Acoustic Method for Detecting Leaks From Submerged Pipelines

Considerable difficulty has been experienced in locating leaks of liquids and, in particular, hydrocarbon liquids such as crude oil, from submerged pipelines, especially such pipelines which lay in open sea or ocean waters. Localization of the leak location is usually accomplished by searching for an oil slick on the water's surface or by seeking some other surface indication of escaping liquids. When some indication of a leak is found divers are employed to search out the exact leak site. Localization in this manner is sometimes impeded because wave action or smallness of the leak prevents surface indication of it from being spotted.

The method of the invention involves the use of sonar to locate leaks of liquids from pipelines. The principles are known and have been applied in the past to show on recordings of reflected acoustic waves the acoustic echos created by hydrocarbon gas escaping from natural seeps in the sea floor or from submerged pipelines. The present invention concerns the detection of liquids leaking from submerged pipelines. The detection function is predicated on the difference in acoustic properties between seawater and the foreign liquid material, such as hydrocarbon liquids (crude oil) escaping from a submerged pipeline. Detection of the leak can be enhanced by introducing into the pipeline a substance or substances, gaseous and/or liquid, which increases the difference in acoustic impedance between seawater and the liquid escaping from the leak. The method of the invention also involves running a "base case" (no-leak conditions) sonar scan of the pipeline to provide comparison of the ocean floor during normal and leak conditions. The comparison of the "no-leak" sonar scan with a later "leak" scan of the pipeline detects the leak location by variations in the scans caused by (1) the leaking fluids themselves (2) a cavity or anomoly formed in the ocean floor by the fluids flowing from the leak in a buried pipeline; and/or (3) sediments that are dispersed by the flow of fluids from the leak. Enhancement of the leak location may be made by restricting flow from the end of the pipeline to cause increased displacement of sediments and increased excavation of a cavity due to increased flow from the leak and/or replacement of the fluids in the pipeline with "nonpolluting" material such as salt water or air and continued displacement of that material through the leak to cause growth of sediment dispersion and increased cavity size. The interface between materials having different acoustic impedances is detectable by known sonar techniques, e.g. side scan sonar, front scan sonar, or direct over-bottom sonar.

Briefly, the invention concerns a method for detecting leaks of liquid being conducted through a submerged pipeline from such pipeline in which acoustic waves are directed along the path of the submerged pipeline; the reflected acoustic waves are recorded and the location of the leak of, for example, liquid hydrocarbons leaking from the pipeline is determined from such recordings. To enhance the detection technique in the detection of liquid, such as crude oil, leaking from the liquid carrying pipeline, gas such as gaseous hydrocarbons, air, nitrogen or CO.sub.2 etc. and/or liquid, such as fresh water, alcohol etc., may be introduced into the pipeline. The introduced gases and/or liquids escape through the leak into the seawater along with the liquid being conducted through the pipeline to provide a greater difference in acoustic impedance between the seawater and the leaking gas and/or liquids than would be provided by the liquid being conducted through the pipeline alone. Gases have sufficiently different acoustic impedance from liquids to enhance detection of the latter. Liquids to be introduced to enhance detection are preselected for acoustic impedances sufficiently different from the liquid being transported to enhance detection of the latter. In addition, sonar reflecting targets or transponders are preferably prelocated along the submerged pipeline to direct the survey path of the sonar along the pipeline. Also, fluids (liquid and/or gas) leaks in a pipeline may be located by comparing "leak" and "no-leak" sonar scans of the pipeline. The flow may be restricted from the end of the pipeline and/or the fluids in the pipeline may be replaced with "non-polluting" material.

FIGS. 1 and 1A illustrate the invention.

