LDD meets unusual piling challenges in the Bahamas using its jacket installation services (JIS).
LDD demonstrated its extensive specialist marine piling experienceand problem-solving skills when it tackled particularly challenging ground conditions found off Grand Bahama Island in the Caribbean. The ground contained difficult-to-drill hard horizons intermingled with groutswallowing voids, and the installation work needed finishing before the rapidly approaching hurricane season. LDD’s answers to these challenges included customised drilling equipment and the first-of-itskind use of specialist grout pile socks as part of its overall JIS. The work was completed safely and on time.
Bahamas Oil Refining Company International Ltd (BORCO) is expanding its storage terminal and required the installation of a jacket with loading arms. LDD was able to advise BORCO, its consultants and the main contractor, Cal Dive International, about the challenges of piling into the vuggy calcarenite ground, which contains hard marble layers and large cavities.
The imminent hurricane season provided additional pressure for the installation to be completed on schedule. The main risks were that the hard layers would delay the drilling of the four large-diameter piles needed to secure the jacket against 100-year storm risks, and that excessive quantities of cement and circulating drilling fluid would be lost in the cavities.
Jason Clark, LDD managing director, says, “There was an unusually long lead time to this job. This type of large diameter drilling was new to all the parties involved, and we had to convince them all that we could complete the work safely and quickly. BORCO’s experience on a previous attempt to install a similar jacket in Freeport had been challenging, so we promoted the use of LDD reverse circulation drilling with our customised drilling tools as an effective solution.
“As the result of very close dialogue over about 18 months, we were able to give BORCO and its engineering consultant, Paloma, confidence that our proposed methods were fit for purpose, safe and reliable. It is very gratifying to see that we met their objectives and that the installation has been completed efficiently,” Clark concludes.
As part of its preparations, LDD attended the site investigation and observed bores at each pile site so the best drilling tools and methods could be developed for the job. In May 2011, Cal Dive International’s Pacific derrick barge placed the 535-t jacket on the seabed. LDD’s sister company CIS has piledriving experience from the area and so provided two 150-kJ hydraulic hammers (one working and one as backup) to drive the four 54-in. sacrificial casings through the overburden to the top of the bedrock as part of the permanent works for this project. Once driven, these sacrificial casings ensure the jacket’s short-term storm survival.
Large -diameter driling
Lee Edwards, LDD project manager, takes up the story: “We then used an LDD 612 offshore drilling rig, which was mounted to the casing top using our in-house-designed gripper can, to drill 18 m below the casing toe. This unique project required specific equipment.
“We designed and built a special bottomhole assembly, a drillpipe stabiliser and a 49-in. drill bit with hydraulically activated side cutters that give 58-in. underreaming capacity. An oversized hole below the driven sacrificial casing was required to provide room for manoeuvring the main insert pile and to give a large enough grouting annulus. We have designed, built and operated a wide range of underreaming systems and consider ourselves to be market leading in this technology.
“The single biggest challenge was the vuggy ground conditions,” Edwards continues. “We mitigated the risk of losing the cuttings flush from the reverse circulation drilling system by having a grouting contractor standing by. Had we experienced loss of fluid circulation, we would have been able to quickly seal the void with cement using specialist spot grouting techniques and then continued by drilling through the grouted plug. Fortunately, grouting was not required, as the drilling tools and the drilling method controlled the ground. Our work was on the critical path; everyone was waiting for us, so completing the work in our time estimate was important.
“Some engineers expressed doubts, based on their experience of this region, as to whether we would be able to drill in such challenging conditions and maintain a straight hole for subsequent pile insertion. This made the completion of the first hole in just 14 hours very satisfying. Even a particularly hard horizon only added 2 hours to our overall predicted drilling time. We did what we said we would do, and that is how to keep customers happy,” Edwards concludes.
Matt Pressel, Cal Dive International project manager, says, “The drilling went very well. It is the first time we had been involved in drilled emplacement of piles. The LDD team was helpful and walked us through the process. They were extremely professional and have some impressive equipment, some of which they designed and built especially for this job.”
Pile socks and grouting
A second challenge relating to the rock’s ultra-high permeability was to prevent excessive loss of cement during grouting. LDD proposed placing geo-membrane “socks” over the piles, which would inflate with grout but prevent it from escaping into the voids. Edwards believes that this is the first application of geo-membranes for a jacket installation.
Pressel says, “The sock worked well for the first three piles. Grouting the fourth pile was more troublesome, but we collaborated with LDD to develop a successful alternative methodology.”
Edwards explains, “We will never know, but I suspect there was a massive fissure at the pile toe into which the membrane grout bubble grew then burst and split the sock. Whatever the cause, we were losing grout and needed to put plan B into action. We decided to use a balanced pressure grouting technique. The pile was sealed at surface and the annulus was dewatered, then we pumped the grout down the pile and released the annulus air pressure in a measured way so that the grout was fed into and held in the annulus in a controlled manner. Using this technique, we were able to form a seal and grout to surface.”
“It is good to work with professional people who are flexible and have extensive experience,” says Pressel. “I have nothing but good things to say about the LDD team members and I look forward to working with them in the future.”