GB1577783A

GB1577783A - Apparatus when used in acid gas exploration transportation or processing

Info

Publication number: GB1577783A
Application number: GB5163/77A
Authority: GB
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1976-04-13
Filing date: 1977-02-08
Publication date: 1980-10-29
Also published as: JPS52124411A; NO770224L; FR2348275B1; AR211954A1; IT1084471B; MX149365A; NO147217B; ATA903976A; BE853481A; CS215084B2; BR7702279A; SE7701171L; DE2616599A1; DE2616599C3; SE437383B; DE2616599B2; NL7613619A; FR2348275A1

Description

(54) APPARATUS WHEN USED IN ACID GAS EXPLORATION, TRANSPORTATION OR PROCESSING (71) We, MANNESMANN AKTIEN GESELLSCHAFT, a joint stock company organised under the laws of Germany, of 4 Dusseldorf I, Mannsesmannufer 2, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to apparatus such as tubes and tube connections manufactured from a material which is resistant to corrosive attacks by acid gas or products thereof.

Hitherto, the material used for apparatus for transporting, conveying and treating acid gas (=natural gas with contents of H2S and/CO2) and acid gas products, e.g. sulphur compounds have been steels which are not alloyed or have a low alloy content and which have been subjected to conventional heat treatments. However, these materials have exhibited insufficient resistance to corrosion when subjected to such use, for example they have inadequate resistance to corrosion caused by general wear, susceptibility to cavity corrosion due to element formation and may be dangerous as they are liable to hydrogen-induced tension crack corrosion. This corrosion can occur individually or jointly. In order to guarantee a sufficient safety margin in operation, when using non-alloyed and low-alloyed steels, various measures have had to be taken, for example, drying of the gas and/or addition of inhibitors and/or raising of the pH value.

When acid gas is conveyed, the materials of the apparatus used are often endangered by large-area local corrosion effects. These corrosion effects are observed on the external surface of tubing being used and are to be seen in conjunction with the introduction of sulphur-dissolving agents or agents preventing the precipitation of sulphur into the annular gap between tubing and casing. The cause of these corrosion attacks is probably the development of large-area macro elements, of which the effectiveness increases with increasing depth of the drilling and with rising temperature, and is no longer observed below a certain temperature which is dependent on the local circumstances and operating conditions, or above a certain depth of drilling.

Furthermore, damage due to corrosion occurs on the internal surfaces of the conveying sections, such damage being caused by corrosion due to general wear and corrosion due to erosion. The corrosion due to general wear is obviously connected with the acidification of the borehole for maintaining the conveying capacity. It is true that this type of corrosion can be reduced by using inhibitors, but it cannot be avoided to a sufficiently reliable extent. In addition, local attacks by erosion corrosion can occur, these being caused by differences in the flow conditions. These are probably to be attributed to the fact that surface layers which are formed during operation and exert a certain protective function, do not develop or are destroyed in specific regions of the tube surface.

The danger of a tension crack corrosion caused by hydrogen, exists in the upper, colder tubing section, more especially when the conveying action is stopped. The occurrence of this type of corrosion is to be attributed to the fact that hydrogen which is formed on the surface of the material by corrosion reactions, does not recombine to hydrogen molecules in the presence of hydrogen sulphide, but is diffused atomically into the structure. Under loads which are far below the elastic limit, this may lead to formation of cracks, and fractures, and thus to the rupturing of the tubing section. The tendency of non-alloyed and low-alloyed steels to hydrogen-induced tension crack corrosion increases with falling temperature and reaches a maximum value at approximately room temperature.

The particular danger to the upper and colder parts of the section which are simultaneously also the parts under highest load during the conveying operation is thereby explained, as is also the additionally increased danger during stoppage because the temperature then falls still further. As a counter-measure, higher strength, lowalloyed heat-treatable steels with an improved resistance to hydrogen-induces tension crack corrosion as compared with hot-rolled or hot-rolled and normalised steels have hitherto been employed, without it being possible, however, completely to avoid damage due to corrosion.

