PPLAPPH Guidelines for Weight-Coating on Submerged
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Transcript of PPLAPPH Guidelines for Weight-Coating on Submerged
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Chevron Corporation H-1 January 1990
Appendix H. Guidelines for Weight-Coating on SubmergedPipelines
Contents Page
H1.0 Introduction H-2
H2.0 Installation Conditions to Be Considered in Design H-2
H3.0 Conditions for the Line In Service to Be Considered in Design H-3
H4.0 Design Objective H-4
H5.0 Design Data Required H-4
H6.0 Weight-Coating Design H-4
H7.0 Weight-Coating Specifications H-6
H8.0 Data for Weight-Coating Control H-8
H9.0 Precast Concrete Weights H-9
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Appendix H Pipeline Manual
January 1990 H-2 Chevron Corporation
H1.0 Introduction
These guidelines cover design and specification development for weight-coating on
submerged pipelines installed at waterway crossings, swamps, and offshore. Further
guidelines for weight-coating are in Section 447 for waterway crossings and
Sections 935 and 953 for offshore pipelines.
When the combined weight of the pipe, corrosion protective coating, and operating
fluid does not provide sufficient stability for the submerged pipeline during installa-
tion and service life, weight must be added by:
Continuous weight-coating of the pipe with a uniform cement-based coating
Individual precast weights attached to or placed over the line at intervals
An economic comparison of alternative combinations of pipe wall thickness and
weight-coating thickness, possibly with alternative protective coatings, may be
necessary. Heavier wall pipe offers greater mechanical strength, possible use of a
lower grade steel, and some insurance against pitting failures. The additional
weight of steel will reduce the need for weight-coating; however, concrete is gener-ally a cheaper way to provide weighting.
For liquid-filled lines the feasibility of constructing the line using water-filled pipe
should be considered. For example, in 1962 a 100-mile, 20-inch crude oil pipeline
was installed in shallow waters from Empire Terminal, Louisiana, to Pascagoula
Refinery, Mississippi. The line was Somastic-coatedwithout weight-coating
and filled with water to submerge it as it was laid from a lay-barge. Of course, the
contents of this line can never be displaced with air or gas.
H2.0 Installation Conditions to Be Considered in Design
Density of water. Water density is seldom significant, but may be a factor inbays where there could be a varying mixture of fresh water and salt water, and
when close control of the submerged weight is critical for the particular
construction method.
Nature of the bottom. Often this is not critical, but may affect buoyant or drag
forces on the line, which may be a consideration for the particular construction
method.
Density of the bottom and/or backfill material. This is a factor if a cohesion-
less material contributes to the buoyancy of the line, either during installation
or after installation as backfill is intentionally placed or is naturally deposited
over the line. Agitation of bottom material under conditions of unusual water
flow or wave action should be considered if the bottom material could become
fluid.
Nature of the backfill material. Besides density, consideration should be
given to possible damage to the line if rock or other hard objects fall intention-
ally or naturally onto the line.
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Pipeline Manual Appendix H
Chevron Corporation H-3 January 1990
Construction method. The method of construction is determined by water
depth, location of the work, and alignment of the line:
Lay barge. The pipe is laid off the end of the barge, usually with a
stinger and sometimes with tensioning, and is lowered to the bottom as
the barge moves ahead. This is a widely used method for all water depths.
Surface pull/push. The pipe is floated into position, and subsequently
dropped to the bottom by filling with water or releasing flotation drums
used to support the line while floated out to position. This is a commonly
used method for lines running from or to shoreline in relatively shallow
water.
Submerged carry. The pipe is carried into position by equipment and
lowered. In shallow river crossings sideboom tractors or backhoes may
traverse the crossing; in deeper water, cranes or winches on barges are
used. This method is used most at crossings.
Off-bottom pull. The pipe is buoyant, usually employing flotation drums,
and is kept submerged by the weight of heavy chains, attached to the pipe
at intervals, which drag along the bottom so that the pipe is off the bottomwhile the lower ends of the chains are on the bottom. This is an unusual
method.
Bottom pull. The pipe is pulled from shore along the bottom into position,
sometimes a distance from the shoreline fabrication site. This is a
commonly used method, both for crossings and offshore.
Conditions may dictate the construction method: deep water requires a lay barge
with stinger, possibly with controlled tensioning during lay. In other cases several
alternative methods may be feasible, at the option of the construction contractor.
