Rational design assessment for classification purpose-application to hull design of large monohulls

5*hInternational Conference on Fast Sea Transportation - FAST ’99 31 Aug -2 Sep 1999 Seattle, WA, USA

RATIONAL

DESIGN

ASSESSMENT

FOR

CLASSIFICATION

PURPOSE

- APPLICATION

TO HULL

DESIGN

OF LARGE MONOHULLS

By Etienne THIBERGE – BUREAU VERITAS – Head of Special Units Service – New

Constructions department

ABSTMCT

The importance of the operational requirements as dej?ned by the )7X

Code, their progressive

working-up and their influence on the design of high speed craft according Bureau

Veritas

experience

are outlined. The main steps of rational design are then described. Last, the Class

structural design assessment is comprehensively presented, with particular emphasis given to large

monohulls and their typical key points of concern, thanks to the significant experience gained by BV

in the last decade.

1 OPERATIONAL REQUIREMENTS

1.1 The operating conditions as per HSC Code The philosophy of the HSC Code [1] is based on management and reduction of risk, as well as passive

protection in case of an accident.

In that view, the Code specifies requirements and guidance not only for the crafl design (chapter 2 to chapter 16) but also for the operation of the craft (chapter

17 to chapter 18).

The Code refers to the following specific conditions.

1.1.1 Normal operation conditions

The normal operation conditions, are K those inwhich the

craft will safely cruise at any heading while manually operated, auto-pilot assisted operated or operated with any automatic control system in normal mode )} (Annex 8- 3.1).

The normal operation conditions are determined in such a

way that actual horizontal acceleration in passenger

spaces remain below a criteria defining a minor effect on safety level (safety level 1- Table 1 of Annex 3).

The resulting effect on safety being just described as <tmoderate degradation )}, one can therefore understand that these conditions typically relate to COMFORT only. In other terms, the normal operation conditions do not quali@ the craft design, but only identifi operating conditions under which no specific operational care is requested.

1.1.2 Worst intended conditions

The worst intended conditions describe a the specified

environmental conditions within which the intentional operation of the craft is provided for in the certification of the craft ))(Chapter 1- 1.4.48).

The Code clearly states that the environmental parameters are dedicated to the area of operation.

In term of performance, the Code specifies that these conditions a are those in which it should be possible to maintain safe cruise without exceptional piloting skill. However, operations at all headings relative to the wind and sea may not be possible>) (Annex 8- 3.2)

The worst intended conditions are so determined that :

● _{actual horizontal acceleration in passenger spaces}

remain below a criteria defining a medium effect on safety level (safety level 2- Table 1 of Annex 3),

● any other cratl motion could not impede the safety of

the passengers,

● the manoeuvrability remains safe, even with reduced

speed,

● the stability remains adequate,

● _{the actual structural load remains within the design}

one.

Passengers should even be requested to be seated when safety level 1 is exceeded.

These requirements obviously relate to safety. They are not only linked with operation of the craft, but also address the craft design itself.

1.1.3 Critical design conditions

The critical design conditions are ((limited specified

conditions chosen for the design purposes, which the craft should keep in displacement mode}) (Chapter 1- 1.4.16). The Code duly outlines that a such conditions should be more severe than the worst intended conditions by a suitable margin to provide for adequate safety in survival condition }}(Chapter 1- 1.4.16).

According their definition, these conditions are only use for crafl design purpose and correspond to the theoretical ultimate environmental conditions in which the survival of the craft is to be achieved with a acceptable safety j), in displacement mode.

It is to be noted that the Code does not define what <(acceptable safety N is.

1.2

1.2.1

The technical specification according BV experience

Operational envelope

The definition of the operational envelope is in fact the result of a three-step process (Fig. 1).

Sl!21W

The very first input in the technical specification are made of objective information and data connected to environmental conditions, which are to be necessarily coped with :

● Intended routes (location, distances, etc...)

● Meteorological data on the route (waves, wind, etc..)

w

In addition to that, operational requirements are also given by the Owner. Theoretically, such parameters may be slightly adapted during the design process. BV experience is that these parameters are generally taken as a firm basis for the design.

The most common Owner requirements typically consist in:

● commercial yearly operation (minimum figure

requested),

● round-trips duration (maximum figure specified,

including harbour operations),

● payload definition (passenger/ freight balance), ● Motion Sickness Incidence (MSI) figures.

WIDJ

The full set of above data are then used as a contractual basis for the working up of the technical specification. At that stage, it becomes the designer task to determine the operational parameters of the craft , in order to comply with all these requirements.

Such parameters are :

● typical wave data (directions, heights and periods

-typical sea spectrum),

● corresponding service speeds, ● possible specific headings, ● possible sailing limitation.

This define in fact the operational envelope.

The second step is based on the Owner / operator experience and the profitability estimation.

The third step is linked to the technical answer which will be proposed for this specific demand.

Operational envelope is strongly depending on the craft itself, whether it is a category A or B passenger high speed crafl, a cargo craft, a catamaran or a monohull, a stabilized craft or not, with or without a bow door, etc... The commonly accepted methodologies which may be used for evaluation of operational parameters are :

●

●

●

hydrodynamic calculations, with the reserve that accurate computation of craft motions and loads with an important Froude number is rather delicate, for the time being. Numerous developments are underway on that topic in the marine community.

Tank tests on a craft model. This method is very much technically profitable, but very time

consuming. In other words, it does not allow for easy step by step optimisation.

