DP systems are normally designed to counteract static and slowly varying forces, and do not respond to first-order wave forces from drag and mass inertia. These forces can be large but have such low periods that the duration of the load, in each direction, is normally too short to initiate any significant movement.

Despite this, some incidents of loss of position are explained by wave loading.

In 2021, a study was conducted of two different DP-operated semis in operational sea states, which concluded that low-frequency wave drift forces can cause a loss of position (Sintef report 2021).

This work was continued in 2022, with a study of the probability of being able to hold position in given sea states (Hs and Tp), for the same DP-operated semis used in the first study.

As the tables in Chapter 5 of the study report show, the probability that the studied semis would be able to maintain position in steep sea states is low.

The results of the work are valid only for the models studied, but the methodology is applicable to related hull forms.

The presented approach is considered to provide a suitable basis for determining interrupt criteria for preventing wave-induced loss of position in DP-operated facilities.