Anti-H2S layer extends operating envelope for sour service flexible
Jeremy Beckman
Editor, Europe
Technip is stretching the sour service boundaries for flexible pipelines and risers. To prevent hydrogen sulfide (H2S) in the wellstream from reaching the annulus of the pipe, the company has developed, in collaboration with IFP Energies Nouvelles (IFPEN), a new thermoplastic material applied as an extra protective layer. This layer contains components that react chemically with the H2S to stop it diffusing to other parts of the pipe and causing potential damage, in the form of sulfide stress cracking or hydrogen-induced cracking. Very high strength steels are particularly susceptible to these types of corrosion.
Currently, the only way to address diffusion is to use sour service steel grades for the vault and armor wire layers surrounding the flexible's pressure sheath. But these steels increase the pipe's weight, and this can impact mechanical performance, particularly in ultra-deepwater applications.
Technip claims that its new material, by halting the spread of H2S, allows use of sweet service steel grades with reduced thickness for the layers comprising the annulus, lowering both the weight and the overall cost of the pipe. Additionally, the "Anti-H2S" layer makes it possible to design flexible pipes for longer-lasting sour service, and for fluids with high H2S content, for which there is presently no solution.
Technip's project engineering division in Le Trait, northern France, has been working on this development for 10 years with IFPEN. A wide range of tests have been successfully conducted at IFPEN's test facilities near Lyon, with full-scale static and dynamic demonstrative tests still in progress. Following presentations at several major oil and gas conferences in 2011, discussions are under way with various operators for potential applications.
Compatibility issues
"We currently produce two types of flexible pipe," said Technip project manager Thomas Epsztein. "These are flexible pipes for sweet service application and flexible pipe for sour service applications. When we have to counter H2S in the pipe during transportation, you have to take into account the fact that H2S, being a small molecule, can diffuse through the polymer pressure sheath."
Epsztein continued: "This diffusion mechanism is activated by temperature and pressure. So we have to consider the compatibility of the steel grade in the annulus to H2S. Until now we have chosen sour service steel grades with higher resistance to H2S, but with lower mechanical performance. This means that we end up using more steel to sustain the flexible pipe's mechanical load, the pressure of the water columns, and axial pressure."
Flexible pipes have been in use offshore for over 30 years. The present construction typically comprises five-six protective layers:
- The innermost layer is a stainless steel carcass in direct contact with the transported fluids, providing resistance to external pressure
- Surrounding this is the pressure sheath (Layer 2), a continuous leak-proof polymer layer
- Layer 3, designed to sustain internal pressure, comprises vault wires spiralled at short pitch
- Layer 4 comprises one or two pairs of helically wound steel armor wires ensuring tensile load resistance
- An external sheath (Layer 5) secures the tightness of the annulus
- Optional syntactic foam tapes (Layer 6) can be added for improved thermal performance.
Layer 2 (the polymer pressure sheath) contains the transported fluids within the pipe bore and protects the steel layers sustaining internal pressure and axial tension against direct contact with the fluids. Over time, water, carbon dioxide (CO2) and H2S will permeate through the polymer, but the design of each flexible takes this into account via a fluid permeation model which allows prediction of partial pressure caused by H2S in the annulus, based on the structure's operating conditions. That predicted pressure is used to select a suitable steel grade for the vault and armor wire layers.
Technip's Anti-H2S layer is placed in between the pressure sheath and the pressure vault (the current layers 2 and 3).
The new material, known as PEZnO, is a composite comprising a thermoplastic polyethylene (PE) matrix compounded with zinc oxide (ZnO) and iron oxide (Fe2O3). Both oxides will react chemically with the H2S after permeation has taken place through the pressure sheath, ensuring the efficiency of the system. The iron oxide gives the material an initial purple tinge that is used as a visual tracer of the reaction taking place, namely:
ZnO + H2S => ZnS + H2O
Reaction occurs within a thin area known as the "reaction front." This serves as a frontier between:
- The PEZnS area on the bore side, where ZnO has reacted with H2S to produce ZnS
- The PEZnO area on the annulus side where ZnO has not been consumed following the reaction with H2S, and which therefore constitutes the remaining protective potential of the Anti- H2S material.
Technip and IFPEN have devised and validated the KilHyS (Kill Hydrogen Sulfide) diffusion/reaction model to predict the position of the reaction area in the Anti-H2S sheath, and the speed at which the reaction will progress. Predictions are based on design and operating conditions, including the operating temperature range and the H2S content in the bore.
Validation of the model is achieved partly via H2S exposure tests on the PEZnO material, and partly via long-term permeation tests on bi-layer tubes. The PEZnO exposure tests involve placing samples of the material in an autoclave filled with the test gas at test pressure and temperature. The samples are periodically removed and cut open to reveal the position of the reaction area, identifiable as the portion where the material changes color from purple to grey. Results to date have demonstrated good correlation with the predictive model.
