Emma Maersk in the Straits of Gibraltar, image (c) Wärtsilä
Wärtsilä hosted a webinar yesterday on the subject of “Performance Optimization for the Merchant Marine Market” and it was moderated by Atte Palomäki, Wärtsilä’s Group Vice President for Communications & Branding.
Speakers included:
A number of topics were discussed including the Energy Efficiency Design Index (EEDI), SEEMP, the Carbon War Room, Ship Vetting and Rightship, ship emissions, the commercial implications of slow steaming, and the technology implications of slow steaming.
We’ve published a number of articles in the past about these topics, however this presentation highlighted a point that I feel deserves a bit more explanation, and that is implications of slow steaming from a technical standpoint.
What is slow steaming?
It’s literally just that, ships are throttling back on their engines and driving slower, and they are doing this for a few reasons:
1) The slower a ship moves, the more time it spends on a voyage, and thus there are less ships available at any one time to transport goods. This helps boost demand in a market that has too much carrying capacity with respect to the global demand for goods. There is an imbalance in the supply/demand equation.
2) It is generally more efficient to drive a ship at a slow speed than it is to drive at a much higher speed and to explain why, it’s critical to understand the major components that make up the overall resistance profile of a ship, and those are skin friction and wave-making resistance. At slower speeds, a vessel creates much smaller waves and the friction of a ship moving through the water is the greatest component of resistance. At higher speeds, a large “wake” is developed which becomes an exponentially greater component of the overall resistance of a ship. Air and appendage resistance are lesser components of this equation.
Hull form optimization is used to balance these components to make a ship as efficient as possible, however in order to optimize a hull form, you need to know what you’re going to use that ship for, and a key element of the design “spiral” is, “what speed will this ship be sailed at?”
Back in 1998 in Dr. Bhattacharyya’s ship design class at the US Naval Academy, we were challenged with creating 4th degree polynomials that would describe a ship’s hull form. Not an easy thing to do, and I mention this to highlight my point that nothing about ship design or ship optimization is a linear problem.
If you change the speed profile of a ship, the hydrodynamics change.
So what changes… specifically
Starting at the bow, you’ll notice that most ships have a bulbous appendage that protrudes out from below the waterline. The bow bulb, as it’s called, was an idea crafted by Rear Admiral David Taylor, the “Chief Constructor” of the US Navy during the early 20th century, and is a design feature that helps reduce the wave-making resistance of a ship.
When a ship travels through the water, you’ll notice that in all cases there is a wave train that develops from the bow, and one that develops from the stern. Just like the sound waves that enter your noise-canceling headphones, these waves can be canceled as well, but only at a specific frequency.
The frequency and amplitude of the bow wave is completely dependent on the speed of that vessel, and the size and shape of the wave needed to cancel that bow wave requires a specifically-designed bow bulb.
That said, when you slow steam a ship, the bow bulb needs to be redesigned so that it works properly for that new speed profile. Efficiency gains of up to 8 percent could be realized by such modifications.
At the stern, you have even more options to increase your propulsion efficiency, as described by Fathom Shipping HERE, but instead of nozzles and various flow optimizers, let’s consider the propeller itself.
It too was designed for a certain speed, RPM, and hull form. A propeller is simply a foil spinning around on an axis. The shape and chord length of that foil works best at a certain angle of attack and at a certain speed. If the angle of attack is too great for the speed of the propeller, you’ll lose flow over the blades and start cavitating. Conversely, if it’s too shallow, you’re not maximizing its potential. I realize that’s a VERY simplified explanation, but at the end of the day, when you start slowing the speed of a vessel down from its design speed, you’re losing propulsion efficiency.
Paint and hull coatings
As I mentioned above, at slow speeds, hull friction becomes a much larger component of hull resistance, thus sleeker new paints have been developed to reduce this skin friction. From a naval architecture standpoint, the molecular speed of water directly adjacent the hull is zero and the speed of subsequent layers of water flowing past the hull increases as you move away from the hull. This is the “boundary layer” and due to the flow gradient as you move outward from the hull, the water is in shear and experiences frictional resistance.
The thinner the boundary layer, the lower the resistance. Barnacle and marine growth are major contributors to hull frictional drag, and due to slow steaming, and thus lower fluid speeds across the hull, coatings that were designed for higher speeds are less effective at shedding such growth and specially-designed paint is needed, and/or more frequent drydockings.
The Ship’s Engine
Just like a ship’s hull form, the engine itself is spec’d out for a certain operating profile. Maersk’s E-class, for example, carries a massive 12 cylinder, 2-stroke diesel engine, but due to the incorporation of slow steaming by Maersk, it no longer needs that much power.
Many other ships face the same issue… their engines are oversized for their operational profile and thus, they are not operated within the most efficient power range. To mitigate this issue, some owners are de-rating their engines by “blanking out” or taking up to half of their cylinders offline so that they can operate the remaining cylinders within a more efficient power range. Other options owners are choosing, as noted by Jasper Boessenkool, Head of Strategic R&D at Maersk Maritime Technology, include reducing the bore size of the engines, updating the nozzles and injectors, or cutting out the turbocharger.
Such updates, although radical in some cases, could result in efficiency gains by up to 15 percent.
At slower engine speeds, another issue occurs… engines don’t run as hot. When engines don’t run as hot or at higher RPMs, different kinds of engine lubricants are needed to deal with issues such as increased acid in the exhaust gases and changes in viscosity. Greater wear will occur to an engine’s cylinders if the incorrect lubricant is used.
Another option is LNG
At the end of the day, all these changes and ideas serve two purposes, and that is to reduce the emissions of greenhouse gases while at the same time making the business of shipping more profitable by lowering the massive overhead of fuel bills.
The widespread use of LNG can do both and many shipowners are excited about it.
As Mr. Boessenkool was describing the de-rating of an engine via blanking out cylinders, I thought to myself, “Why not covert this engine to LNG power and instead of burning HFO, use less powerful LNG to effectively de-rate the engine.”
To get the answer, I spoke with John Hatley, Vice President of Ship’s Power at Wärtsilä North America:
Mr. Hatley commented, “Let’s see how the selection of fuel, traditional or LNG, may alter the engine output performance if “all other things are equal.” Take the 32 series 6 cylinder generator engine rated 3,000kW at 750 rpm on traditional fuel. The alternative 34DF series 6 cylinder generator engine has a rating of 2,700kW at 750 rpm on natural gas.
Why is a larger 34 bore utilized for the gas engine instead of 32 as for the traditional?
Answer, to regain a portion of the otherwise reduced gas engine output within the same engine footprint.
Therefore as you mentioned in theory, an existing engine of otherwise unchanged rpm, bore, stroke; will find a reduction in power output when moving from traditional to gas fuel all other things equal. But in real life, not all other things are equal with advanced automation controls for example, and that is important to note.
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