(This article is republished with EPCM with the kind permission from our marine structure pipeline partner – Krah Pipelines)

Today it is still a reality that, despite the continuous developments in the technology of tangential extrusion on mandrels, some clients, engineers and contractors tend to prefer axially (conventional) extruded, solid wall PE pipes for the marine projects. They are probably not aware, that Krah can present many global successful projects where Krah Marine Pipes (KMP) have been used.

Marine Structured Pipelines KRAH

KMP – Outfall pipeline DN/ID 3400 mm

We carried out marine projects for desalination plants, power plants, sewage treatment plants and other intakes/outlet pipelines. Here we used Krah Marine Pipes with a structured wall and with a solid (pressure) wall.

Nothing speaks against solid wall conventional axially extruded pipes, which have been familiar for many years – as they were the state of art for many years. “Familiarity” means that, in general, no big effort is needed for the design and installation of this well-known “stuff”.

But unfortunately still until today, we, “Krah Pipe Producers, worldwide” need to convince engineers that our pipes are suitable for this application. Especially on the cost point of view, we are a very attractive solution – not only for the pipe but for the complete project including ballasting and sinking. So, we must improve our marketing and technical documentation to move from a niche market-player to the main player.

1 Marine Structured Pipelines KRAH – History

Tangentially extruded pipes theoretically and practically should be equally familiar – companies like KWH, Bauku, Henze (including Chinese copies) and others have been producing several types of ribbed/profiled pipes, low pressure, high ring stiffness, destined mostly to landlines, fittings, manholes and storage tanks application, since the 70s.

In the 80s it was tried to use ribbed/profiled pipes for a few outfall projects, but the pipe available at that time proved to be unsuitable for the use and the prices used to be too high. The main reason was the missing homogenous waterway wall thickness and a proper melting of the profiles, a proper high-quality jointing and a cost-effective price (high extrusion output and the extrusion technology itself).

On the matter, there has been a continuous development of the structural design, covered mostly by the DIN Standards, but, until the late 90s, we were speaking of single-wall pipes with ribs/profiles.

Since the end of the 90s, Krah has developed a new kind of pipe, homogenous, a correct waterway wall thickness and proper jointing. Together with high production output, the pipe was and still is, a very successful solution for all kinds of marine pipes. Many large size projects were realized worldwide by companies like PPA&KRAH, Krah-Pipes, HENZE, FRANK, Krah Estonia, Krah America Latina, Krah Chile, Krah Egypt, Polyplastic, Polypipe, UGPM Oman just to name a few. The largest diameter since today is in Argentina with an internal diameter of DN/ID 3600 mm – all jointed by electrofusion and deployed into the water. Until this time usually, axial extruded PE-pipes were used up to DN/OD1600 and always with SDR17 – due to limits in the production and engineering.

2 Today

Today conventional axial produced pipes are used up to DN/OD3000 and due to new engineering skills, an SDR of 33 can be used. For us, Krah pipe producers, the relevant standard – DIN 16961, last update 2018, is still the base for the standard technical approach. Many other intl. standards used the “old” DIN for a National Standard like ASTM F894, IRAM 13414, JIS K 6780. The normal double-wall pipes, which are covered up to DN/OD 1200 by EN/DIN 13476 or ISO 21138, can´t be used for marine pipe application.

A few engineers started to implement the possibility to use structured wall pipes for marine application, by using the geometrical and mechanical properties of the structured wall (moment of inertia, waterway wall thickness, buckling resistance).

A closed profile for Krah Marine structured Pipes (double chamber wall, super high stiffness)

For the installation in the marine environment, the axial load capacity and bending resistance are more important than the ring stiffness only. It is therefore intuitive why the real double-wall structure (KMP) sections are more suitable for the marine use.

A double-wall structure well calculated can be designed with a requested equivalent SDR (eSDR) and a ring stiffness can exceed the SN values of solid wall pipes with that typical SDR. It is also possible to produce exactly the ring stiffness or pipe stiffness which is needed. The waterway wall thickness can be produced exactly according to applied working pressure. Today a Krah Marine Pipe (KMP) is a tailor-made solution to realize marine pipelines very economically.

