Introduction
 In the summer
of 1994, operators Enserch Exploration partners Ltd completed the
installation of sub sea production equipment, pipelines and jacket for
the Garden Banks GB388 Gulf of Mexico Deepwater project on schedule and
below budget. Block 388 is 250 miles south of New Orleans with proven
reserves of 27 million bbl oil in water depths ranging from 2,100 to
2,400 feet. This precluded conventional development methods. A floating
production facility was selected because such a development approach
would allow initial production in the shortest time possible. (Ref 1) The
export and gathering pipelines were designed by Brown and Root and
installed by McDermott. The various phases of this part of the project
will be discussed in the following sections. (back to top)
Pipeline Design and Material Specifications
The oil and gas gathering pipelines are each 54.4 miles long, 12 inch
nominal diameter, API-5L-X60 pipe, with wall thickness staged in four
steps from 0.5 inches at the shallow water end to 0.688 inches in the
deep end. The additional wall thickness in deep water is required to
resist propagation and buckling. Both pipelines are coated with fusion
bonded epoxy (FBE), and the gas pipeline has an additional 1 inch of 140
pcf concrete coating in water depths less than 450 feet. The concrete is
required for stability against waves and currents during the 100 year
hurricane. All pipeline components are rated for 2220 psi operating
pressure (ANSI Class 900 # system). These pipelines also feature a diver
less pipeline jumper connection system at the sub sea template site.
(Refs 2,3 ).
The oil and gas sales pipelines are 2 and 0.3 miles long, respectively,
conforming to API 5LX60 pipe specifications. The oil line is 8-inch
nominal diameter and 0.438 inches wall thickness, and the gas line is 12
inch nominal diameter and 0.5 inches wall thickness. Both pipelines are
coated with FBE and the gas line has 1-inch of 140 pcf concrete coating,
for stability against waves and currents during the 100 year hurricane.
All pipeline components are rated for 2220 psi, operating pressure (ANSI
CLASS 900# systems). (back
to top)
Pipe Manufacturing
Bids were solicited from pipe manufacturers capable of producing
seamless or Electric Resistance Welded (EPW) API 5L X60 line pipe to
project specifications. Seamless pipe was selected for wall thickness
greater than one-half inch because of commercial reasons and lack of
experience with ERW pipe in the deepest water. The ERW pipe was used to a
maximum water depth of 1250 feet.
Several operators have recently utilized ERW pipe for a number of
offshore applications and there has been some revision of the traditional
attitude which was to specify seamless pipe for smaller diameters and
longitudinal submerged arc welded pipe for the larger sizes. Due to the
deepwater nature of this project notched tensile specimens were taken and
tested during each lot test as an additional measure to check ERW weld
fusion line properties. This "notched bar tensile test" was developed by
the American Gas Association and allows acceptable fusion line ductility
to be specified. (Ref) The results were very satisfactory as the notched
tensile to unnotched tensile ratios were greater than one.
The decision to select ERW pipe is usually made on the basis of
economics as it is usually the cheaper alternative to seamless and in
most instances is cheaper than longitudinal pipe. This is not however,
always the case. Other factors which should be weighed in the decision
are confidence in the ERW mill producing the pipe as quality aspects can
vary enormously from mill to mill, and the service to which the ERW pipe
would be put. There is still some conservatism in the industry about its
use in either sour or high pressure gas service.
The best ERW mills are undoubtedly those operating High Frequency
Induction (HFI) welding (above 100 KHz) which are able to weld up to
around 15 or 16 mm wall thickness with diameters of 20" or more; the
largest being 26" and average 16 or 18". The use of BFI alleviates
concern about possible defects arising in the weld line as a result of
variable electrical contact or inadequate forging upset, which was one
reason ERW was not more widely adopted years ago. In addition most ERW
mills perform a full body or weld line normalizing treatment at around
950 deg C which greatly enhances weld properties. Other major factors are
the quality of the strip, its chemistry and metallurgical condition and
the preparation of abutting edges with machined edges being preferred;
Lower quality versions are sheared or slit. (back to top)
Fitness for Purpose
Two Fitness for Purpose studies were carried out on project line pipe in
two distinct phases of testing. The first phase involved Strain Age
testing on the Seamless pipe to establish the effect of out of
specification Aluminum/Nitrogen ratios of less than two. The second phase
involved the development of alternative defect acceptance criteria for
girth welds in accordance with the project specification.
(back to top)
Strain Age Testing
During Inspection at the Seamless MU it was noticed that certain heats
of pipe were not in accordance with the project specification with regard
to a minimum Aluminum/Nitrogen ratio of two. In order to accept a
specification deviation it was decided to carry out charpy testing on
strain aged samples to ensure that the specification could still be met
as the low ratio has the possibility to produce some strain age
embrittlement during welding.
In order to study the effects of Aluminum/Nitrogen levels less than two
a test matrix was developed for various strain levels and as welded
conditions. The strain levels selected were 0.5 %,I.5 % and 3.0% strain.
