Agrell, Christian (DNV GL Pipelines and Materials) | østby, Erling (DNV GL Pipelines and Materials) | Levold, Erik (Statoil ASA) | Hauge, Mons (Statoil ASA) | Bjerke, Steinar Lindberg (DNV GL Pipelines and Materials)
Design against fracture in pipeline girth welds deals with hypothetical defects. A logical step would be to apply probabilistic assessment procedures including an assumed distribution of weld flaws. The major part of fracture assessments carried out today is semi-deterministic. Even if such an approach might be applicable, it is proposed that it may not always lead to optimum result in terms of combination of safety level and cost-effectiveness. The risk of girth weld fracture is influenced by several parameters like weld defect size, tearing resistance, weld metal mismatch, misalignment, yield to tensile strength ratio, etc. The likelihood of the most detrimental values of each distribution occurring at the same position is usually very low. Further, fracture assessment of pipelines often requires that the different steps in the loading history are considered in the analysis. In this paper we illustrate how probabilistic assessment can be used to obtain the overall reliability of pipelines wrt girth weld fracture. We discuss statistical representation of the different key input parameters and how the analysis can be used to determine characteristic values to be used in design assessment to meet target reliability levels. Further, we illustrate how the procedure can be used to investigate the significance of previous load steps, such as the effect of installation loads on the strain capacity during operation.
The field of strain-based design of pipelines, with special focus on tensile strain capacity due to potential defects, has received considerable attention the last 10-15 years. The ISOPE SBD symposium, having its 10th anniversary at this year’s conference, has been one of the main arenas for exchange of ideas in this respect. In this period there has been made significant contributions and developments within the field. An improved understanding of the main physical features controlling pipeline failure has been established, and several models for estimation of tensile strain capacity have been proposed, (see e.g. Østby (2007), Fairchild et al. (2014), Tang et al. (2014), Wang et al. (2012)). As a part of this development, increased use of FEA, both w.r.t. obtaining new knowledge and to development of quantitative models, has taken place (see review paper by Østby (2015)). There have also been several large-scale testing campaigns to validate models, and in general the models have been found to very well reproduce the average trend in the experimental results. It is underlined that there are still uncertain areas, e.g. embedded defects, where there is a need for a better understanding of the phenomena and how to include this in models. However, in general it is fair to say that we are in possession of quite accurate models for assessing tensile strain capacity of pipelines with defects.
Considering the future development for offshore pipelines, moving towards difficult operating condition and deep/ultra-deep water applications there is a need to understand the failure mechanisms and better quantify the strength and deformation capacity of corroded pipelines considering the relevant failure modes (collapse, local buckling under internal and external pressure, fracture / plastic collapse etc.).
A Joint Industry Project sponsored by ENI E&P and Statoil has been launched with the objective to quantify and assess the strength and deformation capacity of corroded pipes in presence of internal overpressure.
In this paper:
Østby, Erling (SINTEF Materials and Chemistry, Trondheim, Norway) | Nyhus, Bård (SINTEF Materials and Chemistry, Trondheim, Norway) | Hauge, Mons (StatoilHydro ASA, Trondheim, Norway) | Levold, Erik (StatoilHydro ASA, Trondheim, Norway) | Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
Østby, Erling (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry) | Sandvik, Andreas (StatoilHydro ASA) | Levold, Erik (StatoilHydro ASA) | Thaulow, Christian (Norwegian University of Science and Technology)
In this paper the results from SENT testing of two different welding procedures using an X65 base material is presented. The first welding procedure yields close to evenmatch conditions, whereas the second welding procedure gives 10-15% overmatch compared to the base material. Both defects lying in the weld metal and on the fusion line are investigated. It is observed that the ductile tearing resistances in both weld metals are significantly lower than for the base material. The resistance curves measured for the fusion line defects are more similar to the base material curve, however, slightly different crack growth is obtained depending on which side of the defect the measurements are performed. The crack driving force and strain capacity are on average higher in the overmatch specimens. However, a significant scatter is observed, especially for the weld metal defects. For the fusion line defects the scatter is smaller. For the material systems investigated the strain capacity will on average not depend strongly on the crack position.