The developments started to follow two directions: pipe wall construction with single or multi-layer profiles or walls with square or round chambers. Both extruded on mandrels with spirally applied layers (“tangential extrusion” is the technical term in the standards – here Krah production technology).

Many additional aspects and properties of the pipe (KMP) can be found in DIN/PAS 1065:2007-11 “Spirally wound pipes made from polyethene (PE 100) – tangentially extruded – Dimensions, technical requirements and test”.

As the Krah Group, we have decided to proceed on a relatively new way, defining a structured wall which can be easily dimensioned to obtain the characteristics necessary for marine projects.

The KMP – Krah Marine Pipe – is a “real” double wall pipe, meaning that it presents two thick, continuous layers and internal chambers/profiles alternatively circular and rectangular (or combination of both). Today corrugated pipes (produced on a corrugator, not on Krah production technology) are also called double-wall, but they are not “real” double-wall pipes, they have not the availability to handle internal working pressure, they have no pressure-resistant joint and their bending characteristic are not homogenous.

Marine Structured Pipelines KRAH

KMP – wall structure

The Krah-Marine Pipes (KMP) are always with a homogenous and smooth inner layer, it is not a pipe welded by prefabricated square profiles and always has a socket and spigot and is not ending in an open profiled spiral.

The pipe is weldable and fittings and manholes with the same wall structure are easily producible.

Marine Structured Pipelines KRAH

Every pipe has a solid wall electro-fusion socket, and double wall area and a solid wall spigot.

Krah Marine Pipes (KMP) are available from DN/ID800 mm up to DN/ID5000 mm, the standard pipe length is 6 m, but the pipes can be pre-jointed at the production facility, to increase the installation speed. Depending on the client, all pipes can have an inspection friendly inside surface. Krah Pipes, in general, is available from DN/ID300.

3 Calculation

Our standard calculations are based mainly on the following approaches, according to existing standards and covered up by references and experiences. A third-party engineering company can and should always check our proposals.

3.1 Hydraulic capacity

All Krah Marine Pipes are calibrated on inner diameter – so nominal diameter DN is the internal diameter ID. That means the inner diameter is not changing if wall thickness increases. Very often we are faced with misunderstandings in consideration of the nominal diameter and the meaning for inner and outer diameter. Our nominal diameter is the hydraulic diameter.

Tangential and axial extruded pipes are the effect on the hydraulic diameter.

For example:

A Krah Marine structured Pipe (KMP) DN/ID 2000 provides an inner diameter of 2000 mm for all stiffness and pressure classes!

A conventional direct extruded pipe DN/OD 2000 provides always a significant lower inner diameter than 2000 mm and is related to SDR class/wall thickness:

DN/OD2000, SDR 21, PE 100 (stiffness approx. SN 12, PN 8) => hydraulic diameter of ID 1808 mm

DN/OD2000, SDR 26, PE 100 (stiffness approx. SN 6, PN 6) => hydraulic diameter of ID 1846 mm

That means to be competitive in hydraulics to a Krah Pipe DN/ID 2000 a much bigger DN/OD pipe must be considered:

DN/OD2200, SDR 21, PE 100 => ID 1990 mm

DN/OD2200, SDR 26, PE 100 => ID 2030 mm

A difference in inner diameter is affecting directly the velocity and flow characteristics. Together with the quality of the inner surface, the diameter defines the pressure loss for the volume flow rate.

3.2 Working / Operation pressure

Krah Marines Pipe (KMP) are standardized for pressure load capacities according to DIN 16961 and DIN PAS 1065. For decades Krah Pipes have been used for pressure load applications and now finally got respected and described also in the standard.

Instead of mentioning only a “design pressure” we highly recommend differing between operating pressure and real stiffness. The real stiffness must be the result of technical calculations and not only a “choosing” of stiffness class according to ISO9969. It makes no sense to large-scale old realized projects with small pipe diameter. The difference in prices is huge and technically a large-scaling has no positive effect. Reduce the project costs by calculating and not by copy-paste!