Extrapolation of previous fill scale experience, gained on a similar craft. This kind of approach is very much time saving, but the accuracy may be

discussed. It is typically to be confined by one of

Figure 1 DESIGN PROCESS

the other methods.

t-=== _{... ... ... ... ...}

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1.2.2 Environmental conditions limits

From the operational envelope, the Owner and the designer can then elaborate and agree on the so-called worst intended conditions.

The worst intended conditions, in fact, describe the upper

limit of the operational envelope under which the safe operation of the craft is proven and accepted for commercial use, but under which special care are to be

step 1

step 2

step 3

taken for operation (reduction of speed, change of heading, etc...).

Moreover, theworst intended conditions areto include the ultimate weather conditions above which the cratl is not authorised to sail, should the weather forecast exceed these conditions.

These conditions are of paramount importance during the design assessment. Their definition is to be clarified early in the design process.

The following information must absolutely be defined by the designer and accepted by the Classification society:

● Design vertical acceleration level,

● Operational follow-up means for accelerations levels, ● Stabilisation devices (involved in safety or only

comfort related).

The design assessment carried out by the Classification society is based on these worst intended conditions, and the mutual agreement on that set of data is very important before the crafi certification starts.

Bureau Veritas can play a very positive and dynamic role, by highlighting the interaction between these conditions and the craft design or the design of sub-systems.

2 GENERAL DESIGN

2.1 Design

Early in the design stage, the estimated ship behaviour inside the operational envelope is assessed.

As already outline~ model tests or numerical simulation are used for that purpose, and provide the designer with the design parameters, such as :

● vertical acceleration along the craft, ● transverse acceleration,

● other craft motions.

Both global and local loads are then deduced from these parameters.

The vertical acceleration level plays a great role during the design of the hull structure, as it governs the impact pressure on bottom on one hand and the vertical amplification of loads on decks on the other one.

Head-on collision design acceleration is also to be evaluated. It should be noted that the design head-on collision acceleration, as defined by the chapter 4 of the Code, is based on the operational speed parameter, this later being defined as 90% of the maximum speed. The IMO Sub-committee on Ship Design and Equipment, currently working on the revision of the HSC Code, however proposed recently that the impact speed be taken as two-thirds of the operational speed in the next issue of the HSC Code [9].

For instance, the certified environmental limits for the operation of a marine escape system (MES) may affect negatively the theoretical operational envelope of a high speed craft

2.2 Bureau Veritas Class review

The rational design assessment starts with the preliminary design review (review of the specification and the GA against relevant BV Rules [2]) (Fig.2).

2.2.1 Operational aspects

It is noticeable that the Classification society does not take the place of the designer, in defining or issuing requirements for operational envelope.

The design operational envelope is to be formalised and considered as Classification documentation, in so far it is linked to the Class design assessment.

This is to be presented to and understood by the operator as the load limits on basis of which the Class certification is developed and granted. The Permit to Operate, issued by Flag authorities, will take account of limits which are within this Class accepted design operational envelope. The parameters of the operational envelope which concern the hull strength including the worst intended

conditions, will generally form an appendix to the

Classification Certificate.

In case no vertical acceleration level is known, the Rules value is considered for the purpose of the review,

2.2.2 General design aspects

At this early stage of the Class review, the points highlighted in the following table (Tab. 1) are to be paid special attention.

2.2.3 Structural design aspects

In a similar way, some critical points are to be checked very early during the prelimin~ design review, in order to confm the design of the structure.

They are given hereafter (Tab. 2).

Importantly, not only the craft design is to match operational envelope, but also major equipment.

Figure 2

PRELIMINARY DESIGN REVIEW

~

Review of Technical specification&

+

I

*

Table 1 GENERAL DESIGN

Main item Point of concern Rules reference

Collision effect on basis of the so-called design level [2] Passenger (or range of head-on collision acceleration): req. 4.3 – 4.4 – 4.5

. longitudinal location of the accommodation, – 4.6

accommodation design

● seating design and arrangement, Table 4.4.2

● positioning and securing of equipment and baggage in

public spaces and operator’s compartment

General [2] - req. 4.7

Exits and means of

escape Zoning (category B only) [2] - req. 7.11

Sizing of alternative safe area for passengers [2] - req. 7.11 (category B only)

Capacity of survival craft [2] -req. 8.10

Survival craft and

rescue boats Location of survival craft [2] - req. 8.10

( not a class matter)

Number and location of rescue boats [2] - req. 8.10

Location of collision bulkhead [2] - req. C2.1.4

Watertight

bulkheading, below Number and location of bulkheads [2] - req. C2.1.4

tleeboard deck

Number of openings in bulkheads [2] - req. C2.1O.7 +

addendum n“ 1 ( 07-97)

Freeboard arrangement (not a Class matter) MSC/Circ 652

Weathertight doors ( 09-06-94)

Bow doors and associated inner door [3]

Location of inner door [3]

Machinery and Redundancy ( category B ) [1]-preamblell.

auxiliary systems – 2 independent means of propulsion [2] - req. 9.7

arrangement – ability to maintain essential machinery and control in case of [2] - req. 9.8 fme or casualties in any one compartment

Table 2 STRUCTURAL DESIGN Main item worst intended conditions Bulkheading in garage spaces (A40nohull ) Bulkheading above freeboard deck (Catamaran) Bow door

Large openings on side shell

Pillaring

Point of concern

● Adequate specification

. Number and location of transverse bulkheads, including casings, . Correspondence with bulkheading below

