The Ulstein hybrid propulsion concept

by Hans | November 11th, 2009

Norway’s Ulstein Group has designed and built two new anchor handling and offshore support vessels with a new breed of hybrid propulsion system for shipowner Olympic Shipping. The new “Ulstein hybrid propulsion concept” has taken the potential of the rather complex hybrid propulsion concept to completely new levels of performance. The “Ulstein hybrid propulsion concept” combines the best properties of both DE and DM into one system. This includes DM attributes of higher efficiency and better performance at high loads, along with the DE trait of excellent fuel consumption at low loads. Hybrid propulsion is therefore the perfect match for the operation profile of anchor handling vessels like the ‘Olympic Zeus’ and the ‘Olympic Hera’.  In fact, combining the medium-sized DM and DE systems into one large hybrid propulsion system proves to both take up less space and be less expensive than a large DE system.

This article will provide a brief operational profile of the Ulstein hybrid propulsion concept.

 The challenge

In a diesel-mechanical (DM) anchor handling vessel, there is a classical propeller, engine and hull setup. As the design criteria for propellers, engines and hull are optimised for zero speed bollard pull, this normally results in less than desirable efficiency in most other operation modes. Issues often related to this powerful type of vessel include high fuel consumption and pollution.  Ship designers have been working on these challenges for many years. The diesel-electric solutions for anchor handling vessels have been turned down because of excessive costs and large installations not suitable for compact ship designs. In recent years, the common availability and reduced prices on power electronics have opened alternative and competitive roads to solve the challenge. Now ship designers have designed a host of solutions for anchor handling vessel propulsion systems. These range from constant rpm diesel-mechanical propulsion systems to diesel- electric (DE) propulsion systems to hybrid diesel-electric/diesel-mechanical propulsion systems. There are numerous of solutions within each of these three propulsion concepts.     

The DM system seems to be perfect, so what’s the problem?

Dimensioning criteria:

An anchor handling vessel’s selling point is its maximum bollard pull performance, which is witnessed and certified by the class societies. Basically, everything is done by ship designers in order to optimise ship performance for this single test. Maximum bollard pull performance is the main dimensioning criteria for an anchor handling vessel.  

High efficiency:

Under ideal conditions, the diesel-mechanical propulsion system is very efficient. It may have as little as 2.5–3 percent mechanical loss between the diesel engine and main propeller, mainly in the reduction gear. Maximum bollard pull performance utilises maximum prime mover power in the water. An anchor handling vessel with a diesel-mechanical propulsion system also performs very well at the higher end of its power range. In comparison, DE propulsion systems typically have a ten to eleven percent loss between the diesel engine and the shaft of the electrical propulsion motor.   

Waiting and idling:

The nature of an anchor handling job may vary a great deal. Jobs may often include quite a lot of waiting and idle time. These large and powerful anchor handling vessels then have to reduce engine power to a fraction until the next work sequence is started. The potentially large amount of waiting time introduces a number of challenges for anchor handling vessels. During waiting time, there are several factors that contribute to increased fuel consumption, more pollution and more wear and tear than desired.

Zero pitch loss and engines on too low load:

Main propellers running at maximum rpm even at idle, cause considerable zero pitch loss – up to as much as 1,000kW per main propeller on a large anchor handling vessel. Large engines that run very inefficiently while idling cause “carbon buildup” and emit relatively large amounts of polluting particles and gases.

The operation profile for an anchor handling vessel

Ship designers are always eager to know how the ship is going to be used. They study the operation profile to identify the various operating conditions and their duration. This study is also helps in the calculation of the fuel consumption of a ship throughout the course of a year. As the operation profiles may vary a great deal, it is also important to identify the nature of each mode – including the variation of power requirements and duration of sub-activities within the mode.   

Diesel engines and Specific Fuel Oil Consumption (SFOC)

Most diesel engines seem to be most fuel efficient at approximately 85 percent of full load. After collecting fuel consumption data from engine maker data sheets, similarities among a selection of makes and models are revealed:  

  • Fuel economy is at its best between 65 and 90 percent engine load
  • At loads of 50 percent and below, the specific fuel oil consumption increases rapidly
  • At low engine loads, combustion is weak and produces more particles and pollution
  • At lower engine loads catalysers are radically less efficient
  • Low engine loads over long periods of time cause carbon build-up

Key fuel economy elements for anchor handling vessels

Efficient hull

  • Low hull resistance at relevant speeds and drafts
  • Low hull resistance in transit in inclement weather
  • Hull shape optimised for bollard pull with large propellers
  • Good balance between speed and bollard pull

Efficient propulsion system

  • Efficient propeller design for relevant speeds and drafts
  • Efficient propeller design for bollard pull
  • Good balance between speed and bollard pull properties
  • Variable pitch propellers reduce bollard pull/speed compromise
  • Variable propeller rpm reduces “zero pitch losses” when idling
  • Low losses in the power train
  • Low power consumption in idle conditions

Efficient engine configuration

  • Fuel efficiency
  • Good dynamics (handles sudden load variations)
  • Wide range variable rpm/pitch combinator curves
  • Larger number of smaller engines for DE applications for optimum engine loads, leading to fewer engine running hours and less wear and tear

Operation profile

  • Carry out “operation profile” study together with shipowner
  • Identify modes that are dimensioning criteria for ship systems
  • Identify modes that have influence on fuel consumption throughout the year
  • Identify modes that contribute to running hours and wear and tear
  • Identify typical load conditions within each mode

For further information contact:

Ulstein Group, Norway
Email: gr@ulsteingroup.com This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Web: www.ulsteingroup.com 

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