Author: Professor, Dr. Dietrich Stein - TrenchlessOnline

INTERNATIONAL
Motorized Small Boring Units Power
Through Rock on U.K. Pipeline By Desiree Willis
One of the U.K.’s largest pipeline projects is getting
a boost from some high-powered tunneling
machines. The Milford Haven Gas Connection
Project — a 300-km long behemoth stretching
across South Wales, United Kingdom — will require more
than 110 crossings located under roads, major rivers and
environmentally sensitive areas.
Local contractor B&W Tunnelling is currently excavating
24 hard rock crossings ranging from 20 to 80 m in length
for Phase II of the project. B&W is utilizing five hard rock
boring machines, known as small boring units (SBUs), to
excavate the crossings in record time and with minimum
environmental impact.
B&W Tunnelling opted to use two 1.2-m motorized SBUs for the four
longest crossings due to site set up and bore specifications.
Project Overview
The Milford Haven Pipeline will support the United
Kingdom’s increasing requirements for imported gas.
Liquefied natural gas (LNG) will be imported from international
sources to two LNG terminals located in Milford
Haven.Tankers will offload the LNG, where it will then be
converted back to its gaseous form at the terminals before
entering the pipeline. By 2009, the pipeline will transport
up to 20 percent of the U.K.’s natural gas for owner
National Grid.
Constructed in two phases, work began on the pipeline
in early 2006. Phase I involved a 120-km long stretch from
the towns of Milford Haven to Aberdulais. B&W Tunnelling
was sub-contracted by the NACAP/Land & Marine JV to
excavate a total of 38 crossings using SBU technology.As of
August 2007, Phase I is nearly complete, with more than 95
percent of the pipeline in place.
Phase II of the project will extend the pipeline another
185 km from Felindre to Tirley in Gloucestershire. B&W
signed the contract for this phase in December 2006, which
consists of 24 crossings to be bored with SBUs under roadways
and nearly a dozen rivers — 57 trenchless crossings in
total. Many of these crossings are line and grade critical,
requiring careful machine setup and continuous monitoring.
The large number of crossings is one way that general
contractor NACAP/Land & Marine is working to maintain
environmentally sensitive areas. Tunneling under features
such as rivers and woodland minimizes disturbances to the
indigenous flora and fauna found in the region. Vibration
and movement are being closely monitored, and the working
area is restricted at several points to minimize disruption
of the surrounding landscape.
Machine Selection
Two 1.2-m diameter SBU-As are being used for a total of 20
crossings on Phase II of the pipeline project.The machines are
a type of rock boring attachment that utilizes a full-face auger
to excavate crossings for utility installations.The machine is
welded to the lead casing and an auger boring machine (ABM)
provides both torque and forward thrust to the cutting head.
Disc cutters allow the machine to excavate medium to
hard rock from 25 to more than 170 MPa UCS. In mixed
ground conditions, another type of cutterhead can be used.
This cutterhead uses disc cutters, two-row carbide cutters
and carbide cutter bits with larger muck openings to
accommodate soft ground and boulders. The SBU-A, available
in diameters from 600 mm to 1.8 m, is typically used
on drives less than 100 m in length.
Boring for Accuracy
B&W Tunnelling opted to use two 1.2-m motorized SBUs
for the four longest crossings due to the site setup and bore
specifications.“We needed an increased degree of accuracy
given the shaft depths and the line and grade of the crossings,
so we felt the most confident with the SBU-M,” said
Steve Williams, managing director of B&W Tunnelling.
The motorized SBU is a manned-entry, hard rock boring
machine used for longer bores (exceeding 100 m) and for lineand
grade-critical crossings.The machine is used in conjunction
with a standard auger boring machine and is welded to
the lead casing in the same fashion as SBU-As.While both SBU-
As and SBU-Ms are used with ABMs, the SBU-M utilizes a small
invert auger for spoils removal rather than a full-face auger.
The cutterhead is supported by a heavy-duty bearing
housing assembly, which is driven by either a hydraulic
motor or a water-cooled, variable speed electric motor.The
motor provides torque to the drive train and contains a
torque limiter to reduce cutterhead jams in fractured or broken
ground conditions. Power cables in 30-m lengths run
from the machine connection box to a surface-mounted
electrical disconnect cabinet.
Available in diameters from 1.2 to 2 m, the SBU-M can be
steered from an operator’s console inside the machine’s
rear shield. Articulation cylinders and manually adjustable
52 TRENCHLESS TECHNOLOGY September 2007 www.trenchlessonline.com