Design of Ship Structure

By. Ramavtar Rander 1RV08ME077Design of Ship StructureIntroductionThe ship design is divided generally into four parts

Form designArrangement designStructure designFitting design

The design of merchant ships starts with the owners requirements such as kind and volume of cargo, transportation route and time generally. Sometimes the owner has a special requirement such as no bulkhead in hold.Sequence of Structure Design

There is no description of the material, which is one of the most important items in the structural design, because mild steel, which has been a major shipbuilding material.

However, nowadays high tensile steel is becoming more common as a shipbuilding material. When the high tensile steel and other special materials are used as shipbuilding material, the ship structure designer must pay careful attention to these materials.

Structure Design PolicyThe principle of Ship structure design is

a) High reliability b) Good Performance c) Easy Maintenance

High reliability can be achieved by reliable design stated after. Good performance means a well-balanced structure which can carry out the given duty during the given period. Good performance can be obtained by rationalization. Reliability concerns the safety of ship, crew member, and cargo, so as to prevent any failures which could jeopardize the ships safety. Easy maintenance means less repair cost and good accessibility for inspection.Flow Diagram of Reliable Design

Theoretical reliable design aims to keep reliability by gathering data on statisti- cal parameters and calculating failure probabilities theoretically for many kinds of failure modes.The failure probability can be obtained by considering the distribution patterns of the forces applied and the strength of the structure. As shown in fig 3 if probability density functions of forces applied D (hereafter forces applied is to be called demand) and strength of structure C (hereafter strength of structure is to be called capacity) are given with the horizontal axis indicating stress X, the failure probability is given by the following equation:

WherePd{X}: probability density function of demand Pc{X}: probability density function of capacity Qd{X}: probability of demand exceeding certain value Qc{X}: probability of capacity exceeding certain value

Fig4. Probability of fracture and mean safety factorFig3. Probability of fractureDesign FlowThe design of a hull structure is generally carried out in three stages:Basic design: Following the determination of a preliminary structural arrangement in the planning step, the midship section drawings are prepared, followed by the rule-based scantling calculations, strength and vibration calculations, and hull steel weight estimation.Detail design: Following the completion of the detailed midship section drawings, which also include the production method; the bow, stern, engine room, and superstructure are designed in detail. These designs take into account the fitting arrangement and hull block assembly process.Production design: To the above detailed design further information for structure manufacturing is added.

Fig 5CAD /CAM

Fig 6In recent design systems, design data is linked to a production system like CAD/CAM, as shown in Fig.6 and will be integrated in the near future by using a product model like a CIM system. The design data of the old systems was input manually for the production system. By using the product model, so-called concurrent engineering will be successfully applied as shown in Fig. 7, and the design time will be shortened.The organization of the design department and the design schedule depends on the shipyard and the type of ship to be designed, however an example is shown in Fig.8, in which hull structure design is carried out in four stages: (a) basic design, (b) function design, (c) detail design, and (d) part design.

Design Procedure For Ship StructureFig. 7Design Procedure For Ship Structure

Fig. 8Design Spiral

The basic design of a ship is as Fig.9. This sequence starts with the determination of the principal dimensions, followed by the ships speed and engine horsepower, as well as the type of engine, and continues with the determination of preliminary lines, arrangement of compartments, structural arrangement, fittings, engine room arrangement, etc. until a final setting of the list of materials, specifications, and ship price. Thus, starting from a blank sheet, basic ship design consists of making a series of decisions on alternative choices to obtain the outline of a ship possessing, the required characteristics, and performance in the shortest possible time.Fig. 9Step 1 consists of roughly determining the structural arrangementStep 2 the same elements will be examined in more detail to consider strength and vibration. Step 3, the scantlings are determined for the principal members forming the midship structure, and approval drawings are prepared, from which strength and vibration calculations are performed. The resulting data serve to estimate hull steel weight more accurately. Figure 10 shows a ship structure basic design system COSMOS as an example , which includes four modules1. Rule-based scantling calculations 2. Strength calculations :- a. estimation of loads using hull motion calculations b. direct stress calculations using FEM analysis c. evaluation of stress, fatigue strength, and buckling strength3. Vibration analysis 4.Ship steel weight estimation

