Claxton helps ensure first oil from Tombua-Landana at the earliest opportunity
The first of two Claxton contributions to the giant Tombua–Landana project is not that easy to spot, even though it is fundamental to this world-class project. Production facilities weighing 36,500 t are supported on a compliant tower weighing 56,400 t. This stands on a tower-base template of 3,000 t, which, in turn, rests on a levellingpile template. Under this, right on the seabed, there is a 12-slot predrilled template providing nine well and three docking slots designed, supplied and installed by Claxton.
Weighing just 30 t, the small but perfectly formed template provided the pattern for the first wells drilled before the giant platform was installed. It also determines the location of the platform and all 38 wells that will eventually be needed to complete the development.
Claxton won the job of providing the template for the development’s operator, Cabinda Gulf Oil Company Ltd (CABGOC), on the back of lessons learned during the installation of a similar template for the Benguela–Belize platform. Then, the company was called in late in the day to help install the template, which had been provided by another supplier. “We got the chance to demonstrate our expertise in this area on Benguela–Belize and, consequently, were awarded the contract to supply and install the predrilling template on this project,” explains Rowan Patterson, Claxton’s business development director.
The template was installed in two stages. It was initially lowered onto the seabed from an anchor-handling vessel. The Pride Venezuela semisubmersible drilling rig then picked up the template on drillpipe fitted with a Claxton supplied, overshot, double-J running tool and carefully located it over a single well already drilled in the seabed. It was then correctly oriented, landed and locked onto the well’s 30-in. wellhead housing. A further slot was then drilled through the template and a second 30-in. conductor installed, at which point the orientation of the template was effectively fixed.
In all, four wells were predrilled through the Claxton template (of nine possible), each furnished with a 30-in., low-pressure subsea wellhead and an 18¾-in., high-pressure wellhead on 20-in. casing. A docking pile was also installed through an outer, purpose-designed slot on the template before drilling was suspended and the rig was moved away.
This also marked a pause in Claxton’s involvement, during which time some serious construction work was undertaken. A levelling-pile template was fitted over the well template, followed by four levelling piles, their correct alignment being ensured using two guide pin slots in the Claxton template. The main tower-base template was then added and 12 foundation piles were installed; thereafter, there was just the issue of adding the tower itself and then the topsides.
Claxton then returned to carry out another pivotal task in the development: tying back two of the first four wells to the platform. (CABGOC decided to delay tying back the other two wells in favour of drilling further platform wells.) Claxton prepared all the procedures for a process that began with removing the corrosion caps from the 18¾-in., high-pressure wellheads and landing 26-in. conductors on both wells to provide conduits back to the platform’s well deck. “The challenge was to ensure the correct space-out of the 14 conductor centralisers spread along the nearly 40 conductor pipe joints,” says Patterson. “Accurate measurement and careful calculations ensured that, with the conductors locked onto their subsea wellheads, all the centralisers engaged perfectly in the guides situated between the bottom of the tower and the platform.”
For each well, following the installation of the conductor, 133⁄8-in. and 10¾-in. casings were landed and locked into the subsea wellhead. A high-pressure wellhead was fitted to the former at the platform well deck ready to accommodate the production tree.
“There is great satisfaction from being involved in complex projects like this one,” says Patterson, “when everyone is required to play their part to the very best of their ability to ensure the overall success of the venture.” In this case, Claxton’s contributions were essential to the timely completion of the project, both firmly on the critical path to first oil from Tombua–Landana.
The Tombua and Landana oilfields lie in Angola’s Block 14 in roughly 365 m of water. They are being developed by the Chevron subsidiary CABGOC. The fields’ combined production facilities are an integrated topsides structure supported by a 474-m-tall compliant tower. The platform is very similar in concept to the neighbouring Benguela Belize platform.