Test parameters
"We have been through testing at several scales," said Daniel Averbuch, IFP Energies Nouvelles' Progran Manager, Subsea, Umbilicals, Risers and Flowlines. "The process started with lab-scale tests to select the polymer material and metallic oxide, followed by mid-scale testing on something representative of a flexible pipe, and then full-scale testing using real flexible pipes manufactured in Le Trait on an industrial scale in internal diameters of 4.8-in. These full-scale tests are to be considered as demonstrative tests that will give operators confidence in this new technology. It will also demonstrate the ability of the diffusion/reaction model to predict the efficiency of the material over several years."
All tests are being conducted at IFPEN's facility in Solaize on existing test loops, although upgraded in certain cases for this program. Full-scale H2S exposure tests on actual industrial-size flexible pipes started in 2011 and will continue throughout this year. The aim is to confirm the long-term material's efficiency in dynamic and static configurations by early 2013.
"We are already engaged in discussions with several offshore operators on potential applications for this new technology," Epzstein noted. "Feedback has been very positive. We strongly believe this new approach can bring a lot of advantages to flexible pipe use offshore."
Tests are being conducted on seven representative structures, namely one dynamic flexible pipe, one static flexible pipe, and five short pipe samples. All are exposed to a gas mix comprising 93.5% methane (CH4), 5% CO2, and 1.5% H2S, at varying pressures and temperatures, with extended water cooling to induce the required thermal gradient. "We have strict control of the operating environment," Averbuch added, "while at the same time operating stringent safety procedures."
Every six months a short sample of pipe is removed from the test area for dissection and inspection, in order to track the position of the reaction front over time. That position is compared with the results from the KilHyS model prediction – again, a good correlation will confirm the latter's capabilities.
Dynamic conditions will impact the performance of the chosen steel grade. A flooded annulus (caused by diffusion of water condensation from the bore or by damage to the external sheath) will be at risk of fatigue as well as corrosion. The combined impact can be assessed by deriving fatigue curves based on tests conducted in a representative environment (i.e. in sea water with CO2 and H2S at the required partial pressures).
"For us, full-scale tests by IFPEN in Solaize are more valuable than tests in an actual field, as they provide comprehensive monitoring of the effects of temperature and pressure," Epzstein observed. "Moreover, the dynamic conditions applied during the full-scale test are more severe than actual field conditions. But if an operator wanted to test a prototype flexible on a given field, we would be OK with that."
"We are also working on a solution to monitor the reaction taking place between the two sheaths," Epzstein added. "Having a monitoring system in situ offshore would additionally give the client access to how the system is performing."
Long-term contingencies
Eliminating the risk of hydrogen-induced cracking (HIC) or sour service cracking (SSC) allows selection of lower-cost sweet service steel grades with better mechanical properties than sour service grades. Applying high-strength sweet service steel to, for instance, the flexible's pressure vault or armor layers could allow use of wires with thinner dimensions. In some applications, it could render unnecessary the need for a spiral layer between the pressure vault and armor layer. The result could be a weight reduction in the pipe of up to 25%, allowing the use of smaller (and therefore less costly) offshore installation vessels.
But this assumes that conditions in the field are as predicted. There are scenarios where the transported fluid's H2S content turns out to be higher than anticipated in the design phase. A pipe containing an Anti-H2S sheath in this situation would still be able to remain in service for a much longer period – for the entire field life, Technip claims, if the H2S layer is thick enough.
According to Epzstein, "there may be an occasion where an operator tells us that during the first five years in service there was no ingress of H2S, but afterwards it did arise at a pressure above the value considered in the design phase. We would then be able to perform a new modelization to evaluate the efficiency of the Anti-H2S material based on the actual H2S content. The result of this new evaluation would then be shared with the operator to define a new lifetime during which the flexible pipes could be safely used." Averbuch added: "You could also imagine adapting this product for a sweet service application, with respect to possible souring in late field life."
The design thickness of the Anti-H2S material, which can vary from 4-15 mm, is based on the H2S reaction rate and on manufacturing requirements connected to the extrusion process and the sheath's mechanical behavior. "In terms of H2S partial pressure, we cover a large range of applications," Epzstein said. "Every project is different, but current specifications are typically in the range 50-150 ppm of H2S. With our material, however, the system is efficient even with up to 2,500 ppm H2S."
Compared with conventional sweet service flexible designs, Epzstein noted, "we add a layer, so it's a little more expensive. But our solution is much less expensive than existing sour designs because of the lower quantity of steel required."
