Note: Descriptions are shown in the official language in which they were submitted.
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Description
Lubricating Coating Composition
Suitable for Tubular Threaded Joints
Technical Field
This invention relates to a lubricating coating composition suitable for
lubricating coating treatment of tubular threaded joints which are used to
connect oil
country tubular goods (abbreviated as OCTG) to each other.
A tubular threaded joint having a lubricating coating formed by treatment
with a composition according to the present invention can be used to connect
OCTG
io without application of a lubricating grease which contains a large amount
of heavy
metals and hence raises the a concern of causing environmental pollution, even
if the
joint is of the type having an unthreaded metal-to-metal contact portion which
provides the joint with improved sealability but which makes the joint
susceptible to
galling.
Background Art
OCTG are tubing and casing which are used to excavate oil wells. They are
normally assembled on site by connecting steel tubes having a length on the
order of
ten some meters to each other using tubular threaded joints. Conventionally,
the
depth of oil wells has been 2,000 - 3,000 meters, but in recent deep sea oil
fields, it
may reach 8,000 - 10,000 meters.
In its environment of use, a threaded joint for connecting OCTG is subjected
not only to a load in the form of an axial tensile force caused by the weight
of OCTG
and joints connected thereto but also to a combined internal and external
pressure
and subterranean heat. Therefore, it must maintain an airtight connection
without
breakage even in such a severe environment.
During the operation of lowering tubing or casing into an oil well, a tubular
threaded joint which has once been tightened is sometimes loosened and then
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retightened. API (the American Petroleum Institute) requires a tubular
threaded joint
for OCTG to have sufficient galling resistance to make it possible to carry
out
tightening (makeup) and loosening (breakout) ten times for a joint for tubing
and
three times for a joint for casing without the occurrence of galling
(unrepairable
severe seizure) while maintaining airtightness.
A tubular threaded joint having good sealability when used to connect OCTG
is of the pin-box structure capable of forming a metal-to-metal contact seal.
With a
typical tubular threaded joint of this type, a pin is formed on the outer
surface of
each end portion of a steel tube and includes a threaded portion with a male
io (external) thread and an unthreaded metal-to-metal contact portion, while a
box is
formed on the inner surface of a coupling, which is a separate connecting
member,
and includes a threaded portion with a female (internal) thread and an
unthreaded
metal-to-metal contact portion. The tubular threaded joint is tightened by
inserting
the pin into the box and tightening the male and female threads until the
unthreaded
metal-to-metal contact portions of the pin and the box tightly contact each
other to
form a metal-to-metal contact seal. Prior to tightening of the joint, a
lubricating
grease called compound grease is usually applied to the surfaces of the
threaded
portions and the unthreaded metal-to-metal contact portions which are the
engaging
portions of the joint when it is tightened, in order to provide these portions
with
improved galling resistance and airtightness. The engaging portions of a
tubular
threaded joint may be pretreated so as to have an increased surface roughness
by
surface treatment such as phosphating in order to increase the retention of a
compound grease.
However, a compound grease contains a large amount of powder of heavy
metals such as lead, zinc, and copper in order to provide the compound grease
with
sufficient lubricity and corrosion resistance. Therefore, the applied grease
causes
environmental pollution if it is washed off or squeezed out to its
surroundings. In
addition, the process of applying a compound grease worsens the work
environment
and decreases the efficiency of the operation of assembling OCTG. Accordingly,
there has been a demand for a tubular threaded joint which fulfills its
function
sufficiently without application of a compound grease.
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With the aim of providing a tubular threaded joint which can be used without
application of a compound grease, a lubricating coating composition for
forming a
lubricating coating on the engaging portions of a tubular threaded joint and a
lubricating coating or layer formed from such a composition are proposed in
the
following Japanese patent documents:
Patent Document 1: JP 2002-173692 Al
Patent Document 2: JP 2004-53013 Al
Patent Document 3: JP 2004-507698 Al.
Disclosure of the Invention
io Since the OSPAR (Oslo-Paris) Convention pertaining to preventing maritime
pollution in the Northeast Atlantic came into effect in 1998, strict
environmental
regulations have been increasing on a global scale. In the excavation of gas
wells
and oil wells on offshore rigs, in order to minimize the discharge of
causative agents
of marine pollution, there is a recent trend that any substance which is used
on a rig
is and which is capable of being discharged to the environment requires an
environmental impact assessment thereof. If the substance does not meet the
standards in that country or region, use of the substance is prohibited.
The properties to be assessed in this environmental impact assessment are
prescribed in the OSPAR Convention as the HOCNF (Harmonized Offshore
20 Chemical Notification Format). Among these properties, biodegradability
(abbreviated as BOD) is particularly important.
For a tubular threaded joint, an environmental impact assessment of a
compound grease is of course required, since its use unavoidably involves
application and washing operations on a rig. Lubricating coatings and coating
25 compositions for forming such a coating as proposed in the above-listed
patent
documents should also be subjected to such an assessment since there is a
possibility
of such a coating being discharged to its surroundings during washing of a
joint.
However, although the lubricity and anticorrosive properties of lubricating
coatings and coating compositions which have been proposed in the prior art
are
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taken into consideration, their compositions are not designed with
consideration of
biodegradability, which is now an important item requiring assessment, and
they
have not been assessed with respect to their environmental impact. Therefore,
their
use has become difficult under recent conditions in which environmental
regulations
are becoming increasingly strict.
It is an object of the present invention to provide a tubular threaded joint
suitable for use in connecting oil country tubular goods (OCTG) which
eliminates
the above-described problems of the prior art.
