Development of large-diameter line pipe for offshore applications

Transcription

1 Development of large-diameter line pipe for offshore applications Dr. Michael Gräf Europipe Deutschland GmbH, Muelheim, Germany Dr. Hans-Georg Hillenbrand Europipe GmbH, Ratingen, Germany EUROPIPE. The world trusts us.

2 DEVELOPMENT OF LARGE-DIAMETER LINEPIPE FOR OFFSHORE APPLICATIONS Dr. Michael Gräf a Dr. Hans-Georg Hillenbrand b a Europipe Deutschland GmbH, Mülheim, Germany b Europipe GmbH, Ratingen, Germany ABSTRACT The requirements for large-diameter linepipe for offshore applications are becoming increasingly stringent. On the one hand, as a result of the shifting of offshore oil and gas production more and more into the sea, the pipelines have to be laid in deeper waters and over longer distances. On the other hand, for economic reasons, the pipelines need to be operated at higher pressures, thereby requiring higher strength and toughness for the pipe material. Europipe continuously monitors the market requirements and tries to fulfil them by sophisticated research and development activities. This paper presents the latest results on a high strength steel for use as offshore pipelines, paying attention to limit state design, grade X 80 pipelines and heavy wall pipe for deep water applications as well as to materials for sour service like HIC resistant steels, X2 Cr 13 LC resistant to CO 2 corrosion and clad steel for highly corrosive media. 1

3 1. INTRODUCTION Analysis of the market requirements indicates that the demands for pipeline steels, mainly for use in offshore applications, are becoming more and more stringent. Figure 1 shows the demands on offshore pipelines. The gas transport has to be safe even if the pipeline is laid in deeper water or over longer distances. For economic reasons, the pipeline must be capable of being laid easily and need to be operated at much higher pressures. These requirements imply that the pipeline steel has to possess higher strength and toughness, and that the geometry of the pipe has to be optimised. Additional tests are necessary when the pipe is intended for the transport of corrosive media. Figure 1 Europipe tries to fulfil such stringent requirements by means of sophisticated research and development activities. Examples of latest results on high strength steels for non-sour and sour service are presented in the following. 2. LINEPIPE FOR NON-SOUR SERVICE Limit state design (LSD) codes have come into common use over the last twenty years for many types of structures. More recently, this development has been extended to pipelines. These LSD approaches are now appearing in pipeline design codes such as the DNV Offshore Standard. Beside other cost reduction measures like optimization of pipe diameter and use of high strength steel pipe the LSD approach plays an important role (Figure 2). 2

4 Figure 2 Limit state design is a semi-probabilistic design process in which the probabilistic aspects are treated at the code development stage in order to define characteristic values and partial safety factors for load and resistance. They are used to ensure an acceptable low probability of failure across a full spectrum of design cases. LSD is used as a practical method of incorporating reliability methods in the normal design process. This design method is a new challenge also to a pipe producer because it requires that the individual properties determined on commercial pipe should fulfil certain statistical requirements, e.g. the mean value of the yield strength has to be greater than the specified minimum yield strength by at least 2 x standard deviation. Figure 3 shows the production results on one pipe size for a North Sea pipeline which has been designed to LSD. The standard deviation determined, at 13 N/mm², clearly indicates the level of uniformity that can be achieved by controlling the process parameters throughout the production. Another development for increasing transport efficiency with a view to reducing pipeline construction cost is the use of steels of higher grades to avoid large thickness. Figure 4 shows, by way of example, the results of a trial production for an X80 offshore pipeline (36 O.D. x 32.0 mm W.T.). The chemical composition of the steel used for the pipe corresponds to the known MnNbTi steel. This steel has a sufficiently high ratio of Ti to N and is additionally alloyed with molybdenum. The low carbon equivalent ensures good field weldability. 3

5 Figure 3 Figure 4 4

6 The elongation values (A 2 ) are particularly high. The Charpy V-notch impact energy measured at 40 C is in excess of 200 J and the shear fracture area of the DWTT specimens tested at 20 C is greater than 85 %. The forming and welding operations carried out at our Mülheim works on this high strength steel did not cause any problems. Even steels up to grade X 80 with heavy wall are possible with acceptable carbon equivalents. circumferential welding trials indicate that this high grade material can be welded on a laybarge without any problems. A further new challenge to the pipe manufacturer and the pipelaying contractor is the offshore pipeline in deeper waters. A consortium has been carrying out a feasibility study since 1993, for a gas transmission pipeline from Oman to India in a water depth of 3500 m. Figure 5 shows the most important requirements for the pipe. To prevent collapse of the pipeline under an ambient external pressure of about 350 bar, the linepipe to be used has to meet severe requirements. The extreme pipe size consists of a wall thickness of 41 mm at an inside diameter of 610 mm. Figure 5 5

