In 1982, Philippines-Cities Service experienced a wear failure of a cateneary anchor chain mooring used to moor a CALM type buoy. In order to explain the failure, Cities Service conducted a comprehensive failure analysis and model basin test of the failed system, and in addition, performed comparative wear tests on U3 and U4 chain. The results of the investigation indicate that interlink motion and resulting wear are an important design criteria for mooring lines.
In 1982, Phillippines-Cities Service installed a CALM type mooring terminal near Palawan Island in the South China Sea. A 100,000 DWT tanker was soft moored to the CALM for use as a permanent storage facility for the Matinloc Oil field. The system was to remain in service until a rigid arm system under fabrication could replace the CALM type mooring.
As a consequence, the anchor chains were sized for the rigid arm system. Since the CALM buoy did not have sufficient buoyancy to support all six chain legs (standard 6-leg spread with 60° between adjacent chains), only 4 legs were connected. To further reduce weight and maintain adequate safety factors, U4 chain was used in the uppermost portion of the catenary near the buoy. Figure 1 shows the resulting anchor chain pattern which later failed. Table 1 summarizes the important parameters of the mooring at the time of the failure.
Approximately two months after installation a typhoon passed near the Matinclo field generating 40–70 knot winds and 3U ft. maximum waves for a period of about three days. The maximum storm intensity was less severe than the design storm for the tanker connected to the buoy. During the storm two chains failed by excessive interlink wear and the remaining two chains were worn about 50% through. Figure 2 shows one such failed link. On all chains the wear occurred at the junction between the fixed stopper link and the first tree link in the catenary. This point experiences the majority of relative motion between the buoy and the catenary.
Historically, accelerated wear rates have not occurred on similar CALMS; however, the Cities terminal was unique in several ways. Grade U4 chain relatively new product of high strength chain) was used in the wear zone where large interlink relative motions occur. Also, the buoy anchor chain pattern was asymmetric with distinct strong and weak roll stiffness axes and surge stiffness axes. Clearly, either the metallurgical composition of the U4 chain is unusually susceptible to wear as compared to U3 chain; or the asymmetric hookup of the buoy generates such large forces and motions that any anchor chain steel would wear at an accelerated rate, regardless of grade.
A failure analysis and review of existing metallurgical data showed that an experimental wear test program was needed to define the wear characteristics of both U3 and U4 chain. Model basin studies were also performed to quantity the force-motion time history at the wear point.
