Orbital Welding Speeds Riser Construction of Offshore Platforms

Orbital welding speeds riser construction of offshore platforms

An orbital welding machine joins a rise pipe subassembly for a North Sea offshore platform project. An insulating blanket is used to help control the cooling rate.

Strategically situated adjacent to the North Sea oil fields, Haugesund Mekaniske Versted (HMV) has become one of Norway's leading construction yards for the oil and gas-related industry. In keeping with its leadership role in offshore construction, HMV has just completed its first project using advanced orbital GTA welding technology. The project, which was begun in August 1992 and completed in January 1993, was the construction of risers for the Draugen platform, owned by Shell Oil, and the Statfjord platform, owned by Statoil. The risers are the piping that extends from the well head on the seabed up to the platform hull. This hull is supported above the water level by concrete towers.

The riser project was a cooperation between Kongsberg Offshore A/S, Steinsvik Maskinfabrikk A/S and Haugesund Mek. Verksted A/S. Kongsberg Offshore A/S was the principal, responsible for the design, buying of materials and supervision of the project. Steinsvik Maskinfabrikk A/S provided the machining of the connectors, and Haugesund Mek. Verksted A/S supplied the welding, heat treatment, NDE, dimension control and pressure testing. The risers, which consist of a 3-in. pipe running parallel to a 7-in. pipe, are used in maintenance of the oil wells. They were constructed from pipe sections and couplings welded in HMV's welding workshop into subassemblies in lengths of 2 to 15 m. The completed subassemblies were stored in special boxes and transported to the rig. Joining on the rig was accomplished by flanges welded to the end of each subassembly. When fully assembled, the total height of the risers was the same as that of the concrete tower, which for the Draugen platform was 285 m. At Statfjord, the height of the risers would be somewhat less since the water depth is only 146 m.

Fig. 1A - End preparations and weld profile for welds of API 5L X70 to AISI 4130. A similar end preparation was used for welds of API 5L X70 to API 5L X70.

Fig. 1B - Shows end preparation for welds of AISI 4130 to AISI 4130.

The materials specified for the risers presented a difficult technological challenge. Structural steels, the API 5L X70 (modified) chosen for the piping and the AISI 4130 specified for the couplings were selected on the basis of the strength required. These materials required very precise control of the welding thermal cycle in order to retain their favorable mechanical properties after welding. The NACE (National Association of Corrosion Engineers) standard for sour service specified that the material hardness should not exceed a maximum of 248 Vickers hardness (HV) or a Rockwell C value greater than 22. This proved to be a difficult standard to meet in practice because of the materials, high-carbon equivalent and the high criteria for tensile strength. The hardness of welded steel of this type depends upon the amount of martensite in the finished weld. The amount of martensite is determined by the chemical composition of the base material, especially carbon, and the welding thermal cycle. To achieve acceptable mechanical properties required extensive trial-and-error work in order to develop welding procedures that would result in consistently high-quality welds.

Welding Equipment

A key factor in the success of HMV's riser construction project was the purchase of three Model 215 pipe welding systems manufactured by Arc Machines, Inc. (AMI), of Pacoima, Calif. The power supply is a full-function microprocessor-controlled unit that provides precision control of weld parameters including amperage (300 A DCEN), arc voltage, travel speed, wire feed speed and torch oscillation. Weld parameters for each pass of a weld for a given size pipe are entered into the power supply memory via a program operator pendant (POP) and stored for future recall. Once acceptable weld parameters have been developed for a particular material and pipe size, they can be repeated over and over from weld to weld to produce an indefinite number of consistent welds. The company purchased two AMI Model 15 weld heads with a C torch that provides for wire feed in either the clockwise or counterclockwise travel direction. The weld head mounts on a guide ring, which is positioned on the pipe adjacent to the weld joint, and the head travels around the weld joint to complete the weld.

An AMI Model 81 weld head was used for the smaller diameter 3-in. pipe. The smaller pipe turned out to be a nonstandard pipe size. Although 2-in. pipe, OD 60.3 mm, was specified, the pipe received was actually 77.8 mm OD, about a 3-in. OD. It was necessary to modify the Model 81 guide ring to accommodate the nonstandard pipe size.

After purchase of the orbital GTAW equipment, HMV sent two of its welding personnel and one welding engineer to AMI's European office in Gland, Switzerland, for one week of operator training. After delivery of the equipment, four more operators were trained in Haugesund by Teamtrade A/S. Presently they have eight qualified welding operators. As is often the case, several of these operators have become particularly adept at programming and making fine modifications to the weld parameters.

Weld Procedure Qualification

Fig. 2 - Interpass temperatures for each of the 23 weld passes for joining API 5L X70 to AISI 4130.

Welds were done on AISI 4130 welded to itself, API 5L X70 (modified) welded to itself, and the two materials welded together. A weld on piping with a 34-mm wall thickness took 77 passes to complete. It took seven to eight hours to finish a weld of this wall thickness and about 15 h to complete a weld on the heaviest wall thickness, which was 42 mm thick. This included the time required to reach the correct interpass temperature between passes. After completing one or two passes, welding was stopped to allow the pipe to reach the predetermined interpass temperature.