Defects may limit the tensile strain capacity of pipelines. Such defects are mainly found in relation to girth welds. Mismatching in weld metal (WM) stress-strain properties compared to the base material will lead to a modification of the crack driving force as a function of the applied strain. It is common practice to specify overmatch conditions in the weld metal in order to shield or reduce the deformation in this region. However, overmatch can be difficult to obtain in some cases (e.g. for very high strength steels). Another aspect is related to the larger scatter in material properties usually found in weld metals. Also, the ductile crack growth resistance will in many cases differ between the weld metal and the base material. Although not without exceptions, the metallurgical conditions in the weld metal will usually lead to a reduced crack growth resistance compared to the base material of the pipe.
Global buckling of submarine pipelines may happen when the expansion due to temperature and pressure in the pipeline is restrained by the pipe/soil resistance. Global buckling may occur for both trenched pipelines (upheaval buckling) and for exposed pipelines (lateral buckling). Global buckling has been a major offshore pipeline design topic for High Pressure/ High Temperature (HPHT) since the mid 1980s, when the first buckles observed.
In 1996 the Hotpipe project was initiated by Statoil with the purpose of developing design criteria for global buckling. In 2001 the first revision of the Hotpipe Guideline was issued and in 2007 this was published as a recommended practice DNV-RP-F110. In 2002 another initiative was launched with the Safebuck Joint Industry Project to carry out research and develop global buckling criteria for exposed pipelines. This work was intended to complement the work performed by Hotpipe by addressing some of the issues more relevant to deepwater flowlines. The latest revision of the Safebuck Guideline was issued in Dec 2008. Several papers have in parts been discussing global buckling in the past but only two documents provide consistent design guidance; the public DNV-RP-F110 (outcome of the Hotpipe project) and the Safebuck JIP Design Guideline (confidential to JIP participants). In 2009 Safebuck and DNV initiated a process with the purpose of merging these two documents. The advantages of this merger will be several; taking the best parts from the two documents, will remove the confusion in the industry with two alternative design guidelines. The combined document will be published in the public domain as a new revision of DNV-RP-F110.
This paper will discuss the individual advantages of the design concepts in the two different codes and the benefit of combining these. Finally, some elaboration will be given on the structure of the future Safebuck Guideline that may unify how HPHT pipelines will be designed in the future.
The Åsgard Transport System is a 707-km-long, 42-in pipeline for the export of rich gas from the Åsgard field offshore Mid-Norway to the Kårstø onshore terminal on the southwestern coast. The pipeline route traverses the western coast of Norway, including the Norwegian Trench, with a water depth varying from 55 m to 370 m. As a result of high pressure and high temperature in combination with an uneven seabed in the northern section of the pipeline, the potential for large-scale seabed intervention work to control thermal buckling and reduce the effects of trawl pullover loads was initially identified. This paper presents the main aspects leading to the optimised and cost-effective design for control of thermal buckling and expansion. INTRODUCTION The Åsgard Field is located 200 km off the coast of Mid- Norway (Fig. 1). The field comprises 60 subsea wells producing gas, oil and condensate from 3 different reservoirs: Smørbukk, Smørbukk South and Midgard. In addition to the numerous subsea templates and flowlines, field development involves a production ship for oil and condensate (Åsgard-A), an offshore loading and storage tanker for oil (Åsgard-C), a semisubmersible (Åsgard-B) for gas treatment, and an export riser base (ERB), which is the tie-in point for the 707-km-long, 42-in Åsgard Transport pipeline. This runs to the Kårstø terminal. Eight subsea Tee structures have been installed in-line with the 42-in gas trunkline in order to facilitate future connections for potential gas fields along the 684-km-long offshore route. Already, 2 of these subsea connection points have been utilised for tie-in of export gas pipelines from the Norne, Heidrun and Draugen fields. Some of the main technical challenges for the structural design of the Åsgard Transport (ÅT) pipeline were: