Evaluation of Fatigue Design Curves for High Strength Steel Mooring Chain in Seawater
By Yan-Hui Zhang
Chains are used for both temporarily and permanently moored floating units for offshore oil production. The use of high strength steel R5 for mooring chain and accessories in deep water could provide considerable weight, material and cost saving when compared with the lower steel grades R3 and R4 that are conventionally used for mooring chains. However, although R5 has been used in service in a large quantity, the fatigue performance of higher steel grade chains is not well understood and it is possible that the increased static strength may result in an adverse fatigue performance in seawater. Fatigue design recommendations provided by API RP 2SK and DNVGL-OS-E301 are based mainly on fatigue data obtained from steel grades R3 and R4. Furthermore, little information is available regarding the crack propagation behaviour of mooring chain. This information is very important when planning mooring chain inspection intervals.
Industry has therefore expressed the need to obtain fatigue test data of higher steel grade chain under true operating conditions. This report describes work intended to meet this need and to establish reliable and independent fatigue test data for use in design, along with a more fundamental understanding of the potential fatigue cracking locations in mooring chains.
- The limited data found in the literature review did not suggest any significant difference in the fatigue performance between steel grades R4 and R5. The review also revealed a strong effect of mean load level on fatigue performance of mooring chain.
- Fatigue data of six full-scale fatigue tests in free corrosion (FC) seawater, three on grade R4 and three on grade R5, were obtained.
- The three fatigue curves derived respectively from grades R4, R5 and the combined results (R4&R5) agree very well in both the T-N (load ratio versus number of cycles) and ∆S-N (stress range versus number of cycles) plots - no clear effect of steel strength on FC fatigue performance for the steel grades investigated was observed.
- The slopes of these curves were close to 3, the value used in both the API and DNVGL standards. The results confirmed that the existing design curves given in API RP 2SK and DNVGL-OS-E301 can be applied to studless chain links of steel grade R5.
- Fatigue failures were observed at several different locations in a link including the transition point at the inner radius of a link, the crown region, the weld, the stamping mark and the straight bar, with most cracking occurring at the transition point at the inner radius of a link or the weld.
Fatigue test set-up in free corrosion seawater
Long fatigue crack at the transition point at the inner radius of a link
Fatigue cracking at the weld
Fracture surface showing cracking initiation from a 1.3mm deep stamp mark