Fields of Manuscript

Journal of Advanced Marine Engineering and Technology - Vol. 48 , No. 4

[ Original Paper ]
Journal of Advanced Marine Engineering and Technology - Vol. 48, No. 4, pp. 198-206
Abbreviation: JAMET
ISSN: 2234-7925 (Print) 2765-4796 (Online)
Print publication date 31 Aug 2024
Received 24 Jul 2024 Revised 09 Aug 2024 Accepted 19 Aug 2024
DOI: https://doi.org/10.5916/jamet.2024.48.4.198

Bilge pumping performance for cargo holds of large container ships
Kyoung Woo Lee1 ; Dong Su Kim2 ; Kang Woo Chun
1President, Team Sol Inc. (kerry@temsolution.co.kr)
2Ph. D. Candidate, Division of Marine System Engineering, Korea Maritime & Ocean University (ehdtn05@gmail.com)

Correspondence to : Professor, Division of Marine System Engineering, Korea Maritime & Ocean University, 727, Taejong-ro, Yeongdo-gu, Busan 49112, Korea, E-mail: kwchun@kmou.ac.kr, Tel: +82-51-410-4289


Copyright © The Korean Society of Marine Engineering
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Funding Information ▼

Abstract

This study aims to improve the fire safety of container ships by examining the bilge pumping performance necessary to prevent the formation of free surfaces in cargo holds when a water spray system is introduced as an active protection system on hatch covers. The study calculates whether free surfaces would occur in a cargo hold equipped with a water spray system using the bilge pumping system installed on 24,000 TEU-class container ships. The calculation results indicate that the free surface could not be prevented in all cases when only one bilge pump was operated. Even with the operation of two bilge pumps, free surfaces were still observed in all cargo holds except for the No.1 cargo hold. Design modifications, such as increasing the size of the branch bilge pipe, are necessary to prevent free surfaces in the cargo holds. Therefore, it is strongly recommended that when installing a water spray system as an active protection system on hatch covers, the bilge pumping system's ability to prevent free surfaces in the cargo holds of each individual container ship be thoroughly examined.


Keywords: Container ship fire safety, Hatch cover protection, Water spray system, Bilge pumping performance, Free surface

1. Introduction

Fire accidents are among the most fatal incidents that occur on ships, prompting extensive research to mitigate their impact [1][2].

According to the International Union of Marine Insurance (IUMI), between 2000 and 2015, 56 cargo fires on container-ships resulted in an average cost of approximately $20 million per incident, totaling over $1.037 billion in damages (excluding hull damages). Hull damages account for an average of at least $6.5 million per cargo fire incident, with total costs exceeding $0.5 billion between 2000 and 2019 [3].

Marshall Islands et al. and Bahamas et al. proposed measures to improve fire safety in container ship cargo areas, such as re-viewing related requirements to reduce fire risks in cargo areas. Related matters have been discussed since the eighth session of the Sub-Committee on Ship Systems and Equipment (SSE Sub-Committee) [4]-[6].

The European Maritime Safety Agency (EMSA) ordered the CARGOSAFE project, a formal safety assessment (FSA) study on fire safety in container ship cargo areas, and the CARGO-SAFE report was submitted to the 107th session of the Maritime Safety Committee (MSC) [7].

In the CARGOSAFE report, risk control options (RCOs) were categorized into four groups: “Prevention,” “Detection,” “Firefighting,” and “Containment”. A cost-effectiveness assess-ment (CEA) was conducted, leading to the recommendation of two RCOs for each category, as shown in Table 1 [8].

Table 1: 
RCO recommendations across all 3 ship sizes per layer of protection (Table 92 of CARGOSAFE Report)
Fire Mitigation Phase Prevention Detection Firefighting Containment
1st RCO Improved Control of Lashing Portable IR cameras for crew to enhance manual detection Methods for unmanned fire fighting Passive protection to protect from fire spread towards the deck
2nd RCO Container Screening Tool Heat detection looking at individual container temperature rise Manual firefighting tools that increase reach Active protection underneath hatch covers to protect from fire spread towards the deck

In Table 1, “Containment” refers to preventing fires originat-ing in the cargo hold of a container ship from spreading to the open deck area. A60 insulation was recommended as a passive protection RCO, while a water spray system was recommended as an active protection system.

