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Maximizing energy efficiency in the tanker segment

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The carbon-neutral fuels of the future will be expensive. To keep fuel bills and global demand for green fuels under control and ensure ships remain competitive into the 2030s and 2040s, it is critical to minimize energy use and emissions by choosing the right measures for every tanker type and operating pattern.
Efficiency Measures

Unique energy efficiency challenges in the tanker segment

The tanker segment is not exempt from IMO’s target of net zero by 2050 and competitive pressure will likely increase in the coming years. Energy efficiency will play a key role in achieving this and DNV has identified and categorized a wide range of measures that can be taken to enhance operating efficiency. The choice of measures largely depends on key factors such as expected remaining operating time, vessel type – large crude oil carrier, multi-cargo chemical tanker or shuttle tanker – and operating pattern. DNV recommends shipowners to evaluate and coordinate all available options carefully and implement those that have the best cost/benefit ratio for each individual vessel. DNV offers comprehensive advice and resources to support the decision-making and planning process.

Essential, low-investment optimization measures suitable for all tankers

For vessels in operation, most of these measures can achieve significant savings without incurring major capital investment. DNV stands ready to help owners identify additional potential for energy efficiency enhancements.
• Frequent propeller polishing protects the surface and maximizes efficiency.
• An active hull polishing system reduces resistance caused by roughness.
• Optimization of auxiliary systems (electrical, water, low-pressure air, hydraulic, bilge, steam, ballast water, automation and control systems etc.) ensures operation at optimum loads.
• Optimization of auxiliary engine loads reduces the number of engines running.
• Optimization of steam and condensate system for ships with steam-driven cargo pumps as well as those that heat cargo and do frequent tank washing. • Manual main engine tuning to optimize combustion of the bunkered fuel.
• Engine performance testing and tuning.
• Electronic autotuning automates engine performance optimization.
• Optimization of turbocharging at lower engine loads (exhaust gas bypass or “EGB”).
• Autopilot optimization avoids energy wasted by large-angle rudder movements.
• Optimization of controllable propeller pitch settings and speed by applying a “combinator curve”.
• For shuttle tankers loading offshore, optimization of the power generation system during dynamic positioning.

General operational measures: Fleet management

A number of fleet management and navigational measures can be taken to reduce energy consumption and carbon emissions. Planning voyages to avoid waiting time, power variation and excessive speed; maximizing transport work and reducing waiting and lay times to improve the overall utilization of the fleet; optimizing operation planning and execution to improve capacity utilization; optimizing port operations and coordination to minimize time in port; applying weather routing to minimize fuel consumption; and optimizing the composition of the fleet are measures that harbour significant savings potential, especially when combined. For chartered vessels, any measures outside the owner’s control should be coordinated and implemented together with the charterer.

Technical optimization measures for all tankers

Considering rising emission and fuel costs and the expected high prices of carbon-neutral drop-in fuels in the longer term, investments in energy efficiency measures will be paid back within a few years.
• For newbuilds: – Variable-frequency drives (VFDs) for large pumps, fans and motors to control energy use dynamically (sea cooling water pumps, vacuum condenser pumps, ballast pumps). – Waste heat recovery system to reclaim energy that would otherwise be wasted, e.g. utilization of engine jacket water; exhaust boilers fitted to auxiliary engines to utilize waste heat in port.
• All tankers: – Propulsion-improving devices (PIDs) / energy-saving devices (ESDs) – before the propeller: ducts, fins, hull modifications to reduce slipstream losses – after the propeller: rudder bulb, rudder fins, propeller boss cap fins – high-efficiency rudder to reduce drag, increase thrust, regain slipstream losses – A low-friction, antifouling hull coating reduces frictional resistance from marine growth. Other enhancements:
• For newbuilds: Air lubrication to minimize viscous resistance is expected to yield savings in ballast operations but limited savings in laden condition due to higher compressor loads. Performance has not been verified for tankers as yet.
• All tankers: Wind-assisted propulsion systems (WAPS): Must take into account potential added resistance in adverse weather conditions with respect to safe manoeuvring.

