Optimizing Nile Barge Design for Enhanced Efficiency and Capacity

Abstract

This paper has demonstrated the design aspects for inland water units working in the River Nile. The acting loads and stresses induced with in the structural elements of Nile barges are still water bending moment and stresses, local hydrostatic pressure and local stresses. A rational analysis procedure is developed and applied. This approach can end in decreasing the light-weight. Accordingly, this weight will increase the cargo capacity in the Nile barges which can economically have an effect on all terms of transportation business.

Introduction

Regarding inland navigation vessels various design aspects ought to be taken under consideration such as cargo capacity, speed and environment. The design aspects which can be thought-about as a priority vary from one place to another according to various considerations. In Bangladesh, some recommendations were proposed by optimizing the hull form of some existing vessels to scale back the consequences of CO2 emission. In Republic of Poland an enquiry was created concerning the long run of inland water transport relating to the fuel consumption by inland waterway.

It was found that inland water transportation has a lower saving in fuel consumption for the same amount of cargo than that used for other modes of transportations. The expected total sum of inland water transportation by 2027 will be 2.57 million TEU per year.

Basic design includes the choice of ship dimensions, hull form, power, preliminary arrangement of hull and machinery, and major structure. Proper choices assure the attainment of the mission necessities like sensible seakeeping performance, manoeuvrability, the desired speed, endurance, loading capability, and deadweight.

The procedures of design are illustrated with in the Design Spiral, Evans (1959).

In this paper, a redesign of the Nile barges on the basis of the rational structural analysis is applied. The rational structural analysis includes the elastic and plastic stresses analyses of a unit. It composed from different stages of analyses such as primary, secondary and tertiary analysis in addition to the plastic analysis. Applied this approach on an existing barge can result in decrease the building materials that needed for this unit.

Structural Analysis for Inland Water Units

The hull structure has been planned originally by the designer with in an approach that the structure will effectively resist the probable most load which is estimated from previous expertise of failure modes. Therefore, once damage occurs in the structure, it indicates the subsequent facts:

• The magnitude of the load causing the failure is larger than or equals the expected load.
• The particular failure takes place in a very totally different mode from the presumed one.

The following design procedures are adopted in order to consider the structural strength estimation:

• Determine an initial stiffening system of structural members.
• Presume a magnitude, direction and probability of load.
• Assume failure mode of structure due to the load.

There are many failure modes knowledgeable in inland water units. The following modes are significant when dealt with the structural analysis:

• Yielding
• Buckling
• Fatigue
• Material deterioration due to corrosion

When the load applied to an explicit structure exceeds a certain critical value, the elongation would increase rapidly. This elongation is termed yielding. The designer sometimes takes care to take care of the strength of the structure therefore as to not exceed the yielding point.

If a structure is subjected to a compressive load, it should suddenly deflect once the load reaches a critical value. This critical value is called buckling point. When a large deflection takes place, the structure might not recover its original form even once the load is removed.

The structure may be fractured by small loads when they are repeated regularly .The fracture happens because of fatigue. Fatigue sometimes occur from lower loads than yielding strength, especially where the number of cycles is very large. That type of fracture is sometimes caused by vibration, especially if the frequency is very high. Fatigue doesn't have a great effect in the case of Rive Nile units because there are no waves, so the structure is subjected only to cyclic loads resulted from the machinery which could be neglected in that case.

It is the gradual destruction of fabric by chemical reaction with their surroundings like sea water. This must be considered as a mode of failure. Deterioration is not expected to be severe for Nile barges because the vessel is working in fresh water.

Ship structural members are subjected to many forms of stresses evoked by external and internal loads. The hull girder loadings induce stresses referred to as primary stresses in the primary strength members. Strength members of ship structure assemblies are subjected to cargo and external water pressure loadings which induce stresses called secondary stresses. Tertiary strength members are subjected to tertiary loadings which induce a third type of stresses called tertiary stresses. The bottom plating is subjected to further local loadings exerted by the external hydraulics water pressure.

