DR. MICHAEL A. TUCCIO

ANTENNA FOUNDATION F A I R I N G ’ II

ESTABLISHING MOBZLE ACOUSTIC COMMUNICATION SYSTEM (MACS)

SAFETY ON USS NA UTIL US THE AUTHOR

is the Structural Analysis Team Leader in ihe Engineering Mechanics Division of the Naval Underwater Systems Center (NUSC). He has been employed by NUSC for 20 years and has worked in the areas of structural analysis and computer-aided design. He is a Professional Engineer in the state of Connec- ticut and a member of the Society of Experimental Stress Analysis. He received the BSME, MSME, and Ph.D. degrees from the University of Connecticut.

ABSTRACT

USS Nautilus (SSN-571) was decommissioned on 3 March 1980. In her later years in service she served as a test bed for many programs which investigated multiple aspects of future sonar designs. The Mobile Acoustic Communication System (MACS) was one of the last major experiments to be perform- ed in Nautilus. This paper describes a unique data collection and presentation system with large numbers of strain gauges that was used to establish structural safety for the MACS antenna and Nautilus operations. The system utilized real-time processing of digital data with a minicomputer and an instan- taneous display of certain nondimensional quantities on two television monitors. This data system performed excellently and allowed Nautilus safely to conduct deep dive, full speed, and emergency blow operations.

INTRODUCTION

THE MACS (Mobile Acoustic Communication System) antenna system is a mobile acoustic source used to study sound propagation in various areas of the earth’s open oceans. The system is of modular construction and can be installed in or removed from a submarine in a matter

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Figure 2. SSN-571 MACS general arrangement.

of hours. This paper is concerned with the outboard structure shown in Figures 1 and 2. The three dish- shaped antennas were constructed as an open pipe from weldment. The entire 30-foot long by 25- foot high ar- ray was attached to the ship by four pivot assemblies and could be rotated about the pivots through a 45-degree angle. One can imagine that if any sizeable piece of this antenna were swept away by forces that oc- cur during high speed maneuvering a number of pro- blems could occur that would affect ship safety. Not the least of these problems would be impact on the ship screw and an instantaneous change in ship handling characteristics.

The h4ACS antenna was designed to meet the max- imum speed, depth, rates of turn, ascent and descent,

Figure 1. SSN-571 with MACS antenna.

Naval Engineers Journal, September 1983 71

MACS ON USS NAUTILUS TUCCIO

and sea states encountered by Nautilus (the mobile plat- form for MACS). The first sea trip with the MACS antenna installed was therefore a structural test to en- sure that the antenna and ship constituted a safe system. The measurement system described in this paper was designed to display and record structural data to ensure ship safety for the life of the MACS antenna.

Problems associated with at-sea structural testing sometimes requires immediate assessment and im- plementation of corrective action to ensure a safe test. The purpose of this paper is to describe a unique data collection and presentation system for large numbers of strain gauges. The information displayed from this system formed the data base for the determination of structural performance and safety. The system was used in successful sea trials of the MACS antenna system mounted onboard Nautilus.

DISCUSSION Structural testing of large devices normally use strain

gauges and accelerometers to provide information that can be recorded and processed at a later time for perfor- mance evaluation. When it is convenient to control load steps, the data can be reviewed before proceeding in a logical and controlled manner to the next step. In the MACS antenna structural test, there were two loading conditions in which the test engineer had no control over the load magnitude. The first was the response of the system to wave slap. This occurred when surface waves broke over the ship and any appendage. If for some reason Nautilus was required to maintain a surfac- ed position, wave slap loads would not maintain analytically incremented loading. The second loading condition was the vibration response of the pine struc- ture to eddy shedding (von Karman vortices).

Calculation of the eddy shedding frequencies, con- sidering the position of the antenna with respect to the sail or superstructure, proved to be a difficult fluid mechanics problem. It was generally acknowledged that the shedding would be broadband, but the strength and coupling with the structure could not be determined. Therefore, it was required to design a measurement system that would provide instantaneous assessment of system safety.

Data Acquisition System

The MACS monitor system is a computer-controlled data acquisition, display, and recording system that has two primary functions. These functions are:

to display to the system operator measured quan- tities at various points on the MACS antenna system and on the ships hull, so that the safety of the ship is ensured; and to record data from all sensors so that a permanent record is available for later analysis.

There were three types of sensors monitored during the sea trial - strain gauges (82), accelerometers (33), and ship parameters (10). Of these, only the 82 strain 12 Naval Engineers Journal, September 1983

gauges were required to establish ship safety. The ac- celerometer and ship parameters were recorded for later correlation and evaluation. What we were looking for was an indication as to whether or not the strain gauges were showing the presence or development of structural problems.

The interpretation of 82 individual strain gauges can be a time consuming task. The gauges were mounted on five different types of material of various yield strengths so a scan of the raw strain data would not immediately produce significant information with respect to safety factors. Direct conversion to principal stresses was in- sufficient to quickly spot possible problems. Normaliza- tion of the principal stress data so that each stress result was directly comparable, regardless of material, provid- ed a fast means of scanning the data for possible struc- tural problems. Presentation was an important aspect in interpreting these large amounts of data.