A more specific object of the present invention is to provide a tubular
io threaded joint which can be used to connect OCTG without application of a
lubricating grease such as a compound grease and without problems related to
lubricity, which is prevented from rusting and exhibits improved galling
resistance
and airtightness, and which can be used even in countries or regions having
strict
environmental regulations.
Another object of the present invention is to provide a lubricating coating
composition for use in the manufacture of such a tubular threaded joint.
In order to achieve the above objects, the present inventors carried out
investigations on various lubricating coating compositions suitable for a
tubular
threaded joint with respect to biodegradability, lubricity, and anticorrosive
properties.
(A) Biodegradability (BOD)
In order to determine the assessment of environmental impact on the ocean,
the following testing methods are generally employed to assess the
biodegradability
of a substance in seawater:
(a) OECD guidelines for testing of chemicals - 1992 OECD 306:
Biodegradability in Seawater, Closed Bottle Method; and
(b) Modified seawater variant of ISO TC/147, SC5/WG4 N141 1990: BOD
test for insoluble substances.
Among these methods, the one which is more suitable for the particular
sample should be employed. In either testing method, the test result is
indicated in
percent (e.g., BOD = 15%), and the larger the BOD value, the higher the
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biodegradability, which indicates a lower impact on the environment.
The biodegradability assessed by either method is acceptable if the BOD
value measured after 28 days (hereinafter referred to as BOD28) is at least
20% or
"BOD28 >_ 20%". At present, the minimum acceptable BOD value differs between
5 countries or regions, but the criterion "BOD28 >_ 20%" can meet the minimum
acceptable level of BOD for substances which can be used on an offshore rig
according to the regulations in Norway, which are said to be the strictest.
A biodegradability test for use in designing the composition of a lubricating
coating composition can be carried out separately on each of the candidate
io components in the composition. However, in view of the situation of
shipment of
tubular threaded joints as a product, the final judgement of biodegradability
should
be made based on an overall assessment of a lubricating coating composition
which
combines the assessments of the individual components.
(B) Lubricity
A lubricant based on any of a basic sulfonate salt, a basic salicylate salt, a
basic phenate salt, and a basic carboxylate salt is in the form of a grease-
like
semisolid at room temperature and exhibits fluidity under hydrostatic
pressure.
When their lubricity was evaluated by the number of tightening and loosening
cycles
before galling occurred in a repeated tightening and loosening test using
actual
tubular threaded joints, it was found that these basic lubricants exhibit good
galling
resistance even with a relatively thin coating.
(C) Anticorrosive properties
When the anticorrosive properties of sulfonate salts, salicylate salts,
phenate
salts, and carboxylate salts were evaluated by a salt spray test according to
JIS
Z2371, it was found that a basic salt had better anticorrosive properties than
a neutral
salt for each of these salts.
Upon further investigation based on the finding that any of a basic sulfonate
salt, a basic salicylate salt, a basic phenate salt, and a basic carboxylate
salt can
exhibit good lubricity and good anticorrosive properties with a relatively
thin
coating, it was found that this type of a basic lubricant does not have good
biodegradability but can still satisfy the target criterion "BOD28 ~ 20%" by
the
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addition of one or more other lubricant components thereto to form a
lubricating
coating composition while maintaining good lubricity and anticorrosive
properties.
The present invention provides a lubricating coating composition comprising
at least one basic lubricant selected from a basic sulfonate salt, a basic
salicylate salt,
s a basic phenate salt, and a basic carboxylate salt wherein the composition
has a
biodegradability value (BOD) of at least 20% when measured after 28 days in
seawater.
In a preferred embodiment, a lubricating coating composition according to the
present invention further comprises at least one additional lubricant selected
from
io those having a higher (greater) biodegradability than that of the basic
lubricant. The
additional lubricant is preferably selected from a fatty acid metal salt and a
wax, and
more preferably it comprises at least one fatty acid metal salt and at least
one wax.
The fatty acid metal salt is preferably selected from alkaline earth metal
salts of
stearic acid or oleic acid.
15 A preferred chemical composition of a lubricating coating composition
according to the present invention may contain up to 30 inass% of a volatile
organic
dissolving medium (solvent), and the remainder comprises, when the total
amount of
the remainder composition is taken as 100 parts by mass, 55 - 75 parts by mass
of a
basic lubricant, 20 - 25 parts by mass of a fatty acid metal salt, and 10 - 20
parts by
20 mass of a wax.
The present invention also provides a tubular threaded joint constituted by a
pin and a box each having a threaded portion and an unthreaded metal-to-metal
contact portion as engaging portions, characterized in that the surfaces of
the
engaging portions of at least one of the pin and the box are coated with the
above-
25 described lubricating coating composition to a thickness of at least 10
micrometers,
thereby allowing the joint to be tightened without application of a compound
grease.
In the present invention, the term "lubricant" indicates a lubricity improving
agent. A pin is a member of a tubular threaded joint which has a male
(external)
threaded portion, while a box is the other member of the joint having a female
30 (internal) threaded portion.
A tubular threaded joint having a lubricating coating formed from a
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lubricating coating composition according to the present invention has high
biodegradability and still exhibits satisfactory galling resistance and
anticorrosive
properties on the same level as obtained with application of a compound
grease. As
a result, it can be used without application of a compound grease and without
any
concern for environmental pollution even in a country or region having strict
environmental regulations.
