US3454127A - Method of and apparatus for reducing the bubble pulse from underwater explosions and pressure impulses - Google Patents

Description

July 8, 1969 c, L MALME ET AL 3,454,127

METHOD OF AND APPARATUS FOR REDUCING THE BUBBLE PULSE PROM UNDERWATER EXPLOSIONS AND PRESSURE IMPULSES Filed March 22, 1967 I 1 I F 7 IO 20 30 m TIME (MILLISECOND II N PRESSURE AMPLITUDE (ATMOSPHERES) O 2- A l V I l l v T 7 IO 20 3O K40 TIME (MiLLISECOND PRESSURE AMPLITUDE (ATMOSPHERES) O l' l A;

CHARLES I. MALME BILL G. WATTERS, INVENTORS BYRMMRLW ATTORNEYS GAS FIG. 4 3' United States Patent METHOD OF AND APPARATUS FOR REDUCING THE BUBBLE PULSE FROM UNDERWATER EX- PLOSIONS AND PRESSURE IMPULSES Charles I. Malme, Hingham, and Bill G. Watters, Gloucester, Mass., assignors to Bolt, Beranek and Newman, Inc., Cambridge, Mass, a corporation of Massachusetts Filed Mar. 22, 1967, Ser. No. 625,133

Int. Cl. G01v 1/38 U.S. Cl. 181.5 14 Claims ABSTRACT OF THE DISCLOSURE In accordance with the invention, bubble, secondary or cavitation pulses generally following underwater explosions and other pressure impulses are substantially reduced or eliminated by a critically controlled introduction of gas into the bubble, substantially to equalize the hydrostatic pressure outside the bubble.

, The present invention relates to methods of and apparatus for reducing or eliminating the bubble, secondary or cavitation pulse produced by underwater or similar explosions or other pressure impulses.

In seismic and other underwater or similar operations where explosions or other pressure impulses (as from spark, exploding wire, exploding gas, pneumatic, hydraulic, electromagnetic or other acoustic impulse generators) are generated and reflections thereof from various surfaces monitored, it is often important to suppress the bubble or secondary pulses produced following such explosion or impulse generation in view of the clutter or spurious effects resulting from the secondary shock waves or impulses in turn generated by the collapse of the bubble itself.

Attempts have been made through the years to achieve this end as by placing an explosive charge or other pressure wave generator in a pocket of liquid or gas to cushion the bubble collapse effect, or placing the same within a perforated container to damp the bubble oscillations, or by a charge chain or shaped or near-surface charge for creating a gas escape path or blowout to the surface of the water or other fluid medium, or by providing bubblecancelling multiple charges. Unfortunately, none of these proposals has solved the problem of consistently and predictably providing substantial elimination of the bubble pulse and without simultaneous undue attenuation of the generated pressure pulse, as by a surrounding liquid or gas medium, container or other apparatus.

In accordance with discoveries underlying the present invention, however, the above difliculties have been remarkably overcome, providing a novel method of and apparatus for bubble suppression to a degree heretofore unattainable and without substantial attenuation of the available energy in the explosion or other pressure pulse generation.

A further object of the invention is to provide a new and improved pressure-wave generating apparatus of more general utility, as well.

A further significant advantage of the invention as contrasted with the above-mentioned prior art operating in the near-surface blowout mode, resides in the fact that the present invention may be effectively employed at great depths below the surface of the water.

Other and further objects will be later pointed out and more fully delineated in the appended claims.

In summary, however, a method underlying the invention resides in locally introducing into the bubble being produced from the explosion or other impulse, gas pressure additional to any accompanying the generation of 3,454,127 Patented July 8, 1969 the impulse and rather critically at a controlled manner insuflicient in rate to generate its own shock wave but suflicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size to substantially equal the hydrostatic pressure outside the bubble. Preferred constructional and other details are hereinafter set forth.

The invention will now be described with reference to the accompanying drawings,

FIG. 1 of which is a longitudinal section of a preferred apparatus constructed to operate in accordance with the method underlying this invention;

FIGS. 2A and 2B are explanatory wave-form graphs illustrating the operation of the invention and in which the ordinate plots pressure amplitude and the abscissa plots time; and

FIGS. 3 and 4 are views similar to FIG. 1 of modifications.

Referring to FIG. 2A, when an explosive charge or other strong impulse source generates an underwater or similar impulse I, monitoring apparatus will detect reflections such as the signal II reflected from the surface of the water, for example, and spurious signals caused by the bubble pulse III and its surface or other reflection IV. In the example shown in FIG. 2A, the impulse I was generated about thirty-five feet underwater with about 14 grams of explosive powder.

