Total Ross hot-tap uses T-joint, heat pre-treatment

Stolt Comex Seaway loads the over-trawlable protection structure and cap for the hot-tap 6-in. of the Ross Field gas pipeline into the Frigg Line 1 pipeline. [12,194 bytes]


Retrofitting North Sea pipelines

Stolt Comex Seaway uses a welded "T" joint to hot-tap a pipeline.

The Ross field, located in the central North Sea and operated by Talisman Energy (UK) Ltd, is an oil reservoir with a significant volume of associated gas. The field was brought into production by means of a subsea production system and an FPSO which will offload oil to a shuttle tanker. Gas is exported via a new 6-in. pipeline which runs 25 km from the Ross field to the 32-in. Frigg Line 1 gas trunkline, operated by Total Oil Marine, and on to St Fergus in North East Scotland.

A critical part of this development was achieving the connection between the gas export line and the Frigg trunkline. A conventional welded connection to the trunkline would have required the trunkline to be shut down and decommissioned for the duration of the connection process.

This would have posed an unacceptable cost. So, a welded hot-tap, a welded connection made while the pipeline continued to flow gas at full pressure (100 bar), was selected as the most cost-effective solution. Stolt Comex Seaway was selected to undertake the work.

Previous experience

The only previous hot-tap in the North Sea was undertaken by Stolt Comex Seaway in The Netherlands for Wintershall and NAM. The technology of hot-tapping itself is not new, having been used many times in the Gulf of Mexico. However, the technique of establishing a welded "T" joint and then hot-tapping through this into the pipeline uses technology developed and patented by Stolt Comex Seaway.

In order to make a high-quality welded connection onto a pipeline which is flowing high pressure gas, the cooling effect of the gas flowing through the pipe must be couteracted. SCS has developed a technique of using induction heating to maintain a local pipeline wall temperature of 100°C while the weld is being made, which enables hydrogen to be released from the weld.

The hot-tapping operation is performed in the following manner:

  • A branch pipe is welded onto the pipeline.
  • The branch pipe is then reinforced by welding in place a split sleeve section.
  • A three-way Tee valve assembly is welded onto the Tee on the split sleeve.
  • The hot-tapping is performed through the valve assembly, which maintains pipeline integrity, maintaining pressure and flow.
  • Once this is successfully completed, additional connecting pipework and valving can be put in place.
  • The tie-in of the new line is completed.

Commence work

The contract for the work was awarded to SCS in late 1997. Design engineering was completed exactly four weeks later. Procurement, fabrication, and onshore trials were finished in time to take the project offshore on May 20.

The Ross field hot-tap and tie-in operation was carried out in three phases under the control of Total, as owners of the trunk line.

The first phase was to locate the position on the Frigg Line 1 and to check the condition and ovality of the pipe to ensure that the condition of the pipe was within tolerance. Following the satisfactory completion of this operation, the exposed section of the pipe was then wrapped in a concrete mat to provide temporary protection until the hot-tap operation took place.

Full scale onshore trials were then set up for the following purposes:

  • Familiarizing all project personnel with the detail of the operation
  • Undertake the diver/welder qualifications at the National Hyperbaric Centre
  • Verify the finite element analysis of the split sleeve and branch structure that was to be welded to the pipeline
  • Perform a full scale simulation of the welding, hot tapping, and fit up operation.
Welding the branch pipe and split sleeve to the pipeline is only made possible by the use of induction heating of the pipeline, a proprietary process developed by SCS, to overcome the massive heat loss that would otherwise be caused by the continual flow of high pressure gas in the pipeline.

Dry habitat

A subsea welding dry habitat is used to undertake the welding of the Tee and valve assembly. The habitat was then recovered and the hot-tapping rig installed over the pipe. The cutting machinery is introduced to the pipeline through a 12-in. ball valve mounted on top of the welded Tee assembly. Working with a pipeline pressure of 100 bar and a flow rate of 4 meters/sec, the hot-tap machine had to be rated for an internal pressure of 125 bar when cutting and 150 bar when static.

The hot-tapping machine introduces the circular cutter to the pipe by means of a pilot drill which, once the circular cut is completed, holds on to the coupon by means of wire retainers and a "posilatch" system. This is then withdrawn, the ball valve closed, and the cutting machine removed.

Two pipe clamps were then installed to support the additional six inch pipework and valves before an over-trawlable protection structure was positioned over the tie-in point and secured in place by piles.

The ability to perform hot-tap connections in live pipelines may be a critical factor in the economics of bringing a new field on stream. The costs saved by being able to make a high quality welded connection without having to shut down and evacuate the pipeline are considerable and with every hot-tapping project that is successfully completed confidence in the technique is increased.

SCS moved on from the Total hot-tap to tie in the 16-in. gas line from Esso Norge Jotun FPSO into the 36-in. Statpipe trunkline south of Heimdal in the Norwegian sector of the North Sea.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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