21 Novel approach to PE pipe insertion using four-lobe vacuum collapse.pdf

Novel approach to PE pipe insertion using

four-lobe vacuum collapse

L. Ewing, M. Greig and T. Stafford *

The authors have researched, and partially developed, a novel concept for temporarily reducing

the external diameter of a polyethylene (PE) pipe and then allowing recovery to full diameter as a

close fit within a host main. The novelty lies in the four-lobed collapsed pipe geometry, which

provides an efficient reduction of overall outside diameter without excessive strain and in the

achievement and maintenance of this collapsed shape by vacuum. The further attraction of this

approach is that controlled release of the vacuum provides a substantial primary recovery of the

original pipe size and shape. The inventive discovery that makes possible the control of the four-

lobe collapse state involves strict and progressive control of the collapse process, which can, in

principle, be conducted downstream of the extrusion process, making continuous manufacturing,

and indeed, coiling, feasible. Alternatively the collapse process could be made an on site

operation immediately before insertion. The present work was largely conducted using a highly

elastic grade of PE (Dow-DuPont ‘engage’ polymer) that maximised the high shape recovery

potential of the process. Using this tough, rubbery form of PE pipe it was demonstrated that

almost full pipe geometry could be recovered after release of vacuum. The technique is also

applicable to conventional pipe grade PE materials, but with their stiffer viscoelastic properties

recovery would require some internal pressure and perhaps elevated temperature for full recovery

of circularity. The ‘engage’ PE polymers were found to be potentially useful materials for pipe

lining since they are intrinsically tough and appear to be entirely weld compatible with

conventional PE pipes. Engage liners could be electrofused to PE pipe. Iron pipe lined with a

pressurised engage pipe was artificially fractured and subjected to severe angular offset without

damage to the lining.

Keywords: Insertion, Lining, Collapse, Vacuum, Recovery, Polyethylene, Pipe

Introduction

The research project reported here was originally

conceived as a means of developing tough linings for

metallic pipe networks exposed to earthquake or land-

slip risks. At that time the authors were mindful of the

disastrous consequences of metallic pipe failures during

the Kobe earthquake, where gas pipe fractures con-

tributed to the spread of ﬁre and water pipe failures

inhibited the ﬁre ﬁghting response. Of course there are a

number of well established and commercially successful

lining methods but each of them appeared to have costs

and complexities that limited their appeal to pipeline

engineers. What the authors sought was a functionally

simple process for installing a prefabricated PE pipe but

with low capital equipment costs and minimal on site

complexity. For some years the authors discussed the

issue within the authors’ technical partnership,

Plasticpipes, before focussing on what became the two

key concepts of the present work:

(i) temporarily reducing the installation diameter of

the liner pipe but minimising the pipe wall strain

by using a multiple lobe collapse geometry

(ii) obtaining the collapsed state by use of external

air pressure, i.e. creating the temporary diameter

reduction under internal vacuum.

The logic of pursuing these two concepts was that

together they met the requirement for a simple, low

capital cost site operation. By minimising pipewall

strain, the recovery of full pipe circularity and getting

a tight ﬁt within the host main would be more likely. If

this could be achieved from a collapsed state maintained

by internal vacuum, then the installation and recovery

process would be very simple. In effect, the lining could

be ‘self-inﬂating’.

At the time of starting the present work the authors

had become aware of attempts to develop linings for

small diameter water services by expanding, under water

pressure alone, an inserted multilobed tube extruded

Plasticpipes, 24 Chevington Grove, Whitley Bay, Tyne and Wear, NE25

9UG, UK

* Corresponding author, email trevor.stafford@plasticpipes.co.uk

2007 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 4 May 2007; accepted 4 May 2007

DOI 10.1179/174328907X191323

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Ewing et al. Novel approach to PE pipe insertion using four-lobe vacuum collapse

its original diameter. Higher orders of multilobe collapse

do not bring substantially greater reductions of overall

diameter because each successive lobe interferes with its

neighbour restricting the scope for inward collapse.

During searches of previous work a lapsed UK patent

was discovered which also identiﬁed the advantages of

the four lobed geometry but this was maintained by an

internal former with a complex removal system. 2

The authors’ ﬁrst demonstration (to the authors), of

the concept of four-lobe tubular collapse under partial

vacuum conditions required something more sophisti-

cated than an elastic band. So the authors obtained a

PET bottle and a polyvinyl chloride (PVC) plant pot! A

partial vacuum was created in the bottle by putting into

it a small quantity of hot, steamy water and then closing

it off. Under manual constraint it was possible to induce

a four-lobe collapse; slip the plant pot (minus its base)

over the collapsed bottle and then allow air back into the

bottle. Eureka! The authors had a plant pot lined by a

plastic bottle (Fig. 2). So could this principle be made to

work for our industry? To ﬁnd out, Plasticpipes bid for,

and was awarded, feasibility study funds under the UK

Government ‘Smart Award’ scheme. It is the resulting

work and conclusions that the authors report here.