At the 10th session of the SSE Sub-Committee, held in March 2024, the CARGOSAFE report and previously submitted agenda documents were reviewed. It was decided that active hatch cover protection would be discussed further at the 11th SSE Sub-Committee [9]-[13].

Although the seawater flow rate to be supplied by the active protection system featuring a water spray system on the hatch covers has not yet been determined, it is anticipated that the exist-ing SOLAS Regulation II-2/19.3.1.3 will apply.

This regulation currently applies to cargo holds storing class 1 dangerous goods (explosive substances and articles). If the active protection system is introduced, this requirement is expected to be extended to all cargo holds.

SOLAS Regulation II-2/19.3.1.3 outlines not only the sea-water flow rate for the water spray system but also the capacity of the bilge pumping system to effectively discharge the supplied seawater outside the ship. Before applying the water spray sys-tem to all cargo holds, the performance of the bilge pumping system must be reviewed in accordance with SOLAS Regulation II-2/19.3.1.3.

Lee et al. and Seo et al. conducted studies on the performance of bilge pumping systems. They found that, due to logical con-tradictions in the rule requirements of classification societies, the water speed requirement of 2 m/s in the main bilge pipe, as speci-fied in SOLAS Regulation II-1/35-1, is not met in large container ships and bulk carriers [14]-[16].

Given that the seawater flow rate specified in SOLAS Regula-tion II-2/19.3.1.3 is set at 5 L/min per square meter—a relatively high flow rate—the performance of bilge pumping systems in cargo holds requires careful review.


2. Regulations for Bilge Pumping System

The bilge pumping system, which is crucial to ship stability, is one of the most important safety systems on a vessel. The re-quirements for this system are outlined in SOLAS Regulation II-1/35-1, with the key provision being that the water speed in the bilge main should exceed 2 m/s. However, according to the re-search results of Lee et al. and Seo et al., there are cases where the water speed at the bilge main does not satisfy the 2 m/s re-quirement for large container ships and bulk carriers because of logical contradictions in the rule requirements of classification societies [14]-[16].

The failure of the bilge pumping system to meet the 2 m/s re-quirement indicates that its performance is insufficient, prevent-ing the effective discharge of seawater from lower compartments, such as cargo holds.

Therefore, before introducing the water spray system require-ments specified in SOLAS Regulation II-2/19.3.1.3 as an active protection system on hatch covers, it is necessary to review the bilge pumping performance.

Table 2 presents the specifications outlined in SOLAS Regu-lation II-2/19.3.1.3, detailing the required seawater flow rate for the water spray system and the performance criteria for the bilge pumping system. A detailed review is particularly necessary to ensure adequate bilge pumping performance to prevent the for-mation of free surfaces, especially in light of the research find-ings by Lee et al. and Seo et al [14]-[16].

Table 2: 
SOLAS Regulation II-2/19.3.1.3
19.3.1.3 Means shall be provided for effectively cooling the designated underdeck cargo space by at least 5 litres/min per square metre of the horizontal area of cargo spaces, either by a fixed arrangement of spraying nozzles or flooding the cargo space with water. Hoses may be used for this purpose in small cargo spaces and in small areas of larger cargo spaces at the discretion of the Administration. However, the drainage and pumping arrangements shall be such as to prevent the build-up of free surfaces. The drainage system shall be sized to remove no less than 125% of the combined capacity of both the water spraying system pumps and the required number of fire hose nozzles. The drainage system valves shall be operable from outside the protected space at a position in the vicinity of the extinguishing system controls. Bilge wells shall be of sufficient holding capacity and shall be arranged at the side shell of the ship at a distance from each other of not more than 40 m in each watertight compartment. If this is not possible, the adverse effect upon stability of the added weight and free surface of water shall be taken into account to the extent deemed necessary by the Administration in its approval of the stability information.


3. Bilge Pumping Performance

The bilge pumping performance can be determined by how quickly seawater is discharged from a flooded compartment to the outside of the ship. In line with this, SOLAS Regulation II-1/35-1 stipulates that the water speed at the bilge main must be at least 2 m/s.

Various methods exist for evaluating bilge pumping perfor-mance; however, in this study, the concept shown in Figure 1 was applied, following the approach used in the studies by Lee et al. and Seo et al. [14]-[16].