Eighty-two channels of oscillographs or on-line strip chart recorders were considered to be out of the ques- tion because of the large amount of space involved in the relatively tight ship's quarters, and visually scanning this large presentation would be difficult. A high speed printer was, by itself, insufficient from the standpoint of the huge amount of paper that would be necessary to print each value every five seconds throughout the dura- tion of the expected five-day sea trip.

Two TV screens were used (plus two backups) to display the normalized values giving almost instan- taneous information in a concise format. One screen displayed a representative value of normalized stress for all strain gauges. The second screen provided a means of automatically identifying those areas on the structure that showed consistently higher stresses than all other areas.

.--,, Figure 3. Instrumentation flow chart.

TUCCIO MACS ON USS NAUTILUS

THREE SELECTABLE SENSORS: - 6 11 4 2

- 20 70 1 1 115 9 23 12 11 ~-

The flow chart describing the monitor system is shown in Figure 3. The signals from each sensor were first routed through signal conditioning equipment to scale the sensor output. From the signal conditioners, the data was routed through 30 Hz low pass filters. (The 30 Hz cutoff was established based upon the maximum expected shedding frequency and antennas resonant fre- quency.) The signal from each sensor was sampled at a 100 Hz rate, stored in computer memory, and written onto magnetic tape. The operator could request that up to four signals be routed through the waveform display electronics. This d o w e d the operator to observe any spectrum on the spectrum analyzer. The spectrum analyzer was equipped with and X-Y plotter so that a hard copy of the signal was available for on-site study.

The strain gauge signals were subjected to additional computer processing that determined instantaneously if the MACS antenna system was operating in a safe con- dition. This processing included the following steps:

1) The average and peak absolute value of stress was computed for each gauge at five second intervals.

2) Each stress value (derived in step 1) was then divid- ed by the yield strength for the particular material under consideration. In the case of the biaxial gauges, the components were combined according to a von Mises yield criterion and then divided by a yield strength. In this manner, each sensor was normalized with respect to material and loading condition.

3) At the end of each five second interval, the ab- solute values of the mean stress or peak stress ratios from step two were displayed on a monitor via a charac- cter display generator. Each point displayed under peak or mean represented a percentage of the yield strength.

The encoding employed the following symbols for the monitor display: O = stress between 0% and 10% of yield strength. 1 = stress between 10% and 20% of yield strength. 2 = stress between 20% and 30% of yield strength. 3 = stress between 30% and 40% of yield strength. 4 = stress between 40% and 50% of yield strength. 5 = stress between 50% and 60% of yield strength. 6 = stress between 60% and 70% of yield strength. 7 = stress between 70% and 80% of yield strength. 8 = stress between 80% and 90% of yield strength. 9 = stress between 90% and 100% of yield strength. F = stress 100% of yield strength. 0 = stress below operator-defined threshold. - = gauge declared defunct by operator.

The gauges were grouped according to their spatial locations on the ship as shown in Figure 4. This allowed the operator to recognize a problem area and know im- mediately where the problem resides. To avoid clutter on the screen, the operator could select a threshold value, below which no numerical entry was displayed. This was set at one for most of the test duration so that no zeros were displayed.

A second monitor was used to display more detailed information regarding particular gauges (see Figure 5) . This TV monitor displayed the pertinent data from those 12 gauges that exhibited the largest peak stress

3 2 2 5 1 1 2 1 1 3 1 3

1 1 1 2 3 3 2 4 2

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1 1 1

2 1 - 3 . 1 1 1 . 1 1 2

1 1 2 - 1 2

1 1 - 1 1 1 3 1 1 '

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Note the operator can seiect the a s m y 01 Peaks or Means

Figure 4. TV screen layout for 82 strain gauges.

during the previous five-second interval. The data displayed were:

1. RANK - the rank, in terms of peak stress, of the gauge.

2. SENSOR - the identifying number of the gauge.

3. PEAK - the value of the peak stress as a percent of yield strength observ- ed during the five second inter- Val.

RANK SENSOR PEAK MEAN COUNT

1 20 70 11 115 2 52 69 14 116 3 17 68 9 114 4 7 60 7 100 5 1 40 16 91 6 19 35 2 60 7 21 34 10 42 8 22 30 1 10 9 40 29 8 9

10 5 28 7 8 11 61 25 5 6 12 70 36 22 3

NOTE: The 15 gages are made up of 12 gages that have exhibited the largest peak stress during the previous 5-second internal and 3 gages selectable by the operator.

Figure 5. TV screen layout for 15 strain gauges.

