MICROTUNNELLING SUPPORT EQUIPMENT
Pipe jacking and microtunnelling systems require, by their nature, the jacking of a pipe into the ground often using relatively high jacking forces. So the correct choice of pipe with the ability to withstand the required jacking forces during installation and the right properties in terms of final product performance is as important as choosing the right machine to install it.
There is a wide range of pipe materials available for installation using pipejacking and microtunnelling techniques. The choice depends on the requirements of the client, the ground conditions, transportation costs and the length of pipeline. Materials including reinforce and un-reinforced concrete, polymer concrete (concrete aggregate within a matrix of resin), glass fibre/resin-based pipes, vitrified clayware (both glazed and unglazed), steel, ductile iron and also plastic are available as jacking pipe. In the majority of cases the pipe material is either concrete or clayware.
Probably the most important aspects of design in respect of pipes for a pipejack project are the allowable degree of joint deflection and the joint face geometry. In general, the deflection at the pipe joint face should not exceed 0.5°, although deflections of over 1.0° may be permissible for curved drives using appropriate cushioning materials at pipe joints. So, joint manufacture is very important. It is important to be aware that, due to increases in point loading, the maximum permissible jacking load on a given pipe decreases significantly and quickly as the deflection at pipe joints increases. Maintaining, as straight a drive as possible will allow the operator to take full advantage of the design loading of the pipe, should it be required, otherwise the potential for pipe failure in the ground is very real.
An essential feature of pipes for microtunnelling and pipejacking is that the joints do not extend outside the main barrel of the pipe. In other words, the entire joint is contained within the normal pipe wall thickness. For microtunnelling and pipejacking, the advantages of a low-friction external pipe surface without protrusions are obvious. In some instance a pipe design with lubrication ports is also useful to allow lubrication fluids to be injected between the pipe wall and the surrounding ground along the length of the drive to minimise friction losses, which in turn reduces the jacking load requirements.
Pipe length varies according to the microtunnelling system used, the pipe diameter and constraints of space. Typical pipe segment lengths usually range from 1.0 to 2.0 metres, although lengths of 0.75 metres are available for small diameters and longer pipes area sometimes available on special order. Much of the cost of microtunnelling pipe is in the joints, so the use of longer pipe lengths tends to save cost on pipes; on the other hand, this may require larger shafts.
Almost all pipejacks and microtunnels are installed between a drive shaft and a reception shaft. The most notable exceptions are those where the exit point of the shield is either directly out of the ground at a set position or underwater. But even then, a reception arrangement has to be correctly designed.
Drive shaft requirements vary greatly depending on the machine being used, ground conditions, pipe length and material, length of drive and the type of installation. They may be round, rectangular or oval; sheet piled, segmentally lined, caisson constructed, or even unsupported if ground conditions are good enough and local safety rules permit.
One factor common to each drive shaft is that there has to be some form of reaction face for the jacking frame to push against. In suitable ground this can simply be the back wall of the shaft, but this is usually not the case and a thrust wall has to be provided. The thrust wall must enable the jacking frame to exert its maximum pushing force whilst maintaining the integrity of the shaft structure and that of the surrounding ground, so as not to compromise the final pipeline structure.
Shafts are increasingly being built with a soft eye for the launch of machines as well reception capability. This enables operators to exit the cutter head from the shaft without having to expose the ground surrounding the shaft and so risk ground water influx or loss of ground around the shaft, causing instability around the new pipe after installation. Certain microtunnelling systems are designed for use with small drive shafts, and techniques are available, which allow the installation of 1.0 metre long pipes from a shaft of only 2.0 metres diameter.
As a significant number of the systems used in pipejacking and microtunnelling utilise a boring mud or slurry as an integral part of the operation, either as a lubrication for the jacked pipe or as a spoil removal and face support system, the mud mixing and handling systems must be chosen with care. There is a wide variety of mixers, solids separation and recycling systems available and the choice is often best discussed with the pipe jacking shield manufacturer and the mud system manufacturer prior to commencing work. Each boring fluid is usually designed in accordance wiht the site conditions, and accessibility for spoil removal etc. The art of the boring fluid can be very complex and ‘muds’ suppliers can be most useful in advising on the mixture/composition that may be required, as well as advising on design changes in respect of differing ground conditions that may be encountered across the length of any particular drive. The best course of actionis to take advice.
