DESIGN LOADS RULES/CODES FOR CLASSIFICATION OF SHIPS/OFFSHORE INSTALLATIONS

8/16/2019 DESIGN LOADS RULES/CODES FOR CLASSIFICATION OF SHIPS/OFFSHORE INSTALLATIONS

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Assignment 1

JEE332 Structural Analysis

Team 1:

Angus Cameron (192418)

Anwarul Awalludin (188383)

Kevin Bone (428543)

Liam Chan (151795)

Maison Carstensen (429288)

Sukhith Caldera (425818)

Lecturer: Dr Shinsuke Matsubara

Due Date: 20th of May 2016


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Abstract

Bulk carriers form perhaps the most fundamental mode of material transport globally, critically

sustaining global economy demands for offshore trade. Cape size bulk carriers are one of the largest

form of trade vessel, defined not only by size but also their inability to traverse the Panama Canal.

Due to vessel dimension, purpose and global population the consequences presented by the

structural failure of a large bulk carrier are both sever and of significant likelihood. To ensure design

incorporates an appropriate degree of safety classification societies such as Det Norske Veritas (DNV)

and Lloyds Register exist, providing strict rules and guidelines that engineers must adhere to.

This report presents previous research regarding typical loads associated with bulk carrier design,

motions, wave loading and mooring conditions. By examining current and past design characteristics

of bulk carriers using the processes of similar vessel analysis, a typical capesize bulk carrier vessel

was selected. The vessel selected was the 300m long, 50m wide vessel Goliath which has a DWT just

exceeding 200,000 ton. Following this, investigation was undertaken into the methods defined by

DNV and Lloyds Register for calculating global bending moment in still water and vertical wave load

conditions. Upon completion of sample calculations it could be concluded that loading was most

severe in the vertical wave load condition, whilst sagging moments exceeded those in the hogging

state. DNV was seen to overestimate bending moment but incorporate fewer vessel design inputs

compared to Lloyds Register, thus suggesting that Lloyds Register provides a significantly more

accurate calculation.


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Contents

1. Introduction ..................................................................................................................................... 7

2. Literature investigation ................................................................................................................... 8

2.1. Design ..................................................................................................................................... 8

2.2. Structure ................................................................................................................................ 10

2.3. Motions ................................................................................................................................. 13

2.4. Wave loading ........................................................................................................................ 18

2.5. Mooring................................................................................................................................. 22

3. Capesize Bulk Carriers.................................................................................................................. 26

4. Similar Vessel Analysis ................................................................................................................ 27

5. Classification Rules ...................................................................................................................... 33

5.1. DNV ...................................................................................................................................... 33

5.2. Lloyds ................................................................................................................................... 38

5.3. Discussion and Results ......................................................................................................... 44

6. Conclusion .................................................................................................................................... 48

7. Appendix A .................................................................................................................................... 50

8. Bibliography .................................................................................................................................. 52


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Table of Figures

Figure 1. Hatch covers on a bulk carrier at sea (Left), bulk carrier hatch cover removed (top right) and

hatch cover structure showing the stiffened plate (bottom right) (Um & Roh, 2015) ............................ 9

Figure 2. Identification of the dimensional inputs used in the computational analysis (Um & Roh,

2015) ....................................................................................................................................................... 9

Figure 3-Midship cross section comparison of single skin side and double skin side (Ozguc, Das, &

Barltrop, 2005) ...................................................................................................................................... 10

Figure 4-Stress distribution under ultimate state (Hu, Zhang, & Sun, 2001) ...................................... 11

Figure 5. Six degrees of motion freedom with respect to a vessel (Chen, Huang, & Wang, 2013) ..... 13

Figure 6. Relative motion profiles associated with the linear response using a modified Pierson

Moskowitz spectrum (Drake, 2000) ...................................................................................................... 15

Figure 7. Measured and calculated vertical force at bow area with measured wave motions obtained

from the participants of the study (Drummen & Holtmann, 2014) ....................................................... 16

Figure 8. Typical midships stress response spectra of the M/V Stewart J. Cort, showing wave-induced

and springing stresses (Alford & Troesh, 2009) ................................................................................... 17

Figure 9. Vertical bending due to waves (M. Mano et al., Design of Ship Hull Structures, 2009) ...... 18

Figure 10. Proposed Mathematical model (Samsung Heavy Industries Co. LTD 2010) ...................... 24

Figure 11. Bulk carrier similar vessel analysis of Deadweight vs. Year .............................................. 27

Figure 12.Ccomparison of vessel speed in knots and year built ........................................................... 28

Figure 13. Comparison of vessel Deadweight / Capacity versus Year ................................................. 28

Figure 14. Bulk carrier similar vessel analysis, Speed versus Length ................................................. 29

Figure 15. Similar vessel analysis for bulk carrier comparing deadweight with respect to length ....... 30

Figure 16. Bulk carrier comparison, speed versus engine power ......................................................... 30

Figure 17. Block coefficients versus Froude number for the bulk carrier vessels exmained ............... 31Figure 18. Non-dimensional capacity versus beam draft coefficient for the analysed bulk carriers. ... 31

Figure 19. Bending Moment Sign Convention ..................................................................................... 34

Figure 20. Wave bending moment distribution. ................................................................................... 35

Figure 21. Value of kSM against Proportion of Length (DNV, 2015) ................................................. 36

Figure 22 Development of vertical bending stresses along the length of bulk carrier, Goliath. ........... 41

Figure 23. Still water bending moments for sagging conditions for the capsize bulk carrier Goliath,

comparing the predictions obtained using the Lloyds and DNV methods ............................................ 44

Figure 24. Still water bending moments for a hogging conditions for the capsize bulk carrier Goliath,

comparing the predictions obtained using the Lloyds and DNV methods ........................................... 45

Figure 25. Wave condition bending moment for Capesize bulk carrier, Goliath, in hogging condition,

comparing predictions using Lloyd‟s and DNV methods. .................................................................... 46

Figure 26. Wave induced bending moment for hogging condition along the length of the Capesize

bulk carrier Goliath, comparing the predictions using Lloyd’s and DNV methods ............................... 46


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Nomenclature

Symbol Description Units

B Beam m

C B Block coefficient

C W Wave coefficient

D Distance from fwd or aft to point of consideration m

DWT Deadweight tonnage ton

f 1 Ship service factor

f 2,-1 Coefficient considering nonlinear effects applied to hogging

f 2,1 Coefficient considering nonlinear effects applied to sagging

k sm Stillwater bending moment distribution factor along the ship‟s length

k wm Vertical wave bending moment distribution factor along ship‟s length

LOA Overall length of all vessel m

L PP Length between perpendiculars m

LWL Waterline length m

M SO Stillwater bending moment amidships kNm

M SOS Stillwater bending moment amidships sagging kNm

M SOH Stillwater bending moment amidships hogging kNm

M S Stillwater bending moment kNm

M WO Vertical wave bending moment amidships kNm

M W Vertical wave bending moment kNm

M WOH Vertical wave bending moment (hogging) kNm

M WOS Vertical wave bending moment (sagging) kNm

T Draft m

x Forward distance from aft perpendicular m

Δ Displacement tonnes

∇ Volumetric displacement m3σ Permissible combined stress N/mm

2

Maximum hull stress at deck N/mm2 Maximum hull stress at keel N/mm2

Force of hull member above and below the neutral axis N


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Minimum hull section modulus m Design load scenario coefficient


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1. Introduction

Bulk carriers are large vessels used to transport goods between countries and are a vital part

of the world‟s economy. They are able to carry significant quantities of cargo and it is

therefore crucial that they are both efficient and safe. Due to this, there are various design

rules and guidelines put in place to ensure that they are capable of operating in a range of sea

states in a safe manner.

