Pub75_TankerModifications

8/2/2019 Pub75_TankerModifications

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1.0ENERALThis Guide identifies the influential parameters and provides guidelines on improvement ofstructural connections. It also provides for reference illustrations of local structural failuresexperienced in some existing tankers and the corrective measures as compiled by the TankerStructure Cooperative Forum (1986) and from the American Bureau of Shipping's data files.

2.0NFLUENTIAL PARAMETERSFor design of structural details, due consideration is to be given to the following influentialparameters.2.1 Loading Patterns and Nominal Stresses

The load distribution among the surrounding structural members and the correspondingnominal stresses at the location considered should be examined for the combined loadcases specified in 512A3.5.2 of the Rules. It is important to consider the combined effectsof all the simultaneously imposed load components, rather than only one selecteddominating load component. The stress distribution may be obtained from a 3D finiteelement structural analysis as specified in &2A5.3 of the Rules or by other equivalentmeans.

2.2 Stress ConcentrationDue to load transmission and diffusion at a structural connection, it is inevitable that someform of stress concentration is going to occur in the loaded structure. Therefore, particularattention is to be paid to structural notches, abrupt changes in structural properties andexcessive distortions and deformations; such as locations at the bracket toes, cut-outs,terminations of heavily loaded members, the connection of flexible elements with muchstiffer members, the ends of unbalanced structural members, just to name a few. For shipstructures designed with relatively low working stresses and high safety margins, the


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detrimental effects of such stress co ncentrations m ay not b e readily apparent. On the otherhand, for structures designed with relatively high working stresses and low safety margins,damaging effects of such stress concentrations could appear sooner than the anticipatedperiod of time.Appropriate stress concentration factors (SCF's) ob tained from either experimental data orstructural analyses are required to evaluate the d esign of structural details.

2.3 Weld Effects and Fatigue StrengthAnother important factor to be considered is the effects of welding on the properties ofmaterial (heating effects) and on structural continuity (profile of the weld deposit and under-cuts). This factor which highly depends on the welding methods, processes andworkmanship is vital for assessing the fatigue strength of the joint. To date, the weldingeffects on the fatigue strength are primarily determined by experimen tal data presented inthe form of S-N curves and characteristics of the test specimens, as shown in the Append ix5/2AA of the R ules.

3.0ONSIDERATIONS FOR DESIGN OF STRUCTURAL DETAILSIn light of the discussions given in 5/2A5.4, it is apparent that the necessary criterion for thedesign of structural details is simply to offer a well balanced joint which is "compatible" with theanticipated w orking stresses. To this en d, the solution is to be tailored to a specific location of aspecific design. In addition, there is no unique solution to the problem. Many alternatives mayexist. The designers/builders would have to exercise their judgment based on their fabrication

facilities, techniques and experience. The information offered below is provided for reference.

3.1 Structural Configuration and Arrangement of BracketsWhen selecting the structural configurations and sizing the brackets connecting primarysupporting members, due consideration is to be given to the load transmission and the

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relative stiffnesses of the members to be connected. A smooth pattern of load transmissionis essential. The connecting bracket is to be of sufficient size to withstand the highlyconcentrated loads and to shift the critical spots (bracket toes) to the lower stressedregions. An appropriate stiffening system is also required for large brackets to prevent localstructure instability and tripping. Sample illustrations are shown in Fig. 1 and Fig. 2 fordamages experienced in existing tankers with recommended modifications.

3,2 Installation of Brackets and Collar PlateTo prevent local distortions and to minimize the magnifying effects of structural notches,consideration is to be given to additional brackets and collar plates (or lugs) at the criticaljoints and cut-outs, respectively. Sample illustrations are shown in Fig. 3 for local damagesrecorded and the recommended modifications.Alternatively, different stiffening systems may be considered to minimize the critical spots inhighly stressed regions. For example, utilization of the horizontal stiffeners with appropriatetripping brackets for floors in the double bottom, instead of vertical stiffeners wouldeliminate the critical spots at the face bar toes on the flange of longitudinal as shown in Fig.3c.

3.3 Softening Hard SpotsTo minimize the stress concentration at hard spots, such as bracket toes, tapered faceplate, and welded connections of the face plate of transverses and bulkhead plating, thewelded joints may be softened by providing a large radius at bracket toes or a cope holewith proper reinforcement of the surrounding panel to prevent local instability. Somesample illustrations are shown in Fig. 4.To minimize the detrimental effects of knuckles in structural elements, sample illustrationsare shown in Fig. 5.

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Modification Existing

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Figure 1(a) Connection of Bottom Girder and Vertical Web

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LONGITUDINALBULKHEAD

FRACTURE

BOTTOM TRANSVERSEEND BRACKET

AG M . --I

FACE PLATETAPER MINIMUM 1 : 3L C=Ell \\NUELLPENETRATIONINSERTED PLATEVVITH NCREASEDTHICKNESS

TRIPPING

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Figure 1(d) Reinforcement at Connection of Bottom Girder and Oii tight Bulkhead

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FRACTURE BOTTOMSHELL

TRANSVERSE0.7, BULKHEADBOTTOMTRANSVERSE

BOTTOMC.L.GIRDER

FRACTURE

VIEW A - A

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.---.7.._ _ _ _.. - .\",. . - - -\--' TRANSVERSESHELLWEB BULKHEADC' , - -' , . . " , : : : . F c 77 v-":-.-a--NO..76,-A-43 ,e- "p---c.i.e .o k a m s.SIDE LONGITUDINAL

FRACTURE. . . . . . . . . . - - - - - - - - - - - - - - - - . . . . . .CE SHELL. 4 ; . c.,",,1.5 ' l p ? P O R . . . 7-C4 . / . 7 - ,,Y e -Y / C - - , ,4i---fc"- S. / . . A",,, ,F'. C - 7 4, c - . - -. . e . _ O F Y C - Y 7- C . 1 2 . ) .1Mt f I. . :I ihamit AIMEIIIIIIIIIIIIIIIIIIIIIIIIIIFigure 2(b) Connection of Horizontal Girder and Side Longitudinal

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Figure 3(a) Connection of Bottom Girder and Swash Bulkhead

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Figure 3(b) Tripping Bracket on Trans. Web

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Figure 3(c) Connection of Longitudinal to Floor in D.B.

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LONGITUDINAL BULKHE.1,0

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Figure 4( Connection of Transverse Face Plate and Bhd Plating

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Figure 4(d) Bracket Toe on Longitudinal

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Figure 4(e) Sniped Tripping Bracket on Horizontal Girder

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1

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F B

Ground tack

Largeradius

Full penetrationwelds > 75mm

Longitudinal

Figure 7 improved Brackets Toes

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