IntroductionOil and gas reserves in shallow and sweet conditions are becoming increasingly scarce, requiring the development of products able to withstand extremely corrosive oil and gas (sour service).

In early developments, the management of corrosive oil and gas reserves was accomplished by the use of sour service steels and the introduction of chemical inhibitors into the line. As developments have increased in complexity so have the solutions being considered and the recent trend is towards the use of more exotic alloys. However, the availability and cost of these solutions still presents difficulties that can restrict the development of the most challenging resources.

Clad linepipe, carbon steel with a corrosion resistant alloy (CRA) lining, is a material technology that delivers corrosion resistance, pipe strength and an economical use of these costly and scarce alloys. The recognition of the benefits of this mix of materials is resulting in increased demand for longer lengths of clad pipelines. Previous methods for the manufacture of clad linepipe have been slow and more suited to small quantity requirements rather than the larger project quantities now required by the industry. With these factors in mind, Corus Tubes is now offering large scale production of clad pipes. Outside Diameter (inches)

Pip

e Th

ickn

ess

(mm

) inc

lud

ing

CR

A la

yer

15 20 25 30 35 40 450

45

40

35

30

25

20

15

10

5

0

Please refer to sales team for specific size and grade information.

- Process route capable of large scale production- Available in sizes from 16” to 42” outside diameter- Quality plate feedstock sourced from specialist manufacturers- World’s strongest crimp and ‘O’ press combination enables production of thick walled pipe, even in smaller diameters- Flexible production route to accommodate smaller quantities e.g. risers- Increased resistance to liner collapse

Metallurgically bonded UOE clad (DSAW) linepipe

Clad pipe manufactureCorus Tubes UOE double submerged arc welded (DSAW) pipe mill is the only mill capable of manufacturing metallurgically bonded clad linepipe by the UOE method specifically for use in regions of sour service oil and gas. By using our linepipe expertise and UOE process route, we are able to produce high quality clad linepipe ranging in outside diameters from 16 inches to 42 inches. The mill is also capable of supplying very thick wall pipes, even at smaller diameters, due to the power of its crimp and ‘O’ presses. These products are ideally suited to high pressure, high temperature applications as well as deep water and riser applications.

Feedstock manufactureOur clad linepipe is formed from carbon steel and corrosion-resistant alloy (CRA) metallurgically bonded plate providing a highly corrosion-resistant pipe surface. This high quality steel plate feedstock is sourced from a select number of assured manufacturers throughout Europe. This ability to purchase plate on the open market enables us to ensure we provide the best technical solution to meet customers’ bespoke project requirements and to take advantage of technological advances in clad plate manufacture.

Figure 1 below demonstrates a typical manufacturing process for clad plate. Through a controlled process of heat treatment and hot rolling a finished clad plate is produced and despatched to Corus Tubes’ facility in Hartlepool.

Plate can be supplied with a carbon steel backing plate in a range of linepipe grades, API X65 or equivalent being the most commonly requested. The corrosion-resistant alloy layer can be supplied in either 316L or 825, covering the majority of linepipe applications. Prior to despatch to the pipe mill in Hartlepool, all plates are ultrasonically tested and fully inspected.

Pipe manufactureA detailed manufacturing process route for UOE DSAW clad linepipe can be found on page 5.

Plate receipt and preparationPlate is transported by road from receipt at the deepwater port in Hartlepool in accordance with the project schedule. All plates are assigned a unique identification code that is entered into the pipe tracking system to ensure complete traceability. Carbon steel welding tabs are attached to the four corners of the plate before being passed through the edge milling machine. This shapes the plate edges into the optimum profile for welding. The configuration of this preparation is unique to each pipe size and thickness.

After edge milling the plate moves onto the crimping press. This machine bends the edges of the plate so that when the pipe is formed, optimum shape and roundness is achieved around the circumference at the point where the plate edges come together.

