The Netherlands Life cyle design of semisubmersible platforms advanced using HFES

The Norring-4 floating production platform, conceived by Marcon Engineering.

Designers must trade off constantly between the initial cost of their chosen system and its likely in-service running costs. Leiden-based Marcon Engineering recently participated in a project funded by the EC's Thermie programme to research life-cycle design of semisubmersible platforms.

The result was the Heavy Floater Expert System (HFES), a computer-aided design method which allows evaluation of life-cycle cost consequences of decisions early on in the design process to meet present day demands for cost-effective floating production systems. It optimises the cost right through to scrapping of the floating unit.

The Marcon-designed four-column Norring 4 FPS served as the basic design for this project. Marcon's main partner in the project was Registro Italiano Navale of Genoa. Nederlandse Verenigde Scheepsbouw Bureaus, Delft University of Technology, and Germanischer Lloyd also participated as subcontractors.

Design process

The CAD system was set up as a complex conglomerate of miscellaneous programs or modules. The process begins with sizing of the design. Main dimensions and layout are refined to meet client's specific demands, to accommodate practical considerations and to cater for improved characteristics. Determination of local scantlings, weight and initial cost estimates are part of this design stage.

Inspection, maintenance, and repair strategies (IRM) are focussed on fatigue-sensitive locations. Assessment of crack growth requires a long-term stress range distribution (LTSRD). The reliability assessment of the structure at this stress distribution is used to estimate in-service costs of the structure.

The LTSRD is determined by two parallel actions. A global model of the structure is subjected to waves of unit height for a number of frequencies to obtain a response of amplitude operator for the loading of critical joints.

A detailed model of the critical joints is then subjected to unit forces at their boundaries to create a stress response matrix. These conversion operators are then used to translate site-specific wave data into the LTSRD for critical components of the structure.

The reliability approach allows an assessment of future maintenance and repair requirements. The most important phenomenon affecting in-service structural reliability is fatigue crack growth. This growth may be modelled using the Paris-Erdogan equation.

Damage effect can be determined by two different methods. One is based on the higher order moments of the LTSRD. Under the other approach, a Weibull fit to the same data is used as the basis for determination.

Failure is defined as the condition where cracks have grown beyond the critical crack length. At this length cracks become unstable and static loads only lead to progressive cracking, which may result in serious damage to the structure, requiring immediate repair action.

IRM strategy

The probabibility of cracks being present and detected, and the need to repair them, is influenced by the IRM strategy and the platform's basic reliability. Reliability of the structure is characterised by an initial level and degradation in service. In-service inspection and repair can reduce the rate of degradation.

Replacement of critical parts of the construction might actually upgrade the reliability, but this is not considered feasible for the IRM of the monolithic structures under consideration. The IRM policy must be optimised for a given structure before conclusions can be drawn about design aspects.

Initially, the structure may contain significant cracks which were not found before commissioning as well as miniscule cracks which are present in every welded steel structure. Until the first inspection takes place there is a chance that the construction will fail due to growth of these initial cracks.

The cost associated with correcting such a failure is the failure cost. Risk of failure can be expressed in cost terms by multiplication of the cost of failure with the chance that such a failure will occur.

During inspection, cracks may or may not be present, and if they are, they may not be located. The chance of a crack being found depends on the inspection technique used. With Magnetic Particle Inspection (MPI) there is a greater probability of detecting certain cracks than by visual inspection.

In the HFES the probability of cracks being present is combined with the probability that they will be found on inspection. Cost of repair follows on from the combination of probabilities that a crack is present, that it is detected and that it is of a size that requires a repair. The combined probability is multiplied by the estimated cost of the actual repair leading to the expected in-service cost.

Until the next inspection, a situation exists which is similar to the one before the first inspection. However, the actual risk of failure will have increased slightly because of imperfections in the inspection and repair techniques used. The probabilities associated with these events are evaluated on the basis of an event tree.

The probability of repair and the associated cost in the case of multiple subsequent inspections is evaluated using SYSREL software developed by the University of Munich.

A prototype HFES was developed for life cycle, cost-effective design of permanently moored semisubmersible type floaters. Currently, the system allows the systematic assessment of life cycle cost parameters for one floater type only. But the basic configuration is universal enough to allow use for other floater types. Experiments with HFES have shown that the structural design of critical parts of a semisubmersible can be optimised early in the design process.

To evaluate different IRM strategies for the insert plate in the column pontoon connection, the following cases were calculated:

One MPI of the insert plate in the eighth year of service.
Two MPIs during the fifth and tenth years.
Three MPIs during the fourth, eighth and twelfth years.

Results of this reliability assesment clearly show that more frequent inspection in this situation improves the reliability of the structure. The benefit in terms of life cycle cost is derived from the reduced risk, and therefore, the cost of failure. With HFES, this result can be compared to the life cycle cost of changes in design to achieve a similar level of reliability with, perhaps, a different inspection scheme.

Marcon designed its first semisubmersible drilling rig in 1969. The West Venture Norris-5 type rig was built for Smedvig in Stavanger. Since then, Marcon has developed the M-6, a six-column drilling rig as well as the Norring-4.

Since 1991, the company has been associated with Ingenieursburo Veth based in Papendrecht. The two companies offer a range of services from conceptual design through to preparation of drawings: their track record includes work for fixed as well as floating platforms, modules and engineering services to support repair and maintenance.