7 Mechanical strength and geometry of the pipe are the two most important factors influencing the collapse strength. High strength values, which need to be uniform over the pipe circumference, reduce the susceptibility of pipe to collapsing. The material for the pipes is grade X 70 (non-sour). For offshore pipelines, the pipe is often required to meet the requirement for yield strength also in the longitudinal direction. The toughness requirements are quite high at > 200 J average and >150 J individual for the base material and > 100 J average and > 75 J individual for the weld. DWT test and CTOD test are also specified. Outof-roundness of the pipe has a detrimental effect on collapse strength. Hence, the requirement for out-of-roundness reads: as low as possible, but 4 mm maximum. An exclusive contract was awarded in 1995 to Europipe to produce the first 1000 m of pipe for weldability trials. Forming pipe with such an unfavorable diameter-to-thickness ratio places severe demands on the pipe forming equipment, e.g. the crimping press, U-ing press, O-ing press and mechanical expander. Extensive laboratory work involving finite element analysis at Mannesmann Forschungsinstitut was conducted to determine the parameters needed for forming the pipe and the loads occurring on the forming equipment. Figure 6 shows, by way of example, the results of the numerical analysis of the loads occurring on the tools in the O- ing press. As can be seen from the figure, the highest loads occur at the 11 o'clock and 1 o'clock positions. Figure 6 6

8 The variation in pipe strength, the pipe geometry and the Bauschinger effect caused by pipe forming operation all have an effect on the collapse strength. Some results of collapse tests on linepipe are shown in Figure 7. The values shown as calculated collapse pressures were determined by an analytical procedure. The calculated values are greatly dependent on the compressive yield strength, which can be determined experimentally, and on the initial ovality. Figure 7 As can be seen, there is nearly no difference in buckling pressure between pipes of different strength for a given level of ovality with one exeption. The effect of compressive yield strength of the material on the buckling pressure is quite significant in the case of pipes with a thicker wall. Furthermore, it is remarkable that the 28 dia. X 41.0 mm wt pipe subjected to a heat treatment has a higher collapse pressure than does the pipe of the same diameter (not heat treated) but with a higher wall thickness of 44.0 mm. This result can also be explained in terms of the Bauschinger effect. As a result of the thermal treatment (at 200 C) prior to testing, the Bauschinger effect on the mechanical strength of the pipe material was almost completely eliminated. 7

9 3. LINEPIPE FOR SOUR SERVICE Steels to be used for the transport of H 2 S-containing gases need a special treatment in the steelmaking shop. The restriction of the sulfur content to very low values and the addition of calcium to form non-deformable sulfides help prevent the formation of nucleation sites for HIC. If the HIC test environment is a ph3 solution with 1 bar H 2 S, the C and Mn levels of the steel have to be restricted to reduce the centreline segregation and avoid HIC. Figures 8 contains a list of selected orders for sour service pipe executed by Mannesmann/Europipe. The list clearly illustrates the continuous changes that have occurred in the market requirements for wall thickness, material grade and HIC resistance of pipe. As can be seen, the trend has always been towards ever increasing wall thickness, material grade and HIC resistance. Figure 8 This trend is however different from the trend with fit-for-purpose pipe, where other requirements including those for wall thickness and diameter-to-thickness (D/t) ratio predominate. Experience shows that the standard HIC test is capable of differentiating between susceptible and resistant steels. The above-mentioned full-scale tests have demonstrated that the steel suffering certain amount of cracking in the laboratory test can withstand the full-scale test, without cracking, in the same test environment. Therefore, steels exhibiting cracking within the CLR, CTR and CSR limits mentioned above are considered as HIC resistant for conventional 8