Typical girth welds were prepared according to the Lay Contractors
welding procedures using both SMAW and FCAW and samples extracted for
strain aging at 250 deg F. Subsequent to Strain Aging charpy impact
specimens were prepared from the Fusion Line (FL), FL+ 2mm and FL +5mm.
and tested at 32 deg F. Pipe was selected from the heat showing the
lowest Aluminum/Nitrogen level. As can be seen from figure I all the
energy levels for FL+2mm and FL+5mm are extremely high at around 200 Ft
lbs average. The fusion line specimens are lower but still meet the
minimum spec. requirements of 35 ft lbs minimum average. It is interested
to note that energy levels increase from the as welded condition to the
1.5% strain level and drop back down at the 3% strain level for both weld
processes.
The results of the test program indicated that the pipe was Fit For
Purpose and met the requirements of the project specification.
(back to top)
Development Of Alternative Defect Acceptance
Criteria
The inspection of girth welds on the project were in accordance with API
1104 but Enserch required the option of developing Alternative Defect
Acceptance Criteria. This was achieved by using the Engineering Critical
Assessment (ECA) approach in Appendix A of API 1104 and British Standard
PD 6493. The former approach developed Fitness-for-purpose defect
acceptance criteria for girth welds during pipeline installation and the
latter established critical defect sizes for each pipeline to determine
their acceptability should they sustain any damage during service.
The application of the alternative defect acceptance criteria was not
intended to give the contractor any latitude on providing quality girth
welds, but reflected a concern over the risk of performing weld repairs
on a pipeline subjected to high installation stresses. Additionally,
there is considerable quality assurance to be gained by the use of more
stringent inspection, and the extensive weld procedure testing associated
with the application of the ECA approach. However the ECA acceptance
criteria is not used when any of the following apply:
-
Misalignment exceeds 10% of the wall thickness
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There are cracks present
-
Undercut exceeds 10% of pipe wall thickness and/or surface flaw
defect length. Overlapping defects Repair welds.
Girth weld samples were prepared as part of weld procedure qualifications
using both SMAW and FCAW processes for both the ERW (0.5 inch wall) and
Seamless (0.688 inch wall) pipe.
Crack Tip Opening Displacement (CTOD) specimens were prepared in
accordance with BS 5762 and API 1104 in the weld metal and heat affected
zone giving a total of 24 specimens(8 sets). The results ranged from
0.006 ins to 0.031 ins. The test data was reviewed to see if the minimum
values for both weld metal and/or heat affected zone meet or exceed
either the 0.010 or 0.005 inch CTOD values required for the application
of API 1104 Appendix A. The ERW pipe did not provide any alternative
acceptance criteria or relaxation in excess of API 1104 which allows a
defect 2 inches long. The Seamless pipe however provides for allowable
flaw sizes in excess of API 1104 of 5.1 inches long for flaw
heights/depths of approx. 1/8 inch and 2.75 inches long for a defect
depth or height of 0.2 inches.
The test data was reviewed to establish the minimum CTOD value for both
the Seamless and ERW pipe to be used in the calculation of critical
defect sizes by using PD 6493. These values will be utilized after
pipeline installation should any defects be detected.
The results indicated that for the ERW pipe a defect height of 0.275
inch for a complete circumferential surface defect and an 8.7 inch long
through thickness defect was allowable. For the seamless pipe a 0.38 inch
defect height for a completely circumferential surface defect and a 13
inch long through thickness defect was acceptable.
(back to top)
Non Destructive Testing
As the ERW pipe was to be laid in deepwater outside the limits of
previous experience to date, additional precautions were taken during
inspection to provide the highest confidence that premature weld failure
could not occur during the lay operation.
Initially, the pipe was NDT inspected at the mill by the manufacturer
and outside the mill by an independent third party NDT company, Shaw
Pipeline services, Edmonton, Canada (Shaw).The mill inspection included:
(1) Inline UT inspection of ID/OD scarved weld areas, (2) ENH of OD/ID
for surface indications greater than 12.5% and (3) Full body Ultrasonic
testing using an array of 24 45 degree probes. Shaw was contracted to
perform prove-up inspection on the . weld fine utilizing Ultrasonic
equipment with 70 degree probes.
Conventional UT probes (45 degrees) used for inspection of:, ERW weld
seams are susceptible to missing unacceptable weld line defects and
rejecting pipe containing acceptable indications. Research work carried
out by Shaw demonstrated that a substantial amount of experience has been
accumulated 'm ERW weld line inspection using many different transducer
angles and configurations. The selection of the transducer angles used
for this project is based upon that experience.
Generally, ERW defects can be classified as those defects in the fusion
line and those defects contained in the steel or skelp. The type of
fusion line defects include: lack of fusion, weld cracks and penetrators.
These defects are typically positioned vertical in an ERW weld and can be
prominent to the OD,ID, midwall or through wall. Steel defects common to
ERW pipe are often referred to as hook cracks or upturned fibers. They
occur inside the fusion line and are the results of laminar imperfections
in the skelp interacting with the flow line from the ERW process.
As previously discussed, the most detrimental defects tend to be
vertically orientated in the fusion line. For this reason, the highest
angle probes possible were selected for detection of these type of
defects. The transducer selection must also take into account the effects
of pipe diameter and wall thickness.