Conditions for the operating line and the construction method may require that the
line be either empty or filled with water during installation.
The construction method to install the line must take fully into account the weight
and strength of the pipe and the forces on the pipe both during installation and
before the line is filled with the operating fluid.
H3.0 Conditions for the Line In Service to Be Considered in Design
Weight of the pipeline filled with the operating fluid.
Weight of the empty line if the operating fluid is a liquid. This liquid could be
displaced with air or gas, intentionally or inadvertently, during the service life
of the pipeline.
Density of the bottom or backfill material. See Section H2.0.
Effect by hydrodynamic forces on the line during the service life of the pipe-
line. See Section 935.
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Appendix H Pipeline Manual
January 1990 H-4 Chevron Corporation
H4.0 Design Objective
The pipeline must be sufficiently stable on the bottom under all conditions of opera-
tion and exposure to external forces (buoyant, lateral, hydrodynamic). Greater
stability is desirable, but represents higher costs for materials and, very possibly, for
construction because of the greater weight of pipe to be handled.
H5.0 Design Data Required
Data required for design of the weight-coated pipeline has been discussed in
Sections H2.0 and H3.0. These guidelines do not cover specific criteria values to
achieve final stability, such as:
Required submerged weight (negative buoyancy) of the pipe in water or a cohe-
sionless bottom or backfill material, or the equivalent required specific gravity
of the line
Density values for cohesionless bottom or backfill material, and change in
bottom or backfill properties under unusual water flow or wave action condi-
tions
Data to determine hydrodynamic forces on the pipeline. See Section 935.
Risk and consequences of a liquid fill in a line being displaced with air or gas
Establishing design values for most of these criteria will involve prudent investiga-
tion, either by reference to previous installations, search of available geophysical
literature, or field surveys. Other physical data relating to dimensions and weight of
the pipe and corrosion protective coating, weight of the operating fluid, etc., are
readily available.
H6.0 Weight-Coating Design
A weight-coating is a more or less uniform thickness of concrete applied over the
protective coating on the pipe to achieve a combined weight that will give the
desired submerged weight of the pipeline. The density of the weight-coating can be
adjusted within a range of approximately 140 to 190 pounds per cubic foot by selec-
tion of the aggregate used in the weight-coating concrete. Increasing the thickness
of the applied weight-coating will add to the combined weight, but the larger diam-
eter of the weight-coated pipe increases the buoyant force because more water, or
bottom or backfill material is displaced.
The following equations give the weight of weight-coating per lineal foot of pipe,outside diameter of the weight-coating, and the thickness of weight-coating
required to provide a design submerged weight per lineal foot:
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Pipeline Manual Appendix H
Chevron Corporation H-5 January 1990
(Eq. H-1)
where:
Wc = Weight of weight-coating, lb/ft
Ws = Submerged weight of the pipe, lb/ft
Wp = Weight of pipe without weight-coating,
lb/ft
WF = Weight of fluid contents inside the pipe, lb/ft
= 0 for empty pipe
WT = Total weight of weight-coated pipe, lb/ft
= Wp + Wc + WF
Dp = Outside diameter of protective-coated pipe without weight-
coating, in.
Ap = Cross-sectional area of protective-coated pipe without weight-
coating, ft2
= 0.00545 Dp2
Dc = Outside diameter of weight-coated pipe, in.
tc = Thickness of weight-coating, in.
c = Density of weight-coating, lb/ft3
fa = Factor for absorption of water in the weight-coating concrete (see
following discussion)
w = Density of water or cohesionless material,lb/ft3
Wc
Ws w Ap Wp WF+( )+
1w
fac----------
----------------------------------------------------------------=
Dc
13.5Ws facAp Wp WF+( )+
fa
c
w
------------------------------------------------------------------ 1 2/
=
or 13.5Wc
fac---------- Ap+
1 2/
=
tc 0.5 Dc Dp( )=
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Appendix H Pipeline Manual
January 1990 H-6 Chevron Corporation
If weight-coating thickness has already been set, or the weight-coating has been
applied to the pipe, the following equation gives the resultant submerged weight per
lineal foot:
(Eq. H-2)
Two important factors in establishing required weight-coating and specifying
acceptable tolerances for applied weight-coating are:
Absorption of water in the weight-coating concrete
Variations in concrete thickness and density during application of the weight-
coating (discussed under specification tolerances, Section H7.0 below)
Depending on how the weight-coating concrete is applied and controlled during
application, the concrete will absorb water in varying amounts when submerged.The weight of absorbed water can be expected to be 3% to 8% of the weight of the
concrete coating, and should be included as a design consideration. Use of a water
absorption factor of 1.0 is conservative, since absorbed water adds to the stability of
the installed pipe. In calculating on-bottom stability of offshore pipeline, a water
absorption factor of 1.05 is typically used for 140 lb/ft3 concrete. For 190 lb/ft3
concrete, a 1.03 factor is suggested. Water absorption is an important consideration
when the construction method is sensitive to the weight of the pipeline in the water.
