Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 392

The same considerations apply equally to jacket structures, with one important
addition, namely, that a power source to drive the impressed current system is
generally not available until the topside power generation equipment is installed
and commissioned. On large deepwater jackets in the North Sea, this may be a
year or more after installation of the jacket; protection for the interim period is
provided by high-current, short-life sacrificial anodes.
It is strongly suggested that designers contemplating an impressed current
system for North Sea applications provide full sacrificial back-up, with sacrifi-
cial anodes that provide full protection for a minimum of 2 years, plus an allow-
ance for periods of possible impressed current system shut-down during subsea
surveys and maintenance throughout the design life.
The obvious technical attractions of sacrificial systems, as illustrated in
Table 6.3 , make them the type most often chosen for offshore structures.
Also, for many offshore structures, sacrificial systems are the most economical
option for the owner when taking into account both capital expenditure and the
running costs over the design life of the structure. However, generalizing about
economic advantages and disadvantages can be misleading, because they differ
widely for each type and size of structure and according to the design con-
straints imposed by the environmental conditions prevailing at different off-
shore locations. For this reason, economic advantages/disadvantages are not
included in Table 6.2 .
The hybrid system is used in Murchison and Hutton platforms due to the
heavy weight constraints. The CP designers of the Murchison and Hutton plat-
forms in the North Sea carried out detailed assessments of alternative sacrificial
and impressed current designs. The assessments revealed that use of the sacri-
ficial anode is essential but that the heavy weight can be reduced by using
impressed current systems as the primary means of protection on both plat-
forms, despite their vastly different structural configurations, as Murchison is a
deepwater conventional jacket but Hutton was the world ' s first tension-leg

platform.
Thus, a main advantage of using hybrid systems over sacrifical systems is
weight-saving on Murchison and Hutton platforms. In the case of Hutton, the
installed weight of the primary impressed current system, plus supplementary
sacrificial anodes located close to the main node joints, was approximately
60 tons. On the other hand, an equivalent totally sacrificial system would have
weighed around 250 tons. From a practical point of view, the most economical
solution for buoyant structures, such as tension-leg platforms, is the impressed
current system, which is different from the solution for the fixed offshore struc-
ture platforms.
The relatively simple geometry and large, flat surfaces of buoyant structures
are ideally suited for protection to be provided by a small number of high-
current, low-voltage, flush-mounted anodes. Cables to reference electrodes
and anodes can be easily and economically routed through ballast tanks and
human access pathways in the pontoons and columns of the hull.