LDD uses experience and innovation to achieve success on first job
In July this year, Large Diameter Drilling Ltd (LDD) completed its first project. The job was to drill and grout the 42-in. tubular piles for a jacket installation for VAALCO Gabon (Etame) Inc. in Ebouri field, offshore Gabon.
Using newly acquired and fabricated equipment, some of which it developed specifically for the project, LDD installed four through-leg piles raked at 1:8 and 1:10 batter in 81 m of water.
The Ebouri field is part of the larger Etame block, and this platform is the second to have been installed in the block.
LDD was awarded the contract, worth $2 million, following a meeting with VAALCO Energy (VAALCO) in Houston in December 2007. Andy Seager, operations manager, LDD, says, "We walked out of VAALCO's office totally elated, but there was a lot of work to do, as we were expected to be in the field drilling in six months.
"We set out to ensure that we could provide a service beyond VAALCO's expectations by recruiting a team of high-calibre, experienced staff. I had previous experience of working with VAALCO, so we already had an advantage."
The newly formed LDD started work on planning and building the specialist equipment that would be required for this and future projects. Seager says, "From our experience, we knew the biggest challenge was likely to be working from a floating construction vessel onto a fixed platform on the seabed and dealing with the vessel heave motions and forces.
"These would be particularly problematical when inserting and withdrawing the bottomhole assembly, as there is only 1.5 in. of radial tolerance when the drill bit is run through the centre of the pile. This meant that there was potential for the drill to get caught on withdrawal or to touch the base of the pile and damage either the pile itself or the system that attaches the drill rig to the top of the pile when running in."
LDD developed a solution - a passive heave compensator with a 3-m stroke for safe working loads up to 60 t to negate the potential point loading and snagging impact. Using this device, they were able to reduce the point loading to just under a tonne.
"As far as we are aware, no one else has ever used a lift compensator such as this for a drilling and piling operation," says Seager.
That was just the start of the work that was necessary. The company also designed and manufactured the drill bits needed for the varying ground conditions, including two 36-in. drill bits with a 48-in. under-reaming capability that enabled the drill bit to expand beneath the toe of the pile. Spare drill bits were at hand throughout operations, so as to take any maintenance off the critical path.
A special in-drillstring hydraulic power pack and actuating equipment were also developed for working down hole, as well as a 42-in. gripper can featuring a remote, pneumatic gripping connection capable of withstanding the dynamic and passive forces that the drill generates. Heave was likely to be an issue, and, therefore, the gripper can was specifically designed for drill rig lifting in marine environments.
LDD used a 22-t PBA 612 drilling rig that was specially reconfigured for offshore use to enable drillstring running. This had a maximum drill diameter of 2 m and featured 88,000 ft.lb of torque, a maximum thrust of 55 t and a pullback of 65 t. The inherent design of the drill bit was such that it would avoid the cutters getting stuck out beneath the toe of the pile when they were extended by the under-reamer. This is an uncommon but very serious problem during under-reaming drilling operations, which is mitigated by correct under-reamer design and operational experience.
Because of the adaptability of the drill system to varying ground conditions, VAALCO elected not to carry out a site investigation, which would have cost around $2 million, but to use the information gained two years earlier during construction and installation of the Avouma platform.
The operation was successfully performed on time and without incident from Global Industries' heavy lift barge, the DP Hercules. The 123-m piles were transported in three sections that had to be welded together before they were lowered into the 34-m drilled holes.
The soil conditions encountered were not as straightforward as expected from the Avouma experience because of the high levels of clay in places, some of which was unconsolidated and inter-banded with weak mudstone, siltstone and sandstone layers. This was further aggravated by the raked piles. Consequently, drilling took a little longer than planned, despite the excellent performance of the equipment. On completion of the socket and before removal of the drill bit and the bottomhole assembly, the hole was backfilled with a proprietary shaft-stabilising mud product to provide support to the unconsolidated clays and weak layers, which increased the socket stand-up time for safe pile insertion and grouting.
LDD also provided the specialist pressure grouting equipment and supplied 282 t of grout procured and mobilised from Europe to ensure quality of supply. This was used to grout the piles into the rock socket.
"We worked directly on the project's critical path, on a vessel with a very high day rate; therefore our performance and contingency measures had to be first class. We already knew that the sub-base consisted of hard mudstone and sandstone; that is why a driven pile solution was ruled out in the first place. So, we went prepared with drill bits for all the soil and rock conditions that we were likely to encounter.
"We carried an extensive spares inventory so that in the unlikely event of breakdown we would not lose time carrying out repairs. Should we have experienced a drillstring failure, we carried specialist downhole tools to fish for the large-diameter reverse-circulation drillpipe." Seager adds, "We were also prepared for weak, collapsing soil. A spot grouting and under-reaming programme was developed to stabilise the ground if the mud was not sufficient.
"The project was a great success and went exactly as we planned. We coped perfectly with a variety of soils and rocks, and the lift compensator exceeded all expectations. It gave us tremendous control of the lifting and heaving, and really enabled us to expand the operational envelope.
"It is difficult to estimate the savings that this equipment presented directly; however, there were a couple of instances when we would have suffered weather-related downtime, had we not had the compensator to run the bottomhole assembly and relocate the drilling rig," Seager concludes.
"LDD's service added value, and if I ever have to drill and grout piles again, I would not hesitate to call them," says Joe Delahoussaye, construction project manager, VAALCO. "They know their business; they bring certified qualified people; they have the equipment; and they are easy to get along with and knowledgeable. To me, it is refreshing to pay someone to do a job and have them just go out and do it."