Tubes or pipes for transporting acid gas in so-called flow-lines, are most endangered by hydrogen-induced tension crack corrosion on account of the low temperature existing under such conditions, whereas corrosion due to general wear and other local corrosion attacks are of no practical significance. Since the tension crack corrosion caused by hydrogen absorption is linked with an electro-chemical corrosion reaction which is able to take place at low pH values and only when moisture is present, the procedure for avoiding tension crack corrosion used hitherto so as to raise the pH value immediately after the gas emerges from the bore of the tubes or pipes, by the addition of alkalis to pHh8, and thereafter the moisture was removed in a drying plant.

The same corrosion problems as those which occur during the conveying and transporting of acid gas also with plants for preparing and further processing of gas.

We have sought to provide apparatus which can be used for the exploration transport and for the processing of acid gas or products thereof without the apparatus concerned being endangered by corrosion.

Accordingly, the present invention provides apparatus when used for exploration, transportation and processing of acid gas or products thereof, which apparatus is resistant to corrosive attack by said acid gas or products thereof, is formed from a steel having the following composition: 0.001 to 0.12% by weight of carbon 0.2 to 1.5 % by weight of silicon 0.5 to 8 % by weight of manganese 12 to 30 % by weight of chromium 2 to 16 /n by weight of nickel 0.1 to 5 % by weight of molybdenum 0.01 to 1.2 /n by weight of titanium 0.01 to 1.6 % by weight of niobium 0.01 to 3.5 /n by weight of copper 0.01 to 0.35% by weight of nitrogen the remainder being iron and usual accompanying elements and having an elastic limit of at least 440N/mm2 and is subjected during manufacture to cold working to a degree of deformation of at least 3%.

Because of their chemical composition, apparatus according to the invention show a high resistant to corrosion caused by general wear, local corrosion effects such as cavity corrosion and due to chlorides, and particularly against tension crack corrosion caused by hydrogen absorption, under the conditions which prevail during the exploration of acid gas borings, and during the conveying, transporting, working up and further processing of acid gas. Furthermore, they are resistant to corrosion effects produced by element formation.

As a result of the cold-working of the materials, the minimum elastic limits of the highly alloyed steels existing in the solutionannealed state are raised substantially up to the values necessary for their use as piping at great depths and simultaneously exhibiting sufficiently high values for expansion and contraction.

As a result of the invention, the resistance to corrosion due to general wear to cavity corrosion, tension crack corrosion produced by chlorides and penetration of atomic hydrogen into the structure and the resistivity to corrosion effects due to element formation are not impaired. In addition, the measures for protection against corrosion which have so far been used but which generally have been insufficient for a reliable protection of nonalloyed and low-alloyed steels, such as the use of inhibitors with the flushing and acidification of the borehole, alkalisation and drying of the gas, can be dispensed with or at least restricted and thereby the operational safety of the installations is improved.

