Tài liệu PIPELINE PIGGING TECHNOLOGY- P14 doc

Example applications are: a connecting an entrant into a pipeline at its closest point so as to minimize the total pipeline length; b inserting a wye at the base of a riser to tie-in a s

Fig.4 Streamlined wye design.

Recent advances in piggable Y design

d) The web between the incoming branches is kept as long as possible

to maintain the separation between the bores The crotch area,where high stresses would otherwise develop, is machined back andprofiled locally

Manufacture

Scoping calculations show that scaling up existing smaller-diameter

de-signs leads to problems with high weights and thick walls Fig 5 shows a graph

of predicted weight as a function of pipeline diameter for 2500psi pressure

Concerns are that the thicker walls would lead to high costs in manufacture,

inspection and handling The design illustrated in Fig.4 is, therefore, adopted,

with a smooth external profile and thinner walls suited to both forging and

casting manufacture and to ultrasonic inspection This approach also shows

a considerable weight saving, as illustrated in Fig 5

FE analysis for operational loads

The behaviour of the wye under operational loads is determined using

finite-element modelling Pressure containment, loads from the branch

pipework, and temperature differential stresses due to incoming streams at

different temperatures, are evaluated Stress and fatigue levels are kept within

BS5500 allowables

A full-PC version of ANSYS is used Accounting for symmetry planes within

the wye, a quarter model is generated comprising typically 1200 8-noded

brick elements, as shown in Fig.6 A minimum of three elements are used

through the wall thickness High stress gradients occur in the neighbourhood

of the wye crotch, and the mesh is further refined in this area to evaluate the

peak stresses

The behaviour of the wye under pressure is to bend outwards at the

elongated sections where the bores are merging, as shown in Fig.6 The shape

of the cross section is arranged to resist the bending with thicker central walls

This bending movement is also restrained at the crotch, which is

conse-quently the most highly stressed region FE analysis determined that it is

necessary to cut back the area between the bores to relieve stress

concentra-tion Under bending moments in the wye branches the stress intensifies in the

outside of the crotch, which was shown to need reinforcement and a smooth

profile Stresses in the body of the wye were generally very low compared to

code limits, which points to the potential for further design optimization

Fig.5 Weight predictions for wyes.

Fig.6 Finite element meshing for wye piece.

Recent advances inpiggable Y design

The principal use for a wye is to connect two pipelines of the same

diameter such that both can be pigged Example applications are:

a) connecting an entrant into a pipeline at its closest point so as to

minimize the total pipeline length;

b) inserting a wye at the base of a riser to tie-in a second entrant to the

one riser, thus retaining the same number of risers and avoiding the

expense of retro-fitting ones;

c) combining a wye and subsea isolation valve installation;

d) stacking wyes in series, always retaining a piggable inlet to the

pipeline system for future entrants

The alternatives to wyes are risers and tees These are compared in the

following sections Table 2 sets out the broad areas of application for each

First of all, however, a characteristic arrangement for a wye junction (Fig.7)

is considered This would be adapted to suit a particular job, but serves to

illustrate a few points as follows

The offset layout shown in Fig.7 is mainly a function of the installation

method Typically, the main pipeline would be installed with a flanged spool

The wye, valves and protection frame, which would be too big to be laid in

smaller

smaller

same larger

Pigging requirement

infrequent

routine

infrequent or routine infrequent or routine

Table 2 Main applications for riser, wye and tee junctions.

Fig.7 Typical arrangement for wye junction.

Recent advances in piggable Y design

line, would be installed next to it The pipeline spool would be removed and

replaced by dogleg spoolpieces to tie in the wye The pipeline system would

then be leak tested and p re-corn missioned

The two valves on each branch allow either branch to be isolated whilst

the rest of the pipeline system is operational This function could be used, for

instance, during a pipeline repair, for tying-in another pipeline,

decommissioning a branch line, or pressure testing an ESD valve It is always

worth considering, however, whether all the valves are strictly justifiable

At a later date the entrant pipeline would be installed and connected to the

spare branch In the case of a gas line, it would normally be dewatered to a

pre-commissioning valve, a spoolpiece would be connected across to the

wye, tested and blown down, and the entrant pipeline dried prior to

commissioning An entrant to an oil system could avoid the extra

pre-commissioning valve by testing against the wye valves and dewatering back

to the platform Again, there are many variations on this depending on the

relative timing of the main pipe, wye and entrant pipe installations

WYE vs RISER CONNECTION

The main alternative to a wye junction is to connect the second pipeline

via a riser Fig.8 compares the field configurations resulting from wye and riser

tie-ins Several advantages and a few disadvantages arise from having the wye

as opposed to the riser as discussed below First the advantages:

