1. Introduction
A ship’s stability is one of the most important factors to ensure ship safety, and ballast water in turn is essential to secure ship’s stability. The ballasting time is the time it takes to fill a ballast tank with ballast water, and this is important to secure safety and efficient operation of a ship. The fundamental purpose of a ship is to transport cargo and/or passengers. The ballasting and deballasting is an important part of ship’s operation to maintain the ship’s center of gravity during the loading or unloading of cargo. Consequently, if the performance of the ballast system is inadequate to meet the required condition, the loading or unloading time will increase, which will in turn hamper the safety and efficient operation of a ship. The International Maritime Organization (IMO), the United Nations body which administers the international regulatory regime for shipping, had noted the negative impact of nonindigenous organisms transported in the ballast water of ships already in the early 1970s.
Consequently, measures were taken with the aim to minimize invasion of ballast watermediated species through the IMO Marine Environmental Protection Committee (MEPC) Resolutions. As a result of such efforts by IMO, it was determined that an international convention would best meet the needs of the global community, and hence the International Convention for the Control and Management of Ships’ Ballast Water and Sediments (BWM Convention) was adopted in a Diplomatic Conference in 2004 (IMO, 2004). The BWM Convention came into force on 8 September 2017, and all international voyage vessels must install a ballast water management system (BWMS) onboard according to the implementation scheme which is described in Regulation B3 of Annex to the BWM Convention (IMO, 2016;IMO, 2019).
The various researches have been undertaken to study the regulations on BWM Convention and the United States Coast Guard (USCG) (Gollasc et al.. 2007;Čampara et al., 2019), the challenges arising from BWM Convention (Endresen et al., 2004;David et al., 2018), the operational experiences on ships (Bakalar, 2016), the reviews of BWMS technologies (Tsolaki and Diamadopoulos, 2010), the research on global BWMS markets (Hasanspahi´c and Zec, 2017). The BWMS technical studies focused mainly on performance improvement of the system itself, stable operation of the system, and testing and approval of the system of IMO and USCG (Park et al., 2017). Recently, researches on efficiency improvement, remote monitoring, and automation of BWMS have also been undertaken (Akram et al., 2015;Sutherland et al., 2001).
In particular Sutherland et al. (2001) have been studies on UV type BWMS such as a study to investigate the effects of ultraviolet radiation treatment on the abundance and growth characteristics of the natural population of phytoplankton present in the seawater source and a study on changes in performance depending on the UV dose and turbidity (Olsen et al., 2016;First and Drake, 2014). However, there have been very limited studies in the impact to the ships’ operational aspects, directly or indirectly, as a consequence of BWMS installation, such as the change to the ballasting time. After a BWMS is installed on a ship, the discharge flow rate of ballast pumps will decrease due to the flow energy loss from the installation of a BWMS, and there also will be increased in the ballast pumps’ discharge head from the modification of pipelines.
In general, the BWMS manufacturer only provides information on the flow energy losses arising from the BWMS, while the design for installing the BWMS on board the existing ship is performed by the engineering company and/or shipyard. Ballasting time can greatly increase after the installation of BWMS if there is no particular consideration to ballasting. However, information on the increase of ballasting time after the retrofit work of BWMS is rarely disclosed since it is treated as a confidential matter by the engineering company, shipyard and/or shipowners. It is also difficult to find officially released information about the changes in ballasting time.
A BWMS that has less rated capacity than discharge capacity is generally selected, and the flow control valves are installed in front of the BWMS to make the actual flow rate of pipelines less than the BWMS’s rated capacity. The flow rate can be controlled by adjusting the opening rate of flow control valves, which can lead to considerable loss of flow energy.
This kind of flow energy loss means that there would be decrease in discharge flow rate of ballast pump and increase in ballasting time. An excessive ballasting time, when required, would adversely affect the ship’s stability, and the efficient operation including the ships arrival and departure time. Particularly, the flow energy loss would increase due to corrosion in piping, etc. Although there are studies on the increase of corrosion in piping caused by BWMS, studies on the increase in the flow energy loss due to the increased corrosion are not yet known (Song et al., 2014;Eisnor and Gagnon, 2004).
Furthermore, the flow quantity loss arising from backflushing filters of BWMS is not considered in this study. According to the experience of the person in charge of BWMS type test, the flow quantity loss from the installation of a backflushing filter is about 1525 % of a BWMS’s rated capacity during backflushing operation. And the increase in ballasting time arising from such a flow quantity loss should be further investigated in addition to the increase in ballasting time arising from the flow energy loss investigated in this study.
This study provides the relevant parties with helpful information and guidance on the design criteria for the selection of a UV type BWMS, and the modification/addition of a relevant piping system by comparing the increase in ballasting time after the installation of a UV type BWMS.
Further, the specifications and performance of ballast pumps, the size, and arrangement of ballast piping system and the shape and dimension of ballast tank are determined based on the actual data of a 55K DWT class LPG carrier.
2. Flow Energy Loss
There are two factors to consider in flow energy loss. The first is the friction loss arising from the roughness of the pipe wall, and the second is the flow energy loss arising from changes in flow direction.
