The electronic pdf versions of the documents found through http://www.dnv.com/ are the officially binding versions. Copyright Det Norske Veritas.
![[Table of Contents]](/dwicons/b_toc.gif)
|
DNV-OS-C201 Structural Design of Offshore Units (WSD method)
|
SECTION 10
Corrosion ControlSec.10
A. General
Sec.10
A 100 Scope
Sec.10 A
101 Corrosion control of structural steel for offshore structures
comprises:
| — | coatings and/or cathodic
protection |
| — | use of a corrosion allowance |
| — | inspection/monitoring of corrosion |
| — | control of humidity for internal zones (compartments). |
Sec.10 A
102 This section gives technical requirements and guidance for
the design of corrosion control of structural steel associated with
offshore steel structures. The manufacturing/installation
of systems for corrosion control and inspection and monitoring of
corrosion in operation are covered in DNV-OS-C401.Sec.10
B. Techniques for Corrosion Control
Related to Environmental Zones
Sec.10
B 100 Atmospheric zone
Sec.10 B
101 Steel surfaces in the atmospheric zone shall be protected
by a coating system (see D100)
proven for marine atmospheres by practical experience or relevant
testing.Guidance note:
The "Atmospheric Zone" is defined as the
areas of a structure above the Splash Zone (see B201) being exposed
to sea spray, atmospheric precipitation and/or condensation.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10
B 200 Splash zone
Sec.10 B
201 Steel surfaces in the splash zone shall be protected by a
coating system (see D100) proven for splash zone applications by
practical experience or relevant testing. A corrosion allowance
should also be considered in combination with a coating system for
especially critical structural items.
Sec.10 B
202 Steel surfaces in the splash zone, below the mean sea level
(MSL) for bottom fixed structures or below the normal operating
draught for floating units, shall be designed with cathodic protection
in addition to coating.
Sec.10 B
203 The splash zone is that part of an installation, which is
intermittently exposed to air and immersed in the sea. The zone
has special requirements to fatigue for bottom fixed units and floating
units that have constant draught.Guidance note:
Constant draught means that the unit is not designed for changing
the draught for inspection and repair for the splash zone and other
submerged areas.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 B
204 For floating units with constant draught, the extent of the
splash zone shall extend 5 m above and 4 m below this draught.
Sec.10 B
205 For bottom fixed structures, such as jackets and TLPs, the
definitions of splash zone given in 205 to 207 apply.The wave height to be used to determine the upper and lower
limits of the splash zone shall be taken as 1/3 of the
wave height that has an annual probability of being exceeded of
10-2.
Sec.10 B
206 The upper limit of the splash zone (SZU) shall be calculated by:
where:
| U1 | = | 60% of the wave height defined
in 205 |
| U2 | = | highest astronomical tide level (HAT) |
| U3 | = | foundation settlement, if applicable |
| U4 | = | range of operation draught, if applicable |
| U5 | = | motion of the structure, if applicable. |
The variables (Ui)
shall be applied, as relevant, to the structure in question, with
a sign leading to the largest or larger value of SZU.
Sec.10 B
207 The lower limit of the splash zone (SZL) shall be calculated by:
where:
| L1 | = | 40% of the wave height defined
in 205 |
| L2 | = | lowest astronomical tide level (LAT) |
| L3 | = | range of operating draught, if applicable |
| L4 | = | motions of the structure, if applicable. |
The variables (Li)
shall be applied, as relevant, to the structure in question, with
a sign leading to the smallest or smaller value of SZL.
Sec.10
B 300 Submerged zone
Sec.10 B
301 Steel surfaces in the submerged zone shall have a cathodic
protection system. The cathodic protection design shall include
current drain to any electrically connected items for which cathodic
protection is not considered necessary (e.g. piles).The cathodic protection shall also include the splash zone
beneath MSL (for bottom fixed structures) and splash zone beneath
normal operating draught (for floating units), see B202.
Guidance note:
The 'Submerged Zone' is defined as the zone
below the splash zone.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 B
302 For certain applications, cathodic protection is only practical
in combination with a coating system. Any coating system shall be
proven for use in the submerged zone by practical experience or
relevant testing demonstrating compatibility with cathodic protection. Guidance note:
Cathodic protection may cause damage to coatings by blistering
or general disbondment ("cathodic disbondment").---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10
B 400 Internal zone
Sec.10 B
401 Internal zones exposed to seawater for a main period of time
(e.g. ballast tanks) shall be protected by a coating system (see D100) proven for such applications
by practical experience or relevant testing. Cathodic protection
should be considered for use in combination with coating (see also
402).Guidance note:
Internal Zones' are defined as tanks, voids and other
internal spaces containing a potentially corrosive environment, including
seawater.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 B
402 Internal zones that are empty (including those occasionally
exposed to seawater for a short duration of time) shall have a coating
system and/or corrosion allowance. For internal zones with
continuous control of humidity, no further corrosion control is
required. Further, no coating is required for zones that do not
contain water and that are permanently sealed.