Mud mixtures are also used for lubrication purposes along the drive length if conditions warrent such a move. Many pipe manufacturers can supply pipe with specifically positioned lubrication ports in the jacking pipe wall. A mixing unit and pumping arrangement can be fitted in to the microtunnelling arrangement that can be either an automatic, computer controlled feed or manually controlled by the unit operator, to place lubrication fluids along the pipe length during the advance of a drive. Again advice can be obtained from most systems manufacturers or muds suppliers.
HDD & PB Pipe Developments
Whilst the installation and replacement technologies themselves have moved on apace over the past 10 years there have also been developments in the support equipment to go along with these improvementsl. One particular development has been in the pipe sector.
This development in pipe technology has seen use in both the for pipe bursting and HDD sectors. It is the introduction of specialist PE pipes that have a protective outer skin. As pipe bursting generates broken pipe or sharp edges of split pipe that might impinge on the new pipe as it passes through the void created, early installations sometimes found that the new pipe was damaged to the extent that it no longer met the installation requirement. To eliminate this as far as possible some pipe manufacturers developed a multi-layer pipe where the inner pipe was the pipe required for the network operation. The outer skin was designed to be of a harder and sacrificial material that would aborb any damage during the installation procedure and prevent damage to the inner operating pipe. Its use in pipe bursting given the conditions in which it is used is somewhat obvious. However its use in HDD is less obvious given that most bore during pipe pull-in are full of drilling fluid etc. There are however applications form this type of pipe on harder ground formations where transition from one ground type to another may cause damage to the outer skin of the pipe during pull-in or where there is a potential for ground movement in the bore or where sharp soil content may protrude into the bore before pull-in is completed.
In the same sectors the development of coil trailers has also had a signifiant impact on the success of projects. With coil trailers the transport of pipe has become easier the need for on site welding has been reduced and so in turn the impact on the project on the local environment, where pipe strings are no longer laid out across busy streets and walkways and the footprint of the working site has been considerably reduced.
HDD Support Equipment Developments
In the HDD sector specifically there has also been signifiacnt development support equipment, accessories and consumables.
Tracking systems are an example. A decade ago HDD tracking relied on either walkover or wireline tracking systems, with the latter in themselves often being quite complex to use and operate correctly. Now there are a variety of tracking systems which although based on similar technology can give readouts to the operator from a remote monitoring point, do not need the reception antenna to be directly over the transmitter sonde and in the case of wire line system can work in a local magnetic field set up using a secondary electrical circuit, so eliminating errors due to localised electo-magnetic interference.
On the machine itself, considerable physical demands are made of the drill pipes which require sufficient strength to withstand the thrust and pullback forces imposed upon them, enough torsional stiffness to cope with the torque applied, and yet be flexible enough to negotiate changes of direction in the course of the bore. They should also be as light as possible for transport and handling purposes and yet must resist damage due to abrasion and scoring.
The length of individual pipes depends on the type of drilling machine and the space available. Typically, surface launched rigs will use pipes up to 4 or 5 metres long, whilst drill pipes for pit launched machines are often between 1.0 and 1.5 metres in length. Screw joints are most commonly used, although bayonet fittings are found with some systems.
Obviously, it is important to ensure that the drill pipes are wholly compatible with the drilling machine, especially if the rig incorporates an automatic drill pipe handling system, and also with other components such as bore-heads, sondes, and reamers.
Many developments have occurred in joint design, drill pipe material etc and the range available can be fron OEM supplied units to independent manufacturers. In many cases operators run with OEM supplied rods and others choose to go with independently supplied ones. The coice is open and should be discussed fully with suppliers.
Drilling fluid has several functions and can be designed to suit a given set of conditions. Whilst similar to those used in pipejacking and microtunnelling the design use is often very different. Specific uses include: Lubrication and/or cooling of the cutting head and to reduce wear; To soften the ground so that it is easier to drill through; To convey spoil material in suspension out of the bore; To stabilise the bore prior to back reaming; To lubricate the product pipe during insertion; and to power mud motors for drilling through hard ground if such equipment is used.
