Inge Lotsberg

Hot Spot Stress S-N Curve for Fatigue Analysis of Plated Structures

Finite element analysis is being used by designers for fatigue assessment of structures. It is therefore important that a proper link between calculated hot spot stress and fatigue capacity is established. The fatigue capacity is expressed as a hot spot stress S-N curve. This paper presents a derivation of a hot spot stress S-N curve to be used when the hot spot stress is derived from finite element analysis of plated structures. The hot spot S-N curve is linked to the methodology used for finite element analysis of plated structures that is being included in design recommendations for fatigue assessment of welded structures.

Stress concentration factors at circumferential welds in tubulars

Abstract In this paper analytical expressions for stress concentration factors due to fabrication tolerances are derived for a number of design cases based on classical shell theory. These cases include butt welds in tubulars, welds at ring stiffeners and bulkheads in shell structures, welds at conical transitions without and with ring stiffeners and tubulars with colinearity subjected to axial tension in risers and tethers of tension leg platforms.

Fatigue Test Data for Welded Connections in Ship-Shaped Structures

Five different specimens representing different typical welded connections in ship-shaped structures were fatigue-tested by HHI in Korea. The test data from the five different specimens for one loading condition were transferred into one hot spot S-N diagram. It was observed that two of the data series plotted rather high in the S-N diagram as compared with the other three test series. The reason for this was questioned. It was decided to test a few specimens at another test laboratory for verification of the test data. The same types of specimens were fabricated by HHI and shipped to Oslo and fatigue-tested in the laboratories of DNV. The results from this fatigue testing are presented in this paper in addition to the original fatigue test data from HHI.

Overview of the FPSO: Fatigue Capacity JIP

This paper presents an overview of a joint industry project carried out in the period 2001 – 2003 to address the Fatigue Capacity of FPSOs. Numerical investigations and fatigue testing is performed in order to improve the accuracy, robustness and efficiency of finite element modeling and hot spot stress evaluation for typical FPSO details. Design criteria will be developed for fatigue crack growth from the root of fillet welds in typical details in FPSOs including fillet welds around attachments and fillet welds around pipes penetrating deck plates. Fatigue analysis methods for FPSOs with guidelines on determination of side pressures in the water line will be developed. This work will be supported by assessment of actual measurement data from the field and testing in a tank. Methods for planning in-service inspection of FPSOs based on a fracture mechanics and reliability analysis will be developed. Also a procedure for repair of a fatigue crack while the FPSO is on station in the field will be developed.Copyright

Background for Revision of DNV-RP-C203 Fatigue Analysis of Offshore Steel Structure

The last revision of the DNV recommended practice “Fatigue Analysis of Offshore Steel Structures” is from October 2001. During use of this standard some feed back from designers around the world have been received. Also some new research in the area has been performed in the time interval from it was first developed. It is also realised that the document is being used for fatigue design of some other types of details and structures than was thought of when the document was originally developed. Therefore it was now found convenient to revise the document to incorporate the experience gained and new research and developments made in the area of fatigue of offshore structures the last 7 years since the main content for this recommended practice was developed.Copyright

Fatigue life improvement of welded doubling plates by grinding and ultrasonic peening

Fatigue life assessment is important for all floating offshore structures, both not only related to the new building stage but also related to lifetime extensions. Fatigue cracking occurs normally due to uncertainties in estimated fatigue life and this is a well-known problem for floating structures. In this paper, the aim has been to look deeper into the effects of grinding and ultrasonic peening (UP) in order to improve the service life of structural connections. Fatigue testing of full-size fillet-welded doubling plates has been performed. The paper presents results from these tests including as-welded condition and ground and UP improved specimens. The paper also presents alternative S-N curves for improved details. The primary goal has been to assess the possibility for fatigue life improvement of fillet-welded doubling plates where fatigue cracking may initiate from the weld root. The information gained in this project is considered to be important for the offshore industry working with floating structures as well as for development of new fatigue assessment procedures and requirements in standardisation.

Fatigue Capacity of FPSO Structures

This paper presents an overview of a joint industry project, addressing the problem of fatigue capacity of FPSOs (FPSO=ship shaped structures used for floating production storage and offshore loading). Spectral methods are becoming standard methodology for fatigue assessments of FPSOs. This methodology is well suited to numerical methods in combination with finite element representation of the global structure and structural details for response analyses. A proper link between calculated stress and fatigue capacity is required in order to achieve a reliable design. The project has generated significant amounts of numerical data as well as laboratory fatigue test data of typical ship details to improve the design basis for this. An overview of this project together with some links to the main results and recommendations from this project will be presented in this paper.

On the Derivation of Design S-N Curves Based on Limited Fatigue Test Data

Qualification of new characteristic S-N curves for fatigue life assessment of structures is considered to be a significant engineering challenge. First, representative fatigue test data for the actual structural connections have to be derived. Then these test data have to be transferred into characteristic S-N curves that represent a predefined probability of survival. Characteristic S-N curves are also often denoted design S-N curves as these curves are often used directly for fatigue life assessment of structures without application of a material factor. A few large scale tests can add significant confidence to a design S-N curve dependent on the type of structural detail to be designed. The reason for this is that a prototype test specimen can be fabricated in a similar way as the actual connection and it is similar in geometry, material characteristics, residual stress, and fabrication tolerances. In addition it can likely be subjected to a more relevant loading and boundary conditions as compared with that of small scale test specimens. When a limited number of test data are available, it is questioned how a characteristic S-N curve can be derived with a well defined probability of survival. The mentioned issues are further considered in this paper together with some recommendations on how to derive design S-N curves based on limited data.Copyright

Fatigue Testing and S-N Data for Fatigue Analysis of Piles

Design S-N curves in design codes are based on fatigue test data, where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further, it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. In 2006, the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.

A New DNV Recommended Practice for Fatigue Analysis of Offshore Ships

During the last 10–15 years the industry has put significant focus on fatigue analysis methodologies for offshore ships. The reason for this is a larger cost consequence associated with fatigue cracks in these ships compared to traditional tankers. During these years DNV has gained experience from classification of offshore ships, experience from a number of detailed fatigue analyses of ships and FPSO’s and recommendations achieved through the joint industry project: “Fatigue Capacity of FPSOs”. Based on this DNV has found it appropriate to develop a Recommended Practice (DNV-RP-C206) for fatigue design of floating production, storage and offloading units and to link this document to classification services offered by DNV. The methodology can also be applied to other types of offshore ships. Some of the background and content of this Recommended Practice is presented in this paper.Copyright