Directional Drilled Gas Pipeline Crossing of the River Ribble

42-inch dia. Treales to Burscough Gas Pipeline

Author: Ray McGivern 
Project Engineering Manager: Smit Land & Marine Engineering Ltd.
Client British: Gas Transco
Main Contractor: McAlpine-Preussag JV
Contract Period: May 1998 to August 1998
Crossing Data: 42-inch x 2100-ft. steel pipe

Introduction The River Ribble is situated in the North West of England. The source of the river is in the Pennines from an area designated as "The Yorkshire Dales National Park," however the majority of the river flows through the County of Lancashire. Prior to discharging into the Irish Sea, it passes through a dock complex in the City of Preston. During the last decade, many considerable improvements have been made to enhance the environment and a typical example of this is the development of a marina in Preston Docks.

Background
Transco, the pipeline construction arm of British Gas is always conscious of preserving the environment. Its development and management of the countrywide network of transmission lines requires the crossing of many major rivers both at present and in the future. It therefore places great importance on its relationship with the Environmental Agency and the National Rivers Authority.

As with many projects it is essential to begin planning as soon as possible before the proposed construction phase and in this respect Transco commissioned a feasibility study with Smit Land and Marine in 1996 to look into the possibilities of installing this pipeline by directional drilling.

Based upon the information available the study indicated that a crossing by this method was feasible but would require a fuller examination of the geology.

The greater the knowledge of the ground the greater the chance of confirming the feasibility.

Acting upon this advice Transco commissioned Laird Menzies to conduct a comprehensive geophysical and geotechnical investigation. This followed initial surveys when inconsistencies of rockhead levels were discovered.

The results of the studies indicated some very interesting geological features and ground conditions that could not have been predicted from a simple borehole study.

Transco was faced with a contract, where environmental constraints dictated the method of construction to be by Directional Drilling and where geological conditions demanded a very experienced drilling contractor.

Late March 1998 Transco and McAlpine-Preussag contacted Smit Land & Marine to discuss the possibilities of engaging their services for the drilling work and the reassurances that the crossing could be accomplished by the technique of directional drilling.

A letter of intent was issued on 9 April 1998 by McAlpine-Preussag JV who were the main contractors for Transco's 31km Treales to Burscough High Pressure 42-inch diameter pipeline which was required to cross the River Ribble.

After approximately one month for design and procurement, mobilization to the site commenced on 7 May 98.

Geology The Geophysical and Geotechnical Report indicated in general terms that the ground conditions along the route consisted of soft sediments overlying hard rock of the Sherwood Sandstone formation, with the sediments consisting of glacial and post glacial deposits. That is Clay, Till, Glacial Sands, Gravel and Alluvium. The Clay is the most characteristic sediment being Reddish Brown Stiff Sandy Silty Clay. The cross section highlights the varying geology. It shows also an old river bed channel, pressurized sand lenses and the extent of gravel and sandstone layers. Reference was also made in the report to the riverbed contaminates and the possibility of contamination from leachates from a nearby waste tip. The wide variety of materials and contaminates highlighted the need to select a drilled profile that suited these conditions.

Environmental Constraints
The environmental constraints can be listed as follows;

River Training walls to be maintained to prevent flooding to the surrounding areas of land. Access to South bank of river was prevented because of its designation as a Site of Biological Interest (SBI) Possibility of contamination from waste tip on North site to surrounding areas of land. Human health hazard from riverbed contaminates. Human health hazard from subsequent ground water de-watering by transmission of contaminates to ground water.

Grouting Works
Of all the types of ground conditions that can be encountered during drilling, gravel & cobbles are the most difficult and sometimes prove to be impossible for the passage of the drill head and hole-opening tools. The success of drilling through them depends largely upon the amount of cohesive material within the selected band of gravel. Reference has to be made to the material distribution curves in order to establish the chance of success.

If the distribution curves indicate very high percentages of gravel then the designer needs to look at the possibility of either removing the gravels by machine excavation, installing a sleeve pipe and removing the gravel from inside or treating the gravels by injection of grout. Smit Land & Marine in the past have used all of these techniques but since the gravels were at a considerable depth, up to 33-ft, the injection of grout was considered to be the most favored option.

The Gravel layer encountered was described as follows;

"Dense, very sandy, subangular to round, fine, medium and course GRAVEL, cobbles noted."

The grout used was a cement- bentonite mix to give a stiff clay consistency. It was injected by the end of casing and 'Tube à manchette' method in the form of a matrix. Two areas needed to be treated one on the North bank and one on the South bank. The sizes of the grouted areas were; North bank 115-ft x 16-ft x 8-ft (avg.) thickness South bank 184-ft x 26-ft x 9-ft (avg.) thickness Specialist grouting sub-contractor Amec Piling undertook the grouting operations to stabilize 5250-ft3 of gravel, injecting 1250-ft3 of cement-bentonite grout over a four week period, with drilling commencing 10 weeks after the award of contract.

Drilling Technique Because of the extremely bad ground conditions and the need to use a drill motor as opposed to the jetting technique it was decided to use a 9-7/8" diameter tri-cone mill tooth rock bit with a 6-3/4" diameter drill motor. A major part of the drilling was through sandstone with an average hardness in the region of 29 MPa. Prior to the drilling commencing the survey system was calibrated. A special induced DC current grid, which created a known magnetic field, was laid out to surveyed co-ordinates on the surface. This system gives increased accuracy when monitoring the position of the drilling head below the ground. Pilot hole drilling commenced from the North bank along the designed profile and as each 30-ft. length was drilled forward, another length was added into the system at the drill rig. These operations continued until the drill bit and motor exited on the South bank.