Following the unexpected rupture of chains in the chainhawse of the Girassol buoy after only half a year of service a new bending fatigue mechanism of failure has been identified for mooring systems with high pretensions. The premature rupture was caused by bending fatigue of the first free chain link inside the chainhawse. Although the mooring system had been designed according to offshore industry standards, it failed due to this bending fatigue mode. Since this failure, SBM has studied this bending phenomenon in its laboratory and redesigned both the top chain segment size and the hawse connection, which now includes a new Connecting Arm.
The main changes brought about by the Girassol incident are:
Doubly articulated chain stopper,
Use of very low friction bushings in chain stoppers,
Increased lever arm length between articulation and first chain link,
Increased chain diameter.
This paper summarizes the methodology developed to estimate the fatigue damage in the chain subjected to bending. The methodology is then applied to the design of the CCA of the Girassol buoy. Although the methodology accurately predicts the fatigue failures of Girassol, it tends to be overly conservative when applied to other systems. The source of this conservatism is still being investigated and is currently the subject of further testing in the SBM Laboratory.
The Girassol Loading Buoy was installed in September 2001 on Block 17 of the "Girassol" oil field situated offshore Angola, in 1350 m, water depth at Buoy location.
The Buoy allows the export, via 2 mid-water 16" Export Lines, of the stabilized crude oil processed onboard the nearby FPSO, moored 1 nautical mile away, south of the Buoy.
TOTAL E&P Angola operates both Loading Buoy and FPSO systems. SONANGOL is concessionnaire of the field. Block 17 is developped by a Group of Partners involving TOTAL E&P ANGOLA, ESSO ANGOLA, BP, STATOIL and NORSK HYDRO.
The Buoy is anchored to the seabed by means of 3 groups of 3 anchor legs. Before the incident, the anchor legs were composed of co-linear segments of 81 mm studded chains and 130 mm diameter polyester ropes. Within one group, the Legs are spread 5° apart, while each group is spread 120° apart (see Figure 1).
The Legs are numbered B1, B2, and B3 for the West group, B4, B5, B6 for the North group and B7, B8, B9 for the East group. The chain segments are situated at the upper and lower ends of the anchor legs, i.e. respectively at the Buoy connection and at the Anchor Pile connection.
In May 2002, i.e. 235 days after the buoy installation, the anchor legs B4, B5 and B6 broke almost simultaneously, followed one month later by the rupture of the anchor leg B1.
The ruptures in B4, B6 and B1 occurred exactly at the same location, i.e. at the 5th link of the Upper Chain Segment connected to the Buoy inside SBM standard-type "curved chainhawses", while Leg B5 broke in the Upper Rope Segment.