The most detrimental defects tend to be vertically orientated in the
fusion line. For this reason the highest angle Transducers practical were
selected for detection of fusion fine type of defects. Transducer
selection must also take into account the effects of pipe diameter and
wall thickness. The use of focused transducers aids in controlling the
beam profile. This allows the NDT operator the ability to concentrate on
specific areas of the weld profile., In addition these focused
transducers provide superior signal to noise levels over the non focused
versions.
Steel related defects exhibit two reflective surfaces. One reflective
surface is that whose axis is vertical or near vertical depending upon
the amount- of work used in forming the pipe. the other reflective
surface runs parallel to the pipe surface. The geometry of these defects
provides a natural focus or comer effect at the upturn portion of the
imperfection. High angle probes are very suitable to identifying this
type of defect because of the natural focusing effect and the vertical
orientation of the imperfection.
If a 45 degree transducer reference level is calibrated from a N5 notch
or a 1/8 in hole the signal response from the midwall defect would be
non-existent as the returning sound energy would pass behind the
transducer. (See figure 2). A 70 degree transducer does not suffer the
same @s-orientation problems and the response from a mid wall indication
return signal amplitude is very comparable to OD and or ID lack of
fusion.
The consideration to use a 70 deg. transducer for weld line inspection
is based on the API requirement to provide continuous inspection through
the weld line on a 1/16" band on either side A 45 degree transducer can
not accomplish this. A 45 degree transducer can only inspect the OD and
ID while the orientation of the defect must be essentially vertical to
reveal an unacceptable defect. (back to top)
Coating/Cathodic Protection
The pipelines were coated with 15 mils of fusion bonded epoxy (FBE).
Thin film FBE coatings have been successfully used over the last two
decades to protect offshore pipelines which have operating temperatures
up to 230 deg F. Some of the advantages of FBE are that it has excellent
adhesion, good impact resistance, good abrasion resistance, excellent
flexibility, good resistance to cathodic disbandment, excellent
resistance to soil stresses, good resistance to moisture penetration,
good chemical resistance, and is easy to repair. For field joints with
concrete coating Raychem WPC/13 shrink sleeves were used. The void was
filled with polyurethane foam.
Because of slippage problems associated with laying pipe in deepwater it
was decided to develop a FBE "rough" coat on top of the 15 mils film. The
pipe coater in New Iberia, Louisiana developed a method by which they
sprayed dry FBE powder on the 15 mil uncured surface as soon as it exited
the electrostatic spray booth. The Nap-Gard Non-slip coarse coating
performed extremely well during installation with no slippage
experienced.
Problems were experienced during the coating operation because of the
inability to prepare the ERW pipe surface to a satisfactory anchor
profile with a conventional grit blast. As a consequence coated pipe was
failing the cathodic disbandment test. This was attributed to a
decarburized layer which was produced during the accelerated cooling
phase of pipe manufacture. The problem was overcome by using alarger
harder grit in the blasting operation. (back to top)
Cathodic Protection
Cathodic Protection for the pipelines was achieved with Aluminum
Zinc-Indium anodes (Galvalum III). Anode bracelets were attached to a
section of pipe every 520 feet. (back to top)
Installation
The pipelines were layed during May through August 1994 by the McDermott
Lay barge DB 28.
-
Figure A shows the welding of the root pass in the bead stall with
E6010 cellulosic electrodes (E8010-G was used for the hot pass)
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Figure B shows a fill pass station before the first tensioner using
Lincoln Inner shield NR207H.
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Figure C shows the field joint station before entering the Stinger
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Figure D shows a close up of the field joint operation
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Figure E shows the completed -field joint with, polyurethane, foam.
(back to top)
Conclusion
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ERW pipe was layed satisfactorily in water depths of 1250 feet
-
Fitness For Purpose tests and additional NDT gave greater assurance
that the line pipe was Fit For Service while laying in deepwater.
-
Ultrasonic test procedures should be utilized which consider
transducers capable of detecting midwall defects in ERW pipe.
-
A nonslip FBE coating method was developed which proved successful
during installation in avoiding any pipeline slippage in the
tensioners.
-
The surface of pipe produced with accelerated cooling should be
randomly checked to detect any evidence of de-carburization which
could affect the coating operation. (back to top)
References
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Robert D. Pickard, "Gulf of Mexico deepwater project moving toward
1995 start-up", Oil and Gas Journal, June 1994.
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M.M. Beckmann, J.R. Hale, S.W.Davis and C.N.Prescott, Diveriess
Jumper Connection System for a 12-in. Pipeline, OTC conference,
Houston 1994.
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J.R.Hale, S.W.Davis and C.N.Prescott, Garden Banks 388 Pipeline
Jumper Testing and Installation, OTC conference, Houston 1995.
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D.N.Williams and R.J.Eiber, "Notched-Bar-Tensile-Test evaluation of
the ductility of ERW line pipe, AGA technical Meeting on Line Pipe
Research, San Francisco, 1983 (back to top)
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