Brief Description of the Drawings
Figure 1 is a schematic diagram showing a steel pipe for OCTG and a
coupling which are assembled together for shipment.
Figure 2 is a schematic diagram showing a tubular threaded joint having a
threaded portion and an unthreaded metal-to-metal contact portion.
Figure 3 is a schematic diagram showing minute gaps in a threaded portion
and an unthreaded metal-to-metal contact portion of a tubular threaded joint.
Best Mode for Carrying Out the Invention
is In the following description, any percent relating to a chemical
composition is
by mass unless otherwise indicated.
The components which can constitute a lubricating coating composition
according to the present invention are described below individually.
[Dissolving Medium]
A dissolving medium or solvent can be used in order to dissolve or disperse
the basic lubricant and one or more optional lubricants as well as other
additives, if
used, thereby facilitating the formation of a lubricating coating having a
uniform
thickness and a uniform composition in an efficient manner. Therefore, if a
satisfactory lubricating coating can be formed with the lubricant components
alone,
it is unnecessary to use a dissolving medium in a coating composition.
In the present invention, since the principal component of a lubricating
coating is one or more lubricants, an organic dissolving medium is used. There
is no
limitation on the type of dissolving medium as long as it has a high
dissolving power
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for the lubricant components in the composition and easily. volatilizes.
Preferred
dissolving media are petroleum solvents such as those corresponding to
industrial
gasoline prescribed in JIS K2201 and including solvent and mineral spirit,
aromatic
petroleum naphtha, xylene, and -CellosolvesTM. A dissolving medium having a
flash
s point of 30 C or higher, an initial boiling temperature of 150 C or
higher, and a
final boiling point of 210 C or lower is suitable due to its ease of
handling, rapid
evaporation, and short drying time. From the standpoint of biodegradability,
mineral
spirit is preferable.
The biodegradability of an organic dissolving medium is generally not so
io high. Therefore, if it is present in a lubricating coating composition in a
large
amount, the biodegradability of the entire composition is worsened. The amount
of
a dissolving medium is preferably selected, as long as the biodegradability of
a
lubricating coating composition satisfies the conditions for BOD defined by
the
present invention, such that the dissolving medium can improve the wettability
of the
is surface to be coated and the spreadability of the coating composition and
facilitate
adsorption of the below-described lubricity improving agents (lubricants) by
the
surface, in addition to its intended function of dissolving or uniformly
dispersing the
lubricants.
The amount of a dissolving medium in a lubricating coating composition is
20 preferably in the range of from 0% to 30% and more preferably from 5% to
25%. If
the amount is too small, the viscosity of the lubricating coating composition
may be
so high that it is difficult for the composition to form a uniform coating and
exhibit
the above-described adsorbing function. If it is too large, it is difficult
for the
composition to have a desired biodegradability.
25 [Basic Lubricant)
In a composition according to the present invention, at least one basic
lubricant selected from basic sulfonate salts, basic salicylate salts, basic
phenate
salts, and basic carboxylate salts is used as a principal component of
lubricity
improving agents (lubricants). The term "principal component" does not always
3o mean that it is present in the largest amount, but indicates that the basic
lubricant
performs a main role to achieve the desired lubricating performance.
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All the above-described four classes of basic lubricants are a salt formed
from
an aromatic acid and an excess alkali. At room temperature, they are a grease-
like
semisolid substance comprising an oil and the excess alkali dispersed in the
oil in the
form of colloidal microparticles.
The biodegradability of any of these basic lubricants is low and cannot
satisfy
the criterion "BOD28 >_ 20%" by itself. Thus, as the amount of a basic
lubricant in a
lubricating coating composition increases, the biodegradability thereof tends
to
decrease. Accordingly, a basic lubricant is present in a lubricating coating
composition in an amount which is effective at improving the galling
resistance and
io anticorrosive properties of the composition, provided that the composition
as a
whole has a biodegradability satisfying the above criterion. The amount of a
basic
lubricant is preferably in the range of 55 to 70 parts by mass when the total
mass of
the composition excluding the above-described dissolving medium (namely, the
total
amount of all the nonvolatile components which constitute a lubricating
coating) is
taken as 100 parts by mass.
Among the four classes of the above-mentioned basic lubricants, a basic
sulfonate salt is most advantageous in terms of lubricity and anticorrosive
properties.
The sulfonic acid constituting the sulfonate salt may be either petroleum
sulfonic
acid obtained by sulfonating aromatic components of petroleum fractions or a
synthetic sulfonic acid. Examples of the synthetic sulfonic acid include
dedecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and the like.
The
salt of a sulfonic acid may be either an alkali metal salt or an alkaline
earth metal
salt. Preferably it is an alkaline earth metal salt and more preferably a
calcium salt
(namely, a basic calcium sulfonate). Similarly, with respect to a basic
salicylate salt,
a basic phenate salt, and a basic carboxylate salt, an alkaline earth metal
salt and
particularly a calcium salt is preferred.
Below, the present invention will be described with respect to an embodiment
in which the basic lubricant is a basic calcium sulfonate, but the invention
is not
restricted to this embodiment. The following explanation is generally
applicable to
other embodiments in which the basic lubricant is a basic sulfonate other than
a basic
calcium sulfonate or it is a basic salicylate salt, a basic phenate salt, or a
basic
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carboxylate salt, although the amount of the basic lubricant can be adjusted
taking
into consideration the biodegradability and other properties of the
composition.
A basic calcium sulfonate which can be used in the present invention is a
known substance and is commercially available under the trade name Sulfol 1040
s from Matsumura Oil Research Corp. and under the tradename LubrizolTM 5318
from
Lubrizol Corp, for example.