Safety: as can be seen from Fig.8, the wye junction eliminates the need

for an additional import riser on the platform, and is thus a safersolution from the viewpoint of the platform, particularly for gaspipelines

Field layout The wye junction can be sited away from the platform

avoiding seabed congestion around the platform This leaves thefield free to be developed using satellite wells and flowlines, forexample, without being crowded by incoming pipelines from otherfields It also allows the field layout to be planned with greatercertainty, keeping pipelines and flowlines in corridors with safeanchoring areas between, avoiding spoolpieces under boat-loadingareas, etc

Cost The wye will normally show cost advantages over a riser,

particu-larly if the riser has to be retro-fitted, or a cantilever extension has

to be added for the pig receiver If, however, the wye has to be

retro-Fig.8 Comparison of riser and wye tie-ins.

Recent advances In piggable Y design

fitted in an existing pipeline, then the costs could go either way,

depending amongst other things on the pipeline lengths, the

dura-tion of the required shut down, and any penalty associated with

making the new line the same size as the existing

Tie-in: Tying-in at a wye can be done without shutting down the

existing system This has recently been demonstrated by the Gyda

tie-in In comparison, construction work on a platform to tie-in an

entrant is likely to be more disruptive

End of field life: If import risers are used and the original field is

depleted before the end of the pipeline life, it would need to be

maintained as a riser platform, or a subsea junction inserted Using

a wye junction allows the original platform to be isolated and

decommissioned without affecting the rest of the pipeline users

Emergency shut down: If import risers are used and there is an

emergency shutdown on the platform, the upstream fields will also

have to be shut down, whereas a wye junction would keep them

operating independently

Shorter line: A wye junction can be placed to give the entrant the

shortest pipeline route This is particularly so for a retro-fitted wye

Wye junctions also have some drawbacks, and are by no means always the

best solution for tying-in an entrant The main drawbacks are as follows:

Same size line: The wye junction's main use is to connect entrants of

the same size as the original pipeline Whilst it is possible to connect

other sizes, these would not be piggable There is typically a cost and

technical balance for an entrant between having, say, a smaller

non-piggable line to a tee, a larger non-piggable line to wye, or a longer

piggable line to a riser

Subsea valves and protection covers: It would be feasible to have a wye

without valves However, they are normally an operational

require-ment For example, to tie-in an entrant without affecting the rest of

the system would normally need two valves on the branch of the

wye to give double-block-and-bleed isolation For this reason, most

wyes to date have two isolation valves on each branch If subsea

valves are used, it is necessary to have a protection cover

Reverse pigging: Whilst not normally required in operation, it is

sometimes desirable to be able to pig in reverse during

commission-ing, for example in dewatering a line from the shore to the platform

This would cause technical problems at a wye junction which is only

piggable in the convergent directions, and would require some form

of deflector plate for reverse pigging

Fig.9 Retrievable subsea pig trap.

Recent advances inpiggable Y design

Flow limitations: To ensure the passage of pigs through the wye, there

has to be adequate flow in the main line and no reverse flow in the

branch For a pipeline system which needs periodically to be coated

by a slug of corrosion inhibitor held between two batching pigs,

there may be limitations on the flow conditions at the wye to avoid

loss of inhibitor up the second branch

WYE vs TEE

Tees normally have the advantage of being relatively small and light such

that they can be laid with the pipeline and need only a small protection cover

Their main application is for tying-in smaller-diameter pipelines They are not

readily piggable and would require specialist techniques such as gel or foam

slugs, or a subsea pig trap

Fig.9 illustrates a subsea pig trap for a gas pipeline The deployment,

operation and retrieval of this device would be a costly exercise unsuited to

routine pigging It could, however, be justified for intelligence pigging

Overall, the applications of wyes and tees are quite distinct, in that wyes

are suited to a same-sized piggable entrant, and the tee to smaller,

rarely-pigged entrants

CONCLUSIONS

a) The technology for designing and manufacturing piggable wyes is now

maturing This paper details the features to ensure that the junction is reliably

piggable, operates within allowable stress levels, and can be manufactured

b) A successful operational track record for wye junctions has been built

up in the North Sea, and they are now being used in increasing numbers

c) Wyes provide an alternative to import risers for the connection of other

fields to a pipeline system, and in many cases will show cost and safety

advantages both in installation and operation

Recent advances in piggable Y design

M.Rodningen, 1986 Design of piggable subsea components, conference

paper, Subsea pigging technology organized by Pipes & Pipelines

International, Norway.

