Safety through simplicity

In any vessel with electric propulsion and dynamic positioning, continuous availability to the electric power plant is essential for safe and reliable operations. The design of the electric power plant is based on established engineering practices to ensure to the maximum extent that faults in the system will be isolated and lead to a minimum loss of capacity of electric power, propulsion, and thrusters. At the same time, the requirement to reduce fuel consumption and environmental emissions are increasing. In the design, it is necessary to make an optimization of these two, to some extent different criteria.

For drilling vessels with dynamic positioning, the installed power level is high – in the order of 30-40MW, enough to supply a small town. The fuel consumption is a significant cost element for the operations, and also a source of CO2 emissions. Design and operation of the plant to reduce the usage of diesel oil is normally highly valued by the oil company and operators. At the same time, drilling operations are of a nature where loss of electric power is unacceptable, both due to the direct consequences to costs and loss of day-rates, but also the risk of damage to the well and drilling equipment with the added risk of environmental spillage.

There are several approaches to reach a balance between these requirements. One way has been to add dual or layers of monitoring and back-up functions to the system, and advance cross-feeding functions in the network to minimize the consequences of faults, and to ensure optimal loading of the running diesel engines to provide the highest efficiency. The disadvantage of such systems is that as the amount of equipment and parts count increases it may give new and yet unexperienced failure modes. What’s more, with the increasing complexity of the systems, there are limited possibilities for crew and on-shore support functions to really understand the characteristics of the installation, and to operate the system in an optimal and safe way.

Such systems normally will need fairly advanced control systems, made specifically for this installation, and by experts. Even though such systems may function well after installation and tuning, one should consider that they should also be operated and maintained in several decades – long after the design specialists have abandoned the vessel, and following the regular replacement of control and power conversion equipment.

It is my view that safety over lifetime is best obtained by simplicity in design and use of reliable products. Our design should be based as much as possible on industrial standard products and systems, in order to ensure that they are as proven as possible, and with a lifetime support policy from the makers of the main equipment. This is the best way maintainability and support can be safeguarded.

Simplicity in the design means that the design is based on clean segregations within the power and protection systems, minimizing the cross connections and dependencies. In such design, the interconnections and common points between the redundancy segregated parts are kept to a minimum. Therefore, the possibility to analyze, monitor, and protect the remaining common points will be significantly improved. Also, should there be a need for manual interaction, a clean, simple design is possible to understand and control by experienced crew. Our advice in design is to keep it simple and straightforward, utilizing established and well-known engineering practices to enhance safety and optimize availability and fuel consumption. Simple design, utilizing high quality and proven components, together with skilled and experienced engineering resources has been, and will be the best way to obtain a safe installation within the constraints of designing today's advanced vessels.

About the author

Alf Kåre Ådnanes is VP and technology manager with ABB's Marine business unit. He holds MScEE and PhD degrees from the Norwegian Institute of Science and Technology and have had various positions in ABB's Corporate Research Center and in Marine, since start working for ABB in 1991.