A basic calcium sulfonate may be prepared by dissolving a neutral sulfonate
salt in a solvent, which can be suitably selected from aromatic hydrocarbons,
alcohols, and mineral oil, and then adding to the resulting solution an amount
of
io calcium hydroxide required to form a desired basic calcium sulfonate
followed by
mixing. Carbon dioxide gas is subsequently passed through the mixture in an
excess
amount so as to sufficiently carbonate the added calcium hydroxide, and the
reaction
mixture is filtered after addition of a filter aid such as activated clay. The
desired
basic calcium sulfonate is obtained by distilling the filtrate at a reduced
pressure to
is remove the volatile solvent.
In either form of a commercially available product and a synthetic product, a
basic calcium sulfonate is a grease-like semisolid substance which contains
calcium
carbonate in the form of colloidal microparticles which are stably dispersed
in an oil.
The dispersed microparticles of calcium carbonate function as a solid
lubricant and
enable the basic sulfonate salt to exhibit significantly improved lubricity
over
common liquid lubricating oil particularly under severe tightening conditions
having
a large amount of thread interference. When this lubricant works between
frictional
surfaces having minute irregularities (surface roughness), it can exhibit an
even more
improved galling-preventing effect due to the micro lubricating effect by
hydrostatic
fluid pressure of the oil combined with the solid lubrication action of the
microparticles. This effect can be similarly achieved with other basic
lubricants.
As the base number (as specified in JIS K2501) of the basic lubricant which is
used increases, its lubricity (galling resistance) tends to increase, since
the amount of
calcium carbonate microparticles serving as a solid lubricant increases. In
addition,
if the lubricant has a basicity higher than a certain level, it can exert its
activity of
neutralizing an acidic substance effectively, thereby making it possible to
provide a
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lubricating coating with an increased anti-rust ability. For these reasons, it
is
preferred that the basic lubricant which is used in the present invention have
a base
number (according to JIS K2501) of at least 50 mg-KOH/g. If two or more basic
lubricants are used, the base number is a weighted average of their base
numbers.
However, if the lubricant has a base number exceeding 500 mg-KOH/g, its
hydrophilic nature is increased, leading to a decrease in anticorrosive
properties and
easy occurrence of rusting. A preferred range for the base number of the basic
lubricant is from 100 to 500 mg-KOH/g and a more preferred range is from 250
to
450 mg-KOH/g.
[Additional lubricants]
In addition to the above-described basic lubricant which is a principal
lubricant, a lubricating coating composition according to the present
invention
preferably contains one or more additional lubricants as lubricity improving
agents.
Those having better (higher) biodegradability than the basic lubricant are
used as the
1s additional lubricants. As a result, even in the case where a highly basic
lubricant
which does not have such good biodegradability is used as a principal
lubricant, it is
possible to obtain a lubricating coating composition satisfying the condition
of
biodegradability according to the present invention.
Additional lubricants are preferably selected from fatty acid metal salts and
waxes. More preferably, at least one fatty acid metal salt and at least one
wax are
used as additional lubricants.
<Fatty acid metal salt>
An alkaline earth metal salt of a fatty acid is preferably used as a fatty
acid
metal salt, since fatty acid salts with other metals have inferior
biodegradability or
are not preferred from an environmental standpoint. A fatty acid metal salt is
active
as a lubricant, although its activity is lower than that of the above-
described basic
lubricant such as a basic calcium sulfonate.
The fatty acid portion of the salt is preferably one having 12 to 30 carbon
atoms from the viewpoints of lubricity and anticorrosive properties. The fatty
acid
may be either a mixed fatty acid derived from natural fat or fatty oil such as
beef
tallow, lard, wool fat, palm oil, rape-seed oil, and coconut oil, or a single
fatty acid
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such as lauric acid, tridecanoic acid, myristic acid, palmitic acid,
lanopalmitic acid,
stearic acid, isostearic acid, oleic acid, elaidic acid, arachic acid, behenic
acid, erucic
acid, lignoceric acid, and lanoceric acid.
Particularly preferable fatty acid metal salts in terms of biodegradability
are
alkaline earth metal salts of stearic acid or oleic acid, and a calcium salt
of such an
acid is particularly suitable. The fatty acid metal salt may be either a
neutral salt or a
basic salt.
There is no limit on the amount of a fatty acid metal salt in a lubricating
coating composition, and it may be 0%. However, such a salt is usually added
in at
io least a certain amount in order to provide the composition with the desired
biodegradability. A preferred amount of the fatty acid metal salt is in the
range of
from 20 to 25 parts by mass when the total mass of the composition excluding
the
dissolving medium is taken as 100 parts by mass. If this amount is too large,
the
amount of the basic lubricant which is a principal lubricant becomes
relatively small,
leading to a decrease in lubricity.
<Wax>
A wax may be added in order to enhance the biodegradability of a lubricating
coating composition, although its lubricity is lower than that of the above-
described
basic lubricant.
Examples of waxes which can be used include animal waxes such as beeswax
and whale tallow; vegetable waxes such as Japan wax, carnauba wax, candelilla
wax, and rice wax; mineral waxes such as paraffin wax, microcrystalline wax,
petrolatum, montan wax, ozokerite, and ceresin; and synthetic waxes such as
oxide
wax, polyethylene wax, Fischer-Tropsch wax, amide wax, and hardened castor oil
(castor wax). Of these, petrolatum, which is a kind of mineral wax, is
preferred from
the standpoint of biodegradability.
