A valuable property of a turret moored FPSO is its ability to weathervane or self adjust its heading to the varying metocean environment. Still, heading control with thrusters is necessary when a different heading than the natural weathervaning heading is wanted for operational reasons. In particular, in non-colinear metocean conditions it may be desirable to keep the bow of the FPSO against the waves to reduce rolling. It may happen that the weathervaning becomes unstable. In this case, the vessel will go into fishtailing motion, which is a phenomenon that is governed mainly by interaction between sway and yaw. Fishtailing is weather-dependent. When it happens, heading control is needed to stabilize it. One important objective of the study has been to investigate if satisfactory heading control can be obtained using thrusters aft only, or if thrusters forward are necessary. For a given FPSO design, we study weathervaning stability by formulating a linear model in sway and yaw. Based on this model with damping neglected, we develop three simplified criteria for stability. Using heuristic argumentation, we accept the criteria as conditions for sufficiency. The system's eigenvalues give sufficient and necessary conditions for stability. Using metocean hindcast data covering a time span of 56 years, the simple criteria are tested against the eigenvalues of the sway-yaw model. We found good agreement. The heading controller used is essentially a PID controller. It is found that with aft thrusters only, good heading control can be achieved for all the metocean states in the 56 years span, provided the controller gain is moderate. With high controller gain, the thrusters may excite resonance in the turret mooring. Using additional thrusters forward gives the freedom to reduce turret resonance. Time domain simulation with an accurate 6-degree-of-freedom model shows that thrusters aft and forward gives slightly better control than control using aft thrusters only. Still, using only aft thrusters appears to give satisfactory heading control.

The behaviour and characteristics of a turret-moored FPSO subjected to loading from waves, wind and current are investigated. Of particular importance is to find out if fishtailing instabilities may occur, and if such instability can be disclosed by simple criteria involving basic parameters of the system’s mathematical model. Eight cases from model tests are chosen for theoretical study and time domain simulation. Four of the cases involve heading control with thrusters. For the stability study, a simplified linear model in sway and yaw is formulated. It is shown that the inherent characteristics of the model depend on the strengths and relative directions of the metocean processes. The eigenvalues of the sway-yaw model are computed for the eight selected cases to check the stability. A simple approximate criterion for heading stability is derived from the sway-yaw model. It is assumed, but not proven, that the criterion is a sufficiency criterion for stability. Both the experiments and the simulations show that the eight cases are stable. This is also confirmed by the eigenvalues of the sway-yaw model, while the simple criterion wrongly deems several cases unstable. The simple stability criterion is therefore probably conservative, at least when there is significant damping in the system. In one additional hypothetical case with only wave excitation and weak or lacking stability, the simplified criterion agrees well with the model test and simulation. Heading control is necessary when a heading different from the natural weathervaning heading is wanted. The controller used in the experiments and simulations is of simple SISO PID type. With control, the heading variations are reduced significantly.

No abstract

Incidents of loss of position for semi-sumersible vessels have been found to be caused by wave-drift loads. For a semi, the wave-drift loads can be significantly larger than predicted by conventional methods based on potential theory, due to viscous loads on the columns. In addition, the wave-drift load may increase due to the presence of current. A DP system will give much stronger damping than a mooring system of similar restoring stiffness. This will alter the characteristic of the vessel's low frequency motion, which will tend to be exponentially distributed. A consequence of this is that the extreme wave-driven vessel excursions will be large in comparison with the average motion. A marine operation can only be carried out provided the critical variables of response stay below given limits with a sufficiently large probability. To estimate probabilities of limit non-exceedance, simulation of 100 hours of vessel motion is carried out for a number of sea states. Weibull distributions are then fitted to the response data and further used for extreme value calculation. Although the processes of wave frequency (1st order, WF) motion and the low frequency (2nd-order, LF) wave-induced motion are totally different, Weibull distributions could successfully be fitted to the total (LF+WF) motion. The fitted Weibull distributions were close to the exponential distribution, which they should for a strongly damped vessel. From the Weibull distributions quantiles of vessel motion could be calculated for given probabilities of limit non-exceedance. Examples of how this information can be used in interrupt criteria for marine operations are given.

A number of incidents of wave-induced loss of position for dynamically positioned semi-submersible under normal operating conditions have occurred in the Norwegian offshore sector in recent years. A study has been carried out to seek their cause. One hypothesis for the cause has been slamming load from an extremely tall and steep wave that is not effectively counteracted by the DP system due to delays in signal filters and thruster response. Another cause could the loads from a train of tall and steep waves. A linear single-degree-of-freedom numerical model is made for a vessel with DP. The model is essential in that it represents the basic characteristics of the DP system: State observer/filter and feedback control. The model is used to calculate the frequency response and the impulse response of the dynamically positioned semi-submersible. By calculating extreme impulsive load based on published theory it is established that slamming will not cause great vessel excursion. Still, due to delays, the DP system gives more than twice as large excursion as a mass-spring-damper system with identical restoring stiffness and damping. Using an advanced model for vessel with DP, three-hour simulations of stochastic vessel response are carried out for five steep wave states of moderate significant height, The likely cause of large vessel excursion is found to be wave-drift. Due to additional viscous loads on the semi’s columns the wave-drift loads will be significantly larger than predicted with potential theory.