This report investigates and compares two sets of rules; Lloyds Register (LR) and Det Norske

Veritas (DNV). Calculations were made using each set of rules, which were then compared to

one another and discussed. Supporting this, literature reviews examining various aspects of

bulk carriers such as wave loading, mooring, ship structure and motions were investigated

and related back to the overall longitudinal strength of bulk carriers.

For the investigation of the rules, this report focused on one class of bulk carriers, „Capesize‟

carriers. In order to examine the significant loading that these vessels experience as a result of

their size, the carriers need to meet a set of rules and guidelines to ensure sure that they can

operate safely and efficiently.


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2. Literature investigation

2.1. Design

Literature: Propulsion of 200,000-210,000 dwt Large Capesize Bulk Carrier

(Man Diesel and Turbo, 2015)

CO2 regulations imposed by International Maritime Organisation (IMO) are constantly

becoming harder for ship owners to achieve, thus the design of the combined propeller-

engine propulsion system must be optimised to increase vessel efficiency and reduce fuel

consumption. The Energy Efficiency Design Index (EEID) imposed by IMO for vessels

contracted after 1 January 2013, categorises vessels according to Deadweight Tonnage

(DWT), geometric dimensions, fuel type, specific fuel oil consumption and design speed. The

EEDI is quantitatively assessed in units of mass CO2 produced per ton of DWT per nautical

mile travelled.

Large Cape-size bulk carriers typically have a maximum length of 299.3m, beam of 50m and

scantling draft of 17.9-18.4m, boasting a dtw between 205,000 and 210,000 ton. The

expected design operating speeds for these vessels lies between 14-15 knots, however due to

emission regulations these values are likely to reduce in the near future. Increasing propeller

diameter is known to benefit propeller efficiently, thus current focus for the design of future

bulk carriers is to optimise the aft body, hull lines of the ship and ideal operational ballast

condition, such to allow integration of larger propellers with lower optimum propeller speed.

For such vessels gearing is not required to provide increased efficiency over a wide range ofoperational speeds, thus typically the propeller shaft is directly coupled with the engine. At

lower operational speeds this has allowed development of the new two stroke MAN G70ME-

C9.5 compact ultra-long stroke engine, which operates with increased engine and propeller

efficiency at these conditions.

By comparing the currently dominant MAN S70ME-C8.5 series (super-long) two stroke

engine with the proposed ultra-long stroke G70ME-C9.5series, alongside differing propeller

diameters at 14 and 14.7 knots, a greater understanding of the ideal operational conditions

was developed for large size bulk carriers. Clear trends were evident revealing increased

efficiency of 3-8% for G70ME-C9.5 over the currently popular S70ME-C8.5 depending upon

propeller diameter.

Whilst the ultra-long stroke engine does present advantages of efficiency over its super-long

stroke counterpart, it is 1.2m longer and 0.4m wider, thus weighs significantly more. Due to

this, serious consideration must be made to ensure the engine does not exceed a weight which

could prove detrimental to the structure and global bending stresses applied to the vessel.


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Literature: Optimal dimension design of a hatch cover for lightening a bulk carrier.

(Um & Roh, 2015)

Due to constantly increasing international oil and material prices and subsequent operational

and constructional costs, the demand for weight reduction in vessel design is high. Hatchcovers as seen in Figure 1 are integral components of bulk carriers used to protect cargo form

ocean conditions. Hatch covers consist of stiffened plate (Figure 1) and contribute to

approximately 8% of construction costs, however little prior optimisation has been conducted

to improve this vital feature.

Figure 1. Hatch covers on a bulk carrier at sea (Left), bulk carrier hatch cover removed (top

right) and hatch cover structure showing the stiffened plate (bottom right) (Um & Roh, 2015)

The optimisation process was computationally programmed using C++ language, following

the SQP (Sequential Quadratic Programming) iterative method for non-linear design

optimisation. Validation of the developed algorithm was conducted using comparative tests

and the program was applied to the hatch design of an 180,000 ton dead weight vessel.

Optimisation limitations were defined by structural safety standards according to IACS and

Lloyd and geometric limitations were governed by maximum permissible stress and

deflection, minimum thickness of a plate, minimum section modulus and shear area of

stiffeners. The dimensional inputs used in computational analysis can be seen in Figure 2.

Figure 2. Identification of the dimensional inputs used in the computational analysis (Um &

Roh, 2015)

Results obtained for the optimisation of the fwd most hatch cover of the 180,000 ton DW

vessel presented weight reduction of up to 8.5%, however this term is likely to increase for

hatches closer to the vcg as they experience less reaction forces due to acceleration. Via

reduction in hatch mass, the weight and load distribution derivatives of global bendingmoment are subsequently reduced.


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2.2. Structure

Literature: A Comparative Study on the Structural Integrity of Single and Double Side Skin

Bulk Carriers under Collision

(Ozguc, Das, & Barltrop, 2005).

Bulk carriers experience significant fatality rates in the occurrence of accidents, rates which

are far greater than those experienced by other vessel types. As a result, the International

Association of Classification Societies (ICAS) recently announced new safety requirements

for bulk carriers. In addition, the Formal Safety Assessment (FSA) have commenced a study

to investigate whether bulk carriers with double skin side construction are meeting the

structural requirements of SOLAS Chapter XII. There are two main advantages to a double

skin side bulk carrier with respect to the single skin side (See Figure 3). The existence of

redundancy in case of penetration, and the primary structural members are no longer

subjected to corrosive effects from being in contact with cargo loading and unloading

equipment. Double skin side hulls also moderate the effects should the vessel be involved in a

collision, as the cargo will not immediately spill, assuming the inside plating is intact.

However, there is also a chance for a loss of vessel if the inner plating remains intact due to

the additional sectional forces induced due to the accident, collapsing the hull. As a result, it

is important to compare the structural strength in both intact and damaged conditions.

Figure 3-Midship cross section comparison of single skin side and double skin side (Ozguc,

Das, & Barltrop, 2005)

Sixteen different collision cases were studied using ANSYS LS-DATA. The speed of

collision was 10.5m/s and the total collision time was 0.3s. The element was 125mm thick in

contact areas and 375mm in all other areas. It was found that the energy absorption when

rupture of the outer skin occurs is 10% lower for the double skin side (DSS) than the single

skin side (SSS). However, the maximum energy absorbed when the inner shell of the DSS

and outer shell for SSS is 2.2 times more for DSS. It was shown that the DSS bulk carriers

have a higher safety index than SSS bulk carriers in hogging and sagging collision moments.This contributes to an overall greater longitudinal strength of the bulk carrier.


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Literature: Analysis on the ultimate longitudinal strength of a bulk carrier by using a

simplified method

(Hu, Zhang, & Sun, 2001)

The longitudinal strength is the most important element of a ship‟s hull design. This isgenerally represented by the maximum bending moment that can be withstood by the ship‟s

hull cross-section. Over the last 20 years, researchers have found that the linear elastic theory

which was previously employed is now inadequate to estimate the longitudinal strength of a

ship‟s hull. It is necessary to take into account the following considerations; various failure

modes, progressive and interactive behaviour of the failure of structural members, and the

residual strength of members after buckling or collapse. There are three main methods to

calculate the ultimate longitudinal strength of a bulk carrier; the non-linear finite element

method (NFEM), the idealised structural unit method (ISUM) and the simplified method

(SM). Of these methods, the simplified method has proven to be the most straightforward

method with an adequate degree of accuracy. For this reason it has caught the attention of

naval architects. This paper analyses the longitudinal strength of a 34,000 tonne bulk carrier

using the simplified method. Firstly, vertical bending is considered. The values of the

ultimate longitudinal bending moments and the locations of the instantaneous neutral axes at

ultimate states of both hogging and sagging are calculated (See Figure 4). Following this, the

ultimate strength under combined vertical and horizontal bending moments is considered. An

interaction curve was obtained according to the results of a series of calculations for the hull

subjected to bending conditions with different curvature angles. It was found that the

interaction curve was asymmetrical. This was due to the hull cross section not being

symmetrical about the horizontal axis, and the behaviour of the structural members undercompression being different from the behaviour under tension due to the non-linearity caused

by buckling.

