Cold Tapping To Prolong Pipe Life
Authors: Quin Rene 
Published: December 2007

Originally designed as a repair tool, the "cold-tapping" system provides a technique to avoid flooding of a subsea oil or gas line while a section of such a line is changed. Our aim now is to make the system, together with a newly designed connecting tee and branchbarrier valve, the development tool of the near future. Fourteen years of experience in operating large-diameter trunklines in the North Sea have shown that such pipelines will, from a technical standpoint, outlast their original design life, as well as the life of the field for which they were constructed. The point will be reached where marginal fields will have to be connected at minimum cost to existing trunklines. Such connections will have to be performed within the shortest possible shut-down time. The ideal solution will continue to be the hot-tapping technique under hyperbaric conditions, but its offshore application is highly controversial because of technical and safety reasons. Cold tapping, together with a fully piggable, subsea-repairable, connecting tee and branch barrier based on the widely known blowout-preventer technique, should be used in the years to come because it is a technically safe and reliable tool that can bring marginal fields on stream under acceptable economic conditions and simultaneously prolong the life span of existing trunklines.

Introduction

To use the frill capacity of the transportation network, a pipeline operator must have the capability to replace a line section for three reasons: to change the line configuration to insert a slack loop or the connecting tee of a new line; to repair a section damaged by an anchor, corrosion, or internal jamming of a pig; and to avoid pollution problems. problems. The existing technology for repair or modification of subsea gas lines necessitates a production shutdown of about 8 weeks i, e., "killing" all pipeline operations, including depressurization, flooding, changing of line configuration, repair, hydrostatic testing, drainage, drying, and repressurization. For a pipeline operator, the ideal solution obviously is to perform any line-configuration change or repair without interrupting production.

This kind of operation, called "hot tapping" or "plugging," has been practiced onshore for the past 30 years. its offshore application, however, is highly controversial because ofthe danger and difficulty of pre- and postheating to achieve the hardness value required by authorities,difficulty in meeting the welding specifications required for this type of connection,the danger of welding near production lines, andthe danger of weak points in the lines, which are returned to the ocean floor.

Thus, the problem to be solved is how to reduce pipeline shut-down time to the minimum while guaranteeing maximum operating safety, The answer is to work on a depressurized line with plugs to isolate the section to be replaced-i-e-, cold tapping.

General Description

Cold tapping allows work to be performed on a section of pipeline without flooding of the line and subsequent dewatering and drying. The operation is performed on a pipeline where the internal pressure has been pipeline where the internal pressure has been equalized to slightly above the hydrostatic pressure of the seawater. A tap is then pressure of the seawater. A tap is then performed on either end of the pipeline section performed on either end of the pipeline section to be modified, and plugs are inserted into the pipeline and inflated. The section of line to be modified is then removed, and the new section of line is installed by conventional hyperbaric welding. Once complete, the line is repressurized, and the plugs are removed by pigging.

The technique is simple in principle, allows accurate positioning of the plugs, and offers maximum reliability because of efficient monitoring and a wide range of backup procedures. Keeping the line entirely free of extra pieces of equipment after the operation is also a major advantage.

The cold-tapping operation is performed in two steps: pipe boring, which is done by the tapping system, and plug installation, which is done by the launching box. When in use, both pieces of equipment are connected to the saddle, which is clamped around the pipe. The sandwich valve, which is part of the tapping equipment, is permanently bolted to the saddle, allowing the pipe permanently bolted to the saddle, allowing the pipe to be isolated from seawater during the changeover from tapping machine to launching box. The saddle and sandwich valve are fixed to the supporting structure, and the launching box or tapping unit is fixed to the handling structure (Fig. 1). The supporting structure and the handling structure constitute the equipment frames. The plug is installed by the launching box, and a special connector allows its disconnection from the launching box cylinder after it is checked for pressure loss. The cold-tapping equipment is operated by the divers with the hydraulic system and is controlled from the surface by an electric/pneumatic control system.

Equipment

Figs. 2 and 3 show the major items of equipment that make up the cold-tapping system.

The tapping system comprises the tapping machine, the adapter, the cutter, and the sandwich valve.

The launching box is used to install an inflatable plug in a hydrocarbon pipeline through a previously tapped hole in the pipe. It includes two main parts: the pressure vessel and the launching mechanism.

JPT

P. 1314