(for proper strength against racking ) . Openings in transverse bulkheads

( location, sizes, risk of significant secondary bending moments) . Existence of side shell plating in way of transverse bulkhead

( associated date effect)

. Number and-location of “transverse bulkheads . Correspondence with bulkheading below

(for proper strength against mostly pitch connecting moment)

● Openings in ends transverse bulkheads

( location, sizes, risk of significant secondary bending moments) . Structural interference between bow door and fore inner door . Number, size and shape

● _{Location ( risk of excessive stress concentrations)}

. Correspondence with primary structures

● Load transfers

● Local secondary moments

3 STRUCTURAL DESIGN – application to large monohulls

Thanks to the comprehensive knowledge secured with 8 large monohulls which are among the top 20 largest high speed craft, Bureau Veritas Head OffIce specialists developed a tailor-made response to designers and shipyards, both on the design and hull monitoring sides. A summary of this experience is given herafier.

3.1 Design assessment

3.1.1 Major concerns

During the design assessment carried out in the scope of the classification, major concerns governing the structural strength of large high speed monohulls are identified with respect to the structural effects resulting from their failure. When considering the effect categorisation according Annex 3 of the HSC Code [1], we can outline that the

Classification assessment focuses fustly on “hazardous effects”, and then on other “effects”. That means that “major effects” are considered in a second step, and “minor effects” in a third step. It is checked that the corrective arrangements proposed by the designer to remove the “hazardous effect” actually do not upgrade a side “minor effect” or “important effect” to an “hazardous effect”.

A typical example of such a situation should be :

● “hazardous effect” : strength of primary structure

of an aluminium upper deck not adequate,

● corrective arrangement : fitting of pillars

(aluminium) below, in a garage space,

● _{“major effect” : collapse of a single pillar,}

. this “major effect” is in fact upgraded to au “hazardous effect” due to general collapse of aluminium pillaring in case of a fwe in garage space. (therefore, preventive actions are to be taken : either steel pillaring or enhanced sprinkler system).

According to Bureau Veritas experience and R&D, the following major concerns can be check-listed and are to be carefully checked against Rules [2], keeping in mind the single failure concept, according Annex 3 of HSC Code [1]

. Global strength,

● Bottom strength in impact area, ● Waterjet areas,

. Bow and stem doors, . Garage space structure,

● Fatigue life of structural details.

During the design assessment process, the limit operating conditions which are consistent with the structural strength are progressively set out. They are basically based:

● Local strength under impact (key data = vertical

acceleration level),

Table 3

. Global strength (key data= overall longitudinal bending moment).

3.1.2 Global strength

The assessment of the global strength against Rules [2]

is the very first step of the Class design assessment. During this analysis, not only the main hull girder strength is analysed, but also the local strength of plating and secondary longitudinal, contributing to the global strength.

The main characteristics of this step are given hereafter (Tab. 3).

The software “MARSPEED”, developed by BV, is presented in Annex 1.

Global strength analysis

Application throughout the crafi’s length

1 -to check that the actual static longitudinal strength complies with the Rules criteria

2 – to check that the scantlings of plating and Iongitudinals contributing to the Target

overall strength comply with the Rules criteria, under local Rules loads

3 -to identi~ the problematic areas, where design or scantlings need to be modified

1- for each transverse sections, longitudinal stresses induced by Rules overall longitudinal bending moment are calculated and compared with Rules admissible stress levels

Method

2 – for each transverse sections, and for each individual strake and longitudinal, the allowed local bending stress is derived from the actual overall bending stress in the element, and minimum Rules scantling is calculated and compared to the actual one.

Strake : the longitudinal buckling strength of each strake is also checked.

Longitudinal : the shear strength and the attachment to main transverse fiwnes are also checked for each individual longitudinal.

13v tool Rule software “MARSPEED’

Time required ( typical) 2 man-days for a typical 100m – 140m long high speed craft

lle following particular topics are to be focused on :

The adequacy of the Rules [2] value is to be carefully

● Overall longitudinal bending moment :

checked against actual specificities of the design , which could request to adapt the Rule value. In that view, continuous R&D is carried out on that topic,

including springing and whipping effects, where relevant [6].

● Bi-metallic structure (steel/ aluminium) :

In case of hi-metallic structure, the equivalent section principle is used.

The actual Young modulus “Ei”of the material “i” is taken into account, and compared to the Young modulus of the predominant material “E;’.

The individual transverse cross section “Si”of a member built in the material “i” is automatically converted into an equivalent one “SiO”, for the purpose of calculating the overall geometrical characteristics of the craft’s transverse section, and particularly the equivalent overall stress “aiO” in this member.

The actual overall longitudinal stress in this member “d’ is then automatically deduced from the calculated equivalent overall stress “cr~’.

SiO. Si. E@ oi = trio, Em

. Contribution of long superstructures :

The contribution of long superstructures to longitudinal strength is estimated with consideration given to the actual connection of superstructures to the main girder (side walls, internal bulkheads, pillars) which may impose to the supersmcture the same curvature as the main girder below. A contribution factor, between O and 1, is then granted to the members of the superstructure, when describing the crafi’s transverse sections.

The resulting calculated overall stresses in these members are then automatically reduced by this contribution factor,

before comparison with criteria.