The combination of those stresses for a few strength members may reach unacceptable high values of equivalent stresses which can exceed the allowed stresses of the material and cause structural failure.

Hull Girder Bending Stress Components

The hull girder primary stresses are evoked by the subsequent bending moment components:

Stresses due to longitudinal vertical bending moments:

1. Still water bending stresses
2. Wave-induced bending stresses (sagging /hogging conditions)
3. Dynamic stresses

Stresses due to horizontal bending moment:

Wave-induced bending stresses, dynamic stresses and stresses caused by horizontal bending moment don’t affect the structure in the case of River Nile units, because River Nile don't have waves. Still water bending stress is that the solely stress which will be taken into consideration during this case.

The secondary moments and shear forces are the reason for appearing secondary stresses that affects the vessel structural members. These loadings could be determined by analyzing a specified length of the secondary structure assembly like a hold length. Structure assembly comprising a full hold length plus half hold length aft and forward of the selected hold. An example is given for the case when the full hold length is fully loaded and the two half hold lengths are empty. Secondary stresses doesn’t exist during this case as a result of most of the inland water units have only 2 cargo holds. It is illogical to fill one hold with cargo and leave the other hold empty.

In bottom structures, the strength members sustaining the tertiary stresses are the bottom and tank top longitudinals and plating. The following are examples of tertiary stresses of analyses.

Each bottom longitudinal is subject to a load that is cover a rectangular area of length “a” equal to the longitudinal length between two floors, and breadth “b” equal to the sum of the half-spaces between two adjacent longitudinals or between a longitudinal and the adjacent girder. Bottom longitudinals are also subjected to in-plane normal loadings exerted on the longitudinal cross section. These normal tensile or compressive stresses are evoked by hull girder and secondary bending moments.

The bending stress at the attached bottom plating is given by:

σP = (2)

The hydrostatic pressure exerted on the outer shell represents the most local loading on bottom longitudinals. The design loads are defined in terms of bending moments and shear forces and might be calculated by the simple beam theory.

The foremost common method of calculating the strength of ship structure relies on the calculation of each longitudinal and local stresses. The stresses affecting each ship structural member should be estimated according to the compounded stresses imposed on them by the longitudinal bending action of the ship hull girder and the induced bending under the local loads.

Plastic Structural Analysis of Inland Water Units

In the plastic design method of a structure, the whole structure is affected by the yield stress. The plastic stress has occurred because of the actual fact that the ultimate load is found from the strength of steel within the plastic range. The strength of steel beyond the yield stress is totally used during this methodology. It provides placing economy concerning the weight of steel since the sections designed by this method are smaller in size than those designed by the elastic design method.

In reality the exact analysis of a structure can never be carried out. Idealizing a structure is a method of conservatively simplifying the components of the structural system, while keeping the same behavior under loading the same. This is done in order to simplify calculations. Without an idealized structure, design could take a massively longer time.

The Neutral axis (NA) is associated with the fiber that does not undergo change in its length when an element bends and stresses are all less than yield stress. Equal area axis (EAA) is the location of the axis which results in equal compressive and tensile forces when all fibers in a section have reached yield stress.

The following particulars are as follows:

LBP = 67.735 m

LOA = 70 m

LWL = 68.985 m

Breadth = 10 m

Depth =2.8 m

Draft = 1.8 m

CB = 0.899

Light weight = 323.6 ton

In this section, the primary hull girder analysis on the basis of the design rules and a MAXSURF Software is calculated for the case study. The still water bending moment is calculated according to BV rules of the barge is as follows.

MS = 0.273 L2 B1.34 D0.15(1-CB) KN.m (7)

The still water bending moment is = 22302.9 KN.m

The still water bending moment by using MAXSURF software is calculated as follows. Here, the still bending moment is calculated according to three different loading conditions for the barge. The highest value of the still water bending moment will be in the fully loaded condition.

