Chain wears, corrodes, and degrades. It is designed to deteriorate and intended to have an end of service life. But how do we track this? What methods are out there and what are the pros and cons of each? Understanding the current condition and deterioration rate of the entire mooring system is vital for cost-effective and safe operation of any floating asset – today we look at five different methods for measuring and inspecting one of the most common components of a mooring system, chain.
Visual Inspection: Visual inspection of chain is useful for determining if the chain is still attached and gross deformation. More often than not there is a thick layer of marine growth that obstructs the view of corrosion and pitting. There is evidence to suggest that this marine growth can slow the corrosion process by limiting oxygen access to the bare metal. However this is only a factor when compared to areas that are cleaned regularly (consistent rubbing against other structure or touchdown zone moving in and out of the seabed).
Go/NoGo: This has been a widely accepted industry tool for years. A board or steel tool with an opening cut to the size of minimum rejection criteria. The tool is placed over bar stocks and/or intergrip locations to determine if you are beyond rejection criteria. If it goes there has been too much material loss, if it doesn’t there hasn’t. The tool is highly dependent on placement accuracy and often times it is pressed on splaying out the arms.
Mechanical Caliper: As standard as a Vernier caliper for the sea it is operated as such. This tool is a large improvement in terms of data collection and information usefulness. An ROV mounted caliper is capable of determining the actual dimensions of a chain link. This information is useful for tracking material loss over time, determining pit depth, and predicting when the chain is most likely to cross the rejection criteria before it happens.
Optical Caliper: Much like the Mechanical Caliper this tool is useful for gather actual dimensions of chain links. The upside of an optical caliper is threefold: 1) you gain a close visual inspection of the links during measurement taking 2) there is record and certainty that the tool is aligned correctly 3) it is much faster than a mechanical caliper saving boat time
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3D Model of Chain Link with Corrosion
3D Modeling: For the most detailed understanding of link geometry 3D Modeling is the best tool for the job. A highly accurate model is hard to beat in terms of quantifying link strength. Using a 3D Model, the links, intergrip, length, barstock, perpendicular measurements at the crown, cross sectional area, volume loss, and residual strength (using FEA) can be determined.
Pros Cons Visual Inspection Fast, Easy Not much information gathered Go-no-go Cheap Not much information gathered, Often incorrectly placed, Bad data Mechanical Caliper Highly accurate, Useful for any component Depends on placement, Requires light cleaning, Takes more time than Optical Caliper Optical Caliper Fast, Accurate, Visual record, Close inspection Requires cleaning 3D Modeling Highly accurate, Record of full link geometry Can take more time than Optical Caliper
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