Where some doubts exist about the contribution factor, and due to the very fast and user-friendly BV tool, two calculations are performed, granting successively a 0°/0 and a 100% contribution to the superstructure. The second calculation is almost instantaneous, once the first one is carried out.

This doubt may be easily removed by FEM calculation performed at a later stage.

● Influence of windows :

Contribution of windows fitted on side shell or on long superstructures is to be specifically considered, both in term of equivalent metallic thickness to be taken into account in way of glasses, and also for the strength of glasses against the actual overall loads induced by the overall bending of the hull.

A detailed method has been presented by Bureau Veritas in 1995 [5], leading to the parametric expression of the

equivalent thickness of glass and metallic frame work to be input in an overall estimation of global strength. It was clearly shown that neglecting the contribution of the windows on a 100m aluminium high speed monohull was giving an under-estimated stiffness of the top of the superstructure (by 5% of the actual stiffness),

3.1.3 Bottom structure

The assessment of the bottom strength against Rules [2]

is one of the important early step of the Class design assessment. A deep attention is to be paid to the impact zone, typically one third of the craft length forward the midship perpendicular.

The description of the bottom strength analysis is given below (Tab. 4).

The software “STEEL”, developed by BV, is presented in Annex 2.

The following concerns are specifically examined:

● Influence of deadrise angle:

A detailed method has been presented by Bureau Veritas in 1995 [5]. It was explicitly shown that a simple 3D beam model can be used, provided the effect of the bottom plating, working as a longitudinal beam between 2 transverse bulkheads, is properly modelled by a longitudinal beam (Fig.7).

Neglecting this effect could lead to an over-estimation of vertical reactions at ends of bottom primary girders (40% typical over-estimation for a 16° rise of floor). The more the deadrise angle, the more important that effect.

● Impact phenomenon :

Impact is taken into account according Rules formulae [2], using an equivalent quasi-static pressure. It typically depends on the longitudinal location of the area under consideration, the vertical design acceleration at LCG, the deadrise angle and the surface of the area sustaining the impact pressure. As commonly accepted for impact phenomenon, the greater the surface the lower the equivalent quasi-static. In such a situation, the considered surface is edged by

floors and primmy girders.

As stated in the Rules [2], the impact pressure is considered as acting separately on each transverse section, the remaining sections being subject to the hydrostatic pressure associated to scantling draft.

● Buckling problem:

Due to typical longitudinal framing of bottom structure, the buckling strength of bottom plating in transverse direction is rather poor, and need to be carefi.dly and systematically checked.

Transverse compressive stresses are induced in bottom plating by:

- contribution of plating to vertical upwards bending of floors,

- transverse compression of floors due to upwards outside pressure combined with rise of floors.

Where buckling problems are detected, the KARMAN equivalent plate breadth theory is used, in order to assess the need of extra-thickness or additional transverse secondary stiffeners.

. Bottom longitudinal :

Table 4

According BV experience, the sizing of bottom secondary longitudinal is very much influenced by the strength of the attachment to primary transverse structure.

In other words, the actual static strength of welds between longitudinal and floors is to be checked against the Rules [2] criteria. Specific guidance is given in the Rules, depending on the shape of the profile and the design of the attaching welds.

● Spray rail:

Particular attention is to be given to the structural detail in way of the longitudinal spray rail. The strength continuity of the transverse beams associated plate is to be properly designed in order to cope with the transverse stress transfer and to allow a high quality workmanship. Local stiffening on web of transverses may be required to sustain the out-of plane forces wh;ch develop.

Bottom strength analysis

r

Target

=

Method

BV tool

Time required (typical)

3.1.4 Waterjet areas

throughout the craft’s length

1 -to check that the strength of the proposed primary structure complies with the Rules criteria

2 -to check that the connections between secondary structure and primary structure cope with the Rules criteria

3 -to identify the problematic areas, where design or scantlings need to be modified

for each compartment between transverse bulkheads, 3D beam calculation of the primary structure with Rules loads and stresses criteria

BV naval architecture software “STEEL”

1 single man-day for each large compartment

For each waterjet, following loading cases are to be investigated :

A correct and reliable assessment of the structure of waterjet ● LDC 1 : maximum internal hydrodynamic

areas is of primary importance due to difficulties pressure p~ and variations in the built-in nozzle, encountered for possible repairs in service. ● _{LDC2 (*) : horizontal longitudinal force F,l in}

The description of this step is given hereafter (Tab.5). normal service (ahead),

● LDC3 : horizontal transverse force FYand moment

Amplitude of design loads exerted by waterjet propulsion M=during steering operation, system onto the ship hull are to be indicated by shipyard (or

waterjet supplier).

. LDC4 (*): horizontal longitudinal force Fti, vertical The scantlings are to be checked by direct calculations. force F. and overturning moment MYin crash-stop

situation. Except for fatigue strength, the stress criteria are taken

according Rules [2], C3.6. 1.5. (*) depending on the location of the thrust bearing

(generally but not always located aft of the stem, in the The following table (Tab. 6) illustrates the application stator bowl). of each loading case for the scantlings of the various_{components of the waterjet system.}

Table 5

Waterjet compartment analysis

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Application Target Method BV tools Time required Table 6

From transom to fore end of waterjet nozzles

1 -to check that the strength of the primary structure and the bolting on transom complies with the Rules criteria

2 -to check that the constructional details minimize the “hot spots”

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On basis of design loads, 3D beam calculation of the primary structure with Rules stresses criteria (or FEM analysis if required)

