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TWI JIP: Integrity of mooring chains using acoustic emission

Mon, 17 December, 2018

Floating structures such as drilling rigs, spar platforms, and floating, production, storage and offloading vessels (FPSOs) are used extensively in the offshore oil and gas sector.  Increasingly, floating structures are also used for offshore wind turbines in the renewable energy sector.  The mooring chain is one of a floating structure’s most critical components because it is used to anchor it to the seabed and deployed in very deep water.  As such, maintaining and safe-guarding the structural integrity of mooring chains is a high priority because the effects of incremental or immediate damage could result in catastrophic failure of the asset.

One of the ways in which industry is currently tackling how mooring chains efficacy can be improved is with designs that utilise larger links or higher strength materials.  However in parallel, there is a need for technologies that can predict the early stages of damage in mooring chains and shackles.  Therefore, based on many years’ research and development into enhancing the lifespan and performance of mooring chains, TWI has identified that new structural health monitoring (SHM) techniques, specifically deploying acoustic emission, would be ideally suited to addressing the issue.

As a result, TWI is launching a Joint Industry Project (JIP): Structural Health Monitoring of Mooring Chains using Acoustic Emission which will have its inaugural meeting in early 2019.

The project will develop acoustic emission testing (AET) procedures that will be qualified on full-scale, chain tensile and fatigue tests at a realistic loading frequency, backed by extensive material properties characterisation.  The testing will be conducted on TWI’s world-class mechanical test rig facility, both in and out of sea-water, and the project will establish reliable acoustic emission (AE) test data for a more fundamental understanding of the factors affecting chains failure.  Furthermore, the AET procedures will then be used as a tool in mechanical tests to gather information that is not currently available, about crack initiation and growth in chain links.

Tensile tests performed on chains at TWI have indicated that AET is more sensitive to crack initiation than other non-destructive testing (NDT) methods.  Subsequently, the results of the AET procedure qualification could become the basis for a Requirements Specification, incorporated into the production process of bespoke equipment used for testing chains in situ.  Longer term, this would help to reduce costs, and improve the quality, safety and performance of mooring chains, benefitting both users and manufacturers, and meeting the requirements of regulatory bodies.

In summarising the background to the JIP: Structural Health Monitoring of Mooring Chains using Acoustic Emission, Angela Angulo, Team Manager, Condition and Structural Health Monitoring at TWI explains:

“TWI has a strong track record in offshore asset integrity management, having been involved in several floating, production, storage and offloading vessels (FPSO) mooring failure investigations, and instigated a number of projects to understand crack initiation and growth in mooring chains.  One of industry’s current problems is the inability to detect crack initiation, so by the time a crack is identified, typically with ultrasonic NDT or magnetic particle inspection, as two of the most practicable methods used to assess chain links, the cracking is well established.”

“Specifically at TWI, we have in-depth experience of in-service inspection of FPSO mooring chains using advanced ultrasonic and alternating current field measurement techniques.  TWI has also developed a prototype chain climbing robot and remotely operated vehicles for guided ultrasonic wave testing of mooring chains.”

“TWI has expertise in developing acoustic emission testing (AET) procedures for a variety of structural health monitoring (SHM) applications, with particular emphasis on software algorithms for extracting SHM information from noisy acoustic emission (AE) data.  Applications have included experiments in collecting AE data during in-sea water chain tensile and fatigue tests, the methods from which will be formalised into this project as full AET procedures qualified in accordance with guidelines set down by the European Network for Inspection Qualification (ENIQ).”

Angela concluded saying “The project aims to develop AET procedures for detecting crack initiation and growth in mooring chains, based on previous work undertaken on mechanical chain testing rigs in the laboratories at TWI.  These processes will use laboratory equipment but will be developed to a level where they can be used in bespoke chain testing equipment in-situ for real-time SHM of mooring chains.”

For more information about the project and how your company or organisation can become involved, please email: or telephone +44 (0) 1223 899000.

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