Mst= 6155.368 KN

It is noticed that the acting still water moment of the barge is (6155.368 KN), which is less than that calculated by rules (22302.9 KN.m)

Calculation of section modulus for the midship section of the unit.

ZB= (8)

ZB = 3.79 m3

ZD= (9)

ZD = 1.46 m3

Compounding of stresses at bottom plate as mentioned before = 165.73 MPA

Modification of the Barge Structure

As mentioned before, there are no waves in the River Nile and wave bending moment does not exist. There is also a large difference in the values of the still water bending moment as given from the empirical formula from the rules and the actual bending moment calculated by MAXSURF software. In this study, an attempt is made to reduce scantlings of the barge using basic structure methods to save steel weight and assure that the structure is completely safe, as shown in the flow chart.

Some constraints are associated with reducing scantlings of the structure such as:

• The acting stress that affects the structure should be less than the yield stress of the material and the Euler buckling stress.
• The thickness of the plates used in the structure shouldn’t be less than 6 mm.

It is to be noted that the acting bending moment on the structure due to still water, cargo and various loads will produce tensile and compressive stresses on structure members:

• Tensile stresses will cause yielding of strength members. These stresses are less than yield stress of the material.
• Compressive stress will cause buckling of strength members. These stresses are less than Euler buckling stress.

The scantlings of the barge are modified as follows:

• Decrease gradually the structural member scantlings step by step.
• Check primary stresses, tertiary and local stresses.
• If they are acceptable, repeat the first step again and check the stresses, till the scantlings reach the value 6 mm.

The following table shows the scantlings of various structural member of the barge before and after modification:

Structural Member	Before Modification	After Modification	Reduction in Thickness
Bottom Plate	8 mm	6 mm	2 mm
Tank Top Plate	8 mm	6 mm	2 mm
Bottom Longitudinal	80x80x8 mm	80x40x6 mm	2 mm
Tank Top Longitudinal	80x80x8 mm	80x40x6 mm	2 mm
Inner Side Plate	8 mm	6 mm	2 mm
Outer Side Plate	7 mm	6 mm	1 mm
Side Longitudinal	80x80x8 mm	80x40x6 mm	2 mm
Bilge Plate	10 mm	10 mm	-
Sheer Strake	8 mm	8 mm	-
Deck Plate	12 mm	8 mm	4 mm
Weight per Unit Length	44.8 t/m	43.6 t/m	1.2 t/m

Calculation of elastic section modulus of the midship section for the unit after modification

ZB = 3.64 m3

ZD = 1.35 m3

Compounding of stresses at bottom plate after modification = 182.56 MPA

7.3. The plastic section modulus of the mid ship section for the unit is given by:

ZP = 10.31 m3

Here, the plastic moment that required to spread plasticity in the whole section of the inland water unit is (2577500 KN.m). This a greatest value of plastic moment can never reached, because the still water bending moment that can affect the unit in the River Nile is very limited, it is about (6155.368 KN.m).

Calculation of weight of the modified structure

• The weight of the steel used in the building of this unit in the parallel middle body before modification = 44.8 t/m.
• By applying the rational structural analysis, the decrease in steel weight for the case study in the parallel middle body is = 43.6 t/m.
• That means that the weight saving of steel is 1.2 t/m after modification for parallel middle body.
• For bow and stern of the unit, the weight saving assumed is 0.6 t/m.
• The amount of weight saving of steel used in this unit will be 77.4 t.

Conclusion

The main conclusions drawn up from this research work are:

• The still water bending moment calculated by rules is greater by 50% than the actual still water moment.
• The existing inland water units designed according to classification society rules are overdesigned.
• Applying the developed rational structural design procedure will lead to saving in steel weight of ship hull girder without violating structural safety of the unit.
• Accordingly, a reduction of the initial cost of building Nile barges and increasing the cargo carrying capacity of the barges is achieved.