Sandwich assembly Sandwich welding

Separating agent

Clad layer high alloyed

Backing material linepipe steel

Heating

Plasma torch cutting Heat treatment

Separation and ultrasonic testingSurface finishing

and final inspection

Hot rolling

Figure 1: Typical clad plate manufacturing route

CRA clad layer

Figure 1: Typical clad plate manufacturing route

Figure 2: Plate edges are milled to create the optimum profile for welding

Pipe formingThere are two key stages in the transformation of the flat plate profile to the round pipe shape – the ‘U’ press and the ‘O’ press.

The ‘U’ press pushes a bulb shaped head onto the middle of the steel plate and rollers move in from the sides to form a ‘U’ shaped steel plate, from this point on referred to as a skelp.

The skelp then moves through to the ‘O’ press. With a total pressing weight of 50,000 tonnes, Corus Tubes’ ‘O’ press is the strongest in the world over a 12 metre pipe length. This controlled power enables the mill to press clad plates up to 40mm thick.

As the top of the ‘O’ press descends down onto the ‘U’ shaped skelp, the steel is moulded to fit into the size bespoke dies lining the mill. By controlling the speed and power of this press the mill can optimise the shape and configuration of the resultant skelp.

Pipe weldingOnce the clad plate has been formed into a round pipe shape it is passed through to the welding operations of the mill. An initial tack weld is applied to the root of the weld preparation on the carbon steel outer surface of the pipe. This maintains the pipe shape as it passes into the internal welding process.

Preliminary non-destructive testing (NDT) of both the internal and external welds is completed prior to the pipe being transferred to the offline welder. This has been specifically designed for the welding of the CRA layer of clad pipes.

The CRA layer of the clad plate is restored by the electro-slag method of welding. Electro-slag welding allows for optimised welding speed, minimal dilution (less than 10%), an excellent bead shape, no spatter, a ferrite level of less than 10% in the weld bead and good corrosion resistance of the overlay weld. Corus has worked closely with Air Liquide Welding to develop consumables for this process, and can offer matching and overmatching welds for 316L and 825 claddings.

CRA clad layer

MIG welding

Multi-wire double SAW

Electro-slag welding

This Page:

The controlled power of the ‘U’ and ‘O’ presses enables the manufacture of clad linepipe

to precise dimensional tolerances.

Figure 3: After forming, pipe manufacture is completed with a four stage welding process

This overlay weld is then subjected to NDT to prove the integrity of the weld before the pipe is transported to the mechanical expander.

Pipe finishingOnce the pipe has been expanded it passes through to the finishing mill. Within this part of the manufacturing facility, each pipe can be hydrotested and finished in accordance with the client’s project requirements. End faces of the pipes can be bevelled and prepared to the configuration required by the lay contractor.

During the forming process there will have been contact between the carbon steel tooling and the CRA layer. Carbon steel contamination of the CRA surface of clad linepipe can have a detrimental effect to the corrosion resistance of the CRA layer. Following the carbon steel contact during the forming process, in accordance with DNV-OS-F101, Corus cleans the bore with the application of a pickling solution.

Firstly the pipe ends are sealed before feeding a spray nozzle along its entire length, ensuring complete coverage of the CRA with an HF/HNO3 solution while avoiding contamination of the outer carbon steel. After a short time, the acidic solution is rinsed away with deionised water and the pipe is left to dry completely. Ferroxyl tests prove complete removal of carbon steel contamination using this method, and corrosion performance is guaranteed by the passivation resulting after the pickling process.

The expander moves along inside the length of the welded pipe, exerting uniform pressure to ensure consistent dimensional properties around the circumference and along the length of the pipe. Clad pipe from Corus is supplied to the same world-leading dimensional tolerances as our carbon steel pipes, allowing excellent fit up and weldability.

Following final inspection of the bore, protective caps are fitted to the pipe ends in order to protect the CRA surface whilst the pipes are despatched to their delivery point.

Non-destructive testingConventional ultra-sonic and radiographic techniques are not guaranteed to identify all defects in the complex weld structure of a clad pipe. To address this issue, Corus has developed specialist techniques to enable the inspection of all areas of the weld.

Ultra-sonicsConventional ultra-sonic testing uses shear waves to detect defects in the carbon steel weld. Shear waves are formed when the energy is transmitted tangentially to the direction of the wave propagation. Waves of this form become diffused when they cross the boundary between the fine-grained structure of the main weld and body, and into the coarse grained austenitic weld of the CRA overlay. This diffusion causes excessive noise, obscuring any defects in the clad weld.

