Double triumph for HDD in the Middle East
Aug 26, 2010
Carsten Fischbach of DrillTec GUT reports on two difficult undersea HDD projects in the Middle East, both of which have set a record in their own way.
With the successful installation of a 1,500 m long, 760 mm pipeline landfall at Abu Ali island in 2008, oil giant Saudi Aramco achieved the first application of HDD technology for a pipeline shore approach in Saudi Arabia, as well as the first project of its kind in the company's history.
In preference to conventional dredging and trenching, the world's number one oil producer decided to further explore this advanced alternative method and use it for the trenchless installation of a 400 mm oil pipeline over a distance of 2,100 m across a shallow water zone and on to the shore. It would carry crude oil from the Qatif offshore field on to land. German HDD specialist DrillTec GUT, which also carried out the first crossing in 2008, was entrusted with the work.
Horizontal directional drilling (HDD) was chosen as the preferred method of installation. This arose out of concern for protecting marine life on the near shore, but also to safeguard the pipeline against wash-out and exposure to wear and mechanical stresses in the shore and breakwater zone.
The tight schedule and narrow weather window for operating a pipe-laying barge in the relatively shallow waters around Qatif could have endangered the viability of the project. For these reasons, DrillTec and Global Industries, the main contractor and a US-based offshore specialist, decided to pre-fabricate the HDD pipeline section and lay the string on the seabed about two weeks ahead of the planned installation date. This would make the installation independent of the availability of a pipe-laying barge and was a factor, which enabled the construction team to complete the crossing within the scheduled period, despite temporary bad weather.
Although the mainly calcaronite and limestone geology conditions caused only minor difficulties during drilling, the overall length of the bore required regular cleaning and reconditioning of the borehole, especially during the pilot stage. In addition, it was decided to install about 250 m of conductor casing within the onshore entry section to avoid frac-outs of drilling fluid through the relatively soft topsoil layers. The casing also provided support to the drill string as required to generate sufficient thrust in order to penetrate the seabed with the planned angle and accuracy.
To achieve this goal, DrillTec used subsea steering coils at the offshore target area and an intermediate location along the drill path, which enabled deployment of a magnetic-based wireline-steering system. This allowed verification of the drill-head position at those stages and the implementation of necessary corrections to the bore trajectory, as required. Divers were used to install those cable coils ahead of the HDD operation and accurately survey their position after having anchored them to the seafloor using concrete blocks. During drilling, the coils were temporarily energised using a small support boat carrying a suitable power source.
Throughout the operation, fresh drilling mud had to be continuously injected into the borehole to counteract mud losses in the hole; this task was performed by using efficient mud-cleaning systems that enabled reconditioning of the fluid returning to the surface, thereby reducing the amount of bentonite required and the waste mud that had to be disposed.
Finally, and after only 27 days of non-stop operation round the clock, the almost 2.2 km-long section of 400 mm steel pipeline was connected to the drill string by divers after it had been dewatered using caliper pigs powered by compressed air. Dewatering was necessary as, after it had been laid on the seabed, the pipe string was filled with water to prevent it moving as a result of subsea currents. To further stabilise the pipe's position, gravity anchor blocks were placed on the seafloor at both ends of the string.
Abu Dhabi's water and electricity supplier, Transco, required two 400 kVA high-voltage power circuits to be laid between Abu Dhabi's industrial port and the newly developed island of Saadiyat. This was to ensure the strategically important island would have electricity in time for the planned opening of new residential and commercial facilities there.
DrillTec GUT, in association with its long-term joint-venture partner and local, main contractor, NSCC International, had previously installed several 1,220 mm-long water pipelines using HDD for Transco as key elements of earlier major development projects. In early 2009, NSCC-DrillTec in co-operation with UAE-based power specialist Al Barrak, were awarded the challenging Transco contract.
Due to technical restrictions on the production of the 400 kVA cable sections and the necessity to have the underwater crossing installed in one piece without any intermediate joints, both entry and exit points of the HDD borehole had to be located near the shoreline on either side in order to overcome the channel width of about 800 m. This resulted in relatively steep bore angles to allow sufficient coverage above the borehole to safely cross beneath the shoring structures on both banks of the channel.
The restricted space conditions on either side of the crossing also meant that the pilot hole had to be drilled with a very high accuracy to hit the target. This was a challenge in itself since the crossing was situated between a parallel, eight-lane road bridge under construction on one side and two 1,200 mm steel pipelines previously installed on the other side.
Another task for the drilling team was the temporary high loss of drilling fluid downhole and the non-availability of a mud-return option for the first line; this was solved by using two HT400 high-pressure mud pumps, enabling the injection of drilling fluid into the borehole from either end of the drill string. This reduced the amount of mud generated and eliminated the need to transport drilling fluid between the sites.
Before bundling, the pipes were pressure-tested, and then joined to a specially-designed and fabricated pulling head; the string required backhoes and other equipment along its almost 1,000 m length to provide lifting support and ensure the required alignment was maintained.
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