Pipes and pipe coatings

A weight-coated pipe has an innermost anti-corrosion layer, a helically-wound covering of a cementitious sheeting having a reinforcement of organic fibrillated film network arranged as a plurality of superposed layers. This covering is applied in a post-cracked condition so that constituent parts of the next layer of concrete penetrate the micro-cracks and form a good bond. An outer layer of helically-wound sheeting similar to the first-mentioned covering serves to protect the weight-coating of concrete from impact damage.

This invention relates to pipes. 
It has already been proposed in British Patent Specification No. 1,582,945 
to produce sheeting material having a matrix of water-hardened mass (or 
material) with a reinforcement in the form of a plurality of layers of 
fibrillated polypropylene film or other organic material network which has 
the property of fibrillating, such as high modulus polyethylene. More 
particularly, this prior British patent and the cognate U.S. Pat. No. 
4,297,409 disclose an article comprising a water-hardened substance and 
network means of fibrillated organic film in the form of a plurality of 
expanded, non-woven layers embedded within said substance, the fibrils of 
said film providing continuous mechanical bonding to reinforce the 
water-hardened substance so that under excess loading the article exhibits 
multiple cracks. Furthermore, these patents disclose an article comprising 
a water-hardened substance and network means of fibrillated organic film 
in the form of a plurality of layers embedded in the water-hardened 
substance, the effective volume of film in one direction of tensile 
stress, when loaded, amounting to more than 11/2% of the overall volume of 
the article and the fibrils of said film providing continuous mechanical 
bonding to reinforce the water-hardened substance in said direction of 
tensile stress so that under excess loading the article exhibits multiple 
cracks. As mentioned above the preferred reinforcement is polypropylene 
film and a particularly economically advantageous form is laterally 
expanded film arranged as a plurality of layers. In a less preferred form 
the network means is in the form of a weave with a doubled warp and a 
single weft so arranged that at each intersection the weft passes over one 
warp and beneath the other warp; then the warps are twisted after 
insertion of the weft whereby to lock the weft in position. Materials in 
accordance with this earlier dated Patent Specification will be marketed 
under the Trade Marks NETCEM and KALDIN. 
Although the method and material of prior Specification 1,582,945 is 
primarily intended to make use of cement as the matrix, it is also 
possible to use other water-hardened materials such as gypsum. The present 
invention will, in general, make use of cement mortar as the matrix and 
for convenience the term "cementitious sheeting" or "cementitious sheet" 
as used herein is intended to mean a sheet or sheeting made of a 
water-hardened material with a reinforcement in the form of a plurality of 
layers of organic fibrillated film network, such as polypropylene. The 
sheet or sheeting will be in its post-cracked condition when incorporated 
in pipes in accordance with the invention. The prior Specification 
describes in some detail the capability of material in accordance with 
that prior invention of cracking in the overload condition in such a way 
as to form a multiplicity of fine cracks and the present invention makes 
use of this property. The terms "cementitious sheeting" and "cementitious 
sheet" as used herein are therefore intended to refer to sheeting in the 
post-cracked condition. 
The oil and gas industry now work on a world-wide scale making use of 
submerged pipelines and particularly in the case of gas pipelines it is 
necessary to impart a negative buoyancy because motion of the pipeline 
produced by currents can cause serious damage, as any upwards floating of 
the pipeline can lead to eventual rupture with obvious disastrous results. 
At present the concrete weight-coating of such pipelines is securely held 
in place by a steel mesh reinforcement. Impact damage causing superficial 
concrete cracking can result in progressive corrosion of the steel mesh. 
Spalling and subsequent breaking up of the concrete has, in the past, 
resulted from corrosion of the mesh. The various aspects of the present 
invention enable the omission of the steel mesh. 
Submerged pipelines are generally made of high quality steel but it is 
important that this steel shall be completely and permanently protected 
against the very corrosive action of sea water. 
Considerable sums are currently expended with a view to achieving this aim, 
but to some extent such an aim is inconsistent with the application by the 
conventional impingement method of a weight-coating of concrete, since the 
actual application stage of the fresh concrete can lead to puncture of the 
anti-corrosion layer previously applied to the pipes. Even a pin-hole can 
lead eventually to extensive and disruptive corrosion. 
