FULL WELDED ENCIRCLEMENT SLEEVES

 MINIMUM THICKNESS FOR REPAIR GIRTH WELDS IN CORRODED

The minimum weldable thickness is numerically assessed in this work, as a function of pressure during the welding operations of a corroded gas pipeline, according to the approach by Battelle. The minimum weldable thickness is found to increase when the flow rate of the transported gas in the section being repaired increases. Integrtity of the reapairs is assessed, and alternative measures to momentarily increase the flow in the area of the repair are evaluated. .

1.- INTRODUCTION

Companies transporting natural gas have thousands of miles of buried pipes, which date back to the 40’s and 50’s. At that time, protection techniques against corrosion (e.g. protective coatings and cathodic protection) were not well developed, and the companies commonly face defects due to corrosion degradation. Worlwide the most common way to repair gas leakage is to change the defective portion of the pipe. To do this, however, it is necessary to stop pumping gas and vent the affected portion of the line. Where there are no loops to deviate the gas flow, doing this means stopping provision of gas to some areas, with the consequent losses to the users and the transporting company. One of the alternatives available to minimize service losses and restore serviceability of corroded lines is the use of full encirclement sleeve repairs.

Full encirclement welded sleeves are used to repair defects in underground gas pipelines, see Figure 1. The reinforcements consist of two half sleeves welded lengthwise, which are also welded circumferentially to the pipe if there is a gas leakage or other severe defects. Standard designs are found in API RP 1107 [1] . These reinforcements habitually are carried out in areas where local loss of thickness or gas leakage are detected, generally due to corrosion. When a through-the-thickness defect is detected, reinforcements with O'ring and venting valve are used to prevent gas form reaching the welding operations. This kind of repair requires a circumferential (girth) fillet weld to the pipe, to prevent gas leakage during subsequent service. The possibility of repairing gas leaks is probably the most important advantage of welded sleeve repairs over competeing techniques, such as clock springs [2].

In-field welding of these sleeves is normally a difficult task. Usually, short times and poor soil or weather conditions make cutting, handling and welding the sleeves to the buried pipes require especially trained personnel and equipment. It is no surpris e, therefore, that several weld repairs fail in different ways. These failures have in many cases been the driving force for changes and improvements in the fabrication of the sleeves, field welding procedures and non destructive testing of the repairs.

If not properly trained, due to the fear to burn through the pipe, welders tend to minimise the welding time and use high depostion rates, which are obtained with cellulosic electrodes. The use of cellulosic-coated  electrodes is a common practice in the construction of pipelines but for welds made onto in-service pipelines there is a serious risk of reduction in ductilility and toughness by Hydrogen Cracking [3] the specification of low-hygroscopic basic electrodes is now recommended, along with specified preheats and heat imputs to avoid high cooling rates and burn through [4] .

The occurrence of burn through during the circumferencial reinforcement-to-pipe weld is governed by the thickness of the pipe being welded and the penetration of the molten pool during welding. Weld penetration depends on weld heat input, heat dissipation along the pipe and the reinforcement, and the dissipation through the gases inside (natual gas) and around (air) the pipe. This last dissipation depends upon temperature and flow parameters, pressure and flow tate. The controlling parameter is the gas flow rate.