Innovative engineering
solves subsea pipeline tie-in challenge
Custom hot tapping machine plays pivotal role
Michel
Courbat
T.D. Williamson S.A.
T.D. Williamson S.A.
Technip was recently
contracted by Burullus Gas Co. (Burullus) to tie in an expansion to its
existing subsea West Delta Deep Marine (WDDM) facilities. To accomplish this,
it was necessary to tie in a new 36-in. gas trunkline pipeline, which is part
of the Phase VII project, to the existing system under pressure without
shutting down production. To perform the tie-in, Technip retained T.D.
Williamson S.A. (TDW) to carry out three subsea hot tap intervention
operations.
Two traditional
16-in. hot tap operations would be completed on a 26-in. pipeline, and one
innovative 20-in. hot tap on a 36-in. pipeline. To ensure that the hot tap
interventions would be successful, it was necessary to engineer, install and
pre-commission two hot tap assemblies, including one capable of cutting through
a blind weld-neck "tappable flange" made of duplex stainless steel on
the 36-in. line.
Hot tap machine
In preparation for
the operation that would take place in depths to 95 m (311 ft), TDW worked with
a Belgium-based engineering and construction specialist to produce the special
hot tap tool known as a "cutter." This special tool would be used for
the 20-in. hot tap and would need the ability to effectively cut the duplex
plate. Since the duplex has a very high mechanical strength – meaning that it
has a high elongation before reaching breaking point and a high level of
hardenability – the cutting process employed must be very rigid and
vibration-free while using the TDW Model 936D subsea tapping machine.
TDW's customized subsea tapping machine onboard the dive
support vessel Wellservicer.
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Working at TDW's
facility in Nivelles, Belgium, a series of engineering, design and preliminary
tests was performed. The first step involved engineering several alternative
designs. The first alternative consisted of using either a proven cutter
design; or that proven design updated with various teeth geometry. However,
this option was not pursued because it could not penetrate the duplex stainless
steel.
TDW's customized subsea tapping machines onboard the dive
support vessel Wellservicer
prior to the operation.
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A second option
involving removable teeth and welded teeth support was considered, but this was
rejected due to its inability to resist vibration.
Ultimately, the
design selected for fabrication featured a subsea electro-pump to supply
adequate hydraulic power, a pilot drill with bronze plates to reduce vibration,
and a specially manufactured set of cutters with removable cutting teeth that
would be able to penetrate duplex stainless steel without breaking.
Preliminary
trials
After the design was
finalized, materials were procured and the prototype was fabricated and made
ready for the first phase of testing: the internal preliminary trials.
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A diver prepares for the vertical hot tapping operation.
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During a period of
eight weeks, the prototype was subjected to rigorous testing associated with a
number of capabilities. The cutter's ability to make deep cuts on a plate of
the same type of duplex stainless steel as the blind weld-neck "tappable
flange" on the pipeline was an initial challenge.
The team made
material and design improvements, ultimately achieving a prototype that could
produce a smooth and satisfactory cutline. In addition, special bronze guides
were developed and installed on the pilot drill to control vibration.
By the end of the
four-month trial period, several renditions of the prototype had been used to
complete four tapping operations. Before and after each cutting trial, visual
and nondestructive examinations (NDE) of the cutters and pilot drills were
carried out. The final prototype, which featured dual sets of cutting teeth and
the pilot drill with the bronze guides, performed well. As the hot tap machine
would be required to operate at an average pressure of 100 bar, pressure tests
were undertaken to satisfy the requirements of the forthcoming factory
acceptance test. The decision was then made to proceed to the second stage: the
official trials.
Official trials
During the official
trials three tapping operations were carried out with the custom machine. Two
hot taps were completed on duplex plate, and one cold tap was executed through
equal duplex tappable plate. These tapping operations revealed that the
equipment endured the rigorous process, remained properly aligned and cut the
duplex steel plate effectively. These operations took place as part of a
requisite system integration test (SIT), which confirmed the following:
- The teeth accurately cut the duplex stainless steel
- The pilot drill remained rigid and vibration-free
- The design of the cutter was improved by adjusting the teeth support.
It also proved that
the tapping machine could be unset in the middle of the cut and reset while
reaching the cut back without causing the tapping machine to be misaligned or
moved out of proper position.
With the official
trials of the custom hot tap cutter successfully completed, the system received
approvals from Burullus, Technip, and the Burullus Independent Verification
Authority to perform the subsea operation well in advance of the project
mobilization. In preparation for the impending operation, two hot tap machines
were produced in order to guarantee 100% back-up of this critical piece of
equipment.
Maintaining gas
pressure
For three weeks, TDW
worked from Technip's dive support vessel Wellservicer to
carry out all three hot taps. Throughout the process, a prevailing pressure of
100 bar (1,450 psi) was successfully maintained in the existing gas export
system. The innovative hot tap on the duplex tappable flange required just six
days to complete.
In spite of the fact
that the hot tap intervention was carried out subsea, making it more
complicated to mobilize and install equipment than when working onshore, the
operation was carried out by skilled divers as intended, with no lost time
incidents or production downtime.
Much of the success
was attributed to the investment in planning and pre-operational equipment
trials and testing. TDW worked with the Technip and Burullus teams to ensure
that the operation would proceed like clock-work, and that the customized
cutting tool would operate effectively on duplex stainless steel. As a result,
the operation provided three tie in points, preparing the way for Technip to
successfully tie-in the new 36-in. gas trunkline for the WDDM Phase VII
development.
The author
Michel Courbat is
offshore project manager for T.D. Williamson S.A.
11/01/2011
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