GB2275539A - Method and composition for detecting leaks. - Google Patents

Abstract

Leaks in hollow underwater structures are detected and located by introducing a solution comprising an optical brightener into the inside of the hollow underwater structure, followed by scanning the outside surface of the structure with an ultraviolet lamp. Optical brightener leaking from the underwater structure is caused to fluoresce by the ultraviolet lamp thereby revealing the presence and location of leaks in the underwater structure. Optical brighteners were previously known for use in laundry detergents, but are especially useful for use in this leak detection method because they are colourless, fluorescent and non-toxic. The invention also provides solutions comprising an optical brightener, a biocide and an oxygen scavenger for use in the above method.

Description

METHOD AND COMPOSITION FOX NG LEAKS The present invention relates to a method and composition for detecting and locating leaks in hollow underwater structures such as pipelines.

Within the offshore oil exploration and production industry there exists a continuing requirement for leak testing and for pressure testing of underwater pipelines, risers and a wide variety of associated structures such as platform "jacket" legs and the legs and compartments of semi-submersible drilling rigs. The testing is typically carried out at the end of construction and installation procedures and prior to commissioning, but may also occur in the event of work-over operation and additions or alterations to existing installations.

Existing methods for detecting and locating leaks include methods according to which a tracer dye is introduced into the hollow underwater structure, typically at a concentration of about 20 mg/l in sea water, and leakage of the dye from the structure is then detected by inspection of the outside of the structure by a diver. The normal practice is to use Rhodamine B or fluorescein dyes, which are detected at any leaking points in the underwater structure by their visible fluorescence when illuminated with a hand-held ultraviolet light source carried by the diver. Fluorescence detection of this kind is preferable to simply trying to detect the optical absorption (colour) of the leaking dye, since the latter method cannot be relied upon at depth in murky marine conditions.

The use of rhodamine B and fluorescein dyes as above is effective, but presents environmental problems. The first problem is that the rhodamine B and fluorescein dyes are strongly coloured. Even quite low concentrations of these dyes in sea water are visible to the human eye and thereby betray the presence of discharges during and after testing. This is unacceptable from a public relations point of view, and generates particular protest when the discharge takes place in estuaries or close to shore.

The second problem caused by the existing method is that rhodamine B and fluorescein dyes are toxic towards marine organisms. Both dyes are placed in Category 2 of toxicity by the DTI and MAFF. That is to say, their marine LC50 at 96 hours versus certain specified organisms is in the range of from 10 to 100 mg/l.

Accordingly, it is an object of the present invention to provide a method of detecting leaks in hollow underwater structures that can at least partially overcome the above problems. It is a further object of the present invention to provide fluorescent compositions for use in the method of the invention.

The present invention provides a method of detecting a leak in a hollow underwater structure comprising the steps of: providing a solution of an optical brightener inside the hollow underwater structure, and illuminating an external surface of the underwater structure with an ultraviolet light source to cause visible fluorescence of any optical brightener leaking from inside the underwater structure.

The term "optical brightener" is a term given to a number of fluorescent dyestuffs widely used in the laundry detergent art. The characteristics of optical brighteners are as follows. First, the optical brighteners are substantially colourless. that is to say, aqueous solutions of optical brighteners at concentrations of 1 to 500 mg/l are colourless to the human eye when illuminated with ordinary white light. Second, optical brighteners fluoresce at visible wavelengths when illuminated with ultraviolet light in the 200-400 nm wavelength range. Finally, the optical brighteners are substantially non-toxic to marine organisms, being placed in Category 1 or O of toxins.That is to say, the marine LC50 at 96 hours versus the same specified marine organisms is greater than 100 mg/l and frequently greater than 1000 mg/l in commercially presented form for both discharge registration and use.

The optical brighteners have been specifically developed to bond to the surface of certain fabrics during the washing process. Once bonded to the fabrics they produce a fluorescent whitening effect in daylight. The low toxicity of the optical brighteners is a requirement because of the large amounts of these materials that routinely flow into the domestic effluent stream.

The optical brighteners in common use have been described, for example, in "Surfactants in Consumer Products", J. Falbe ed. pp 279-284 Springer-Verlag Heidelberg (1987). They fall broadly into the classes of stilbene derivatives, coumarins, quinolones, diphenyl pyrazolines, benzoxazoles and benzimidazoles. Specific categories include 4,4'-bis-(triazinylamino)-stilbene-2,2'disulfonic acid derivatives and salts thereof, 4,4'-bis-(v triazole-2-yl) -stilbene-2, 2 '-disulfonic acid derivatives and salts thereof, stilbenyl naphthotriazole derivatives, 4,4'distyrylbiphenyl derivatives, 1,3-diarylpyrazoline derivatives, coumarin derivatives, quinolone derivatives, 2styrylbenzoxazole derivatives and 2-styrylnaphthoxazole derivatives.

