Double triumph for HDD in the Middle East

Shore approach for 400 mm crude trunk line at Qatif, Saudi Arabia

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.

Furthermore, the extent of shallow water that had to be crossed made HDD commercially viable, compared to the alternative of dredging and/or cofferdam construction. Also, deploying conventional pipelaying barges was not an option given the shallow water.

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.

Following several months of intensive engineering and preparation, both onshore and offshore, drilling began in June 2009 using one of DrillTec's advanced 350 t-capacity HDD rigs. Drilling extended from the beach area towards the pre-determined offshore exit point at a depth of up to 20m below the seafloor.

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.

Accuracy was a major challenge on the project, given that the landfall section of the HDD section was followed by a T-junction in the pipeline less than 100 m from the borehole exit point. This required the drilling contractor to punch the bore out of the seabed - after having undertaken 2.1 km of underground drilling - to an accuracy of less than +/- 3 m lateral offset from the designated position.

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.

Once the pilot borehole was successfully completed within 15 days, and after verifying the achieved punch-out location, the borehole was reamed up to a final diameter of 600 mm to provide enough space to later house the 400 mm pipeline. The tools used for reaming and cleaning the bore were connected to the drill string offshore, with support provided by a crane barge and industrial divers. The reaming process was conducted in reverse mode from offshore to the onshore drilling rig.

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.

DrillTec employed a multinational crew, comprising mostly European expatriate key personnel, and a well-trained labour force from India and other Asian countries. Among the problems they had to cope with were the remote location of the site, the rising temperatures, regular sandstorms sweeping across the site and the quality of local supplies. Furthermore, the burden of excessive bureaucracy related to the maximum security needed in strategically important industrial areas, such as those around Qatif, did not make life any easier for the construction team.

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.

Less than 15 h later, the pull head emerged from the ground onshore, making this job a success and representing a state-of-the-art application of HDD in the Middle East. As such, the project is sure to attract the attention of future clients in and around the Arabian Gulf. Hopefully, it will also demonstrate the efficiency of HDD and its environment-friendly nature.

Sea-channel cable crossing at Mina Port, Abu Dhabi, UAE

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.

Although Transco had previously used HDD for installing its largest power cables over short distances, it was the almost 1,000 m length to be drilled under the sea channel that made this particular undertaking a state-of-the-art project. When complete, it would represent the longest ever HDD installation in the Mid East for this type of cable installation (two 400 kVA cables installed in heavy-duty HDPE conduit bundles comprising seven pipes each for every circuit). Therefore, Transco decided to give the task to trusted hands, based on previous experience.

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.

Special attention had to be given to the entry side of the crossing. Here, NSCC-DrillTec had to set up their huge, 350 t-capacity HDD rig on a restricted site in the congested industrial area at the edge of one of the port's main access roads. After conducting forward reaming for 30 m, NSCC-DrillTec installed a 1,000 mm-diameter HDPE sleeve under the road to avoid settlement in the road base, which could lead to possible interruption of local traffic. Such an eventuality would have to be avoided at all costs throughout the continuous, 24 h/d operation.

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.

In both cases, clearance was no more than 10 m in some places, which meant that steering engineers encountered severe magnetic interference with their guidance systems at certain stages. However, a combination of various guidance systems finally enabled the multinational crew to drill the parallel boreholes for both cable circuits accurately within a 10 m-wide utility corridor, while maintaining the required 5 m clearance between the two lines.

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.

While drilling works were ongoing, welding could take place. The HDPE conduit pipes for both circuits were welded and bundled on Saadiyat island, which was a project in itself, given the other ongoing major construction works there and the excessive interference that those activities caused along the required stringing area.

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.

Despite the difficulties outlined above, NSCC-DrillTec and AL Barrak jointly managed to have both circuits successfully installed beneath the seawater channel within less than four weeks from the start of drilling. This meant that the longest section of 400 kVA cable circuits installed by HDD in the Middle East could finally be handed over to a satisfied client ahead of schedule.

>> This article is presented with permission of Trenchless World / Aspermont UK. <<