Hybrid Power Retrofitting Study for a Platform Supply Vessel

This paper takes a platform supply vessel as the parent vessel and introduces its power system configuration and working conditions; to meet the requirements of carbon emissions reduction and improve economic efficiency, a preliminary program of marine hybrid power retrofitting of diesel generator sets assisted with Lithium battery module is proposed on the base of analyzing its working conditions, the fuel consumptions and carbon emissions are compared between before and after the retrofitting, etc. According to the calculation results, the paper concludes that the investment payback period of the refitting is about 6.38 years, and this refitting will have good economic benefits and social benefits.


Introduction
In 2020, China promised to strive to "reach the carbon peaking by 2030 and achieve carbon neutrality by 2060" [1] .In 2021, the State Council issued the Action Plan for Carbon Peaking by 2030, requiring the shipping industry to promote the low-carbon transformation of vehicles [2] .In this context, energy conservation and emission reduction have undoubtedly become one of the hottest topics in the shipping industry [3] .
In recent years, Lithium batteries have significantly improved their capacity and service life, laying a foundation for their application in power systems [4] .The potential of energy conservation and emission reduction by traditional means, such as ship-type optimization, has been almost dug out [5] .In the context of the implementation of the "dual carbon" strategy, oil-electric hybrid vessels with low energy consumption and low carbon emissions have become a new way of energy conservation and emission reduction for vessels.
To implement the "dual carbon" strategic requirements and improve economic efficiency, China National Offshore Oil Group Co., Ltd.proposes to carry out the oil-electric hybrid power retrofitting for a platform supply vessel.To determine whether this retrofitting has economic prospects, this paper analyzes the fuel consumption of the vessel based on the collected data and estimates the investment payback period of this retrofitting in combination with economic costs and carbon emissions, to provide a reference for the decision-makers of the project.

Figure 1. "DONG FANG HAI" platform supply vessel
The parent vessel used in this study is the "DONG FANG HAI" of China National Offshore Oil Corporation.The appearance of the parent vessel is shown in Figure 1 and it is a platform supply vessel, which was built in August 2016, and the specific design parameters are shown in Table 1.This vessel adopts an electric propulsion system equipped with two sets of 1,650 kW main thrusters and two sets of 690 kW bow thrusters.The main thrusters are driven by converters, and the side thrusters are driven by soft starters.The power plant is installed with four sets of 1,200 kW Cummins diesel generator sets.At economic speed, two sets of diesel generator sets are used to supply power; three sets of diesel generator sets are used to supply power during normal operation, and one set of diesel generator sets is used for backup.The main switchboard has an automatic power management system to enable automatic power distribution and zoning of the power supply.A single-line diagram of the vessel's power grid is shown in Figure 2.

Fuel consumption measurement and data analysis
To obtain the actual fuel consumption data of the parent vessel, this paper adopts the way of tracking measurement to investigate the fuel consumption of the target vessel, and the measurement instrument used is the external clamping ultrasonic flowmeter, as shown in Figure 3.The parent vessel generally covers six working conditions in each mission, which are high-speed sailing, low-speed sailing, arriving in and leaving the port, leaning on the platform, oil field guarding, and operation.The energy of the power system is all derived from 0# diesel oil, and the oil density is 0.835 g/ml.The information related to the number of working conditions, duration, fuel consumption rate, and total fuel consumption for each condition is shown in Table 2.For the same conditions, the lower the load factor of a vessel's generator sets, the farther away it is from its optimum operating point and the higher its fuel consumption rate.The fuel consumption rate of the four generator sets configured for this vessel is 206 g/kwh at its optimum operating point.As can be seen from Table 2: the fuel consumption rate of the parent vessel diesel generator sets is higher than 225 g/kwh under all working conditions, indicating that its load rate is low and it is not operating at the optimum operating point of the diesel generator sets.In particular, the fuel consumption rate is as high as 238 g/kwh in the arrival and leaving the port working condition, which is much higher than 206 g/kwh, indicating that there is still a large potential for energy saving.

