More and more operators are asking their drilling contractors to operate their DP units in closed-bus mode, in order to save on fuel and maintenance costs and to reduce the environmental footprint. It also enables a significant reduction in engine hours, with less wear and tear on the engine, and expanded windows for maintenance in a cold engine room.
Dynamic positioning operations are strictly regulated, as loss of position can have dire consequences. Loss of power is a main risk, and complete redundant design including generators and thrusters has been required. This comes at great cost during operations, and the industry has for many years worked on smarter solutions with equivalent safety.
The traditional systems are designed for open bus mode, meaning completely separated power systems and when operated in closed bus mode the closed bus-ties may create a failure propagation path, undermining fundamental design requirements defined in IMO and classification rules. A closed bus system is a much more complex and tightly integrated system, which is demanding to build, verify and operate safely.
“Despite being the subject of discussion in expert DP circles for years, the industry has never come to a clear agreement on how to safely use closed bus configurations,” says Aleks Karlsen, DNV GL’s principal specialist in DP. The discussions have revolved around the necessity of short-circuit tests, and the potential for damaging equipment on units not designed with this in mind.
To manage these risks, DNV GL is now issuing an Offshore Technical Guide providing guidance on how systems based on closed bus-ties configuration can be designed and verified with additional protection and monitoring facilities, ensuring integrity and robustness. The guidance addresses the critical issue of testing by recommending test requirements that safeguard equipment from damage during testing, while at the same time obtaining sufficient evidence of robustness.
Approval of a closed bus DP unit requires validation of the fault tolerance of the connected system, including live short-circuit testing of worst case failure modes. Extensive experience with operations and testing proves that live earth fault and short-circuit tests are necessary. Major equipment manufacturers confirm that such testing is not only possible, but necessary. Other stakeholders, including flag state authorities, coastal state authorities, and major oil companies have also made it clear that proper testing of all failure modes is necessary.
“This is a critical safety issue. As stated in the IMO’s (International Maritime Organization) DP guidelines, a loss of position is not to occur in the event of a single failure of any active or static system or component, including failure caused by fire or flooding. The solution we provide through our Offshore Technical Guidance sets design requirements that enable live short-circuit tests to be carried out with minimum risk of damaging the equipment. Once engineering and calculation methods have matured to become more reliable and accurate compared to testing data, our requirements to testing may be adjusted accordingly. Right now, only testing provides sufficiently reliable data,” says Aleks Karlsen.