P.G.Brown, J.Ritchie, K.McKay and AJ.Grass, 1990 Piggable pipeline wye

connection - Development and design, Advances in subsea pipeline

engineering and technology, Kluwer Academic Publishers, pp 207-228.

REFERENCES

Pigging through Yfittings

PIGGING CHARACTERISTICS OF

CONSTRUCTION, PRODUCTION AND

INSPECTION PIGS THROUGH PIGGABLE

WYE FITTINGS

RESULTS OF pigging tests are presented for various construction,

produc-tion and inspecproduc-tion pigs which demonstrate their pigging characteristics

while passing through a lOin x lOin x lOin piggable wye fitting Detailed

results are presented for inflatable and soluble spheres, a dual-diameter

scraper pig, squeegee(cup-type) pig, foam pigs, dual-diameter gauging pig

and an intelligent pig Details of the test facility, procedures, and

data-reduction techniques are also presented and discussed

INTRODUCTION

Piggable wye fittings used for high-pressure, underwater pipeline

applica-tions were introduced in the North Sea nearly ten years ago Since then, other

areas such as the Gulf of Mexico, Adriatic Sea and Middle East have also seen

applications The main reason for using piggable wye fittings is to allow lateral

connections to trunklines that can be pigged from either the lateral side or

through the trunkline

There are several reasons for designing a piggable lateral connection For

oil pipeline applications, the main interest has been to allow scraper pigs to

be used where accumulated paraffin deposits can lead to plugging of the

lateral pipeline For gas or two-phase liquid/gas transmission applications, the

interest is usually to allow running pigs for removal of liquids that increase

pressure losses or cause internal corrosion There is also a growing interest in

the use of inspection pigs that can be used to examine the lateral pipeline

Prior to the introduction of piggable wye fittings, on many gathering

systems it was necessary to bring the pipeline to a platform, up a riser and into

Fig.l Symmetric piggable wye.

a pig receiver The product was then inter-connected with another pipeline

and a launcher was used to allow the next segment of pipeline to be pigged

In some instances, the sole purpose of the platform and risers is to allow two

pipelines to be inter-connected while maintaining the piggability of both

pipelines One of the biggest future uses for piggable wye fittings will be the

elimination of such high-cost facilities associated with gathering systems

The feasibility of manufacturing piggable wye fittings for high-pressure

applications is now well established However, there has been very little

information published relative to the performance of typical construction,

production and inspection pigs required to pass through piggable wye

fittings The availability of such performance data on the characteristics and

limitations of piggable wye fittings will be useful for designing and evaluating

future applications

The results presented in this paper give quantitative performance

charac-teristics for the following specific pigs:

1 TDW Redskin foam pig

2 Knapp Polly Pig foam pig

3 F.H.Maloney inflatable sphere

4 Select Industries soluble sphere

5 TDW dual-diameter (14 x 10) scraper pig

6 Knapp Polly Pig dual-diameter (14 x 10) gauging pig

7 S.U.N.Engineering squeegee (cup-type) pig

8 VetcoLog intelligent pig

Pigging through Y fittings

Fig.2 Non-symmetric piggable wye.

Qualitative results are also presented and discussed in relation to general

observations and pigging characteristics that may be useful in designing and/

or operating pipeline systems with piggable lateral connections Design of the

piggable wye fitting as a pressure vessel and structural element of the pipeline

system is not within the scope of this work; that topic has been has been

discussed previously[5]

GEOMETRY CONSIDERATIONS

Several types of internal geometries have been proposed for

high-pres-sure, forged piggable wye fittings The symmetric wye geometry is shown in

Fig 1; in a symmetric wye, the inlets are located symmetrically relative to the

outlet This geometry minimizes the angular turn that a pig has to make as it

passes through the wye fitting Another type of internal geometry that has

been considered for high-pressure, forged piggable wye applications is the

non-symmetric geometry, shown in Fig.2, which has the advantage that pigs

passing through the straight run do not have to negotiate a turn However,

pigging through the lateral inlet requires negotiating an angle that is twice as

severe as for the symmetric wye fitting for a given angle between inlets

It should be noted that several other geometries have been used for

fabricated wye fittings For example, Ref 6 describes a 12-in application for