There is no limit on the amount of a wax in a lubricating coating composition,
and it may be 0%. However, at least certain amount of wax is usually added in
order
to provide the composition with the desired biodegradability. A preferred
amount of
the wax is in the range from 10 to 20 parts by mass when the total mass of the
composition excluding the dissolving medium is taken as 100 parts by mass. If
this
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amount is too large, the amount of the basic lubricant which is a principal
lubricant
becomes relatively small, leading to a decrease in lubricity.
[Tubular threaded joint]
A tubular threaded joint is constituted by a pin and a box each having a
threaded portion and an unthreaded metal-to-metal contact portion as engaging
portions. A lubricating coating composition according to the present invention
can
be applied to the surface of the engaging portions of at least one of the pin
and the
box.
Typically, a tubular threaded joint is shipped in the state shown in Figure 1.
io Namely, a steel pipe A for oil country tubular goods (OCTG) has a pin with
a male
threaded portion 3 formed on the outer surface at both ends thereof, and a
coupling
B has a box with a female threaded portion 4 formed on the inner surface at
both
sides thereof. Before shipping, the coupling B is connected to one end of the
steel
pipe A. In this figure, for simplicity, an unthreaded metal-to-metal contact
portion is
is omitted.
A tubular threaded joint is not limited to the type illustrated in Figure 1. A
different type of tubular threaded joint called an integral type can also be
used. This
type, which has a pin with a male thread on the outer surface at one end of a
steel
pipe for OCTG and a box with a female thread on the inner surface at the other
end
20 of the pipe, does not need to use a coupling for tightening. In addition,
it is also
possible to form a box at both ends of a steel pipe for OCTG and a pin on both
sides
of a coupling. Below, the present invention will be described with respect to
an
example of a tubular threaded joint having the form shown in Figure 1.
Figure 2 schematically shows a cross section of the connecting portion of a
25 tubular threaded joint. In this figure, 1 is a pin, 2 is a box, 3 is a male
(external)
thread portion, 4 is a female (internal) thread portion, and 5 is an
unthreaded metal-
to-metal contact portion of each of the pin and box. The male and female
threaded
portions 3 and 4 and the unthreaded metal-to-metal contact portions 5 of the
pin and
box serve as engaging portions of the tubular threaded joint and form
frictional
30 interfaces during tightening of the joint. According to the present
invention, a
lubricating coating composition is applied to the engaging portions of at
least one of
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the pin and box (i.e., to the threaded portion 3 or 4 and the unthreaded metal-
to-
metal contact portion 5) so as to form a grease-like semisolid lubricating
coating.
The pin and the box are shaped so as to interfit with each other, but when
they
are observed in detail, as shown in Figure 3, there are minute gaps 6
particularly
between mating male and female threads. In the absence of these gaps between
mating threads, tightening of a tubular threaded joint becomes practically
impossible. In addition, a larger minute gap 6 usually exists between the
unthreaded
metal-to-metal contact portions and the thread portions of the pin and box as
depicted. A lubricating composition such as a compound grease is retained in
these
io gaps and can seep out to their surroundings under the pressure exerted
during
tightening and thereby prevent galling, so these minute gaps contribute to
lubrication. A lubricating coating formed from a lubricating coating
composition
according to the present invention is semisolid like a compound grease and
thus can
seep out, thereby providing improved lubricity and airtightness to the joint.
Also like a compound grease, a lubricating coating composition according to
the present invention has improved anticorrosive properties. As a result,
after a
tubular threaded joint has been shipped in the state shown in Figure 1, the
engaging
portions of the joint to which the lubricating coating composition is applied
are
protected from rusting.
[Thickness of lubricating coating]
A primary purpose of the formation of a lubricating coating in a tubular
threaded joint is prevention of galling even under severe lubrication
conditions
which may be accompanied by plastic deformation as encountered when the
pressure
applied to the joint is locally excessive due to misalignment or inclination
of the
joint caused by some problems in assembling the joint for tightening or due to
incorporation of foreign matter. For this purpose, it is essential to
introduce
lubricants (lubricity improving agents) into the frictional interfaces and
maintain the
lubricants therein.
Accordingly, a lubricating coating composition must be applied in an amount
sufficient to fill the minute gaps 6 such as those between mating threads in
the
engaging portions of a tubular threaded joint. If the applied amount is too
small, it
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becomes impossible to expect the lubricants to seep into frictional interfaces
or seep
into a gap from other gaps under the action of the hydrostatic pressure
generated by
tightening. For this purpose, the thickness of a lubricating coating is
preferably at
least 10 micrometers.
5 Since the engaging portions of a box and a pin contact each other when a
tubular threaded joint is tightened, in the interest of achieving lubricity,
it is
sufficient to form a lubricating coating on the engaging portions of only one
of the
pin and box. However, in order to also provide the engaging portions with
anticorrosive properties, it is necessary to form a lubricating coating on the
engaging
io portions of both the pin and box. The minimum thickness necessary for
anticorrosive properties is also 10 micrometers. Therefore, a lubricating
coating
with a thickness of at least. 10 micrometers is preferably formed on the
engaging
portions of both members. However, when a coupling is connected to one end of
a
steel pipe for OCTG as shown in Figure 1 before shipping, the pin and the box
on
15 the connected side are protected from rusting by forming a lubricating
coating on the
engaging portions of only one of the pin and the box. Even in such a case, on
the
opposite non-connected sides of the pin and the box (the left-hand pin and the
right-
hand box), it is preferable that both the pin and box have a lubricating
coating.