Retrofit objectives
There are various ways to retrofit a vessel with hybrid power [6] .The retrofitting solution described in this paper refers to the use of a combined diesel-electric propulsion concept with a containerized Lithium battery pack connected to the vessel's power grid.When diesel generator sets are running at a low load rate, and the fuel consumption rate is high, the connected Lithium battery pack can be used as a load and charged by diesel generator sets to increase the load rate of diesel generator sets, so that diesel generator sets can often run at the optimum fuel consumption point.At the same time, under working conditions where the vessel requires less power, diesel generator sets can be stopped, and the vessel can be powered by the Lithium battery pack to reduce the overall fuel consumption rate.The containerized Lithium battery pack is used to achieve energy savings by optimizing the energy supply strategy of the vessel.

Retrofit program
The single-line diagram of the retrofit program is shown in Figure 4.All equipment selected has met the relevant requirements of the China Classification Society and the designed system has met vessel safety requirements.To meet the requirements of the retrofitting, the project team chose a 2 MWh container-based Lithium iron phosphate battery pack, and its theoretical service life is 7~8 years [7] .

Introduction to integrated power usage scheme
(1) Four diesel generator sets and the Lithium battery pack are all put into use under high-speed sailing conditions.
The total power capacity of diesel generator sets is 4×1,200 kW, and the power fluctuation range, which can be taken by the container-based Lithium iron phosphate battery pack, is 500~1,000 kW.Before the high-speed condition, the Lithium battery pack is fully charged.During the voyage, the Lithium battery does peak shaving to smooth out grid fluctuations and keep diesel generator sets running at the optimum fuel consumption point.
(2) Diesel generator sets (4 or 3 sets) and the Lithium battery pack are used under the working conditions of leaning on the platform, oil field guarding, and operating.
The power sources connected to the grid include 4 sets × 1,200 kW or 3 sets × 1,200 kW and the Lithium battery pack.Firstly, the Lithium battery pack is charged; when charging is complete, the diesel generator sets stopped, and the power supply mode is switched to the Lithium battery pack.
(3) The Lithium battery pack and one diesel generator set are used under the working conditions of low-speed sailing and arriving in and leaving the port.
When the SOC (status of Charge) of the Lithium battery pack system is low, diesel generator sets are switched on to supply power and charge the Lithium battery pack with a power of around 1,000 kW.

Economic benefits analysis
For the same type of vessel with different power systems, the vessel's revenue, crew's wages, additional benefits, charges of port usage, and other costs are the same generally [8] .The main cost differences before and after the hybrid power retrofitting of the target vessel are as follows: (1) The capital cost of the hybrid power retrofitting 0 Y .
The payback period is the time required to recover the project investment in terms of the net proceeds of the project and is generally measured in years.The shorter the payback period, the more profitable and risk-resistant the project is [9] .In this paper, the payback period of the capital cost for hybrid power retrofitting is selected as the main indicator to measure the evaluation of the economic benefits of this hybrid retrofitting.The specific values of the four main costs affecting the payback period are calculated as follows:

The capital cost of the hybrid power retrofitting 0 Y
As shown in Table 3, the capital cost budget for this retrofitting is RMB 9.8 million yuan.

Y
The newly added containerized power battery pack and the grid-connected control cabinet can be recycled at a market price after reaching the end of their service life thereby generating proceeds 1

Y
(in RMB million).After consulting with professional recyclers, this paper uses the straight-line depreciation method to estimate the recycling cost of newly added equipment based on the service life of the Lithium battery pack [10] , and establishes a relationship between the Lithium battery pack recycling proceeds 1 S (in RMB million) and the Lithium battery pack service life X : The equation for the relationship between the grid-connected control cabinet recovery proceeds 2 S (in million RMB) and its service life X is:

