Nace CP 1, Course Manual

Page: 325 / 535

RP0176-2003
NACE International
3
Structure-to-Electrolyte Potential: The potential difference
between the surface of a buried or submerged metallic
structure and the electrolyte that is measured with reference
to an electrode in contact with the electrolyte.
Submerged Zone: The zone that extends downward from
the splash zone and includes that portion of the structure
below the mudline.
Subsea Template: A structure intended to be placed on
the sea floor to accommodate oil and/or gas wells, related
production equipment, and/or pipelines.
Thinner: A volatile solvent used to lower the viscosity of a
coating material.
Thermoplastic: A material capable of being repeatedly
softened by heat and hardened by cooling.
Thermosetting: A material that undergoes a chemical reac-
tion from the action of heat and pressure, catalysts, and
ultraviolet light, leading to a relatively infusible state.
Tie Coat: A special-purpose intermediate coat, usually org-
anic, used to act as a bridge between a primer and finish
coat to overcome either incompatibility or application
problems between the primer and finish coats. Urethane: A chemically cured coating consisting of a solu-
tion vinyl, vinyl acrylic, or acrylic base reacted with isocyan-
ate converter to form a tough, durable, glossy coating.
Valve Reach Rod: A long valve stem extension that en-
ables personnel on the surface to open and close a ballast
control valve located near the bottom of the structure.
Vinyl Acrylic: Vinyl resin modified with acrylic resin in sol-
vent solution.
Vinyl Coatings: Vinyl resin dissolved in a solvent. Vinyl sol-
ution coatings may be air dried or baked.
Voltage: An electromotive force, or a difference in electrode
potentials, expressed in volts or millivolts.
Voltage Drop: The voltage across a current-carrying resist-
ance in accordance with Ohm’s Law.
Wash Primer: A thin, inhibiting primer, usually chromate
pigmented, with a polyvinyl butyral binder.
Wear Plate: A sacrificial member attached to the structure
in the splash zone to protect it from anticipated corrosion
and/or erosion caused by ice and/or high-velocity, silt-laden
water.
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Section 3: Structural Design for Corrosion Control
3.1 Introduction
3.1.1 This section recommends design features that
simplify corrosion control of structural steel in the at-
mospheric, splash, and submerged zones of offshore
structures. The structural design parameters that must
be considered for an offshore structure to resist dy-
namic and static loads are the responsibility of civil and
structural engineers and are not a part of this standard.
3.2 Splash Zone
3.2.1 This is the zone of an offshore structure that is
alternately in and out of the water because of the influ-
ence of tides, winds, and seas. Excluded from this
zone are surfaces that are wetted only during major
storms. In the Gulf of Mexico, the splash zone typically
covers an interval of about 2 m (6 ft); in Cook Inlet,
Alaska, the interval approaches 9 m (30 ft); during win-
ter storms the splash zone in the North Sea can be 10
m (33 ft).
3.2.2 A structure should be designed to minimize the
surface area of steel in the splash zone. Intersecting
“T,” “K,” and “Y” joints should be avoided in the splash
zone.
3.2.3 Methods for controlling corrosion in the splash
zone are presented in Section 10.
3.2.4 Added metal thickness, either as a weld-applied
wrap or as a heavier-wall tubular member, may be
used to compensate for anticipated splash zone corro-
sion over the life of an offshore structure.
In determining the required level of added wall thick-
ness the following factors should be taken into account:
•
Required life of the structure
•
Use of lifetime minimum or no maintenance coat-
ings (e.g., thermal-sprayed aluminum)
•
Maintenance philosophy if there is coating dam-
age (how long is it likely to be left to corrode before
the coating is repaired)
•
Corrosion rate of carbon steel in seawater at the
location the structure will be installed
3.2.5 Piping, etc., passing through the splash zone,
that is to be removed after installation, should prefer-
ably be clamped to the structural member, not welded
to it.