10 sour service linepipe. It becomes however increasingly difficult, or impossible, to fulfil these requirements as the market demands ever increasing strength and/or special pipe geometries, e.g. a low D/t ratio. The application of the standard HIC test therefore results in discarding pipe that would perform satisfactorily in actual service. In such cases, it is therefore necessary to adopt a fit-for-purpose approach in selecting the test conditions for the laboratory test. Two examples of such fit-for-purpose testing and their backgrounds are presented below. The first example concerns a project inquiry for 42 OD x 31.5 mm WT linepipe in grade X 65 or, if practicable, in grade X 70 and HIC requirements. A lean chemical composition that is typically used for the production of linepipe intended for sour service could not be used here because of the heavy wall. The chemical composition was optimised for most part to fulfil the requirements for mechanical properties. Attention was paid to each and every production step with a view to improving HIC resistance of the steel. Different HIC test variants were tried out to work out a procedure that is close to the predicted service conditions and that is not difficult to implement. Figure 9 shows the behaviour of the 31.5 mm thick grade X 70 material, produced on a trial basis, in the various HIC test variants. The HIC index shown on the Y-axis is a measure of the extent of cracking. The higher the value of the index, the larger is the extent of cracking. The ph of the test solution was 3. The first row beneath the bars indicates the types of specimen used. Standard HIC specimens and large plate specimen were tested. Figure 9 The plate specimens were hydrogen charged by placing a glass tube on the specimen and filling it with the test solution. The second row beneath the bars shows the number of specimen sides exposed to the test solution. Only one-sided hydrogen charging was used in the case of plate specimens. The third row shows the partial pressure of H 2 S in the H 2 S + N 2 gas mixture with which the test solution was saturated at atmospheric pressure. The figure clearly demonstrates the effect of test conditions on the HIC index. 9

11 As this type of steel is not characterised by very low C and Mn contents, it is easier to produce pipe with excellent mechanical properties. Also, there is a big potential for the development of higher material grades like X 80 for slightly sour service. The second pipeline project in question, which is entitled Blue Stream Project, concerns a sea bed pipeline between Djubga in Russia and Samsun in Turkey crossing the Black Sea over a length of about 370 km. The specified requirements for the pipe are listed in Figure 10. The special feature of this project is the combination of extremely deep waters the pipeline had to pass through over a large length and the ambient corrosive environment mentioned above. At water depths in excess of 2200 m, the ambient water pressure exceeds the internal pressure on the pipe. Consequently, the pipe had to be designed to ensure that it should not fail by plastic collapse. Figure 10 This, in turn, entails narrow tolerances on out-of-roundness for the pipe. The pipe should not deviate from the true circular shape by not more than 3 mm. Moreover, the D/t ratio of the pipe is relatively low. As the D/t value decreases, the deformation experienced by the outside surface during pipe forming increases. As a result, the HIC specimens may exhibit severe cracking in the zone close to the pipe outside surface. It is technically not feasible at present to produce pipe of the given geometry with reasonable productivity and pass the conventional HIC test to NACE Standard TM Therefore, the HIC test conditions selected had been modified taking into account the real service conditions. 10

12 By means of trial production, it could be demonstrated that it would be possible to produce pipe which would meet the requirements for HIC resistance under the modified test conditions as well as the severe requirements for mechanical properties (Figure 11). Figure 11 For the transportation of CO 2 -containing wet gases, low-alloy carbon steels cannot be used, because they suffer heavy weightloss corrosion. Corrosion resistant steels with 13% Cr have been developed. Not only the corrosion behaviour but also special steel production and rolling techniques and welding procedures were evaluated in our labs. In the meantime, we have produced new large diameter pipe (24" x 13.3 mm WT) from X2 Cr 13 LC material. Figure 12 shows a macrograph of the submerged-arc weld in the pipe. A duplex welding wire (25Cr - 9Ni - 3Mo) was selected to achieve best results. The mechanical properties are summarised in Figure 13. Strengths and toughness properties are on a good level. The hardness values are lower than in previous trials. This is a result of the modified lean chemistry. 11

13 Figure 12 Figure 13 The use of high alloy materials for the linepipe often represents the only solution to combat corrosion in situations where inhibition and gas processing is not practicable. Clad pipe combines the excellent corrosion behaviour of high-alloy austenitic materials such as Incolloy 825 and the high strength of carbon steels. Europipe has developed the technology of producing clad pipe from clad plate, which can be produced by sandwich-type rolling or by rolling of explosion-clad slabs. In both cases, a metallurgical bond between the substrate and the cladding is ensured. 12

14 Figure 14 shows production results on a clad linepipe X 65 / Incolloy 825 (24" dia. x 20.5 mm wt). The substrate steel has a very good toughness and a low transition temperature in the DWT test. The pipe was welded from the inside using the electro-slag strip welding process which leads to very little dilution from the parent material. Figure CONCLUSION The paper gives an overview of the current requirements and the associated development of linepipe for offshore applications. It could be demonstrated that the pipe manufacturer had to launch a number of R&D projects as a means of fulfilling these requirements. Only through a pipe production step that is integrated into the entire chain of production steps and R&D activities it is possible to develop such demanding products. 13