The simplest drilling fluid is water, and it may be unnecessary to use anything more sophisticated for short bores of small diameter through good ground.
A mixture of Bentonite and water is the most common type of drilling fluid or ‘mud’. In addition to simple water/bentonite fluids, there are polymer-based materials and a wide range of additives, which are used to tailor the properties of the drilling fluid to suit the soil conditions and the nature of the project. For example, the viscosity should be low enough to flow through the system at reasonable pressures, but sufficiently high to prevent significant loss into the ground.
As with pipejacking and microtunnelling, the formulation of drilling fluids is a complex science and is an area where specialist guidance should be sought, especially when dealing with difficult ground conditions. The design of mixing, pumping, filtration and recycling plant is also a major consideration, especially for large-scale projects, and again advice should be sought from experienced contractors or manufacturers.
Although most attention is focused on major items of equipment, there are numerous accessories and ancillaries that play an important part in the success of an HDD project.
Various types of towing heads for polyethylene pipes are available, including pressure tight heads and versions aimed specifically at directional drilling that are designed to prevent the ingress of drilling fluid or debris into the product pipe.
Swivel connectors are an essential component during the backreaming and pipe-pulling operation, and should be designed to prevent the entry of mud and debris to the bearings. Models capacities from less than 5 to over 200 tonnes are available.
‘Breakaway connectors’ are also available to protect the product pipe. The connectors have a series of pins designed to break under a predetermined load, and are set according to the permissible tensile load on the product pipe.
Other important ancillary equipment may include butt-fusion machines for jointing polyethylene pipe, pipe support rollers and cable pullers.
One of the most useful recent developments as a planning tool for engineers using or considering using HDD for an installation is Bore Planner software. This software enables engineers to put into a computer the parameters of a bore and the Bore Planner program will produce a ‘best fit’ route and depth.
Data acquisition software has also been developed which enables steering data to be utilised to complete an ‘as-built’ drawing for the contractor and/or the client based on the actual pilot bore run.
From a safety point of view several OEMs now also provide with their machines, either optionally or as standard, Safety Cut out or Lock out systems which protect operators when working on the machine. As well as the general use of ‘Faraday’ cages to, protect operators in the event of a cable strike, safety systems now exist that provide for power shut down should the operator not remain seated at the controls. Other systems have been developed which allow the machine to be locked out to ensure that when work is being carried out on the pipe side of a bore (a position often out of line of site of the drilling rig), the rig cannot be started or operated without the correct clearance signal being given.
There are also of course a wide variety of manufacturers for down hole tooling such as drill rods, cutterhead for soft, medium and even rock work. Each item should be investigated fully with the manufacturer to establish its suitablility for the project in hand.
Health & Safety Tools and Training
Because many of the situations and work sites associated with trenchless technology involve the workforce in confined spaces, be it in existing pipelines or working in a jacked pipe string to repair a cutter or change cutters at a face, it is vital that all personnel likely to be involved in such work is fully trained in and conversent with all national and local safety rules and regulations. It is also vitally important that they are aware and know how to correctly use all on site safety equipment. In the case of underground works this would involve gas detection equipment, rescue equipment and breathing apparatus. There are numerous safety and regulatory training course available and in many countries now it is a mandatory requirement that certain workers be trained, qualified and certificated in various aspects of such works and the use of safety and rescue equipment. Check all local requirements before commencing work.
Whilst most of the equipemnt mentioned above is an integral part of the trenchless system or systems with which it works there is of course a wide variety of equipment that would be found on both trenchless and non-trenchless construction site. These would include the likes of Pumps, Air Compressors, Hydraulic power systems, hosing cabling etc. These in the main are supplied as standard on most sites and in many instances with many of the basic equipment spreads as part of the purchase package but in relation to trenchless technology each requirement should be examined carefully if it is to purchased or rented separately from a standard base kit. Items such as shaft depth length of drive etc must be taken into account before choosing an item of equipment because in underground situations it is quite reasonable to expect that the workload may well be more than might be expected on a surface located site.