GB1588160A

GB1588160A - Riser for offshore oil or gas fields

Info

Publication number: GB1588160A
Application number: GB4123477A
Authority: GB
Original assignee: Constructors John Brown Ltd
Current assignee: Worley Field Services Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-04-15

Description

(54) RISER FOR OFFSHORE OIL OR GAS FIELDS (71) We, CONSTRUCrORS JOHN BROWN LIMITED, a British Company, of CJB House, Eastbourne Terrace, Paddington, London W2 6LE, 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: The present invention relates to risers for use in offshore oil and gas fields.

Of the equipment used in offshore oil and gas field development, risers are about the most vulnerable, being exposed to the worst environmental conditions, operated at high pressure, frequently at high temperature, and for convenience fabricated from pipe the same as that used for the submarine pipelines; the operating conditions of the latter are, of course, quite different.

The protection of risers is vital for not only are they in themselves expensive to replace, but their failure can result in severe damage and loss of production and has even resulted in loss of life. The main cause of riser failure has been generally due to their corrosion by sea water.

Hitherto, organic coatings have been used to protect risers from sea water corrosion.

However, these coatings, as well as being expensive, are open to mechanical abuse both in the construction yard and in their subsequent handling. Moreover, special slings are required to install these coated risers in their supporting structures.

Two failures in connection with risers in the North Sea used in oil production have been well documented and it has been suggested that excessive temperatures have been primarily responsible in both cases.

The first failure was in connection with a riser which was coated with a bituminous bonded gravel filled coating approximately Q" thick which is extruded over the pipe.

The coated pipe was sheathed with reinforced concrete. The pipe in question was connected to a step out well which was -under test. It was known that the concrete had been damaged but the condition of the coating below the concrete was not known.

The pipeline failed by explosion due to corrosion penetration of the pipe wall which was 15 or 16 mm thick in the course of a few months. It is understood that cathodic protection was applied to the structure supporting the riser but that no anodes were mounted on the riser itself. The general arrangement had the following defects: (a) reinforced concrete is an unsuitable sheathing for vertical pipework.

(b) the coating used is normally made with bitumen rather than coal tar enamel and the former may absorb a certain amount of water.

(c) if the concrete is damaged and its steel reinforcement makes contact with the steel of the pipe, then the pipe is shielded from any cathodic protection it may have received.

(d) the amount of cathodic protection such a riser will receive depends on circuit resistances, anode disposition and the effect of the connected sub marine pipeline.

(e) rates of corrosion increase substan tially with temperature rise and com pared to ambient rates it has been suggested that they may double for every 10 C rise in temperature. Cur rent densities required for cathodic protection are generally high.

The second failure was in connection with several coated risers each supported by a coated structure. Each structure was cathodically protected by zinc anodes, but no anodes were attached to the risers themselves. Coating failure in the form of blistering occurred due to the operating temperature exceeding the coating specification temperature. In spite of the risers indicating a protected potential, rapid corrosion occurred below the blisters. Emergency repair of the risers involved removing the coating at the blisters and attaching additional sacrificial anodes to the risers themselves.

Other methods which have been adopted for riser protection have involved the use of monel sheathing through the splash zone, an organic coating overall, and an insulated wear sleeve at the points of support. Alternatively, additional wall thickness may be applied to the pipe at these locations.

The first of these other methods should be fully effective but is very expensive, complicated and involves flexible bonding connections. The latter may be adequate for the service life of the riser; the problem is that it is difficult to predict the corrosion rate of the steel as it is chiefly influenced by the actual operating temperature and oxygen supply.

From the foregoing, it will be apparent that for some time there has been a need for a riser which does not suffer from the problems associated with coated risers. We have now found that such problems can be avoided or reduced and that risers having a life expectancy at least equal to that of the oil or gas field in which they are to be used can be produced without prohibitive expense.

According to the present invention there is provided a riser for use in an offshore oil or gas field, which riser has sea water corrosion resistant sleeves secured thereto in such a manner as to prevent sea water from penetrating any area inside the sleeves and adjacent the riser, the sleeves being in electrical contact with the riser, and the riser, in use, being cathodically protected.

