On commission from the Petroleum Safety Authority Norway (PSA), Sintef has carried out a study of wave loading that can cause a loss of position in DP-operated facilities.
DP systems are normally designed to counteract static and slowly varying forces, and respond to a limited extent to first-order wave forces. These forces change direction and, over a wave period, the mean tends to zero. The duration of the load, in any one direction, is normally too short to initiate any significant movement. Despite this, some incidents of loss of position are explained by wave loading.
The study includes analyses of two different DP-operated semis in operational sea states, and concludes that low-frequency wave drift forces can cause a loss of position. The largest wave drift forces often coincide with groups of high waves passing through the facility, which is why this effect is also referred to as a wave train. The wave drift forces are a nonlinear effect of sea states consisting of multiple harmonic waves of different amplitude and period.The study shows that the effect of wave drift forces should be taken into account when establishing operational criteria.
