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.

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 line- and 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 hydraulic shoes allow for continuous control of line and grade, while a laser target system provides instantaneous measurement of boring accuracy.

SBU Technology & Microtunneling
Many of the crossings on the pipeline are using microtunneling machines (MTBMs) — 11 crossings utilized MTBMs for excavation during Phase II of the project, mostly in soft soil to mixed ground conditions with expected water ingress. MTBMs are superior in jobs below the water table or in soft ground; however, they can be highly inefficient in hard rock conditions. The machines use tungsten carbide drag bits on the cutting head, which tend to break off in rock above 75 to 100 MPa UCS. The broken bits require expensive replacements, therefore slowing progress and increasing man-hours.

In dry or near dry conditions, when an auger is used in conjunction with the MTBM for spoils removal, the microtunneling method is still more expensive than an SBU machine with ABM. An MTBM using an auger in this fashion requires that all rock chipped from the face be further crushed for removal, resulting in slower excavation rates. SBU systems, in contrast, use interchangeable single disc cutters to size the rock chips directly at the face.

While MTBMs are superior in some conditions, they are much more expensive for hard rock excavation than either an SBU-A/ABM combination or an SBU-M/ABM setup. Purchasing a typical MTBM will often cost five to 10 times more than using an SBU-A, and two to three times as much as purchasing an SBU-M of the same size.

Advantages of SBU Technology
Williams based his decision to use Robbins SBUs on past jobsite experience. B&W originally purchased two Robbins SBU-As in 2002 for a similar natural gas pipeline through siltstone. This previous project involved six hard rock crossings along a 70-km pipeline extending from St. Fergus to Aberdeen in Scotland, United Kingdom.

&W Tunnelling sees its use of SBUs as a competitive advantage in the United Kingdom, where the product is not well known compared to the United States. “These machines can definitely compete with microtunneling machines given that they can use concrete pipe and sacrificial casings. SBUs also do not require slurry — microtunneling machines are dependent on slurry, which can often be messy and inefficient,” explained Williams.

Excavating the Crossings
As of summer 2007, SBU excavation has gone extremely well. Average rates of SBU-M excavation are 1.5 to 2.0 m per hour and none of the machines have required rebuilding between crossings.

B&W began excavating the crossings using five Robbins machines — three 1.2-m diameter SBU-As and two 1.2-m diameter SBU-Ms. The machines utilize 6.5-, 9.5- or 11.5-in. diameter disc cutters (depending on the rock hardness) to bore through geology between 70 and 200 MPa  UCS.  Many of the crossings are in ground consisting of hard rock interbedded with clay and gravel, necessitating careful cutterhead design.

Each of the crossings is different, and the launch pits for each have various set ups, depending on the type of crossing (such as a river or road), as well as whether line and grade were critical. These variations resulted in both the SBU-A and the SBU-M being utilized on the project. SBU-As are being used on crossings of shorter length and that are not line and grade critical, while SBU-Ms are boring on line and grade critical crossings, which contractually have a 50-mm tolerance. The SBU-M crossings require that 10.5-m diameter shafts be constructed, which range from 10 to 30 m deep and are lined with concrete-bolted segmental rings. Launch pits (24-m long x 3-m wide) are being used on all of the SBU-A crossings.

At the onset of the bore, the machines are welded to a 1.2-m O.D. steel sacrificial pipe casing, which is removed after completion of the crossing. The sacrificial pipe will save time during the assembly of the final 1.2-m O.D. steel pipe used for the pipeline. The gas line will need extensive welding and weld testing of each pipe length, which can be performed at the bottom of each launch shaft. The large 10.5-m diameter of the shafts then enables welding of the 6-m lengths of steel pipe to take place. A semi-automatic welding bug is used, while a winch in the opposite shaft draws the newly-welded pipe lengths through the tunnel until the crossing is complete. The entire pipeline is due for completion by 2009.

Desiree Willis is a technical writer for The Robbins Co., which is headquartered in Kent, Wash.