The inner (waterway) wall of Krah Marine Pipes is designed according to the real operating and pressure conditions. For intake lines, we face no inner pressure, for outfall lines the Maximum Operating Pressure (MOP) is decisive! We can design the Krah Marine Pipes for any inner pressure, there is no limitation. But with increasing pressure resistance, the wall thickness of the inner wall is increasing, and this makes it less efficient.

Remember: the increasing of the wall thickness of KMP is not affecting the hydraulic diameter of the pipe!

It is commercially and technically recommended to design the inner wall regarding the real working conditions. Don’t ask for a high “design pressure” when you have special needs for the sinking process or backfilling of the pipe.

Marine Structured Pipelines KRAH

Extract DIN16961:2018, Attachment D

3.3 Pipe stiffness

In the past, we realized many projects where complete pipe-system have been installed underground, with a very difficult ground situation and very complicated backfilling. We can, and we do consider the backfilling and installation of the pipeline after the sinking process. We provide a full structural calculation of the pipes, according to international standards.

Marine Structured Pipelines KRAH

Testing of the pipe stiffness and flexibility (30% deformation)

3.4 Buoyancy

To avoid buoyancy, it is typical to use concrete ballast blocks for Polyethylene Pipes.

Of course, this is also available and standard for Krah Marine Pipes. Normally the concrete blocks are positioned every 6 meters. An advantage to direct extruded pipes is the slightly profiled outer wall, especially at the joint. That reduces the risk of slipping to zero percent, even at big changes in temperature and because of that a shrinking of pipe diameter.

Marine Structured Pipelines KRAH

Here you see concrete blocks mounted in the pipes

Cheap blocks without any screws can be used, the blocks are linked to the pipe-structure by using concrete.

A very cost-effective solution for the blocks

Another smart and patented solution is the partial filling of profiles with concrete/grout. The Krah Marine Pipe structure gets filled with concrete/grout in the second spiral. That means that the pipe is still semi-flexible and can be bent easily, whilst at the same time the pipe is heavy, to reduce or to eliminate concrete blocks. The second “load” spiral can be filled partly. The filling can be done at the factory or short-time before installation.

Marine Structured Pipelines KRAH

A smart solution – a partial filled KMP

The bending radius is depending on the wall structure and filling but reaches values of 2 times of the unfilled Krah Marine Pipe profile. In case that all chambers are filled (one spiral solution) the pipe has to be handled as a fix and rigid beam.

Because of the partially filling with the defined distance, the Krah Pipe stays still semi-flexible and provides all the benefits for typical S-curve installation.

3.5 Bending

Krah Marine Pipes are designed for a bending radius of 50 x pipe diameter. In design, we consider the strain of outer fibre as well as buckling resistance. Because of the very stiff pipe wall structure, Krah Marine Pipes provide the highest safety against buckling.

Many finite element analyses were carried out – including the bending behaviour on the joints.

Marine Structured Pipelines KRAH

FEM – Analysis of a bent Krah pipe

Krah Marine Pipes are produced out of high-quality PE 100 with the best characteristics in strength and stability, so these proven values are used in all our calculations.

A bent Krah pipe

Our nice combination of flexibility and stiffness will give to the Krah Marine Pipeline a perfect behaviour to seismic ground movements – often happens after installation. Several tests are done, details and independent test reports can be requested at Krah-Pipes in Germany.

3.6 Allowable pulling forces

At Krah Pipes, the axial wall thickness gets jointed homogenously by an electrofusion process. The joint is designed for the same load as the pipe wall itself. There is no weak point in the complete pipe string and no separate reduction factor must be considered. For the pulling force only, the minimum axial connected wall thicknesses are considered, even because a “spiral” in our case a double spiral can handle pulling forces too – so we consider this aspect as a special safety to our marine solution.