According to the present invention in a first aspect there is provided a 
method of producing a combination of a metallic pipe and a cementitious 
covering for the pipe comprising the steps of winding cementitious 
sheeting (as hereinbefore defined) around the pipe with the sheeting in 
its post-cracked condition and securing the wound sheeting in position in 
relation to the pipe. 
Further according to the invention in the same aspect there is provided a 
pipe incorporating at least one helically-wound cementitious sheet (as 
hereinbefore defined) the cementitious sheet being in a post-cracked 
condition with a multiplicity of micro cracks in at least one surface 
thereof. 
Contrary to the generally accepted understanding of persons skilled in the 
art of cement-based products, the material referred to as "cementitious 
sheet" or "cementitious sheeting" has sufficient flexibility or ductility 
both in its green (unhardened) and hardened states to be wound around a 
core into a coil for storage and transportation to its place of use. When 
coiled in its green state, a thin sheet, for example polyethylene foil, is 
interposed between the turns to separate the layers as the material will 
usually harden during storage. 
When the material is wound in its hardened state to form the coil, multiple 
cracking or crazing will occur at the outer surface of the material, as 
wound. Conversely, when the material is unwound on the coil, if it was 
initially wound in the green state, a multiple fine cracking or crazing 
will occur at the surface which was the inner surface when wound on the 
core. Owing to the nature of the reinforcement this multiple fine cracking 
does not even disturb the integrity of the material in contrast to other 
cement-based materials. 
In the preparation of pipes for submersion great care has been taken to 
ensure that the pipes are fully protected and one method which has already 
been adopted is the application of a layer of the order of 200 to 500 
microns thickness of a suitable epoxy-resin. The method involves heating 
the pipe to 180.degree. to 200.degree. C. and spraying epoxy-resin powder 
on to the pipes where it fuses to form a tough, corrosion-resistant, 
coating. While the presence of concrete in contact with this corrosion 
protected surface does not impair its effectiveness, there is some risk 
during manufacture that when using high-speed "throwing" techniques 
impingement of sharp aggregate particles in the concrete mix is liable to 
damage the anti-corrosion protective coating. 
In accordance with the method and apparatus of the first aspect of the 
invention the cementitious sheeting is wound on to the anti-corrosion 
material and preferably the sheeting is secured with the aid of a 
compatible adhesive directly to the coating. 
Since the anti-corrosion coating of the pipe may be smooth, it will 
generally be desirable during manufacture to apply the epoxy adhesive 
either to the pipe or to the inner face of the inner layer of hardened 
sheeting so that there is proper adhesion between them. 
The impinging concrete can be applied without risk of damage to the 
corrosion protection coating since the cementitious sheeting is well 
capable of withstanding the impact of concrete particles during its 
application stage. 
Small-diameter pipelines, displaying no buoyancy when empty, need no 
weight-coating but may require mechanical protection of the rather 
vulnerable epoxy coating. A layer of helically-wound cementitious sheeting 
applied round the anti-corrosion coating ensures protection against impact 
and abrasion. Such pipes are sometimes laid from a reel ship where the 
deformation of the pipe is such that the yield point of steel is exceeded. 
In order to accommodate the compressive strains in the cementitious 
sheeting, it will be desirable to wind with a small gap between the turns 
and to select an epoxy formulation of adequate elasticity, e.g. one with a 
polyamide curing agent. On bending, the cementitious sheeting then retains 
its integrity on the compressed side of the pipe. 
In accordance with a further development of the invention, the concrete is 
applied while a layer of cementitious sheeting is wound at a spacing from 
the pipe surface and this layer forms a permanent shutter. The 
helically-wound layer will be in the post-cracked condition. To ensure 
concentricity a helical spacer mcmber or members may be provided between 
the helically-wound layer and the pipe. 
It is possible to make use of the post-cracked condition of the sheeting to 
improve keying to the concrete so that the more liquid portions of the 
concrete mix will tend to flow into the cracks during the manufacturing 
stage. 
If no layer of cementitious sheeting is applied, the problem of adhesion 
between the concrete weight-coating and the anti-corrosion coating remains 
the same, as it is not possible to increase adhesion simply by applying an 
adhesive to the outer surface of the anti-corrosion coating. There is a 
particular risk that even if initial adhesion is tolerably good, the 
weight-coating concrete can slide relative to the pipeline itself when 
clamps are gripping the concrete as it is being dispensed from a laying 
barge, because the considerable weight of hanging pipe beneath the water 
surface will set up high shear stresses. 