Figure 1: 
Schematic of bilge pumping system [14]-[16]

As illustrated in Figure 1, to effectively discharge seawater from the compartment to the outside of the ship, the combined suction head of the bilge pump and the head of the accumulated seawater, due to the water level in the compartment, must exceed the flow energy loss within the bilge piping system. Therefore, to assess bilge pumping performance, it is essential to evaluate the flow energy loss in the bilge piping system.

In this study, friction loss was assessed using the Darcy-Weisbach equation, as shown in Equation (1), which is one of the various methods for evaluating flow energy loss [17]-[19]. The friction factor required for this calculation was determined using the Colebrook-White equation, as in Equation (2), rather than relying on the Moody chart [20][21].

P=fL+LEDρV22(1) 
1f=-2log ϵ3.7D+2.5Ref (2) 

Here, P represents flow energy loss (Pa), f is the friction loss coefficient, L and LE denote the length of pipe (m) and equivalent length of valves and fittings (m), respectively, D is the internal diameter of pipe (m), ρ is the fluid density (kg/m3), V is the velocity of the fluid in the pipe (m/s), and ε is pipe wall roughness (m). The pipe wall roughness, required to calculate the friction factor in Equation (2), was set to 1.0 mm, consistent with the values used in the studies by Lee et al. and Seo et al. [14]-[16].

The flow energy loss from the pipe fittings and valves is calcu-lated using the equivalent length method, as specified in NFPA Code 13 and shown in Table 3. However, Table 3 provides equivalent lengths only for pipes up to 300 A, while 400 A pipes are used in the bilge pumping system of 24,000 TEU-class con-tainer ships. Because this study aims to compare the flow rate between cargo holds, the results are valid as long as the same conditions are applied across all cargo holds. Therefore, the equivalent length values for 300 A pipe fittings and valves are applied to the 400 A pipe fittings and valves [22].

Table 3: 
Equivalent schedule of the 40 steel pipe length chart (NFPA Code 13, p.13-237, Table 23.4.3.1.1)
Fittings and Valves Fittings and Valves Expressed in Equivalent Meter of Pipe
15mm 20mm 25mm 32mm 40mm 50mm 65mm 80mm 90mm 100mm 125mm 150mm 200mm 250mm 300mm
45° elbow - 0.3 0.3 0.3 0.6 0.6 0.9 0.9 0.9 1.2 1.5 2.1 2.7 3.4 4.0
90° standard elbow 0.3 0.6 0.6 0.9 1.2 1.5 1.8 2.1 2.4 3.0 3.7 4.3 5.5 6.7 8.2
90° long-turn elbow 0.2 0.3 0.6 0.6 0.6 0.9 1.2 1.5 1.5 1.8 2.4 2.7 4.0 4.9 5.5
Tee or cross (flow turned 90°) 0.9 1.2 1.5 1.8 2.4 3.0 3.7 4.6 5.2 6.1 7.6 9.1 10.7 15.2 18.3
Butterfly valve - - - - - 1.8 2.1 3.0 3.7 2.7 3.0 3.7 5.8 6.4
Gate valve - - - - - 0.3 0.3 0.3 0.3 0.6 0.6 0.9 1.2 1.5 1.8
Swing check - - 1.5 2.1 2.7 3.4 4.3 4.9 5.8 6.7 8.2 9.3 13.7 16.8 20.0

The density and viscosity of seawater are essential for evaluating flow energy loss, using Equations (1) and (2), respectively. Although these conditions can vary, the study uses the physical properties of standard seawater at 20°C, as provided by the International Towing Tank Conference (ITTC). The applied density and viscosity are 1,024.8103 kg/m3 and 0.001077 Pa∙s, respectively [23].


4. Bilge Pumping System of 24,000 TEU-class Container Ships

In this study, calculations were performed using the specifica-tions of the bilge pumping system for cargo holds of 24,000 TEU-class container ships to determine whether the bilge pump-ing system meets the bilge pumping requirements of SOLAS Regulation II-2/19.3.1.3. A bilge pumping system with the same specifications as those used in the study by Seo et al. was utilized [16]. The hull shape of the fore side of the container ship changes rapidly toward the bow, as shown in Figure 2, causing the width of the cargo holds to narrow. The volume and depth of each car-go hold are presented in Table 4.