3.7 Marine Structured Pipelines KRAH: Jointing

The real Krah Marine Pipe double-wall structure gets homogeneously jointed by integrated Krah electro-fusion process. The Krah Marine Pipe electro-fusion joint guarantees a strong connection between the pipes. Because inner and outer wall are joined by electrofusion a perfect load transition is assured for all occurring stresses/loads. Also, the stiffness of the jointing area is higher than the stiffness (requirements) of the pipe itself. Another large benefit of an integrated electro-fusion joint, compared to butt-fusion, is the aspect that KMP has no NO-sagging of pipe – to provide a better, safer and quicker installation.

Marine Structured Pipelines KRAH

Krah welding equipment for large pipes

As you can see in the picture above, the KMP joints need “only” little E-Fusion devices and no expensive butt-fusion machines. For every joint made the machine can print a detailed jointing report, to have a full set of QC-documentary.

It is funny, that Krah-Electro-Fusion fittings were used, to repair conventional PE-pipes – when the using of butt-fusion machines wasn´t possible. So, it seems, even our competitors are trusting our electro-fusion joint and they are happy.

3.8 Marine Structured Pipelines KRAH: Fittings and specialities

All kinds of fittings (bends, reduction, branches, diffusers, etc.) and manholes/intake towers are produced with Krah Marine Pipes with the same technology and same jointing. This was one of the core businesses for these kinds of pipes since the 80s. To fabricate these fittings usually stronger wall constructions are used, to handle the welding factors and other needed properties.

An integrated stub-end for a flange connection

A stub-end (for flanges the pipe strings to each other) can be a direct part of the pipe.

Marine Structured Pipelines KRAH

Even the flanges can be tested by FEM

It is kind of funny that many conventional PE-pipe producers are using Fittings, Manholes, Stub-Ends produced by tangential extruded technology – mainly Krah pipes (and they don´t mention this in their specs). So even our competitors are trusting the homogenous wall structure of our fittings, for sensitive elements of the pipe strings.

Marine Structured Pipelines KRAH

Fittings (bend) for an outfall project

Intake chamber design

3.9 Sinking

During the planning and calculation of the sinking process, other effects in the design can occur, like string lengths, waves, currents, wind, speed of installation, using floaters, etc. This will affect the upper mentioned calculation conditions and will be considered there.

The photo above is showing the floaters used during the deployment by PPA&Krah in a project in Peru

4 Marine Structured Pipelines KRAH: Production technology

By the way, the same Krah machines can also produce solid wall pipes in maximum diameter of DN/ID 5000mm; with the same normalization – EN12201 – of axially extruded pipes, besides structured wall KMP pipes. It is easy to reach a homogenous solid wall thickness of 200 mm to 300 mm (with no sagging, no voids and no blisters). The production technology can be extended to all interested Krah pipe producers who will produce Krah Marine Pipes under Krah supervision and license. The latest production line called KR800-max is controlling the homogenous wall structure continuously.

Production line KR800 – MAX – able to produce KMP up to DN/ID 4000mm

The needed area of a production area is only 20m x 30m x 6m– so it is quite easy to move or to set up a plant close to the project site. This will increase the speed of the installation, lower the transportation costs and the possibility of “forgotten” parts.

5 Marine Structured Pipelines KRAH: Summary

Krah-Marine-Pipes (KMP) are available from DN/ID1000 up to DN/ID4000 mm in the length of 6m. The pipes are produced out of polyethene PE100 and can have a co-extruded yellow inspection friendly inside surface. The jointing of the pipes is done by an integrated special electro-fusion joint. The pipes can be designed according to the project requirements in respect of working pressure, stiffness, bending ratio and pulling force – to guarantee the price and quality optimum for a marine project.

Price- and quality-wise Krah Marine Pipes (KMP) are a very attractive alternative to other marine pipe materials, like GRP, steel, concrete and of course to conventional solid wall PE pipes. The market of large diameter pipes for a marine application is huge, so the Krah Group will concentrate its network (today approx. 50 Krah-Pipe producers worldwide) to provide a good solution for the clients for their desalination-, power-, treatment- plants or any other marine application in the future. Complete design software for Krah Marine Pipes (KMP) should be available in the year 2019 and we will have a tight and close relation to several international engineering companies, to support the final customer and to build a solid fundament of technical experience and documents. We will join the experience and documents from our network partner and we will be more present in global projects.