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Asset Integrity Management System for

Ships and Offshore Units Hull Maintenance

Philippe Renard, Bureau Veritas, Paris/France, philippe.renard@bureauveritas.com

Abstract

Marine offshore platforms (offshore platforms, ships, etc.) are exposed to a hostile environment. To

ensure their integrity, a complex Asset Integrity methodology is requested, with the objectives of man-

aging the uncertainties and the risks for the people, the environment and the very expensive assets

throughout their service life. Based on the considerable R&D efforts in this area, the objective of the

HLC-AIMS project is to develop an innovative decision support system for the online maintenance of

platforms, resulting from the integration of Inspection campaigns, Risk Based Inspections (RBI), Fi-

nite element Model and Hull Condition Model based on an innovative neutral and standard exchange

file system, with a graphical interactive database using a 3D geometric modeller. In addition to in-

creased platform safety, the expected benefit for the users is a single and on-line access to all in-

service follow-up tools.

1. Applicability This paper is derived from a project proposed to the Eurogia European cluster. The project applies to multi-purpose marine offshore platforms dedicated to energy production, either fossil energy (oil and gas FPSOs, jackets, semi-submersibles, column type units, methane hydrates extraction platforms) or renewable energy (windmills, solar panels, waves energy, tidal energy platforms), Figs. 1 to 4.

Fig. 1: FPSO

Fig. 2: Windmill platform

Fig. 3: Semi-submersible unit Fig. 4: Tidal energy platform

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There are many difficulties and profitability issues involved in developing renewable energy plat-forms: one fossil energy plant produces about 1000 MW, whereas a modern windmill of 90 m propel-ler diameter produces 2 MW only. Nevertheless, our purpose was to push forward an asset integrity management (AIM) tool and not to study the design of those platforms as such. In the same way, this project could be extended to high-value ships, such as gas carriers in ice conditions or any futuristic kind of hotel or artificial leisure island, Figs. 5 to 7.

Fig. 5: Arctic gas carrier Fig. 6: Floating town Fig. 7: Floating hotel The nature of the platform remains rather independent of the various types of energy plants it must accommodate. The platforms will be exposed to a hostile environment which must be taken in consid-eration from the loading and maintenance perspectives. Salty water, waves, tropical climate or icy conditions will damage the structure and in all cases those platforms are submitted to corrosion and large internal strains. 2. Expected benefits for the users

Operators do not want to overlook any future degradation, which could potentially lead to the interrup-tion of energy production. Prevention of production interruption, through timely repairs, is their main objective. Environmental damages, although happening seldom, must also be avoided, to favour good relationship with the neighbouring countries. In view of these concerns, this system will provide:

• A single and on-line access to all in-service follow-up tools, with a common format for entry of data. By opposition, the current procedures are dispersed and not computerised, and gathering in-formation can take weeks for decision making, starting from inspection campaigns;

• Increased platform safety, by providing a permanent easy and transparent access to all platforms structural data;

• Coherence of measurements between successive measurements campaigns, because the meas-urements are in a structured electronic format;

• Applicability to any kind of platform, because the tool is based on a real 3D model and not on pre-drawn sketches.

3. State of the art

• When a platform is initially designed, the structural elements are calculated to ensure the struc-

ture’s integrity in all circumstances, but, during life at sea, the structural elements are submitted to coating degradation, corrosion and fatigue cracks. Periodical measurements and inspections are performed to continuously monitor the condition of the structure. However, beyond those specific measurements, a more global view of the structure's condition is required to assess its condition and take maintenance decisions. Asset integrity management (AIM) refers to the continuous proc-ess of allowing decision making, to ensure the efficient operation of the asset, while checking its integrity.

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The degradation phenomena develop in a random environment, therefore probabilistic methods and risk analysis are at the heart of AIM tools. Failure scenarios start from an initial event and pro-pagate to a final event which may lead to production interruption, costly repairs or environmental consequences. Sometimes, mathematical utility functions are used under constraints (reliability, safety, fitness-to-purpose). The final decision though, involves the operator's personal criteria.

• The basic idea of this project is to develop a set of integrated and interrelated AIM tools using a common standard format for exchanging data and information between the main partners of the AIM process: class societies, owners and thickness measurement ™ companies. AIM tools are usually disconnected: they will be connected in the project to achieve optimal decisions and will use a common database, the hull condition model (HCM) standard.