Figure 4-Stress distribution under ultimate state (Hu, Zhang, & Sun, 2001)


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Literature: Statistical properties of bulk carrier longitudinal strength

(Campanile, Piscopo , & Scamardella, 2014)

Ship structures are continuously exposed to age related damages such as fatigue, cracking,

localised dents and corrosion. These all relate and contribute to a vessel‟s loss of life and itsimpact on the environment. While corrosion effects can be managed, thickness reductions

need to be considered for old vessels operating beyond their service life, such as bulk carriers.

Corrosion wastage was not considered up until the 1980‟s, the scantlings of merchant ships

were determined by more or less empirical formulas, implicitly accounting for corrosion

safety margins. To harmonize the safety margins, ICAS decided to prioritise the development

of Common Structure Rules (CSR) in 2006. These rules are based on the “net scantling

approach” to address the corrosion effects that a bulk carrier is likely to experience in its

operating lifetime. The method takes into account corrosion based on explicitly defined

corrosion additions for one side of each structural element, depending on both the

compartment category and the structural member such as platings or primary supporting

members. The rules of scantling compliance depend on the considered structural

requirements (local strength, hull girder strength, fatigue assessment) and analysis type

(buckling and collapsing, thickness and hull cross section). This „net scantling approach” is

easy to be applied in the building process, however it does not take into consideration the

annual corrosion rate. The paper focuses on statistical properties of time-variant hull girder

section modulus and ultimate bending moment capacity, determined by a Taylor series

expansion method and Monte Carlo simulation.

Hull girder section modulus and ultimate bending moment capacity follow the normaldistribution, not only under the assumption of uncorrelated variables, to which the

Lindeberg – Feller Central Limit Theorem applies, but also when full correlation exists.


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2.3. Motions

Literature: Bulk Carrier’s Mot ion Analysis with Sloshing Effect in Water Ballast Cargo Hold

(Chen, Huang, & Wang, 2013)

Roll motions combined with the lateral wind loads can cause dangerous heeling angles, or in

extreme cases even capsizing. Ship motions are defined by six degrees of freedom that a ship

may experience and can be seen in Figure 5. Of these motions, heave pitch and roll are

oscillatory, whilst surge, sway and yaw are not. Oscillatory motions are typically produced by

wave excitation, however any instantaneous excitation will result in some degree of

oscillatory motion until motion damping produces equilibrium.

Figure 5. Six degrees of motion freedom with respect to a vessel

(Chen, Huang, & Wang, 2013)

For bulk carriers, encountering severe environmental conditions during a voyage is ordinary.

The main objective is to try and reduce the damage incurred during such circumstances. One

method of motion reduction is to vary the mass distribution with ballast water by permanently

or temporarily distributing mass distribution of the vessel during the voyage. These methods

are known as passive and active stabilisation respectively. Loading a suitable amount of

ballast water can reduce the harm caused to the vessel by abnormal waves, and increase a

ship‟s safety and stability. By adding ballast water and increasing the draft, prevention of

cavitation effects on propellers and reduction of slamming effects may be reduced. To

confirm and validate motion predictions, model and computational analysis is required.

To investigate the ship motions, different loading conditions are carried out; during normal

ballast condition, heavy ballast condition and design load condition. The deadweight capacity

of the bulk carrier is 9300 MT and the water ballast is located amidships. The ship speed for

all tests was taken to be 5 knots. After the ballast water is loaded, numerical computations are

simulated in the frequency domain by using the commercial code HydroSTAR, developed by

Bureau Veritas. To validate the obtained data from HydroSTAR, it was compared with the

software SMP (Ship Motions Program). SMP is based on strip theory and is widely known to

be reliable in simulating ship motions. The strip theory introduced by Korvin-Kroukovsky

and Jacobs in 1957 has been revised and is still being used today to predict the six different

motions.


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Results show that the HydroSTAR results are quite similar to those obtained from SMP,

except for the roll motion around resonant frequencies. Due to this, practical tests are

required to achieve further information into vessel roll option about the free damped

frequency, where resonance will occur. Practical tests may be used to develop understanding

of motion in regular or irregular seas if tested in regular seas. This is achieved by applyingthe linear superposition principle in correspondence to some idealised wave spectra which

suits the operational conditions.

Practical and computational tests have provided conclusive evidence suggesting that vessel

seakeeping severity is increased in the heavy ballast condition compared to standard ballast

conditions, when specifically operating in 5 metre height waves and beam seas. Conversely,

the seakeeping performance is generally improved when the ballast tank is partially filled.

However, precautions must to be taken to ensure the free surface within the tank does not

jeopardise either the tank structure or the overall vessel stability. Further investigation into

the optimum ballast load is required, due to the correlation between the global bending loadacting on the vessel due to both motions and vessel ballast weight distribution.


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Literature: Transient Design Waves for Green-Water Loading on Bulk Carriers

(Drake, 2000)

Extreme relative motion occurs at the forward hatch cover location when a bulk carrier

encounters steep fronted waves with pronounced asymmetry. This can result in green wateron the deck and high loads acting on the hatch covers, which in certain cases they have been

unable to withstand. From the investigation conducted, it has been identified that extreme

immersion of the bow can be attributed to the vessel pitching downwards into a steep fronted

wave, most notably when that wave has pronounced asymmetry. Furthermore, extreme

midship bending has be associated with instantaneous symmetric wave profiles where a crest

or trough is located at midships, an example of the asymmetric wave profile is shown in

Figure 6, and shows the extreme wave as it encounters the vessel.

Figure 6. Relative motion profiles associated with the linear response using a modified

Pierson Moskowitz spectrum (Drake, 2000)

This investigation examined the influence of extreme vessel motions due to the shape of the

encountered waves. For this investigation Drake, implemented linear wave profiles, linear

transfer functions, as well as calculating the vessel‟s non-linear motions. From this study it

was concluded that due to relative extreme motion between the wave and vessel, the highest

loads were seen to occur when the vessel was close to being level and encountered a steep

fronted wave with pronounced asymmetry.


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From this article it can be identified that in the design and development of a bulk carrier,

investigation into the sea states in which it will operate and the waves that it will encounter

will be essential to ensure the vessel can withstand the bending moments applied.

Furthermore, care should be taken to ensure that the motions and loading experienced by the

vessel are accounted for and that the vessel will be able to withstand them. From this article,with respect to the documentation produced by the classification societies, the regulations are

simple cases and all endeavours need to be made to ensure that the vessel not only complies

with the regulations, but is also able to withstand all conditions that it may encounter.

Literature: Benchmark Study of Slamming and Whipping

(Drummen & Holtmann, 2014)

Vessel such as Bulk carriers, travelling in harsh environments can experience significantSlamming. The slamming motions results in significant loading on the vessel, commonly

attributed to a change in acceleration due to the interaction between a water surface and the

vessel. According to Drummen and Holtmann (2014), when slamming occurs in vessels over

200 metres in length or longer, a transient dynamic structural response known as whipping

can occur. The vertical bending moments induced through whipping when combined with

wave bending can be double that of wave bending on its own. Slamming is aspect of vessel

motion and loading that requires significant investigation to ensure that it is adequately

accounted for.

