Issue 38

T. Lassen et alii, Frattura ed Integrità Strutturale, 38 (2016) 54-60; DOI: 10.3221/IGF-ESIS.38.07 55 I NTRODUCTION ffshore loading buoy units are often constructed with a large central turret in order to allow the floating unit to weathervane. The geostationary moored turret has a turn table with a bearing arrangement in the top. The turn table can be based on a sliding bearing concept, but for larger turrets a roller bearing design is usually preferred. The wheels are rolling on a circular rail mounted on the top of the floaters deck structure, see Fig. 1. The durability of these bearings during a typical target service life of 25 years is a matter of concern. During inspection of rails in service, surface cracks have been detected on former installations. These cracks may eventually obstruct the rotation that allows the floating unit to weathervane. The cracks are thus considered as a hazard due to the risk of encountering unforeseen loading condition if the buoy should be locked in one direction. Furthermore, the bearings are so huge that replacement in-situ will be very cumbersome and expensive. This makes the fatigue Safe Life Limit (SLL) a major design criterion. Further details are given in [1]. Figure 1 : Wheel Rail connection, Ref. [1]. The objectives of the present work are: • Study the fatigue resistance of the Martensitic-Austenitic stainless steel S165M particularly when subjected to multiaxial stress situation typical for Rolling Contact Fatigue (RCF). • Apply the Random Fatigue Limit Method (RFLM) as a supplement to conventional stare case methods that in the authors’ opinion are outdated. • Demonstrate the ability of the Dang Van multi-axial stress concept to reduce scatter in the fatigue limit. This is demonstrated by small scale testing with specimens subjected to pulsating and alternating tension. • The Dang Van based fatigue limit can be used to ensure safe life condition during the service for the offshore installation in question. This is verified by full scale fatigue testing. The practical design based on the obtained model is verified by full scale testing and the reader is referred to [2] on this subject. T HE DANG VAN STRESS CONCEPT FOR MULTIAXIAL FATIGUE he sub-surface stress situation in the contact point between wheel and rail is multi-axial and may lead to fatigue cracking under repetitive loading. It is a common hypothesis that it is the shear stress amplitude that is the key variable to the fatigue crack initiation, but also other stress contributions may play a role. The multi-axial fatigue criterion according to the Dang Van approach reads, [3, 5]:     a DV h e t t a t max          (1) where τ a is the acting shear stress amplitude (deviation from mean shear stress) at any time, whereas σ h is the simultaneous acting hydrostatic stress. τ e is the shear stress amplitude strength (fatigue endurance limit) when no other stress O T