Page 621 - Corrosion Engineering Principles and Practice

584 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 585

metal surfaces of the structure. Therefore the internal circuit resistance
equations and the BE equations representing the seawater can be
solved together to predict the potentials and currents flowing in the
overall system.
In the application example presented here the investigation
focuses on an offloading SPM and its anchor chains. The anchor
chains present an interesting CP problem because there is significant
attenuation of the current flowing along the chains due to very high
internal resistance caused by the chain link to chain link contact.
Therefore near the SPM the chain will receive significant current from
the anodes but this will drop off significantly within 25 to 30 m. The
model of the SPM and its anchor chains is shown in Fig. 13.47.
The 3D model of the connectors, seen as cylinders in Fig. 13.47,
needs to include the internal geometry because the anodes not only
need to protect the outer metallic surfaces but also the internal
structure. The connectors are hollow and the anchor chains enter into
the connectors and are connected at the far end of the connector
cylinder. In the model of the complete system the lines connecting to
the FPS would also be included as well as the FPS and the subsea
systems.
The predicted protection potentials provided by the CP system is
shown in Fig. 13.48. One possible design change is to create holes in
the connectors so the CP can provide better protection to the internal
structure and chain. Computer modeling can help by predicting the
effectiveness of different options.

Single point
mooring system Sea surface

FIGURE 13.47 Model of the SPM and the anchor chains. (Courtesy of Robert
A. Adey, Computational Mechanics BEASY)