Figure 2: 
Schematic arrangement of cargo holds [16]

Table 4: 
Specifications of cargo holds [16]
Cargo Hold Net Volume (m3) Depth (m) Average Horizontal Area (m2) (Volume/Depth)
No.1 41,526.4 31 1339.56
No.2 47,151.0 31 1521.00
No.3 51,099.2 31 1648.36
No.4 52,365.5 31 1689.21

Figure 3 shows a schematic of the bilge pumping system in-stalled on 24,000 TEU-class container ships, and the detailed specifications for each cargo hold are shown in Tables 5, 6, 7, and 8.


Figure 3: 
Schematic of bilge pumping system [16]

Table 5: 
Arrangement of bilge pumping system (No.1 C/H) [16]
Node Node ND (Sch.40) Length of pipe (m) Fittings and Valves
100 101 400A 1 Check Valve
101 102 400A 2 Butterfly Valve
102 110 400A 246.5
110 111 200A 2 Butterfly Valve
111 112 200A 2 Check V/V, Elbow

Table 6: 
Arrangement of bilge pumping system (No.2 C/H) [16]
Node Node ND (Sch.40) Length of pipe (m) Fittings and Valves
100 101 400A 1 Check Valve
101 102 400A 2 Butterfly Valve
102 103 400A 218
103 110 150A 10 Tee (branch)
110 111 150A 2 Butterfly Valve
111 112 150A 2 Check V/V, Elbow
103 120 150A 10 Tee (branch)
120 121 150A 2 Butterfly Valve
121 122 150A 2 Check V/V, Elbow

Table 7: 
Arrangement of bilge pumping system (No.3 C/H) [16]
Node Node ND (Sch.40) Length of pipe (m) Fittings and Valves
100 101 400A 1 Check Valve
101 102 400A 2 Butterfly Valve
102 103 400A 188
103 110 150A 17.2 Tee (branch)
110 111 150A 2 Butterfly Valve
111 112 150A 2 Check V/V, Elbow
103 120 150A 17.2 Tee (branch)
120 121 150A 2 Butterfly Valve
121 122 150A 2 Check V/V, Elbow

Table 8: 
Arrangement of bilge pumping system (No.4 C/H) [16]
Node Node ND (Sch.40) Length of pipe(m) Fittings and Valves
100 101 400A 1 Check Valve
101 102 400A 2 Butterfly Valve
102 103 400A 141.5
103 110 150A 19.7 Tee (branch)
110 111 150A 2 Butterfly Valve
111 112 150A 2 Check V/V, Elbow
103 120 150A 19.7 Tee (branch)
120 121 150A 2 Butterfly Valve
121 122 150A 2 Check V/V, Elbow

Figure 4 shows the performance curve of a bilge pump in-stalled on a 24,000 TEU-class container ship. Two power bilge pumps were installed on the ship. The suction head of each bilge pump was calculated using Equation (3) through curve fitting of the performance curve.


Figure 4: 
Bilge pump performance curve [16]

Tables 9, 10, 11, and 12 show the research results of Seo et al., which indicates the calculation of the average speed while discharging all the seawater within the cargo hold, assuming that each cargo hold is fully filled with seawater [16]. The average water speed at the bilge main did not satisfy the 2 m/s require-ment of SOLAS Regulation II-2/35-1, except for No.1 cargo hold. In addition, when the water level in the cargo hold is low, all cargo holds fail to satisfy the 2 m/s requirement.

SH=6.20183×10-7Q2+0.00142Q+1.65441(3) 
Table 9: 
Calculation results for No.1 cargo hold (actual arrangement, 400A-200A) [16]
Water Level Flow Rate (m3/h) Water Velocity at Bilge Main (m/s) Pump Suction Head (SH, m) Flow Energy Loss (dH, m) Static Head by Water Level (LH, m)
0% 321.63 0.758 2.18 2.18 0
10% 519.74 1.225 2.56 5.66 3.1
20% 657.88 1.55 2.86 9.06 6.2
30% 770.57 1.816 3.12 12.42 9.3
40% 868.18 2.046 3.35 15.75 12.4
50% 955.51 2.252 3.58 19.08 15.5
60% 1035.26 2.44 3.79 22.39 18.6
70% 1109.11 2.614 3.99 25.69 21.7
80% 1160.00 2.734 4.14 28.10 24.8
90% 1160.00 2.734 4.14 28.10 27.9
100% 1160.00 2.734 4.14 28.10 31.0
Mean Velocity (m/s) 2.121