A lubricating coating formed from a lubricating coating composition
according to the present invention does not need to be made extremely thick
since it
contains a basic lubricant such as a basic calcium sulfonate which has a
significantly
high lubricity. Too thick a coating not only wastes materials, but works
against the
goal of preventing environmental pollution, which is an important object of
the
present invention. Although the upper limit of the coating thickness is not
limited, it
is preferably approximately 200 micrometers.
A more preferable thickness of the lubricating coating is in the range of 30
to
150 micrometers. However, when the surface roughness is increased as described
below, the thickness of the lubricating coating is preferably larger than the
value of
the surface roughness (Rmax). The thickness of a lubricating coating formed on
a
3o rough surface is defined in the present invention as the mean value of the
smallest
thickness and the largest thickness.
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When a lubricating coating composition according to the present invention
contains a dissolving medium, the composition itself can be in the form of a
liquid
having good applicability rather than a semisolid at room temperature, and it
can be
applied as is without heating. Once such a composition is applied to the
engaging
portions of a tubular threaded joint, the dissolving medium, which is
generally
volatile, vaporizes from the applied coating and leaves a semisolid
lubricating
coating. Application of the lubricating coating composition can be carried out
by
any suitable coating method such as brush coating, dip coating, or spray
coating.
When the viscosity of a lubricating coating composition is too low for it to
be
io applied at room temperature as in the case where the amount of a dissolving
medium
is small or even zero, it can be applied after heating to a temperature
sufficient to
lower the viscosity to such a degree that the composition can be applied
easily.
[Surface roughness of engaging portions]
With a tubular threaded joint treated with a lubricating coating composition
is according to the present invention to form a lubricating coating on the
surfaces of its
engaging portions such as a threaded portion and an unthreaded metal-to-metal
contact portion, if the surface roughness of the engaging portions, which is 3
- 5
micrometers in an as-machined state, is increased by a suitable means, its
galling
resistance is further improved. This is because the functions of lubricants of
seeping
20 into frictional interfaces or seeping into a gap from other gaps under the
action of the
hydrostatic pressure generated in the engaging portion by tightening are
caused by
lubricants confined in minute indentations of the surface roughness. The
intensity of
these functions depends on the magnitude of the surface roughness regardless
of the
method of forming the surface roughness. A range of surface roughness suitable
for
25 improvement in galling resistance is 5 - 40 micrometers expressed as Rmax.
If the
surface roughness exceeds 40 micrometers Rmax, sufficient sealing cannot be
obtained in the peripheries of indentations, and the desired hydrostatic
pressure is
not generated so that sufficient lubrication is not obtained. A more
preferable range
of Rmax is from 10 to 30 micrometers.
30 <Method of surface roughening>
Although there is no restriction on the method of surface roughening, the
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17
following methods are possible:
(1) Blasting with sand or grid as abrasive grains: The surface roughness
which is obtained can be controlled by the size of the abrasive grains.
(2) Etching with an acid: The surface is roughened by immersion in a
strongly acidic solution such as sulfuric acid, hydrochloric acid, nitric
acid, or
hydrofluoric acid.
(3) Phosphating: A chemical conversion coating with a phosphate such as
manganese phosphate, zinc phosphate, iron-manganese phosphate, or zinc-calcium
phosphate is formed. As the phosphate crystals deposited on the surface by
io phosphating grow, the roughness of the crystal surface increases.
(4) Electroplating: Copper plating or iron plating is suitable. Electroplating
occurs preferentially in protruded portions on the surface, leading to a
slight increase
in surface roughness.
(5) Dry-process impact plating: This is a plating method such as zinc blasting
or zinc-iron alloy blasting in which particles having an iron core coated with
a
metallic material for plating (such as a zinc or a zinc alloy) are blasted
onto a surface
to be plated using centrifugal force or air pressure.
It is easier to apply these treatment methods for surface roughening to the
surface of a box, but they can be applied to the surface of a pin or the
surfaces of
both the pin and box. Methods (3) through (5) result in the formation of an
undercoat layer having an increased surface roughness, which prevents direct
metal-
to-metal contact in the engaging portions after a lubricant coating has been
lost, so
these methods are preferred in that galling resistance and anticorrosive
properties are
simultaneously improved. In particular, a coating of manganese phosphate is
preferable since it is made of bristling acicular crystals and hence can
easily achieve
a large surface roughness, which is good at retaining a large amount of
lubricants.
Some materials of a steel pipe for OCTG such as high alloy steels cannot
undergo phosphating directly. In such cases, phosphating can be performed
after the
iron plating described in (4) above is initially applied. When an undercoat
layer is
formed as in methods (3) to (5) above, the thickness of the undercoat layer is
preferably greater than the surface roughness of the undercoat layer, since
the layer
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18
has good retention of lubricants and good adhesion. Thus, the thickness of the
undercoat layer is preferably in the range of 5 - 40 micrometers.
Examples
The following examples are presented to further illustrate the present
invention. These examples are to be considered in all respects as illustrative
and not
restrictive.
In the examples, lubricating coating compositions were prepared and their
biodegradability was determined using the following two methods (a) and (b),
which
are commonly employed for evaluation of biodegradability of a substance in
io seawater in environmental impact assessment:
(a) OECD guidelines for testing of chemicals - 1992 OECD 306:
Biodegradability in Seawater, Closed Bottle Method; and
(b) Modified seawater variant of ISO TC/147, SC5/WG4 N141 1990: BOD
test for insoluble substances.