Particularly preferred optical whiteners for the practice of the present invention are those sold under the Registered Trade Mark PHOTINE by Hickson and Welch Ltd., those sold under the Registered Trade Mark TINOPAL by Ciba Geigy Ltd. and those sold under the Registered Trade Mark LEUCOPHOR by Sandoz Ltd.

The solution of the optical brightener is normally premixed and then pumped into the hollow underwater structure. The sensitivity of leak detection can be greatly increased by applying hydrostatic pressure typically of 120 bar to the solution inside the hollow underwater structure, as this forces the optical brightener solution out through small leaks in the structure that might otherwise go undetected. The concentration of optical brightener in the test solution may vary within wide limits.

The lower limit is determined by the detection limit of the ultraviolet emission/detection device and is typically a few part per billion. The upper limit is determined by the solubility limit of the optical brightener. However, for reasons of expense, the practical upper limit is of the order of a few grams per litre (tests are normally carried out on hollow underwater structures having internal capacities of from 5m3 to 5,000,000m3. The concentration of optical brightener is preferably in the range of 0.1 to 1000 mg/l, more preferably 1 to 100 mg/l still more preferably 5 to 50 mg/l and most preferably about 20 mg/l. The solution is normally an aqueous solution, and may be a solution in sea water if the underwater structure is submerged at sea.

The general leak detection procedure is similar to that for the existing rhodamine B or fluorescein dyes. The duration of the test may be from 48 hours to 12 months depending on the magnitude of the underwater structure and the test protocol. Typically, the underwater structure having the solution of optical brightener therein is allowed to equilibrate for 24 hours and then the exterior surface of the underwater structure is inspected by divers using handheld ultraviolet emission/detection devices. Suitable emission/detection devices are supplied by Chelsea Instruments Ltd. under the Registered Trade Mark AQUATRACKERS.

The solution containing the optical brightener preferably also contains a biocide and/or an oxygen scavenger. The purpose of both the biocide and the oxygen scavenger is to maintain a sterile, abiotic environment inside the hollow structure. This is particularly important for the long-term tests and for tests carried out on pipelines where the cleanliness of material subsequently to be pumped through the pipeline is important. Additionally, the prolonged growth of anaerobic organisms causes a buildup of acidic metabolites such as sulfuric acid and H2S that can corrode the underwater structure. The oxygen scavenger is preferably a bisulfite salt preferably at a concentration of 100 to 1000 mg/l. The oxygen scavenger takes up elementary oxygen dissolved in the solution. This deprives the contained volume of solution of all free oxygen and hence leads to the rapid demise of all aerobic species including bacteria, crustacea, fish and phytoplankton. The biocide then eliminates all anaerobes such as sulfate reducing bacteria that might otherwise remain viable.

Preferred biocides include VANTOCIL (Registered Trade Mark) available for ICI, at a concentration of 20-150 mg/l, and AGMA PROCEINE 40 (Registered Trade Mark) available from Agma PLC, at a concentration of 50-200 mg/l.

In use, the optical brightener biocide and oxygen scavenger are combined in solution in such a fashion that the concentration of each individual component preferably does not exceed 10,000 mg/l. This is to prevent any undesirable reaction between the oxygen scavenger and the other components. The solution is them pumped into the hollow underwater structure and testing is carried out as described above.

Claims (11)

1. A method of detecting a leak in a hollow underwater structure comprising the steps of: providing a solution of an optical brightener inside the hollow underwater structure; and illuminating an external surface of the underwater structure with an ultraviolet light source to cause visible fluorescence of any optical brightener leaking from inside the underwater structure.

2. A method according to claim 1 wherein the solution inside the hollow underwater structure is under applied hydrostatic pressure.

3. A method according to claim 1 or 2 wherein the solution of an optical brightener further comprises a biocide.

4. A method according to claim 1, 2 or 3 wherein the solution of an optical brightener further comprises an oxygen scavenger.

5. A method according to any preceding claim wherein the optical brightener comprises a stilbene derivative, a coumarin, a quinolone, a diphenyl pyrazoline, a benzoxazole or a benzimidazole.

6. A method according to any preceding claim wherein the optical brightener comprises a 4,4'-bis-(triazinylamino)stilbene-2,2'-disulfonic acid derivative or salt thereof, a 4,4'-bis-(v-triazole-2-yl)-stilbene-2,2'-disulfonic acid derivative or salt thereof, a stilbenyl naphthotriazole, a 4,4'-distyrylbiphenyl, a 1,3-diarylpyrazoline, a coumarin, a quinolone, a 2-styrylbenzoxazole or a 2styrylnaphthoxazole.

7. A method according to any preceding claim wherein the concentration of the optical brightener in the solution is from 1 to 100 mg/l.

8. A method according to any preceding claim wherein the concentration of the optical brightener in the solution is from 5 to 50 mg/l.

9. A composition for use in a method according to any preceding claim comprising an optical brightener, a biocide and an oxygen scavenger.

10. A composition according to claim 9 wherein the oxygen scavenger is a bisulfite salt.

11. Use of an optical brightener for the preparation of a solution for use in a method according to any of claims 1 to 8.