In the maritime world considerable resources have been invested in the prediction of the

loads resulting from slamming, however very little investigation as to the accuracy of the

results obtained has been conducted. To analyse the accuracy of the prediction methods used

to predict the loads induced by slamming a number or research facilities and classification

societies were involved in a benchmarking analysis. This investigation compared the results

of towing tank data and finite element analysis data across the institutions, as can be seen in

Figure 7. It was concluded that in general good agreement of the results using similar

methods at different institutions resulted and there was a high level of correlation to the

experimental data obtained. However, it was observed that some complex methods used

within finite element analysis may incur additional uncertainties.

Figure 7. Measured and calculated vertical force at bow area with measured wave motionsobtained from the participants of the study (Drummen & Holtmann, 2014)


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Therefore it can be identified that in order to effectively design and develop vessels such as

Capesize bulk carriers, the various motions that the vessel will undergo must be examined

and means of calculation utilised. From this study it can be seen that the theoretical methods

available to the engineer and naval architect can be effective in predicting the loads resulting

from extreme motions, but in-depth understanding of the methods is required for accurateresults.

Literature: Generating Extreme Ship Responses using non-uniform Phase Distributions

(Alford & Troesh, 2009)

To produce an effective design it is paramount that estimates of loads that will be applied to

the vessel are accurately obtained. However, according to Alford and Troesch (2009)

estimating ship responses and loads considering only extreme waves can have a negative

effect. This is because this methodology does not take into account the dynamic responses

and behaviour of the vessel. Therefore, it is suggested that through the methods developed byLindgren, Boccotti and Taylor, utilising linear wave theory, the maximum extreme wave

value is inserted into the desired wave train to provide an extended irregular wave record

(Alford & Troesh, 2009). This will allow for investigation to examine the correlation between

extreme waves and extreme vessel motions.

This method was utilised to predict the springing of a Great Lakes bulk carrier. “Springing

occurs when the encounter frequency of the waves, or a harmonic of the encounter frequency

of the waves, excites the ship at its two-nodal excitation frequency” (Alford & Troesh, 2009,

p. 644), according to Alford and Troesch when springing occurs the bending moment of the

vessel can significantly increase. The stress response spectra of a Great Lakes bulk carrier is

shown in Figure 8, this plot shows the relationship between the extreme responses. This

behaviour is especially important in very large carriers due to the implications that it can have

on structural integrity. When springing occurs, due to the elastic nature of the behaviour,

there is a change in the extreme response, as well the shape of the wave train, producing large

responses. Wave induced bending varies with frequency, at lower frequencies it relates to the

rigid body dynamics, while at high frequencies it is related to the effects of springing (Alford

& Troesh, 2009).

Figure 8. Typical midships stress response spectra of the M/V Stewart J. Cort, showing wave-induced and springing stresses (Alford & Troesh, 2009)


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Therefore from the work of Alford and Troesch (2009) it can be identified that there are a

number of different phenomena and behaviour that go into the motion of a bulk carrier. It is

also noted that the excitation forces of the waves must be understood effectively, in order to

provide the most accurate predictions. Furthermore, the effect of springing must be

acknowledged by the designer to ensure effective mitigation of the inherent risk that it causes.

2.4. Wave loading

Literature: Design of Ship Hull Structures, Chapter 2 Structural Design Loads

(Mano, Okumoto, Takeda, & Okada, 2009)

All ships are subjected to different types of loads; such as hydrostatic pressure, hydrodynamic

pressure and environmental loads caused by wind, currents, and waves. Among the variety of

loads, accumulative wave induced hydrodynamic pressure can have a significant effects on

the ship structure and eventually contribute to the structural collapse. Subsequently, it isobligatory for every naval architect to analyse, in depth, all loads a vessel hull structure may

experience and, with emphasized significance placed on wave induced loads.

In conjunction, global loads act on the entire hull girder while the local loads are more

involved with panels, girders, beams, and stringers. When looking at a global deformation

perspective, a ship will behave like a beam due to its slender shape as a wave generates a

vertical and horizontal bending moment which acts to provide buoyant force unevenly

distributed along the vessel length resulting in “hogging” and “sagging” bending moments

(Figure 9). Hogging is the condition where amidships is deflected upwards, so the top

longitudinal layer of the ship experienced tensile stress whilst the below the neutral axis the

vessel experiences compression. On the other hand, sagging is the condition where the

amidships is deflected downwards. Nonetheless, the hogging and sagging bending moment

act interchangeably on the hull girder along the ship across the progressive waves, increasing

the global stresses applied to the vessel due to cyclic loading and subsequent fatigue.

Figure 9. Vertical bending due to waves (M. Mano et al., Design of Ship Hull Structures,

2009)


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Each classification society, which is tasked to provide analysis methods to ensure all vessels

are designed safely, quantify the degree of the global load in terms the strength. The

International Association of Classification Societies (IACS) standardized the rules in the

Unified Rule Requirement of 1989 which has been recognized by all classification societies.

It was stated that the minimum allowable bending moment a vessel must withstand is roughlyequal to the worst case scenario or one in 20 year extreme wave load, with a probability of

10-8 that wave induced loads will exceed such value.

Literature: Loads for use in the design of ships and offshore structures

(Hirdaris, et al., 2014)

In recent years, since bulk shipping has proved itself as the most cost and energy effective

way of transporting a great deal of cargo at once, the number of bulk carriers has dramatically

increased. Such demand has also gradually enlarged the size of existing bulk carriers and

transformed their shape, pushing the limits of structural design. The problem involving suchtrend is the influence of the wave loading that becomes larger with the increase of the hull

size. Subsequently, particulars of new bulk carriers could exceed the valid ranges of methods

used to predict vessel wave loading, thus increasing the need for new quality experimental

data, particularly pertaining to the measurement of global loads from model tests. Evaluation

of wave induced loads is consistently conducted using partly nonlinear methods whilst

comparing predictions with model tests. Furthermore, increasingly complex analysis

techniques are becoming popular, allowing fully nonlinear analysis, particularly using CFD

based methods. The potential flow formulation of slamming problems has continued to also

raise interest. Recent studies have focused on the evaluation of slamming loads on symmetricand asymmetric sections using the Wagner approach. Furthermore, hybrid CFD based

methods (e.g. RANS or SPH) have become commercially available and significantly more

popular.

When it comes to the maximum longitudinal strength of the hull, severe abnormal waves

should be taken into consideration. Moreover, fatigue caused by continuous wave loads could

have a larger impact on the hull girder than the relatively less frequent higher magnitude

extreme wave loads.

Literature: Effect of ship length on the vertical bending moments induced by abnormal waves

(Foneseca, Guedes Soares, & Psacol, 2007)

The most significant component of interaction between abnormal waves and vessel loading is

the high extent of non-linearity. The main focus of this article is to evaluate the influence of

the ship dimension on the vertical bending moment generated by the rogue waves. The

magnitude of global loads induced by these abnormal waves compared to the hull strength

will depend on the relationship between the wavelength and the ship length. The investigation

was carried out on geometrically similar type of bulk carriers between 100m to 350m. The

analysis was then performed using the time domain sea keeping program which solves

equations of motions and structural loads.


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It is usually very hard to predict the maximum wave induced loads on ships during their

lifetime, and there are a couple of uncertainty predictions made; one of which is associated to

the stochastic nature of the waves and the other basis is examining excessive wave conditions

which approach the restrictions imposed by numerical methods due to highly nonlinear vessel

response.

Many approaches have been taken to calculate the design wave loads on ships. If the loads

are basically linear, then it is possible and efficient to apply the linear long term distribution

method. The other procedures that have been taken into account are based on the assumption

that the linear seakeeping model is able to accurately identify the conditions in which the

extreme wave loads occur. The focus is to quantify the worst conditions that the ship will

encounter in a long period of time by applying similar linear sea keeping methods and

conducting more accurate and time consuming nonlinear time domain simulations only for

selected range of extreme conditions.

