TEEKAY and Wärtsilä have unveiled designs for a new ultra-high efficiency Shuttle Tanker capable of burning waste compounds from crude cargoes as an engine fuel
With regulations tightening on vessel pollution, efforts have been piling into technologies which can help to reduce emissions. In addition to powering ships using LNG and reducing fuel consumption through hybrid battery technologies, other efficiency gains are being made to squeeze additional fuel from resources that would otherwise be wasted.
One such technology forms the centrepiece of a new Shuttle Tanker design concept from marine engineering group Wärtsilä and ship operator TEEKAY. In addition to running on its primary fuel supply of LNG, the group has designed a system to capture the volatile organic compounds (VOCs) which escape from the crude as the tanker loads cargo. As this VOC mix is defined as a clean product, similar to a LPG mix, it can be used as subsidiary fuel to power the vessel.
Currently, recovered VOCs are largely considered a waste product and mixed back in with the cargo or discharged at port. According to its designers, this capture system enables tankers to almost return to shore powered by this secondary fuel mixed with LNG - reducing the total bunker fuel required by up to 30%
Wärtsilä senior technical adviser for gas solutions Knut Brødreskift explained to InnovOil that the technology had its roots in an ambitious Statoil project initiated in 2000 to reduce ship emissions from tankers working between the North Sea and Rotterdam. A potential application for the technology resurfaced around two years ago, when TEEKAY sought to develop its next class of shuttle tanker, in preparation for older vessels retired under Norway’s ever-stricter operation and environmental regulations.
The typical operating profile of a shuttle tanker includes periods of loading and offloading (requiring accurate dynamic positioning [DP] adjustment), as well as transit and ballast adjustments, all of which place different demands on vessel equipment. That has led to inefficiencies in the way this equipment is used. Most shuttle tankers are equipped with direct propelled 2-stroke diesel engines, which run on heavy fuel oil (HFO) or marine gas oil (MGO). These are mainly used for propulsion during transit, with a 4-stroke auxiliary plant providing power for the thruster system used during the dynamic positioning operation.
With lower emissions as its goal, TEEKAY had already opted to use LNG as the primary vessel fuel, but replaced the 2-stroke configuration with full electric propulsion, based on four generator sets driven by Wärtsilä 4-stroke dual-fuel (DF) engines. With these engines as the only power plant on board, total installed power capacity has been reduced from roughly 26 MW to 23 MW, meaning increased system efficiency and corresponding reduced fuel consumption.
During fuel loading, Wärtsilä’s VOC recovery plant uses compression and cooling phases to reduce the compounds to -45°C, liquefying the heavier hydrocarbons to so-called liquid VOC (LVOC). This LVOC is then stored under pressure of about 8 bar in ambient conditions in a storage tank on the deck of the vessel.
LVOC contains gaseous hydrocarbons such as propane and butane with a low methane number (MN) of 25 and hence is not suitable to be burnt directly in the gas engines. However, mixing the LVOC with LNG (with an MN of between 70 and 90) ensures a satisfactory MN for the gas engine to be viable at any required power.
The lightest hydrocarbons such as methane that escape to the recovery plant and are stored as and surplus VOC (SVOC) in gaseous form, and are treated as fuel for a smaller gas turbine generator set – power which itself is sufficient to operate the VOC plant. “It’s actually quite extreme emission conditions, which go up to 16,000 cubic metres per hour of gas stream[ing] out of the cargo tanks, so it’s quite a big short-term event in terms of these emissions,” Brødreskift added. This configuration therefore offers 100% hydrocarbon emission reductions.
Assuming the shuttle tanker services a typical North Sea platform, loading a crude oil cargo of around 850,000 barrels, Wärtsilä says its system will recover in average 100 tonnes of LVOC and 10 tonnes of SVOC. That 100 tonnes could represent up to 30% of the vessel’s total fuel consumption, and across an average of 32 trips per year, it says CO2 emissions can be reduced from 43,000 tonnes to 25,000 – a 42% reduction compared to more traditional shuttle tanker vessels.
At the time of writing the average price of intermediate fuel oil (IFO 380) in Rotterdam was US$370 per tonne, around 10% less than the global average. Based on a shuttle tanker’s annual consumption of around 9,000 tonnes, InnovOil estimates that a 30% reduction through using a VOC system could represent savings of US$1.1 million per year per vessel.
Power distribution is also made more efficient via a so-called Low Loss Hybrid system (LLH). Integrating the electrical system means there is less excess plant capacity required, and with four gen-sets reduces the impact of any plant failure.
Two 250-kWh battery packs installed on board are also able to adjust dynamic load variations, smoothing out peaks and enabling the vessel to work under higher load conditions without requiring additional generators. Wärtsilä says this vessel is the first ship of this size using batteries to improve efficiency during peak load operations.
“By having 4 dual-fuel gen-sets on board, and everything else electrically driven, you have a huge advantage in terms of managing your electricity with even better regularity. You can ensure that your DF gen-sets will run at optimal fuel consumption conditions at all times, with the battery taking the peak loads,” Wärtsilä Business Manager Stein Thorsager explained.
Using this system, the expected total energy consumption in the new Shuttle Tanker concept will go from 110 GWh to 75 GWh per year, compared with a traditional tanker.
So far, TEEKAY has four firm orders and two options for the new vessel design with Samsung in South Korea. But with estimates suggesting that at least half of all UK and Norwegian Continental Shelf production will be transported to land using shuttle tankers, there will continue to be a sizable market. Moreover, around 40% of this existing fleet is scheduled for replacement during this period, owing to a 20-year age limitation for new tankers.
At this stage Thorsager concedes that the additional VOC plant is “relatively high cost” but that the company is looking to reduce this substantially through equipment optimisation. Moreover, by the time that operating and emissions regulations come into force, he is confident that ship owners will be able to secure ROI through savings of 30% or more in fuel.
As emissions regulations do begin to inform ship contracting, the designers are confident that the VOC capture system will be able to satisfy even the most stringent authority requirements expected within 2030 – which could make this Shuttle Tanker a particularly sound investment indeed.