Driving, drilling and socks

Bahamas Oil Refining Company is expanding its storage terminal and required the installation of a jacket with loading arms. LDD was able to advise 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, that excessive quantities of cement and the circulating drilling fluid would be lost in the cavities.

Jason Clarke, LDD managing director, says, “There was an unusually long lead time to this job. This type of large-diameter drilling was new to Cal Dive International and we had to convince the company that we could complete the work safely and quickly. The clients experience on a previous attempt to install a similar jacket in Freeport was problematic and we promoted the use of reverse circulation drilling as an effective solution to the challenge. As a result of close dialogue over a period of around 18 months we gave the client and his engineering consultant Paloma, confidence that our proposed methods were safe and reliable. It is very gratifying to see that we met their objectives and that that the installation has been completed efficiently. As part of its preparations, LDD attended the site investigation and observed bores at each pile site. In May 2011, Cal Dive International’s Pacific derrick barge placed the 900-t jacket on the seabed. LDD’s sister company, CIS, has pile-driving experience from the area and for this project provided an S-150 hydraulic hammer to drive the four 54-in. Piles as part of the permanent works. When driven, these piles ensured the jacket’s short-term storm survival.

Large-diameter drilling
Lee Edwards, LDD project manager, takes up the story: “We then used our LDD 612 drilling rig, which was affixed to the pile top using our in-house-designed gripper can, to drill 18 m below the pile toe. The project was unique and required specific equipment. We designed and built a special bottom-hole assembly, a drill-pipe stabiliser and a 49-in. drill bit with hydraulically activated side cutters that give a 58-in. Under-reaming capacity. An oversized hole was required to provide room for manoeuvring the insert pile and to give a large enough grouting annulus. We have designed, built and operated a wide range of under-reaming systems, and consider ourselves to be market leaders in this technology.

“The single biggest challenge was the ‘vuggy’ ground conditions. 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 and continue by drilling through the grouted plug. Fortunately, grouting was not required at this stage. Our work was on the critical path; everyone was waiting for us, so completing the work in the time we estimated 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. This made the completion of the first hole in just 14 hours intensely satisfying. Even a particularly hard horizon only added 2 h to our overall predicted drilling time. We did what we said we would do, and that is how to keep customers happy.”

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 a 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 come up with 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 pressure grouting technique. The annulus was dewatered, and 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 within 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.”