Full scale measurements of wave induced loads

-1-FULL-SCALE MEASUREMENTS OF WAVE-INDUCED LOADS J Lundgren Full-scale measurements

Investigations during recent years of wave-induced motions and loads on ships has given special attention to container

ships. These are generally built with extremely large hatch openings. Large hatch openings reduce the torsional rigidity of the hull girder and thus wave-induced torsion becomes

an important feature.

In

L1]

results are presented from calculations and measu-rements of wave-induced motions, hydrodynamic hull pressures

and stresses on MS Nihon, a

35.000

dwt containership, for particulars see Table I. Stresses were measured with 8 strain gauges distributed around a transverse section in a way to make it possible to separate stress components from vertical

and horizontal bending and torsion, respectively. The hydrodynamic pressures were measured with 7 pressure transducers around a

midship transverse section and with 9 pressure transducers around a forward transverse section. Furthermore pitch,

roll angles and accelerations forward and amidships were mea-sured.The sea states were measured with wave buoys at some of the measurements. The results of the measurements were com-pared with results from theoretical calculations using the mea-sured wave data in association with a strip-theory program, LR

257 0

(Lloyd's Register), a structural response program, LR

257

K and a pressure response program, LR 316. The result of

the investigation indicates that the theoretical methods used

predict the full-scale response behaviour satisfactorily,

parti-cularly the ship motions. The structural responses are, however, highly affected by the component of vertical bending which

appears to be considerably overestimated in the theoretical cal-culations for stern and quartering seas. The calculated pressures were in good agreement with those measured.

In

t2I

some results are presented of the measurements of wave-induced motions and stresses on MS Taimyr, a

15.000

dwt

cargo liner ("open" ship), for particulars see Table I. In

the forward end hydrodynamic pressures with four pressure trans-durcers and the vertical acceleration were measured. Stresses were -measured amidships and at the aft quarter length. At the last

mentioned position four strain gauges were located around the sec-tion in a way to make it possible to separate the stress components from vertical and horizontal bending and torsion, respectively. Furthermore all six modes of motion were measured at the aft part

of the ship. Measurements were carried out every eight hours and each sample period lasted for about 20 minutes. About 130 recordings have been obtained. The measurements will continue throughout

1975

and more detailed results of measurement are planned to be published later. TZ;;;;OlliiiE

_=VA:anal

Laboratorium voor Scheepahydromechanica Archiet Mekelweg 2,2628 CD Den

-2-Current full-sclae measurements

In cooperation between the Swedish Ship Research Founda-tion and American Bureau of Shipping measurements of wave-induced motions and stresses are being carried out on MS An-tonia Johnson, a 14,700 dwt containership, for particulars see Table I. During the first voyage measurements were carried out with 82 strain gauges, four accelerometers and two

pres-sure gauges. The strain gauges were mainly located around four transverse sections and in a way to make it possible

to separate the stress components from vertical and horizontal bending and torsion. One accelerometer and the pressure gauges were located forward and three accelerometers amidships. At

this voyage 12 measurements, each of 30 minutes duration, were ,carried out. The results of these measurements are shown in

L3-I (not published). After the first voyage the measurements

have continued for about 2

1

year with 30 of the gauges. To-tally about 170 measurements have been carried out. Measure-ments have been carried out only in moderate and rough seas. After each period of 30 minutes recording a computer on board has calculated RMS value and mean period for each gauge and

carried out calculations for separation of the three earlier mentioned stress components. The results of the measurements will be compared with results from theoretical calculations.

Recommendations for further experimental investigations

As regards future progress

in

the prediction field there is a vital need to examine thoroughly the hydrodynamic theory involved in the strip theory in the zero encounter frequency region. It is believed that the superposition theory breaks down in this region. It is recommended to examine model tank results for both regular and irregular wave systems. Further

correlation with full-scale data is also necessary, especially for cases where reliable wave measurements are obtained.

_

Ship particulars

Ref Ship type Lpp B .D

m m m.

Container ships .

E1.1 Ms Nihon

257.6

_32.21

_23.90

[33

MS Antonia Johnson

157.2

_23.75

_15.85

Dry cargo liner

(open-type)

REFERENCES

1 J Lundgren and K V Taylor, Ships with Large

Hatch Openings. Wave-induced Motions and Stresses. Calculations and Full-Scale Measurements on MS Nihon, The Swedish Ship Research Foundation Report No 111, 1975, 85p.

2 K Lindemann, A First Approach to a Hull Surveillance

System, Norwegian Maritime Research 1 (1973) 4, p7-12.

3 J Lundgren, Ships with Large Hatch Openings.

Full-scale Measurements of Wave-induced Motions and Stresses on MS Antonia Johnson, The Swedish Ship Reserach