Qualification of High Strength Carbon Steel Wires for Use in Specific Annulus Environment of Flexible Pipes Containing CO2 and H2S

2006 ◽  
Author(s):  
Carol Taravel-Condat ◽  
Nicolas Desamais
Keyword(s):  
Carbon Steel ◽  
Corrosion Fatigue ◽  
High Strength ◽  
Full Scale ◽  
Small Scale ◽  
Annular Space ◽  
Flexible Pipe ◽  
Corrosion Rates ◽  
Steel Wires ◽  
Flexible Pipes

Flexible pipes have been used for many years in offshore applications for the transportation of crude oil, gas and water. Such structures are subjected to mechanical loads due to tension, high internal pressure and dynamic motions which are sustained by the use of high strength carbon steel wires. The steel wires are located in the annular space that may contain water and acid gas (CO2, H2S) which can be detrimental for steels. For that reason, risks of Sulfide Stress Corrosion Cracking (SSCC) and Hydrogen Induced Cracking (HIC) shall be considered. Moreover, for dynamic conditions, presence of corrosive environment in the annulus could significantly reduce the fatigue performance of flexible pipe compared to air environment. The annulus composition is calculated using a permeability model that has been validated with medium scale tests, full scale tests and field cases [1]. In parallel, corrosion and corrosion fatigue studies have shown that the annulus of a flexible pipe is a very confined space with over-saturation in iron and no renewal of oxygen. This results in a higher pH, much lower corrosion rates and less HIC and SSCC than one would expect based on normal environments. This paper presents, in a first part, the different results obtained during small scale corrosion and corrosion fatigue tests demonstrating the beneficial specificity of the annular space. In a second part, the paper presents the results of pH measurements and corrosion rates obtained during a full scale dynamic corrosion fatigue test in CO2/H2S environment validating the previous results. This test was conducted on a 6” pipe between 1999 and 2003. In a third part, the paper describes how the annulus specificities should be used today to determine the suitability of carbon steel wires for use in flexible flowlines and risers considering SSCC, HIC, corrosion and corrosion fatigue.

H2S Consumption and the Derivation of a New Annulus Prediction Model for Offshore Flexible Pipes

2016 ◽  
Author(s):  
Marie Haahr ◽  
Jonas Gudme ◽  
Jacob Sonne ◽  
Sten Overby ◽  
Torben Nielsen ◽  
...  
Keyword(s):  
Prediction Model ◽  
Scale Validation ◽  
Cost Savings ◽  
Small Scale ◽  
Annular Space ◽  
Flexible Pipe ◽  
Independent Verification ◽  
Outer Sheath ◽  
Flexible Pipes ◽  
Sour Service

This paper presents the outcome of investigations on the effects of H2S consumption in the annulus of a flexible pipe. Low-molecular gases, such as CH4, H2S, H2O and CO2, permeate slowly from the bore through the inner liner into the annular space between the inner liner and outer sheath of a flexible pipe. This space is densely packed with carbon steel armour wires leaving a very limited free volume. In the presence of water, a corrosive environment for the armour wires is generated and a risk of sour service cracking is introduced. H2S concentration in the annulus is traditionally calculated by balancing the inflow through inner liner and the outflow through outer sheath and vent valve. In order to assure H2S resistance of the armour wires towards calculated H2S concentrations, pipes for sour service are typically designed with lower strength wire grades of larger dimensions compared to the possibilities of sweet service pipes. Over the last decade, more and more offshore data has been obtained indicating considerably less H2S in the annulus than predicted by the traditional annulus models. This observation has triggered in-depth investigations of the complex corrosive H2S environment inside a flexible pipe annulus exposed to sour service conditions. An extensive small-scale test program has been conducted and showed that at permeation rates typical for flexible pipes, the consumption of H2S in the corrosion processes occurring in the annular space lowers the concentration and hence criticality of the H2S so significantly that it leaves the traditional models overly conservative to an extreme extent. Using this knowledge of consumption of the corrosive gases in the annulus has become an increasingly important topic with the focus on deeper waters, cost savings and service life extensions without compromising flexible pipe integrity. Based on experimental data obtained, a new annulus model for prediction of H2S pressure in annulus has been derived. Data is presented in this paper to illustrate the methodology for an annulus prediction where the consumption of H2S is included. The data presented covers laboratory tests with variations and effects of gas flux, H2S concentration and total pressure. A full-scale validation, led to an Independent Verification Agency certification of the model. With the introduction of this new annulus prediction model, a wider range of wire products becomes available for the pipe designers. Lower weight pipes with stronger armour wires render optimizations for both cost savings and applications at deeper waters possible.