WHAT WE CLAIM IS: 1. Apparatus when used for exploration, transportation and processing of acid gas or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. caused by hydrogen, exists in the upper, colder tubing section, more especially when the conveying action is stopped. The occurrence of this type of corrosion is to be attributed to the fact that hydrogen which is formed on the surface of the material by corrosion reactions, does not recombine to hydrogen molecules in the presence of hydrogen sulphide, but is diffused atomically into the structure. Under loads which are far below the elastic limit, this may lead to formation of cracks, and fractures, and thus to the rupturing of the tubing section. The tendency of non-alloyed and low-alloyed steels to hydrogen-induced tension crack corrosion increases with falling temperature and reaches a maximum value at approximately room temperature. The particular danger to the upper and colder parts of the section which are simultaneously also the parts under highest load during the conveying operation is thereby explained, as is also the additionally increased danger during stoppage because the temperature then falls still further. As a counter-measure, higher strength, lowalloyed heat-treatable steels with an improved resistance to hydrogen-induces tension crack corrosion as compared with hot-rolled or hot-rolled and normalised steels have hitherto been employed, without it being possible, however, completely to avoid damage due to corrosion. Tubes or pipes for transporting acid gas in so-called flow-lines, are most endangered by hydrogen-induced tension crack corrosion on account of the low temperature existing under such conditions, whereas corrosion due to general wear and other local corrosion attacks are of no practical significance. Since the tension crack corrosion caused by hydrogen absorption is linked with an electro-chemical corrosion reaction which is able to take place at low pH values and only when moisture is present, the procedure for avoiding tension crack corrosion used hitherto so as to raise the pH value immediately after the gas emerges from the bore of the tubes or pipes, by the addition of alkalis to pHh8, and thereafter the moisture was removed in a drying plant. The same corrosion problems as those which occur during the conveying and transporting of acid gas also with plants for preparing and further processing of gas. We have sought to provide apparatus which can be used for the exploration transport and for the processing of acid gas or products thereof without the apparatus concerned being endangered by corrosion. Accordingly, the present invention provides apparatus when used for exploration, transportation and processing of acid gas or products thereof, which apparatus is resistant to corrosive attack by said acid gas or products thereof, is formed from a steel having the following composition: 0.001 to 0.12% by weight of carbon 0.2 to 1.5 % by weight of silicon 0.5 to 8 % by weight of manganese 12 to 30 % by weight of chromium 2 to 16 /n by weight of nickel 0.1 to 5 % by weight of molybdenum 0.01 to 1.2 /n by weight of titanium 0.01 to 1.6 % by weight of niobium 0.01 to 3.5 /n by weight of copper 0.01 to 0.35% by weight of nitrogen the remainder being iron and usual accompanying elements and having an elastic limit of at least 440N/mm2 and is subjected during manufacture to cold working to a degree of deformation of at least 3%. Because of their chemical composition, apparatus according to the invention show a high resistant to corrosion caused by general wear, local corrosion effects such as cavity corrosion and due to chlorides, and particularly against tension crack corrosion caused by hydrogen absorption, under the conditions which prevail during the exploration of acid gas borings, and during the conveying, transporting, working up and further processing of acid gas. Furthermore, they are resistant to corrosion effects produced by element formation. As a result of the cold-working of the materials, the minimum elastic limits of the highly alloyed steels existing in the solutionannealed state are raised substantially up to the values necessary for their use as piping at great depths and simultaneously exhibiting sufficiently high values for expansion and contraction. As a result of the invention, the resistance to corrosion due to general wear to cavity corrosion, tension crack corrosion produced by chlorides and penetration of atomic hydrogen into the structure and the resistivity to corrosion effects due to element formation are not impaired. In addition, the measures for protection against corrosion which have so far been used but which generally have been insufficient for a reliable protection of nonalloyed and low-alloyed steels, such as the use of inhibitors with the flushing and acidification of the borehole, alkalisation and drying of the gas, can be dispensed with or at least restricted and thereby the operational safety of the installations is improved. WHAT WE CLAIM IS:

1. Apparatus when used for exploration, transportation and processing of acid gas or

products thereof, which apparatus is resistant to corrosive attack by said acid gas or products thereof, is formed from a steel having the following composition: 0.001 to 012 /n by weight of carbon 0.2 to 1.5 % by weight of silicon 0.5 to 8 /n by weight of manganese 12 to 30 /n by weight of chromium 2 to 16 % by weight of nickel 0.1 to 5 % by weight of molybdenum 0.01 to 1.2 /n by weight of titanium 0.01 to 1.6 % by weight of niobium 0.01 to 3.5 % by weight of copper 0.01 to 0.350/, by weight of nitrogen the remainder being iron and usual accompanying elements and having an elastic limit of at least 440N/mm2 and is subjected during manufacture to cold working to a degree of deformation of at least 3%.

2. Apparatus substantially as hereinbefore described.

3. A process for manufacturing apparatus as claimed in claims 1 or 2, wherein during manufacture of the apparatus, the steel is subjected to cold working to a degree of deformation of at least 3%.