Inspection of the entire weld can be achieved by the use of compression waves. These waves transmit the energy from particle to particle in-line with the direction of travel by a compressive action. These waves can be used to penetrate the CRA weld area, crossing the boundary without diffusion.

A combination of shear and compression probes is the best option to inspect the whole weld in sequence. Shear waves are used to check the majority of the weld and the junction between the CRA layer and the main body and weld. Compression waves are included to inspect the coarse grain overlay weld structure. These probes are mounted within Corus’ existing test equipment.

Figure 4: The clad pipe is expanded to achieve its final dimensions

Single crystal (pulse echo and cross reflect) shear waveprobes, for carbon steel and

boundary defects

Twin crystal (send and receive) compression waveangle beam probes, for austenitic clad

weld defects

Pickling/Passivation

CustomerAcceptance

Plate stock Edge preparation milling

“O” press “U” pressInternal & external DSAW

End-facing and bevelling

Mechanicalunload

On-line

01-02-03

Hydrostatic testing

01-02-03

Mechanical expander

01-02-03

01-02-03

Weld seam ultrasonic testing or X-ray

01-02-03

01-02-03

Edge crimping

01-02-03

Electro-slag overlay welding

Off-line

Weld seam ultrasonic testing or X-ray

01-02-03

Clad pipe process route

Quality managementDuring pipe manufacture, as part of Corus Tubes quality management philosophy, production tests are taken to comply with the customer’s specification. Corus Tubes has a dedicated materials testing laboratory, which completes all mechanical and toughness testing prior to product release. For more intricate fracture toughness and sour service testing, the facilities at Corus Research, Development and Technology are also available. Examples of clad specific tests available are tests for bond strength between the CRA and carbon steel, corrosion testing of the CRA to ASTM G48 or ASTM A262 and ferroxyl testing for cleanliness of the CRA layer.

Corrosion testingA key challenge for materials producers and specifiers is how to test and guarantee the corrosion resistance of the CRA layer. There are a number of test methods available which each have their own advantages, however a common drawback among all is that none of the laboratory tests can mimic accurately the service conditions of the pipe. The principal international standards deal with this question in various ways.

DNV-OS-F101 This standard recognises that the corrosion resistance of the CRA is primarily defined by chemical composition: and states, “Unless otherwise agreed, corrosion testing of roll bonded clad pipes or any longitudinal weld seams is not required.” Where this testing is required DNV references ASTM G48 “Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless steels and Related Alloys by the Use of Ferric Chloride solutions,” Method A.

API 5LDThe current (1998) version of API 5LD recommends the intergranular corrosion test as per ASTM A262 Practise E. The proposed new edition of API 5LD recognises that: “The purpose of this [corrosion] test is to assure proper manufacturing procedures for austenitic steel and Ni-base alloys. It is not a test to determine susceptibility for use with a particular environment.” It also states that corrosion testing should be carried out as part of the MPQ: “The testing procedure shall conform to the requirements of the latest edition of ASTM A262, Practice E, Practice B or ASTM G28 or ASTM G48, Method A (Section 8) whichever is suitable for the cladding or liner material and as agreed between the purchaser and manufacturer.”

Referenced in both DNV-OS-F101 and API 5LD, the ASTM G48 test has become the industry standard reference for determining corrosion integrity. When using this test, acceptance criteria is not specified and must be agreed between the purchaser and manufacturer for each project. The test itself involves removing the CRA layer from the carbon steel base then machining a test coupon from this CRA. This test coupon is then immersed in a ferric chloride solution at a specified temperature for a set period of time. Acceptance criteria of the test are normally based on the presence or absence of pitting, or degree of weight loss measured during the test. The nature of this test means that the end result is highly dependent on the initial condition and machining of the test coupon.

It should be noted that with all tests for corrosion, it is extremely difficult to simulate infield service conditions in the laboratory environment.