Prevention and Monitoring of Fatigue-Corrosion of Flexible Risers’ Steel Reinforcements

2007 ◽  
Author(s):  
Antoine Fe´lix-Henry
Keyword(s):  
Corrosion Fatigue ◽  
Corrosion Behaviour ◽  
High Strength Steels ◽  
High Strength ◽  
Annular Space ◽  
Monitoring Methods ◽  
Steel Wires ◽  
Service Production ◽  
Sour Service ◽  
Fatigue Corrosion

Flexible pipelines used for oil production on floating platforms are subject to dynamic loads and are designed to ensure no fatigue failure over their service life. Their reinforcing layers are located in a very confined environment called the annular space and are made out of high strength steel wires. If these steel wires are subjected to a wet corrosive environment, corrosion fatigue can become an issue, especially for sour service applications with the presence of H2S and CO2. Over the past years, Flexi France has dedicated a lot of effort not only to predict the composition of the environment in the annular space and the steel wires’ fatigue-corrosion behaviour, but also towards the prevention of corrosion and the monitoring of this annular space. Several innovative solutions recently developed and tested in Flexi France are presented in this paper concerning prevention of annular space flooding by seawater ingress or by diffused water. The paper also presents new designs for integrating monitoring sensors into the pipe structure and corrosion fatigue mitigation methods. Inspection and monitoring methods by integration of sensors or conduits within the pipe structure during its manufacture are now commercially available. These technologies, some of which are already applied on offshore fields, extend the usage limits of high strength steels and increase the fatigue life of dynamic risers. This is of primary importance for demanding applications such as deepwater, high pressure and/or sour service production.

MOLDI™: A Fluid Permeation Model to Calculate the Annulus Composition in Flexible Pipes — Validation With Medium Scale Tests, Full Scale Tests and Field Cases

2003 ◽  
Author(s):  
Carol Taravel-Condat ◽  
Marc Guichard ◽  
Joseph Martin
Keyword(s):  
Temperature Gradient ◽  
Element Model ◽  
Steel Part ◽  
Full Scale ◽  
Shielding Effect ◽  
Flexible Pipe ◽  
Fluid Mixture ◽  
Medium Scale ◽  
Steel Wires ◽  
Flexible Pipes

In flexible pipes, the steel wires in the annular space can sometimes be in contact with a corrosive environment containing condensed water or seawater and acid gases (CO2 and H2S), coming from the bore by permeation through internal polymer sheaths. In order to choose the most suitable steel wires to avoid hydrogen embrittlement and to evaluate their resistance to corrosion and corrosion fatigue, it is necessary to know the annulus composition. A finite element model named MOLDI™ has been developed in order to precisely calculate this environment. This model is composed of two major modules: The first one describes the time dependent permeation of fluids through the pipe layers. Concentration and pressure versus time are calculated assuming Fick and Henry’s laws. The shielding effect of steel wires on the plastic sheaths and the temperature gradient through the structure are taken into account. The second module uses a thermodynamic flash algorithm to calculate the phase equilibrium in the annulus, including water condensation. In order to validate this model, a permeation testing program has been initiated in 1997. It is based on medium and full scale permeation tests and field cases. For each test, the pressure build-up and the composition inside the annulus are measured and compared with the ones calculated with MOLDI™. Medium scale prototypes consist in a polymer tube surrounded by a steel part simulating the annulus geometry of a flexible pipe (shielding, free volume). One of the prototypes has been specially designed in order to be able to maintain a chosen temperature gradient between the bore and the external side. The full scale permeation tests are conducted on a piece of manufactured flexible pipe. At the beginning of the test, the bore is filled up with a pure gas or with a fluid mixture (gas–water–crude oil) under pressure and temperature. The permeation rate from the bore to the annulus is determined by recording the pressure build-up in the annulus versus time. The gas composition in the annulus is measured using a Gaseous Chromatography device. Field cases have also been used to validate MOLDI™. To do that, the annulus of flexible risers has been monitored on site and the measurements compared with MOLDI™ simulations. The validations conducted at the time being, using different polymer sheath materials and fluid mixtures, have shown that our permeation model predictions are in good agreement with experimental results. Consequently, MOLDI™ is considered as validated and is now used to calculate the annulus composition from the field conditions.

An Experimental and Numerical Investigation of the Effect of Axial Thermal Gradients in Flexible Pipes

2017 ◽  
Author(s):  
Dag Fergestad ◽  
Frank Klæbo ◽  
Jan Muren ◽  
Pål Hylland ◽  
Tom Are Grøv ◽  
...  
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Cross Section ◽  
Measurement Techniques ◽  
Full Scale ◽  
Model Complexity ◽  
Flexible Pipe ◽  
Element Analysis ◽  
Flexible Pipes ◽  
Full Scale Testing

This paper discusses the structural challenges associated with high axial temperature gradients and the corresponding internal cross section forces. A representative flexible pipe section designed for high operational temperature has been subject to full scale testing with temperature profiles obtained by external heating and cooling. The test is providing detailed insight in onset and magnitude of relative layer movements and layer forces. As part of the full-scale testing, novel methods for temperature gradient testing of unbonded flexible pipes have been developed, along with layer force- and deflection-measurement techniques. The full-scale test set-up has been subject to numerous temperature cycles of various magnitudes, gradients, absolute temperatures, as well as tension cycling to investigate possible couplings to dynamics. Extensive use of finite element analysis has efficiently supported test planning, instrumentation and execution, as well as enabling increased understanding of the structural interaction within the unbonded flexible pipe cross section. When exploiting the problem by finite element analysis, key inputs will be correct material models for the polymeric layers, and as-built dimensions/thicknesses. Finding the balance between reasonable simplification and model complexity is also a challenge, where access to high quality full-scale tests and dissected pipes coming back from operation provides good support for these decisions. Considering the extensive full scale testing, supported by advanced finite element analysis, it is evident that increased attention will be needed to document reliable operation in the most demanding high temperature flexible pipe applications.

