Bi-metal, CRA-lined pipe employed for North Sea field development

Fabrication progresses of bi-metal pipe and ancillary supply and control lines into the 44-in. carrier pipe (Fig. 1).

• This is one of the hydraulic expansion presses, with top die and end-loading rams in open positions, used to manufacture bi-metal-lined pipe (Fig. 2). [45,451 bytes]
• Production process for bi-metal mechanically bonded pipe (Fig. 3 [95,736 bytes]
• Here, bi-metal pipe components have been assembled and are about to undergo hydraulic expansion (Fig. 4). [23,258 bytes]
• Typical seal-weld configuration (Fig. 5) [44,564 bytes]
• Testing regime for bi-metal-lined prpe, Bruce Phase II (Fig. 6) [109,919 bytes]
• British-Borneo Petroleum Syndicate plc's Norpeth TLP is the world's first mini-TLP (tension-leg platform) located in the Gulf of Mexico's Ewing Bank Block 921, in 1,670 ft of water. This Atlantia Corp. SeaStar mini-TLP design has a smaller hull, but its tensioned tethers and basic concepts are similar to a conventional TLP. Alliance Engineering Inc. designed the topsides facilities, capable of handling 35,000 bo/d and 42 MMscfd. (Photo courtesy of Alliance Engineering Inc.) [27,903 bytes]

Significant cost savings on capital expenditure for production flow lines were achieved by the use of corrosion-resistant alloy (CRA) lined pipe, instead of solid CRA and metallurgically clad pipe options.

Presented here are the materials philosophy of the project team and the rationale for selecting bi-metal-lined pipe along with a description of the manufacture, fabrication, and installation of the flow lines.

The bi-metal, CRA-lined pipe was supplied by United Pipelines (U.K.) and H Butting (Germany). The flow line was fabricated and the bundle installed by Rockwater Ltd. by controlled depth tow method.

Clad pipe

Corrosion resistance to the process environment is provided by the internal CRA clad layer, typically 2.5-3.0 mm thick, while the less expensive carbon backing steel provides the line pipe with the required strength and toughness to maintain the mechanical integrity.

Clad line pipe can be manufactured by various techniques that include the forming and welding of hot roll bonded plate, hot co-extrusion, weld overlay, and manufacture of lined pipe. The details of the various methods of manufacture are well documented.^{1, 2} For Bruce Phase II development, manufacturing of the CRA-lined pipe was by a process involving hydraulic expansion of a CRA liner pipe inside a carbon-steel outer pipe. The finished product is supplied as bi-metal-lined pipe.

A more-detailed description of the manufacturing process occurs presently.

Manufacture of bi-metal-lined pipe is based upon use of traditional manufacturing routes for the primary materials, carbon-steel outer pipe, and CRA liner pipe.

Combined with the innovative hydraulic expansion process, the result is a mechanically bonded, lined-pipe product that can be manufactured economically compared with solid CRA and metallurgically clad product routes.

The cost effectiveness of bi-metal-lined pipe is demonstrated by the fact that bi-metal-lined pipe has been used extensively in the U.K., Norwegian, and Dutch sectors of the North Sea and also in the Gulf of Mexico.

Bruce Phase II development

The field lies across blocks 9/8a, 9/9a, and 9/9b in the Northern North Sea, approximately 340 km northeast of Aberdeen. BP Amoco plc is operator; partners are Elf Exploration U.K., Total Oil Marine, BHP Petroleum, and VEBA Oil Nederland.

The eastern side of the field was initially developed in 1993. A subsequent study concluded that the western side of the field should be developed with a subsea scheme, designated BP Bruce Phase II Development Project.

The Phase II field has been developed with subsea production facilities consisting initially of eight subsea wells, a subsea production manifold (manufactured in solid 22% Cr duplex stainless steel and lined with rubber) connecting an 18-in. production flow line which transports well stream products to a new compression-reception platform.

The subsea facilities were developed in an alliance among BP Amoco, Kvaerner Oil & Gas, Kvaerner FSSL, Rockwater Ltd., and Heerema Marine Construction.

The eight subsea wells are grouped around a single two-head manifold structure and are operated via a multiple electro-hydraulic subsea control system.

The subsea manifold structure houses manifolds and headers, subsea control modules, and jumper connections to the production well cluster. The production manifold is connected by a pipeline bundle to a platform tow head close to the compression/reception platform.

