Pipeline padding protection
A properly backfilled pipeline trench can considerably extend a pipeline’s lifespan, making it an integral part of the construction process. Pipelines International asked major construction company Nacap why padding is so important and what it looks for in a padding machine to ensure that pipelines are protected.
Soft soil is used to surround and protect a pipeline before and after it is laid into a trench. This soil is often derived from excavated material which has been run through a padding machine. The padding machine sifts out hard, potentially damaging materials such as rock and clay, leaving fine soil to act as a barrier between the pipeline and its harsher surroundings.
Nacap’s Mark Bumpstead says that while matters such as pipe coating protection and cathodic protection are major inputs into pipeline integrity, it is essential that appropriate backfill is sourced, placed and compacted to ensure the pipeline is safely laid and correctly supported in the trench.
“From a constructor’s perspective, rework during any defects liability period is costly, so doing it correctly the first time is the only way to go,” says Mr Bumpstead. He also points out that poorly backfilled pipelines can reflect negatively on a contractor’s work quality and reputation, reducing the chance of repeat business.
Buying versus renting
Mr Bumpstead heads up Nacap’s operation in Australia and South East Asia. He says that currently, Nacap Australia owns two padding machines: the Ozzie OP300 padder and the recently-purchased Superior SPD 450 padding machine. The company chose to purchase the machines because this provided greater certainty of resources for project execution than hiring.
“Often mobilisation times are short and we need to know these machines are available when we need them,” explains Mr Bumpstead. “But we also have a good working relationship with the padding machine providers and often supplement our own padding fleet with externally hired machines if necessary.”
Despite the major advantage of availability and convenience that ownership affords, Mr Bumpstead admits that padding machines are costly to maintain. “These machines work hard and are consistently used each day with many moving parts, so we have undertaken many major rebuilds of our padding fleet,” he says.
A padder to suit the project
Mr Bumpstead says that generally the size and nature of a project will determine which padding machine is used. He referred to the QSN 3 Project that Nacap Australia is currently working on in Queensland, Australia, involving the construction of a 938 km, 18 inch diameter gas pipeline. Nacap has been completing construction at a rate of approximately 5 km/d.
“To maintain such a high average speed it is necessary to have a series of large-volume padding machines that have the capacity to convert large volumes of screened material from the excavated trench spoil,” says Mr Bumpstead.
“On this particular project, we have a smaller padding machine providing bottom padding – “÷bedding’ – with the larger units providing the larger volumes of sand required for top padding – “÷shading’.
“In certain circumstances, the use of sandbags, or foam or polystyrene pillows, required as alternatives to bedding. In slower moving and shorter pipeline projects, it is possible to use a single, smaller machine.”
In addition to its padding machines, Nacap Australia owns a number of small screen pans and excavator screen attachments. These can be used to supplement the larger machines, which the company generally uses for mainline production.
Mr Bumpstead says that screen size can be varied to suit the geotechnical profile of a project: however the screen size selection needs to be considered along with the pipe coating material and thickness as a primary concern.
“For more elaborate and robust coating systems a larger screen can be used. For this reason, construction contractors should always consult the project design engineers to ensure the coating system is compatible with the geotechnical conditions and the method of construction.”