- BV naval architecture software “STEEL” (or FEM analysis if required) - hand checking for bolting

No typical figure (very much depending on actual design)

component LDC1 LDC2 LDC3 built-in nozzle - plating x (1) x (2) -bending behaviour ship transom x (2) x bolting on stem x (5)

(l): - thickness under lateral pressure - fatigue strength

LDC4

x (3)

x (4)

x (5)

(2): -buckling (1OO’XOof F, transferred by built-in nozzle in case of thrust bearing aft of the stem) (3): - ratio of MYdirectly sustained by the built-in nozzle to be estimated on basis of relative stiffhesses, (4): - ratio of MYdirectly sustained by the transom structure to be estimated on basis of relative stiflhesses,

(5): - bolting calculation taking account of the actual pre-tension in bolts

General principles are to be strictly followed for such structures subject to cyclic loadings. They are listed hereafter:

● continuous welding,

● shear connections between stiffeners and transverse

frames,

● soft toe brackets, ● no sniped ends,

● no termination on plate fields, ● no scallops in critical areas,

● no start and stop of welding in comers or at ends of

stiffeners and brackets,

. possibly grinding of toes of critical welds.

3.1.5 Bow and stem doors

The strength assessment of bow and stem doors is also a

key step, and it is carried out against specific Rules [3]. The description

(Tab. 7).

The table ( Tab. 8 ) summarises the range of BV ● Respective location of bow door and inner door :

intervention,

Bow door and inner door are to be arramzed so as to The following particular topics are to be paid special preclude the possibility of the bow door cau;ing structural

attention: damage to the inner door or to the collision bulkhead in

the case of damage to or detachment of the bow door.

● Longitudinal location of inner door:

The inner door is considered as part of the collision bulkhead and, in that respect, must be located within the limits specified for the collision bulkhead.

Table 7

Bow and stern doors strength analysis

Application

Target

Method

BV tool

Time required ( typical)

item Bow door + Inner door Stem door Vehicle ramp BV Rules [3] [4] [2]

Bow doors and bow visor – stem ramps/ doors

1 -to check that the strength of the proposed structure complies with the Rules criteria under local loads (at sea and during loading /unloading operations)

2 -to check that the strength of locking devices complies with the Rules criteria under sea loads

3 -to identify the problematic areas, where design or scantlings need to be modified

1 – Rule calculation for secondary structure

2 – 3D beam calculation for primary structure and locking devices under Rules

loads and with Rules stresses criteria

BV naval architecture software “STEEL”

2 man-days for large ramps/ doors

5 man-days for shaped bow visor or bow doors

Table 8

design review

● _{Strength of structure and locking devices} ● Operating device : no Class requirement ● See item “vehicle ramp” if relevant

● Strength of structure and locking devices

. Operating device :

- review if manoeuvring under load - no Class requirement if not

● See item “vehicle ramp” if relevant

. Strength of structure

● Operating device : no Class requirement

Tests and trials

. No Class request . Operating manual requested ● No Class request Q Operating manual requested . No Class request

3.1.6 Garage space structure

The assessment of the garage space structure against Rules

[2] is then carried out in flame of the Class design assessment.

The description of this analysis is presented hereafter (Tab. 9).

The racking analysis is performed for checking strength of structure against lateral horizontal inertia effects induced by rolling motion, which cannot be neglected on a monohull. The racking analysis is to be performed where no complete transverse bulkheads efficiently restrain the transverse inertia loads. This mostly applies to passenger ferry or ro-ro above the lowest garage deck.

The simplest and most efficient methodology is a three-step calculation, hereafter described (Tab. 10).

The design rolling parameters are :

● _{either determined by tank tests or fill scale}

measurements,

● _{or estimated. For that purpose, the vertical location of}

rolling centre may be considered as the vertical cen(re of gravity when no information is available (0.8 x T about for monohull - T about for catamaran).

The type and category of the craft is to be considered, in a view to downgrade the stress criteria when the sailing conditions used for determining the rolling behaviour are unlikely to happen :

● if the craft is a B category, beam sea condition with

O speed is not realistic,

● _{in such a case, using the rolling parameters}

measured in these conditions will allow to use a less severe stress criteria (typically the yield stress) The distributed masses are either estimated or requested from shipyard.

During the 2D beam analysis of step 2, and if transverses bulkheads connect the upper passenger decks, it is highly recommended to consider these upper decks as a single longitudinal box beam, with rotation about a longitudinal axis filly restrained. That is the combined effect of the high torsional inertia of this box beam and the high inertia of the vertical support provided by side shells in garage space.

It may also be cost-effective to combine step 2 and step 3 in one single step, using a 3D beam analysis, if the size of the structure allows it.

Table 9

Garage space strength analysis

Application

Target

Method

BV tool

Time required

throughout the garage

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1 -to check that the primary structure complies with the Rules criteria under vertical local loads (design loads for vehicles or Rules loads for sea loads and accommodations)

2 -to check that the primary structure complies with the Rules criteria under transverse racking loads

3 -to identify the problematic areas, where design or scantlings need to be modified

1- 3D beam calculation of the primary structure above the lowest garage deck with Rules loads and stresses criteria

2 – 2D beam analysis of the structure contributing to racking strength

BV naval architecture software “STEEL”

1 – 4 man-days ( typical figure) for the garage space

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step 1 2 3 Table 10 main muwose

- Calculation of transverse horizontal forces induced by rolling for each deck above the lowest deck

- Distribution of transverse horizontal forces ( step 1 ) on vertical structural members efficiently acting against racking. - Calculation of transverse horizontal deflections at deck levels

- Detailed analysis of transverse structures involved in racking strength

3.1.7 Fatigue life of structural details

In 1998, BV issued a guidance Note for assessment of fatigue strength of conventional vessels [7].