The BP Bruce Phase II bundle consists of a 44-in. OD carrier pipe (Fig. 1) which contains an 18-in. production flow line and an 8-in. test pipeline, both of which were manufactured from bi-metal CRA lined pipe, supplied by United Pipelines and H Butting.

In addition to the two bi-metal-lined flow lines, there is a 10-in. carbon-steel gas reinjection line also supplied by United Pipelines and H Butting from Mannesmann and a subsea manifold umbilical. The Bruce II bundle is approximately 5.7 km long.

Development

Each alliance member contributed its particular expertise to the process of the development, management, design, fabrication, installation, hook-up, and operations. Each partner took a proportional share in the risk and the rewards of the overall capital expenditure.

The process operating conditions of the Bruce II development which governed the materials selection and were the design basis for the subsea flow lines were the following: temperature, 85° C. maximum; pressure, 295 bar maximum; CO2, 2.2 mol % maximum; H2S, 3 ppm maximum; chloride, 47,000 ppm.

Materials philosophy

The initial feasibility study looked at the option of using carbon steel with inhibition. The calculations showed that for the temperature profile of the flow line, carbon steel was only suitable with an unusually large corrosion allowance for straight sections.

The large corrosion allowance was deemed impractical for bends, low points, and where slugging occurred. This was because the relatively high amounts of carbon dioxide in the process water would produce carbonic acid that would in turn corrode the carbon steel. The initial study therefore indicated that a corrosion-resistant alloy would be required to satisfy the 25-year design life.

The principal requirement of the candidate CRA materials is their resistance to CO2 corrosion. It was recognized that 22% Cr duplex stainless steel (DSS) and 316L austenitic stainless steel would provide the necessary corrosion resistance in the process fluids. (The 316L can be used in this chloride-containing environment because of the absence of oxygen in the process fluids.)

A solid 316L material was unacceptable because the low proof strength of the material would have led to high wall thicknesses, thus a significant increase in the weight of the flow lines.

The CRA materials options that were evaluated were solid 22% Cr duplex stainless steel, metallurgically bonded X-65/316L, and bi-metal-lined pipe X-65/316L.

Calculations based upon prices obtained from the open market showed that the bi-metal-lined pipe was significantly less expensive compared with the 22% Cr DSS option. Although metallurgically bonded clad materials of the type X-65/316L are well proven for production flow line applications, their cost structure is only marginally better than 22% Cr DSS.

The through-life costing exercise carried out by Kvaerner Oil & Gas on the various material options showed that the most cost effective materials selection for the Bruce II development production flow lines was bi-metal mechanically bonded lined pipe.

Through-life cost calculations showed the X-65/316L bi-metal-lined pipe option to be more economic than inhibited carbon-steel line pipe with an 8-mm corrosion allowance. Therefore the decision was taken to use X-65/316L bi-metal mechanically bonded lined pipe for the production flow lines.

Following are the details of the X-65/316L bi-metal flow line dimensions:

• 18-in. production flow line: 18-in. OD x 22.18 mm W.T. + 2.5 mm, X-65/316L
• 8-in. test line: 8-in. OD x 11.09 mm W.T. + 2.5 mm, X-65/316L.

Using bi-metal-lined pipe

Bi-metal-lined pipe material combinations are specifically manufactured to satisfy requirements of individual project demands, and a typical range of material options is detailed in Tables 1 [15,853 bytes] and 2 [21,706 bytes].

From the typical range of material combinations shown, it can be seen that bi-metal-lined pipe allows materials engineers and design engineers the opportunity to satisfy demands of the particular project's process environment, even for high-temperature, high-pressure applications, while at the same time optimizing the overall cost effectiveness.

In consequence, bi-metal-lined pipe combinations can be supplied to optimize the following advantages.

• Resistance to chloride and sulfide stress corrosion cracking (SCC) in environments containing CO2/Cl/H2S.
• Resistance to localized corrosion (pitting and crevice corrosion).
• Resistance to general corrosion even in aggressive acid environments.
• Resistance to erosion corrosion and corrosion fatigue.
• Ease of fabrication, using combinations of automatic plasma/gas tungsten arc welding (GTAW) for the seam weld of the liner, automatic GTAW for the seal welds, and plasma/GTAW for the girth welds.
• High mechanical strength, to reduce pipe wall thickness based upon the choice of outer carbon-steel pipe materials and the respective design code.
• Excellent coating adhesion, for corrosion and insulating coatings.

