Page 272 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Design index 12/249
levels are high and a corrosive environment exists, either inside from other pipelines, offshore platforms, or shore structures.
or outside the pipe wall. Note that seawater is a corrosive envi- When isolation is not provided, joint cathodic protection of the
ronment for metal and higher stress levels are common in structure and the pipeline should be in place. Score this item as
offshore operations. described in Chapter 4.
Score this item as described in Chapter 4.
AC interference
C2. Cathodic protection (weighting: 25%)
This variable will often not apply for offshore pipelines, except
On pages 74-76, we discuss some basic concepts of galvanic perhaps at shore approaches. The evaluation can he based on
corrosion and common industry practices to address the corro- the same criteria as discussed in Chapter 4. Because the AC
sion potential. These apply equally to offshore pipelines. interference is normally not an important risk indicator for off-
Because of the strong electrolytic characteristics of seawater shore pipelines, those possible points can be distributed to other
(uniform conductivity), cathodic protection is often achieved variables where there is a belief that other variables play a larger
by the direct attachment of anodes (sometimes called bracelet role in the offshore pipeline risk picture.
anodes) at regular spacing along the length of the pipeline.
Impressed current, via current rectifiers, is sometimes used to C2. Coating (weighting: 250/,)
supplement the natural electromotive forces. Attention should
be paid to the design life of the anodes. As a primary defense against corrosion, the pipe coating is
Score this item as described in Chapter 4. intended to provide a barrier between the pipe wall and the elec-
trolyte. Because concrete is often placed over the anticorrosion
Test leads coating for buoyancy and/or mechanical protection, it can he
evaluated as part of the coating system. The concrete should be
The effectiveness of the cathodic protection is often monitored compatible with the underlying coating during installation and
by measuring the voltage of the pipe relative to a silvedsilver long-term operation. Metal reinforcing within the concrete can
nitrate reference electrode in the water in the same fashion as interfere with the cathodic protection currents and should be
the copper/copper sulfate reference electrode is used in onshore designed for proper performance.
analysis. The use of test lead readings to gauge cathodic protec- Offshore coatings must often be designed to withstand
tion effectiveness has some significant limitations since they more forces during installation, compared with onshore
are, in effect, only spot samples of the CP levels. Nonetheless, installations. Coating properties such as flexibility, moisture
monitoring at test leads is the most commonly used method for uptake, and adhesion may be more critical in the offshore
inspecting adequacy of CP on onshore pipelines. A discussion installation.
oftest leads for onshore lines @ages 79-82) applies in theory to Some amount of coating degradation is to be expected with
offshore lines as well. Offshore lines normally provide few the aging of a pipeline. A pipeline operated at higher tempera-
opportunities to install and later access useful test leads. tures may cause more stress on the coating.
Therefore, it is thought that this item does not play as Score this item as described in Chapter 4. Points can be
significant a role as it does in the onshore case. When pipe- awarded based on
to-electrolyte readings are taken by divers or other means at
locations along the pipeline, points may be awarded here or as a Quality of coating
type of close interval surveys. Quality of application
Quality of inspection
Close interval survey Quality of defect corrections.
A close interval survey (CIS) technique for offshore lines can
involve towing an electrode through the water above the line IV. Design index
and taking continuous voltage readings between the pipe and its
surroundings. Another technique involves the use of remotely The design environment for an offshore pipeline is quite dif-
operated vehicles (ROVs) and/or divers to follow the pipeline ferent from that of an onshore pipeline. The offshore line is
and perform a visual inspection as well as pipe-to-electrolyte subjected to external pressures and forces from the water/
readings. Because the reference electrode must be electrically wavekurrent environment that are usually more dynamic and
connected to the pipeline, limitations in the practical use of often more severe. As previously noted, the pipe is being placed
these techniques exist. When conditions allow, spot checking in an environment where man cannot live and work without the
by divers can also provide information similar to the close inter- aid of life-support systems. The difficulties in installation are
val survey. numerous. Many of the risk-related differences between
Score this item as described in Chapter 4. onshore and offshore pipeline systems will appear here in the
design index. Related to this, see also the construction portion
Currentjlow to other buried metals of the incorrect operations index.
It should be assumed that the industry will continue to move
When the density of foreign pipelines or other metallic struc- into more challenging environments such as deeper water,
tures is high, the potential for cathodic protection interferences more extreme temperatures, and arctic conditions. This pres-
is correspondingly high. In scoring this item, the evaluator ents new problems to overcome in design, construction, and
should note the isolation techniques used in separating piping integrity monitoring.