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.
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