1 Sketches produced by tracing proﬁle of deformed rub-

ber band illustrate effective reduction of overall dia-

meter by collapse into multiple lobes

using a tough but elastic PE copolymer. 1 The material

chosen by Connor and Daniels to develop that tech-

nology (and typiﬁed by the Dow-DuPont ‘engage’ poly-

mers) appeared to be eminently suited to the authors’

investigation since it offered a combination of low

stiffness, high elastic recovery and toughness while being

potentially weld compatible with conventional PE pipes.

In considering the principal of insertion diameter

reduction by pipewall collapse, the ﬁrst useful modelling

tool was a humble ‘rubber band’. By taking an elastic

rubber band and progressively deforming it from a

circular shape through a series of multiple lobed collapse

shapes the authors could immediately perceive that the

most efﬁcient shape was that of four-lobe collapse.

(Fig. 1)

Two-lobe collapse, which represents the fundamental,

unconstrained collapse mode of a tube under excessive

external pressure, is the form already used in some

insertion techniques. The collapsed diameter actually

exceeds the original diameter and in practice, for

insertion applications, a pipe requires further distortion

and banding constraints before it can ﬁt into a host pipe.

Three-lobe collapse offers no advantage since overall

diameter remains approximately the same. Four lobed

collapse does provide the possibility of an easy ﬁt within

Preliminary studies

Greater understanding of the stress–strain relationship

in four-lobe collapse of polymeric cylinders was

obtained through a programme of numerical modelling

and analysis that was conducted by Boot and Naqvi at

Bradford University. Their mathematical model, based

on 100 mm diameter pipe, took into consideration a

number of combinations of pipe wall thickness, material

properties, and extent of internal vacuum (external

pressure). A plane stress ﬁnite element model of the pipe

was created in the plane of cross-section, with standard

four-node elements. The internal face of the pipe was

deﬁned as a self-contacting surface (i.e. it could not pass

through itself during collapse), and large deformation

theory was taken to be applicable. Material properties

were input for three potential pipe materials; a conven-

tional MDPE/PE80 pipe grade and two alternative

2 First four - lobe vacuum collapse and recovery demonstration

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3 Examples of Bradford University FE model predictions

grades of Dow-DuPont engage PE co-polymer.

Mechanical properties of the polymers were evaluated

by tensile testing and the most signiﬁcant parameter in

this application, secant elastic moduli, were evaluated

as: PE80, 200 MPa; ENG8540, 80 MPa; ENG8100,

9 MPa.

The Bradford work generated valuable numerical

information relevant to the design of a practical system

of controlled collapse. As expected, at standard dimen-

sion ratio (SDR) of less than 60 it was predicted that

four-lobe collapse would not be achievable with

evacuated PE80 pipe at ambient temperature. The

ENG8540 material could potentially be collapsed into

four-lobe form at practically viable pipe geometries. The

ENG8100 could potentially be collapsed as a low SDR

(thick wall) pipe but its ﬂexibility and strength were

probably inadequate for lining applications. The

Bradford work also provided guidance on the conditions

required to constrain and promote the collapse into the

four-lobe state. It was predicted that a four-lobe pre-

ferential collapse could be achievable using four equally

spaced constraints, each providing initial displacement

radially inwards of 10 mm (10% of diameter). Vacuum

collapse into four lobes was predicted, for example, as

occurring at 0?4 bar internal pressure (5 0?6bar

external pressure) into a shape shown in Fig. 3.

These early theoretical considerations were, on the

whole, encouraging and took the authors’ experimental

plans well beyond the ﬁzzy drinks bottle stage. One

problem was the non-availability of tubular material of

appropriately low pipe ring stiffness to investigate

controlled collapse at ambient temperature. The authors

had thought that plasticised PVC or elastomeric tubes

might be useful but commercially available products

were either too small or too ﬂexible. Dow-DuPont were

prepared to make polymer available to the authors in

trial quantities but

prohibitive until more precise speciﬁcation of pipe

material, diameter, and thickness could be established.

In fact the authors’ ﬁrst experimental trials were con-

ducted on short lengths of pipe obtained by rotational

moulding conducted at Queen’s University, Belfast.

Subsequently the authors felt able to commit to a small

scale extrusion run of engage polymer pipe, with the

cooperation of the development unit at Uponor

(Hilcote). Dow-DuPont kindly donated the polymer

for this purpose.

Experimental

The value of practical experimentation for obtaining

a basic understanding of novel concepts was soon

demonstrated. The ﬁrst Plasticpipes test rig was devised

as a static constraint unit, applying the initial four-lobe

distortion via four metal bars in parallel with the pipe

axis (Fig. 4), while internal vacuum was created by a

small pump. The result was not the simple four-lobe

collapse predicted by the Bradford model. As the

external–internal pressure differential became effective,

the pipe surface moved inwards, freeing itself from the

constraining bars. Once this occurred the now uncon-

strained pipe reverted immediately to its naturally

preferred two-lobe collapse state. This was a project

threatening concern that remained a worry as the

authors moved to a dynamic constraint rig that was

planned as a prototype for creating continuous four-

lobe collapse in long pipe lengths. This rig utilised a

combination of constraint rollers and internal vacuum

piston seals (Figs. 5 and 6).