From investigations it can be concluded that the bending moments are maximized when the

ship length is roughly similar with the wavelength of the abnormal waves. Results show that

the relationship between the abnormal wave height and the maximum moment is almost

linear, however when the significantly increased wave height, linearity breaks down. Results

also confirmed that increased wave height yields higher values of global bending moments ,

whilst increased ship lengths has a similar influence on bending moments.

Literature: Experimental and Numerical extreme motions and vertical bending moments

induced by abnormal waves on a bulk carrier

(Vasquez, Fonseca & Guedes Soares, 2013)

An issue which arises occasionally, is the degree of breakdown of a bulk carrier when

exposed to severe wave loads. Nowadays ships try retain maximum stability when faced with

severe sea conditions. This article exhibits an investigation which concentrates on the global

structure loads and motions produced by abnormal waves on a bulk carrier.

A systematic study was done on the wave generated vertical ship motions and structural loads,

showing that the global loads are mainly effected by the combination of the bow flare and the

wave steepness. To foresee the response of the ship, the strip theory method based with a

non-linear time domain is used. A simulation on a real storm situation (New Year Wave)

which included extreme freak waves is replicated at the seakeeping model test tank and by

the numerical code to test the ships. After which the gathered experimental and numerical

data were compared.

The two real storm wave situations considered for the investigation were the New Year Wave

(NFW) and the Single Freak Wave North Alwyn (SFWNA). The experimental data was then

compared with the time domain simulations from the bulk carrier model tests in head sea

conditions at a static condition (Fr=0) and slow ahead (Fr=0.1).The magnitude for both the

waves was the same.


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Results show even though the SFWNA had a smaller height the sagging moment was much

greater when compared with the NFW since the length was a lot closer to the length of the

ship, resulting in the encounter frequency approaching free damped frequency of the vessel,

subsequently inducing resonance. Analysing both the linear and non-linear simulations, the

data obtained was seen to be fairly close, comparing well with measured values.

Literature: Ultimate hull girder strength of a bulk carrier under combined global and local

loads in the hogging and alternate hold loading condition using nonlinear FEA.

The failure of a bulk carrier hull is often associated with severe wave loading and certain hold

loading conditions which can cause the hull to fail due to degree breakdown. The loads

caused by the interaction between large wave loading can cause elasto-plastic deformation of

the ships hull. There are two types of loads that act on the ship structure; local loads

(experienced by stiffened panels, girders and beams) and global loads (which act on the

“ships girder”, the vessel as a whole).

When a bulk carrier experiences hogging the mid-ship of a ship experiences an upwards

deflection. This deflection results in the top longitudinal layer of the ship experiences tension

whilst the bottom layer of the ship experiences compression. The main cause of this load case

is the alternate hold loading (AHL) condition. This condition is caused by high density cargo

being loaded into all odd numbered holds leaving the even holds empty.

To analyse this problem and incorporate its implications in design, finite element analysis

(FEA) with the ABAQUS software is used to study combined AHL conditions effect on the

ultimate hull girder strength of a bulk carrier. The reference vessel used for this investigation

was an old model a design modification factor (DMF) of 1.4 was used to correlate the

findings with the new common structural rules (CSR).

Calculations were completed using the ultimate strength under just longitudinal bending in

the hogging condition. These results were then compared with Smith‟s method also known as

the simplified method. This method produced ultimate longitudinal strengths which were

similar but generally smaller than the FEA method which was used calculate the bulk carrier

strength under AHL conditions.

Results have shown that irregularities and imperfections in vessel construction, such as poor

welds, and residual stresses can alter the hull girders ultimate strength. Furthermore, it was

found that the DMF has a larger influence on the ultimate bending capacity under combined

global hogging moment and local load than pure hogging and bending loads. This evidently

shows that the bending capacity under combined global and local loads can be optimised by

strengthening bottom panels.


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2.5. Mooring

Literature: Movements of Moored Ships in Harbours

(Elzinga, Iribarren, & Jensen, 1992)

The motions of a moored ship at a berth can be categorised as either horizontal (surge, sway,

and yaw) or vertical (roll, pitch, and heave). The vertical movements of a vessel can be

classified as an independent section to the mooring line but on the other hand, the horizontal

movements rely significantly on the berthing line. The movement of a vessel could cause

unsynchronised forces to act on the mooring line and fenders between the vessel and berthing

area. The Permanent International Association of Navigation Congresses (PIANC) therefore

authorised a special working group to investigate the developments of moored ships in

harbours with the intent to produce new criteria for ship movements in safe working

conditions (i.e. at the point when operations must be minimised or even stopped), and

additionally for safe mooring conditions (i.e. at the point when ships need to leave the berths).

The working group explains the rules as issued by different arrangement social orders that

concern the number and type of mooring lines to be carried on board. The rules ought to

mirror that a moored ship is a dynamic framework having uncommon necessities as for safe

working and additionally safe mooring conditions. Accordingly, the mooring operations of

bulk carriers could be enhanced by utilizing delicate spring lines for ships which are

presented to long stretch waves.

Literature: The vessel in Port; Mooring Problems

(Schelfn & Ostergaard, 1995)

A safe mooring should be capable of resisting the forces imparted on the moored ship by

wind, swell, and other vessels. It is fundamental to establish operational standards which

appropriately account for the forces experienced by the ship and mooring. The magnitude of

the environmental forces imparted on a moored ship change dramatically at different ports. It

is important to know the route that will be operated by a ship as this will determine the

typical forces which will be imparted on the ship while moored. The design of the mooring

system should take the varying magnitudes of environmental forces into account but should

also have enough headroom in order to be able to safely deal with adverse conditions.

International regulatory authorities have not developed strict rules regarding the design of

mooring systems since it is hard to allow for the variation in conditions experienced by

different ships. It is critical for a ship to have an adequate mooring standard practice for the

safety of the ship, mooring infrastructure and the environment. When developing mooring

standards it is important to have a good understanding of the environmental forces which will

act on a ship and the mooring infrastructure so standards can be developed accordingly.

At most berths the mooring system is designed to the maximum wind and current strengths

experienced at its location, as well as forces imparted from different sources including waves.

For each of these circumstances and mooring configurations, static examination strategies can be employed to determine the correct mooring line strength for that particular situation.


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Due to their size, bulk carriers are often required to berth at deeper open water moorings

which can experience greater environmental forces than protected shallow water berths. Due

to the severity of the environmental forces experienced by the moored ship that it is critical

that they are correctly calculated. It is possible to use three-dimensional potential streams to

develop a simulation for the first-order hydrodynamic forces and subsequent high-recurrencemovements experienced by a moored ship. These are then able to be used to ensure that the

correct standards are employed to prevent the breakage of mooring lines when ships undergo

extreme motions, preventing damage to the ship and mooring structure. A safe mooring

should be capable of holding forces from the surroundings such as wind, swell, and from

other vessels. To set up an operational standard that guarantees the ship is appropriately

prepared for safe mooring, it is fundamental, tangent upon the trading route, to decide the

environmental forces that would typically be experienced by the moored ship. At exactly that

point can the ship be legitimately outfitted with a mooring arrangement of adequate

restriction capacity to oppose these forces and to permit some level of adaptability inside

ordinary security resistances. Thus, strict rules for mooring systems are not specified by

international regulatory authorities such as the United Requirements of the IACS. A

proficient mooring framework is fundamental for the safety of the ship, the terminal and the

environment. In managing the issue of improving the moorings with adequate restriction

ability, it is important to focus on the environmental forces applied on the ship, general

standards that decide how the forces are conveyed to the mooring lines, and utilisation of

these standards to build up a decent mooring course of action.