Specifically, the biodegradability of each component in a coating composition
after 28 days (BOD28) in seawater was determined by one of the above methods
selected suitably therefor. More specifically, method (a) was used for mineral
spirit,
petrolatum wax, and polyethylene resin powder (used in a comparative example),
while method (b) was used for basic calcium sulfonate and calcium stearate.
As the final judgement of biodegradability, the overall biodegradability of
the
lubricating coating composition as a whole was determined by combining the
results
of the individual components. The value of the overall biodegradability was
calculated as a weighted average of the values of biodegradability of the
individual
components by considering their contents in the composition.
The lubricity of each lubricating coating composition was tested using a
tubular threaded joint constituted by a pin formed on the outer surface at
both ends
of a steel pipe for OCTG (outer diameter of 17.8 cm =7 inches) made from
either
carbon steel or 13Cr steel having the compositions shown in Table 1 and a box
formed on the inner surface of a coupling of the same steel material.
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All the tubular threaded joints used for testing were of the type having a
threaded portion and an unthreaded metal-to-metal contact portion on each of
the pin
and box and capable of forming a metal-to-metal seal. In the following
description,
the engaging surfaces of a pin, which include the surface of its threaded
portion and
unthreaded metal-to-metal contact portion, will be referred to as the "pin
surface",
and the engaging surfaces of a box which includes the surface of its threaded
portion
and unthreaded metal-to-metal contact portion will be referred to as the "box
surface".
The tubular threaded joints underwent the following surface treatment for
io surface roughening. The pin surface of a tubular threaded joint of the
carbon steel
was treated by zinc phosphating, and the box surface thereof was treated by
manganese phosphating. The box surface of a tubular threaded joint of the 13Cr
steel was coated with copper plating, and the pin surface thereof remained as
machined without surface treatment. The 13Cr steel is a kind of high alloy
steel and
is more susceptible to galling than the carbon steel.
A lubricating coating composition to be tested was applied by brush coating
only to the box surface of a tubular threaded joint which had been surface
treated as
described above so as to form a lubricating coating having a thickness of 30
micrometers.
The tubular threaded joint having a lubricating coating formed from the
composition on the box surface was subjected to a repeated tightening and
loosening
test with a tightening torque of 20,000 N-m for up to ten cycles, and the
lubricity of
the lubricating coating composition was evaluated by the number of tightening
cycles before galling occurred in the test.
The anticorrosive properties of each lubricating coating composition were
evaluated by a salt spray test (SST) specified in JIS Z2371 using a test sheet
with
dimensions of 50 nun x 100 nun and 2 mm in thickness made of the carbon steel
or
the 13Cr steel having the compositions shown in Table 1. A lubricating coating
to
be tested was formed to a thickness of 30 micrometers from each composition on
the
test sheet as machined without surface treatment. The coated test sheet was
subjected to the salt spray test for 1000 hours, and the presence or absence
of rust
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was determined visually.
The compositions of the lubricating coating compositions which were tested
and the test results are shown in Table 2.
Table 1
C Si Mn P S Cu Ni Cr Mo
Carbon steel 0.24 0.3 1.3 0.02 0.01 0.04 0.07 0.17 0.04
13Cr steel 0.19 0.25 0.8 0.02 0.01 0.04 0.1 13 0.14
(Contents in mass%, the balance being Fe and incidental impurities)
Table 2
Example 1 Example 2 Comparative Conventional
Example Example
Mineral spirit 25% 15%
Lubricating Basic Ca sulfonate 50% (66.7)* 65% (65)* 72% (84.7)*
coating Grease
composition Calcium stearate 17% (22.7)* 23% (23)* specified by
(mass%) API
Petrolatum wax 8% (10.6)* 12% (12)* 8% (9.4)*
Polyethylene powder 5% (5.9)*
Ease of application(* 1) Good Fair Good Good
Biodegrad- BOD28 (*2) 26% 36% 12%
ability Poor (*4)
(in seawater) Assessment (*3) Good Good Poor
Lubricity (*5) Carbon steel 10 cycles 10 cycles 10 cycles 10 cycles
13Cr steel 10 cycles 10 cycles 10 cycles 10 cycles
Anticorrosive Carbon steel No rust No rust No rust No rust
properties in
SST 13Cr steel No rust No rust No rust No rust
*The numerals in parentheses are parts by mass based on 100 parts by mass of
the total amount of
the composition excluding the dissolving medium (mineral spirit).
* 1: Ease of application:
Good: applicable at room temperature;
5 Fair: applicable only after lowering the viscosity of the composition by
heating.
*2: Overall assessment of the results of the individual components measured by
either of the
following methods: "OECD guidelines for testing of chemicals - 1992 OECD 306:
Biodegradability
in Seawater, Closed Bottle Method" and "modified seawater variant of ISO
TC/147, SC5/WG4
N141 1990: BOD test for insoluble substances".
io *3: Assessment of biodegradability:
Good: BOD28Z20% (reached the target value for the present invention)
Poor: BOD < 20% (did not reach the target value for the present invention)
*4: Cannot be used irrespective of its BOD value due to the presence of heavy
metals such as lead
contained therein.
15 *5: Number of tightening cycles before galling occurred in a repeated
tightening and loosening test
up to ten cycles.
