Speed was critical when Statoil wanted to use the West Elara jackup rig to install a high-pressure drilling riser for two wells in 132m of water at a Gullfaks satellite field, Norway. The riser had to be ready for installation in eight months to meet the rig’s operational schedule.
Claxton Engineering, an Acteon company, managed the project with support from sister companies 2H Offshore, Pulse Structural Monitoring and Subsea Riser Products, who manufactured sections of the riser and supplied the riser spider. The companies jointly delivered a detailed drilling riser solution, bespoke engineering work and a monitoring system to track performance.
This project was in unusually deep water for jackup rigs, as Darren Bowyer, project manager, Claxton, explains: “For jackup operations, deep water is over 80m. Leg length determines the depth that jackup rigs can operate in but average jackup rigs like the West Elara are for 80–100m of water, so 132m was challenging. The main issue is scaling up to the needs of deeper water. Depth increases the current and wave deflection of the jackup rig and the drilling riser, and increases loading on the whole system.”
According to Bowyer, “The industry has little experience of jackup rigs in depths greater than 100m and in harsh environments. This makes for conservatism in design and operations. Furthermore, during high-pressure operations, well control considerations require a large blowout preventer, which adds a stiff component to the drilling riser and changes the dynamics of the string.”
Drilling riser designs aim to minimise the risk of failure and meet design code and standard safety margins, but often have a large built-in “comfort factor” for coping with complex and unpredictable loading.
The loading that a riser will face is difficult to model and is usually simplified for the design process because some of the factors are not well understood. For example, engineers must make assumptions about weather patterns in their model’s input data. The fatigue information used is often based on standard industry codes rather than component tests and monitoring. Consequently, designers can find it difficult to calibrate their models with data gathered during operations.
For this project, 2H improved the model for riser loading, as experience shows that detailed modelling delivers an answer more than 30% closer to reality. The project team also recommended changing operational procedures to reduce riser deflections in the splash zone caused by waves and currents.
The modelling indicated the potential for very high fatigue in the original riser design, so it was necessary to change the riser, the rig and the air gap.
The alterations to the drilling riser design included adding a vortex-induced-vibration suppression system; using forged rather than welded joints in high-stress and fatigue-prone areas; and upgrading the tensioning system to 500t.
Bowyer says, “Adding special fairings to reduce vortex-induced-vibration and drag was a first for a high-pressure drilling riser in the North Sea.”
The rig design changes included increased load capacity for the Texas deck. The rig’s overshot capacity was also reviewed, but it was found to be fit for purpose.
This project was the first use of Claxton’s NT-2 tool for deploying a riser and enabled it to be pressure tested before make-up to the blowout preventer.
Monitoring and integrity management have become key focus areas, so Pulse developed a riser monitoring system to gather field data; enable integrity management; verify the predicted models; and enable the operator to drill safely. The system included topside and subsea sensors for recording movement, load and dynamic bending parameters.
This project has provided valuable insights into the use of jackup systems in deeper water. Bowyer says, “This project had a challenging time frame and about 90% of the riser was changed after the contract award. By drawing on successful projects like this, it should be possible to extend jackup drilling deeper.”
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