A reasonably reliable method of determining the water absorption factor is to weigh
several joints of pipe in air and again after submerging in water for a sufficient time
to allow water absorptionusually at least 48 hours. Concrete samples may give an
approximation, but are not likely to be representative because of their small size.
H7.0 Weight-Coating Specifications
Specifications for weight-coating should contain three sections. The first two
sections need to be developed for the particular project, giving consideration to
conditions during installation and for the line in operation. The third section can
incorporate standard specifications suitable for the application method, as follows:
Description of pipe to be weight-coated, nominal weight-coating thickness and
density, and coating application method
Tolerances for thickness, density, and weight of weight-coating; methods for
measurement and calculation of these during application; means to controlapplication to meet the tolerances
Quality of weight-coating material components and applied concrete, consis-
tent with the particular application method selected
Rejecting weight-coated pipe that does not conform with specifications is very
costly, not only to the weight-coating applicator (since the purchase order contract
normally makes him responsible for all costs to correct weight-coating that does not
Ws WT wW
cfac---------- Ap+
=
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Pipeline Manual Appendix H
Chevron Corporation H-7 January 1990
meet specifications) but also to the Company, because of the delay to remove and
re-do the concrete coating and the possible damage to the pipe or the corrosion
protective coating and attendant delays.
There is usually agreement that the concrete quality must conform to the specifica-
tions, and, normally, reputable weight-coating applicators have established proce-
dures that produce a good product. However, for both commercial application
methodsimpingement and compression coatthe thickness of concrete and the
density of the concrete will vary slightly during application, and will affect the
submerged weight of the individual pipe joints. Close control of thickness is diffi-
cult, and a fraction of an inch may have a significant effect on the submerged
weight. Also, the weights of the protective-coated pipe joints before weight-coating
vary. This influences the total weight of the weight-coated joint, and is not within
the control of the weight-coating applicator. Weight and dimensional tolerances
must be clearly defined in the specification, and understood and agreed to by
Company and the weight-coating applicator before award of the purchase order
contract. Specifications must be realistic to get an achievable product.
Weight tolerances defined in weight-coating specifications are often the basis forinformation included in pipeline construction specifications, and when the
Company furnishes the weight-coated pipe to the construction contractor, the
contractor has valid claim for recourse if the weight-coated pipe does not conform
to weight data stated in the construction contract. In one instance, an effluent line
was to be pulled empty on the bottom in a shallow bay. A submerged weight of 10
pounds per lineal foot was specified, and a 10% tolerance on the specified
submerged weight was specified in the weight-coating purchase order and again
stated in the construction contract. This represented a hypothetical control of
weight-coating to within 1 pound per lineal foot on 3.4-inch-thick concrete coating,
which weighed 425 pounds per lineal foot on a 36-inch pipe. This was in no way
achievable.
The description section of the specification should include:
Size and total length of pipe, type and thickness of corrosion protective coating
on the pipe, average pipe joint length, and minimum and maximum joint
lengths
Thickness and density of weight-coating to be applied, application method, and
length of hold-back of weight-coating from the end of the pipe (to allow for
application of protective coating at the girth welds)
Shipping and storage information and instructions
The section on tolerances should recognize the tolerances desired to comply withdesign and construction requirements, practical limitations on control of thickness
dimensions and density inherent in the particular application method, and the adjust-
ments available during application to achieve the specified tolerances. In devel-
oping this section of the specifications, input is needed from the weight-coating
applicator either in discussion before soliciting quotations or as specifically
requested information with the quotations. This information from the applicator
should include not only values for proposed tolerances, but also how and when
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Appendix H Pipeline Manual
January 1990 H-8 Chevron Corporation
measurements of weight, outside diameter, and concrete density are taken, and the
ability to make adjustments during application to keep the product within toler-
ances.