• Major classification societies and several engineering companies are proposing AIM services to their customers. They cover specialized fields such as plants, electric equipment, pressure vessels, pipelines and sometimes the platform itself. The core of the service deals with failure modes, chains of failures, effect and criticality analysis. Some R&D efforts (e.g. the MonitAS JIP lead by MARIN) try to derive the condition of the platform from the stress measured by gauges installed on board. Although the measured stresses give a useful estimate of the deterioration of the struc-ture in a wide area around the gauges, they are not a proxy for a map of the structural elements' condition and therefore cannot be used alone to specify the maintenance actions.

• There is little public documentation on detailed risk analysis of each individual structural member as well as the follow-up of detailed structural measurements in current AIM software tools. Like-wise, it remains unclear to what extent the specific constitutive software modules (e.g. finite ele-ment program and the risk based inspection module) are integrated and to what extent the data capture can be done only once. The analysis usually consists of a methodology performed by a human using a set of computerized tools, but there is no exhaustive software for RBI, for instance. All existing AIM software is proprietary; thus no data or result can be read by another company's software. The software is not centred on a standard format for the description of both structure and assessment data.

• Tools like HLC-AIMS are, in the first place, a requirement from the operators, who want to be helped to manage their installations and ask for more complete information, with quicker and eas-ier access to this information. What is relevant is the mastering of the incurred risk to or caused by the offshore platforms. For instance, for offshore oil production, there are in the whole world about 100 units big enough to justify AIM services. One of those units represents a capital cost of about 400 million €. Stopping its oil production would cost in the order of 10 million € per day. Finally, it would potentially cost 200 million € in damages and interests, in case of major pollution reaching nearby coasts.

4. Hull Condition Model (HCM) central database The basic data necessary for all AIM tools are the same. All those tools need a description of the platform's structure and the measurements values. Whence the idea of building our AIM system with a database in the centre, containing the platform's structure and the measurements values, which will exchange information with all surrounding AIM tools. The user will have a coherent set of tools at his disposal, applying the same logics all along the process, thus minimizing interpretation errors and training requirements. The selected central database is the HCM standard, resulting from the EC project called CAS (2005-2008), which was initially designed to support ships' full thickness measurements process. CAS resulted in the publication of the HCM standard, which contains, in XML format, the geometrical description of the platform geometry and the associated measurements. The Open HCM consortium was recently created to deal with the evolutions and the promotion of this standard. With HCM in its centre, this AIM tool becomes an open system. It is open to other AIM tools, because their HCM standard database can be read by HLC-AIMS: the platform's operator will not suffer any interruption,

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e.g. in case of a change of owner. HCM is also open, by principle, to all TM companies, which will easily input their measurements into HLC-AIMS. In this project, HCM will need expansion to cater the needs of an exhaustive set of AIM tools. Those improvements will eventually be proposed to the Open HCM consortium. The starting point will be the Open HCM standard, as currently published by the Open HCM Consortium and expansion to this Open HCM standard will be provided:

• to model beam-type structures, such as FPSOs' topsides, jacket units structure or the beam-type structure of semi-submersible units, between deck and floats, in addition to typical ship struc-tures;

• to take care of thickness measurements, coating parameters, cracks, reports, pictures and films, instead of taking care of thickness measurements only;

• to accommodate all processes supported by the new AIM integrated tool.

5. Platform model from shipyard

In this context, the existence of a 3D model for the platform is critical. It is always possible to build from scratch, when the platform is in service, a 3D model good enough for the in-service monitoring, but it is much quicker and cheaper if the shipyard model, used for building-up the platform, can be exported towards HCM, Fig. 8. It is, however, too early to know whether the shipyards will consider this possibility:

• either as a commercial opportunity to expand their services to the owner, supplying an electronic 3D model in addition to the traditional paper drawings documentation, thus developing added value and privileged relationship with their customers;

• or as a breach of design confidentiality, because some basic structural features are incorporated in HCM and could be communicated to third parties. However, the HCM model is so simplified that a competing shipyard would certainly not be able to build a sister ship from it.

The final business model for HCM based tools, either dedicated to a few sophisticated users or becom-ing the new state of the art, will thus depend to a large extent on the level of participation from the shipyards.

Fig. 8: Export HCM from shipyard

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6. General architecture The HCM database collects all input in an homogenous format and shares the information between the specialised modules. All modules needed for asset integrity management of a platform are organised coherently around this HCM database. The individual modules are discussed in the following:

Fig. 9: HLC-AIMS schema

• NDT input module: This module deals with entry of thickness measurements, cracks and coating condition. This module will fit the various operational modes which are expected to be used by TM companies all around the world.