By the technique underlying the construction of FIG. 1, however, the bubble pulse effect III (FIG. 2B) and any spurious or cluttering reflections IV resulting therefrom have been found to be reduced to substantial elimination (from the practical point of view) and certainly a factor of at least three (10 db or more) under the amplitude of the bubble pulse III of FIG. 2A. In accordance with the embodiment of FIG. 1, an electrically detonatable charge 1 is mounted within a sealed terminal section 2 of a nonattenuating container 3, as of thin-walled nondynamically constraining plastic tubing containing a fast propellant or high explosive 4, with the charge 1 extending also within an inner sealed plastic tube 5 carrying a deflagrating slow charge 6. Through proper control of the nature and voltune of the charges 4 and 6, the initiating of the detonation at 1 will cause the fast explosive 4 to generate the main pressure impulse I, starting the formation of the bubble thereabout in the water or other fluid medium (hereinafter often generally referred to as underwater). During the bubble formation, the slow charge 6, simultaneously initiated, will slowly develop gas pressure within the bubble additional to that generated by the explosive 4 generating the impulse I. In accordance with the invention, this is done at a controlled rate related to the deflagration rate of the charge 6, insufficient to generate its own shock wave (which would produce deleterious impulses of its own), but in sufficient quantity, related to the relative volume of the charge 6, to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thus dissipating or suppressing the bubble without any substantial generation of secondary pressure pulses, as at III, FIG. 2B. If desired, the deflagration of the charge 6 could be commenced after the detonation of the explosive 4 instead of simultaneous operation, as by the later ignition of the portion within inner container 5.

-As an example, a detonator containing about five grains of PETN high explosive has been thus successfully employed, as shown in FIG. 1, within an outer thinwalled plastic container 3 about one inch in diameter and 3 /2 inches long containing a fast explosive propellant powder of the smokeless pistol powder type, and an inner substantially coaxially disposed plastic tube inch in diameter and 2% inches long containing a slow deflagrating charge of pelletized black powder, the weight of the latter being about equal to that of the former and having a pressure development rate of about 03x10 p.s.i. per second. The maximum bubble size for these experiments was reached about 14 milliseconds after the detonation; i.e., a total bubble period of about 28 milliseconds with the explosion being generated about 35 feet underwater. It is, of course, to be understood that for commercial seismic or related applications, this apparatus may be scaled in dimensions.

Apart from the empirical results just presented, an approximate relationship has been found for predetermining the relative volumes or weights of the charges required as follows:

where W and W are respectively the weights of the impulse producing and suppressing gas generating charges 4 and 6, F is an empirically determined specific force of the gas-generating charge 6, c is the volume of gas and solids per unit weight of charge 6, P is the hydrostatic pressure opposing bubble expansion, Q is the energy released per unit weight of the impulse-producing charge 4, and K is the fraction of such released energy available for bubble oscillation. If the main charge 4 is TNT, for example, and smokeless deflagratory powder is used for the bubble-suppressing charge 6, then at depths of 500 fathoms or less, W /W may be calculated to have a value of the order of magnitude of about 0.5.

It has further been determined that at certain operations of the above described character, the suppression gas produced by the deflagration of charge "6 should be evolved preferably within a time interval of at least onetenth the bubble period such that this secondary, additional or repressurization gas evolution can be a subsonic process for charge weights of 10- pounds and up.

While the technique embodied in the apparatus of FIG. 1 is preferred for many applications, the modifications of FIGS. 3 and 4 may be used to achieve at least some of the advantageous results thereof. In FIG. 3, the above-mentioned controlled and critically timed production of additional gas pressure during the bubble formation is attained with the aid of a volatilization-rate-controllable solid or liquid medium 10 adjacent the charge such as CCI F (Freon-ll), that permits the shock wave from the detonation at 1 to pass outward substantially unattenuated (in view of the initial solid or liquid state of 10), but, during the bubble growth, absorbs heat from the detonation and volatilizes at a controlled rate, dependent upon the nature and size of the medium 10, adding suflicient gas to that of the normal detonation to prevent the bubble from collapsing under the pressure of displaced water. In actual successful tests, it was found desirable to use a black-walled polyethylene container 3 /8 inch by 3 inches) to prevent the heat pulse from the detonating explosion of anumber six de-tonator 1 from escaping too fast before suflicient exchange is developed to warm the volatilizable Freon-11 medium filling the containerthe boiling point of the medium 10 being about 74.8 F. in this instance. Clearly other similar media 10 and heat-escape-containing mechanisms may also be employed if operated in the manner stated above.

In the modification of FIG. 4, a pressurized controlledcondensible gas cartridge 3' mounted by mechanical means 15 on the de-tonator 1 is caused to rupture upon detonation, again permitting substantially unattenuated outward transmission of the shock wave before the release of the pressurized gas within the cartridge 3' (as contrasted with prior art systems having initially present attenuating gaseous or similar media or rigid containers and the like), such release again being at the necessary controlled rate and quantity predetermined by the pressurization and volume of the cartridge 3. Carbon dioxide cartridges of the type commonly used to inflate life vests (of about 10 cc. volume and pressurized to about 400 atmospheres) have thus been successfully used to suppress the bubble pulse. While in the embodiment of FIG. 4 the pressurized gas is stored adjacent the impulse genera-tor, it is to be understood that the invention has decided utility where the gas is stored at a remote location, but its release or introduction is effected within the bubble in the region adjacent the impulse generator.