The BV methodology used for conventional vessels focuses on the longitudinal overall bending stresses, and is shown hereafter (Fig. 3).

This methodology is the one computerised in VERISTAR Hull tool developed by BV for design and assessment of complete structure of conventional vessels.

In that procedure:

● BV Rules are used for determination of variation and

distribution of longitudinal stresses, with 4 rules conditions :

- filly laden condition - et% of time (head sea condition 50°A - oblique sea condition 50°!o) - ballast condition - ~% of time (head sea condition 50% - oblique sea condition 50?ko).

- ct and ~ are tabulated depending on types of vessels, withct+~=l,

● Long term distribution of stresse ranges is based on a

two-parameter Weibull distribution ( Fig. 4 ). The shape parameter “k” is generally taken as a function of the ship’s length,

. The notch stress approach is used, considering the peack stress “q” in a notch such as the toe of a weld or the edge of a cut-out

CSl= Kf ~~ where Kf : fatigue notch factor

a~ : hot spot stress coming either from a fme mesh FE analysis or from the nominal stress amplified by a stress concentration factor, . The expected life time is generally taken as 20 years.

sub-stem - design rolling parameters - distributed mass of each deck - calculation of horizontal inertia transverse forces

- determination of efficient vertical transverse structure

- 2D beam analysis in longitudinal symmetry plane, including transverse structure and decks

- 2D beam analysis in transverse vertical plane, on basis of transverse deflections calculated in step 2

Unfortunately, several assumptions used for conventional-vessels are not verified for High Speed Craft and need to be replaced by appropriate theories. Some examples are given below :

● Weibull fimction for the long term distribution of

stresses (due to dynamic additional global loadings),

● Design fatigue life for high speed craft (the actual

navigation conditions being more frequently close to the design conditions than for conventional vessels),

● Additional critical loadings (e.g. bottom impact)

For that reason, and in order to turn its experience on large high speed monohulls to advantage, BV recently embarked on an ambitious R&D program, aiming at setting up au appropriate procedure for fatigue assessment of large high speed craft.

Some very first steps have already been completed:

● ● ● ● ● ● ●

Detailed analysis of global stress, impact stress and behaviour measurements at sea, with the very helpfid participation of the fiench Owner SNCM [6], Characterisation of various structural response to wave loads : “quasi- static” response , “dynamic” response (springing) and “impulsive” response

(whzpping) and validation of BV instrumentation and

measuring methods,

Applicability of the BV hydrodynamic software SHIPLOAD (quasi static response on wave loads) for medium and high Froude numbers,

Applicability of the BV hydrodynamic software SHIPLOAD (frequency of water impacts on bottom and side shell),

Predominant structural failure mode analysis, Validation of fatigue notch factor for aluminium, SN curves and design SN curve for aluminium

The following steps are under way :

. Typology of critical loadings (characterisation, criteria of appearance),

● Demonstration of applicability to HSC of methods

already developed by BV for numerical analysis of whipping,

. Evaluation of influence of 20 year fatigue life on design.

Beside this R&D program, a simple method dedicated to high speed monohulls has been successfully experimented on some craft, starting from this simple observation : On large high speed monohulls, the experience clearly illustrates the need for 2 main investigations early in the design assessment process :

- analysis a: longitudinal at bottom and in upper decks, in the area of maximum overall bending moment,

analysis b : crossing of longitudinal and bottom transverses in bottom impact area.

The corresponding two procedures are described hereafter ( Fig.5 and Fig.6 ), with following footnotes:

(1) - the working up of craft speed versus wave height in head sea conditions is based on two simultaneous

conditions :

- the actual vertical acceleration at LCG is not to pass beyond the design value,

- the overall wave bending moment ( “quasi static” response ) must be kept below the design value

(2) - the sea states data on site are re-distributed in the “n;’ series of Tp used for experimental determination of Mwave = f(V) curves,

(3) - using BV hydrodynamic software SHIPLOAD (4) - local pressure is derived from draft, wave height and heave motion ( using BV software SHIPLOAD),

(5) - using BV software SHIPLOAD.A typical probability chart ( Fig.7 ) is shown hereafter for one longitudinal location on board, one sea condition and one head speed,

(6) – using the OCHI-MOTTER impact occurrence criteria [1O] :

- emergence of the hull,

- relative velocity greater than the threshold velocity ( 3m/s for a 120m LBP craft)

Figure 3

FATIGUE STRENGTH - PROCEDURE

Selection of structural details + I I - FE global analysis - FE local analysis h 1 I I

I

1

I

I

Upon completion of this R&D program, the fatigue calculations will be inserted in the existing MARSPEED software, in order to offer the marine community a user-fi-iendly, time saving and cost effective tool.

The FE fme mesh analysis and the hot spot procedures are also very much in use in BV, for comparative calculation of fatigue life of a detail in basic and improved configurations.

For one given detail, and without any change in the loading history and nominal response, the fatigue life is roughly proportional to the hot spot stress to the power of 3. Therefore the comparison of the hot spot stresses on basic and improved details is a very good gauge of how efficient is the improvement with respect to the fatigue

life (e.g.: crossing of bottom longitudinal and bottom floors in the slamming area).

