TWI Collaborative Research Knowledge Summary

'ChainTest' - automated inspection of mooring chains

by Stephen Williams

Introduction
Objectives
Technical approach
Planned inspection route
Further information

EC collaborative project; title:
Autonomous robotic system for the inspection of mooring chains that tether offshore oil and gas structures to the ocean floor

Introduction

Floating platforms for plant, machinery and accommodation are being used increasingly in offshore oil and gas production. Semi-submersible drilling rigs have for some time been common in offshore exploration, but now floating structures are to be found in 'Tension Leg' platforms, 'Spar' platforms, 'Floating Production Storage and Off-loading vessels' and support structures such as 'Flotels' (accommodation) and crane barges. As oil fields are opened up in deep water at the margins of the continental shelf, production platforms that stand on the sea bed are no longer feasible; moreover, floating platforms can be moved from one field to the next, allowing smaller, marginal fields to be exploited.

Floating platforms must be moored to the sea bed. The integrity of the mooring system is critical, because the consequences of failure could be oil pollution on a catastrophic scale. Floating structures are designed, in common with all offshore structures, to survive the 15m waves derived statistically from the '100 year storm'. However, there is evidence of much larger freak waves. Also, climate change suggests that storms will become more frequent in the future, increasing the risk of damage to offshore structures. Recorded instances of damage to the mooring system of floating offshore structures already indicate a problem. The current frequency of cable chain breaks is as high as one break for every 3 years of operation. Breaks often go unnoticed, increasing the probability of multiple breaks and total mooring system failure.

There is, therefore, an urgent need to improve the integrity management of mooring systems. One issue is the inspection of chain links. Regulatory visual inspection is currently done with the cable chain brought up on deck, or, for greater sensitivity to defects using non-destructive tests (NDT), with the cable chain taken on-shore. Increasingly, interim 'swim-by' inspections of the chain in-situ are being conducted from Remote Operated Vehicles. The inspection, however, is unreliable and susceptible to human error, because of the hundreds of metres of chain involved and the short times allowed for inspection. Industry recognises the need for more effective inspection.

The EC project 'ChainTest' proposes to address these problems. As suggested in the project's full title Autonomous robotic system for the inspection of mooring chains that tether offshore oil and gas structures to the ocean floor, the approach is to develop a robotic crawling vehicle for testing the chains in situ, below water. The vehicle will be automated, and sensitive to fatigue cracks, corrosion, bent chains and abrasion.

Objectives

The scientific and technical objectives are:

• To overcome the limitations of current inspection working practices on chains used in the mooring systems for floating offshore oil and gas production platforms by -
  
  

  - obviating the need for human inspectors, therefore increasing inspection reliability;
  
  

  - developing an autonomous vehicle that will crawl along the chain below water, carrying out inspection tasks while the chain is in-situ, eliminating the need to bring the chain on board;
  
  

  - developing a cleaning system to remove sufficient marine growth and rust scale from the surfaces of the chain for inspection;
  
  

  - developing a vision system that can measure chain link dimensions and conduct visual inspection;
  
  

  - using novel non-destructive techniques that will detect fatigue cracks and corrosion with the minimum of surface preparation and probe scanning and on surfaces hidden between chain links.

Technical approach

A concept drawing of the vehicle is shown in Figure 1. The vehicle will deploy novel inspection techniques developed as part of this project to enable inspection of mooring chains in situ. The chains may be covered with marine growth and corrosion scale, but the on-board cleaning techniques will be able to remove enough of this material to allow NDT to be carried out.

The inspection vehicle will be able to inspect mooring lines, in particular the chains for:

1. Corrosion and abrasion at surfaces hidden between interlocking chain links;
   
   
2. Fatigue cracks;
   
   
3. Bent chains;
   
   
4. Elongated chains.

Furthermore, the system will have defect sizing capability and will be able to detect and size defects.

Fig.1. Concept drawing of Chain-test

The lengths of chain which require most inspection are:

• near the turret of floating platform mooring systems, around which the vessel rotates with tides and where most damage is caused by abrasion;
  
  
• in the splash zone where wave action and corrosion is most prevalent;
  
  
• near the anchor points where, again, abrasion is common.
  
  

The project will focus on tackling the problem of cracks, and corrosion will not only be detected but also its severity will be assessed so that a decision can be made as to whether to replace the mooring immediately, carry out a repair, or leave in place with a monitoring regime.

The inspection will provide a 'fingerprint' of the mooring line including data from innocuous flaws as well as unacceptable defects. This can be compared with future inspections to indicate any degradation.

Planned inspection route

The inspection techniques to be employed will fall into three categories: visual inspection; Advanced Alternating Current Field Measurements (ACFM); and Ultrasonic Guided Wave (UGW) testing or low frequency resonance techniques.

Visual inspection is currently used to reveal gross defects such a chain link distortion. A general standard specifies the limits of length within which a 5-link section of chain should fall (under a specific calibration loading). It will be the responsibility of the on-board vision system to detect this. The advantage this has over existing procedures is that 100% of the chain can be inspected. However, the main innovations of this project concern the deployment of advanced NDT methods.

An ACFM probe and system will be developed that can detect surface fatigue cracks on the weld of each chain link below thin marine deposits. The main innovation is increasing the capability of the technique to detect cracks on the chain surface and measuring the depth of these cracks without electro-magnetic interference from other parts of the chain in close proximity; see Figure 2.

Fig.2. Stud-less chain link illustrating planned inspection route (arrows indicate path of guided waves)

The large surface area and complex shape of a chain link makes it impracticable to attempt 100% coverage with ACFM. A non-specific 'sorting' method is needed that will determine whether a significant structural change has occurred. If this is the case, the link will be subject to visual inspection and ACFM. If the chain gives a 'normal' response, the crawling vehicle will move onto the next chain link. For this purpose either UGW testing or first order modal vibrations (low frequency resonance testing) are to be employed.

UGW testing is generally used between 10kHz and 300kHz and its present commercial application is pipe testing. In this project, the technique will be modified for the application of rod waves which are relevant for chain links. The research will begin with numerical modelling of a single link and continue with experiments to investigate the response of the link to different excitations to determine the optimum test conditions. The model will then be refined in stages so that eventually the behaviour of a multi-link system under loading is known.

The ability to detect defects in the region where two links overlap, which would be inaccessible by shorter-range methods, will be assessed. Whether UGW or first order modal vibrations are to be employed, both will provide information about the whole link from a single point as illustrated in Figure 2.

Further information

TWI's collaborative projects

Non-destructive testing at TWI

NDT Research at TWI

NDT technology group at TWI, email: ndt@twi.co.uk