Consistent, repeatable orbital welds depend upon consistent and uniform end-preparations. For orbital pipe welding, the pipe end was machined in the workshop to a 25-deg bevel with no radius between the sides of the joint and the land. Because of the demand for a minimum inside diameter, an end preparation with a 2-mm machining of the inside diameter was used. However, the inside diameter was not machined for welding AISI 4130 to AISI 4130 - Fig. 1.

Once initial weld parameters had been established, HMV began to qualify the operators. The qualification of the procedures for the project were according to the DNV (Det Norske Veritas) rules for submarine pipeline systems. Welds were sectioned and submitted to a bend and tensile test. Charpy impact tests at minus 40 degrees C were done several times, always with satisfactory results. If the hardness values were in the gray zone (Rockwell C around or above 22), an accelerated corrosion test was performed to test the welds for resistance to stress corrosion cracking. The corrosion test results met all requirements.

However, weld procedure qualification (WPQ) was not a simple matter. Passing the WPQ required two months of trials before all the specification requirements were met. It started by doing a hardness measurement on a test weld and if the hardness value was too high the welders would make another weld and try again. Only if the hardness test was acceptable would they continue testing by doing a tensile test. If the welds were accepted, the rest of the test program was carried out. The degree of hardness of the finished weld depends upon the amount of martensite in the weld and HAZ. The amount of martensite depends on the chemical composition of the material being welded and the chemical composition of the filler metal, especially the amount of carbon or the carbon equivalent, as well as the preheat and postheat temperatures and cooling rate. The carbon content of the API 5L X70 (modified) was 0.10, while that of the AISI 4130 was 0.33.

A lower preheat temperature resulted in higher hardness values, while a higher preheat produced a softer weldment but with lower tensile strength. By using time-temperature-transformation (TTT) curves, also called isothermal transformation (ITT) curves, for steels of a particular carbon content or carbon equivalent, it is possible to predict and thus control the amount of martensite in the finished weldment and thereby achieve the desired strength and hardness. To obtain a martensitic structure, the cooling rate must be sufficiently rapid to miss the "nose" of the curve and reach the Ms temperature at which the transformation to martensite begins. Increasing the carbon content shifts the nose of the curve to the right. For a maximum Rockwell Hardness of 22, a slower, controlled cooling rate was indicated.

In order to gain control of this process and consistently achieve welds with the desired mechanical properties, precise electronic control of preheat temperature, interpass temperatures, and postweld heat treatment was an essential requirement. This was provided by equipment already in use at HMV, a model KM450XP and programmable electrical elements for preheat and postheat treatment, which were documented with a printout of each weld. Insulating blankets were used to retain the heat and control the rate of cooling. This equipment provided accurate, repeatable control of heat parameters. This control in combination with the precise control of welding parameters was a vital component for the achievement of successful welds on these sensitive materials.

Passing weld qualification was greatly facilitated by orbital welding, as the heat input into the weld affects the temperature and cooling rate of the pipe. Manual welding techniques do not offer this consistent control of weld parameters. Orbital welding equipment allows welding technicians to hold the weld parameters constant while varying the preheat temperatures and/or the interpass temperature or cooling rate. For example, on welds of API 5L X70 to itself, the preheat was held to a minimum of 150 degrees C, the interpass temperature was 200 degrees C for the root pass, and the postweld heat treatment (stress relieving temperature) was 640 degrees C for two hours. After stress relief, the pipe was cooled at a controlled rate of 150 degrees C /h. Different interpass temperatures were used for the various passes as shown in Fig. 2.

Production

All three power supplies were used in production welding (Fig. 3) around the clock, three shifts per day, operated by nine or ten welding technicians. Temperature monitors were kept on the pipes for the duration of the fabrication with racks of chart recorders running constantly to monitor the temperature cycles. The client was interested in maintaining similar records of weld parameters from the welding power supplies, but when amperage and other weld parameters showed no variation from day to day, they decided to discontinue what they felt was unnecessary documentation. Some weld repairs were necessary, particularly at the start of the project, but with a few changes to the geometry of the end preparation the failure rate dropped almost to zero.

Fig. 3 - The orbital welding machines were in constant use during construction of the risers.

Weld quality was generally high with a failure rate of only 1.5% on the AISI 4130. Another advantage of machine welding was that shrinkage was predictable and held to a minimum. The maximum allowable shrinkage was 0.5 mm in a 15-m line, which was easily accomplished. A total of 574 welds were completed on the riser project including 287 on the 3-in. pipe and 287 on the 7-in. pipe. Finished risers were pressure tested to 10,000 Ib/square inch.

Haugesund Mek. Verksted is one of the few companies in the world capable of handling a project such as this. It has the facilities, the technical knowledge and the progressive attitude that drives it to attempt and to be successful with new advanced technology.

The company has also been successful in the orbital welding of alloys such as duplex stainless steels, 6-Mo super austenitic stainless steel, and 316L stainless steel pipe as well as X52.

BY Barbara K. Henon, Ph.D., Arc Machines, Inc.

Acknowledgments

Grateful acknowledgment goes to the following: Arnfinn Gabrielsen, manager of the welding department, HMV; Eivind Veim, European welding engineer, HMV; Einar Wathne, welding technician, HMV; and Jorgen Levesen, manager of the welding division, Teamtrade A/S.