Shuttle tankers - safe, flexible, efficient
Shuttle tankers are an alternative to subsea pipelines. Typically for large water depths. For harsh weather operation the use of shuttle tankers is a necessity. DNV GL has incorporated the latest findings related to their design, operation and safety in an updated shuttle tanker paper.
Shuttle tankers transport crude oil from offshore oil fields to terminals where use of subsea pipelines are not feasible. Major deployment areas include the North Sea and the Brazilian offshore fields. The global fleet has been growing steadily for decades, comprising 88 ships by the end of 2016. Two owners, Teekay Corporation and Knutsen NYK, account for 62 per cent of the fleet, and 64 per cent of all shuttle tankers are DNV GL-classed. Ship sizes vary between 95 and 155,000 dwt, where the larger sizes are typically operate in Brazil.
Nine newbuilds are scheduled for delivery in 2017/2018, and one vessel on average is scrapped annually. 32 vessels are more than 16 years old and will require replacement in the near-to-medium term. Nearly 50 per cent of newbuilds have been built by Samsung Heavy Industries in South Korea.
Shuttle tankers differ from “standard” crude oil tankers. To increase the regularity during loading operations and for the purpose of collision avoidance, they are equipped with dynamic positioning (DP) systems, which typically includes azimuth and tunnel thrusters both forward and aft. North Sea shuttles typically have a twin-screw propulsion system for redundancy and dynamic positioning purposes. In order to improve the position-keeping and manoeuvring capability in ballast condition, it is not uncommon that shuttle tankers have an increased ballast tank volume compared to standard crude oil tankers.
Vessels designed for operation in rough sea areas such as the North Sea feature a hull design with increased fatigue life. As shuttle tankers load offshore, they all have a bow loading arrangement. Some existing shuttle tankers were built with submerged turret loading (STL) systems required for serving specific North Sea oil fields.
Shuttle tankers operating on the Norwegian Continental Shelf may need to comply with Norwegian regulations for emissions of non-methane volatile organic compounds (NMVOC) and install complex vapour recovery process systems for that purpose. Significant technological developments are ongoing with respect to utilizing the VOC as fuel for e.g. power generation purposes. Recent North Sea shuttle tankers use electrically rather than steam-driven cargo pumps. The consequence is that they typically have larger auxiliary engines, smaller boilers and inert-gas generators as opposed to flue gas systems.
North Sea shuttle tankers are typically provided with state-of-the-art nautical safety systems and bridge designs, and typically comply with enhanced fire safety and pollution prevention standards.
Shuttle tankers load directly from floating production/storage units or various types of offshore loading systems/buoys. Loading time may vary from 24 hours to more than one week, while the voyage itself is typically short. Therefore the loading and discharging frequency is comparatively high, with up to 50 cycles a year per ship.
Some shuttle tankers spend 25 to 50 per cent of their operating life in loading mode at the field. The North Sea is a harsh environment where significant wave heights up to 5.5 metres, wave periods of 12 seconds, wind speeds up to 19.7 metres per second and current speed of 0.5 to 1 metres per second can occur. These operating conditions result in a very specific set of design specifications, especially for position-keeping. Although similar conditions may apply in certain offshore oil fields in Brazil, the weather conditions in Brazil are generally less harsh. Note however that current speed is generally higher in Brazil than in the North Sea.
State-of-the-art shuttle tankers are either equipped with a bow loading system (BLS) for loading from offshore loading systems/buoys, FPSOs or FSOs, or a submerged turret loading system (STL). STL loading is currently used at very few offshore installations, notwithstanding the fact that it allows loading in more severe weather conditions than BLS, supporting a significant wave height (Hs) of 16 metres.
Dynamic positioning (DP) systems are an essential component in today’s shuttle tanker technology and must be capable of maintaining the tanker in position in harsh weather conditions. Today most cargo owners specify that new shuttle tankers shall satisfy IMO dynamic positioning Class 2 requirements.
New shuttle tankers operating in both the North Sea and Brazil appear to have adopted the DNV GL’s class notation DYNPOS(AUTR) as the required minimum. Historically requirements have gradually become more stringent, a development that is likely to continue and may lead to frequent use of more advanced notations such as DYNPOS(E) and DYNPOS(ER). These notations ensure reliable and robust yet flexible DP systems which can be run in more cost-efficient modes with a smaller environmental footprint compared to traditional redundant DP systems.
DNV GL has also issued rules for the use of batteries in hybrid DP systems to further support industry efforts to deliver efficient, eco-friendly and incident-free DP operations.
Battery power on shuttle tankers
In a recent joint industry project, four ship types with selected operational profiles were analysed to quantify the fuel, emissions and reliability benefits of using hybrid power for dynamic positioning, drilling, propulsion and backup power.
The study found that hybrid power architectures are technically feasible, with a viable return on investment (ROI) and payback periods of zero engine hours for shuttle tankers and up to 7,700 for other ship types. In the case of the shuttle tanker selected for the study, using battery power increased efficiency by 38 per cent. The result is a multifaceted value proposition: operational efficiency is improved by balancing diesel engine loads and avoiding wasteful idling periods; reducing engine running time also cuts CO2 and other noxious emissions.
Redundant engines may be dispensable if the battery system functions as a spinning reserve. Avoiding cycles of extreme engine loads reduce engine wear and maintenance costs and may allow maintenance cycles to be extended. What is more, the ability to close the tie switch between buses can greatly improve the hybrid value proposition. Batteries can be optimized either for fuel efficiency or for backup power, depending on the given application.
In hybrid DP operations, batteries can supply load for approximately one-third of the operating time, reducing generator cycles and responding faster than a generator set. As for backup power applications, economic feasibility depends on the ratio of investment cost versus the desired duration of backup power availability.
Furthermore, fire safety is a key concern for battery rooms, which must be designed with fully independent ventilation, cooling and fire suppression systems and a sophisticated, integrated control system. As an indispensable element of the hydrocarbon value chain, shuttle tankers must keep up with the evolution of technology to satisfy today’s and tomorrow’s safety and environmental requirements, and DNV GL does its part to make sure they will. The updated DNV GL brochure “Shuttle Tankers“ describes recent technological advances in detail.