As an alternative, therefore, the helically-wound inner sheeting can be 
applied in the manner of a screw, so that in axial cross-section the pipe 
has a castellated form and the keying provided by the internal layer 
adhered to the anti-corrosion coating by an epoxy-adhesive will render 
slipping of the weight-coating concrete relative to the pipe itself more 
unlikely. 
In accordance with either aspect hereinbefore described, the cementitious 
sheeting may be wound around a pipe in its "green" unhardened condition or 
wound to form the inner or outer skin of the pipe in the green condition. 
Summarizing, the aspect of the invention first considered namely 
weight-coating of the pipelines provides for the following possibilities: 
(a) protection of an anti-corrosion coating on a steel pipe by a single 
layer of cementitious material applied directly to the anti-corrosion 
coating; 
(b) a complete weight-coated pipe incorporating a layer of cementitious 
sheeting which serves both as a permanent shutter and as a protection 
against handling during installation and abrasion/impact when installed in 
submerged conditions; 
(c) the provision of a partial protection for an anti-corrosion protected 
pipeline by means of a helically-wound layer of cementitious sheeting with 
the turns spaced from one another so that externally applied concrete will 
be effectively keyed to the pipe. 
According to the present invention in another aspect there is provided a 
method of producing a pipe comprising helically winding cementitious 
sheeting (as hereinbefore defined) on to a mandrel, applying spacing means 
to the outer surface of the cementitious sheet on the mandrel, winding a 
second cementitious sheeting on to the spacers and simultaneously filling 
the space between the two layers of sheeting with a concrete or mortar 
mix. 
Concrete pipes are well known but at present have the disadvantage that 
costly moulds have to be provided and these have a relatively short life 
owing to the highly abrasive nature of concrete. This second aspect of the 
present invention provides a method which enables such pipes to be formed 
with the aid only of a simple mandrel which will be in contact with 
relatively less abrasive cementitious sheeting (as hereinbefore defined) 
and the outer layer of cementitious sheeting acts as a permanent shutter 
thus avoiding the necessity of any mould. 
As for the first aspect of the invention, the cementitious sheeting will be 
stored on coils and either the inner or the outer surface will be in a 
post-cracked condition depending on whether the material is initially 
hardened or is in a green condition. 
Although the method in accordance with the second aspect of theinvention 
will normally make use of concrete, mortar can also be used for particular 
purposes. Again as for the first aspect of the invention the post-cracked 
condition of the layers will be used for keying purposes and provided 
adequate vibration is ensured the finished pipe can be used for the 
conveyance of gas or liquid since the cracks in the cementitious sheeting 
will be effectively filled thus rendering the pipe fully impervious.

Referring now to FIG. 1, a coil 10 of cementitious sheeting 12 (as 
hereinbefore defined) has been wound on to a reel or drum of appropriate 
diameter and if initially in a hardened condition the sheeting will crack 
to form multiple fine cracks on the surface, which when wound on the reel 
will be outside. If the cementitious material is initially "green", that 
is urhardened, then cracks will not normally be formed during tbe 
winding-on process but will arise during unwinding and will then occur on 
the inner face (as wound) of the sheeting. The sheeting will, as indicated 
in FIG. 1 be helically-wound on to a pipe 14 and winding will continue for 
the full length of that pipe. Although not shown, the pipe will have an 
anti-corrosion layer of epoxy-resin on its outer surface and prior to the 
winding of the sheeting on to the pipe, epoxy adhesive will have been 
applied to the anti-corrosion epoxy coating. As illustrated, the 
post-cracked surface when coiled in the green state will lie outermost on 
the pipes so that good keying is ensured with the concrete mix to be 
applied subsequently. 