Y
As shown in Table 2, before the hybrid power retrofitting, the fuel consumption rate of diesel generator sets under all working conditions is higher than the optimum fuel consumption rate of 206 g/kWh at the optimum fuel consumption point due to the low load rate of diesel generator sets.If the parent vessel is retrofitted with hybrid power, the fuel consumption rate of diesel generator sets will be reduced for highspeed sailing, leaning on the platform, oil field guarding, and operating conditions.Diesel generator sets can be stopped during low-speed sailing and arriving in and leaving the port conditions, meanwhile, the Lithium battery pack takes responsibility for the power supply to achieve saving fuel costs.In each mission, the fuel consumption in the retrofitted vessel is: where 1 ≤ i ≤ 4.
A is the fuel consumption rate before the retrofitting of the i-th working condition; A is the fuel consumption rate after the retrofitting of the i-th working condition; 1 i B is the total fuel consumption before the retrofitting of the i-th working condition; 2 i B is the total fuel consumption after the retrofitting of the i-th working condition.
The amount of fuel that can be saved in each mission for the four working conditions mentioned above can be expressed by equation (5).
The specific calculations are shown in Table 4.The amount of fuel saved in every mission is: where D is the fuel consumption for low-speed sailing conditions before the retrofitting.
J is the fuel consumption for the arriving in and leaving the port working conditions before the retrofitting.
The above analysis shows that the fuel saving for every mission is approximately 1.97 tonnes.With 60 times per year, the annual fuel saving is approximately 118.2 tonnes.The official website of the National Development and Reform Commission shows that the maximum retail price of 0# diesel in Tianjin is RMB 8,630 yuan per tonne on 3rd February 2023, so the annual fuel cost saving 2 Y for the target vessel after retrofitting is RMB 1.02 million yuan.

Y
Since June 2013, China has established carbon emission trading markets in seven provinces and cities and launched trading [11] .The reduced carbon emissions from the hybrid power retrofitting of the parent vessel can be traded on the market and thus generate benefits.
The annual carbon reduction benefit 3 Y can be calculated according to equation (7): where: B is the reduced quantity of fuel consumption, kg.Referring to the data from Tianjin Emission Rights Exchange, the average price of the last seven days of the national bulk agreement on carbon trading was RMB 57.5 per tonne.As a result, the hybrid power retrofitting will yield an annual benefit of RMB 21,000 yuan in terms of the number of CO2 emissions reduced.2.1 The calculated data for main factor amounts are shown in Figure 5.The payback period (in years) can be calculated from equation (8).
The calculation shows that the payback period for the capital cost of this retrofitting is 6.38 years.Compared to the service life of 7 to 8 years of the containerized Lithium battery pack in this retrofitting, the retrofitting has good economic prospects and social benefits.

Conclusion
To evaluate whether the hybrid power retrofitting of the platform supply vessel has economic prospects, this paper introduces its power load profile, the objectives, and program of the retrofitting, the integrated power usage scheme, and the fuel consumption status.By analyzing its fuel consumption, annual fuelsaving benefits, and carbon-saving benefits after the retrofitting, the payback period is derived.Calculation results show that this retrofitting can achieve cost recovery in a relatively short period of years.Retrofitting is a good guide to energy saving and emission reduction in existing vessels with low loads and high fuel consumption.

Figure 2 .
Figure 2. Single-line diagram of the power system of the "DONG FANG HAI"

Figure 4 .
Figure 4. Single line diagram of the hybrid retrofit program for the "DONG FANG HAI".
recovery proceeds from additional equipment are:

3 1000
average price of carbon trading transactions YB   =

Table 2 .
Working conditions of the target vessel

Table 3 .
Budget for the hybrid power retrofitting of "DONG FANG HAI".

Table 4 .
Comparison of total fuel consumption in a typical mission.
 is the CO2 emission factor of the diesel, 3.0959 kg CO2/kg.

Table 5 .
Calculated data for main factor amounts.