Friction loss in a piping system can be estimated by using the Moody Chart (Fried and Idelchik, 1989). And the flow energy loss at pipe fittings and valves can be estimated by using the flow energy loss coefficient or by using equivalent length, which is equal to the length of straight pipe that represents the same flow energy loss (Steidal et al., 1996). Although the internationally recognized design criteria for the flow energy loss coefficient and equivalent length have not been established, the method of equivalent length which is determined in an industry standard was used in this study to estimate the flow energy loss at pipe fittings and valves. The flow energy loss was calculated per the DarcyWeisbach equation, as in the formula (1) below. Regarding the use of the friction loss coefficient, ColebrookWhite’s equation was applied for this study instead of the Moody chart, as in formula (2) below (Casey 1992).
where ΔP is pressure loss (Pa), f is friction loss coefficient, L is pipe length (m), L_{E} is equivalent length of fittings and valves (m), ρ is fluid density (kg/m^{3}), V is fluid velocity (m/s ), ϵ is roughness of pipe wall (m), D is pipe inner diameter (m), and Re is the Reynolds number. For calculating the friction loss coefficient, pipe wall roughness of 0.5 mm, which is applicable to general ship’s steel pipe, was used. Although there are many standards for equivalent lengths of pipe fittings and valves, the standard set out in NFPA Code 13 of National Fire Protection Association (NFPA, 2013) which is known as widely applicable to actual design work, was used for this study, as shown in Table 1 below.
In addition, the density and viscosity of seawater should be considered for the calculation of flow energy loss, and these parameters have different values depending on the seawater temperature and salinity. For this study, the standard seawater properties at 20°C in the standards of seawater set out in the International Towing Tank Conference (ITTC, 2011) were applied. Thus, the applied value of density is 1,024.8103 kg/m^{3}, and the applied value of viscosity is 0.001077 Pa · s .
When a BWMS is installed on a ship, additional flow energy loss results from the installation of the BWMS and from the use of flow control valves. Information on flow energy loss as a result of the installation of BWMS could be provided by a BWMS manufacturer. However, information on flow energy loss arising from the flow control valves is not easily available without undertaking actual measurements, even though much flow energy loss is expected since the opening rate of the valves is adjusted at the site. Thus, this study was undertaken only to compare the ballasting time before and after installing UV type BWMS. It should be noted that the flow energy loss arising from the use of flow control valves was not taken into account quantitatively in this study.
According to the design data provided by a manufacturer of the UV type BWMS used in this study, the flow energy loss is 0.7 bar when the flow rate is 1,500 m^{3}/h. And, the flow energy loss of the UV type BWMS corresponding to the flow rate can be calculated using formula (1). From the formula (1), f, L and L_{E} can be eliminated because these terms are variables for flow energy loss, however, the flow energy loss by the UV type BWMS for a specific flow rate is determined by design data of the UV type BWMS. In addition, D and ρ are constant. Hence, all terms except V can be expressed as a constant factor and this constant factor was expressed as K in this study.
The factor in formula (3) was calculated based on the design data and D and ρ of formula (3). The nominal diameter of the UV type BWMS is 300A (internal diameter is 297.9 mm) and the density of seawater is 1,024.8103 kg/m^{3}. And, the flow energy loss by the UV type BWMS is 0.7 bar (= 6.9628 m) when the flow rate is 1,500 m^{3}/h (velocity is 5.978 m/s ). Hence, the calculated factor K is 0.1948362.
3. Calculation Condition
The performance curve of pumps, the arrangement of the piping system and the shape of the ballast tank are necessary to calculate the ballasting time. In this study, there were two ballast pumps with a nominal discharge flow rate of 750 m^{3}/h, and the quadratic function is driven by using these pumps’ measuring results at manufacturer’s shop, as shown in Figure 1 and formula (5) below.
where H is discharge head (m), Q is discharge flow rate (m^{3}/h). The ballast piping system itself has complex engineering arrangements, as do the ballast tanks. However, the shape of the ballast piping systems and ballast tanks was simplified as much as possible in this study since the purpose of this study is to review the difference in ballasting time before and after installation of a UV type BWMS, although it would be ideal for calculation of ballasting time if it were possible to reflect the actual arrangement of ballast piping systems and ballast tanks. Figures 2 and 3 show simplified ballast piping systems and ballast tanks. In Figure 2, the number on the figure is node number used for expression to the specification of the ballast piping system. Figure 3 shows the crosssection of the ballast tank used in this study and it is assumed that the ballast tank is arranged at the front of the engine room forward bulkhead. Table 2 shows the detailed ballast piping system. In this study, the ballasting time calculation was carried out by using the data of a gas carrier having 55,000 deadweight class and having 40.8 m of the actual length of the ballast tank.
4. Calculation Procedure
Calculating ballasting time is a nonlinear and timedependent problem in principle, since the back pressure is different per the ballast water level of the ballast tank, as is the case for the discharge head and the discharge flow rate of ballast pumps.
Also, a boundary condition is necessary for all numerical calculations. In this study, the ballast water level was used as the boundary condition because the discharge head and the discharge flow rate of ballast pumps are changeable as per the ballast water level of ballast tanks.
The discharge head and the discharge flow rate of pumps were calculated for each ballast water level, and thereafter, ballasting time was calculated by using the changed values of the ballast water level and the discharge flow rate of the pump.
In this study, the interval of ballast water level was assigned at 5 mm to calculate ballasting time, and the following procedures were applied for the calculation:

