second to second.

On the Wärtsilä 26 engines, Wärtsilä has tested and verified that they can cope with a 43% load step in two seconds, and the system is thus capable of meeting the needs of the 180 tbp vessel size in dynamic positioning mode. Similarly, a gradual power increase from 0 to 100% load can be executed in well below 20 seconds. In addition, the Wärtsilä 26 powered generating sets also have excellent black-out recovery. This capability is designed to cover a situation where we have only one generator set in operation, and it cuts out. If this occurs, the second gen-set can be on standby and take over the load within less than 10 seconds, giving a significant additional safety benefit to the whole vessel.

Environmental friendliness

The AHTS vessels are subject to emissions regulations, which set limits on oxides of sulphur (SOx) and oxides of nitrogen (NOx), with especially strict limits when vessels are in designated Emission Control Areas (ECAs).

These are mainly coastal waters near centres of population or areas of environmental sensitivity.

AHTS vessels usually use low sulphur distillate fuels, which mean they have no problems with SOX emissions. In terms of NOX, all Wärtsilä engines are tuned to work with our SCR aftertreatment units, and this is the form of NOx reduction we will use on the AHTS vessels to comply with IMO Tier III in ECAs. Wärtsilä engines used on the AHTS applications will have IMO Tier III certificates according to requirements.

Future conditional

What future holds for AHTS ships is hard to predict, but it is clear that the business environment is changing. It is becoming more and more competitive and challenging, fuel prices are much more volatile than in the past, and emission requirements are getting progressively stricter.

These factors are drivers for greater efficiency, flexibility, and low emissions, and Wärtsilä is developing its products not only to meet these needs. The new Wärtsilä 31 with 610 kW per cylinder is a perfect solution for anchor handlers.

Where fuel efficiency, operational flexibility, high power density, long intervals between overhauls, and high levels of safety are of paramount importance.

 The aspiration is to be the best, and, with the help of efficient technologies, Wärtsilä intends to remain the most valued business partner in the market.

 

AHTS

Ship Design 

Press release

More information: nico.hoglund@wartsila.com

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