Whilst a riser which is or is to be cathodically protected by an impressed current alone is within the scope of the present invention, we have found that in practice problems can arise. There is a danger that no current or insufficient current will reach the main source of corrosion, i.e. the galvanic cell created at the junction of the riser and the sleeve, if physical barriers such as pipes or structural members situate themselves therebetween.

It is therefore preferred to cathodically protect the riser by the use of sacrificial anodes adjacent the sleeves. Any sacrificial anode can be used, although aluminium alloy is preferred. Furthermore, the method by which the sacrificial anode is fixed to the riser is not critical. However, some methods of fixing sacrificial anodes to the riser are more preferred than others. For example, anodes which are cast onto the riser withstand stresses and strains better than do anodes which have been welded directly onto the riser or onto a support which is welded onto the riser. Replacement of fractured or consumed anodes in possible, although not unexpectedly problems can arise.

More preferably, cathodic protection of the riser is obtained by the use of sacrificial anodes supplemented by an impressed current. An advantage of this arrangement is that the impressed current can be used as a back-up system in a situation where the current from the sacrificial anode becomes insufficient to protect the riser.

In a preferred embodiment the sacrificial anodes are located on either side, within say 1 m., of each sleeve, and if necessary additional anodes are located at points between the sleeves.

The sleeves of the risers may be made of any material provided that the three requirements of the sleeve which are referred to above are satisfied. However, we have found that copper-nickel alloys such as 90 Cu/lONi and 70 Cu/30Ni are to be preferred since they are particularly resistant to corrosion and pitting. Preferably, the sleeves are welded onto the riser.

Instead of being closely fitted to the riser, the sleeves may be connected and sealed at one end thereof (the lower end) to the riser and the annulus so formed filled with a suitable thermosetting filler, such as a high temperature pipeline enamel. Using this method the wear sleeves can be made to any appropriate thickness and fitting is relatively simple.

In use, the riser of the present invention should be electrically insulated from the submarine structure on which it is supported so that it potential can be monitored independently. Whilst the choice of the electrical insulator is not critical, we have found neoprene to be a satisfactory material.

The method by which the riser is connected to the submarine pipeline is not critical, provided that the electrical insulation between the riser and the supporting structure is maintained.

Also, in use, the riser splash zone is preferably sheathed, for example, from -3 m to It 10 m with respect to the mean sea level with monel 400. In a preferred embodiment cast sacrificial anodes are located just below (in the range of 1-2 m) the termination of the monel sheath.

In practice, the riser of the present invention would be suspended from a cross frame well above the water level. Furthermore, appropriately located reference electrodes would be attached to the structure and wired back to the monitoring panel, to which a lead would also be taken from the seaward side of the riser insulated support, to enable riser potentials to be read and recorded.

It will be appreciated that the longer the lengths of riser pipe that can be pre-fabricated the better, since it is beneficial to keep the number of circumferential welds that will be necessary to assemble the complete riser to a minimum.

The advantages of the riser corrosion control system discussed above are as follows:- (1) There is no temperature limitation on the operation of the riser.

(2) By avoiding the requirement of coat ing the riser, considerable cost is saved both at the construction yard in actual shot blasting and coating and also in the installation of the riser in the structure since special slings are not required.

(3) The cathodic protection requirement can be predicted with considerable accuracy as distinct from unknown deterioration rates of coatings, and the riser would be fully cathodically protected as soon as the supporting structure had been installed in the sea.

(4) By arranging electrical connections from the riser to the monitoring and control unit on the structure and by utilising structure reference electrodes at certain levels, the potential of any particular riser can be continuously recorded. In addition, if any particular reason, for example damage to the submarine line to which a riser is connected, necessitates an increase in riser potential to provide protection to the submarine line some distance from the structure, this can be accom plished by biasing the riser more negatively with respect to the struc ture than is required for its own pro tection. This would somewhat increase the load on the main cathodic pro tection system but it is unlikely to be significant.

