Page 376 - Pipelines and Risers

Pipeline Inspeciion, Mainienance and Repair 341

STABILISATION MATTRESS
IN INSTALLED POSITION

LEADING MATTRESS
EDGES SCOURED INTO
THE SEAEEO

Figure 18.3 Stabilization mattress type stability method.

Cathodic Protection Repairs
The cathodic protection facilities of the pipeline system may need to be repaired or enhanced
if the system performance is shown to be inadequate This ineffectiveness may be due to a the
anodes being damaged or being prematurely depleted as a result of bad CP design or
unexpected and severe corrosion coating breakdown.

The introduction and connection of anode “sledges” may be utilized to achieve extra cathodic
protection. These anode “sledges” are connected at specified intervals along the pipeline
system and at a minimum stand-off distance from the line, both requirements being optimized
for a given situation.

Electrical connection between the end of the anode “sledge” cable and the pipeline is typically
achieved by employing mechanical screws or by “wet” welding onto an in-situ doubler plate
from an original anode. The use of screwed connections, although simpler in concept, have
been known to loose their electrical contact over time. The technique of “wet” welding onto
an in-situ doubler plate or strap is therefore recommended as the preferred method of
providing electrical contact.

Pipeline Span Rectification
Within the pipeline system’s design life unacceptable freespans may develop due to a number
of factors which include scouring action or the passage of sand waves. It is usual practice,
during the pipeline design phase to calculate the permitted spans of the system for all phase of
installation and operation. With the pipeline full of water, air or gas allowable spans are