Page 620 - Corrosion Engineering Principles and Practice

582 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 583

CP systems, causing significant currents (and therefore voltage drops)
to occur in the flowlines connecting the components.
The following example pertains to a floating production system
(FPS) connected to a single point mooring (SPM) system by an
offloading line (Fig. 13.46). The FPS may be protected by an ICCP
system and the SPM by a sacrificial anode system. If these systems
are not electrically connected then there is unlikely to be interference
as the only electrical return path is through the seawater itself.
However, if they are connected by the offloading lines an electrical
circuit is created which allows current to flow from the ICCP anodes
on the FPS to the SPM and current to return along the offloading
lines. This can be expressed mathematically in Eq. (13.7) for the SPM
and Eq. (13.8) for the FPS.

I = I + I (13.7)
flowline connected to SPM SPM anodes SPM metal surfaces

I = I + I (13.8)
flowline connected to FPS FPS anodes FPS metal surfaces
For equilibrium the current flowing along the flowline to either
the SPM or the FPS must balance. The currents flowing into or out of
the metal surfaces on the SPM and the FPS can be determined by
integrating the normal current density calculated by the BE over the

Semisubmersible
floating production system Single point
mooring system

Mooring system

Flexible catenary
High temperature & Riser tower
Subsea pressure jumper
wells,
manifold Drilling riser

FIGURE 13.46 Example of a modern floating deepwater oil and gas production
system.