Table 10: 
Calculation results for No.2 cargo hold (actual arrangement, 400A-150A) [16]
Water Level Flow Rate (m3/h) Water Velocity at Bilge Main (m/s) Pump Suction Head (SH, m) Flow Energy Loss (dH, m) Static Head by Water Level (LH, m)
0% 258.98 0.610 2.06 2.06 0.00
10% 422.31 0.995 2.36 5.46 3.10
20% 536.10 1.263 2.59 8.79 6.20
30% 628.92 1.482 2.79 12.09 9.30
40% 709.30 1.672 2.97 15.37 12.40
50% 781.22 1.841 3.14 18.64 15.50
60% 846.89 1.996 3.30 21.90 18.60
70% 907.70 2.139 3.45 25.15 21.70
80% 964.59 2.273 3.60 28.40 24.80
90% 1018.24 2.400 3.74 31.64 27.90
100% 1069.15 2.520 3.88 34.88 31.00
Mean Velocity (m/s) 1.766

Table 11: 
Calculation results for No.3 cargo hold (actual arrangement, 400A-150A) [16]
Water Level Flow Rate (m3/h) Water Velocity at Bilge Main (m/s) Pump Suction Head (SH, m) Flow Energy Loss (dH, m) Static Head by Water Level (LH, m)
0% 240.97 0.568 2.03 2.03 0.00
10% 393.98 0.928 2.31 5.41 3.10
20% 500.56 1.180 2.52 8.72 6.20
30% 587.49 1.384 2.70 12.00 9.30
40% 662.77 1.562 2.87 15.27 12.40
50% 730.13 1.721 3.02 18.52 15.50
60% 791.63 1.866 3.17 21.77 18.60
70% 848.58 2.000 3.31 25.01 21.70
80% 901.86 2.125 3.44 28.24 24.80
90% 952.10 2.244 3.57 31.47 27.90
100% 999.77 2.356 3.69 34.69 31.00
Mean Velocity (m/s) 1.650

Table 12: 
Calculation results for No.4 cargo hold (actual arrangement, 400A-150A) [16]
Water Level Flow Rate (m3/h) Water Velocity at Bilge Main (m/s) Pump Suction Head (SH, m) Flow Energy Loss (dH, m) Static Head by Water Level (LH, m)
0% 237.95 0.561 2.03 2.03 0.00
10% 389.18 0.917 2.30 5.40 3.10
20% 494.52 1.165 2.51 8.71 6.20
30% 580.43 1.368 2.69 11.99 9.30
40% 654.84 1.543 2.85 15.25 12.40
50% 721.41 1.700 3.00 18.50 15.50
60% 782.19 1.843 3.14 21.74 18.60
70% 838.48 1.976 3.28 24.98 21.70
80% 891.14 2.100 3.41 28.21 24.80
90% 940.80 2.217 3.54 31.44 27.90
100% 987.91 2.328 3.66 34.66 31.00
Mean Velocity (m/s) 1.630


5. Calculation and Review

The studies by Lee et al. and Seo et al. assumed that the cargo hold is filled with seawater and calculated the average water speed at the bilge main while discharging all seawater in the car-go hold, as it is not possible to predict how much seawater will flow into the cargo hold during flooding. The purpose of this study is to determine whether the bilge pumping system can pre-vent the formation of free surfaces in cargo holds when the water spray system is activated. Additionally, if a free surface occurs, this study aims to assess the level of seawater accumulated in the cargo holds. Therefore, the calculations were carried out accord-ing to the procedure shown in Figure 5 [14]-[16].


Figure 5: 
Calculation Procedure

The amount of seawater supplied to the cargo holds should be calculated in accordance with SOLAS Regulation II-2/19.3.1.3 to determine whether a free surface occurs during the operation of the bilge pumping system and to identify the water level when the free surface does occur. The amount of seawater supplied to the cargo hold is 125% of the combined capacity of the flow rate of 5 L/min per square meter of the horizontal area of the cargo holds and the required number of fire hose nozzles, in accordance with SOLAS Regulation II-2/19.3.1.3.

