Koninklijke Vereniging - Société Royale



Shuttle tankers –safe, flexible, efficient

DNV GL has taken a look at the shuttle tanker and in particular hybrid battery powered dynamic positioning (DP) systems.*

The global fleet has been growing steadily for decades, comprising 88 ships by the end of 2016, varying in size between 95,000 dwt and 155,000 dwt. The larger sizes typically operate offshore Brazil and the smaller in the North Sea.

Two owners, Teekay and Knutsen NYK, account for 62% of the fleet, and 64% of all shuttle tankers are DNV GL-classed.

Nine newbuilds are scheduled for delivery in 2017/2018, and on average, one vessel is scrapped annually. Some 32 vessels are over 16 years old and will require replacement soon.

To increase the regularity during loading operations and for collision avoidance, shuttle tankers are equipped with DP systems, which typically includes azimuth and tunnel thrusters fitted both forward and aft.

North Sea shuttles typically have twin-screw propulsion systems for redundancy and DP purposes. To improve the position-keeping and manoeuvring capability in ballast condition, it is not uncommon that shuttle tankers have an increased ballast tank capacity, compared to standard crude oil tankers.

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.

Developments are ongoing regarding the use of VOC as a fuel for eg, power generation purposes. Recent North Sea shuttle tankers use electrical rather than steam-driven cargo pumps and as a result, they typically have larger auxiliary engines, smaller boilers and inert-gas generators, as opposed to flue gas systems.

Loading time from FPSOs/FSOs or various types of offshore loading systems/buoys, may vary from 24 hours to more than a 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- 50% 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 m, wave periods of 12 secs, wind speeds up to 19.7 m per sec and current speed of 0.5 to 1 m per sec, can occur.

In Brazil, the weather conditions are generally less harsh, however, current speed is generally higher in this area than in the North Sea.

Loading systems
Today’s shuttle tankers are either equipped with a bow loading system (BLS) 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 m.

As for DP, most cargo owners specify that new shuttle tankers should satisfy IMO DP 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.

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 DP, drilling, propulsion and backup power.

The study found that hybrid power systems were technically feasible, with a viable return on investment (ROI) and payback periods of zero engine hours for shuttle tankers.

In the case of the shuttle tanker selected for this study, using battery power increased efficiency by 38%.

The result is a multi-faceted 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 reduces engine wear and maintenance costs and may allow maintenance cycles to be extended. In addition, the ability to close the tie switch between buses can greatly improve the hybrid value proposition. Batteries can be optimised either for fuel efficiency or for backup power, depending on the given application.

In hybrid DP operations, batteries can supply load for about 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.

Fire safety is a key concern for battery rooms. These rooms must be designed with fully independent ventilation, cooling and fire suppression systems and a sophisticated, integrated control system.          


*This article is a taken from a paper produced by Olav Tveit, DIVV GL's senior principal engineer and ship type expert.



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