Many DP vessels designs date back to a time when there was limited understanding of how to make closed bus power plants fully fault tolerant, resulting in preventable risk and a number of potentially costly incidents. DNV GL Class had previously only allowed this configuration for DP Class 2 vessels. The poor incident record eventually led to many operators preferring to operate with open bus-ties, especially for critical DP operations. In addition to cost and fuel efficiency burdens, open bus-ties can impose other restrictions on a vessel’s ability to carry out its industrial mission, often resulting in otherwise avoidable downtimes.
More recently, there has been an added disadvantage to the use of open bus systems, as Steve Cargill, technical authority for DP at DNV GL’s Noble Denton marine services unit, explains: “Industry requirements to reduce emission levels have increased significantly in recent years; particularly, in important territories, such as the Gulf of Mexico and the Arctic. Operating with bus-ties open in such regions risks expending an operator’s emission budget before work can be completed. Clearly, a solution needs to be found.” He believes high integrity DP Class 3 closed bus power systems may be able to provide an acceptable solution for many types of DP operation. Such solutions are now available from a number of sources, as a result of the work undertaken by DNV GL - Maritime to develop new rules for Dynpos AUTRO, and pioneering work by DNV GL’s customers in recent years to develop new safeguards and techniques for testing and analysing power systems.
Live testing: no longer a controversial option
Classification societies, DP industry bodies, equipment vendors and customers have cooperated to create guidance on methods for testing and analysing closed bus power systems. Most recently, this also includes live fault ride-through testing on power-plant switchboards – a technique once resisted by the industry for its perceived risks. It represents a major step forward as Steve explains: “Failure modes and effects and analysis (FMEA), proven by testing and supplemented by a range of supporting studies including validated mathematical models, allows Class to achieve the required level of confidence that the power plant is single-fault tolerant and to approve this configuration for DP Class 3.”
Steve adds, however, that although the new procedures are intended to prove the robustness of closed bus power systems, DP vessels that are designed, analysed and tested to meet requirements for Dynpos AUTRO (closed bus-ties) are more robust whether operating with open or closed bus-ties. “For closed systems, in particular, fault ride through testing supplemented by validated computer simulation will play a crucial role as the industry begins to increase its use of closed bus configuration in light of requirements for lower emissions and fuel consumption,” he says.
Robust designs deliver confidence
Verification schemes are considerably more onerous for DP Class 3 closed bus power systems, Steve explains. “Typically they will include, for example, the need to analyse, test and mathematically model all possible failure modes that can propagate by way of the bus-ties, as well as to verify that power-plant systems are able to withstand the severe disturbance associated with clearing a fault. This requires additional studies and analysis into possible failures, existing and potential protection functions, and backup protection. Such designs are ‘built to be tested’, adding significantly to confidence levels.
“Maintaining integrity in these closed bus systems throughout the vessels’ lifecycles will be the next big focus, and techniques for establishing that is something that we’re working on now with our partners,” he adds.
DNV GL is able to provide the mathematical modelling services required to undertake this and similar work: “Testing does subject a power plant to a reasonable, although measured, amount of stress,” says Steve, “and, it’s therefore impractical to test every conceivable power level and configuration; however, this can be addressed using validated mathematical models. Classification societies have agreed a level of testing that they will accept as a starting point, and our mathematical models are validated using that data in order to demonstrate fault tolerance in a full range of approved operating conditions.”
“This type of verification process results in a very robust design, which is crucial to industry confidence” Steve adds. “It is better than any previous verification process. It’s a more holistic approach which requires additional effort, but the prize is worth having.”
Further information on DP vessel design philosophy and rule requirements is available in recommended practices DNV-RP-E306, DNV-RP-D102 and offshore technical guidance DNVGL-OTG-10 Edition April 2015. Click here for further advice and detail.