The feasibility study of conversion of a Natural Gas (CH4) pipeline analyzes if piping and the main components of the system can support a change with pure hydrogen or with a mixture of 20% hydrogen and CH4.

This study is carried out in accordance with the international standards for the design of piping and pipe components that have been treated with hydrogen as process fluid. Generally, a conversion study is carried out through the following stages:


1. Identifying the chemical/physical damage mechanisms, which could be triggered by in-service hydrogen;

2. Identifying piping, its components and construction material belonging to each pipeline type;

3. Defining mechanical specifications and chemical/physical properties, which the examined material should have in accordance with the relevant international standard, in order to avoid the previously mentioned damage mechanism;

4. Defining if technical specifications on materials have or do not have an impact on the project;

5. Defining possible further testing and/or Post Weld Heat Treatment for piping and in-service components with hydrogen in accordance with ASME B31.12.

Identifying damage mechanisms is therefore the starting point in order to carry out the feasibility study. It is also the object of this in-depth analysis in order to try to provide an indication of the main physical and chemical phenomena, which occur using hydrogen as an energy carrier.

Damage mechanisms for the station components are to be identified with reference to the standards API 581 RP and can be divided in two macro-categories:

1. Chemical Damage Mechanisms: in other words, hydrogen-induced cracking in pipelines;

2. Physical Damage Mechanisms: energy carrier physical leakage from piping.

Damage phenomena, whether physical or chemical, are to be assessed separately in accordance with the standard API 571 RP, in order to define temperature and pressure ranges, where the phenomenon becomes significant for in-service hydrogen components. The following Table 1 lists the main damage mechanisms identified for this study:

Table 1: Damage Mechanisms for piping and pipe components under analysis according to API 581 RP

HIC

The damage mechanism of HIC occurs in wet hydrogen sulfide (H2S) environments and can be found in carbon and low alloy steel.

Hydrogen blisters can appear at various depths from the steel surface, in the center of the plate or around the weld area.

Occasionally, neighboring or adjacent blisters at slightly different depths may cause cracks that link them to one another.

Cracks causing linkage of blisters have often step-wise pattern. HIC is therefore sometimes referred to as “step-wise cracking”.

The main factors influencing this damage mechanism are:

• Environmental Conditions (pH, temperature, H2S level, contaminants and their temperature);
• Material properties (hardness, microstructure, mechanical strength);
• Traction stress level (PWHT-applied or residual).

In the case under assessment, the wet H2S concentration – in the composition of the new hydrogen-based energy carrier – is below 50 ppm. Therefore, in accordance with API 571 par. 5.1.2.3.3, this damage mechanism should not occur. However, we recommend that it should be considered for a more conservative analysis.

BLISTERING

In wet H2S environments, hydrogen blisters can be found in carbon and low- carbon steel. Hydrogen blisters may form as bubbles on the external or internal surface, or in the pipe gauge.


This bubble is caused by hydrogen atoms that form during sulfide corrosion on the steel surface. The atoms diffuse into the steel and form a discontinuity in the steel crystal structure, such as inclusions or laminations.


Hydrogen atoms combine to form hydrogen molecules that are too big to diffuse to the surface. The pressure therefore increases and form a local deformation, thus forming a bubble. Blistering is caused by hydrogen generated by corrosion and moisture (in highly aggressive environments).

In the case under assessment, the wet H2S concentration – in the composition of the new hydrogen-based energy carrier – is below 50 ppm. Therefore, in accordance with API 571 par. 5.1.2.3.3, this damage mechanism should not occur. However, we recommend that it should be considered for a more conservative analysis.

HE

The diffusion of atomic hydrogen can cause brittle fractures due to the loss of ductility in high-strength steel. HE can occur during welding, manufacturing or due to processes that cause the diffusion of hydrogen into steel in an aqueous, corrosive or gaseous medium.

Materials that are affected by this damage mechanism are commonly carbon steel, low alloy steel and some high strength nickel alloys. The main factors that influence this damage mechanism are:

• H concentration in steel/alloy;
• Strength level and microstructure of the component, i.e., embrittlement resistance (UTS> 800 MPa).

The hydrogen source may be constituted by services with gas atmospheres at high temperature (molecular H2 dissociates to form atomic hydrogen that can diffuse into steel) as well as by production, welding or cleaning processes.

LEAKAGE

Hydrogen is colorless, tasteless and highly explosive, with low viscosity and low molecular weight. It is an asphyxiant and explosive gas. Due to these properties, appropriate measures are necessary to absolutely avoid leakage in the system.

This damage mechanism, a physical one, affects valves (the body as well as the valve seat), gaskets and threads. Therefore, due to H2 use in this kind of system, special materials/zero-leakage operating and treatment procedures are to be planned.

HIGH TEMPERATURE H2S Corrosion

Hydrogen in hydrocarbon flows with H2S increases the severity of high-temperature sulfidic corrosion at temperatures above 260°C, according to API 571 RP.

This particular damage mechanism can be influenced by many factors, but the most significant one is temperature.

Therefore, since every class of the station pipelines were found to be in lower design temperature ranges, this mechanism can be considered not applicable for the present study.