DNO International was drilling offshore Oman when a short-notice change in the drilling program brought forward the start of work on the West Bukha 4 well.

The design for the well called for the installation of four centralisers on the conductor to provide structural integrity but only two were available at the time of drilling.

However, the schedule change meant there was no time to order the necessary additional platform centralisers before drilling began.

DNO could have opted not to install the additional centralisers but this was undesirable owing to the fatigue damage that can result from the movement of the conductor and the damage to the platform guides that the conductor repeatedly hitting them could cause.

Claxton Engineering found a way around the problem.

Far East business development manager Nick Dale said Claxton usually expected to install the platform centralisers at the same time as the conductor system.

This time, though, it did not have the option.

"We suggested retrofitting the additional centralisers during drilling as this would enable the rig to move onto the well and start operations while we designed and fabricated the items in parallel," Dale said.

Analysis of the well by Claxton's sister company 2H Offshore showed the conductor would require the additional centralisers - one located about 10m subsea and the other in the splash zone.

"DNO decided to use our retrofit solution because it would deliver a well-engineered design that could be installed off the critical path to save both offshore rig time and costly replacement in the longer term," Dale said.

Claxton provided a complete packaged solution, including platform and rig surveys, design, analysis and fabrication of subsea and surface retrofit centralisers.

It also engaged and managed the activities of third-party abseiling and diving teams to help with installation.

Mid-way through the drilling program, during the drilling of the long 133/8-inch casing section, there was sufficient space on the rig to let Claxton mobilise the installation team, abseilers and divers and complete the subsea installation of the centraliser with no adverse effect on drilling operations.

The system Claxton supplied was a 31in conductor, a 40in conductor guide and retrofit hinged centraliser complete with polyurethane buffers, quick-release collapsible hinges and profiled fins to let the centraliser interface with the platform guide funnels.

The centraliser was fabricated from AISI 4130 steel and had a corrosion-resistant, thermally sprayed aluminium coating.

The time to completion, discounting offshore delays, was about six months.

It included the three months from initial discussions to contract award, 1.5 months of fabrication work, 1.5 months of planning, surveying and writing procedures and one week offshore to complete the installation process.

Dale said retrofit solutions such as this gave the added flexibility to allow drillers to address fatigue concerns on existing wells before damage occurred and minimise the risk of expensive repair work.