Sec.10 B
403 Tanks for fresh water shall have a suitable coating system.
Special requirements will apply for coating systems to be used for
potable water tanks.
Sec.10 B
404 To facilitate inspection, light coloured and hard coatings
shall be used for components of internal zones subject to major
fatigue forces requiring visual inspection for cracks. Regarding
restrictions for use of coatings with high content of aluminium,
see D101.
Sec.10 B
405 Only anodes on aluminium or zinc basis shall be used. Due
to the risk of hydrogen gas accumulation, anodes of magnesium or
impressed current cathodic protection are prohibited for use in
tanks.
Sec.10 B
406 For cathodic protection of ballast tanks that may become affected
by hazardous gas from adjacent tanks for storage of oil or other
liquids with flash point less than 60°C, anodes on zinc basis are
preferred. Due to the risk of thermite ignition, any aluminium base
anodes shall in no case be installed such that a detached anode could
generate an energy of 275 J or higher (i.e. as calculated from anode
weight and height above tank top). For the same reason, coatings
containing more than 10% aluminium on dry weight basis
shall not be used for such tanks.
Sec.10 B
407 A corrosion allowance shall be implemented for internal compartments
without any corrosion protection (coating and/or cathodic
protection) but subject to a potentially corrosive environment such
as intermittent exposure to seawater, humid atmosphere or produced/cargo
oil.Any corrosion allowance for individual components (e.g. plates,
stiffeners and girders) shall be defined taking into account:
| — | design life |
| — | maintenance philosophy |
| — | steel temperature |
| — | single or double side exposure. |
As a minimum, any corrosion allowance (tk) to be applied as alternative
to coating shall be as follows:
| — | one side unprotected: tk = 1.0 mm |
| — | two sides unprotected: tk = 2.0
mm. |
Sec.10
C. Cathodic Protection
Sec.10
C 100 General
Sec.10 C
101 Cathodic protection of offshore structures may be effected
using galvanic anodes (also referred to as "sacrificial
anodes") or impressed current from a rectifier. Impressed
current is almost invariably used in combination with a coating
system. Guidance note:
The benefits of a coating system (e.g. by reducing weight
or friction to seawater flow caused by excessive amounts of anodes)
should also be considered for systems based on galvanic anodes.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 C
102 Cathodic protection systems in marine environments are typically
designed to sustain a protection potential in the range - 0.80 V
to - 1.10 V relative to the Ag/AgCl/seawater reference
electrode. More negative potentials may apply in the vicinity of
impressed current anodes.Guidance note:
The use of galvanic anodes based on aluminium and zinc limits
the most negative potential to - 1.10 V relative to Ag/AgCl/seawater.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 C
103 Design of cathodic protection systems for offshore structures
shall be carried out according to a recognised standard.Guidance note:
Recommendations for cathodic protection design may be found
in DNV-RP-B401.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 C
104 Cathodic protection may cause hydrogen induced stress cracking
(HISC) of components in high strength steels that are exposed to
severe straining in service It is recommended that the welding of high strength structural
steels is qualified to limit the hardness in the weld zone to max.
350 HV (Vicker hardness). The use of coatings reduces the risk of
hydrogen embrittlement further and is recommended for all critical
components in high strength structural steel.
Guidance note:
There is no evidence in the literature that structural steels
with SMYS up to 550 N/mm2 have
suffered any cracking when exposed to cathodic protection in marine
environments at the protection potential range given in 102.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10
C 200 Galvanic anode systems
Sec.10 C
201 Unless replacement of anodes is allowed for in the design,
galvanic anode cathodic protection systems shall have a design life
at least equal to that of the offshore installation. For ballast
tanks with access for replacement of anodes and any other such applications,
the minimum design life should be 5 years.
Sec.10 C
202 Anode cores shall be designed to ensure attachment during
all phases of installation and operation of the structure. Location
of anodes in fatigue sensitive areas shall be avoided.
