Using horizontal directional drilling technology, DrillTec GUT GmbH, a German based contractor, mobilised to South Taranaki in January 2007. After initial site and equipment set up, drilling of the first hole commenced on 1 February 2007.
The work scope required two parallel holes being drilled from the proposed gas plant location to a length of 2,200 m terminating offshore.
Each initial pilot hole of 250 mm OD was increased in diameter to 450 mm and 750 mm OD to accommodate the 270 mm and 500 mm OD sleeve pipes that were installed.
Although Horizontal Directional Drilling is now accepted as the preferred and proven method for pipeline installations, the variants employed for the Kupe Project are still not common place. The steep cliffs along the Taranaki coastline left few installation options for the gas flowline and umbilical. DrillTec also used a retractable microtunnelling machine for the installation of DN1200 mm steel support casing in the first 200 m of the shore crossing.
Offshore weather conditions in the region required that all hole opening and casing installation procedures were carried out from the shore based drill rig, rather than positioning equipment offshore on barges.
Detailed engineering and planning was required from DrillTec to ensure the design criteria could be achieved.
Assuming ground conditions were compatible with the provided geological data, other elements could only be proven once drilling commenced.
Accuracy for the exit position offshore was a major concern. With only 300 metres of surface navigation available, 1,900 m would be drilled using the downhole survey system only. The accuracy of long HDD projects has for many years been reliant on a secondary survey system to fine-tune the downhole system. Despite this secondary system not being utilised for the Kupe Project exit, the accuracy for both pilot holes was excellent and both drills were within seven metres of the required target area. For DrillTec their confidence in the downhole surveyors employed for the project was rewarded.
Information provided to DrillTec with regard to the formations expected recommended that a complex drilling mud procedure was employed.
After a few hundred metres of drilling on the initial pilot hole DrillTec decided a basic Bentonite fluid without additives would be equally as effective.
The basic Bentonite design remained in place for the entire project, offering a considerable cost saving to the client and allowing no adverse effects to drilling performance.
Like the pilot hole drilling, the forward reaming operations progressed at an excellent pace for both holes. Previous concerns over drill pipe deflection or buckling over such distance in a large diameter hole proved to be unfounded.
The main areas of concern during the forward reaming process were the formation was so soft that numerous 9.5 m drill pipes could be run in at full speed. In the soft formation the risk of leaving the existing hole increases, however in this case the enlarged stabilisation head of the hole opening tools proved their worth. The tools had been manufactured by DrillTec and based on the designs it had used on the Otway Gas Project.
Installation of the threaded casing sleeves into the 450 and 750 mm OD reamed holes also proved to be an anti-climax in terms of the perceived risk of dead-pushing such a long length of casing pipe. Both 270 and 750 mm OD casing had been selected on wall thickness to achieve neutral buoyancy when installed, which is a standard procedure for HDD works.
It is common in the HDD industry that calculated or designed installation loads do not always correspond with actual loads due to variables such as geology, mud weight, borehole integrity. However, the casing installation loads for the Kupe Project needed between 7 and 10 tonnes of thrust from the drill rig to complete the 2,200 metres of 20 in. casing which closely matched the calculations.
With both casings installed by 25 April 2007 DrillTec had come in under budget and six weeks ahead of programme. The fabrication of the DN300 gas flowline is planned for June and installation will be completed in July 2007.