TransCanada Corp.’s natural gas supply network in Alberta and British Columbia, Nova Gas Transmission Ltd. (NGTL), will continue growing — but at a reduced rate — after completing additions now under way.

For a forecast C$570 million (US$428 million), the next NGTL facilities construction program will fulfill gas transportation contracts for 2.7 Bcf/d as of 2018, TransCanada announced Monday.

NGTL is currently building or awaiting National Energy Board (NEB) approval for C$2.7 billion (US$2 billion) in facilities for up to four Bcf/d, with completion targets of late 2016 and the fall of 2017.

Additions of pipe, compressors, inlets, outlets and other hardware are annual events on the NGTL grid. The installations respond to evolution of production fields and gas user demand, translated into shipper contracts requiring construction.

The current main drivers of change and additions to the 25,000-kilometer (15,000-mile) NGTL web of pipelines are growing fuel use by Alberta thermal oil sands and power plants and migration of supply development into northeastern BC.

Annual NGTL construction programs do not always increase total gas deliveries across the system, which currently average about 10 Bcf/d. Instead, the traffic pattern has changed, with Alberta gas use rising while supply collection declines for out-of-province shipments by TransCanada’s Mainline to the United States, Quebec and Ontario.

TransCanada credited the 2018 construction program to shale gas drilling in northwestern Alberta and northeastern BC, serving the still-expanding western Canadian industrial and electricity markets.

In a statement Monday on the 2018 agenda, TransCanada president Russ Girling reminded stock exchanges “Our NGTL System is sitting on top of extensive natural gas supplies, making it well-positioned to unlock the resource and reliably and efficiently link it to growing markets.”