(1) Discharge head (P1) of pumps was assumed to be a certain value.

(2) The relevant discharge flow rate from the pumps’ performance curve was calculated by using the pumps’ assumed discharge head (P1).

(3) The pressure loss (DP1) from pumps to the bell mouth in the ballast tank was calculated on the basis of the pumps’ discharge flow rate.

(4) The back pressure (DP2) applicable to the ballast water level was calculated.

(5) If the calculated value of DP1 + DP2 – P1 was appropriate within the convergence condition, the process goes to the next step; otherwise, a new value for the pumps’ discharge head (P1) was assumed, and steps (2) to (4) are undertaken iteratively.

(6) The ballasting time of each interval was calculated by the use of the changed volume (DV) applicable to ballast water level, and the discharge flow rate of pumps converges as above.

(7) Total ballasting time was calculated by adding up each ballasting time at each interval.
The formula (6) was applied to the convergence condition for the calculation procedure (5).
where i is the iteration number of iterative calculation.
In principle, “DP1 + DP2  P1 < acceptable error” is correct as shown in calculation procedure (5), but when convergence condition is applied, it is not possible to converge when the discharge head of the pump is fixed at a constant value due to the flow control function. Therefore, formula (6) is applied as the convergence condition. Also, formula (6) is considered as same as the convergence condition of the calculation procedure (5) because the flow energy loss (DP1) depends on the discharge flow rate of the pump and the discharge flow rate of the pump is determined by the discharge head of the pump.
5. Calculation Results and Discussion
As described above, one reason for installing a flow control valve is to prevent the BWMS from taking in more seawater than its rated capacity. It is anticipated that there would be considerable differences between the case when only the flow energy loss from the installation of the UV type BWMS is considered and the case where the loss from the installation of both the UV type BWMS and the flow control valve are considered.
The purpose of this study is to provide informative data to the relevant parties by comparing ballasting time before and after the installation of a UV type BWMS based on calculation results. In this study, the following three cases were simulated for the calculation of ballasting time.

(1) Case 1: Ballasting time before the installation of a UV type BWMS

(2) Case 2: Ballasting time after the installation of a UV type BWMS without the application of the flow control function

(3) Case 3: Ballasting time after the installation of a UV type BWMS with the application of flow control function (only the function, without flow energy loss due to the flow control valve)
As described above, the rated capacity of the UV type BWMS installed on board in this study is 1,500 m^{3}/h. In the case of the flow control function, it is calculated that the pumps’ discharge flow rate is 1,500 m^{3}/h and their discharge head is 20.73 m if the pumps’ total discharge flow rate is calculated as 1,500 m^{3}/h as above.
Tables 3, 4 and 5 show the accumulated ballasting time, pump discharge flow rate and pump discharge head when the ballast water level of the ballast tanks changes from 0% to 100% in each case above.
Table 6 shows the comparison data of ballasting time when the ballast water level is at 100% in each case. Based on the simulation parameters and calculations used in the three cases described above, there is a 20.4% difference in ballasting time between the case before the installation of the UV type BWMS and after the installation of the UV type BWMS together with the flow control function.
In Table 5, the flow rate was limited to 1,500 m^{3}/h, and there is a difference in total head loss (11.90 m) and the pumps’ discharge head (20.73 m) with a 0% ballast water level. However, the total head loss is the same value of 20.73 m, which is the pumps’ discharge head, to meet condition (5) in section 4 above. This difference occurs because the flow energy loss arising from the flow control valve is not considered in this calculation.
Therefore, it is assumed that the flow energy loss from the flow control valve could be at least 8.83 m (=20.7311.90) if a flow control valve is installed in front of the UV type BWMS and the same simulation parameters and calculations are applied.
In a typical ship, the capacity of the ballast pump has a considerable margin. According to the experiences of authors, the shipyard’s general procedures for selecting ballast pumps are as follows:

(1) Calculate the total volume of all ballast tanks (V).

