Fatigue of Mooring Chain in Air and Water - Results and Analysis

Abstract

Abstract The paper will present the results of the joint industry study into the fatigue resistance of large diameter anchor chain', and discuss the interpretation of the results. The study represents the largest body of well documented corrosion fatigue tests on a variety of different chain grades and sizes. The analysis & the results use the available fatigue data, including the life of tested but un-failed Iinks, to determine suitable fatigue curves for mooring lines of different lengths. Many of the findings were to be expected, based on past data, but there were some surprises with respect to chain grade. Fracture surface examination indicate that the fatigue cracks had propagated across a large percentage of the section prior to final tire. Post test inspection of un-failed links indicate that many cracks had initiated prior to failure. Introduction In 1989 Noble, Denton and Associates, Inc. (NDAI) launched a joint industry study into the fatigue of mooring anchor chain. At the time there was very little fatigue data available, and almost no consistent sets of data comparing different parametersat realistic mean loads and dynamic load ranges. The main study reached completion in December 1992 when National Engineering Laboratory (N EL) completed as much of the test program as they could within the budget provided. The program consisted of sets of air and water fatigue tests. All of the air tests, and two of the corrosion tests were carried out in a 'Pre-Test' program. This was set up to determine priorities for the main test program performed at O.7Hz, except for two performed at 0.2 Hz. The main test The sea water tests were benchmark tests that were program entailed only sea water tests. In total, there were seven air tests and nine sea water tests, producing 49 fatigue failures and 66 test run out It appeared from the last test of the main test program that there was a disproportionately low life for the 4' K4, in water, which had not been expected based on the results of the pretest program. In the pre-test program, conducted in air, it appeared that it would be reasonable to plot the fatigue life of K4 on the same Iine as ORQ chain, but using K4 break in strength. (This is contrary to what is said in API RP 2FP12, which was, it was assumed, being conservative because of a lack of data.) By contrast, the corrosion fatigue test results from the main program suggest that the fatigue life of K4 is at the 'lower end' when it is plotted with respect to ORQ chain breaking strength. For this reason, it was suggested that additional tests be run on 3' and 4' K4 chain, The extension study entailed a further four corrosion fatigue tests.