When TECO Peoples Gas and SeaCoast Gas Transmission needed to extend their 80 km SeaCoast Pipeline, a large challenge was faced: the additional 2.5 km of length they required had to be laid beneath a river.

TECO Peoples Gas hired drilling company Mears Group to undertake the task.

To complete the project, Mears drilled a 2.26 km pilot hole beneath the St Johns River at a depth of 40 m below river bottom to install the 16 inch steel pipe.

The bore was constructed by Mears using horizontal directional drilling (HDD), and the project was undertaken within a design-build framework.

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Design-build versus design-bid-build

Project Manager for Mears’ sub-consultant J.R. Giese Operations John Fluharty says that the design-build process is relatively new to the HDD industry.

Design-build offers a subtle but important difference from the typical procedure for HDD projects. In the typical process, a company such as TECO would hire an engineer to develop plans, procure a construction company to implement those plans and construct the project. This process is often called design-bid-build and requires multiple contracts. However, with a design-build process, a single company is hired to engineer and to build the project, all under a single contract.

Susan Hines of the Design-Build Institute of America says that utilising the design-bid-build procedure can create disputes between a project’s engineer or designer and the company that constructs the project. She says that it creates a disconnect between the two parties that can often turn contentious because of concerns of liability and litigation.

For instance, Ms Hines explains that when the construction company finds changes that need to be made to designs during construction, it takes paperwork and dialogue for the changes to be approved and implemented.

“It slows things down a lot,” Ms Hines says.

In the end, design-build projects help to improve two important aspects of any project – money and time.

The St John’s River bore

Mears drilled the feasibility pilot hole for the project in November 2009 and product pipe was pulled in April 2010.

Mears Project Superintendent Mike Maxwell said the crew drilled from both sides of the St Johns River and intersected the two holes under the river – a waterway lined with stately homes and used mostly for recreational purposes.

A 33,069 pound HDD rig was used for the pilot hole on one side of the river and a 13,999 pound rig was used to drill from the opposite side.

Mr Maxwell says that the crews reamed the bore to a 24 inch diameter and then swabbed the hole before pulling the product pipe. The crew drilled through sandy silt before making it to clay for the drill’s running depth. Once this was completed, Mears utilised a 50,045 pound rig to perform the reaming and to pull the product pipe through the hole.

“It was fairly easy going; we were able to use a jetting bit all the way through,” Mr Maxwell says.

Mr Maxwell says the length of the drill combined with the unique aspects of the job made it a satisfying accomplishment.