Alternatively, if an independent automatic impressed current system connected to the riser only is the ultimate protection system and it is considered that due to installation of equipment delays, it may be about 1218 months before it can be operated after the structure has been installed in the sea, a short life sacrificial system can be installed to cater for this contingency in the following manner: (i) The riser is connected to the structure electrically by a bond located at the anchor point or other convenient and accessible site.

(ii) Sacrificial anodes are mounted on the riser supports in such a manner as to provide protection to those areas of the riser subject to galvanic attack.

(ill) The riser sleeves of monel and cupro nickel can be painted to reduce the potential galvanic attack on the steel and the demand for cathodic protec tion from the short life anodes.

This arrangement also makes use of any structure mounted sacrificial anodes near to the riser itself. When the impressed current system is to be energised the electrical bond to the structure must of course be removed.

(5) This system overcomes problems of different pipeline dimeters caused by changes in wall thicknesses which might restrict use of cleaning scrapers, pigs or spheres.

(6) Provided structural requirements are satisfied, the riser can be constructed of the same line pipe as is being used for the submarine pipeline.

In order that the invention may be more fully understood, reference should be made to the accompanying drawings in which: Figure 1 shows a submarine structure on which risers of the present invention are supported; Figure 2 is a side elevation of part of a riser of the present invention; and Figure 3 is a view along III-III of Figure 2.

In Figure 1 there is shown an oil riser 1 supported along its length by horizontal cross-members 2 and diagonal cross members 3 of a submarine structure 4. Also shown in a gas riser 5 supported along its length by a vertical member 6.

Located at each submarine point on the risers where there is attachment of the riser to the structure is a sacrificial anode (not shown). With regard to the oil riser 1, the arrangement of the anode varies with the type of cross-member to which the riser is attached. Figures 2 and 3 show the arrangement of a sacrificial anode at the point where the riser is attached to a diagonal cross-member 3. The riser pipe 7 has therearound a copper-nickel alloy sleeve 8. For reasons of clarity neither the riser pipe 7 nor the sleeve 8 is shown in Figure 3. A stand-off tube 9 is clamped by means of one half of a clamp 10 around the corresponding half of the sleeve 8. Around the other half of the sleeve 8 is clamped by means of the other half of the clamp 10 a hollow aluminium alloy anode 11. The anode 11 is welded to steel members 12 which are welded to one half of the clamp 10. Around the clamp 10 are vertical and horizonal stiffeners 13. Bonded to the inside of the clamp 10 is a lining 14 of neoprene which electrically insulates the riser from the supporting structure.

WHAT WE CLAIM IS: 1. A riser for use in an offshore oil or gas field, which riser has sea water corrosion resistant sleeves secured thereto in such a manner as to prevent sea water from penetrating any area inside the sleeves and adjacent the riser, the sleeves being in electrical contact with the riser, and the riser, in use, being cathodically protected.