Literature: Passing Ships Effects on Moored Capesize Bulk Carriers

(Hall, van der Molen, & Scott, 2013)

Numerous ports worldwide are being expanded to handle larger volumes of materials, and

inland ports are beginning to reach their capacity as there is a higher number of ships using

existing shipping channels. With this increase in shipping traffic and ship sizes in world ports,

passing ship interactions are becoming more of an issue. Passing ship interactions can cause

large vessel motions which can damage the ship, the berth, loading and unloading equipment

and endanger lives.

Through the use of a numerical model developed using PASSCAT it is possible to evaluate

the pressure field created by the passing ship. This can be used to determine the forces andmoments imparted on the moored ship. To show the PASSCAT results were reliable,

physical model tests were undertaken using 1/100 scale physical models of the same ships

used in the numerical simulation. The model testing was completed using defined limits for

passing speeds, as such the accuracy of this model at passing speeds outside these limits

could not be verified. Figure 10 shows the mathematical model used to investigate passing

vessel effects on a moored ship.


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Figure 10. Proposed Mathematical model (Samsung Heavy Industries Co. LTD 2010)

This study has limitations due to the model developed. This simulation was only validated for

a passing distance of 200 meters in an empty port. More importantly, the results for this test

are only for a specific port and would require alteration to be applied to other ports.

The study proved the simulation to be reliable and proved very successful with interesting

results. Despite this simulation only being applicable for the tested port, it was proven that

with further study and the correct alterations it can be accurately applied to other ports around

the world.

It is crucial to consider the mooring forces exerted on a moored bulk carrier due to passing

vessels. The mooring force contributes to the load distribution acting on the vessel adding to

global bending moment the structure must withstand.

Literature: Open Water Berths and Single Point Moorings

(Mehaute & Chiu, 1980)

Liquid bulk carriers have the ability to load and unload their cargo through flexible transfer

lines. This allows these ships to complete unloading and loading despite relatively large ship

motions. With a thorough understanding of the dynamics of a berthed ship when acted upon

by waves, winds and currents and the methods used to secure ships at open sea berths, it is

possible to construct these terminals in relatively exposed locations without breakwaters.

The use of a single point mooring and flexible dolphin is the best and safest mooring for

ships to use in exposed berths. Many bulk carrier terminals around the world utilise open sea

berths as opposed to single point moorings. In these cases it is important to understand how

the motions of a ship are affected by normal and adverse weather conditions in order to

correctly design the mooring systems and ensure correct mooring standards.

The forces in mooring lines and fenders, and the amplitude of motions of a moored ship when

exposed to waves include a mixture of subharmonic, harmonic and superharmonic

oscillations. Harmonic motions are the oscillations of a ship in its six degrees of freedom

which correspond to the frequencies of the waves. These oscillations primarily affect the roll,

pitch and heave of the moored ship. Subharmonic motions on the other hand predominantly


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affect the surge, sway and yaw of the ship and make up a substantial part of the ships total

motions since the natural frequency of these systems are low.

Liquid bulk carriers, unlike other ships which are often moored tight to fenders in smooth

water ports and can therefore be considered practically motionless, are often moored using

flexible loading arms. Due to the flexibility of this type of loading arm the moored ship is

able to move relatively freely. Fixed dolphins are no longer designed for open berths since

the most effective way to prevent excessive forces from mooring lines and fenders being

transferred in to the moored ship is to add flexibility.

Through mathematical models and model testing it was found that the use of a flexible

mooring system combined with the use of non-symmetric constant tension winches allow for

the berthing of ships at open sea berths without exceeding specified safety criteria.

Literature: Restricted water effects on berthed ship – passing ship interaction

(Denehy, Duffy, Ranmuthugala, & Renilson, 2015)

Whilst underway, a ship generates a pressure field which imparts forces and moments on

objects in its surroundings, including ships at berth. This interaction causes the berthed ships

to move on their moorings. There have been a number of instances where excessive motions

experienced by a berthed ship have caused damage to mooring infrastructure and even loss of

life. Berthed ship motions can also affect a ships ability to load and unload safely.

Many studies have been performed by a number of authors to better understand the

interactions between passing and berthed ships. These studies have used both empirical andsemi-empirical methods to investigate and predict the interaction forces and moments

imparted on the berthed ship. However most empirical methods are only able to be applied

when passing interactions occur with a water depth to draft ratio of greater than 1.1 and with

no lateral restrictions, and the interaction moments and forces change with the square of

passing ships speed.

In order to more accurately model passing ship interactions, two channel widths and two

under keel clearances we investigated experimentally, empirically and numerically. The

different methods were then validated against each other. All of the tests we conducted using

the channel widths of 3.04 B and 8.25 B and under keel clearances of 1.04 T and 1.2 T with Band T being the ships Beam and Draft respectively. All test were conducted with a constant

passing distance.

It was noted that in previous experiments the tow rigs used for the passing ships did not

restrict the sway of the passing ship or measure this motion. As a result the magnitude of

these forces and moments were unknown. To correct for this, a new tow rig was developed

which constrained the surge, sway and yaw of the passing ship which provided a constant

lateral separation between the models.

Since these ship interactions occur at low speed it is possible to neglect the wave-makingdissipation of the passing ship. As a result, using CFD software it is possible to determine the


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velocity field developed around the passing ship using potential flow with the free surface

modelled as a rigid wall. The interactions of the forces and moments imparted on the berthed

ship from the velocity field around the moving ship were estimated using an inviscid double

body numerical model. Due to the complex hull design a simplified model was used for this

method. The effect of this simplified model on the simulation was not quantified in the study.

Upon comparing the results of the different models it could be seen that surge force

magnitude was not significantly affected by the reduction in under keel clearance. The sway

force and yaw moment imparted on the berthed ship increased considerably when the under

keel clearance was reduced. Once scaled to represent a 300m long ship, the experimental

results, inviscid CFD predictions and existing empirical predictions were used as inputs into

numerical simulation software to predict the motions of the moored ship. The predicted

motions using the forces and moments calculated using CFD correlated well with the

experimentally measured interaction forces for the surge and sway motions.

3.

Capesize Bulk Carriers

In recent decades the need to the transport large amounts of raw materials around the globe

has increased dramatically. Subsequently the demand for materials has resulted in an

increased global fleet of large sized bulk carriers. Due to the reduced cost of buying materials

in bulk, combined with the astonishing load to required power ratio of large vessels, the

average size of bulk carriers has increased significantly to ensure ship owners are receiving

maximum return on their investment. (Fei, J. 2015)

Bulk carriers come in a range of sizes depending on the cargo to be shipped and the route to be operated by the ship. One of the largest size of bulk carriers are known as Capesize bulk

carriers.

Capesize bulk carriers are designed primarily to handle raw materials from deep water

terminals. The raw goods most commonly transported by Capesize ships include iron ore,

bauxite, coal and other commodities. A typical Capesize bulk carrier may exhibit a

deadweight of approximately 100-200,000 tonnes and a draft of 17 metres. However

Capesize bulk carriers can also include larger sub-categories VLCC, ULCC, VLOC and

ULOC which can have a deadweight of up to 400,000 tonnes. (Lloyd's Register Foundation,

2014)

The overall size of a bulk carrier ultimately determines the route it must travel between ports.

Even with recent upgrades to the Suez and Panama Canals, Capesize bulk carriers still exceed

the maximum size limits for passage of these canals, thus they must transit around Cape Horn

and the Cape of Good Hope in order to deliver materials.

