Building the case for battery powered OSVs

07 Aug 2017

Battery technology is set for increasing application in the offshore sector, according to the authors of a recent study by researchers at Marintek and the Norwegian School of Economics.

One option for reducing the emissions and climate impacts in shipping is through hybrid power technology. In this context, hybrid means adding electric battery capacity to the conventional power setup, facilitating a power production more adapted to the demand in various operating modes. It is particularly promising for vessels operating under varying conditions, and we conduct this analysis on offshore vessels because of their great variation in actual loads and their great need for potential load at short notice.

Offshore support vessels (OSVs) have multiple combustion engines and dynamic positioning (DP) systems to ensure that they can perform their duties with a high reliability at nearly any sea state. The DP mode must keep power resources available at any time sufficient to handle peak loads caused by extreme waves and wind variations, even in combination with failure of one of the main vessel engines. This has resulted in a general operational pattern with vessels running multiple engines simultaneously even at calm sea conditions when serving the oil and gas installations (inside a radius of 500m).

When engines operate at lower power, fuel consumption per unit of output (per kWh)produced increases. This increase makes a small impact compared to the total cost of the operation. In contrast, for emissions, low loads yield a greater increase in emissions of exhaust gases such as NOx and aerosols such as black carbon (BC).

Batteries have many advantages. First, they can compensate for load fluctuations, enabling engines to run at a more constant – and optimised - load. Second, operation of engines at very low loads is avoided and the engines can run more at medium to high power with lower specific fuel consumption and lower emissions. Third, batteries engage instantly and can provide any peak power required by the DP system. Fourth, they enable the vessel to abort its DP operation safely supposing all engines should stop and not start again. For these reasons, installing batteries may enable a reduction of the number of main combustion engines currently installed, namely from four or three to three or two.

Application and analysis

Figure 1 shows a typical annual operational profile with the associated power demands for a supply vessel operating in the North Sea. It indicates that the vessels will stay around 25% of time in port (loading, unloading and waiting). Around 40% of the time will be used in transit to and from the oil fields where the speed reflects voyage priorities, like urgency, focus on fuel savings or the scheduled arrival time. 35% of the time will be used serving the oil and gas installations either waiting in standby mode or in DP mode serving the platform. The large share of travelling at economy speed, 10 knots instead of 12 or 15 knots, reflects the overcapacity of offshore support vessels and the slowdown of the whole oil and gas sector. Standby vessels will spend less time in transit and in ports than typical supply vessels, anchor handlers will be in DP mode less often, and subsea vessels, which operate (remotely) underwater vehicles serving oil and gas installations at the seabed, might spend more time in DP mode. However, these variations in operational profile do not affect the main conclusions of the present study.

Power and propulsion

To handle large variations in power demand and strict DP requirements, offshore support vessels are equipped with multiple engines, advanced control systems and a propulsion systems consisting of propellers and thrusters. This article considers three alternative engine setups for a typical offshore support vessel. The first is the standard setup consisting of four diesel generator sets, which may be of equal size or with two slightly larger and two slightly smaller ones. The second alternative setup is to add batteries to the standard setup, one for each part of the switchboard. The third option is to replace two of the engines from the standard setup with batteries, keep one of the remaining engines unchanged and double the installed power of the last one.

These setups enable the vessel to meet the requirements of classification societies for operation in DP mode, even if the generators connected to one part of the switchboard, or the switchboard itself, stop working. The explanation is that the main engines or batteries connected to the other part of the switchboard will have sufficient capability to continue the DP operation. With a conventional setup, two or three engines will have to run when the vessel is operating in DP mode to assure power delivery in the event of a blackout of one or even two engines simultaneously. With the battery options, it is sufficient to have one engine running, because the batteries will provide the required power instantly if the engine stops. When two or three engines are supporting a low overall power demand, each of these engines will be lightly loaded, resulting in higher fuel consumption per kWh and significantly higher emissions per kWh.

The best performance, namely the lowest specific fuel consumption, for engines is achieved at around 80% power, while at 30% power output fuel consumption increases by around 5% for a variable speed setup and around 10% for a fixed speed setup. Since 30% power is a typical operational point per engine running when vessels run multiple engines in DP mode, the potential savings with batteries will be the difference in specific fuel consumption between 30% and 80% power. In addition, batteries will give savings for other operational modes like at port and standby, as well as for some transit speeds, since these imply lower or higher than 80% power.

Fuel consumption

Combining operational profile with fuel oil consumption enables us to calculate annual fuel consumption for four distinct options based on a combination of engine type (constant versus variable speed) and engine setup (standard versus hybrid). The hybrid setup here refers to both battery alternatives presented, because they will have the same fuel consumption and emissions.

