Page 14: of Maritime Reporter Magazine (February 1974)

Esso Norway Tests— (Continued from page 14)

Structural response caused by pressure sys- tems on tank boundaries must be compared to a condition with zero pressure on tank boundaries in order to get absolute values. This meant that, under conditions of zero trim, the internal pressures on the boundaries of tank groups Nos. 2, 3, and 4 (except for the forward boundary of No. 2 and the after boundary of

No. 4) must be matched by the external pres- sures governed by the draft. Expressed other- wise, this meant that the height of water bal- last in all tanks in groups

Because this combination of drafts and tank loadings formed the base with which all read- ings of instruments were compared, this con- dition was called a "base run."

Ideally, if the structure were affected only by pressures, only one base run would be needed. In reality, however, there are a num- ber of other influences which affect the in- strument readings which measure structural response. Some of these are: 1. Thermal Influence. The phenomenon of thermal stress is well-known, and the fact that it is powerfully affected by radiation from the sky had to be considered in planning the test program. Time limitations led to the de- cision to operate the test schedule on a con- tinuous round-the-clock basis, without regard to the ambient weather conditions at the time of the instrument reading. In addition to the temperature effects on the steel of the struc- ture, temperature obviously has some effect on the cabling of the instrumentation system itself. 2. Instrument Drift. Instrument systems normally show some drift, that is. continuous change with time, even with the best com- pensation available. Thus, in comparing the readings of a test run with those of a base run, the shorter the time interval separating the two runs, the more reliable the results should be. 3. Shakedown. The structure itself in some areas is subject to sequential instabilities, often characterized by the words snap-througii or oil-canning. This means that a nominally plane plate may exhibit a bulge to one side at one time, and at another time under exactly the same conditions of loading may exhibit a bulge of another character, usually on the oth- er side. Phenomena such as this characterize structural "shakedown" and may be presumed to gradually disappear as the structure under- goes a series of loadings and unloadings and finally assumes its ultimate form.

These three influences and some others less important led to the decision to intersperse among the sequence of test runs a number of base runs instead of depending on a single base run.

Strain Gage Locations

Most strain gages were located in stations spaced along straight lines spanning regions of expected high stresses, particularly high shear stresses. Most of the lines of such gage stations spanned the depth of the instrumented member in order to get continuous stress maps over the depth. Some emphasis was placed on gaging locations which had exhibited diffi- culties or unexpected behavior, particularly bulging or buckling, in previous ships in serv- ice. Three transverses, one oiltight bulkhead, one swash bulkhead, 'the centerline girder, and one wing-tank longitudinal girder, were so in- strumented. The locations and identifying numbers (circled) of strain gage sections are shown in Figures 2, 3, and 4. Wing-tank gird- er is not shown since the results are not used in this paper.

A total of about 1,400 measurement strain gages were used. 'Ideally, each strain gage sta- tion should consist of six gages, three on each side of the plate, arranged in three-gage ro- settes, to get the complete state of stress. In locations in which the principal stress directions were clear from the configuration, only gages aligned with one or both of those directions were used and, in locations in which buckling or bulging was inhibited or prevented by ad- jacent structure, gages were placed on only one side of the plate.

The general aim was to get, by means of the measured strains, the so-called gross stress- es. Some consideration was given to gaging in regions of expected high-stress gradients, such as in the vicinity of cutouts for longitudinals in transverses. This was abandoned in view of the very large number of gages which would have been required, and the somewhat ques- tionable nature of possible results.

Deflection Measurements

The deflection measurements can be grouped in three categories: 1. Deflections identifying longitudinal and transverse hogging or sagging a't deck. 2. Vertical deflections of the platefield which forms the bottom structure of the center tank. These could be important as indications of the effect of web buckling or bulging on bottom deflection, if any occurs. 3. Lateral buckling or bulging of individual plate panels. Here, direct measurement was judged to be impossible. Instead, reliance was placed on a scheme of measuring pretest bulg- ing along important lines of strain-gage sta- tions. Only those sections were so measured which either were expected to show high stressing or which indicated before the tests started that sizable fabrication bulges existed.

The intention was, of course, to relate these bulges with the plate bending stresses which the gage system would reveal.

Conclusions

The conclusions derived from the analyses and interpretations are listed together for con- venience, without the justifications and expla- nations that are given in the paper. 1. The effects of thermal changes on stress- es were smaller than anticipated. The same conclusion applies with respect to instrument drift and shakedown. 2. Vertical hull girder deflections at deck were about as computed at bulkheads but showed distinct flattening (i.e., reduced curva- ture) between bulkheads. 3. For runs which involved the same hy- draulic l-oading on 'them, the oiltight and swash bulkheads at deck displaced vertically about the same amount, although the loading is of course applied at bottom and the in-plane stiff- nesses of the two are quite different. Analysis of stresses at bottom shows they cannot dis- place vertically the same amount there, due to differences in vertical in-plane strains. 4. Lateral elastic plate deflections under in- plane loading were generally small, not over one inch. Growth in bulging over a two-year service period was perceptible (j4 to Yz inch). 5. In analyzing vertical shear forces trans- mitted by deep thin-webbed beams of tapered form (such as brackets in bottom transverses), the flange must be accounted for. Use of an "effective flange" technique, with the Vieren- deel formula, was well verified by the tests. 6. Safety factors based on averaged coordi- nate shear stresses against yield were gener- ally satisfactory, but the use of the 2-D equiva- lent stress criterion (von Mises) showed some areas where safety factors were close to unity.

Gages placed along raw edges of the lower openings in transverses in wing tanks showed very low safety factors. 7. Safety factors against buckling in some panels of deep webs are low for some runs, even less than unity, especially where in-plane stressing normal to the axis is accounted for.

No buckling failures occurred. 8. The use of the "shadow rule" with 60- degree triangular shadows gave good correla- tion between bottom forces and structural sup- port forces in the center tank. 9. In the center tank, with the bottom under heavy hydraulic loading, the longitudinal com- ponents support 20-25 percent of the total load.

Of that part of the loading carried by the trans- verse system, about 75 percent is transmitted to the longitudinal bulkhead and about 25 per- cent to the center girder. 10. In the middle wing-tank strut there was consistently good agreement between meas- ured axial forces and those computed by a simple method, but in the bottom strut the measured forces were generally much lower than those computed by the same method. r-r-T

DUCT

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Figure 2—Strain gage station on transverse bulkheads.

Looking Forward ©FR.90 -©FR.90 (g>| j

FR90 (g)®FR.90 J ~ FR.90

Figure 3—Strain gage station on transverse web frames.

Looking Port Side

CENTER-TANK

No. 3

CENTER-TANK No. 3 t© 1§> ®

CENTER-TANK

No. 2 92 0T.-BHD SWASH-BHD

Figure 4—Strain gage station on center girder.

February 1, 1974 17