Welding Requirements of a
Steel Catenary Riser (SCR) by:
Gordon A. Aaker, P.E.
Design Consideration
The design of a SCR will
include non-fatigue sensitive regions and fatigue sensitive regions.
Non-Fatigue Region - The non-fatigue sensitive regions of the SCR will
be welded and inspected according to ANSI B31.3 for normal service condition.
Fatigue Region - The fatigue sensitive regions of the SCR will be fabricated
and inspected according to ANSI B31.3 for serve cyclic service and following requirements.
Pipe Manufacturing Considerations
The objective
in the fabrication of the SCR is to develop a welding plan, which optimizes SCR
fatigue life and minimizes the difficulties of field fabrication and NDE inspection
of the SCR welds. The primary concern in achieving these objectives is the quality
of the pipe that is received for fabrication. An important factor in fatigue life
is internal misalignment. · When line pipe is furnished according to the
minimum requirements of API 5x, the ovality of the pipe ends and/or the variation
in pipe wall thickness are problems. This problem can be minimized by negotiating
tighter tolerances with the pipe manufacturer to achieve higher quality pipe.
· Measures the thickness ovality of the pipe ends. This data can be evaluated
to determine the best combination of pipe fit-ups to achieve minimum internal
offset. · Machining the pipe ends prior to fit-up. Experience indicates
that: (1) every effort should be made to receive the best quality of pipe that
is possible; (2) attempt to minimize the internal misalignment by measuring the
pipe ends for best combination of fit-ups and, (3) machine the inside diameter
to achieve a maximum of 1/32" of mismatch at any location around the ID circumference.
Welding Considerations
The design bases for the flow line portion of the riser are ANSI B31.4 and ANSI
B31.8. The design bases for the casing is ANSI B31.8. The design codes specify
that the minimum requirements for welding are to be performed according to API
1104. Welding will be performed according to the requirements of API 1104, latest
edition for Procedure Qualification Record and Welder Performance Qualification
except that the defect tolerance limits for production welding shall meet the
minimum requirements of ANSI B31.3 for severe cyclic conditions (see ANSI B31.3,
Table 341.3.2A). This requirement shall apply to the flow line and the casing.
The nondestructive examination (NDE) that is performed on production welding will
include 100% Radiography (RT) and 100% Ultrasonic (UT) on each weld.
Welding will be performed using a GTAW root and hot pass. The remaining portion
of the weld shall be performed with the SMAW process. For our purposes the GTAW
root pass and hot pass shall be defined as "root area" and shall be taken to be
a total of 0.25" thick divided equally between the two with 0.125" thickness for
the root pass and 0.125" thick for the hot pass. Any weld repair that requires
metal removal to a depth within 0.25" of the pipe ID surface shall be considered
to be a "root area" defect and shall result in a weld cut-out rather than a localized
repair. An intermediate RT may be considered after welding the "root area" and
one or two fill passes to ensure the "root area" is free of defects before continuing
with the welding operations. If any root area defects are found at this stage
of welding, the weld shall be cutout just as would happen if roots were defects
are found during final RT.
Weld Reinforcement
(O.D. Weld Caps)
The weld cap shall be ground smooth if fatigue analysis indicate it is necessary.
When required, the weld cap should be ground down or machined to 1/32" maximum
above pipe OD surface. A flapper wheel sander can be used to smooth out the weld
cap and blend the weld toe smoothly into the parent pipe and weld cap. Final sanding
marks shall be oriented parallel to the longitudinal axis of the pipe. The cap
shall be visually inspected for nicks, steps or other sudden changes in the weld
cap. Magnetic particle (MT) shall be performed. Radiography (RT) All welds shall
be radio graphed using standard techniques and a sensitive fine grain film (e.g.,
AgfaD2). Acceptance/rejection shall be based on ANSI B31.3 for severe cyclic service.
Ultrasonic (UT) All welds shall be ultrasonically examined. Acceptance/rejection
shall be based on ANSI B31.3 for severe cyclic service. Ultrasonic examination
may be performed either manual or automatic.
The objective
in the fabrication of the SCR is to develop a welding plan, which optimizes SCR
fatigue life and minimizes the difficulties of field fabrication and NDE inspection
of the SCR welds. The primary concern in achieving these objectives is the quality
of the pipe that is received for fabrication. An important factor in fatigue life
is internal misalignment. · When line pipe is furnished according to the
minimum requirements of API 5x, the ovality of the pipe ends and/or the variation
in pipe wall thickness are problems. This problem can be minimized by negotiating
tighter tolerances with the pipe manufacturer to achieve higher quality pipe.
· Measures the thickness ovality of the pipe ends. This data can be evaluated
to determine the best combination of pipe fit-ups to achieve minimum internal
offset. · Machining the pipe ends prior to fit-up. Experience indicates
that: (1) every effort should be made to receive the best quality of pipe that
is possible; (2) attempt to minimize the internal misalignment by measuring the
pipe ends for best combination of fit-ups and, (3) machine the inside diameter
to achieve a maximum of 1/32" of mismatch at any location around the ID circumference.
The design bases for the flow line portion of the riser are ANSI B31.4 and ANSI
B31.8. The design bases for the casing is ANSI B31.8. The design codes specify
that the minimum requirements for welding are to be performed according to API
1104. Welding will be performed according to the requirements of API 1104, latest
edition for Procedure Qualification Record and Welder Performance Qualification
except that the defect tolerance limits for production welding shall meet the
minimum requirements of ANSI B31.3 for severe cyclic conditions (see ANSI B31.3,
Table 341.3.2A). This requirement shall apply to the flow line and the casing.
The nondestructive examination (NDE) that is performed on production welding will
include 100% Radiography (RT) and 100% Ultrasonic (UT) on each weld. 
The weld cap shall be ground smooth if fatigue analysis indicate it is necessary.
When required, the weld cap should be ground down or machined to 1/32" maximum
above pipe OD surface. A flapper wheel sander can be used to smooth out the weld
cap and blend the weld toe smoothly into the parent pipe and weld cap. Final sanding
marks shall be oriented parallel to the longitudinal axis of the pipe. The cap
shall be visually inspected for nicks, steps or other sudden changes in the weld
cap. Magnetic particle (MT) shall be performed. Radiography (RT) All welds shall
be radio graphed using standard techniques and a sensitive fine grain film (e.g.,
AgfaD2). Acceptance/rejection shall be based on ANSI B31.3 for severe cyclic service.
Ultrasonic (UT) All welds shall be ultrasonically examined. Acceptance/rejection
shall be based on ANSI B31.3 for severe cyclic service. Ultrasonic examination
may be performed either manual or automatic.