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[Example 1]
A lubricating coating composition was prepared which contained 25% of
mineral spirit as a dissolving medium, 50% (66.7 parts) of basic calcium
sulfonate
having a base number of 400 mg-KOHlg as a basic lubricant, and 17% (22.7
parts)
of calcium stearate and 8% (10.6 parts) of petrolatum wax both as additional
lubricants. The parts in parentheses are the amounts of the respective
components in
parts by mass based on 100 parts by mass of the total amount of the components
in
the composition excluding the dissolving medium.
This lubricating coating composition contained a dissolving medium and thus
io had a low viscosity and a high spreadability, so it was easy to apply and
could be
applied by brush coating to the box surface of a test threaded joint to be
used in a
repeated tightening and loosening test and the surface of a test sheet for a
salt spray
test while remaining at room temperature. On the other hand, due to the
presence of
a dissolving medium which does not have good biodegradability, the
biodegradability (BOD28) of the entire lubricating coating composition in
seawater
was 26%, which was higher than the minimum acceptable value of 20% but was
lower than that of the composition of Example 2 containing no dissolving
medium.
In the repeated tightening and loosening test using a test threaded joint
having
a lubricating coating formed from the composition, 10 cycles of tightening and
loosening which are required for a joint for tubing could be performed without
the
occurrence of galling both with a joint made of the carbon steel and one made
of
13 Cr steel. In addition, in the salt spray test for 1000 hours using a test
sheet having
a lubricating coating formed from the composition, no rust was found on either
the
carbon steel or the 13Cr steel.
[Example 2]
A lubricating coating composition was prepared which did not contain a
dissolving medium but contained 65% of the same basic calcium sulfonate as
used in
Example 1 as a basic lubricant, and 23% of calcium stearate and 12% of
petrolatum
wax both as additional lubricants.
Since this lubricating coating composition did not contain a dissolving
medium and thus had a high viscosity at room temperature, it was previously
heated
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22
to 60 C to lower its viscosity and then applied by brush coating to the box
surface
of a test threaded joint to be used in a repeated tightening and loosening
test and the
surface of a test sheet for a salt spray test. On the other hand, due to the
absence of a
dissolving medium, it had good biodegradability, and the biodegradability
(BOD28)
s of the entire lubricating coating composition in seawater was 36%, which
greatly
exceeded the minimum acceptable value of 20%, and was higher than that of the
composition of Example 1 containing a dissolving medium.
In the repeated tightening and loosening test using a test threaded joint
having
a lubricating coating formed from the composition, 10 cycles of tightening and
io loosening which are required for a joint for tubing could be performed
without the
occurrence of galling both with a joint made of the carbon steel and one made
of the
13Cr steel. In addition, in the salt spray test for 1000 hours using a test
sheet having
a lubricating coating formed from the composition, no rust was found on either
the
carbon steel or the 13Cr steel.
15 [Comparative Example]
A lubricating coating composition was prepared which contained 15% of
mineral spirit as a dissolving medium, 72% (84.7 parts)of the same basic
calcium
sulfonate as used in Example 1, and 8% (9.4 parts) of petrolatum wax and 5%
(5.9
parts) of a polyethylene resin powder (which is described in JP 2002-173692 Al
as a
20 preferable lubricating additive due to its effect on improving galling
resistance) both
as additional lubricants. The parts in parentheses are the amounts of the
respective
components in part by mass based on 100 parts by mass of the total amount of
the
components in the composition excluding the dissolving medium.
This lubricating coating composition contained a dissolving medium and thus
25 had a low viscosity and a high spreadability, so it was easy to apply and
could be
applied by brush coating to the surfaces of a test threaded joint to be used
in a
repeated tightening and loosening test and a test sheet for a salt spray test
while
remaining at room temperature. However, due to the presence of the dissolving
medium which does not have good biodegradability, a high content of the basic
30 lubricant, and the presence of a polyethylene resin powder, the
biodegradability
(BOD28) of the entire lubricating coating composition in seawater was 12%,
which
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23
was below the minimum acceptable value of 20%.
In the repeated tightening and loosening test using a test threaded joint
having
a lubricating coating formed from the composition, 10 cycles of tightening and
loosening which are required for a joint for tubing could be performed without
the
occurrence of galling both with a joint of made of the carbon steel and one
made of
the 13Cr steel. In addition, in the salt spray test for 1000 hours using a
test sheet
having a lubricating coating formed from the composition, no rust was found on
both the carbon steel and the 13Cr steel.
Thus, in this comparative example, the desired galling resistance and
io anticorrosive properties could be achieved since the composition which was
used
contained a large amount of a basic calcium sulfonate having good lubricity
and
anticorrosive properties along with polyethylene powder having good lubricity.
However, the composition could not satisfy the desired biodegradability and
therefore cannot be used in a country or region having strict environmental
regulations.
[Conventional Example]
The performance of a compound grease as prescribed in API specifications
BUL 5A2 which contained a large amount of heavy metal powder was evaluated as
a
conventional example. In the repeated tightening and loosening test using a
test
threaded joint having a lubricating coating formed from the grease, 10 cycles
of
tightening and loosening which are required for a joint for tubing could be
performed without the occurrence of galling both with a joint made of the
carbon
steel and one made of the 13Cr steel. In addition, in the salt spray test for
1000
hours using a test sheet having a lubricating coating formed from the grease,
no rust
was found on either the carbon steel or the 13Cr steel.
Thus, it was confirmed that the galling resistance and anticorrosive
properties
of a lubricating coating composition according to the present invention which
has
good biodegradability are as good as those of a compound grease which contains
a
large amount of harmful heavy metals such as lead and hence cannot be used in
a
3o region having strict environmental regulations.