Naval Engineers Journal, September 1983 73

MACS ON USS NAUTILUS TUCCIO

Figure 7. Antenna frame structure - looking forward.

cos e = Az/s e = mass density of sea water v = ship velocity (in x direction)

Mi = fixed and bending moment at each end of member, i = x, y, z C,, C, = normal and tangential drag coeffi- cients Fi = normal drag force, i = x, y, z

The fixed end moment, from statics, is M = sN/12, and

MX = 0

My = M cos e , sin e cos e , sin e

D = diameter of member

MZ = M

Q = evV2 dynamic pressure on a surface perpendicular to the x axis P = periphery of member

Normal Force = N = C,sQD sin2 8

Tangential Force = T = C,sQPcos2 e Nsin e + Tcos 8 At each joint F, =

2

F, = (Tsin 8 - Ncos e x)

and Fz = (Tsin e - Ncos e x)

cos e , sin e cos e sin e

Figure 8. Finite element model of MACS antenna.

74 Naval Engineers Journal, September 1983

The stresses due to the above levels were determined using the NASTRAN finite element computer program. The results indicate the positions of maximum stress for purposes of positioning some of the strain gauges. The position for the remainder of the gauges was determined from a free vibration anaylsis, which will be described later.

2) Proximity of a Strain Gauge to a Welded Joint: The MACS structure was fabricated from standard pipe sections using appropriate welding techniques. Calcula- tions of stress at a joint were modified to include the joint discontinuity. Marshall [3] reviews, analyzes, and evaluates the design criteria of the codes that govern construction of offshore drilling platforms. The results of Marshall [3] show that although the complete stress picture at a joint is complex, the concept of punching shear is quite useful in correlating test data and for- mulating design criteria. Based upon these results, a safety factor of 1.8, with respect to punching shear stress, was used but modified by the relationship that shear yield stress is equal to 0.5 x axial yield stress, the so-called “Maximum Shear Theory of Failure.” The resulting “knock-down” factor for the strain gauge was 3.6 based on axial yield stress.

3) Effects of Fatigue Stress: Eddy shedding from the round pipe at the maximum design speed was calculated to vary from 10 to 54 Hz because of the various pipe sizes and variation of the Strouhal number. This was in the range of structural resonant frequencies, so a correc- tion was made for fatigue damage based upon the Modified Goodman Diagram.

The shedding freqency for a circular member was calculated in the standard manner, i.e.,

TUCCIO MACS ON USS NAUTILUS

f = sv 9

D S = Strouhal number, V = Velocity normal to cylinder, and f = Vortex shedding frequency (Hz).

The important Reynolds number, R,, regions for the MACS antennas are given by Church [4] as -

Subscritical 500 < R, < 2 x 10.5 Supercritical 2 x 105< k< 2 x 106 Transcritical 2 x 106 < R,

S = 0.2 0.2 < S < .45 S = 0.27

The Subcritical and Transcritical regions are characterized by periodic vortex shedding. However, in the Supercritical region the wake changes from periodic to random. Fortunately, in the Supercritical region, the flow is too random to provide in-phase loading cor- related of any significant length.

The lowest resonant frequency of the MACS anten- nas was calculated to be approximately 4 Hz. Therefore, excitation by eddy shedding forces was a distinct possibility. Strain gauges were placed at the position of maximum relative stress for each mode between 4 to 54 Hz as determined by the free vibration analysis.

As a worst case limit the 54 Hz frequency was chosen to represent the exciting frequency. The test was

Figure 9. Modified GOODMAN diagram for area away from pipe joints for HP-9-4-20 material.

scheduled to last five days, which for 54 Hz totals about 25 x 106 cycles. For example, Figure 9 was used for the HP 9-4-20, which was one of the five construction materials. The endurance limit was used in lieu of the fatigue strength at 25 x 106 cycles to provide an added cushion for ship safety.

A safety factor of 2 was used for those gauges away from joints and a factor of 3.6 for those near joints to establish the areas of limits for strain gauges on both the endurance strength and yield strength. The shaded area of Figure 9 shows the safe operation area for a strain gauge away from a joint. For example, the peak stress indicator was set at 45 ksi and the average stress in- dicator was set at 75 ksi.

SUMMARY

The data system used for at-sea structural testing of the MACS antenna safely allowed Nautilus to execute the test plan.

The presentation of experimental information in the manner described herein provided the Test Engineer and Commanding Officer with clear and unambiguous data during all aspects of the deep dive, full speed, and emergency blow operations.

During the sea test, only one of the 82 strain gauges failed. This is an excellent record and a tribute to the ex- pertise of General Dynamics/Electric Boat, installers of all the sensors. The entire system performed as expected except for one unforeseen problem. During the transient period between surface running and complete submergence, the capacitance of the external cabling changed sufficiently enough to cause the conditioning equipment to show a shift of the zero position. For- tunately, the amount of shift was small enough not to affect overall safety during this transient period.

REFERENCES

[I] Autonetics North American Rockwell, “Mobile Acoustic Source Antenna,” Report No. C73/208/201, 1973.

[2] NASA SP-222(03), The NASTRAN User’s Manual, 1 March 1976.

[3] Marshall and Toprac, “Basis for Tubular Joint Design,” Welding Research Supplement, May 1974.

[4] Church, Gwin, Wilder, “Self-Induced Vibrations of the Project MACS Actuator Structure,” Contract No. N00024-71-C-0264, Electric Boat DivisiodGeneral Dynamics.

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Naval Engineers Journal, September 1983 75