What is claimed is:

1. A method of eliminating bubble pulses that normally follow the production of a strong pressure pulse, that comprises, producing an energy impulse in a fluid medium, during the expansion of the bubble resulting from said impulse locally introducing within the bubble gas pressure additional to any accompanying the production of said impulse, and controlling the introducing of said additional gas pressure at an insuflicient rate to generate its own shock Wave but sufiicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thereby to prevent cavitative collapse of the bubble, the said introducing of gas pressure being effected by vaporizing a volatilization-rate-controllable medium adjacent the region of impulse production within the bubble but only after the commencement of the formation of the same.

2. A method of eliminating bubble pulses that normally follow the production of a strong pressure pulse, that comprises, producing an energy impulse in a fluid medium, during the expansion of the bubble resulting from said impulse locally introducing within the bubble gas pressure additional to any accompanying the production of said impulse, and controlling the introducing of said additional gas pressure at an insutficient rate to generate its own shock wave but suflicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thereby to prevent cavitative collapse of the bubble, the said introducing of gas pressure being effected by actuating a deflagrating charge stored adjacent the region of impulse production.

3. A method as claimed in claim 2 and in which the said impulse producing is efiected by detonating an impulse-producing exploding charge at said region.

4. A method as claimed in claim 3 and in which the ratio of Weight W. and W of the impulse-producing and deflagrating charges, respectively, are adjusted to conform substantially to the relationship:

where F is the specific force of the deflagrating charge, a is the volume of gas and solids per unit Weight of the same, P is the hydrostatic pressure opposing the expansion of the said bubble, Q is the energy released per unit weight of the impulse-producing charge and K is the fraction of such released energy available for bubble oscillation.

5. A method as claimed in claim 4 and in which the said ratio is of the order of about 0.5.

6. A method of eliminating pulses that normally follow the production of a strong pressure pulse, that comprises, producing an energy impulse in a fluid medium, during the expansion of the bubble resulting from said impulse locally introducing within the bubble gas pressure additional to any accompanying the production of said impulse, and controlling the introducing of said additional gas pressure at an insufficient rate to generate its own shock wave but suflicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thereby to prevent cavitative collapse of the bubble, the said rate being controlled such that the said introducing step is performed over a time interval during the bubble period such that it is elfected subsonically, said time interval being at least approximately one-tenth of the said bubble period.

7. Apparatus having, in combination, means for producing within a fiuid medium a strong energy impulse that would normally give rise to the generation of a bubble pulse, means disposed adjacent the producing means for introducing gas pressure additional to any accompanying the producing of said impulse and locally within said bubble as it expands, said last-named means being adjusted to introduce said gas pressure at a controlled substantially subsonic rate insufiicient to generate its own shock wave but sufficient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pres sure outside the bubble, thereby to reduce tendency toward cavitative collapse, means being provided for en abling the said introducing means to introduce said gas pressure over a time interval at least approximately onetenth that of the bubble period.

8. Apparatus having, in combination, means for producing within a fluid medium a strong energy impulse that would normally give rise to the generation of a bubble pulse, means disposed adjacent the producing means for introducing gas pressure additional to any accompanying the producing of said impulse and locally within said bubble as it expands, said last-named means being adjusted to introduce said gas pressure at a controlled substantially subsonic rate insuflicient to generate its own shock wave but suflicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thereby to reduce tendency toward cavitative collapse, the impulse-producing means comprising a detonatable exploding charge disposed within a nonattenuating container.

9. Apparatus as claimed in claim 8 and in which the said introducing means comprises a deflagrating charge supported adjacent the exploding charge.

10. Apparatus as claimed in claim 9 and in which means is provided for initiating the charges substantially simultaneously.

11. Apparatus as claimed in claim 9 and in which means is provided for initiating the deflagrating charge after that of the exploding charge.

12. Apparatus as claimed in claim 9 and in which the deflagrating charge is supported in a thin-walled nonattenuating container disposed substantially coaxially within the first-named container.

13. Apparatus having, in combination, means for producing within a fluid medium a strong energy impulse that would normally give rise to the generation of a bubble pulse, means disposed adjacent the producing means for introducing gas pressure additional to any accompanying the producing of said impulse and locally within said bubble as it expands, said last-named means being adjusted to introduce said gas pressure at a controlled substantially subsonic rate insuflicient to generate its own shock wave but suiiicient in quantity to cause the pressure within the bubble when it has substantially reached its maximum size substantially to equal the hydrostatic pressure outside the bubble, thereby to reduce tendency toward cavitative collapse, there being provided adjacent the producing means a volatilization-rate-controllable medium comprising the said introducing means and in heat-exchange relationship with the energy produced by the said impulse in order to generate said gas pressure by its vaporization.

14. Apparatus as claimed in claim 13 and in which means is provided for containing the release of heat energy produced by the said impulse in order to aid in the said vaporization.

References Cited UNITED STATES PATENTS 2,599,245 6/1952 Finn 181-.5 2,619,186 11/1952 Carlisle 181.5 3,292,140 12/1966 Angona et al. 181 .5 3,371,740 3/1968 Loper 18l-.5

BENJAMIN A. BORCHELT, Primary Examiner.

G. H. GLANZMAN, Assistant Examiner.

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