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 also worked to develop a similar type of code for double hull tankers, which involve huge amounts of measurement information.
Performing those inspections efficiently requires processing measurement information on a real-time basis, resulting in cost savings because fast assessment of the ship's condition and decision-making could be done while the ship is still in the dock for maintenance. Measurement information consists of thickness measurements, visual assessment of coating and detection of cracks. In the existing situation, because there is no standardisation of data, the information is recorded manually on ship drawings or tables, which makes handling very difficult. Measurement information takes a long time to report and to analyse, leading to some repairs being performed at the next docking of the ship.
Reporting these structural measurements efficiently requires processing measurement information on a real-time basis. This would result in cost savings because a faster assessment of the ship’s condition and quicker decision-making could be done while the ship is still in the dock for maintenance.
Reliability of the analysis of the measurement reports could also be significantly improved by the use of electronic displays, associated with automatic warning devices in case of excessive deterioration of the structure. This is to be compared with today’s existing paper measurement reports, which are checked manually, page by page, by surveyors.
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 system developed is applicable to any ship type, but, due to the current focus on tankers and bulk carriers, these ships are used as the main case studies.
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.
Systematic comparison and consistency checks of measurement campaigns will trigger electronic alerts. Repair decisions and residual lifetime of the structure will be calculated with modern methods of risk-based maintenance modelling, with the interesting feature that the model will be updated after each measurement campaign.
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 photo realistic 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 assessme
The main innovations consist in the fact that the resulting HCM 3D model is:
- a standard, and is dedicated to all actors in the condition assessment process, and especially thickness measurement societies and classification societies;
- a simplified model, good enough for ships-in-service maintenance, by opposition of the existing huge shipyards CAD models used for building the ships.
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.
It is expected that those connections will be operational around the year 2011.
Other implications are:
- The proof that the complete condition assessment process can be achieved using HCM as the only interface between the involved actors.
- The feasibility of software tools development for:
- a generator for simplified ship's hull models which can easily be exported to HCM;
- visualisation and virtual reality of the ship's hull and measurements using HCM;
- condition assessment using only HCM;
- automatically exporting the HCM simplified 3D ship's model, to be used during the whole operational life of the ship, from the shipyard CAD detailed 3D ship's model.
- The scantling prediction and statistics-based algorithms developed in the project for condition assessment purposes, which could be the foundation for future condition assessment tools, using HCM or not.
- The Risk-based inspection methodology developed for the project, based on a segmentation of the structure, can be the base for the development of future Risk-based inspection modules.
- The main features of a robot dedicated to thickness measurements on a ship's outer shell, including the sensors configuration and the positioning system. The robot itself is not patented, however the robot software, in charge of positioning the robot on the ship's hull is being recorded at the Programs Protection Agency.