Cost effective inspection and structural maintenance for ship safety and environmental protection throughout its life cycle
For oil tankers to be more environmentally friendly throughout their life cycle, the IMO (International Maritime Organisation) has set forth a condition assessment scheme "CAS" for single hull tankers and is now working to develop a similar type of code for double hull tankers, which involve huge amounts of measurement information. The CAS system is built around the design of an exchange standard format to describe, in a neutral way, the structural data and associated measurements. All tools used in the ship hull monitoring process are expected to have this exchange standard format incorporated. The system includes such innovative features as: - the development of a simplified and flexible ship electronic model which can be refined to fit the needs of classification inspections; - additional measurement information into the ship model; - automatic updating of the measurement information into the ship model; - the integration of robotics; - easy handling of measurement information using virtual reality; - immediate worldwide access. The re-engineering of the process was to include the definition of a standard exchange database, called Hull Condition Monitoring (HCM). The HCM was to be tuned up by all the partners intervening in the process, and, as studies and tests would be performed by the partners, in the course of the project, new features would be incorporated into the standard. The HCM design was especially focused on: - measurement data, corrosion, cracks and painting; - geometrical information required for the definition of plates (planar, curved and corrugated) and profiles; - the possibilities for the export out of a current CAD geometrical ship's model, used by the shipyards to build the ship, towards an HCM model. The major evolutions of the HCM data model during the course of the project were: - change from thickness-measurement-centric to be hull-condition-centric; - addition of inspection results beyond thickness measurements (coating condition, cracks, etc), repair data and corrugated bulkheads. Interfaces between existing 3D model generators and HCM are possible: The 3D model generators from two classification societies in the project, the 3D model generator developed in the course of the project and a shipyard CAD software could successfully produce HCM files and vice versa. Regarding the geometric input tool, a prototype 3D model generator was implemented to produce a ship 3D model in accordance with the HCM standard. From the geometric point of view, the model does not need to have high accuracy, because a simplified geometric description, based on linear approximations of both curves and surfaces, was adopted in the HCM. However, all the plates and stiffeners to be inspected during a campaign must be present and identifiable on the model. So, it was assumed that the simplified model should be feasible from the information on the drawings commonly existing on board the ship, such as the general arrangement, body plan, midship section, etc. For each of the structural systems (bulkheads, decks, etc), generating templates were defined. Each generating template defines a family of structural system members, with similar shape and scantlings. The visualisation tool developed could read the XML files in the HCM format and visualise the structural elements in 3D. The virtual reality tool could export the 3D model to a 'ray tracing' format, to generate photorealistic images. Different appropriate measurement technologies for thickness measurements and cracks detection were carried out: A procedure for the application of eddy current crack detection was provided; the standards for the inspection of corrosion prevention measures were analysed, including the inspection of coatings as well as the inspection of cathodic protection systems and leading-edge coating assessment technologies were tested. The condition assessment tool such as the calculation of the equivalent age of the vessel, based on statistical methods, was implemented, and the calculation of inertia modulus for any ship's section and the consequences of the entry into force of the common structural rules for oil tankers and bulk carriers, was documented. The risk-based inspection (RBI) methodology adapted to HCM was developed and applied to the final demonstration. The procedure comprised a segmentation of the ship structure in areas of similar degree of risk, an estimation of the risk and an adjustment of inspection planning. A demonstration of the CAS process in realistic conditions was performed in a European repair shipyard, on board a 150 000 DWT oil tanker, in dry dock. The ship's renewal survey measurement campaign was carried out using the CAS process and prototype tools, as developed in the project. The demonstration proved that it was possible to build a 3D model of the whole ship, where the areas to be measured are represented with greater detail than the rest of the structure. Measurements on board consisted of measurements taken inside the ship by two UTM operators and on the outer shell by a robot. The combined measurements were entered into the HCM model and analysed using the post-processing tools developed in the project. Globally the project has attained its objectives: The HCM standard was published and all prototype software tools used in the condition assessment process, from the initial thickness measurement to the final assessment of the ship's structure, were developed. The project has elaborated the core conceptual principles upon which all improvements of the condition assessment process of ships in service in the next years are expected to be based. However, some prototype tools features, although still in accordance with the project principles were not very easy to use: for instance, the entry of thickness measurements into the 3D model was done by typing in pop-up windows. In the final demonstration, more advanced features were supplied by some pieces of software developed by partners on the sides of the project, enabling for instance the use of a touch screen to enter more conveniently the measurements into the model. Two classification societies involved as partners in the project have already developed their own commercial tools based on HCM. They are now considering the integration of those tools with other specialised modules in their own information systems. HCM applies to ordinary ships of all types, but also to FPSOs (offshore oil floating storage) and jacket type offshore platforms. HCM is very appropriate for connection with: - finite element models to calculate the stress levels in the hull structure; - risk analysis modules to calculate the criticity of failures in the hull structure; - condition assessment modules, visually showing the structural elements to be repaired; - inspection/measurement robots.