On The Beneficial Influence of a Very Low Supply of H2S on the Hydrogen Embrittlement Resistance of Carbon Steel Wires in Flexible Pipe Annulus
Authors: Desamais Nicolas , Taravel-Condat Carol 
Published: July 2009

Flexible pipes have been used to transport oil and gas in offshore applications for more than 30 years. With the development of fields in ever deeper water, the mechanical loads in flexible risers have been significantly increased and, consequently, very high strength wires are often needed. However, more of the developed fields contain H2S which generally is not compatible with the use of high strength steels due to their sensitivity to hydrogen embrittlement. Therefore, understanding the relationship between H2S in the bore and its impact on the wires in the annulus is critical for designing flexible pipes for ultra deep sour service fields.

Today, the approach used to calculate the annulus composition does not address the extremely low rate of H2S entering the annulus by permeation from the bore through the sheath, compared to the ability of the large amount of steel wires in the annulus to consume the H2S. This phenomenon has already been observed in full scale experiments with high level of H2S in the bore.

The standard corrosion and SSCC/HIC testing methods, generally use a large flowrate of gas compared to the surface of steel, i.e. H2S partial pressure remains constant and is not influenced by the corrosion reaction. For the specific case of the steel located in a flexible pipe annulus a new programme was initiated in 2007 to study the influence of the very low supply of H2S on the hydrogen embrittlement. The approach is based on using calibrated flowrates of H2S/cm2 steel, the methodology requiring very accurate control and precise measurements of H2S into the solution.

The paper will present the experimental methodology being used as well as the first results that have been obtained. The initial results indicate that by reducing the flowrate of H2S/cm2 steel by a factor 100 (compared to standard tests), very high strength steels with SMUTS=1200MPa and more could be proposed instead of steels with SMUTS=850MPa which are used today. An example will be presented to illustrate the impact this finding has on the feasibility and competitiveness of flexible risers for sour ultra deepwater developments.

Introduction

In a flexible pipe, carbon steel armours are not in direct contact with the fluid circulating inside the bore but are located in the annular space. This space is mainly filled with carbon steel wires giving a very high confinement. The acid gas contained in the crude oil environment such as CO2 and H2S and water reach the annular space by permeability through the polymer pressure sheath. This diffusion process is slow and more often the H2S content in the bore is limited (partial pressure below 100 mbar). Consequently the flowrate of H2S in the annulus is extremely low compared to the one applied in standard testing methodologies which are used to characterized the susceptibility of carbon steel wires to hydrogen embrittlement in presence of H2S (SSCC/HIC). This is particularly true if the H2S flowrate is given as a function of the quantity of steel surface available in the annulus (mL of H2S/min/cm2 of steel). Indeed, during a corrosion process in presence of H2S, a part of the H2S or all is consumed to form corrosion products such as iron sulphide.

After a short description of the flexible pipe structure, this paper will present in a first part, the methodology used to calculate the annulus composition. In particular, flowrate of H2S in the annulus (mL of H2S/min/cm2 of steel) will be given for different projects and compared with flowrate of H2S in small scale and full scale tests.

In a second part, we will present the experimental procedure we have used to study the influence of the very low supply of H2S on the hydrogen embrittlement for two high strength carbon steel wires. The effect of H2S flowrate on the saturation of the solution and on the presence of HIC cracks in the carbon steel wires will be presented and discussed.