The Effect of High Pressure High Temperature and Sour Service Environment on Subsea Dissimilar Joints

Introduction

There are many challenges to designing, manufacturing, and fabricating subsea equipment or pipeline networks, especially when the equipment is operating in harsh environments and under high pressure high temperature (HPHT) working conditions. It becomes even more complicated when dissimilar joining materials and cathodic protection are added to the mix.

Dissimilar joints are a vital part of subsea oil and gas systems, where clad forged fittings, such as hubs or tees, are joined to corrosion resistant alloy pipe. When operating under high pressure, high temperature and in sour environments, the probability of material degradation is high. That is why proper material selection is so important.

This article discusses the failure of a dissimilar joint between SA-29 Gr.8627 or SA-29 Gr.8630 forged and machined into a tee and then buttered with alloy 625. In this study, an equal 18” diameter pipeline tee connected to three weld neck flanges in locations A, B & C (see Figure 1) showed many blisters and surface cracks in three locations near the weld areas.

In this article, the author will discuss the root causes of the failure, along with potential solutions and preventive actions that would not compromise the welding production rate. He will also discuss the importance of proper material selection for high pressure, high temperature, and sour environments, coupled with cathodic protection to avoid failures or cracks, like those observed in the study.