The amount of seawater supplied to the cargo holds by the wa-ter spray system varied depending on the horizontal area of the cargo holds, which varied with the depth according to the hull shape of the fore and aft sections of the container ship.

In this study, considering that the horizontal area of the cargo hold was not constant, the value obtained by dividing the total volume by the depth was applied as the average horizontal area. The value is the average horizontal area in Table 4.

The flow rate should be determined based on the number of fire hoses required. Because container ships generally carry dan-gerous goods, four jets are required for fire hoses. However, the flow rate of a single jet is not specified for the SOLAS and IMO instruments and the flow rate may vary for each container ship. In this study, by referring to IACS UI SC 163 Rev.2, the flow rate of one jet was determined as 23.5 m3/h [24].

Table 13 presents the combined capacity for each cargo hold of a 24,000 TEU-class container ship, considering the water spray system and the required number of fire hoses, in accord-ance with SOLAS Regulation II-2/19.3.1.3.

Table 13: 
Required capacity of seawater when fire occurred in cargo holds
Cargo Hold Average Horiz. Area (m2) Capacity of Water Spray System (m3/h) Capacity of 4 Jets (m3/h) 125% of Combined Capacity (m3/h)
1 1339.56 401.87 94.0 619.84
2 1521.00 456.30 94.0 687.88
3 1648.36 494.51 94.0 735.64
4 1689.21 506.76 94.0 750.95

The calculation results of the accumulated water level for each cargo hold are listed in Tables 14, 15, 16, and 17. In these cases, the required amount of seawater, as shown in Table 13, was supplied to the cargo hold, and the bilge pumping system was operated to discharge the seawater from the cargo hold to the outside of the ship.

Table 14: 
Calculation results for No.1 cargo hold
No. of Bilge Pumps 1 2 1 2
Size of Branch Bilge Pipe 200A 200A 250A 250A
Water Level for Free Surface (m) 5.27 0.00 1.52 0.00
Water Level for Free Surface (%) 17.00% 0.00% 4.91% 0.00%
Flow Rate of Each Pump (m3/h) 619.84 321.63 619.84 468.14
Flow Rate at Bilge Main (m3/h) 619.84 643.25 619.84 936.28
Water Speed at Bilge Main (m/s) 1.46 1.52 1.46 2.21

Table 15: 
Calculation results for No.2 cargo hold
No. of Bilge Pumps 1 2 1 2
Size of Branch Bilge Pipe 150A 150A 200A 200A
Water Level for Free Surface (m) 11.54 1.41 2.53 0.00
Water Level for Free Surface (%) 37.21% 4.56% 8.18% 0.00%
Flow Rate of Each Pump (m3/h) 687.88 343.94 687.88 458.69
Flow Rate at Bilge Main (m3/h) 687.88 687.88 687.88 917.39
Water Speed at Bilge Main (m/s) 1.62 1.62 1.62 2.16

Table 16: 
Calculation results for No.3 cargo hold
No. of Bilge Pumps 1 2 1 2
Size of Branch Bilge Pipe 150A 150A 200A 200A
Water Level for Free Surface (m) 15.77 2.46 3.49 0.00
Water Level for Free Surface (%) 50.86% 7.93% 11.26% 0.00%
Flow Rate of Each Pump (m3/h) 735.64 367.82 735.64 446.23
Flow Rate at Bilge Main (m3/h) 735.64 735.64 735.64 892.45
Water Speed at Bilge Main (m/s) 1.73 1.73 1.73 2.10

Table 17: 
Calculation results for No.4 cargo hold
No. of Bilge Pumps 1 2 1 2
Size of Branch Bilge Pipe 150A 150A 200A 200A
Water Level for Free Surface (m) 16.97 2.75 3.47 0.00
Water Level for Free Surface (%) 54.76% 8.88% 11.18% 0.00%
Flow Rate of Each Pump (m3/h) 750.95 375.48 750.95 457.29
Flow Rate at Bilge Main (m3/h) 750.95 750.95 750.95 914.58
Water Speed at Bilge Main (m/s) 1.77 1.77 1.77 2.16

In this study, calculations were performed separately for sce-narios where one bilge pump was operating and where both bilge pumps were operating. These calculations were based on the installed branch bilge pipe as well as the branch bilge pipe of the next larger size.