• Modeller: This module defines the geometry of the structural elements. It will include the modelling of beam-type structures, such as FPSOs' topsides, jacket units’ structure or the beam-type structure of semi-submersible units, between deck and floats.

• Viewer/Navigator: This module translates the HCM database into a 3D model display of the platform. All inspection data (thickness measurements, coating condition, cracks, inspection reports, pictures, video films), can be visualized at the same time on this 3D model. Miscella-neous mechanical equipment (pumps, valves, etc) and means of access (ladders, platforms, etc), will be displayed as objects taken out of a library of 3D objects. Visualization facilitation functions, such as colour codes to show the thicknesses, removal of some elements to have a better view of other elements and wire-line representation will be provided. A tree representa-tion of the structure will ease the navigation inside the model.

• Survey data: "Flags" will be displayed on the 3D model to attract the user's attention, Fig.10. Clicking on the "Flags" will trigger the display of the inspection data previously attached to this location. The user will adjust the level of details to be shown on the screen, and the type of information he wishes to see. The display may also show all inspection data recorded be-tween two given dates.

• F.E.M. module: The finite element module (F.E.M), will provide the level of stress in the structure, corresponding to the real corroded scantling of the structure, when submitted to class-defined loads. This updating of stresses (to be made in accordance with the real cor-roded scantling) is sometimes required by the operator at the time of the platform's classifica-tion certificate renewal. The input of measurements will be automatically interfaced with the finite elements calculation. The HCM based geometrical model, made of plates and stiffeners, will be automatically translated into a finite element model, Fig. 11. This finite element mo-del is of a very different nature than that of the geometrical model, because it consists of fi-nite elements for the calculation of stress and not of physical plates and stiffeners.

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Fig. 10: "Flags" to locate inspection data

Fig. 11: Geometric to finite element model

• RBI module: The risk matrix (i.e. probability of failure vs. consequence of the failure of a struc-tural element) will be established from:

- the wastage, stress or level of fatigue of each structural element, as recorded in either HCM or the Finite Element Model, providing the probability of failure of the structural element;

- the level of gravity of the consequences of failure for each structural element.

Each structural element will thus be positioned in one cell of the risk matrix. The most critical cells (in red in Fif. 12) contain the most critical structural elements, which need to be surveyed more often than the others. An RBI plan can thus be deducted from this matrix. The most critical structural elements (red and yellow on the sketch) will appear on the 3D model display. The degradation of those elements will be carefully followed-up, with alarms popping-up in case of large or accelerated degradation.

Fig. 12: RBI matrix

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• Rules Assessment: This module checks the conformity of the platform's structure, in its present condition, with the classification rules, Fig. 13. In particular, the thickness of each plate and stiff-ener will be checked in relation to the rules requirements for this type of platform and classifica-tion notations. Traditional checking, based upon the structural wastages, as well as the new Common Structural Rules (CSR) checking, with wastage allowances expressed in millimetres, will be dealt with. This module will show the elements with substantial corrosion and to be re-paired, which will be also displayed with colour codes, . Buckling criteria will be taken into ac-count. Predictive probabilistic simulations will provide the future condition of the platform as calculated in 5 years, 10 years, etc.

Fig. 13: Rules assessment display

References

CABOS, C.; JARAMILLO, D.; STADIE FROHBÖS, G.; RENARD, P.; VENTURA, M..; DUMAS, B. (2008), Condition assessment scheme for ship hull maintenance, 7th Int. Conf. Computer Applica-tions and Information Technology in the Maritime Industries (COMPIT), Liège/Belgium, pp. 222-243 JARAMILLO, D.; CABOS, C. (2006), Computer support for hull condition monitoring with PEGA-

SUS, 5th Int. Conf. Computer Applications and Information Technology in the Maritime Industries

(COMPIT), Oegstgeest/Netherlands, pp. 228-236 JARAMILLO, D.; CABOS, C.; RENARD, P. (2005), Efficient data management for hull condition

assessment, 12th Int. Conf. on Computer Applications in Shipbuilding ICCAS, Busan/Korea RENARD P.; WEISS P. (2006), Automation of the ship condition assessment process for accidents

prevention, 5th Int. Conf. Computer Applications and Information Technology in the Maritime Indus-tries (COMPIT), Oegstgeest/Netherlands, pp. 403-408