A new study by Kongsberg Maritime demonstrates how operational data can devise integrated hybrid solutions for LNG-powered vessels, delivering savings through optimised energy storage, power take-off and digitalisation
To meet the growing global demand for LNG, Qatar Petroleum is undertaking the largest LNG carrier (LNGC) shipbuilding programme ever, reserving construction slots at South Korean and Chinese shipyards to build more than 100 vessels. Under the signed agreements, approximately 60% of the global LNGC shipbuilding capacity will be reserved for the shipbuilding programme over the next seven to eight years.
These LNGC newbuildings will be powered by either low-pressure, Otto-cycle dual-fuel X-DF or high-pressure, diesel-cycle two-stroke dual-fuel ME-GI engines, suggesting steady near-term growth in the LNG-powered fleet.
Kongsberg Maritime undertook a study – learning from operational data gathered from existing LNGCs – to propose ways in which LNG-powered vessels can maximise both efficiency and sustainability, saving both fuel and emissions.
Getting the best from LNG
When compared with traditional marine fuels, diesel and MDO, LNG yields significant environmental benefits, lowering CO2 emissions by as much as 24%, SOx by 99% and NOx by 87%. Its purity and minimal sulphur content produce negligible ash and particulates, making LNG a favoured option for reducing greenhouse gas (GHG) emissions.
But simply switching fuel is only one action that can be taken to make maritime operations more sustainable. New research carried out by Kongsberg Maritime using data from low-pressure, dual-fuel LNG engines shows that taking a holistic view of onboard power generation and use can deliver fuel savings of up to 3.8% for LNG and 56% for pilot fuel.
In addition, Kongsberg’s data-driven approach addresses the issue of methane slip: the amount of unburnt fuel that is trapped by the piston rings and escapes each cylinder when both its inlet and exhaust valves are open. A common challenge for all internal combustion engines, it is particularly serious for LNG as methane is a potent greenhouse gas. The proposals developed through this study minimise methane slip, yielding fuel savings and emissions reductions of up to 7.2% CO2e.
These efficiency improvements can deliver relative annual opex savings of up to 8%. This translated to an annual cost savings of US$483,000 for the LNGC that was the subject of the study.
Comparing efficiencies
Data analytics were key factors in delivering opex savings. This is where Kongsberg Maritime’s K-IMS information management system for LNG carriers comes into play.
Designed to enable continuous access to vessel data both from on board and on shore, one of the functions of K-IMS is to log and collate information for performance analysis. If the customer chooses, this data can be anonymised and shared with Kongsberg to help with product development, which provides a great resource for optimisation projects such as this one.
Kongsberg Maritime’s answer to delivering efficiency savings is to look at the loading on the available on board sources of power, identifying scenarios in which those sources are not being used efficiently. The vessel used for this analysis – an LNGC operating in the Far East – was fitted with two 16-MW, low-pressure, two-stroke, dual-fuel propulsion engines, two 3.7-MW four-stroke, dual-fuel main generator sets and two 2.8-MW four-stroke, dual-fuel auxiliary generators.
Two-stroke engines are inherently more efficient than four-stroke engines – for example, at 85% load, a low-pressure, two-stroke engine will typically consume around 7% less energy than an equivalent four-stroke, dual-fuel engine. This discrepancy is even more pronounced at lower loads, reaching around 15% at 50% load and 36% at 25% load.
Methane slip is also smaller – a low-pressure, two-stroke dual-fuel engine will produce between 20 and 50% less methane slip per kWh delivered than an equivalent four-stroke engine. As a result, maximising the use of the two-stroke propulsion engines in the LNGC in the study made the most sense.
While steaming laden at the vessel’s usual cruising speed of 16 knots, the low-pressure, two-stroke LNG engines used for propulsion on this vessel were operating at around 55% capacity. In ballast at 15 knots, this dropped to 50%. The four-stroke, dual-fuel LNG engines used for auxiliary power generation were shown to be running at between 42% and 48% capacity when under way, reducing to between 33% to 45% when in ballast.
Based on its operational profile, LNGC steams 84% of the time, so the recommendations made by researchers focus on performance both laden and in ballast, although time spent loading, unloading and at anchor were also considered.
Using spare capacity
There is clearly significant unused capacity on both the propulsion engines and the generator sets. This led Kongsberg Maritime to propose shifting some of the power generation load to the more efficient, two-stroke propulsion engines. To do this, a power take off (PTO) system would need to be installed, with an inline generator on each shaft delivering a total of up to 3.4 MW. Although this is relatively small compared with capacity of the existing generator sets, the power is in theory sufficient to support the average auxiliary loads when laden (3,127 kW) and in ballast (3,050 kW).
To avoid the risk of a power blackout from a generator set failure and to provide redundancy, shipowners tend to run more generator capacity than is strictly required to service the loads. In a four-generator set installation, the gensets will usually be configured to operate in pairs, meaning there will be two separate feeds to the vessel’s energy distribution system, both of which will need to be powered to provide redundancy.
Installing a PTO provides another power input to the power distribution system, making it possible to provide redundancy without running unnecessary generator sets. By taking advantage of excess capacity from the propulsion engines, the vessel’s power requirements can be met more sustainably, while minimising the use of auxiliary power.
