Shuttle tankers have been in service in the North Sea since the 1970s. Their design and deployments have evolved and are being increasingly guided by economics, stringent emissions legislation, and the promise of electrification
2018 was a seminal year for shuttle tankers, with 10 contracts placed on the orderbook, noted DNV business director tankers, Catrine Vestereng, who gave the keynote presentation of Riviera Maritime Media’s Shuttle Tanker Tech & Ops webinar day: North Sea shuttle tanker operations: economics, emissions and electrification. Norwegian classification society DNV was the premier partner of the event, with sponsorship by technology group Wärtsilä.
The orderbook translated into 14 shuttle tanker deliveries in 2020, mainly for operations in the North Sea and Brazil, regions that have very different requirements. The North Sea shuttle tankers have a high level of environmental technology compared to those for the Brazilian sector, but that is likely to change, said Ms Vestereng.
That is not to say shuttle tanker technology is standing still. “We (DNV) are now exploring, with several owners, remote witnessing of the DP operation to increase the efficiency and quality of DP testing utilising the data gathered,” she said.
The North Sea is particularly challenging for shuttle tanker operators. In a poll on the challenges of operating shuttle tankers in the North Sea, 26% listed compliance with stricter environmental regulations, 16% pointed to crew competence in operating increasingly complex ships, and 19% listed cost reductions while maintaining safety. However, 39% of those surveyed chose all of the above.
Wärtsilä senior integration expert Kjell Storelid was involved in developing the Altera (formerly Teekay) shuttle tankers for the North Sea. The sixth of the Altera shuttle tankers is now on the way from the shipyard in South Korea, but the process started in 2015. “There were three key criteria,” said Mr Storelid, “The energy efficiency of the ship should be optimised, it should set the new standard for emissions reductions, and it should be safe, for the crew, for all the equipment, and for the installations. And it should also safeguard the investment from future changes in regulations and emissions restrictions.”
The starting premise for the Altera North Sea shuttle design was to examine the problems the vessels were meant to solve. “Because a shuttle tanker is a unique vessel, it should be an efficient oil tanker, but at the same time, have the capabilities of an advanced offshore vessel when it comes to station keeping and so on,” said Mr Storelid.
Models of every different mode of operation, from station keeping, loading, offloading, ballasting and so on, were built and scrutinised. These operational mode models were the basis for examining the different concepts put forward as solutions. An important element was dealing with the second source of emissions, the cargo (the primary source being the propulsion system). “From each cargo loading, there is an average of 100 tonnes of oil loss and one of the key basic elements for going forward with this project was that we should utilise this 100 tonnes of boil off as fuel on board,” he said.
Many solutions were examined, but the most elegant was to use the volatile organic compounds (VOC) from the cargo boil-off as fuel together with LNG in the dual-fuel engines. By using LNG as the primary fuel and supplementing this with the energy recovered from the VOC, these vessels will lower their emissions of CO2 equivalents by 30 to 35%, a minimum of 30,000 tonnes per year, compared to conventional oil-fuelled shuttle tankers.
“These vessels are not just dual-fuel engines, they are actually triple-fuel engines. It can run on diesel, it can run on LNG, and it can run on a mixture of LNG and cargo boil-off,” said Mr Storelid. What of future fuels? This is covered, too, with an ongoing project on using ammonia as a fuel in the same engines.
In a poll, LNG was the number one choice (53%) of fuel for shuttle tankers in the next decade. In second place was hydrogen (23%), then LPG (13%), methanol (8%) and VLSFO (3%).
The VOC from the cargo is composed of various substances. Some are tapped off, liquefied and stored in a tank. The VOC containing methane is also stored in a tank and run through a gas turbine, converted to electricity and sent into the shuttle tankers’ electric grid. The design of the electric grid enables a new approach to power distribution. A traditional shuttle tanker has typically two sources of power with limited overlapping functionality. Two big main engines are connected to the main propellers for transit operation and a diesel-electric power station for station keeping. Typically installed power is between 30 and 40 MW.
For the Altera shuttle tankers, there is one common power plant and with 20 MW of installed power. Electric motors on the propellers and the battery system supports the power distribution system for the first four vessels. These have battery safety notation.
The final two shuttle tanker have battery power notation, with increased battery sizes and the battery systems are part of the DP system, a natural development of the electric propulsion process.
Norwegian state energy company Equinor is the client for the shuttle tankers and laid down the criteria. Equinor platform technology, ship technology specialist Frank Aksel Svanes noted that the new North Sea shuttle tanker represent a balance between the power required and the reduction in emissions. That, he noted, is one aspect, but the other is the ability to understand and operate the complex technology on board the new North Sea shuttle tankers. This extends to the software, too, which is newly developed, and the crew training.
“The challenge lies in the balance between the complexity when it comes to the technical equipment and developing simplicity and competence in the operational procedure,” he said.
The development of the Altera shuttle tankers in the North Sea demonstrates an approach of starting with the problem, creating models to encompass the problem, and finding a concept that fits the boundaries of the solution. The alternative approach is to incrementally develop and optimise an existing solution. Both approaches focus on shipping’s obsession with energy efficiency.
In a poll: What do you have as primary focus for energy-efficient solutions for the shipping industry within your business area? Hull and machinery (34%) and low-carbon fuels (32%) were the leading concerns. Hull and propeller optimisation scored 17%, and operational measures 12%, with ‘other’ making up the remainder (5%).
Is the uncertainty of alternative fuel retrofit options delaying your secondhand purchase? Ask the experts at the Marine Propulsion Webinar Week (6-9 April 2021). Register for all Riviera events here.
North Sea Shuttle tankers webinar panel. From left to right: DNV business director tankers Catrine Vestereng, Wärtsilä senior integration expert Kjell Storelid, Equinor platform technology, ship technology specialist Frank Aksel Svanes