Finn Kirkemo

Structural Design of Metallic Components in ISO 13628-7

ISO 13628-7 (ISO-7) for completion and workover (C/WO) riser systems was published in 2005 and adopted back by API as API RP 17G 2nd edition 2006 (identical). ISO-7 gives requirements and recommendations for the design, analysis, materials, fabrication, testing and operation of C/WO riser systems. This paper provides a brief introduction and background to some of the design and material requirements in ISO-7. The main focus is on requirements for calculation of static and cyclic (fatigue) capacities of metallic components in the C/WO riser system subjected to pressure and external loads. Some differences between ISO-7, API 6A (6A), API 6X (6X), API 17D (17D), ASME VIII Div. 3 (Div.3) and ASME VIII Div.2 (Div.2) are also included.Copyright

Fatigue Capacity of Steel Cylindrical Bodies and Conduits Subjected to Cyclic Pressure

This paper presents a code review (API 17G, ASME VIII Div 2 and ASME VIII Div 3) for addressing the fatigue capacity of steel cylindrical bodies and conduits subjected to cyclic pressure only. The fatigue capacity for pipes with a yield strength of 75ksi and 90ksi and with rated working pressures (RWP) ranging from 5ksi to 30ksi have been considered using both the S-N approach and the fracture mechanic approach. The S-N based fatigue lives from API 17G are found to be much longer than the corresponding S-N based fatigue lives from ASME VIII Div 2 and Div 3 and by the fracture mechanic (FM) approach as required in ASME VIII Div 3 for vessels where a leak-before-break condition can not be demonstrated. The S-N predicted fatigue lives are found to decrease with increasing RWP while the FM based fatigue lives are found to be rather independent of the RWP. The S-N based fatigue lives from ASME VIII Div 2 and Div 3 for free corrosion conditions are found to be shorter than the corresponding FM based fatigue lives for RWPs ≥ 25ksi and ≥ 20ksi, respectively. Based on this work, it is recommended to establish the fatigue capacity of steel cylindrical bodies and conduits subjected to cyclic pressure using either ASME VIII Div 2 or Div 3. The FM approach is considered to give a lower bound fatigue life as the number of cycles to initiate a crack is disregarded. Guidance on when it is considered applicable to use the API 17G criterion is given in the conclusion section of this paper.Copyright

Fatigue Assessment of Threaded Segment Specimens Based on the Weakest Link Theory

In this paper, the weakest link approach has been used to estimate the fatigue life of threaded segment specimens based on fatigue test data of smooth specimens of the same material. The segment specimens are cut outs from an ACME threaded component with a thread geometry typically used in subsea connectors.Fatigue life estimates of the threaded specimens were also made by means of the local stress approach which is often the prescribed method in standards and recommended practices, such as DNV-RP-C203. This method uses the von Mises stress range at the most severe location of the specimen when estimating the fatigue life.The estimated S–N curve based on the weakest link approach shows good correspondence with fatigue test results of the threaded segment specimens. The estimated S–N curve based on the local stress approach is well below the S–N curve for the threaded segment specimen. A factor between two and three on stress is observed. Hence, the inherent level of conservatism related to the local stress approach when assessing the fatigue strength of components with very high stress concentration factors is considered to be high.Copyright

Fasteners in Subsea Applications: End User Experiences and Requirements

Fasteners used for critical applications subsea represent a challenge in terms of material selection, quality control, traceability and documentation for manufacture and delivery to ensure adequate performance, integrity and avoidance of costly failures. The definition of critical subsea fasteners are both pressure containing and primary load bearing members, but may in a wider sense also include fasteners which may affect functionality of valves, equipment and instruments for safe, reliable and cost effective operation.The major failure mechanism experienced during the last decades is so-called hydrogen induced stress cracking (HISC) or hydrogen embrittlement (HE) primarily resulting from the hydrogen charging conditions of the cathodic protection (CP) system. The main bulk of fasteners used subsea is of low alloyed steel grade type (e.g. ASTM 320 Grade L7, L7M, L43 bolts with ASTM 194 Grade 4, 7M, 7 nuts) which relies on electrical continuity to the cathodic protection system to avoid sea water corrosion, but may be prone to HISC/HE if the fastener hardness and strength level exceed the specified and established limits. If higher strength materials are needed from design, either cold formed or precipitation hardened CRA’s are usually selected (e.g. Grade 59, 660, 625, 686, 718, 725), but these may also be susceptible to HISC/HE depending on strength level and manufacturing practice.The present paper will give some field experience and typical examples of recent HISC failure case histories for low alloyed steel grades, but also for some CRA type of subsea fasteners. Furthermore, some in-house low alloyed steel grade (L7, L43) fastener HISC test results will be presented to indicate the robustness of today’s hardness and strength level restrictions, and at which loading and pre-tension level a given strength grade may be prone to HISC failure. Finally, some experience from manufacturing stage as well as focus areas in materials specification, quality assurance / quality control and design will be discussed.Copyright

Fasteners: Strength and Quality Requirements

Heavy hex nuts may be selected with similar hardness to the studs to avoid stripping of nut threads for pressure containing (closure bolting) and primary load bearing bolting. However, industry specification for subsea production systems, e.g. such as API Specifications 6A, 16A, 16C, and 17D, allows the use of low strength heavy hex nuts, e.g. ASTM A194 Grade 2HM or 7M, to be applied together with high strength studs, e.g. ASTM A320 Grades L43 or L7. In the refining and chemical industries the common practice is to use ASTM A193 B7 studs with 2H nuts. Calculations and testing of nuts have been performed and capacity formulas have been established for nut structural capacities. Guidance is given on selecting nut strength/hardness to avoid stripping of nut threads. ASTM specifications give minimum quality requirements during manufacture. A brief review of standard quality requirements is given and guidance for additional requirements for high integrity fasteners in order to have equivalent quality as pressure containing forgings is given. The results from this paper may be used as background for requirements in code updates and purchaser specifications.Copyright