This simple method is both easy to work and very much informative, provided the mesh of the common parts is not modified fi-om basic to improved detail.

Figure 4

Probability density fi.mction of the Weibull distribution

0.05 0.04 0.03 0.02 0.01 0 0.0 20.0 40.0 60.0 80.0 100.0 Stress N/mm2 . 260

Figure 5

SIMPLIFIED PROCEDURE FOR FATIGUE STRENGTH OF MONOHULL ( OVERALL BENDING )

r-J-l

curve in head sea (1)

➤

1

‘-f

operational routes

I

I

reduction factors on curves (2)

Mwave= f(V) for “ni” sets of

( Hs, Tp ) for various headings b 4

(3)

v

1 1

short terms global load history for all headings

E

1 MARSPEED model l—~

I I

*

short term structural _I I response history I

I

Modification of

- Hot spot stresses design

- Fatigue notch factor _A

- Cumulative damage ratio

I

Yes

Figure 6

SIMPLIFIED PROCEDURE FOR FATIGUE STRENGTH OF MONOHULL ( IMPACT )

7

*

I

curve in head sea

1

impact probability calculation

(5) b

+

v

short terms impact _I load history

STEEL ₄ I

o“ v

short term structural response history

+

FE local analysis

Modification of

- Hot spot stresses design

- Fatigue notch factor - Cumulative damage ratio

Yes 4

End

Figure7 Tp=8.5s -Hs=3.00m- V=40knots 1.OE+OO 1 8= M -= m H u I I i I mi m w -3,5 -3 -2.5 -2 -1.5 -1 -0,5 0 depthunderfreesurface(m) 1.0E41 1.OE-02 1.0E43 3.2 Hull monitoring

The operation of the large HSC must take into account the practical restrictions in terms of wave heights and of vertical accelerations, which are turned into limits by the Classification Society or by the flag Authority. In fact, these limits can be exceeded either in case of a quick degradation of the sea state - HSC are mainly operated in coastal waters where short notice forecasts are unreliable - or in case of a confhsed sea where groups of higher or steeper waves are likely to be met at random intervals. In these situations rare but severe slammings are likely to occur.

On the contrary of conventional cargoships, where the relatively modest propulsive power automatically reduces

the speed when sailing in adverse condition, the HSC forces her way without other warning than spray and vibrations of the hull. An experienced crew will reduce the speed or alter the course accordingly, based on a qualitative estimation and by referring to their experience.

This situation has two drawbacks :

● optimizing the course and speed is out of reach, ● the Officers newcomer to the large HSC need a

learning period of time.

In that context, BUREAU VERITAS has produced a dedicated instrument - SAFENAV II - for providing the crew with real time information on the severity of the sailing conditions. The information come ftom two strain gauges on shell Iongitudinals in the region of the

hull submitted to hydrodynamic impacts and fkom one On the contrary of conventional ships, these impacts accelerometer located at the bow (Fig,8). induce stresses of high energy and of extremely short duration, with a non-conventional time-distribution (Fig.9). Figure 8 [0. N/--l 6 5 . . . , 0 -7

A

evaluation of their severity is definitely inaccurate without the impacts (Fig. 11). the assistance of an instrument. The accelerometer provides

related to the occurrence of

The frequency of the vertical acceleration is slow enough to be correctly perceived by the crew and the association accelerationhpact stresses allows for a better assessment of the actual sailing condition.

The information is processed and displayed on a small panel (Fig. 10) located on the navigation bridge.

Figure 10

This display consists in two bright colour-coded columns, split into GREEN, ORANGE and RED segments. The RED one corresponds to the worst

intended conditions and it invites the crew to take a

corrective action.

Additionally the information is stored on a diskette, for an in-depth fiwther exploitation. A sample of a one-day sailing in severe conditions (Fig. 12 and Fig. 13) shows the short duration and the quick change of these conditions.

These displays highlight the need for the crew to evaluate correctly the combination sea state/heading and to react promptly at any change. By providing objective information, SAFENAV II proved to be helpful in this context.

The positive experience gained fkom three years of exploitation of SAFENAV II justifies a series of improvement to be carried out. They are oriented toward the Owner with a tool dedicated to the statistical exploitation of the sailing conditions, as recorded by SAFENAV II, and toward the naval architect and the Classification Society, through a systematic long duration storage of the information about the structural loads.

Figure 11

100 so m 70 ~: 40 30 20 10 0 Figure 12

SAFENAV II —Sigma MAX Pcii —.-S@na N1/10 Pal 8 Ma ‘1997 — — — — --l---

l---l

07CCI Os?xI 09W low ll:W 1200 1300 14:00 15DI 1600 17:W 1600 19CHI T- ~h.mm] Figure 13 1600 1400 1200 400 2(KI o

WFENAv II _{—AadwMEi-P4xd Wlllo}

8Msy1997 — — — ---- ---

1

1---

:.::1[:---

—

—

offering designers and Owners the evidence of Bureau

4 CONCLUSION Veritas expertise in that field.

In particular, on basis of this increasing knowledge and Bureau Veritas is proactive with respect to the _{improvement of the understanding of the loads on large} International Regulation applicable to high speed craft, _{high speed crafl, BV will confm its position as one of} their interpretations and the development and application _{the leading Classification Society for large high Speed} of comprehensive and dedicated Rules and tools. _monohulls.

Additionally, rational design assessment methods are continuously developed and up-dated in Bureau Veritas, taking benefit of the experience in service, and thus

REFERENCES

[1] International Code of Safety for High-Speed Craft - IMO Resolution MSC.36(63) adopted on 20 May 1994 [2] BV Rules for the Construction and Classification of High Speed Craft - NR396 UNITAS RO1- 1997

[3] BV documentary Note” Steel bow door and inner door on high speed craft”- Tentative Rules ND DT1/308 Rev A -1998

[4] BV Rules and Regulations for the Classification of ships - NR412 DNC RO1 -1997

[5] “Aluminium High Speed Craft and Bureau Veritas” – the 2ndInternational Forum on Alurninium Ships 1995 – E. Thiberge

[6] “High Speed Monohulls – Experimental determination of loads on structure by models and full scale tests” – FAST’97- E. Thiberge – JF Leguen – G. Babaud

[7] BV guidance Note” Fatigue Strength of Welded Ship Structures” - NI 393 DSM RO1 -1998

[8] “Navires rapides et impacts hydrodynamiques” – training course report – BV R&D centre CRD 1996 – JJ Fotzeu

[9] “Report to The Maritime Safety Committee” – IMO Sub-Committee on Ship Design and Equipment – DE42/1 5 – 25 March 1999

[10] “Numerical estimation of slamming risks” – Bulletin technique du Bureau Veritas July-August 1979 – N. Guyen Ket – B. Duval – M. Huther

[11] Soflware development department DV2 - New Buildings Management- Bureau Veritas - Fax 33147145350-Phone :33147145356 – E.mail : pascal.bege@bureauveritas.com

ANNEX 1-

MARSPEED is an interactive software for the design and verification of High Speed Craft scantlings. It applies to both monohull or multihull designs and deals with all combinations of metallic materials (steel, rolled or extruded aluminium) in the same section.

This soflware calculates the scantlings of plating and longitudinal stiffening

The input of data is made are very simple and consists in: . basic ship data, only once per craft (notations, main

dimensions, still water bending moments, material catalog, etc...) (Fig. 3)

● transverse section data (longitudinal location, geometry,

actual materials, specificities, etc.) (Fig. 4)

MARSPEED includes a user-friendly modelling tool of hull structure and loadings, based on:

● graphic-assisted description of the geometry,

. database of rolled profiles and materials,

● automatic Rules loading, including bottom impact

and wheeled loads. MARSPEED computes :

● _{the geometric properties of each described}

transverse section,

● the weight of the structure, ● _{the global strength criteria,}

● the detailed scantling of strokes (Fig. 5) and

stiffeners,

b the buckling strength of strakes.

MARSPEED also calculates and displays the ratio of strake and stiffener scantlings as against Rules scantlings, thus allowing a visual optimisation of the structure (Fig. 6).

This software is the ideal tool for quick verification and adjustment of scantlings with reference to Rules.

It is also filly appropriate for control and optimisation of hull weight balance.

This software is distributed free of charge upon request to the software development department~l 1].

-Figure Al. 1 BASIC SHIP DATA

k *..” ....=.. __... =... _-=_ . . ...-... -_.w... =_.. —’---”--r ~1 1~=...~.,... ..’ Figure Al.2

TRANSVERSE SECTION DATA

Figure A 1.3

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1

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,

H

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23./

DETAILED SCANTLING OF LONGITUDINAL

Figure A1.5

STRAKE RATIO AGAINST RULES SCANTLINGS

ANNEX 2-

“STEEL” is a 3-D beam analysis program developed by Bureau Veritas and is dedicated to shipbuilding, in steel or in aluminium.

The software includes user-friendly windows application allowing the following :

● easy pre-processing for modelling and loading, ● _{efficient post-processing for good understanding of}

structure behaviour,

● efficient post-processing for stress distribution analysis, ● enhanced interactivity with printing and graphical

options.

The easy pre-processing allow numerous and simple cross checking of the model and the loads. High graphic options allow to make comfortable, even with complex 3D structure.

The very simple and easy post-processing make the structure behaviour quite understandable and increases the demee of confidence given to the calculation.

A high interactivity level is provided for, for easy modelling :

. plane modelling,

. duplication of 2-D rings in space,

● _{creation of a symmetrical model by mirror effect,} ● intensive use of graphic checks at every step of the

program.

Results outputs are either numerical or graphical. The limits of the software areas follows:

● 360 nodes, ● 500 beams,

● 80 different types of beams, ● _{100 internal beam releases,}

● 10 loading cases (with 100 groups of forces for

each loading case),

● 131040 degrees of freedom.

“STEEL3” is typically one of the best time-saving tool for assessment of primary structure behaviour.

Figures A2. 1 and A2.2 illustrate a typical 3D calculation of the primary structure of a large high speed ferry.

In addition, it makes the structure optimisation really simple Quotation and software may be obtained from the

and fast. software development department [11].

FAST99- 1 UNLOAOED STRUCTURE i i0399

i30m Monohul 1 FR 42 to F59

STEEL 14 B Oureau Varltas OEMO t3ureau Kw-i t as

Figure A2.1 3D BEAM MODEL

FAST99- 1 LOAOING 3. + Dynamic 5ea pressure 090699 DELTA II EO

CA

‘ i30m Monohul 1 FR 42 to F59

STEEL 14 B Bureau Veritas DEMO But-eau Veri tes

Figure A2.2 DEFLECTED STRUCTURE