Existing large crude oil tankers: Operational and technical measures

Large crude carriers typically have a standard operating pattern and can feasibly adopt many of the optimization measures that apply to all ships. Uniquely, their power needs for discharging operations in port are very high, with steam-driven cargo pumps accounting for a significant proportion of energy consumption. Furthermore, these vessels also use steam to heat their cargo to reduce cargo viscosity. Optimizing steam plant operation can therefore generate substantial savings during discharging operations. For Suezmax/VLCCs built more than 15 years ago and designed for a 16-knot cruising speed, engine derating is an additional optimization option (turbo cut-out; cylinder cut-out; alternative mapping). Major technical upgrades may include a waste heat recovery system, auxiliary engine exhaust gas boilers for steam generation or water heating, autotuning or a shaft generator retrofit.

Large crude oil tanker newbuilds: Technical measures

At the design stage it is important to verify that all options have been weighed and scrutinized with overall system efficiency and the operating pattern in mind. Standard solutions proposed by shipyards may have to be challenged. In view of the high steam demand on board large crude carriers, an optimized steam plant design is a key objective. This includes the sizing of the auxiliary boilers and the capacity and control of associated draft fans. Furthermore, the auxiliary engine sizes should be optimized at the design stage for maximum efficiency. A shaft generator (power take-off [PTO]/power take-in [PTI]) for the hotel and sea load may be advisable to minimize generator operating times. Another important decision is the selection of an exhaust gas boiler vs composite boiler for the main engine. As an option for large tankers with steam-driven cargo pumps, a small topping-up inert gas generator might improve energy efficiency.

Multi-cargo (oil and chemical) tankers: Technical measures

Chemical tankers, typically smaller than 50,000 dwt, either use electrically powered hydraulic pumps with the hydraulic unit located in the engine room, or directly driven electric pumps with the motors located on deck. Hydraulic pumps, while more costly, may have benefits in terms of more efficient auxiliary engine operation. Instead of using a very large auxiliary engine that will only work at full load when discharging in port, there is the option to size the auxiliary engine for the power needed during voyages only, and to use small, dedicated diesel engines that drive hydraulic aggregates. For tankers with deep-well pumps, the difference in fuel consumption between hydraulically vs electrically driven pumps should be calculated to select the best solution. Shore power is of interest for tankers with hydraulically or electrically driven cargo pumps. Cargo heating and tank cleaning with water requires boiler operation. Heating is either carried out by using heating coils in each tank or deck heaters. The latter require simultaneous pump operation and are therefore less economical. On oil product and chemical tankers, cargo tank cleaning with water typically requires heating of the washing water. This is performed either by heating water in slop tanks or using dedicated cargo wash water heaters. During tank cleaning, separate tank washing pumps may be used. However, cargo pumps are typically also in operation for circulating washing water from cargo or slop tanks.

Multi-cargo (oil and chemical) tankers: Technical measures

Operators of multi-cargo tankers, especially medium-range tankers, that frequently wash their cargo tanks should consider ways to make tank cleaning more efficient. This can be a major competitive advantage. A shaft generator, typically operated by a fuel-efficient two-stroke main engine, is an economical solution. Since shaft generators are best dimensioned for the power need at sea only, separate auxiliary engines must be on board for cargo operations in port (refer to slide 7 above).
Shuttle tankers: Energy challenges

Shuttle tankers transport crude oil from the well to the shore or to another vessel. Their high energy need is a business and operational challenge. Energy-intensive activities include: • Dynamic positioning (DP) to keep the vessel in position at the offshore loading points.
• Cargo heating for discharging.
• Pump operation when discharging. Constantly changing loads during DP operation require a high-performing, flexible power system. DP also requires high redundancy to ensure safe operation even in case of a power failure. It is therefore important to minimize consumption during voyages, and to optimize the machinery to deliver peak power when needed without wasting power in transit.

Shuttle tankers: Solutions

Improving energy efficiency while minimizing emissions:

• Enhance energy efficiency and flexibility of diesel-electric systems through variable engine speed/frequency control.
• Use electric cargo pumps rather than steam pumps when discharging cargo in port to benefit from the high-performing hybrid power system that is needed for DP.
• Replace a conventional split electrical system with an advanced, safe closed bus-tie system to reduce energy consumption substantially.
• On shuttle tankers that heat their cargo, reduce boiler energy consumption during cargo heating through standard optimization measures.
• Hybridize by adding a diesel-electric system and batteries for peak shaving: – Plug-in (using shore power) system for ships with frequent port calls and electrically driven cargo pumps – Non-plug-in (charged during voyage) system for peak shaving, load optimization and zero-emission operation in sensitive marine ecosystems
Source: DNV