Manufacturing

The first one was commissioned approximately 8 years ago and can produce bi-metal-lined pipe in 6-m lengths (double random lengths with jointers), in the size range of 4-30 in. OD with up to approximately 40-mm W.T.

The second, commissioned in October 1995, constitutes an investment of approximately $20 million and is the largest press of its kind in the world. The press can produce bi-metal-lined pipe in 12-m lengths without jointers in the size range of 6-24-in. OD, with up to 40-mm W.T.

Fig. 3 shows a flow diagram of the manufacturing process for the bi-metal-lined pipe.

The manufacturing process initially involves manufacture of the carbon-steel outer pipe, which can be produced by seamless or welded manufacturing processes. The carbon-steel outer pipe for the Bruce II development was manufactured by a seamless route.

The CRA liner pipe is produced separately by the welded method of manufacture using welded strip or plate methods of production in accordance with the respective codes and standards.

The CRA liner pipe is telescopically aligned inside the carbon-steel outer pipe, and the assembly is placed inside a die tool consisting of two half shells and hydraulically expanded at ambient temperature with a radial pressure up to 2,500 bar (Fig. 4).

At the same time, simultaneous compression of the pipe is induced by an axially operating force of up to 2,500 kN on each end of the pipe assembly.

The simultaneous action of operating radial and axial forces in the pipe assembly contained within the die tool causes the inner CRA line pipe to be expanded by approximately 2-5% until it touches the inside wall of the outer carbon steel pipe.

The operation is then followed by a combined expansion of both the inner and outer pipe by approximately 0.5-1.0%, with the outer pipe being ultimately constrained by the closed die tool.

Consequently, bi-metal-lined pipes are produced with high dimensional accuracy so that optimum tolerances and straightness of the bi-metal-lined pipe are achieved.

Seal welds

The reason for seal welding the pipe ends is twofold:

1. The weld effectively seals the interface between the CRA liner and carbon-steel outer pipe, thus holding out dirt or moisture during transportation, storage, and subsequent site welding.
2. The weld forms an integral part of the pipe-end weld preparation at the mill.

Fig. 5 shows the typical seal-weld configuration. The choice of filler metal is made upon the need to overmatch the strength of the carbon steel outer pipe and to optimize the corrosion-resistance performance of the girth weld relative to the liner grade of material.

Table 3 [11,666 bytes] presents a selection of the filler metals that have been used for both the seal welds and girth welds on various combinations of carbon steel outer pipe and CRA liner. There are usually several suitable filler metals which can be used for a particular combination of carbon-steel outer pipe and CRA liner grades.

The filler metal chosen for the seal welds on the BP Bruce Phase II bi-metal-lined pipe, material combination X-65/316L, was a nickel-based consumable ER Ni Cr Mo-7 modified. This filler metal had been successfully used on previous bi-metal-lined pipe projects.

ER Ni Cr Mo-7 yields mechanical properties which overmatch the strength of the X-65 carbon steel, and the weldment posseses the necessary ductility and fracture toughness to resist crack growth at the carbon-steel weld-to-liner interface.

The filler metal selected has low susceptibility to microstructure-segregation, thus enhancing the corrosion resistance of the weldment while reducing susceptibility to weld-metal solidification cracking.

It was a requirement of the project to control the depth of penetration of the seal weld into the liner to between 10% and 75% of the liner thickness. The use of the automatic GTAW process enables accurate control of the welding parameters.

This ensures that the seal-weld joint dimensions in terms of depth of penetration around the circumference of the carbon steel-CRA interface and into the CRA liner can be closely controlled. All seal welding was carried out in accordance with the requirements of ASME IX.

In production, the seal welds were examined at a frequency of one examination for 50 pipes. Macro examinations and hardness measurements were taken at the 3, 6, 9, and 12 o'clock positions in order to confirm the quality and integrity of the seal weld.

Specifications

The carbon-steel outer pipe was manufactured via a seamless route with the plug mill process and in accordance with API 5L.

The project specification imposed several additional requirements that included stringent control of chemical composition, an upper limit on the yield strength, and a Charpy impact-test requirement of 50 Joules minimum average at -50° C.

There was also a requirement for strain aging test to be carried out on samples strained 3% and then aged for 1 hr at 250° C. The acceptance criteria were according to the requirement of the unaged Charpy specimens.

The carbon steel had to comply with the sour-service hardness requirement of 248Hv10 maximum.

