Design Under Arctic Conditions: A Summary of the Arctic Materials Project Guideline

The main goal of the 10 years Arctic Materials KMB project run by SINTEF (2008–2017) and supported by the industry is to establish criteria and solutions for safe and cost-effective application of materials for hydrocarbon exploration and production in arctic regions. The objective of the arctic materials project guideline (PG) is to assist designers to ensure safe and robust, yet cost-effective, design of offshore structures and structural elements in arctic areas through adequate material testing and requirements to material toughness. It is well known that when the temperature decreases, steel becomes more brittle. To prevent brittle fracture in the Arctic, the structure needs adequate toughness for the loading seen at low temperatures. None of the common offshore design codes today consistently address low temperature applications. In this respect, arctic areas are defined as minimum design temperatures below what current international standards have considered per today, i.e. −10 °C to −14°C. For practical applications, the PG defines arctic areas as minimum design temperature lower than −10 °C. It is acknowledging that design standards to a certain degree are based on operational and qualitative experiences gained by the offshore industry since the 1970’s. However, for arctic offshore facilities, limited operational experiences are gained by the industry. The basis of the guideline is that safe and robust design of structures and structural elements are ensured by combining standard industry practice today with learnings and findings from the 10 years Arctic Materials project. This paper is concerned with the rationale behind the material and test requirements provided in the arctic material guideline. The material requirements will be discussed in detail with emphasis on toughness requirement, constraint effect, thickness effect, acceptance criteria and material qualification criteria.

Yield Point Determination Using Cyclic Loading in Polymer

Defining the yielding point of semicrystalline polymers is a matter to be established in the literature. ASTM D638-14 and ISO 527-1 standards define the yielding point as the point where there is an increase in strain without an increase in stress, which coincides with the beginning of necking of the test samples. The literature has been reevaluating this matter, taking into account the methods used and their respective damage generation in the material. Polymer materials are used in the oil and gas industry, for example, in risers. The understanding of the transition between the elastic and plastic regions is necessary, as well as the understanding of the damage done in both regions. This study is about the effects of cyclic loadings with triangular and sinusoidal loading, with different strain levels and their effect on the mechanical behavior of a fluorinated polymer(Halar ECTFE). The cyclic loading tests were strain-controlled and done with frequencies around 0.1Hz, equivalent to a strain rate of 0.2 and 0.4%/s, and strains up to 2%, with the effects on the transition from the elastic and on the stress relaxation being observed. The results show that up to strains of 0.5% the material has elastic behavior, irrespective of the loading. When the strains are greater than 0.75%, the material shows relaxation on all loadings cycles. Between 0.5% and 0.75%, the triangular loading led to cyclic hardening, while the sinusoidal lead to stress relaxation. The stress relaxation is then related to the damage accumulation on the structure of the material, while the hardening to the chain orientation.

A Modified Engineering Critical Assessment Approach for Offshore Pipelines

Environmentally assisted sub-critical static crack growth can occur in offshore pipelines exposed to aggressive production environments. Recent advances in fracture mechanics testing methods have shown that slow static crack growth rates can be reliably measured in sweet and sour environments under constant stress intensity factor (K) conditions. This has potential implications for the engineering critical assessment (ECA) of pipe girth welds subject to low cycle fatigue loading with long periods of operation under constant static load between cycles, e.g. lateral buckling. This paper demonstrates the influence of including static (i.e. time dependent) crack growth as well as fatigue crack growth in a modified pipeline ECA approach.