In summary, Corus Tubes offers the following corrosion testing on 316L cladding

• ASTM G48 testing with no pitting on ID surface at 15 deg. C • ASTM A262 Method E tested with no cracking visible after sensitisation • Ferroxyl tests prove no carbon steel contamination on final CRA surface layer

Dimensional control Dimensional control of clad linepipe is of utmost importance to ensure that the material can be efficiently welded in the field. This facilitates repeatability and quality in field girth welds, thereby reducing timely and costly fit up delays.

Through its extensive experience of manufacturing carbon steel linepipe for projects all over the world, Corus Tubes has developed the capability to produce linepipe with superior tolerances and is able to apply this technology to the manufacture of metallurgically bonded clad linepipe.

The following histograms depict tolerances achieved on typical projects for carbon steel linepipe. Reference to carbon steel pipe manufacture allows us access to a greater body of data than clad projects alone; the forming process being identical and material responses being essentially the same.

Diameter

The distributions achieved on projects demonstrate that the measurements taken, for the vast majority of the pipe, are within a range of +/-0.5mm around a mean. However, the normal distribution imposed on the histograms shows that there is potential for some pipes to lie outside this range, which would need to be accounted for either in pipe yield or identified and accepted for a project.

The control of ovality in pipe ends is high within the Corus Tubes production process with the majority of values lying within the region of 1mm. The infrequent, wider ranging results take the limits out to 2mm.

Measurements of pipe body ovality are difficult to take during the manufacturing process, however the UOE process has been proven to give equal control at both pipe end and pipe body.

PeakingDuring the UOE forming process the edges of the plate have to be preformed by the crimp press. This ensures that the curvature of the pipe body is consistent even at the weld area. By careful control of this part of the forming process Corus is able to produce linepipe with minimal out of roundess in this key region of the pipe.

Diameter

Freq

uenc

y

546.0 546.5 547.0 547.5 548.0 548.5 549.00

6000

5000

4000

3000

2000

1000

0

Ovality

Freq

uenc

y

0.3 0.6 1.2 1.5 1.80.0

2300

2000

1300

1000

300

0

2.11.0

Ovality (pipe ends and pipe body)

Tensile results Mean Rt0.5 / MPa Mean Rm / MPa Mean Elongation % Mean Rt0.5/Rm Ratio / 50mm Tranverse 485 558 44 0.87

Longitudinal 506 558 42 0.91

All Weld Tensile (ID) 555 633 15 0.88

All Weld Tensile (OD) 602 671 25 0.89

Transverse Weld Tensile 579

Example charpy results Position Temp °C Average Energy Average Shear Weld Centre Line Mid-Thickness -30 166 98

Weld Centre Line Outside Diameter -30 170 100

Weld Centre Line Inside Diameter -30 169 100

Fusion Line 50/50 Mid-Thickness -30 193 83

Fusion Line 50/50 Outside Diameter -30 240 73

Fusion Line 50/50 Inside Diameter -30 253 95

Body Mid-Thickness -30 446 100

Body Outside Diameter -30 432 100

Body Inside Diameter -30 445 100

Tensile, charpy and drop weight tear testing (DWTT) results show the same excellent performance as Corus carbon steel pipes. We can guarantee X65 tensile performance and our combination of world leading plate suppliers and welding know how allows us to offer upper shelf charpy and DWTT performance at –30°C, despite the reheating effect of our electroslag overlay weld.

Bond Shear StrengthOur average shear strength of the bond between CRA and carbon steel is >400MPa.

Drop weight tear testing: example transition data Temperature Shear 1 Shear 2 Shear Av. -10 100 100 100

-40 100 100 100

-50 100 100 100

-60 95 0 48

-65 10 0 5

-70 0 0 0

Hardness is controlled allowing us to comply fully with customer specification requirements.

Hv10 Hardness (316L cladding) Body Haz Weld CRA Minimum hardness (actual) 180 160 200 180

Maximum hardness (actual) 198 211 228 253

Mean hardness (actual) 186 182 213 207

Mechanical properties

Corus Tubes can supply metallurgically bonded CRA clad linepipe in a wide range of diameter and wall thickness combinations. The UOE process enables Corus to meet project demands for large volumes of pipe to precise dimensional tolerances.