Upon reviewing the calculation results, it was confirmed that the operation of a single bilge pump could not prevent the for-mation of a free surface in all cargo holds.

The occurrence of a free surface is depicted in Figure 1, where the sum of the water head (LH) due to the water level in the cargo hold and the pump suction head (SH) has the same value as the flow energy loss (dH) occurring in the bilge piping system. For instance, in the No.4 cargo hold, a free surface formed at 54.76% of the cargo hold depth; however, when the water level was low-er than 54.76%, dH exceeded the sum of LH and SH, indicating that the bilge pump could not discharge the seawater supplied to the cargo hold (via the water spray system and number of jets) to the outside of the ship.

Additionally, even when two bilge pumps were operated, the formation of a free surface could not be prevented in the case of the currently installed branch bilge pipe (150 A), except in the No.1 cargo hold. To address this issue, two bilge pumps should be operated, and a branch bilge pipe of larger diameter than the one currently installed should be used, in accordance with the rule requirements of ship classification societies, to prevent the formation of a free surface.

While the prevention of free surfaces, as required by SOLAS Regulation II-2/19.3.1.3, is intended to prevent accidents caused by sloshing, it is important to note that in most cases, the cargo holds of container ships are always loaded with containers, re-ducing the likelihood of accidents such as hull damage caused by sloshing.

Therefore, it is necessary to evaluate whether allowing a free surface up to a certain water level is acceptable or if it is more reasonable to prevent free surfaces through design modifications to the bilge pumping system. Considering the calculation results, because the size of the branch bilge pipe—part of the bilge pump-ing system—needs to be increased by only one step, it is reason-able to uphold the free surface prevention requirement with min-imal design modifications for 24,000 TEU-class container ships.

Furthermore, similar cases are anticipated for container ships of other sizes, where it is believed that free surface prevention can also be achieved with minor design modifications, such as increasing the size of the main, common, or branch bilge pipes.

However, bilge pumping performance is affected by flow en-ergy loss. This study did not include data on flow energy loss related to the strainer fitted on the pump inlet, nor did it account for the mud box at the end of the branch bilge pipe. Future stud-ies would incorporate these factors to provide a more compre-hensive assessment of bilge pumping performance.


6. Conclusion

In this study, a review of the free surfaces in cargo holds was conducted using the bilge pumping system installed on 24,000 TEU-class container ships, along with the introduction of a water spray system in accordance with SOLAS Regulation II-2/19.3.1.3. This regulation serves as an active protection system for hatch covers to enhance the fire safety of container ships. The findings of this study are as follows.

  • (1) Two bilge pumps should be operated when operating the water spray system because a free surface cannot be pre-vented when only one bilge pump is in operation.
  • (2) Even when two bilge pumps were operated, a free sur-face occurred in cargo holds No. 2, No. 3, and No. 4 with the currently installed bilge pumping system. To prevent this free surface, design modifications, such as increasing the size of the branch bilge pipe, were required.
  • (3) When the free surface occurred, the water speed at the bilge main did not satisfy the 2 m/s requirement of SO-LAS Regulation II-1/35-1.

Considering the calculation results, this study found that when a water spray system is installed on the hatch cover as an active protection system in accordance with SOLAS Regulation II-2/19.3.1.3, it is necessary to evaluate whether the bilge pumping system can effectively prevent the formation of a free surface in the cargo holds of each container ship.


Acknowledgments

This paper was supported by the Ministry of Oceans and Fisheries of Korea through the Korea Institute of Marine Science and Technology Promotion (KIMST) Grant No. 20220603.

Author Contributions

Conceptualization, K. W. Lee, D. S. Kim, and K. W. Chun; Methodology, K. W. Lee, D. S. Kim, and K. W. Chun; Validation, K. W. Lee, D. S. Kim, and K. W. Chun; Resources, K. W. Lee, D. S. Kim, and K. W. Chun; Writing - Original Draft Preparation, K. W. Lee and D. S. Kim; Writing - Review and Editing, K. W. Chun.


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