Feature article: the importance of mooring chain measurement

Why is chain measurement important?

Mooring chains are designed with specific corrosion allowances to account for corrosion and wear throughout their service life. Design corrosion allowances are generally 0.2-0.4mm per year, but often higher in tropical waters. This amounts to 4-8mm total corrosion allowance for a typical FPSO mooring system with a 20-year design life. Monitoring chain diameter is a de facto performance standard for every mooring system and the accuracy and precision of measurement tools is critical when corrosion allowances are so minute.

When chains corrode and wear beyond the applicable tolerance, they may no longer meet their basic strength requirements and require replacement. Chain link strength is more sensitive to wear and corrosion in certain areas than in others, so corrosion allowances may be consumed in some areas without chain link strength being significantly impacted.

Which areas on a chain are most likely to see degradation?

Corrosion can happen anywhere on the surface of the link, but wear is generally isolated to the intergrip (double diameter) area. Corrosion rates used in the mooring design are in fact combined corrosion and wear rates, applying primarily to the intergrip area where diameter reduction is reasonably proportional to strength reduction.

Which areas of chain links are most sensitive to corrosion and wear?

There is considerable variation in tensile stress based on the geometry of the chain link. Maximum tension is developed at the crown of the link, then followed by the inner shoulder area.

As chain links wear and corrode at the intergrip area, tensile stress increases at the crown until a crack initiates. This is the most likely area for failure, therefore intergrip diameter measurement (in the plane of intergrip wear) is the most crucial measurement for tracking and managing the residual strength of mooring chains.

Which areas do you assess during a chain measurement survey?

Regular measurement of intergrip and sidebar diameter is the best way to confirm adherence to design corrosion and wear rates, as required by Class and industry best practice.

Intergrip measurements are taken in the axial direction, across the wear plane. Due to the chain manufacturing process, the axial diameter is always smaller than the perpendicular diameter, often by as much as 5%, so perpendicular diameter isn’t a critical measurement until intergrip corrosion/wear allowance is consumed.

Sidebar diameter measurements are used to track bulk corrosion rate, independent of wear. The sidebar area is easily accessible for measurement, and the rate of corrosion is usually representative of the chain link as a whole. Sidebar diameters should be measured in the plane of the link (IP) and out of plane (OP) so that cross-sectional area can be approximated. The weld area should be avoided as diameter can be 10-15% higher than the nominal rate in this area.

Older inspection codes require link length measurements to identify link elongation. For permanent mooring systems designed to 100-year return periods and modern safety factors, link elongation is not a typical concern.

What are the advantages of Ashtead Technology’s chain measurement methods?

Mooring chains can be measured with a variety of equipment, including mechanical calipers, optical calipers, and using 3D modelling/photogrammetry.

Ashtead Technology’s Chain Measurement System (CMS) is an ROV-deployed optical caliper using HD cameras and machine vision to measure critical chain link dimensions with real time results, ensuring chain corrosion and wear rates are within expected tolerances.

The optical CMS can collect intergrip length, sidebar diameters, and link length measurements all in a single ROV dive. With over 25 years of commercial use and continuous improvement, the CMS is considered the most reliable and repeatable method of monitoring mooring chain integrity offering a reduction in measurement time by a least 50%.

Ashtead Technology’s optical CMS provides:

1. Measurement of all critical dimensions in a single ROV dive (no need to reconfigure the tool between intergrip and sidebar diameter measurements)
2. Accuracy of +/- 0.5mm and high degree of repeatability (precision)
3. Data collection time as little as seconds per chain link with real-time results
4. Able to be deployed under conditions of high chain movement
5. Capable of measuring chain inclination simultaneously for verifying mooring tension and load sharing between lines

What is the recommended schedule for chain measurement activities?

Class societies require mooring chains to be measured at five-year intervals for renewal surveys. However, better practice is to perform measurement during 2.5-yearly intermediate surveys or annually on select lines, such that each mooring line is surveyed on a five-year rolling schedule. In some cases, focusing on just one line per bundle can be acceptable if all mooring lines in the group have similar dynamics, degradation, and robust safety factors.

This article was provided as a Technology Spotlight feature article for Hart Energy in April 2022 – view