Table 1: Annual fuel consumption as a function of engine and battery technology   
     Annual fuel consumption (tonnes)
Operational modeAnnual hoursAverage power (kW)SFOC fixed speed (g/KWh)SFOC variable speed (g/kWh)Fixed speed engine Variable speed engine Fixed speed engine and batteryVariable speed engine and battery
DP2,4001,600225210864806768756
Standby6001,500220205198185180177
Port2,270225290265148135102101
Transit eco (10 knots)3,0002,3002052001,4151,3801,3801,359
Transit (12 knots)4003,300200197264260264260
Transit max (15 knots)906,000204204110110108106
Total8,760   3,0002,8802,8002,760

The main observations from Table 1 are: annual fuel consumption with constant speed engine and standard setup is around 3,000 tonnes, and replacing it with a variable speed engine reduces the fuel consumption with 120 tonnes per year, or 4%. Furthermore, combining a constant speed engine setup with batteries reduces fuel consumption with 200 tonnes compared to the standard constant speed engine setup and with 80 tonnes compared to the standard variable speed setup. Finally, combining batteries and variable speed engine generator sets gives only a marginal additional reduction compared to fixed engine speed hybrid setups (40 tonnes). Given the fact that combining variable speed engine and generator setups with DP systems is novel technology, and hence comes at a high cost, we will in the remaining of the article focus on fixed speed engine and generator power trains.

Environmental assessment

To estimate the cost of a standard power setup, we can say that typically, around 8,000kW have been installed on offshore support vessels built for operation in the North Sea, which means four main engines at 2,000kW each with a total cost of US$7 million including the generators. Adding batteries to this standard setup raises costs by US$1.25 million (for a battery capacity of 500kWh including the battery-management systems). Removing two engines and then doubling the capacity of one of the remaining two engines, gives a total engine power of 6,000kW, for a cost of US$5 million. With less installed power, there is a need for a larger battery capacity of 1,500kWh at a cost of US$2.5 million, resulting in a total cost of US$7.5 million for this hybrid alternative. These costs can be compensated at first by the annual fuel saving of US$100,000, resulting from a saving of 200 tonnes at a current price (2016) of around US$500/tonne for marine gas oil (MGO). There will also be savings on lower maintenance cost due to fewer engines and fewer generator running hours, but these will be offset by the costs related to the additional training and knowledge required on board. Table 2 shows capex, savings and payback based on the current fuel price and the 2014 price- US$1,000 per tonne, for the three alternative power setups.

Table 2: Capex, savings and payback     
   Annual saving (US$ millions)Payback time (years)
 Capex (US$ millions)Fuel (tonnes)Fuel cost US$500/tonneFuel cost US$1,000/tonneFuel cost US$500/tonneFuel cost US$1,000/tonne
Standard setup7     
Adding batteries8.252000.10.212.56.3
Two engines and batteries7.52000.10.252.5

The main observation from Table 2 is that with the present fuel cost the payback time of 12.5 years is too long if the batteries are added to the standard engine setup - even longer than the expected duration of the batteries. Combining batteries with two engines gives a shorter payback time, 2.5-5 years, but is only possible for new vessels.

Figure 5 summarises the annual fuel consumption, emissions and abatement cost for the standard and hybrid setups, which implies that the standard power setup will combust 50% of its fuel at high power and 50% at low power, while hybrid power option will combust all at high power.

Conclusions

Combining batteries with combustion engines reduces local pollution and climate impact. Hybrid power setups give a 40–45% reduction in annual global warming potential in Artic areas and around 20% reduction in the North Sea. When the focus is on reducing local pollution, the reduction of harmful exhaust gases enabled by a hybrid solution is in the magnitude of 25–30%.

When the focus is on financial advantage, based on current fuel prices for marine gas oil, the economics is more dubious. When retrofitted on an existing vessel the payback will be 10–15 years (12.5 years with 200 tonnes of saved fuel and an increase in capex of US$1.25 million). Unless the shipowner is very concerned about the environment and has the financial resources, or the customer demands it, or there are regional incentive schemes available, batteries will not be retrofitted on existing vessels. For newbuildings, hybrid prospects look brighter, since batteries can replace one or two of the four main engine and generator sets. With only US$500,000 in incremental capex compared to the present standard power setup, payback time is estimated to about five years for a new-built vessel, which will be found favourable by many.

NOTE: The above article is an edited version of the original research paper. For the full study, search for: Haakon Elizabeth Lindstad, Gunnar S. Eskeland, Agathe Rialland, 'Batteries in offshore support vessels – Pollution, climate impact and economics', Transportation Research Part D: Transport and Environment, Volume 50, (2017).