The ﬁrst trials with this equipment, using 90 mm

extruded tube and drawing it through the rollers, while

vacuum was maintained, unfortunately produced the

same result as the static rig. Uncontrolled two-lobe

collapse appeared to be unavoidable as the pipe lost

constraint from the rollers.

the cost of extrusion would be

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Ewing et al. Novel approach to PE pipe insertion using four-lobe vacuum collapse

4 Static constraint and collapse test rig

It was at this ‘low point’ of the authors’ efforts that, as

sometimes thankfully happens in R and D, a ‘break-

through’ occurred. Separate, small scale observations of

the piston sealing behaviour during pipe collapse were

being conducted using short lengths of pipe between the

seals. This led to the chance discovery that at short

lengths, a four-lobe collapse could be achieved naturally.

This occurs probably because the end constraints of the

piston seals inhibit lower order collapse. Indeed it was

possible to trace a pattern of two, three and four-lobe

collapse with decreasing length between the end seals

(Fig. 7).

Once the natural four-lobe collapse had been obtained

in a short length it acted as its own constraint allowing

6 General arrangement for hauling pipe through dynamic

constraint rig

progressively longer collapse lengths by simply moving

the end seals. This phenomenon held when transferred

over to the continual production rig, and longer, stable

lengths of

four-lobe collapsed pipe were achievable

(Fig. 8).

This effectively met the authors’ target of demonstrat-

ing a feasible production route towards a pipe in a four-

lobe collapse state maintained by vacuum. However, this

was not the limit to Plasticpipes investigation of the

materials and the process for developing a new lining

technology.

Peripheral studies

It was recognised from the outset that any novel

material proposed for pipe lining could not be practical

unless

it could be jointed into conventional pipe

5 Dynamic roller constraint and sealed vacuum collapse

prototype rig

7 Four-lobe collapse found in short pipe length

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Ewing et al. Novel approach to PE pipe insertion using four-lobe vacuum collapse

8 Controlled and stable four-lobe collapse demonstrated

over longer length in dynamic rig

networks. One of the attractions of the engage type of

polymer was that as a PE co-polymer it might be weld

compatible with conventional PE pipe. Accordingly

trials were made of the possibility of joining our 90 mm

extruded pipe using conventional PE electrofusion

couplers (Fig. 9). The results were good, leading the

authors to conclude that a liner pipe could be jointed to

conventional medium density polyethylene (MDPE) or

high density polyethylene (HDPE) pipes.

The highly elastic nature of the engage co-polymers

(they are routinely used as thermoplastic elastomers) led

us to believe that they could be characterised as tough,

with long term resistance to crack propagation, and

would have high, elastic recovery from deformation.

Toughness was proven by the absence of crack growth

in environmental stress cracking (ESC) tests using the

Virole consistent strain method. A more practical

demonstration of toughness was to artiﬁcially crack and

displace an iron pipe ﬁtted with a liner carrying water at

pressure. This experiment was conducted at Sunderland

University (Fig. 10) and gave a dramatic demonstration

of the effectiveness of such lining in its resistance to

failure of the host pipe induced by ground movement.

Elastic recovery was of great interest since a four-lobe

lining product would need to recover its pipe circularity

after storage or transport in the collapsed state.

Plasticpipes therefore conducted a small scale experi-

mental programme using a novel test rig. Samples of

engage and conventional HDPE pipe material were bent

and constrained in a jig with a level of deformation

consistent with that of the four-lobe collapse geometry

(Fig. 11) . Samples were ﬁxed in this way for various

timescales and then recovery rate and extent were

10 Fracture of host pipe: no failure of lining (Sunderland

test)

11 Constraint for recovery test: clamped sample

12 Recovery test equipment

measured using a sensitive linearly variable displacement

transducer (LVDT) in the arrangement illustrated in

Fig. 12. Much comparative data was obtained but the

general conclusion was that we could expect faster and

more complete recovery of the elastic PE co-polymer

than the HDPE material (Fig. 13).

Finally, Plasticpipes gave a little consideration as to

whether the four-lobe collapsed pipe could be trans-

ported in coiled form. Figure 14 shows a very simple

manual demonstration (perhaps reminiscent of the ﬁzzy

drinks bottle), that four-lobe geometry pipe can be

potentially further collapsed ﬂat and coiled. In such a

form the internal vacuum might be preserved even if end

seals had failed.

Conclusions

Using simple, low cost capital equipment, it has been

demonstrated that continuous and controlled four lobed

9 Electrofusion joint in engage pipe

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