Background and New Revision of DNVGL-RP-F108

DNV-RP-F108 [1] was first issued in 2006. The Recommended Practice was developed to provide guidance on testing and analyses for fracture control of pipeline girth welds subjected to cyclic plastic deformation, e.g. during installation by the reeling method, but also for other situations where pipelines may be subjected to large plastic strains. The Recommended Practice was based upon a Project Guideline developed within the Joint Industry Project “Fracture Control for Installation Methods Introducing Cyclic Plastic Strain - Development of Guidelines for Reeling of Pipelines”. The new revision is based on the extensive experience and knowledge gained over the years use of the previous versions, as well as new knowledge from recent R&D projects. The main content of Appendix A of DNV-OS-F101 (now DNVGL-ST-F101) [2] have been transferred to DNVGL-RP-F108. Only the requirements relative to ECA and testing have been retained in DNVGL-ST-F101 [2]. The new revision has got a new number and new title, i.e. DNVGL-RP-F108, “Assessment of Flaws in Pipeline and Riser Girth Welds”. This paper lists the fundamental changes made in the new RP from the old Appendix A of the previous DNV-OS-F101 and discusses some of the changes, although within this paper it is not possible to cover all changes. The focus is on clarification of use of S-N versus the fracture mechanics approach for fatigue life computation, classification of fatigue sensitive welds, calculations of more accurate crack driving force by re-introduction of the plate solution, for which a new Lr,max (plastic collapse) calculation and a modified way to account for residual stresses have been specified. The RP presents new assessment procedures pertaining to use of finite element analyses for fracture mechanics assessments. A unique feature of the new RP is the guidance on sour service testing and assessments included in the Appendix C of the document to support pipeline/riser ECAs to develop flaw acceptance criteria for NDT.

Improved Bend Over Sheave Durability of HMPE Ropes for Deep Sea Handling

The use of synthetic fiber ropes for subsea installation is extending, as the offshore industry explores deeper waters, but there are few data available to evaluate the lifetime of these materials. In a previous OMAE presentation the authors described results from the first phase (2010–2013) of a JIP aiming to understand the mechanisms controlling the long term behavior of HMPE fibre ropes [1]. This presentation will describe the results from the second phase of this study (2014–2018) in which predictive models have been developed and applied to a range of improved braided rope materials. Two modeling approaches will be discussed, an empirical method based on residual strength after cycling, and a numerical approach using finite element software specifically adapted to fibre materials [2]. An extensive test program, which has generated a database of CBOS (cyclic bend over sheave) results for various grades of HMPE and different constructions, will be described. Comparisons have been made with steel wire handling lines in order to quantify the benefits of fibre ropes for these deepwater applications.

High Cycle Fatigue Damage Evaluation of Steel Pipelines Based on Microhardness Changes During Cyclic Loads: Part II

The hardness of a material shows its ability to resist to microplastic deformation caused by indentation or penetration and is closely related to the plastic slip capacity of the material. Therefore, it could be significant to study the resistance to microplastic deformations based on microhardness changes on the surface, and the associated accumulation of fatigue damage. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. Here, Berkovich indentation tests were carried out in the samples previously submitted to high cycle fatigue (HCF) tests. It was observed that the major changes in the microhardness values occurred at the surface of the material below 3 μm of indentation depth, and around 20% of the fatigue life of the material, proving that microcracking is a surface phenomenon. So, the results obtained for the surface of the specimen and at the beginning of the fatigue life of the material will be considered in the proposal of a new method to estimate the fatigue life of metal structures.

Hydrogen Induced Stress Cracking of Superduplex Steels: Effect of Operation Temperature