Setting a minimum on the weight of each weight-coated joint is practical if the
construction method is not sensitive to the weight of the pipe in water, since the
coating applicator can reasonably produce weight-coated pipe that meets or exceeds
the specified minimum. However, when the construction method is critically depen-
dent on the weight of the pipe in water, the setting of maximum and minimum
weights must be carefully considered, and water absorption taken into account.
This is typical for surface pull/push and bottom pull methods.
Because of the difficulty in closely controlling the weight-coating on each joint of
pipe, practice is to specify tolerances for weight and thickness for averages of a
number of joints, often ten, recognizing that when welded and laid, a considerable
length of line will act together in the water and on the bottom. Thus, the specifica-
tion for weight-coating should include weight and thickness tolerances for indi-
vidual joints and closer tolerances for averages of any 10 consecutive joints.
The section on quality of material components and the weight-coating concrete
should pertain to the particular application method, and usually can utilize standard
specifications with current updating as available from specialists in the Materials
Division of the Chevron Research and Technology Company.
H8.0 Data for Weight-Coating Control
Measured data are needed to determine that weight-coating is within specification
tolerances and to control the ongoing application process. The data that must be
taken, at appropriate intervals, are:
P = Weight of the protective-coated joint before weight-coating, lb
J = Weight of the weight-coated joint, lb
Dc = Average outside diameter of the weight- coated pipe joint, as
determined by measuring the circumferences at a number of
places along the length of
the joint, in.
L = Length of the pipe joint, ft
h = Lengths without weight-coating, such as hold-back from the ends
of the pipe (to allow for application of protective coating at girth
welds), ft
a = Lengths without weight-coating for any other purpose (anode
bracelets, branch connections, etc.), ft
The calculated weight Jcalc of weight-coated joint is:
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Pipeline Manual Appendix H
Chevron Corporation H-9 January 1990
Jcalc = L Wp + (L 2h a) Wc(Eq. H-3)
where Wc is based on specified concrete density and thickness to give a specified
weight per lineal foot, and L, h, and a are as indicated above. The value for Wp can
either be calculated for the pipe steel plus the protective coating, or based on actual
weights P and lengths L. This calculated weight can then be compared with themeasured weight J.
The approximate submerged weight Ws of the joint withoutaccounting for waterabsorption can be calculated as follows:
(Eq. H-4)
and the approximate concrete density as follows:
(Eq. H-5)
Scales for weighing the weight-coated joints should be calibrated and certified
before start of weight-coating and, for large orders, should be checked periodically.
Calculations to be made will depend upon the tolerances set for a particular design
and construction method. Data from calculations can be used to adjust concrete
thickness and/or density during the day if the applicator is set up to respond
promptly.
Accurate records of the measured data and calculations should be made available to
the construction contractor and Company field engineers.
H9.0 Precast Concrete Weights
Bolt-on precast concrete weights may be useful:
For shorter sections of pipeline, such as waterway crossings
In muskeg or swampy terrain, where additional weighting is needed for inter-
mittent lengths but can only be determined during construction
In locales where continuous concrete coatings are not available or are uneco-nomic
The spacing between weights can be determined using the submerged weight of
each precast unit. If the line is to be installed by a bottom-pull method, special
measures should be taken so that the weights do not move along the pipe as the
pipe is dragged over the bottom, and that the drag forces on the weights do not
WsJ 2h a+( )Wp
L 2h a+( )------------------------------------- 0.00545 wDc2=
c
J L WpL 2h a+( )-----------------------------
1
0.00545 Dc2 Ap
------------------------------------------=
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Appendix H Pipeline Manual
January 1990 H-10 Chevron Corporation
damage the protective coating. Because of this, continuous weight-coating is prefer-
able.
For buried lines crossing seasonally flooded ground where construction is done
during the dry season, set-on pre-cast weights can be used, carefully placed in
position over the pipe, followed by backfilling. Additional protection should be
provided to prevent damage to the protective coating under the precast weights,
usually rock-shield or equivalent heavy flexible padding. Some precast weights
have a felt or burlap shield cast into their interior surfaces.