In the method illustrated in FIG. 3 a pipe is used which may or may not 
have been provided with a layer of cementitious sheeting to protect the 
anti-corrosive coating and additionally a helical spacer 16 has been wound 
on to the outside of the pipe. The single continuous spacer could be 
replaced by a plurality of individual spacers such as spheres arranged 
like beads on a string. It is then possible to wind a second or additional 
layer 22 of cementitious sheeting and substantially simultaneously an 
appropriate concrete or mortar mix 18 fills the space between the two 
layers of sheeting delivered through a nozzle 20. Compaction to achieve a 
high density of the concrete can be effected in the conventional way by 
vibration, for example by a poker vibrator or by vibrating the pipes at 
the point of filling. Conveniently, the pipe will be rotated and 
translated, the reel of cementitious sheeting and the concrete injection 
nozzle remaining stationary. In this instance it would be preferable that 
the outermost surface of the outer sheeting will be in a non-cracked 
condition and that the inwardly directed surface will be cracked so that 
good keying is ensured with the concrete filler. 
The completed pipe is illustrated in FIG. 2 where all the layers are 
individually shown including the anti-corrosion protection 24 on the 
immediately outer surface of the pipe which is shown with a greatly 
exaggerated thickness. 
In a practical example such a pipe having a diameter of 1 m. and 13 m. 
length would make use of inner and outer cementitious sheeting layers 12 
and 22. The sheet would be about 3 mm. thickness (it could be as small as 
2 mm. and as large as 6 mm.) and the width is 1.2 m. (it could be as 
little as 0.3 m.). 
The weight-coating of the pipe can be modified by incorporating short 
chopped lengths of fibrillated polypropylene twine in the concrete to 
improve its impact resistance. Alternatively, a reinforcing network of 
fibrillated polypropylene can be incorporated in the concrete or mortar 
mix in order further to improve impact resistance. 
Instead of winding the inner and/or outer layer of cementitious sheeting 
from a single coiled sheet, two or more narrower sheets can be app1ied as 
a multi-start helical winding. 
Although epoxy-resin has been referred to as the preferred anti-corrosion 
coating, other coating materials such as bitumen or epoxy bitumen can be 
used. 
In its broadest aspect the invention can be considered as extending to a 
pipe, not necessarily of circular cross-section, which incorporates 
cementitious sheeting (as hereinbefore defined) in a post-cracked 
condition. 
EXAMPLE 1 
A test specimen (A) was made consisting of a slab of concrete on a 
substrate in the form of rigid steel plate 3 mm. in thickness with a 
sheeting of a cement mortar mix having a plurality of layers fibrillated 
polypropylene network incorporated therein. The specimen was made in a 
mould measuring 0.5.times.0.5 M/ and 50 mm. thickness. The concrete mix 
used for this slab had the following composition in parts by weight: 
Cement 1; 
Free water 0.51; 
Sand (5 mm.downwards) 2; 
Thames gravel of 5-10 mm. 1; 
Thames gravel of 10-20 mm. 2; 
After hardening, this concrete had a compressive strength of 45 
MN/m.sup.2., which is in the range of strengths required for submerged 
pipelines. The sheet was one of the composites referred to in British 
Patent Specification No. 1,582,945. The sheet was placed in the mould 
while the concrete was being vibrated to ensure proper compaction. The 
sandwich of steel, hardened concrete and cementitious sheet represents the 
system of weight-coating where the sheeting serves as a permanent 
shuttering. The specimen was tested under conditions simulating those met 
by weight-coated pipes in practice when handled, being laid or in a 
vulnerable position on the sea bed. 
A specimen (B) was made in a similar way to test specimen (A), but the 
concrete mix included a reinforcement of chopped, short lengths of 
fibrillated polypropylene twine. Such concrete is known in itself and is 
known under the trade name "CARICRETE". It has a high impact resistance 
and can be used in conditions where such resistance is necessary as in 
pile driving. In the particular control specimen, commercial twine of 
twisted fibrillated polypropylene cut into 35 mm. staple was used. 0.5% by 
volume was added to the concrete. This specimen incorporated cementitious 
sheeting as specimen (A). 
A control specimen (C) was prepared in the form of a slab of concrete of 
the same dimensions as specimen (A). 
A further control specimen (D) was prepared in the form of a slab of 
concrete of the same dimensions as specimen (A) but with reinforcement as 
in specimen (B). No cementitious sheeting was included. 
