Mooring Line Components With Semi-Brittle Behavior: Verification of Fitness for Purpose

Abstract

Low impact energy for Charpy V Notch (CVN) specimens and associated low Crack Tip Opening Displacement (CTOD) values have occurred on several occasions in high strength steel offshore mooring components. In the present work an Engineering Critical Assessment (ECA) for shackles has been carried out to demonstrate fitness for purpose. Typical values for CVN and CTOD are 17 Joules and 0.01 mm respectively at design temperature. The purpose of the present work is to demonstrate that even in the case where normal quality requirements are not met, the shackles may still have enough structural integrity and fatigue durability to withstand the load spectrum in the field during the planned target service life of typically 20 years. The ECA is based on applied fracture mechanics as outlined in the BS7910 document. Shackles made of QR4 high strength steel with different geometries and loading modes are analyzed. Extreme load cases and fatigue load spectra are treated and fracture mechanics modelling is discussed. A CTOD value as low as 0.01 mm may give critical surface crack depth close to 1% of the diameter in an ultimate limit state condition. For normal ductile steel behaviour the critical crack depth is usually close to 15% of the diameter. However, under the assumption that the pre-existing crack depth is 0.25 mm, the predicted fatigue life based on fracture mechanics analysis is still over 100 years. For a target service life of 20 years this corresponds to a Design Factor Fatigue (DFF) of 5. This is close to the requirement given by DNV for mooring chains. Based on the present analysis it can be concluded that the shackles are fit for purpose as manufactured even under unfavourable and unlikely assumptions.