As seen in FIG. 1 a crude oil carrying pipeline 10, laid on the ocean floor 11 beneath a body of seawater 12, has connected to it a series of spaced-apart sonar relector buoys 13. A towing and recording vessel 15 has attached to it a submerged "fish" which is the sonar bearing vehicle 16. The scan area (side scan sonar is illustrated) is indicated at 17 and a leak of liquid and gaseous hydrocarbons from the pipeline is indicated at 18. In practice, the recording vessel 15 tows fish 16 along the path of the pipeline as directed by the sonar reflecting targets or acoustic transponders 13. Transducers in the fish or other sonar bearing vehicle emit short acoustic fan-shaped pulses perpendicular to the ship's track and the pipeline direction. When the scan area passes over leak 18 the recording of the reflected waves will show a difference on the recording because of the difference in acoustic properties between seawater and crude oil escaping from pipeline 10. As an example, the acoustic impedance of crude oil is approximately 1.2 .times. 10.sup.6 kilograms per second meter squared (Kg/sec m.sup.2) and the acoustic impedance of seawater is approximately 1.5 .times. 10.sup.6 Kg/sec m.sup.2. In that manner leak 18 is located and localized and divers may be sent down to repair the leak.

Gas, such as methane, air, carbon dioxide or nitrogen, etc. may be injected into the pipeline to enhance the detection of the escaping crude oil or other liquid from the pipeline. Each of those gases has a substantially lower acoustic impedance than crude oil and facilitates detection of the crude oil. For example, the acoustic impedance of air is 1.31 .times. 10.sup.3 Kg/sec m.sup.2 and that of methane is 0.94 .times. 10.sup.3 Kg/sec m.sup.2 and that of carbon dioxide is 1.54 .times. 10.sup.3 Kg/sec m.sup.2. All gases have acoustic impedances drastically lower than that of seawater and all will, consequently, be excellent reflectors. The column of bubbles is detectable along with the oil on the recorded reflected waves. Other liquids having acoustic impedances lower than seawater, such as fresh water, (1.44 .times. 10.sup.6 Kg/sec m.sup.2) and ethyl alcohol-95% (1.01 .times. 10.sup.6 Kg/sec m.sup.2), may be introduced into the pipeline instead of or together with the gas to enhance detection of the crude oil leak. Liquids with acoustic impedances higher than that of seawater can be used as effectively as liquids with lower impedances than seawater. In any case the intensity of reflection will depend on the magnitude of the difference of the acoustic impedance. An example of a heavier liquid is 20% sodium chloride solution (1.84 .times. 10.sup.6 Kg/sec m.sup.2 acoustic impedance).

Further, leaking fluids escaping from a pipeline, and particularly a buried pipeline will stir up considerable sediment, which would serve as an acoustic reflector. Also, the escaping fluids will excavate a crater than will be shown on the sonar. Thus, as shown in FIG. 1A a leak 18a in a pipeline 10a (to which is attached sonar reflector buoys 13a) covered or buried by sediments forms a crater or cavity 19 in the sediments and displaces the sediments as indicated at 20. Both the cavity 19 and the dispersed sediments are detectable by the sonar scan. Increased displacement of sediments 20 and increased evacuation of cavity 19 is achievable by restricting flow from the end of pipeline 10 thereby causing increased flow from the leak. Also, increased growth of sediment dispersion and increased cavity size are achievable by replacing the fluid in the pipeline with "nonpolluting" material such as salt water or air and continued displacement through the leak. In addition, leaks of fluid in a pipeline may be located by first running a "base case" survey of the path of the pipeline with a sonar device during normal nonleak conditions. That "base case" sonar scan is later compared with a sonar scan of the path of the pipeline during leak conditions. Variations in the two scans will indicate escaping fluids and/or, with respect to buried pipelines, displacement of sediments and cavities.

The recording may be done on a ship or on a helicopter with the recorder connected to the sonar "fish" equipment.

While the preferred embodiment described herein is directed to locating leaks of crude oil being transmitted through and escaping from submerged pipelines the invention also contemplates locating leaks of liquids other than crude oil which are being transmitted through and escaping from submerged pipelines. Also, as described, the invention contemplates in another preferred embodiment locating leaks of gases, such as methane or natural gas, as well as of liquids.

Changes and modifications may be made in the specific illustrative embodiments of the invention shown and/or described herein without departing from the scope of the invention as defined in the appended claims.