Corrosion Fatigue Behavior of Mooring Chain Steel in Seawater

Abstract

With a majority of the reported chain failures related to fatigue, this phenomenon is one of the main topics to be studied as part of Mooring Integrity Management. Present fatigue design is mainly based on fatigue curves for chains under tension-tension loads in seawater. However, the applicability of these curves for different loading modes and specific environments remains unclear. This paper studies the fatigue behavior of the material used on chains as it builds the baseline for the performance of these mooring components. It includes uniaxial fatigue tests that were undertaken on R4 and R5 steel grades obtained from actual chains after all their manufacturing steps. Samples were not only tested in air and in synthetic seawater but different corrosion related parameters were also studied: frequency, temperature and cathodic protection. From the results of these tests, separated SN curves were obtained. Subsequently, these curves were analyzed and compared against present recommended design curves for material. Fractographic examination was undertaken to assess the effect of corrosion and cathodic protection and comparison between material and component response was also addressed. Results showed the strong synergy between corrosion and fatigue. Also, the improvement from fatigue design curves to actual response of the materials was quantified.

Avaliação de Juntas Soldadas de um Aço Fundido de Alta Resistência para a Indústria Offshore

“…As amarras são um elemento importante das linhas de ancoragem de plataforma de petróleo e, são submetidas a diversos tipos de mecanismos de desgaste que podem promover a fratura do componente [8,79]. Como a substituição de um único elo pode ser uma tarefa complexa, foram desenvolvidos procedimentos de soldagem de recuperação de elos por soldagem pelo processo eletrodo revestido com desempenho satisfatório em fadiga [21,23], que é o mecanismo mais determinante na fratura dos componentes das linhas de ancoragem [44,80].…”

Multiaxial Fatigue Analysis of Mooring Chain Links Under Tension Loading: Influence of Mean Load and Simplified Assessment

“…are compared with the alternated fatigue strength derived from the data of Fernandez et al [170] (whose data were interpolated using a slope m=3) and Arredondo et al [35]. Table 6.2 reveals that there is an important difference between the fatigue properties obtained from specimens tested under sea water flow and pre-corroded specimens.…”

“…Once the fatigue properties of steel accounting for corrosion are known, they can be used for the implementation of a fatigue criterion. Arredondo et al [35] performed fatigue testing of R4 and R5 specimens in both air and seawater at 0.5 and 10 Hz. The tests were carried out at room temperature of about 20 °C and 27 °C.…”

“…The predicted fatigue lifetime by the Dang Van fatigue criterion depends on the value of the alternated fatigue strength used for its calibration. As an example, the predicted fatigue lifetime using Perez Mora's data [8] is 14,4 times higher than predicted using Arredondo et al [35] for a test frequency of 0.5 Hz.…”

“…Different fatigue properties of R5 steel accounting for corrosion can be found in the literature. Fernandez et al[170] and Arredondo et al[35] presented fatigue testing under sea water of small specimens of R5 grade at frequencies varying between 0.5 and 10 Hz. These tests were conducted with a R ratio (defined as the minimum peak stress divided by the maximum peak stress) of 0.1.…”