Duplex stainless steel has been used on subsea facilities since the mid 80-ties. The experiences with these materials have been relative good and only a few failures have been reported. However, BP and Shell experience some serious cracking of duplex steel in the mid 90-ties and in beginning of the century. The root cause of these failures was identified to be Hydrogen Induced Stress Cracking, HISC, where the hydrogen source was the cathodic protection system of the subsea facility. These and other similar failures resulted establishment of Joint Industry Projects, JIPs with financial and technical contribution from leading oil companies, contractors, material suppliers and research institutions as TWI, SINTEF and DNVGL. The objective of the JIPs was to establish practical usage limits for duplex stainless steels. The JIPs resulted in a recommended practice “DNV-RP-F112 - Design of duplex stainless subsea equipment exposed to cathodic protection.” This document minimized the failure rate of duplex steel components used subsea. However, since duplex steels components have been used on subsea facilities long before the guidelines and recommendations were issued, there are lot of components presently in use that may be overloaded compared to guidelines and recommendations. As a part of life time extension of one of Statoil’s long time producing fields, a HISC re-calculation of spools connecting SPSs to infield pipelines showed that many of the spools were exposed to stresses above the recommended stresses given in DNV-RP-F112. Since these recommendations were primarily based on testing at ambient seabed temperature (4°C), Statoil, together with SINTEF, started in 2016 a project where the aim was to evaluate the resistance against HISC as an effect of the operation temperature. The results of this project show that the critical net section stress/AYS (HISC resistance) increases with increasing temperature. Based on this, the before mentioned spools can be considered safe even though the spools are exposed to stresses above the recommendations in DNV-RP-F112. Further, the investigations show that the guidelines and recommendations given in DNV-RP-F112 may be conservative for temperatures above 4°C. It is therefore recommended to perform more testing to confirm and incorporate the findings from the present investigation in future revision of DNV-RP-F112.

In-Situ Investigation of Initiation and Propagation Behavior of Bainitic Steels in Sour Environment

Hydrogen induced cracking (HIC) occurs by the poisoning effect of hydrogen sulfide (H2S) which promotes hydrogen absorption and entry at steel surface. Therefore, it is important for linepipe steels to have sufficient HIC resistance in sour environments. The HIC resistance is usually evaluated by measuring cracks after the standardized immersion test such as NACE TM0284. However, the general evaluation method cannot investigate HIC initiation and propagation behavior separately. It is necessary to understand the effect of metallurgical factors on the cracking behavior of sour service linepipe. In this study, in-situ ultrasonic inspection equipment was applied to the HIC test for the several linepipe steels with bainitic microstructure in order to clarify crack initiation and propagation behavior quantitatively. The three dimensional (3-D) distribution of cracks in the specimen was successfully captured as time sequence, and the temporal change of the crack area ratio (CAR) was investigated. It was revealed that the CAR-time curves are consist of four stages with different CAR increment rate. The first stage is the incubation of crack initiation. In the second stage, cracks occur and grow, and adjacent cracks coalesced rapidly. Regarding the first and second stages, sensitivity for the HIC initiation was well correlated with the hydrogen diffusion coefficient and the density of crack initiation site, such as MnS and Nb inclusions. In the third stage, the coalesced cracks propagate along the center segregation region. From the investigation of individual crack behavior, the crack along harder region showed higher propagation rate. In the fourth stage, the crack propagation rate was decreased to be in stasis. It can be stated that crack growth in the final stage is strongly affected by the hardness of base material and the crack easily propagate when HIC occurs in high strength steels.

Validation of Adaptive Gaussian Process Regression Model Used for SIF Prediction

The main aim of this paper is to perform the validation of the adaptive Gaussian process regression model (AGPRM) developed by the authors for the Stress Intensity Factor (SIF) prediction of a crack propagating in topside piping. For validation purposes, the values of SIF obtained from experiments available in the literature are used. Sixty-six data points (consisting of L, a, c and SIF values obtained by experiments) are used to train the AGPRM, while four independent data sets are used for validation purposes. The experimental validation of the AGPRM also consists of the comparison of the prediction accuracy of AGPRM and Finite Element Method (FEM) relative to the experimentally derived SIF values. Four metrics, namely, Root Mean Square Error (RMSE), Average Absolute Error (AAE), Maximum Absolute Error (MAE), and Coefficient of Determination (R2), are used to compare the accuracy. A case study illustrating the development and experimental validation of the AGPRM is presented. Results indicate that the prediction accuracy of the AGPRM is comparable with and even higher than that of the FEM, provided the training points of the AGPRM are aptly chosen.