This paper presents some of the challenges encountered during the welding of the Amoco CATS pipeline. The 36-in. diameter high pressure gas pipeline was installed from an onshore terminal at Teesside, England to the North Everest Riser Platform located in the Central Graben Development Area in the North Sea. The off- shore pipeline was installed by McDermott/ETPM using the lay barge LB200. Submerged arc welding (SAW.) was used for the double joints and ETPM's proprietary Saturne 8T gas metal-arc welding (GMA W) system for both the mainline welds and the root pass of the double-joint welds. The CATS pipeline has undergone rigorous testing, not only when the line pipe was being fabricated, but also when establishing welding procedures and throughout the pipelaying process. This rigorous testing and monitoring has resulted in a pipeline of very high quality.
A two-year pipelaying program has been completed in the North Sea, using primarily automatic GMAW techniques. The CATS Pipeline and Riser Platform was installed to join the gas fields in the North Sea to the industrial area of Teesside, England as shown in Fig. 1. A power generating plant is being completed, and other industries in the Northern sector of England will benefit from this gas. The total length of the pipeline is 404.5 km (251,3 miles) and the riser length is 115 meters. The pipeline is 36 in. in diameter, API 5L Grade X65 fabricated with double submerged arc welds to form the longitudinal seams. The wall thickness was 28.4 mm (1 .12 in.) with a small portion 33.9 mm (1 .33 in,) wall thickness at onshore and shore approach sections. The line was designed for dry sour service. The design pressure is 2600 psig with a 1500 ppm/4 psi max. hydrogen sulfide. The design oxygen content was 10 ppm/0.03 psia max. and the carbon dioxide was 25 mol%A/654 psia max.
The line pipe was fabricated by Mannesmann to conform to sour service performance requirements. The CATS pipeline represented an order for 260,000 tons of steel. The technical requirements for the line pipe were based on the API 5L specification for line pipe1 with certain additions, deletions, and modification made to conform to Amoco requirements with mutually agreed manufacturing Iimits2. For example, the product analysis chemistry of the pipe material was subject to the limits shown in Table 1. The two carbon equivalent measurements specified were the IIW and Pcm suchthat:
(Mathematical equation available in full paper)
Charpy V-notch impact testing was also modified from API 5L requirements to state that a set of three specimens would be taken from a pipe from each heat, transverse to the weld, from each of the following locations:
Weld Metal - Notched on weld centerline
Heat Affected Zone - Fusion line centered on
(HAZ) notch
Base Metal - Notch on base metal
It was also stated that the top of the Charpy V-notch specimen would be taken 2 mm from the outside pipe surface. The acceptance criteria for the toughness are shown in Table 2.