Higher loads for better efficiency
In addition to the inherent higher efficiency of the two-stroke engines, this approach permits them to operate closer to optimum load. At 85% load, an engine of this type will typically consume around 3% less energy per kWh delivered than at 50% load – a small but significant efficiency gain. For the four-stroke generator sets, this difference is even more pronounced. Efficiency data for these engines shows that increasing the load from 50% to 85% reduces energy consumption per kWh delivered by about 9%.
Higher loadings can be achieved by running fewer generator sets. By careful analysis of the energy requirements in different modes of operation, Kongsberg Maritime was able to show that, even without a PTO, it would in most cases be possible to shut down one or more generator sets while still allowing sufficient extra capacity to handle peak loadings. Additional redundancy could be maintained by the addition of an energy storage system (ESS). These recommendations are shown in Table 1, with the figures in brackets indicating the prior use measured by K-IMS.
Table 1 - Energy requirements in different modes of operation |
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Loading |
Laden |
Anchor |
Unloading |
Ballast |
Main (3.7 MW) generators |
0 (1) |
1 (1) |
1 (1) |
2 (2) |
1 (1) |
Auxiliary (2.8 MW) generators |
2 (2) |
0 (1) |
0 (1) |
0 (2) |
0 (1) |
It is telling to note that, under current conditions, two generator sets are being used while under way when one would suffice. Similarly, when loading and unloading, one or more generator sets are being used than have been shown to be necessary.
Plugging the energy gap
Adding a PTO without battery backup would in theory allow all four gensets to be shut down while under way. However, this does not address the need for redundancy. Kongsberg proposed a battery solution to maintain redundancy while allowing excess generation capacity to be shut down.
By using the data collected by K-IMS, Kongsberg were able to calculate the optimum size of the ESS required to provide up to an hour’s redundancy when steaming with no generator sets operating. A data-led approach here is essential, as correct sizing of the ESS ensures sufficient redundancy while keeping capital expenditure as low as possible. Power provision would be handled by the PTO and the ESS, with the ESS being charged using spare capacity from the PTO at times of lower load.
In other modes of operation (loading, unloading and at anchor) the generators would still be used, meeting the bulk of the demand while running closer to their full capacity, improving their efficiency. The battery would handle peak loadings and provide redundancy.
Even without the PTO system, adding a battery is of benefit to cope with spikes in demand and to allow the generators to be more closely matched to the average power required.
Assessing the options
As a result of this analysis, five options were identified. The first is simply to make best use of the existing installation by matching generator use to power demand, while providing redundancy. The second is to add a battery to the existing system, supplying sufficient redundancy to allow some generation capacity to be shut down where possible.
The other three options involve installing the PTO system with one of the following configurations:
In these scenarios, Kongsberg calculated the generator requirements (shown in Table 2)
Table 2 - Generator requirements |
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Mode |
Speed (kts) |
Reference |
Battery only |
PTO |
PTO + battery |
PTO + battery |
Loading |
- |
1MG + 2AG |
2AG |
1MG + 2AG |
1MG + 1AG |
1RG |
Laden |
16 |
1MG + 1AG |
1MG |
PTO |
PTO |
PTO |
Anchor |
- |
1MG + 1AG |
1MG |
1MG |
1MG |
1RG |
Unloading |
- |
2MG + 2AG |
2MG |
2MG + 2AG |
2MG |
2RG |
Ballast |
15 |
1MG + 1AG |
1MG |
PTO |
PTO |
PTO |
MG = main generator (3.7 MW); AG = auxiliary generator (2.8 MW); RG = replacement generator (4.8 MW)
Clearly, the PTO and battery solutions address different problems. The PTO results in significantly fewer generator set hours while sailing but has no effect on static operations such as loading or unloading. Adding a battery without a PTO benefits all operations and adds redundancy, but still cannot take advantage of spare capacity from the propulsion engines while under way. The best solution to minimise reliance on auxiliary engines is a hybrid PTO/battery approach.
Additionally, this approach addresses the issue of methane slip, using two methods. First, by shifting power generation to the two-stroke propulsion engines – which have inherently lower methane slip – and second by shutting down unused engines.
Conclusions
Even discounting the option of changing the existing four generators for three larger units – a step which would most likely be deferred until a scheduled refit to minimise capital expenditure – the numbers are still compelling. Installing a PTO and battery can reduce the running hours on auxiliary engines by up to 87% when measured across all modes of operation, making better use of the efficiency and lower methane slip of the two-stroke propulsion engines by operating them at a higher loading.
Because the vessel spends most of its time underway, the greatest gains are made by adding a PTO. But to maximise sustainable operation – cutting emissions while at the same time reducing both fuel usage and opex – a fully integrated, hybrid system with advanced power management to balance power demands between the auxiliary generators, PTO and battery-based energy storage is required. Operator guidance – for example in making decisions about shutting down generator sets – is also key to gaining the greatest benefits from a hybrid system and is provided as part of Kongsberg’s digital solution. In the future, Kongsberg Maritime plans to extend this system, using smart integrated control methodologies to further lower methane slip.
Digitalisation through K-IMS has revealed that even without extra equipment it is possible to make changes that will reduce both costs and emissions while maintaining safety and redundancy. By intelligently examining each point in the system to identify which resources can be operated more efficiently and where new technologies will be of most benefit, a holistic, dynamic solution can be developed to ensure sustainable, cost-effective operation at all times.
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