Structural integrity assessment of shackle in subsea tether arrangements

Abstract

Inspection images gives an impression that the H-link shackles are subjected to a combination of pitting, crevice and fretting corrosion. All these relate to localized corrosion. There are several coating breakdowns areas and which might be experiencing severe corrosion. However the difficulty of detecting material degradation and uncertainty/variance of corrosion pattern proves difficult for providing a generalized guideline for the integrity assessment of similar types of systems The overall goal of this study was to find out how corrosion effect the structural integrity if the shackle connected to a tether system. The theory part of the report includes a study of basic different types of corrosion, contact mechanism, and detecting yielding and fatigue under different corrosion condition. This was done by drawing a first model represent the fabricated shackle .This model is used to observe how the shackle react to the forces prior of any corrosion. The method used to simulate the corrosion was to change the dimension of the shackle according to the amount of material lost. A third model was made to demonstrate the effect local corrosion combined with uniform corrosion. A numerical and analytical analysis was done to compare the results. The crack formation is the most critical one, showing that the number of cycles to failure is greatly reduced and the pin loses 90 percent of its capacity. This means that if the shackle in field is design to withstand 20 year in service life. The shackle can fail in 2 year in presence if cracks according to the results obtained in this work