2. A riser as claimed in claim 1 which, in use, is cathodically protected by an impressed current.

3. A riser as claimed in claim 1 or claim 2 in which each sleeve has adjacent thereto

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. (2) By avoiding the requirement of coat ing the riser, considerable cost is saved both at the construction yard in actual shot blasting and coating and also in the installation of the riser in the structure since special slings are not required. (3) The cathodic protection requirement can be predicted with considerable accuracy as distinct from unknown deterioration rates of coatings, and the riser would be fully cathodically protected as soon as the supporting structure had been installed in the sea. (4) By arranging electrical connections from the riser to the monitoring and control unit on the structure and by utilising structure reference electrodes at certain levels, the potential of any particular riser can be continuously recorded. In addition, if any particular reason, for example damage to the submarine line to which a riser is connected, necessitates an increase in riser potential to provide protection to the submarine line some distance from the structure, this can be accom plished by biasing the riser more negatively with respect to the struc ture than is required for its own pro tection. This would somewhat increase the load on the main cathodic pro tection system but it is unlikely to be significant. Alternatively, if an independent automatic impressed current system connected to the riser only is the ultimate protection system and it is considered that due to installation of equipment delays, it may be about 1218 months before it can be operated after the structure has been installed in the sea, a short life sacrificial system can be installed to cater for this contingency in the following manner: (i) The riser is connected to the structure electrically by a bond located at the anchor point or other convenient and accessible site. (ii) Sacrificial anodes are mounted on the riser supports in such a manner as to provide protection to those areas of the riser subject to galvanic attack. (ill) The riser sleeves of monel and cupro nickel can be painted to reduce the potential galvanic attack on the steel and the demand for cathodic protec tion from the short life anodes. This arrangement also makes use of any structure mounted sacrificial anodes near to the riser itself. When the impressed current system is to be energised the electrical bond to the structure must of course be removed. (5) This system overcomes problems of different pipeline dimeters caused by changes in wall thicknesses which might restrict use of cleaning scrapers, pigs or spheres. (6) Provided structural requirements are satisfied, the riser can be constructed of the same line pipe as is being used for the submarine pipeline. In order that the invention may be more fully understood, reference should be made to the accompanying drawings in which: Figure 1 shows a submarine structure on which risers of the present invention are supported; Figure 2 is a side elevation of part of a riser of the present invention; and Figure 3 is a view along III-III of Figure 2. In Figure 1 there is shown an oil riser 1 supported along its length by horizontal cross-members 2 and diagonal cross members 3 of a submarine structure 4. Also shown in a gas riser 5 supported along its length by a vertical member 6. Located at each submarine point on the risers where there is attachment of the riser to the structure is a sacrificial anode (not shown). With regard to the oil riser 1, the arrangement of the anode varies with the type of cross-member to which the riser is attached. Figures 2 and 3 show the arrangement of a sacrificial anode at the point where the riser is attached to a diagonal cross-member 3. The riser pipe 7 has therearound a copper-nickel alloy sleeve 8. For reasons of clarity neither the riser pipe 7 nor the sleeve 8 is shown in Figure 3. A stand-off tube 9 is clamped by means of one half of a clamp 10 around the corresponding half of the sleeve 8. Around the other half of the sleeve 8 is clamped by means of the other half of the clamp 10 a hollow aluminium alloy anode 11. The anode 11 is welded to steel members 12 which are welded to one half of the clamp 10. Around the clamp 10 are vertical and horizonal stiffeners 13. Bonded to the inside of the clamp 10 is a lining 14 of neoprene which electrically insulates the riser from the supporting structure. WHAT WE CLAIM IS:

1. A riser for use in an offshore oil or gas field, which riser has sea water corrosion resistant sleeves secured thereto in such a manner as to prevent sea water from penetrating any area inside the sleeves and adjacent the riser, the sleeves being in electrical contact with the riser, and the riser, in use, being cathodically protected.

one or more sacrificial anodes which when the riser is in use, cathodically protect the riser.

4. A riser as claimed in claim 3 in which additional sacrificial anodes are located between the sleeves.

5. A riser as claimed in claim 3 or claim 4 in which the sacrificial anodes have been cast onto the riser.

6. A riser as claimed in claim 3 or claim 4 in which the sacrificial anodes have been welded directly onto the riser or onto a support which has been welded onto the riser.

7. A riser as claimed in any one of claims 3 to 6 in which the sacrificial anodes comprise an aluminium alloy.

8. A riser as claimed in any one of the preceding claims in which the sleeves are closely fitted to the riser.

9. A riser as claimed in any one of claims 1 to 7 in which the sleeves are connected and sealed at one end thereof to the riser and the annulus so formed is filled with a thermosetting filler material.

10. A riser as claimed in any one of the preceding claims in which the sleeves comprise a copper-nickel alloy.

11. A riser as claimed in claim 10 in which the alloy is a 90Cu/lONi or 70Cu/ 30Ni alloy.

12. A riser as claimed in any one of the preceding claims having clamped thereto means for attaching the riser to the supporting structure.

13. A riser as claimed in claim 12 in which said attaching means is a stand-off tube which, when the riser is in use, is welded to the supporting structure.

14. A riser as claimed in any one of the preceding claims which is electrically insulated from the supporting structure.

15. A riser as claimed in claim 14 in which electrical insulation is provided by neoprene.

16. A riser as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.