(2) Calculate the required flow rate (Q = V/t) of the ballast piping system with the total volume of the ballast tank and the required ballasting time (t, 24 hours in general).

(3) Calculate the required inner diameter of the ballast piping system with the reference speed (v, 2–4 m/s in general).

(4) Select a pipe size (ND, e.g., 300 A) with a larger inner diameter than the required inner diameter.

(5) Select a pump that is compatible with the required flow rate and selected pipe size.
While following the above procedures, the reference speed is set by shipyards empirically, taking into account the flow energy loss at the ballast piping system and the increase of back pressure per the rise of the ballast water level. The reference speed has enough margin and is significantly larger than actually required margin since the ballast pumps are selected as per the above procedures.
However, if a BWMS is selected without considering the actual ballast pumps’ operating conditions, ballasting time could significantly increase by the flow energy loss from the installation of the BWMS, additional ballast piping system, and flow control valve. Of particular note, the substantially simplified piping system was applied for this study to review the difference in ballasting time before and after installation of a UV type BWMS. It was found that this increased ballasting time by about 15–20%, as shown in Table 6, despite the application of a simplified piping system.
Also, a BWMS could be installed on the first or second deck in the engine room because it is difficult to secure enough space in the top of the double bottom tank. The flow energy loss could increase significantly more than the result of this study due to an additional ballast piping system with a very complex arrangement, since BWMS and additional ballast piping system should be installed in the engine rooms, bypassing existing equipment and pipelines.
Considering the result of this study and the same simulation parameters and calculations are applied, it is estimated that the ballasting time following the installation of a UV type BWMS could increase by at least 30% in comparison with the current ballasting time if actual conditions could be taken into account.
Especially, for bulk carriers and tankers, which need more ballast water than other types of ships, could encounter significant delays during their loading and unloading work due to this increase in ballasting and deballasting time.
And, the increase in deballasting time may differ from the increase in ballasting time depending on whether or not it needs to pass through the filter. The most of UV type BWMS do not use filters during the deballasting operation. Thus, in this study, the focus was given to finding the difference in ballasting time before and after the installation of UV type BWMS since the flow energy loss could significantly decrease due to bypassing filters during the deballasting operation. However, for some existing ships, the ballast water may need to bypass through the filter of BWMS when deballasting operation due to the practical reason.
Figure 4 shows a diagram of a general ballast piping system and the path of the ballast water during ballasting and deballasting operation. And, the filter for BWMS should be fitted at point “A” in Figure 4 to avoid the passing through the filter during deballasting operation. However, in most of existing ships, the valves in Figure 4 (V1, V2, V3 & V4) are fitted near the ballast pumps and the filter cannot be fitted at point “A” due to the lack of space. Hence, the filter and UV reactor of UV type BWMS should be installed in other spaces of the engine room and the ballast water may be passing through the filter during deballasting operation. Furthermore, the installation of additional pipes and valves may prevent passing through the filter, but, this work requires additional cost and special consideration for actual operation.
And, the increase in deballasting time may differ from the ballasting time although the ballast water passes through the filter during deballasting operation because the water velocity may differ from water velocity during the ballasting operation.
6. Conclusion
In conclusion, this study shows that the concerned parties should consider the capacity of existing ballast pumps when installing UV type BWMS on ships to minimize the increase in ballasting time by taking into account the existing ballast pump’s performance and the flow energy loss arising from the installation of UV type BWMS and an additional piping system.
In particular, this study has found that the ballasting time following the installation of UV type BWMS increased by about 20% based on the sample calculation carried out using the specification of 55,000 DWT class gas carrier and its existing ballast pump. And, it has been estimated that the ballasting time following the installation of a UV type BWMS could increase by at least 30% in comparison with the existing ballasting time if actual conditions could be taken into account.
In addition, various measures are needed to minimize the increase in ballasting time due to BWMS installation. First, it is recommended that the BMWS with the capacity that is larger than the capacity of the ballast pump is selected. This would reduce the increase in the ballasting time because the flow control valve may not need to be installed and the flow energy loss occurring in the BWMS will be reduced because the flow rate passing through the BWMS is reduced.
Based on the calculation results, it was found that the ballasting time of 55,000 DWT Gas Carrier was 2.152 hours without considering the installation of UV type BWMS and flow control function. The ballasting time was increased to 2.457 hours after the installation of UV type BWMS and it was increased to 2.591 hours considering both its installation and flow control function. The ballasting time was increased by 14.2% after its installation and it was increased by 20.4% considering both its installation and flow control function.