The corrosion-resistant 316L austenitic stainless-steel liner was manufactured in accordance with the requirements of API 5LC. The project specification also imposed several additional requirements for the liner.

The principal amendment was modification to the chemical composition specified in API 5LD LC1812, which was a minimum molybdenum content of 2.5%.

The testing philosophy incorporated into the project specification was one of lot testing of the carbon-steel outer pipe at the carbon-steel mill and lot testing of the CRA liner pipe before the hydraulic expansion operation.

On the basis of this extensive testing requirement, the project required only manufacturing-procedure qualification tests on the finished bi-metal-lined pipe.

Fig. 6 summarizes the testing regime adopted for the Bruce Phase II development.

Testing

Table 4 [21,733 bytes] shows the results obtained for the finished 18-in. bi-metal-lined pipe. Also included for comparison purposes are the results for the same pipe in the prehydraulic expansion condition. On the basis of the results, the manufacturing route was considered qualified.

On this project, the bi-metal-lined pipe may operate up to the maximum design temperature of 85° C. As a result of the different amounts of thermal expansion between the CRA liner and carbon-steel outer pipe at the maximum design temperature, compressive forces would be introduced into the CRA liner.

Rockwater determined the compressive load that could develop at the maximum operating temperature owing to the differences in the coefficients of thermal expansion.

In order to verify that the bi-metal-lined pipe could withstand the compressive loads envisioned in the bundle, a full-scale axial compression test was undertaken by an independent research center.

A test sample of the bi-metal-lined pipe approximately 1.5 m in length was taken from a production pipe. The sample was subjected to an axial compression test: the loading on the end face of the pipe. A series of strain gauges were attached to the test samples at various planes and locations along the inner and outer surfaces of the bi-metal-lined pipe.

The test sample was then subjected to increasing increments of compressive loading, and the performance of the liner was fully monitored during all stages of the loading.

Analysis of the test results led to the conclusion that no buckling of the CRA liner had occurred at the loads which would prevail in the bundle.

Coating; fabrication

Conventional production routes, which ensure a good surface finish on the outside of the pipe, manufacture the carbon-steel outer pipe of the bi-metal-lined pipe.

This advantage of the bi-metal-lined compared to solid CRA meant that for the Bruce Phase II development, the coating was undertaken with use of standard manufacturing procedures, with standard blasting techniques, and avoided using special blasting mediums.

In all, 514 pipe joints were successfully coated.

Fabrication of the Bruce Phase II bundle was completed by Rockwater Ltd. at the company's Wick, Scotland, plant, adjacent a beach to facilitate launching the bundle into the North Sea.

The 18-in. bi-metal-lined pipe flow lines were initially fabricated into 24-m double joints manufactured with a combination of manual GTAW and submerged arc welding (SAW) welding processes.

The GTAW process was used for the root and hot passes. The welds were filled and capped with the SAW process.

The filler metal used to fabricate all the joints was a duplex grade, 2209. The duplex wire was selected by the fabricator because it was considered an ideal choice for the bi-metal-lined pipe materials combination of X-65 and 316L because its corrosion resistance matched that of the 316L liner and its strength matched that of the X-65 carbon-steel outer pipe.

The SAW welding was carried out with a compatible flux designed to maintain the correct austenite-ferrite phase balance.

The 24-m double lengths were then welded into the line over five manual GTAW welding stations.

The 8-in. flow lines were welded with the manual GTAW process. A firing line was established with up to four welding stations producing 500-m length spools.

When entire sections of the bundle were completed, the bundle was gradually moved up the fabrication site on a track, until the entire 5.7-km bundle was fully fabricated and connected to the tow heads.

The on site welding at Wick was carried in accordance with the requirements of BS4515 1996.⁴ All girth welds were subjected to 100% inspection with gamma radiography. Evaluation of the radiographs was carried out in accordance with the requirements of BS4515.

In the case of the 8-in. x 11.1 mm + 2.5-mm test line, the total number of welds carried out was 479. The number of repairs was eight with a total repair length of 280-mm plus one cut-out. This situation constituted a repair rate of 0.29%.

In the case of the 18-in. production flow line, the total number of welds carried out was 511. The number of repairs required was 13, with a total repair length of 650 mm plus two cut-outs. This constitutes a repair rate of 0.48% for the production flow line.

