Edison Welding Institute (EWI) and Enterprise Products Operating LP (Enterprise) worked together to develop an in-service welding program. The objective of this project was to relax flow restrictions on current in-service welding procedures to allow for welding onto liquid pipelines with flow rates outside of current flow limits. Enterprise’s current products include liquid propane, liquid ethane, and propane and ethane mixes in addition to other refined products. The current Enterprise in-service welding procedures restrict welding onto liquid pipelines with a flow rate between 1.3 and 4.0 ft/s (0.4 and 1.2 m/s). The minimum flow rate of 1.3 ft/s (0.4 m/s) was used because it was Enterprise’s minimal operating flow rate. The maximum flow rate of 4 ft/s (1.2 m/s) was grandfathered into the procedures. When welding onto an in-service pipeline to repair a damaged section of pipe or to install a branch connection (i.e., hot-tapping) there are two main concerns (burnthrough and hydrogen cracking) and both concerns needed to be evaluated for both flow conditions. The results from the project allow welding onto no-flow liquid pipelines with wall thicknesses between 0.25 and 0.5 in. (6.4 to 12.7 mm). Even though welding onto a no-flow thin-walled liquid pipeline [i.e., less than 0.25 in. (6.4 mm)] would not increase cracking susceptibility, the risk of burnthrough and eutectic iron formation would make the procedure unacceptable. The results of this project also indicated that acceptable welds can be made onto a high flow liquid pipeline [up to 12 ft/s (3.7 m/s)]. It was recommended, however, that Enterprise only use the temper bead welding procedures for such applications. Proper use of the temper bead welding procedures (i.e., proper heat input, weld toe spacing and stringent low hydrogen welding practice) has been shown to produce acceptable, crack-free welds. It is important to note that none of the welds showed signs of cracking, but the hardness levels of the heat input control procedures all exceeded the critical hardness level for their intended carbon equivalent materials. Increasing the flow rate from 4 to 12 ft/s (1.2 to 3.7 m/s) does appear to increase the cooling effect but it is not possible to determine the magnitude of the effect from the results of this work.

An amine tower was inspected and was shown to have wall loss over the majority of the circumference. A repair plan was developed which included welding onto the tower while the tower remained in operation. The two step repair plan first required welding two Inconel 625 rings to the SA516-70N steel tower and then welding a stainless steel sleeve to the Inconel 625 rings encapsulating the corroded area. Since the repair welds could be exposed to an amine environment if the steel tower corroded through, API Recommended Practice 945 (API RP 945) was used to aid in the qualification of the welding procedure. API RP 945 recommends a post-weld heat treatment (PWHT) to reduce hardness and relieve stress, but since the planned repair was to be made in-service, PWHT was not preferred. To address the hardness aspect a temper bead technique was used to successfully qualify a welding procedure, without PWHT, in accordance with 2006 NBIC and 2004 ASME Boiler and Pressure Vessel Code Section IX with hardness values below 200 Brinell. The temper bead welding procedure used Inconel 182 SMAW electrodes and required strict welding heat input control and weld bead placement. The heat input could be monitored by controlling the welding parameters or by using the run-out ratio diagram. The temper bead passes needed to be deposited in such a manner that the weld toe of the temper bead was no more than 3/32 in. (2.4 mm) away from the weld toe of the initial layer. To address the residual stress aspect the procedure qualification weld was thermo-mechanically modeled to predict the residual stress distribution on the inside surface of the amine tower. The repair procedure was performed on the operating vessel to emplace the straps and the sleeve. Shortly afterwards, the tower wall was breached and the internal environment reached the annulus inside the sleeve. Operation continued for several months until the replacement vessel was available. Once the vessel was removed from service, a section of the repaired area was examined for residual stresses and hardness in the carbon steel. The peak residual stresses were lower than predicted by the analysis from the qualification stage. However, the measured heat affected zone (HAZ) hardness was well above the desired level of 200 Brinnell. Analysis showed that the increased hardness level correlated with improper temper bead placement [i.e., temper bead to weld toe spacing greater than 3/32 in. (2.4 mm)] along with other indications of deviations from the qualified procedure.

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The two broad categories of fiber-reinforced composite liner repair and deposited weld metal repair technologies were reviewed and evaluated for potential application for internal repair of gas transmission pipelines. Both are used to some extent for other applications and could be further developed for internal, local, structural repair of gas transmission pipelines. Evaluation trials were conducted on pipe sections with simulated corrosion damage repaired with glass fiber-reinforced composite liners, carbon fiber-reinforced composite liners, and weld deposition. Additional un-repaired pipe sections were evaluated in the virgin condition and with simulated damage. Hydrostatic failure pressures for pipe sections repaired with glass fiberreinforced composite liner were only marginally greater than that of pipe sections without liners, indicating that this type of liner is generally ineffective at restoring the pressure containing capabilities of pipelines. Failure pressure for pipe repaired with carbon fiber-reinforced composite liner was greater than that of the un-repaired pipe section with damage, indicating that this type of liner is effective at restoring the pressure containing capability of pipe. Pipe repaired with weld deposition failed at pressures lower than that of un-repaired pipe in both the virgin and damaged conditions, indicating that this repair technology is less effective at restoring the pressure containing capability of pipe than a carbon fiber-reinforced liner repair. Physical testing indicates that carbon fiber-reinforced liner repair is the most promising technology evaluated to-date. Development of a comprehensive test plan for this process is recommended for use in the next phase of this project. iv 41633R47.pdf

Welding onto an operating pipeline, or in-service welding, for modification and repair has been used safely by pipeline operating companies for many years. The two primary concerns when welding onto carbon steel pipelines are the safety of the repair crew and the integrity of the pipeline after the in-service welds have been completed. However in-service welding has been limited for some products that could react at the pipeline operating pressure if they reach a sufficient temperature that can occur during in-service welding. Even with this additional risk mitigation approach, some companies have historically been extremely cautious or have not allowed welding onto pipelines that contained some products. One such product is ethylene. Welding onto ethylene pipelines has been performed in the past but has more recently been considered a product for which in-service welding should not be undertaken due to the potential of an ethylene decomposition reaction that is exothermic and could result in a pipeline failure. The weld trials performed during this project were to test the hypothesis that an ethylene decomposition reaction that could result in a pipeline failure would not be produced as a result of in-service welding onto an ethylene pipeline. This was based on the background literature search and industry survey that showed in-service welding onto ethylene pipelines has been performed safely but generally there has been no detailed reporting of how those in-service welds were deposited. If an ethylene decomposition reaction does not result in a pipeline failure, then this work will be used to develop a relationship between welding heat inputs, pipeline wall thickness, and pipeline operating pressure under which in-service welds could safely be deposited onto an ethylene pipeline.