Despite Capesize vessels not having the restrictions on the maximum length, beam and draft

of other ship classes, they still have limits due the size of ports around the world. Due their

large size, in particular their large drafts, there are only a small number of deep water ports

and terminals which have the infrastructure to accommodate Capesize bulk carriers greater


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than 200,000 tonnes. Therefore bulk carriers must be designed around the maximum size

limits of the ports and subsequent routes their owners intend to use. (Bhaskar, P. 2015)

4. Similar Vessel Analysis

Since Capesize bulk carriers can have a deadweight of anywhere from 100-400,000 tonnes, a similar

vessel study was performed to determine if any trends exist in the designs of these vessels. A random

selection of more than twenty Capesize bulk carriers were selected, covering a broad range of vessel

commission dates and sizes. Each vessel‟s dimensions, deadweight, launch year, engine power and

operational speed were tabulated and used to compare all of the vessels. Both dimensional and non-

dimensional relationships were used to develop the comparison plots below. A tabulated version of

the full vessel particulars used for these calculations is included in Appendix A.

The design of large bulk carriers has evolved over the last twenty years. To investigate the trends in

design which have changed, a number of different vessel characteristics were plotted against the year

the vessel was launched. Such can be seen in the Figure 11.

Figure 11. Bulk carrier similar vessel analysis of Deadweight vs. Year

As seen in Figure 11, in our selection of Capesize bulk carriers there has been an increased demand

for bulk carriers between 150-200,000 tonnes. This may be attributed to a number of factors including

demand for larger bulk carriers, particularly oil carriers, reducing. This is in part due to the impact of

marine incidents involving these ships. The main reason for this trend however, is that ship owners

are looking to increase the profitability their ships. This is best done through versatility. Since there

are very few ports which can handle ships of over 200,000 tonnes, owners are opting for ships very

close but just within the maximum permissible port dimensions, subsequently increasing the number

of possible trade routes the vessel may service.

Figure 12 shows how the operational speeds of bulk carriers has changed over past decades. It can be

seen that the required operational speeds for bulk carriers has reduced. This is due to a number of

global factors. The most likely factor influencing this trend is the dramatic increase in the cost of fuel.This is critical as this is the largest component of a vessel‟s operating costs. There is also a correlation

0

50000

100000

150000

200000

250000

300000

350000

1985 1990 1995 2000 2005 2010 2015 2020

D e a d w e i g h t ( t o n s )

Year


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between the length of a bulk carrier and its designed average operating speed. This is further

investigated and discussed in Figure 13.

Figure 12.Comparison of vessel speed in knots and year built

Figure 13. Comparison of vessel Deadweight / Capacity versus Year

To evaluate the how the efficiency of hold design has changed over years of development, the

ratio of deadweight to capacity was plotted against year. It can be seen that deadweight to

capacity has reduced since Capesize bulk carriers were introduced. This shows that the design

of the holds of these ships has evolved to use less volume more efficiently.

0

2

4

6

8

10

12

14

16

1985 1990 1995 2000 2005 2010 2015 2020

S p e e d ( k n o t s )

Year

0

0.5

1

1.5

2

2.5

1985 1990 1995 2000 2005 2010 2015 2020

D e a d w e i g h t / C a p

a c i t y ( t o n s ) / m 3

Year


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From the above figures it can be seen that the current trends for Capesize bulk carriers are for

increased numbers of smaller ships which are designed to operate at lower speed than in

previous decades. It can also be seen that the design of the holds on this size of ships has

become more efficient. However, due to the costs associated with manufacturing a new ship

bulk carriers are often in service for over two decades. For this reason it is important tounderstand the general design trends for Capesize bulk carriers and not just current trends.

To get a better understanding of these general design trends a number of dimensional and

non-dimensional comparisons were investigated.

Dimensional Comparisons:

Figure 14, shows that for Capesize ships there are two interesting trends that occur when

speed is compared against length. The first expected trend is that in general, as a ship‟s lengthincreases, its speed also increases. The other interesting trend is that for ships between 285m

and 295m there is a large range of operational speeds. This is due to the popularity of this

size of ship and the different requirements of ship owners in this size range.

Figure 14. Bulk carrier similar vessel analysis, Speed versus Length

To investigate how the deadweight capacity of Capesize ships changes with length, each

vessel‟s deadweight and length were plotted against each other as seen in Figure 15. It can be

easily seen that as a ship‟s length increases, so does its deadweight capacity. This can be

understood, as larger ships have a greater displacement allowing them to carry more.

0

2

4

6

8

10

12

14

16

270 280 290 300 310 320 330 340 350

S p e e d

( k n o t s )

Length (m)


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Figure 15. Similar vessel analysis for bulk carrier comparing

deadweight with respect to length

It was only possible to find information the engine power of 12 of the Capesize bulk carriers

investigated (As seen in Figure 16), however it was decided that this smaller sample would

still provide valuable insight into this relationship. This sample included ships of between

288.15m and 300m manufactured between 1999 and 2015. The downward trend observed in

the plot can be linked to improved efficiencies of engines and the design optimisation of hulls

in this size range since 1999.

Figure 16. Bulk carrier comparison, speed versus engine power

0

50000

100000

150000

200000

250000

300000

350000

270 280 290 300 310 320 330 340 350

D e a d w e i g h t (

t o n s )

Length (m)

0

2

4

6

8

10

12

14

19000.00 20000.00 21000.00 22000.00 23000.00 24000.00 25000.00 26000.00

S p e e d ( k n o t s )

Engine Power (Hp)


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Non-Dimensional Comparisons:

To remove the influence of the ships dimensions on observed trends, a number of non-

dimensional relationships were investigated.

The block coefficient is a measure of the fullness of the ship‟s hull form. When plottedagainst Froude number it can be seen how close the ship‟s hull design is to the recommended

block coefficient for that vessel and is shown in Figure 17.

Figure 17. Block coefficients versus Froude number for the bulk carrier vessels exmained

To evaluate how efficiently each vessel hull was designed to maximise load capacity, the

hold capacity against its block volume was plotted against beam to draft ratio and is seen in

figure 18. It could be seen that as a ships B/T ratio increases the efficiency of its carrying

capacity decreases. This is as a result of the hull having a relatively larger volume.

Figure 18. Non-dimensional capacity versus beam draft coefficient for the analysed bulk

carriers.

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15

B l o c k C

o e f f i c i e n t

Froude Number

0.00

0.20

0.40

0.60

0.80

1.00

1.20

2.40 2.50 2.60 2.70 2.80 2.90 3.00 3.10 3.20

C a p a c i t y / L B T

B/T Ratio


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Selected Average Capesize Bulk Carrier;

Using all of the above information we were able to select a suitable average ship which will

be used to compare the design standards developed by Lloyd's and DNV.

The particulars for the Capesize bulk carrier selected can be seen in Table 1.

Table 1. Selected Capesize Bulk carrier for investigation, from similar vessel analysis

Vessel Name Goliath

Deadweight (ton) 209 537

LWT (ton)

28 939

Displacement (ton) 238 476

Length (m) 300

Beam (m) 50

Draft (m)

18.43

Block Coefficient (CB)

0.84Engine Make Man B&W

Grain Capacity (m ) 227 362

Speed (knots) 10.70

Year 2015

Class ABS

There are a number of reasons why Goliath was deemed to be the best representation of an

average Capesize bulk carrier. Within the random sample of ships selected it could be seen

that Goliath was the ship which most closely met the current trends for Capesize vessels. Itcould also be seen that for other relationships investigated, the Goliath was always close to

the trend and around the mean vessel. The Goliath's geometric specifications and deadweight

are also quite close to the average ship dimensions for Capesize bulk Carrier.


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5. Classification Rules

5.1. DNV

DNV is an international certification body which regulates the design and production of

maritime vessels, to ultimately ensure all the elements of vessel design incorporate the

appropriate factors of safety. Part B of Section 5 of DNV Rules for Classification of Ships

(Det Norske Veritas) details the processes required to calculate the global still water and

wave induced bending moments for vessels. Although the method described below is only

used for the preliminary design stage, the quoted probability of the vessel exceeding

calculated values is 10-8.

