Reimagining the shuttle tanker
Vessels ordered recently by the world’s largest shuttle tanker company, Teekay, will offer a new level of ecological and economical solutions, writes Stein Thorsager, business manager, Wärtsilä Gas Solutions.
In mid-2017, Teekay Offshore contracted for two newbuild vessels (and two options, taken up in November) of a new shuttle tanker design with Samsung Heavy Industries in South Korea. The new ship concept emerged from a partnership between Wärtsilä and Teekay.
The concept allows tankers to operate both on liquefied natural gas (LNG), as the primary fuel, and volatile organic compounds (VOC) – the environmentally harmful gas evaporating from the oil cargo tanks – as secondary fuel. The solution aims to reduce annual emissions of CO2 equivalents by up to 42% compared to a conventional shuttle tanker- equal to the emissions from 22,000 cars per vessel per year.
The main task for a shuttle tanker is to transport oil from offshore fields to land-based oil terminals. As shuttle tankers operate in offshore conditions, these vessels are the most advanced merchant vessels with unique varieties in operation modes including loading in dynamic positioning (DP) condition, transit in laden, unloading and ballast conditions.
All of these operating modes have their own requirements, but traditionally these requirements have led to equipment installed on board not being used efficiently in the various operation modes.
The new shuttle tanker will have three advantages over a conventional shuttle tanker:
Reduced emissions: VOC emissions from the cargo will be eliminated, NOx from the engine exhaust will be reduced by 84% (to well below IMO Tier III levels), SOx emissions will be practically eliminated, and particulate matter will be reduced by more than 96%.
Operation flexibilities: An efficient use of the installed machinery and propulsion systems in the vessel secures an unmatched manoeuvring capability, while the built-in system redundancies ensure an inherent system robustness when managing unexpected events.
Improved economics: A 22% reduction in total fuel consumption and, with the use of VOC as fuel, a considerable reduction in bunkering needs, combined with fewer running hours hence lower maintenance costs for machinery.
During operation, conventional shuttle tanker designs feature high levels of emissions, mainly from the emissions of VOC to the atmosphere during loading, storage, and transportation of crude oil. For instance, the storage and loading of crude oil onto ships is responsible for more than 50% of Norway’s VOC emissions. The Norwegian authorities have thus implemented stringent emission reduction regulations for all shuttle tankers loading crude oil from offshore processing platforms located in the Norwegian continental shelf.
The loading operations take place when crude oil is loaded to the shuttle tanker directly via a loading hose connected to a floating production and storage offshore (FPSO) unit, fixed platform or via a loading buoy. This operation can take place in harsh environments and with difficult sea states. The crude oil needs to be loaded quickly to ensure the DP operation can be finished before weather changes.
During the offshore crude oil loading, the VOCs are emitted from the crude oil cargo tanks and must be captured by a VOC recovery plant, avoiding harmful emissions to the atmosphere. Wärtsilä has long experience in developing VOC recovery plants that can satisfy the requirements from the Norwegian authorities. With these experiences, Wärtsilä has now designed a new generation of VOC recovery plant that will satisfy also the more stringent authority requirements expected from 2030.
The Wärtsilä VOC recovery plant uses compression and cooling phases to liquefy the heavier hydrocarbons to liquid VOC (LVOC) that is stored in a tank on the deck of the vessel. The lighter hydrocarbons that are not liquefied, referred to as surplus VOC (SVOC) - which mainly consists of methane gas - will be burnt in a gas turbine for electricity generation. A gas turbine is two times more efficient than the boiler with steam generator traditionally used for this purpose.
From a typical North Sea platform installation, each crude oil loading of an 850,000 barrel cargo will recover 100 tonnes of LVOC and 10 tonnes of SVOC. Assuming 32 loadings each year, stopping these VOCs escaping into the air means that annual CO2 emissions are reduced by 42%, from 43,000 to 25,000 tonnes.
VOC AS FUEL
Today’s shuttle tankers are usually equipped with direct propelled two-stroke diesel engines running on heavy fuel oil or marine gas oil. Recovered LVOC has so far been considered as a waste product. The two-stroke engines are mainly used for propulsion during transit, and the four-stroke auxiliary plant is providing power for the extensive thruster system used during DP operation.