Impact tests were carried out by dropping a steel ball of 80 mm. diameter 
and of 2 kg. weight from heights up to 4 m. on to the test slab. The 
damage to the control slabs (C) and (D) resulting from repeated drops on 
one spot was observed and the load (20N) was multiplied by height in 
meters and number of drops to obtain the energy in Joules needed for this 
damage to occur. Hair cracks in the concrete layer which were visible 
under a magnifying glass developed in both controls at 80J, while severe 
cracks, that is branched cracks, were clearly visible to the naked eye at 
160J in plain concrete. Control specimen (D) showed no severe cracking 
even after 8000J was expended in ten drops from 4 m. height. 
The falling ball experiments on specimen B, covered with cementitious 
sheeting exhibited indentation and slight abrasion of the sheeting surface 
with cracking of the concrete underneath. These cracks when inspected, 
after removal of the steel substrate, were similar to those in the 
corresponding control slabs but the cracked concrete was in all cases 
restrained and protected by the cementitious sheeting. 
EXAMPLE 2 
A steel pipe without any concrete weight-coating, but having an exposed, 
corrosion-protected coating was helically-wound with sheeting in 
accordance with the invention (see FIG. 1 and related description) for 
mechanical protection of the coating. The pipe had been factory-coated at 
about 180.degree. C. with an epoxy powder compound which had cured to give 
a hard, smooth finish on the pipe. The outer diameter of the pipe was 170 
mm. It was wrapped with cementitious sheeting of 250 mm. width, and, 
during the winding of two-component fluid epoxy adhesive, at a rate of 
covering 400 g/m.sup.2 was applied to the inside of the sheeting. Steel 
wire strapping temporarily secured the sheeting at the beginning during 
the hardening of the adhesive at ambient temperature. The pipe which had 
been protected for half of its length with a helically-wound sheeting was 
tested in the following ways: 
Falling steel balls were used to test impact strength, each ball weighed 2 
kg. and had a diameter of 80 mm. Another ball used weighed 0.225 kg. and 
had a diameter of 38 mm. The smaller ball falling from a height of 2 mm. 
on to the unprotected epoxy coating caused a slight dent but falling from 
3 m. it chipped the coating to show some bare steel where corrosion of the 
steel pipe could set in. The same ball falling from 3 m. on to the 
wrapping caused no damage even after repeated drops on the same spot. 
The larger ball chipped the epoxy coating after one fall from a 2 mm. 
height. Falls on the sheeting proved harmless from a height of 2 m. and 
from a height of 3 m. when impacting upon the sheeting. Slight abrasion 
and formation of powder on the surface of the sheeting was the only effect 
after repeated drops on the same spot from 3 m. height. 
The adhesion of the wrapped sheeting was tested by glancing hammer blows 
and the sheeting itself was locally destroyed and then an attempt was made 
to peel the sheeting from the coated surface. No loss of adhesion was 
observed. 
In practice, such small diameter pipes, as tested, are twice deformed 
beyond the yield point of the steel, first when being reeled and later 
when unrolled from the reel. The ability of the cementitious sheeting to 
follow such deformations was tested in the manner described hereinafter in 
Example 3. 
EXAMPLE 3 
Cementitious sheets having a width of 0.25 m. and a length of 0.5 m. were 
adhered on to each side of a 0.5.times.0.5 m. steel plate 3 mm. thick. One 
sheet extended across the plate parallel to the two sides, while the other 
sheet was adhered diagonally. A two component fluid epoxy adhesive as used 
in Example 2 was spread at a coverage of about 300 g/m.sup.2 on the 
sheeting, which, after curing at ambient temperature, bonded it firmly to 
the steel. 
In a four-point bending test, the plate produced in the immediately 
preceding paragraph was bent to a radius of curvature of 0.5 m. This 
condition simulated the bending of the pipe described in Example 2 with an 
outer diameter of 170 mm. around a hub of 18 m. diameter corresponding to 
the reel. 
Another test represented the bending of a 400 mm. outer diameter pipe 
around the same hub. Then the radius of curvature of the steel place in 
the test was 0.2 m. 
In both tests the tensile strains on the convex sides of the test piece 
were well within the permissible elongations of the sheeting and no damage 
was observed. 
The compressive strains on the concave side of the bent plate were 
calculated in this test to be of the order of 1% to 2% respectively, which 
is in excess of the permissible strain for hardened cement. This caused 
the matrix to fail by local loss of adhesion and slight crumbling but the 
sheeting still had sufficient integrity to provide protection.