pplapph guidelines for weight-coating on submerged

7/29/2019 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


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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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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(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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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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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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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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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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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.

pplapph guidelines for weight-coating on submerged

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