The study is part of Havtil’s work related to the topic of loss of position for floating offshore facilities.

We have previously investigated incidents involving loss of position in connection with drilling operations (West Mira and West Hercules) and gangway operations (Floatel Endurance). Audits have also been carried out among industry operators to follow up on their efforts to ensure safety in connection with DP-operations.

In addition, the following studies related to this topic have been carried out and published:

In the 2022 study on interrupt criteria, the parameter probability of success was introduced and examined as a criterion to support decisions on whether to continue or abort operations. In the industry, the Most Probable Maximum (MPM) value, or sometimes the Expected Maximum (EMAX), is often used when establishing operational limits. In our view, the use of these parameters may in some cases lead to a wrong understanding of risk, as they are exceeded in 63% and 43% of cases, respectively, for relevant dynamic processes.

The use of probability of success and thruster utilisation factor is demonstrated in this study by analysing two different marine operations:

  1. Station-keeping for a semi-submersible drilling unit
  2. Gangway operations between a floatel and a moored FPSO

The two case studies illustrate fundamentally different exceedance mechanisms:

  • The drilling operation is capacity-limited, governed by the thrust margin against environmental forces
  • The gangway operation is precision-limited and sensitive to lateral and vertical relative motions that first-order waves can typically excite

The report also provides guidance and recommendations for establishing simulation models, selecting environmental conditions, and defining simulation durations to ensure statistically reliable results.

The handling of weather forecast uncertainty, as well as the effects of sea states with multiple wave components (wind sea and swell), is also discussed.