The two sets of weld repair rates clearly demonstrate the excellent weldability of the bi-metal-lined pipe.

U.S. MMS looks at near-term regulatory action offshore

He spoke in March to the 2nd International Deepwater Pipeline Technology Conference in New Orleans.

Cathodic protection

Included were members of oil and gas industry, academics, vendors, and regulators. These special design needs were identified from recent deepwater-project experiences in water depths 1,000-5,000 ft.

The group concluded that the anode chemical composition and spacing requirements must be modified from what is typically done for shallow water because lower temperatures in deepwater result in lower electrical conductivity. These conditions prompt the need for special chemical composition and more stringent anode spacing to provide adequate protection.

Oynes said MMS' gulf regional office initiated the process for publication of a safety alert to inform all Offshore Continental Shelf (OCS) operators of these considerations. The safety alert will likely be published this month.

Flushing, abandonment

MMS may commission a study to establish risk-based flushing and abandonment requirements for OCS pipelines. Based on the results of the study, the 1 and 5-year requirements may be changed depending upon the type of service and/or age of the pipeline.

The study will be completed in 12-18 months.

Regulatory compatibility

Beginning later this year, the two agencies will be reviewing their respective regulations to "facilitate compatible regulatory requirements" for all OCS pipelines whether under DOI or DOT jurisdiction.

Other issues, according to Oynes, include the need for development of tool standards for pipeline intelligent pig inspections and reporting requirements. Such standards are needed to maintain an effective pipeline-inspection program.

Also, MMS is seeing increased cases in which conflicts have arisen between pipeline activities and OCS drilling and development. MMS prefers companies to resolve those conflicts among themselves, recognizing the multiple-use concept of the OCS.

Acknowledgments

The authors would like to thank the directors of the particular companies for permission to publish this article.

References

1. Smith, L., Celant, M., Practical Handbook of Cladding Technology, Casti Publishing Inc., U.K., 1998.
2. Smith, L., Engineering with Clad Steel, Nickel Development Institute, Technical Series No. 10064, 1992.
3. Specification for CRA Clad or Lined Steel Pipe. API Specification 5LD, First Edition, Jan. 1, 1993.
4. Specification for Welding of Steel pipelines on land and offshore. British Standard BS 4515, 1996.

The Authors

Mark Spence is technical manager for United Pipelines, Warrington, U.K., where he has worked since 1996. After joining Mather & Platt in 1984, he advanced from trainee metallurgist to technical manager. Spence has 15 years' experience in corrosion resistant alloys for offshore and onshore applications. He holds a BS (1989; honors) in metallurgy from Manchester Polytechnic.

Ces Roscoe is managing director for United Pipelines, which he founded in 1994. Previously, he held several positions in companies supplying pipeline packages and specialist engineering equipment to the offshore industry. Roscoe is a chartered engineer and holds a PhD (1987) at the University of Manchester Institute of Science & Technology where he specialized in the physical metallurgy of duplex stainless steels.

Karl Schafer is managing director of H Butting GmbH & Co., Hanover, for which he has worked for the past 20 years. He trained as a metallurgist and studied Eisenh?ttenkunde (the study of iron processing) in Clausthal, Germany. He graduated with a doctoral degree in 1966. Schafer has also worked for Krupp Stahl AG for 8 years.

John Hutchison is a principal subsea engineer with BP Amoco in Houston. He joined BP in 1981 and has worked on numerous offshore projects around he world. He holds a bachelors degree (1969) in mechanical engineering from Glasgow University and an MBA (1973) from the Scottish Business School. Hutchison is also a chartered engineer in the U.K.

Alan Foxton is principal materials and welding engineer with Halliburton Subsea Service in Aberdeen, which he joined in 1993. Previously, he held several positions in metallurgy and welding with a number of engineering contractors serving the offshore industry. He holds a BS (1979) in metallurgy from Teeside Polytechnic and a post graduate diploma in welding technology. Foxton was formerly the chairman of the Teeside branch of the Welding Institute.

Charles Barraclough is principal metallurgist and welding engineer for Kvaerner E&C in London, which he joined in 1987. Previously, he was the metallurgist for W.S. Atkins for 7 years. Barraclough has more than 20 years' experience on projects for construction, oil and gas, and offshore industries. He holds a BS (1975) in metallurgy from the University of Surrey and an MBA from Kingston Business School (1993). He is also a chartered engineer (U.K.), a European Engineer, and a registered welding engineer.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.