Fig.10General ArrangementRatio of ship length and depth (L/D)The arrangement of longitudinal and transverse bulkheads.Midship section shapeType of bulkhead in cargo hold Transverse (frame) space and longitudinal spacePosition of superstructure, deck house, deck machineryEngine particulars, machinery arrangement, propeller particulars and rough lines should be checked by the structure designer from strength and vibration points of view.Concept of optimum structure design applying genetic algorithm

Fig. 11

Design Of Hulk Structure

Design of BeamsDesign of GirdersDesign of PillarDesign of PlatesDeflection of Hulk StructureWeldingHulk Structure Vibrations

Design Of BeamsConsidering the strength, the stiffened panel will be assumed to be a collection of beams which include some parts of the attached plate. The breadth of this plate is called the effective breadth or effective widthThe effective breadth is represented as follows:

Be: effective breadth B: distance between stiffenersx:stress of longitudinal direction along with max: maximum stress at connection line of a stiffener

Fig 12 :Effective breadth of stiffened plate2. Span Point of Beams Yamaguchi proposed a formula for bending which was obtained theoretically and experimentally

Fig. 13 Span points for shearing deformation3. Design of Cross section4. Optimal Design of Beam Section

a) Balanced girder b) with plate

a) Elastic Design b) plastic designDesign of GirderA girder is a structural member which supports lateral forces imposed by beams. Beams, stiffeners, frames, longitudinal, etc. are called secondary members. Girders, web frames, transverse webs, etc. are called primary members for which the shearing force has to be taken into consideration during design

M: bending moment F: shearing force t: thickness of web plate I: sectional moment of inertia m1: moment of the part farther than distance y1 from the neutral axisa) Web onlyb) With plate and faceDesign of PillarPillar supports the axial force and generally supports the axial compression but sometimes it supports the axial tension.A slender pillar under a compressive load will break by bending when the load exceeds some limit which is called buckling. The stress at buckling is decided by the slenderness ratio of the pillar. The slenderness ratio is given by l/k , where l is length of pillar and k is radius od gyration

Fig.14 Slenderness and buckling critical stress of pillarDesign of PlatesPlates make up the main hull structure such as shells, decks and bulkheads, in con- junction with secondary supporting members such as stiffeners and primary supporting members such as girders

w: deflection at plate center a: coefficient q: intensity of uniformly distributed load a: length of shorter edge of plate b: length of longer edge of plate D: flexural rigidity of a plateE:Youngs modulus :Poissons ratio t:plate thickness

Deflection of Hulk Structure1. Deflection in Hulk Girdera. Expansion and contraction of pipes and rods fitted in longitudinal direction on the upper deck or bottom.b. Increase of draft caused by deflection of hull girder. c. Generation of secondary stress by the deflection of hull girder. d. Flexural vibration of hull girder, whipping.

2. Deflection in Beam

Fig.14 Deflection of Optimum BeamWelding

Before the use of welding methods in hull construction, the steel plates were joined by rivets

Water Stopping Welding

In case of tension

In case of bending

Fillet welding Hulk Structure Vibrations

The vibration itself cannot be not clearly defined, unless these three parameters direction, frequency and amplitude are specified.

Concept to minimize hull viberationPrevent resonanceReduce exciting force

Problem in Ship Design

The change in boundary conditions of ship vibrationsboundary conditions caused by energy savingProblem in Ship Design

Progress in Ship DesignIn the hull structure this framework is most important and should be defined before longitudinal, transverse and local strength. The strength of this framework is called basic strength hereafter. In Fig. 15 the technical conditions, which led to the scaling up of ships, are shown.

Fig. 15Software used for Ship Structure Designing CATIA V5 RINA MARINENupas CadmaticReferencesDesign of Ship Hull Structures by Yasuhisa Okumoto Yu Takeda Masaki Mano Tetsuo OkadaShip Design and Construction by The Society of Naval Architects and Marine Engineers 601 Pavonia Avenue Jersey City, NJ 07306Millennium Class Tanker Structural Design From Owner Experience to Shipyard Launching Ways --James Read, Arne Stenseng, Rod Hulla and Darold PoulinAnalysis and Design of Ship Structure by Philippe Rigo and Enrico RizzutoCOMPARATIVE STUDY OF SHIP STRUCTURE DESIGN STANDARDS by Ship Structure Committee RADM Craig E. Bone U. S. Coast Guard Assistant Commandant, Marine Safety and Environmental Protection Chairman, Ship Structure Committee