Raising the bar for drilling risers
Understanding and anticipating the market means we are often working on solutions before clients ask for them.” That Claxton holds one of the largest drilling riser stock inventories in the UK, with API NACE compliant systems up to 12,200 psi and a full inventory of supporting equipment, helps its fast response. “This is a real bonus for clients,” says Bowyer, “We have more pressure options, connection types and overall complete system options in-house than any other supplier. In addition, our ancillary items for riser handling, installation and tensioning have all been developed through years of experience and all with the aim of providing safe and efficient systems that reduce critical path rig time. So clients can be sure we are offering the best fit riser for their drilling campaign, not just a system we want to push.”
For oil and gas operators managing the equipment costs is a key consideration, but equally important is the need to ensure that they are procuring the most appropriate systems. Claxton responds to changing client needs by offering flexible commercial options. “We can provide risers on sale or rental terms to suit client projects and experienced crews to run the system offshore if necessary,” says Bowyer. “Being fully independent means that we focus on what the client actually needs and recommend the most effective solution. As part of the Acteon group of companies, we can draw on support from specialist sister companies such as 2H Offshore and Subsea Riser Products for access to combined levels of riser knowledge and expertise that no one else can match.”
Claxton’s riser expertise also played a key role in the success of a new development in the UK North Sea in water depths of approximately 90 m. This development includes a floating production, storage and offloading vessel, which was specially modified to meet the field’s requirements; six wells; and a 12-km gas export pipeline connected to the BP Central Area Transmission System pipeline to enable gas export to the UK. The production facilities will have a production capacity of about 30,000 bbl/d of oil.
Once again, Claxton’s contribution to this project met a tight deadline. Bowyer says, “We supplied a flanged high-pressure riser system package for the project, including interface connectors subsea and at surface. The project took six months from start to finish and was on time and on budget. The installation phase was completed smoothly, notwithstanding some early technical challenges interfacing the tension system, also supplied by Claxton, to the rig’s existing systems.”
The Claxton approach at this field during this project was driven by its significant previous deployment experience, which enabled it to offer a range of operational enhancements. For example, Claxton suggested having double riser joints racked back in the derrick and deploying the subsea trees with the riser system delivered significant time savings and so reduced operational costs. At the project wash-up meeting, the client’s team rated Claxton’s equipment and the services that the company had provided as “excellent”.
Claxton had to work as part of a large team and carry out the work scope in association with other specialist contractors. This called for a high level of flexibility, collaboration and coordination to ensure that the drilling riser was delivered and installed on time. Planning and executing the various tasks was a complex challenge, as several key players each made key contributions.
The drilling manager for the project stressed the importance of teamwork and attributed the success of the project to excellent interaction across the team and a willingness to learn from the performance coaches.
“Throughout most of the project there would have been more than 50 people actively engaged in delivering the drilling programme, with as many as 20 service companies being represented,” he said. “Understandably, every company focused on its own specialist tasks, so managing this extended team to deliver the project on time and on budget was crucial,” Bowyer continues. “The performance coaches played a vital role by standing back from the operation and taking an impartial view of what was going on. This enabled them to capture all the lessons learned from the project execution and feed them back to the team to promote continuous improvement.”
Analytical methods and operational procedures
Drilling riser designs are created to minimise the risk of failure and, in addition to meeting design code and standard safety margins, often have a large “comfort factor” built in. This helps to ensure they can cope 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 relevant factors are not well understood. For example, engineers cannot accurately predict weather patterns and must make assumptions in the input data they use for the model. The fatigue information used in models is often based on standard industry codes rather than component tests and monitoring. Consequently, designers can find it difficult to calibrate the models they have created with the data they gather during operations. Given the number of unknowns and estimates, it is unsurprising that most designs take an extremely cautious approach.
For this project, 2H set about developing a more complete and accurate model for riser loading. Experience shows that detailed modelling can deliver a modelled loading response that is more than 30% closer to reality. The project team also recommended some changes to operational procedures that would reduce riser deflections in the splash zone caused by wave and current action. The challenges overcome on this project prompted Statoil to set out exactly what it would require and expect in terms of modelling for future work.
Changes to riser and vessel
Modelling indicated the potential for a very high level of fatigue in the original riser design. This made it necessary to change the riser, the rig and the air gap. The alterations to the drilling riser design included adding a vortexinduced- vibration suppression system; the use of forged rather than welded joints in high-stress and fatigue-prone areas; and an upgraded tensioning system.
Bowyer explains, “Vortex-induced-vibration suppression and drag reduction were achieved by adding special fairings to the riser. This was the first time this approach had been applied to a highpressure drilling riser in the North Sea. In addition, this 24-in. riser was much larger than a standard application (usually about 133/8 in.). All the fairings used in the project were recovered intact. We also recommended a significant upgrade for the tensioning system. A typical North Sea jackup system has a 200-t rating, but Claxton experience and the 2H analysis suggested that a system rated up to 500 t would be required for this operation.”
The rig design changes included increased load capacity for the Texas deck. For this project, the West Elara rig required increases to its vertical and horizontal load capacity. Subsequently, the West Elara’s sister rigs under construction at the time were also upgraded. A review of its overshot capacity was also necessary. Increasing the overshot capacity would help to restrain lateral motion in the riser and to cope with significant bending loads during storm conditions. However, detailed analysis of the rig’s overshot capacity indicated that it was fit for purpose.
Claxton has more than 20 years’ experience of deploying risers. This was the first use of the NT-2 tool for deploying a riser and it enabled the riser to be pressure tested before make-up to the blowout preventer. The tool can perform a wellbore pressure test of 7000 psi while holding 308 te in tension.
Ivar Traeen, senior subsea engineer, Statoil said, “We were very pleased with the Claxton equipment and operations, and particularly with the clarity of their procedures. The supervisors involved during the installation and monitoring operation both displayed excellent, proactive attitudes and participated fully in the ongoing operations.”
Monitoring and integrity management
Monitoring and integrity management have become key focus areas for offshore operators in recent years. Pulse developed a riser monitoring system that would 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. The data the monitoring system gathered was supplied in real time through a traffic light alarm format. This showed the status as green when all parameters were in normal limits; amber when one parameter was outside normal operating limits and required adjustment; and red if any parameter was outside ultimate operating limits, in which case disconnection was recommended.
This project has provided valuable insights into the use of jackup systems in deeper water. Bowyer says, “The time frame on this project was extremely challenging and included many design iterations: 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. Can we do this elsewhere in 150 m of water? Yes. But the key to success will be early planning and detailed analysis.”
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