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.

Gjøa was the largest field development project in Norway in 2010. Gjøa was proven in 1989 and are now being developed together with nearby Vega satellites. The combined reserves are estiThe recent Gjøa field development in the North Sea has many features that are relevant for the oil and gas developments north of Western Australia. While the field location is not very similar to the north of Western Australia, the field development solution is very relevant. Several subsea clusters are tied back to a semi-submersible platform with export of gas and condensate via pipelines to shore. Other aspects to the project that are relevant to Western Australia are split location engineering between Norway and India, fabrication of the hull in Korea and subsequent heavy lift transport to the assembly yard, pre-installation of the mooring system, and tow to field with ocean going tug boats. The semi concept, which was used for the Gjøa development, is a mature technology with few technical challenges on a conceptual level. On the other hand the building of an oil and gas platform for A$2 billion has many challenges, both economical and technical, that have to be solved to have a successful project for both the client and the contractor.