The average of 100 tonnes of LVOC recovered at each loading could represent up to 30% of the total fuel consumption of the shuttle tanker, if we could use it as fuel. Teekay has therefore replaced the conventional two- and four-stroke engine configuration with fully electric propulsion with generator sets driven by Wärtsilä four-stroke, dual-fuel (DF) engines.
With electric main propulsion motors and DF generating sets as the only power plant onboard the vessel, flexibility and overlapping functionality is achieved. This power distribution concept is part of Wärtsilä’s Low Loss Hybrid configuration, reducing the total installed power on board from 26 to 23MW. This leads to further reductions in fuel consumption and an increased overall efficiency of the vessel.
To achieve full SECA and NECA compliance, the new shuttle tanker will be further equipped with a Wärtsilä LNGPac fuel gas handling system to enable operation in gas mode with LNG as primary fuel for the engine.
With both LNG and LVOC onboard, Wärtsilä started to develop and performed testing of the possibility of mixing LNG with LVOC in gas form for potential valuable fuel for the engine. LVOC comprises heavier gas hydrocarbons such as propane and butane that have a relatively low methane number (MN) of 25, making them less suitable as fuel for gas engines. By mixing the LVOC with LNG with an MN between 70 and 90, we achieved an acceptable MN for the gas engine at any required power. Depending on the MN of LNG, the engines will run at a maximum continuous rating (MCR) of less than 100%. This de-rating mode is acceptable for Teekay due to the operational flexibility built into the new concept.
Using LNG as the primary fuel and LVOC as a secondary fuel, the tanker enables the utilisation of 100% of the recovered LVOC as fuel for electric power generation. The power distribution system will include batteries for further fuel savings, peak shaving and added overall system redundancy.
Wärtsilä’s Low Loss Hybrid concept – an integrated system featuring energy storage and power management – also minimises the impact of a failure during DP operation. Along with the required trial speed, the vessel’s power requirement in DP condition determines the size and functionality of the power plant. While a traditional electrical distribution system could lose more than 50% of installed power and several thrusters in the event of a DP failure, the Low Loss Hybrid system will lose only 25% of the installed power and not more than one thruster.
The increased efficiency also improves DP manoeuvring capability. A conventional system would use 60% of thruster power, whereas in the new tankers only 40% will be used, giving much better margin for manoeuvring.
The required total mechanical installed power will be reduced by 14% compared to conventional tankers, improving overall fuel performance compared with traditional power distribution concepts. As a result, total energy consumption on the new tankers will go from 110GWh to 75GWh per year.
The electric equipment room will also be more compact compared to a conventional electric room. The power distribution system uses a low voltage arrangement. Combined with the reduced installed power, this makes operations simpler for the crew onboard.
The hybrid energy system – and in particular the energy stored in the batteries - will enable the engines to operate at a load where fuel consumption is optimal. The batteries will handle the dynamic load variations and the engines will get a stable load. Engines can therefore operate in a higher load area without the risk of having to start additional generators due to transient load variations. The tanker is the first ship of this size to use batteries to improve efficiency during transit operations.
For crude oil offloading operations at the onshore terminals, Wärtsilä will supply either electric-driven pumps for pump room installation, or electric-driven deep well cargo and ballast pumps that enable a distributed pump solution, eliminating the need for a separate pump room and interconnecting pipelines in the cargo holds.
The new concept represents a game-changer for the shuttle tanker sector. The concept enables shuttle tankers to travel from the oil fields on its own waste gasses rather than releasing them into the atmosphere. Both emissions and bunkering requirements will be considerably reduced and the chosen propulsion solution will increase overall efficiency compared with conventional systems. The overall running hours will also be lower, which will mean lower maintenance costs for machinery.
We estimate that half of all offshore oil production in Norway and the UK will be transported to land using shuttle tankers. About 40% of the existing tanker fleet is ready for replacement due to the 20-year age limitation for new shuttle tankers.
We see an exciting time ahead for similar contracts in the near future for this ecological and economical shuttle tanker concept developed in cooperation between Teekay and Wärtsilä.
This article was originally published in Wärtsilä’s stakeholder magazine and website 'In Detail'.
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