For ships with a small block coefficient, high speed and large flare, the hull girder buckling

strength in the body needs to be considered based on the distribution of still water and

vertical wave bending moments. The particularly applies to ships larger then 120m and with a

speed of larger than 17knotts.

For ships with large deck openings (width of hatch openings in a transverse section exceeding

65% of ships breadth or the length exceeds 75% of total ship length) the longitudinal strength

needs to be considered.

In addition to this, consideration needs to be given to ships with the following:

-CB=0.6

-L/B≤5

-B/D≥2.5

Ballast:

Ballast loading conditions involving partially filled peak and or other ballast tanks at either

departure, arrival or during intermediate conditions are not permitted to be used as design

conditions unless:

-design stress limits are satisfied for all tank levels.

Where multiple tanks are intended to be partially filled, all combinations of empty, full or in between intended levels shall be investigated.

Prior to conducting calculations, it is essential to convey the sign convention. DNV states that

downwards load is assumed to be taken as positive values, which are to be integrated from

the aft perpendicular in the forward direction as shown in the figure 19 below.


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Figure 19. Bending Moment Sign Convention

Wave Load Conditions

Vertical wave bending moment amidships

(1) – (2)

(3)

Where:

For seagoing conditions For harbour and sheltered water conditions (enclosed fjords, lakes, rivers)CB is not taken less than 0.6.

And,

CW is the wave coefficient and is dependent on vessel length as shown below:

(4) (5) (6)When required in connection with stress analysis or buckling control, the wave bending

moments at arbitrary positions along the length of the ship are normally not to be taken less

than:

(7) Where:

Values of k WM may also be obtained from the Figure 20 below.


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Figure 20. Wave bending moment distribution.

Still Water Bending Moment

The design still water bending moments amidships (sagging and hogging) are normally not to

be taken less than:

(8) – (9) – (10) Where:

When required in connection with stress analysis or buckling control, the still water bending

moments at arbitrary positions along the length of the ship are normally not to be taken less

than:

(11)

Where:

Between specified positions shall be varied linearly from 0.15 to 1.0 as shown in thefigure 21 below.


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Figure 21. Value of kSM against Proportion of Length (DNV, 2015)

Sample Calculations:

Vertical Wave Loading:

The vessel parameters are defined in Table 1

Table 2:Wave Coeff icient for DNV

L CW

L > 100

0.0792L

100 ≤ L ≤ 300 10.75 - [(300 – L)/100].

300 ≤ L ≤ 350 10.75

L > 350 10.75 - [(L-350)/150] .

Vertical Wave Bending Moment:

Finding the maximum bending moment about amidships:

From Table 1,

.

Where,


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For sagging,

–

For hogging,

The distribution factor is equal to 1 as the calculation takes place amidships, thereforethe wave moment for sagging is:

Hence, the wave moment for hogging is:

Calculations for still water

Calculation of the maximum still water moment about amidships for sagging:

–

For Hogging:

–

As the vessel is being calculated about amidships, the distribution factor, is equal to 1,hence the still water sagging moment is:

For hogging:


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5.2. Lloyds

Lloyds Register, Marine is one of the oldest and largest Classification Societies. Lloyds

works to ensure that the international safety and environmental standards of a ship are

maintained throughout its life (Alderton, 2004). During the design and development of a

vessel which will be entered into class, it is paramount that many considerations are

employed to ensure it meets the society‟s stringent regulations. For Lloyds, these are set out

in the Rules and Regulations for the Classification of Ships. The requirements employed by

Lloyds, with respect to the longitudinal bending moments are documented in Part Three,

General Ship Structures, and Part Four, Ship Structures (Ship Types) of the document. Key to

this investigation is Part Three, Chapter Four, Section Five. This section documents the

longitudinal strength requirements of vessel requiring Lloyds classification (Lloyds Register

Group, 2016).

Under Lloyds regulations, all ships greater than 65m in length are required to undergolongitudinal strength calculations in order to determine the hull girder strength, calculations

must be conducted for the range of load and ballasting conditions proposed. If a

superstructure is fitted that is greater than 0.15L and the span of it extends 0.5L amidships,

then the requirements for the longitudinal strength in the hull and superstructure that is

erected, need to be considered for each individual case (Lloyds Register Group, 2016).

Within the Lloyds Classification

Of special consideration within the Lloyds Regulations, in ships between 120 and 170 metres

in length and a service speed of greater than 17.5 knots, that also have a bow shape factor of

greater than 0.15, the hull midship section modulus and the distribution of longitudinal

material in the forward half-length needs to be specially considered (Lloyds Register Group,

2016).

Direct calculation procedures:

The Lloyds Register direct calculation method involves the derivation of response to regular

waves, the short term response to irregular waves, and long term response predictions using

statistical distributions of sea states. Other direct calculation methods should usually include

all three elements and produce similar results to that of the Lloyds Register method.


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Individual consideration with the direct calculation procedure will be needed for ships that

have the following characteristics;

An overall length greater than 400m

Unusual type or design

Unusual hull weight distribution

L/B ratio of less than or equal to 5

B/D ratio of less than or equal to 2

Large deck openings, or areas where warping stresses exceed 14N/mm²

A CB of 0.6

Carriage of heated cargo

For an accurate assessment of the longitudinal strength of a ship to be made, the following

information is required:

The general arrangement of the ship, including the volume and centre of gravity of all

tanks and compartments.

Bonjean data (tables or curves) for a minimum of 21 equally spaced stations along the

length of the hull. May also require a lines plan.

Calculated lightweight and its weight distribution.

Loading manual.

The centre of gravity and weights of all deadweight items for each of the loading

conditions for individual ship types. This information can be submitted in the form of

a preliminary loading manual that includes the still water bending moments and shear

forces.

Design vertical wave bending moments:

The hogging or sagging design hull vertical wave bending moment at amidships is given by

the following formula:

(kNm) (12)The overall wave loading is predicted using the formula:

(13)Where:

is the wave bending moment factor, shown in table 2 at the aft end of L and is the design vertical bending moment between 0.4L and 0.65L from aft at the forward end of L, intermediate values can be obtained through linearinterpolation


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Ship service factor for unrestricted sea going service, should not be less than 0.5. for sagging (negative) moment

for hogging (positive moments) (14)

For vessels operating in sheltered waters or on short voyages, reduced still water bending

moments may be applicable. For a vessel operating in sheltered waters:

(kNm) (15)For a vessel operating on short voyages:

(kNm) (16)

Still water bending moment [MS]:

Sagging and hogging are the maximum moments that are calculated under specific loading

conditions. Still water bending moments are calculated along the length of the vessel and

must satisfy the following relationship:

s s M M

(17)

The permissible still water bending moments for sagging and hogging are to be taken from

the following calculations and the lesser of the two used for the calculations:

(18)

(19)Where:

the permissible combined stress in N/mm2

are reduction factors for the calculation of longitudinal stiffeners


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Permissible hull vertical bending stresses:

To calculate the permissible vertical bending moments acting on the hull, the vertical bending

stress is required and can be seen in Figure 22. The methods for calculating the stress along

the length of the hull varies with the following formula:

(a)

Stresses within 0.4L amidships:

*

+ (20)(b)

Outside of 0.4L amidships:

(

) *

+ (21)

Where d is the distance (in metres) from A.P or F.P

Figure 22 Development of vertical bending stresses along the length of bulk carrier, Goliath.

Local reduction factors:

Maximum hull stresses at the deck (σD) and keel (σB) are calculated by the following

equations;

̅ * + (22)

̅ * + (23)

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

180.00

200.00

0 50 100 150 200 250 300

V e r t i c a l b e n d i n g s t r e s s [ N / m m 2 ]

Length from APP [m]

DESIGN LOADS RULES/CODES FOR CLASSIFICATION OF SHIPS/OFFSHORE INSTALLATIONS

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