Dual bypass, mechanical plugging move into large diameter pipelines

The layout on the ocean floor shows the mechanical plugging tee and bypass system. [26,319 bytes] A method of mechanically clamping a pipeline in order to install a tee without stopping production flow has been used successfully on a very large pipeline in the US Gulf of Mexico. The method avoids welding operations on the seafloor.

Pipeline production continues while tees installed

A method of mechanically clamping a pipeline in order to install a tee without stopping production flow has been used successfully on a very large pipeline in the US Gulf of Mexico. The method avoids welding operations on the seafloor.

The subsea mechanical plugging tee manufactured by T.D. Williamson (TDW) was recently deployed by TDW Services working with Ceanic (American Oilfield Divers). Two 16-in. tees were used in connection with tying-in a 16-in. pipeline with a 24-in. line at a depth of 150 ft.

The plugging tee was developed two years ago in conjunction with Big Inch Marine Systems (BIMS) of Houston, pioneer of the mechanical clamping mechanism that eliminated the need for welding operations on the ocean floor. TDW modified the neck of a BIMS tapping tee and installed a Lock-O-Ring flange. This permitted insertion of Stopple(R) plugging heads into the line to stop flow, and installation of a completion plug after work was finished so that the tapping valve could be removed.

Project planning

This type of project is preceded by weeks and months of planning. Of major concern during the planning is whether or not the pipeline was to be shut down during the work. Costs and risks of keeping it in service were balanced against those and other factors involved in shutting in the line. On this job, the customer elected to keep both lines in service by using a bypass.

This was the first time the plugging system had been used on a line this large. It was also a first for the mechanical plugging tee. Divers working on the bottom installed the fittings and tapping and plugging equipment, then operated the machines while in continual voice communication with TDW technicians on the diving barge. Since the equipment is operated in near zero visibility, extensive training operations are conducted before going to sea.

The job at hand called for joining the 16-in. crude oil transmission line with a 24-in. line where the two crossed. The 16-in. line would then be cut, remaining oil removed and the line filled with seawater and abandoned. The 16-in. tie-in would be done in two sections. After the first section was tied in to the 24-in. line, a 6-in. bypass was installed, connecting the old 16-in. line with the new section, keeping both the 16-in. line and 24-in. line in service while the remainder of the work was being done.

Jobsite layout

Fourteen Ceanic divers were used on the job. Divers could remain on the bottom about one hour, using standard industrial hard hat diving equipment. Because of the large amount of diving time required, divers worked two at a time from two different stations on the barge.

A TDW Model 1200 tapping machine, configured for underwater use, was used to make the three 16-in. taps required to tie the two lines together. The first was on the 24-in. line through a 24 by 16-in. BIMS Mechanical Tap Tee, necessary for tie-in of the 16-in. line. The others were through the two mechanical plugging tees installed on the 16-in. line. Stopple plugging machines isolated the section between, which was purged, cut and blanked off. A 6-in. side outlet on the upstream (eastern) plugging tee provided connections for a temporary bypass line, which kept the 16-in. line flowing while all connections were being completed.

In order to minimize work on the ocean floor, as much assembly work as possible was done on the barge deck. When tapping the 24-in. line, The tapping machine with cutter and pilot drill installed was assembled to a 16-in. ball valve, then both to the 24-in. mechanical tap tee.


This combination was pressure tested, and with the tap tee clamps in the open position, lowered to the ocean floor, placing the clamps around the 24-in. line in the horizontal position. The tapping tee clamps were placed around the pipe then closed and tightened.

Not an easy task, it took more than six hours (24 hours of clock time because of weather factors and diver rotation) to tighten and tension the body bolts, tighten the collet flanges, and set the seals. Sandbags were placed under the valve and end of the tapping machine to minimize stress on the pipeline.

On completion of pressure testing, the submersible power unit was lowered, the hydraulic hoses were connected and the tap commenced. Four hours later, the tap was complete and the valve closed. Before recovery, the tapping machine was purged of oil. Two hoses were sent down and connected to the tapping machine adapter, which housed the cutter. Water was pumped through one and the oil purged to an on-deck container through the other. The tapping machine, with coupon, was then recovered.

During the next few days, divers installed the first section of 16-in. line to the outlet on the valve at the 16-in. tap tee. This section contained a pre- installed 6-in. in-line fitting and valve for later connection of the 6-in. bypass line.


The next phase involved installation of, and tapping through, the mechanical plugging tees. These were installed in the same manner, except they were rotated to the vertical position before tightening the clamping mechanisms. On deck, they were assembled to TDW Sandwich valves and the tapping machine.

The tapping machine-plugging tee combination was lowered to the bottom in the horizontal position and the open clamp placed over the pipe. After the clamp was bolted up loosely, a series of lift bags were attached to the top of the tapping machine. The bags were inflated slowly, giving the end of the tapping machine positive buoyancy and rotating the fitting, valve, and tapping machine to the vertical position.

A vertical tap was selected to eliminate potential problems the effects of gravity could have on a horizontal plugging operation. In the meantime, divers continued to work on installation of the 6-in. bypass, connecting it to the side outlet on the plugging tee and making it ready for operation.

The tap was completed three hours later, cycling through three divers. Completion of this tap placed the bypass in service. Partial Bow now went into the plugging fitting, into the 6-in. bypass, through the newly installed 16-in. line and into the 24-in. line. The second plugging tee was installed and the line tapped without incident.

Bypass in service

When the plugging machines were ready and tested, each was lowered into the water and installed on its respective Sandwich valve in a vertical position. It was critical that the machines be installed with the plugging head facing the section of pipe to be isolated. If one bolt hole was out of alignment, the plugging head would not have functioned properly. To make sure they were aligned correctly, a weld bead was placed on the flanges of both the plugging head housing and valve. The diver could then align the two beads by touch as the plugging machine was being installed.

Two bolt-on 4-in. fittings and valves were prepared for installation on the 16-in. line to be isolated and cut. These would be tapped through and used for purging the oil from the line before it was cut. Like the other taps, the first 4-in. fitting was bolted up to the valve and the tapping machine on deck. After the first tap was completed, the valve was closed and the tapping machine recovered. It was then installed on the remaining fitting and valve, lowered, installed and the tap made.

The plugging heads were lowered into the line, isolating the section to be removed and placing all flow through the temporary bypass. Once a seal was confirmed, hoses were connected to the 4-in. fittings and oil was purged from the isolated section. The line was then cut and the two pipe ends blanked.

While the remainder of the new 16-in. line was being lowered into place and connected, pressure was equalized on the downstream plugging head and it was retracted into the housing.

After closing the valve, oil was purged from the plugging head housing and it was recovered. The tapping machine was used to install a plug in the plugging tee flange. The tapping machine and valve were recovered, and a blind flange installed, completing work at the downstream location.

After the new 16-in. line was completed, the upstream plugging head was retracted, placing flow through the new line. The valve was closed and the Stopple machine was recovered. A plug was installed in the plugging tee flange, the valve removed, and a blind flange installed. The 6-in. temporary bypass line was shut down, purged and removed, leaving the job complete.

Thirty-six days after starting the job, the jack-up barge and all personnel were gone. The tie-in and abandonment had been completed with no environmental impact, no down time, and no lost production time.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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