Finite Element Analysis of Flexible Pipes Under Compression

2014 ◽  
Author(s):  
Eduardo Ribeiro Malta ◽  
Clóvis de Arruda Martins
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
High Strength ◽  
Element Model ◽  
Large Displacements ◽  
Flexible Pipe ◽  
Element Analysis ◽  
Flexible Pipes ◽  
Compressive Loads ◽  
Surface Vessel

Axial compressive loads can appear in several situations during the service life of a flexible pipe, due to pressure variations during installation or due to surface vessel heave. The tensile armor withstands well tension loads, but under compression, instability may occur. A Finite Element model is constructed using Abaqus in order to study a flexible pipe compound by external sheath, two layers of tensile armor, a high strength tape and a rigid nucleus. This model is fully tridimensional and takes into account all kinds of nonlinearities involved in this phenomenon, including contacts, gaps, friction, plasticity and large displacements. It also has no symmetry or periodical limitations, thus permitting each individual wire of the tensile armor do displace in any direction. Case studies were performed and their results discussed.

Thermal Effects on the Anchoring of Flexible Pipe Tensile Armors

2015 ◽  
Author(s):  
Olaf O. Otte Filho ◽  
Rafael L. Tanaka ◽  
Rafael G. Morini ◽  
Rafael N. Torres ◽  
Thamise S. V. Vilela
Keyword(s):  
Finite Element ◽  
Finite Element Model ◽  
Thermal Effects ◽  
Current Model ◽  
Element Model ◽  
Small Scale ◽  
Flexible Pipe ◽  
System A ◽  
Flexible Pipes ◽  
Better Than

In the design of flexible pipes, predict the anchoring behavior on end fittings is always challenging. In this sense, Prysmian Surflex has developed a finite element model, which should help the end fitting design as well the prediction of the structural behavior and the acceptable maximum loads. The current model considers that the contact between armor-resin is purely cohesive and has been suitable for the design of end fittings [1] and [2]. But tests and new studies [3] and [4] indicate that only cohesive assumption would not be the best approach. Experimental data from prototype tests also show that the current model would not predict acceptable results for loads higher than those used in previous projects. This document will describe a study developed considering the friction and thermal contraction, instead of the cohesive phenomenon in the anchoring behavior analysis. Small scale tests were conducted in order to understand the anchoring relation between the resin and the wire used in the tensile armor. For this purpose, a special test device was developed to simulate an enclosure system. A parametric study was also performed to identify the cooling temperatures, coefficients of friction and contact properties parameters taken from small scale tests. The finite element model considers the thermal effects during exothermic curing. Using the new parameters obtained, a second model was developed. This model consists of only one real shaped bended wire inside an end fitting cavity. To validate the model, samples were tested on laboratory according anchoring design. The results of this round of tests were studied and corroborate the argument that use friction and thermal effects is better than use only the cohesive condition.

Review of Some Data on Corrosion of Stainless Steel and Higher Alloys in Pulp Mill Equipment

CORROSION ◽  
1961 ◽  
Vol 17 (6) ◽  
pp. 9-13
Author(s):  
Harold C. Templeton
Keyword(s):  
Carbon Steel ◽  
Black Liquor ◽  
Pulp Mill ◽  
High Strength ◽  
Case Histories ◽  
Good Resistance ◽  
Green Liquor ◽  
Corrosion Rates ◽  
Alkaline Pulping ◽  
Mill Equipment

Abstract Allowable corrosion rates for equipment with low corrosion tolerance are discussed. Corrosion case histories in the various pulping processes are presented. Sulfite pulping solutions are most frequently-handled in 316 material. Stainless 316 is used because of good resistance to local attack under scale or deposits. In alkaline pulping, soda process digesters are usually made of carbon steel with heater tubing of either carbon steel or Type 316 alloy. Kraft digesters are commonly carbon steel, though many units are partially to wholly lined with alloy materials. Type 316 most frequently. Stainless 316 is used for heater piping, strong black liquor piping and green liquor piping. Carbon steel is most frequently used for weak black liquor. Corrosion in groundwood pulping varies greatly depending on wood being ground, water supply and treating chemicals added. Stainless 304 and 316 alloys are used frequently, both to resist corrosion and reduce contamination. Chlorine and hypochlorite bleaching usually pose severe corrosion problems. Hastelloy C alloy is most commonly used for high strength solutions. In more dilute solutions the molybdenum-bearing. stainless steels are satisfactory. Chlorine dioxide is even more corrosive to metals than hypochlorite, and some operators have replaced Hastelloy C with titanium. 8.5.3

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