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DETECTION OF SAFETY CRITICAL CRACKS AND CORROSION IN SHIPS USING NOVEL SENSORS AND SYSTEMS BASED ON ULTRASONIC LINEAR PHASED ARRAY TECHNOLOGY

Final Report Summary - SHIP INSPECTOR (Detection of safety critical cracks and corrosion in ships using novel sensors and systems based on ultrasonic linear phased array technology)

Structural failure is a major cause of the wreckage of ships, vessels and tankers and causes loss of life and pollution of the seas and coastal waters of Europe as follows:

In 2004, the total amount of oil and oil products transported by sea reached the two billion-tonne / year mark (more than 40 % of total maritime traffic). The European Union (EU) accounts for 27 % of this traffic, with 90 % of Europe's oil arriving by sea. 2.5 billion tonnes of oil is used around the world every year, and 3 million tonnes is discharged every year into the oceans as a result of oil carrying vessel and tanker failures. Tanker accidents typically account for 12 % of all oil pollution.

Structural health monitoring (SHM) of safety critical engineering structures has seen a significant rise in recent years and guided wave ultrasonics (GWU) technology has been a valuable tool of choice. Inspection techniques employing GWU have been developed for tubular structures using various transducer types and are currently applied successfully in pipes. The focus of SHIPINSPECTOR is placed on large stiffened naval structures, such as in double hull oil tankers, and the development of defect detection methods for these large stiffened steel plates.

Project context and objectives:

Shipyard work has an injury accident rate more than twice that of construction and general industry.

Currently, inspections are largely carried out manually in dry dock periods by operators in the 9000 small and medium-sized enterprises (SMEs) working in these hazardous conditions. Current methods of inspection of these welds for cracks and other large areas for corrosion have major drawbacks:

- They require the vessel, ship or tanker to be dry docked, emptied and cleaned with a consequent two week disruption. Each day at dry dock currently costs around EUR 50 000 to the ship / vessel / tanker operator.
- Feedback from the industry indicated that prior warning of corrosion in ship plating before a vessel is dry-docked for general refurbishment and repair will lead to savings by ensuring that replacement plates and fabrication crews are available as the vessel is docked and the down-time of the vessel may be minimised.
- The inspections are mainly visual and therefore subjective with no reliable hardcopy results. They are also prone to operator fatigue. Operators and surveyors often have to work in confined conditions and can also be exposed to hazardous conditions e.g. toxic gases, working through abseiling on ropes and via scaffolding - owners / operators require to receive status reports of the area monitored in order to identify when degradation is occurring in service. It will certainly add value to know the potential areas of concern for detailed examination when the relevant parts of the vessel are accessed for local inspection.

The test is conducted manually, is slow and only provides a limited amount of data, namely about whether the weld is acceptable or not to the design specification. Information about trends in the quality of welding that may be indicative of future defects is not available and in any case cannot be gathered soon enough to influence the next weld.

It was reported in 2006 that each year over 400 ocean going ships sink, many as a result of weakened structures due to corrosion and inadequate / poor welding quality. Therefore, the importance of the inspection and monitoring of defect prone areas to detect and highlight abnormalities adds significant value directly to the ship maintenance industry.

The main aim of the SHIPINSPECTOR project was to apply long range ultrasonic testing (LRUT) methods to develop a monitoring system for in order to detect defects in ship hull structures. Sensor arrays permanently attached to the surface storage tanks may be activated periodically while at sea by an automated system. Ultrasound will propagate through the structure and interact with volumetric features, such as gross corrosion at the waterline, welds, stiffeners and so forth. This interaction will be monitored over time. This continuous monitoring method will allow small changes in the structure to be detected, thereby providing the operator with an indication of the condition of each tank, without the need for it to be emptied or entered.

This information could be used by the ship operator to better prepare for the dry-docking process, by targeting maintenance tasks more efficiently, and allowing replacement components to be arranged ahead of time. It is foreseen that this will significantly reduce the time ships spend in dry docks, with an obvious associated cost saving

To accomplish the project objectives, the work activities have been organised into a number of discrete work packages (WPs). Those were divided into research (WPs 1 to 5), demonstration (WP6), dissemination and exploitation (WP7), development of guidelines (WP8) and management (WP9).

The description for those WPs are shown below:

WP1 - Requirements and characteristics from SMEs and research and technological development (RTD) performers
WP2 - Develop advanced UT flaw detector and UT linear phased array
WP3 - Develop methods for linear phased array techniques
WP4 - Signal analysis and software development
WP5 - Design and analysis of load application apparatus for linear phased array
WP6 - Experimental study and evaluation
WP7 - Training / dissemination / exploitation of enhanced LRUT testing systems
WP8 - Development of guidelines
WP9 - Project management.

Project results:

The details of technical and scientific results for each WP are presented as follows:

WP1 'Requirements and characteristics from SMEs and RTDs is 100 % complete at the end of the project. There were two deliverables re-submitted for this WP: D1.1 - Survey report of SME service company requirements and D1.2 - Identification of test pieces and ships. Both deliverables were prepared with the collaboration of consortium - specifically the ship classification societies (effectively the end users) and SMEs. The overall objective of this WP was to arrive at detailed specifications for all the other WPs in the project. It was essential that the project met the needs of large numbers of European SMEs and thus of the SME-AGs that represent them and are members of the consortium. Thus these bodies had a major role in finalising these specifications.