Sec.10 C
203 The documentation of cathodic protection design by galvanic
anodes shall contain the following items as a minimum:| — | reference to design code and
design premises |
| — | calculations of surface areas and cathodic current demand
(mean and initial/final) for individual sections of the
structure |
| — | calculations of required net anode mass for the applicable
sections based on the mean current demands |
| — | calculations of required anode current output per anode
and number of anodes for individual sections based on initial/final
current demands |
| — | drawings of individual anodes and their location. |
Sec.10 C
204 Requirements to the manufacturing of anodes (see 205) shall
be defined during design, e.g. by reference to a standard or in
a project specification.
Sec.10 C
205 Galvanic anodes shall be manufactured according to a manufacturing
procedure specification (to be prepared by manufacturer) defining
requirements to the following items as a minimum:| — | chemical compositional limits |
| — | anode core material standard and preparation prior to
casting |
| — | weight and dimensional tolerances |
| — | inspection and testing |
| — | marking, traceability and documentation. |
Sec.10 C
206 The needs for a commissioning procedure including measurements
of protection potentials at pre-defined locations should be considered
during design. As a minimum, recordings of the general protection
level shall be performed by lowering a reference electrode from
a location above the water level.
Sec.10 C
207 Manufacturing and installation of galvanic anodes are addressed
in DNV-OS-C401 Sec.5.Sec.10
C 300 Impressed current systems
Sec.10 C
301 Impressed current anodes and reference electrodes for control
of current output shall be designed with a design life at least
equal to that of the offshore installation unless replacement of
anodes (and other critical components) during operation is presumed.
It is recommended that the design in any case allows for replacement
of any defective anodes and reference electrodes (see 304) during
operation.
Sec.10 C
302 Impressed current anodes shall be mounted flush with the object
to be protected and shall have a relatively thick non-conducting
coating or sheet ("dielectric shield") to prevent
any negative effects of excessively negative potentials such as
disbondment of paint coatings or hydrogen induced damage of the
steel. The sizing of the sheet shall be documented during design.
Location of impressed current anodes in fatigue sensitive areas
shall be avoided.
Sec.10 C
303 Impressed current cathodic protection systems shall be designed
with a capacity of minimum 1.5 higher than the calculated final
current demand of the structure.Guidance note:
Impressed current cathodic protection provide a more non-uniform
current distribution and are more vulnerable to mechanical damage
which requires a more conservative design than for galvanic anode
systems.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 C
304 A system for control of current output based on recordings
from fixed reference electrodes located close to and remote from
the anodes shall be included in the design. Alarm functions indicating
excessive voltage/current loads on anodes, and too negative
or too positive protection potential should be provided. A failure mode
analysis should be carried out to ensure that any malfunction of
the control system will not lead to excessive negative or positive
potentials that may damage the structure or any adjacent structures.
Sec.10 C
305 Cables from rectifier to anodes and reference electrodes should
have steel armour and shall be adequately protected by routing within
a dedicated conduit (or internally within the structure, if applicable).
Restriction for routing of anode cables in hazardous areas may apply.
Sec.10 C
306 The documentation of cathodic protection design by impressed
current shall contain the following items as a minimum:| — | reference to design code and
design premises |
| — | calculations of surface areas and cathodic current demand
(mean and initial/final) for individual sections of the
structure |
| — | general arrangement drawings showing locations of anodes,
anode shields, reference electrodes, cables and rectifiers |
| — | detailed drawings of anodes, reference electrodes and
other major components of the system |
| — | documentation of anode and reference electrode performance
to justify the specified design life |
| — | documentation of rectifiers and current control system |
| — | documentation of sizing of anode shields |
| — | specification of anode shield materials and application |
| — | commissioning procedure, incl. verification of proper
protection range by independent potential measurements |
| — | operational manual, including procedures for replacement
of anodes and reference electrodes. |
Sec.10 C
307 Manufacturing and installation of impressed current cathodic
protection systems are addressed in DNV-OS-C401 Sec.5.Sec.10
D. Coating Systems
Sec.10
D 100 Specification of coating
Sec.10 D
101 Requirements to coatings for corrosion control (including
those for any impressed current anode shields) shall be defined
during design (e.g. by reference to a standard or in a project specification),
including as a minimum:| — | coating materials (generic type) |
| — | surface preparation (surface roughness and cleanliness) |
| — | thickness of individual layers |
| — | inspection and testing. |
For use of aluminium containing coatings in tanks that may
become subject to explosive gas, the aluminium content is limited
to maximum 10% on dry film basis.
Guidance note:
It is recommended that supplier specific coating materials
are qualified by relevant testing or documented performance in service.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.10 D
102 Coating materials and application of coatings are addressed
in DNV-OS-C401 Sec.5.
![[-]](/dwicons/i_colpse.gif)