DNV Introduces New Rules for Ageing Drilling Rigs

Many mobile offshore units have now reached an age of 20 years or more, thereby exceeding their originally planned lifetime. DNV has introduced new rules and calculation tools that allow owners to systematically demonstrate to operators and regulators that their units can remain safely in operation for many more years beyond the originally planned lifetime.

The measures introduced include calculation of a Fatigue Utilization Index (FUI) for each unit; FUI being defined as the ratio between effective operational time and documented fatigue life. For units with FUI above 1, depending on the defect history, additional provisions have been introduced such as focusing on thorough repair and improvement of critical design details, with requirements for increased frequency of NDE inspections and leak detection monitoring if attention has not been paid to such details.

The measures have now successfully been in place for two years, and DNV has concluded that there are few indications that operation of offshore units beyond their calculated fatigue life constitutes an imminent safety problem if the new measures are followed.

For more than 20 years, DNV has also provided their classed units with In-service Inspection Programs, which are specific for each unit and its particular arrangement and circumstances.

To further assist rig owners with lifetime expectancy calculations for their rigs, DNV has also developed a tool enabling preparation of tailor-made inspection programs based on risk-based inspection principles.

Traditional planning of in-service inspection takes basis in prescriptive rules and leaves limited possibility to adapt the inspection effort to the actual condition of the unit. By application of probabilistic crack growth models based on the use of structural reliability models for prediction of future deterioration, additional information about the actual deterioration may be introduced and greatly enhance the precision of the predictions.

The tool is named RB-IIP (Risk based In-service Inspection Program) and makes it possible to:

  • optimize the inspection activity

  • focus the NDT on critical locations

  • confidently operate after exceeding documented fatigue life, provided adequate actions are taken.