The SHIPINSPECTOR project's mission statement was: 'To continually monitor the 20 mm stiffened plate floor of VLCC and ULCC vessels' storage tanks for the presence of 40 mm diameter, half wall pit defects'. The process for this crucial committee decision began even before the kick off meeting, with a discussion between the end users and RTDs. Initial opinions were voiced to specify target vessels at ABS's premises. The minutes of this meeting were circulated before the kick-off meeting, where a wider discussion took place to tie down specifications. Other additions have been made where they have arisen in other discussions over the course of the project. After the above decision making process, the consortium came to the conclusion that very large crude carriers (VLCCs) and ultra large crude carriers (ULCCs) were the target vessel. These are the largest classes of ships, which pose the most difficult inspection challenges. VLCC tankers are typically 350 m long with a 20 m draft and 60 m beam, whereas ULCCs are 415 m long. A geometry was decided upon that should be generically representative of ship structures of interest. Currently, there are certain areas of these ships (identified in a similar deliverable report) that can only be inspected in a dry dock.

This is an incredibly costly process - firstly the docking charges can be hundreds of thousands of Euros per day, but also the lost earnings from assets and personnel can be significant too. The above is of course true for all sizes of ship, however the costs scale up and so the largest ships will be affected most.

WP2 - Develop advanced UT flaw detector and UT linear phased array was 100 % complete in month 26. The objective was to develop an optimised procedure for the inspection scenarios defined in WP1. The work covered the array design, including consideration of the transducers to be used, the ultrasound propagation characteristics across the stiffened plates, the surface preparation required and the imaging of the responses from the plate area tested. The large scale (12 x 6 m) specimen for later studies was also produced in this WP.

WP3 - Develop methods for linear phased array techniques were 100 % complete in month 25. The objective of WP3 was to produce and to test a sensor array to the requirements defined in WP2, together with the associated electronics. This work is complete and was reported in month 25, April 2011 (deliverable report D3.1). The work consisted of setting up a linear array for initial experiments on the large-scale specimen, implementation of full matrix capture procedures for data gathering and development of imaging algorithms for display of data. The requirement for a multiplexer device for handling large numbers of sensors was also defined.

WP4 - Signal analysis and software development was 100 % complete in month 25. The objective of this WP was to develop methods of conditioning the ultrasonic signals in order to improve the performance for defect detection. The work was completed and was reported in month 25, April 2011 (Deliverable report D4.1). The work involved assessing the effectiveness of a number of signal conditioning procedures for signal quality enhancement, including averaging, enveloping, band pass filtering and cross-correlation. A software package was developed to allow these to be implemented. The effectiveness of these processes for imaging defects was studied, both using simulated data and data collected from the large scale specimen.

WP 5 - Design and analysis of load application apparatus for linear phased array was 100 % complete in month 25 and was reported as deliverable D5.1 (also in periodic report 1 and 2). The aim of this WP was to engineer robust prototypes capable of resisting harsh environments representative of those in real ships. This work was identified as behind schedule in the first periodic report (10 % complete against 50 % planned). Additional work was done in the second period; the work package was completed on schedule and reported in month 25, April 2011 (deliverable D5.1).

Designs for sensor protection enclosures were produced and examples manufactured and tested.

Studies were also carried out on the attachment of the sensors themselves and recommendations made for sensor attachment for the long-term tests planned in WP6. A multiplexer device for switching between sensors, as identified in WP3, was designed and built for use in the experiments.

WP 6 - Experimental study and evaluation is 100 % complete in month 40. The objectives of this WP were to prepare for large scale trials, to evaluate the performance on a laboratory scale and to carry out the final large-scale monitoring experiments. This work is completed and outcomes were demonstrated during the final meeting of the project.

Extensive tests were performed on the performance of different designs of encapsulation for the sensors. These resulted in a final design being produced for a field-usable prototype. An uprated multiplexer was manufactured for switching between the sensors in the array. Following evaluation of the performance of the arrays and defect visualisation procedures, revised transducer arrangements were evaluated and recommendations made for the configurations to be used for the monitoring trial.

A long-term trial, over a three month period by collecting 600 000 data sets, showed that the target 40 mm diameter x 10 mm deep defect could be detected, provided close control was exercised over the test conditions at all times and that the external influences on test results, particularly temperature, were measured and compensated for.

Potential impact:

The project will contribute reducing the risk to which inspectors are exposed whilst working on ships. It has been noted that shipyard and marine work has an injury-accident rate more than twice that of construction and general industry. This project will contribute to reduce injuries and deaths to SME workers in the ship maintenance and inspection industry.

The SHIPINSPECTOR technology will help operators, classification societies and regulatory agencies worldwide to manage risk more effectively. There are 12 000 SMEs involved in the 50 billion inspection and maintenance sector. The SHIPINSPECTOR consortium will disseminate the technology and associated training to the SMEs represented by the participant SME AGs.

The main purpose of the dissemination activities is to raise awareness of the project results in order to promote the widest dissemination of knowledge gained during the SHIPINSPECTOR project.

The project website (see http://www.shipinspector.eu/ online) has been created and updated with news and project activities throughout the project term. Further dissemination is planned to continue via the SHIPINSPECTOR project website.

The project flyer was designed and printed in Bulgarian, French, German, Greek, Italian, Japanese, Spanish, Russian and Ukrainian in order to increase public awareness in different markets in Europe.

A video discussing both the developed monitoring system and the project in general can be found on the project website or can be accessed on the web service YouTube following this address http://youtu.be/5OeiNpJRFGw This has been published to ensure the continuity of dissemination even after the project is completed.

At the end of the project, a webinar was held in order to attract industrial attention to the development. The webinar provided a great platform to access an appropriate audience and to disseminate the project achievements. There was a good response from some of the industry players and these interests will be pursued after the completion of the project. 18 companies including Shell and Maersk have attended the webinar session held on 30th July 2012. The webinar was recorded for those who could not join the presentation on the day and uploaded onto the project website http://www.shipinspector.eu/news/index.jsp

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