Advanced Quantitative Hydrogen Sensors for Characterization of Wrought Metal and Weldments for Offshore Structures
Authors: Lasseigne Angelique Nicole , Koenig Kamalu M. , Jackson Joshua E. , Scott Joseph , Moline Keith 
Published: May 2011

Hydrogen plays a critical role in the long-term integrity of offshore structures, and substantial efforts and costs are used to maintain the lowest hydrogen concentration possible, particularly in welded structures. In spite of these efforts, hydrogen still accumulates through a variety of methods, leading to damage and ultimately failure. Unfortunately, there is no existing method for accurate determination of the through-thickness hydrogen content that would be able to detect and monitor such damage before it reaches critical levels. The future of characterization of wrought and weld metals in offshore applications lies in the use of advanced non-destructive sensors to monitor real-time material properties, of which hydrogen is among the most important. Common electromagnetic sensors already utilized for for detection of cracks, defects, and wastage can be modified to measure material properties such as microstructure, residual stress, interstitial contents, etc. Tools that can monitor material properties throughout design, processing, and service-life of components can then be used to proactively prevent the formation of cracks and defects that lead to failures.

The use of electromagnetic sensors to provide non-contact, through-thickness quantified hydrogen concentration measurements in the wrought and weld metal enables a mapping of the amount and location of hydrogen accumulation. The effect of hydrogen on materials properties is critical, as hydrogen is notorious for causing hydrogen embrittlement or hydride embrittlement in many metals and alloy systems. Hydrogen concentration and location monitoring will enable proactive maintenance on components, such as the use of techniques to remove hydrogen (dependent on the metal and solubility) or replacement of parts well before catastrophic failure occurs. These next-generation non-destructive sensors offer users the capability to simultaneously improve their operating efficiency, safety, materials integrity, and costs. The use of non-destructive sensors that quantitatively assess materials properties to perform proactive maintenance will greatly improve the safety and integrity of offshore structures and reduce the risk of catastrophic failures.

Introduction

To operate at higher pressures and temperatures than ever before, the offshore industry continues to strive to use higher strength metals with higher strength-to-weight ratios. The existing infrastructure is also rapidly aging and improved methods to monitor and protect these high-value assets are needed for better integrity management. The use of higher strength alloys in offshore structures causes further concern because of the threat and sensitivity of the alloys to hydrogen embrittlement and/or stress corrosion cracking especially when dealing with structures that are cathodically protected. There are a variety of possible sources for hydrogen production that occur both during fabrication and operation. For materials to safely operate under such extreme conditions, it is imperative to have non-destructive sensors that can achieve a full quantitative materials property assessment. The material's sensitivity to hydrogen embrittlement and cracking becomes even more important for welded joints because of the microstructural, compositional, and residual stress gradients surrounding it. Hydrogen assisted cracking and/or hydrogen embrittlement traditionally occurs in the weld heat-affected zone or in the weld deposit dependent on the final residual stress distribution.