DICAS - A New Mooring Concept for FPSO's

Abstract

Abstract PETROBRAS has developed a new mooring system for production and storage tankers moored in offshore Brazil. The system is denoted DICAS (Differentiated Compliance Anchoring System), and is basically a spread mooring system with different stiffness at the bow and stem of the ship. This difference in stiffness allows the ship to weather vane partially under environmental conditions. Due to the characteristics of this system, the design has to take into account the varying direction of the weather and the best layout of the production risers and mooring lines. Since DICAS is a partial weather vaning system, it makes the production swivels and turrets unnecessary. Therefore, the cost for production systems based on existing tankers will be drastically reduced, so it shall be very useful for low production and marginal fields in mild environments. This paper presents some results from experiments and simulations, and discussions on the design criteria for such systems. Important factors to be included in the design are the damping coefficients, namely the wave drift damping, the mooring line damping and the linear and quadratic viscous damping. The effect of interaction between current and waves is also important. FSO in Spread Mooring System In 1993, the first PETROBRAS FSO in an eight-lines spread mooring system was installed in Caravela Field, Santos Basin, offshore Santa Catarina. The design criteria for this location, at 195 meters water depth, was based on a temporary system to be in operation for less than 2.0 years, with one year recurrence period of disconnection. It means that the tanker was allowed to leave the vosition once a vear in mean. To help the disconnection operation, one slip hook was installed in each mooring line, as well as the riser had a quick connect and disconnect system (QCDC). At beginning, the system was minded to be symmetrical and with no differences on the stem and bow mooring lines weight and pretension. The mooring system was very tight and allowing changes in the heading of the ship only up to 5 degrees. It is convenient to mention the most frequent weather comes from North-East direction and the most severe one comes from SoutNSouth-West direction. So the tanker was installed heading South-West and the off-loading operation was done bow-to-bow, or optionally stern-to-bow, in tandem. In May 1994, a 100-year storm coming from East obliged the tanker to leave the location. There was no problems and, specially, no oil spills into the water. Learning from that experience, the DICAS idea was born and, after extensive calculations, the stem lines were installed more slacked, with different azimuth angles, giving the system more flexibility for changing the tanker heading[1]. This changes on the mooring lines decreased the probability of disconnection to once in three years period. Therefore, the Alagoas FSO, a 28,000 dwt tanker, in Caravela Field may be considered as the first DICAS prototype. Data acquired from Alagoas were fundamental to develop a DICAS for a permanent production and/or storage tanker. Based on this experience, PETROBRAS started to develop the DICAS following the design flow presented in Fig. 1