Creating safe buckling zones in deepwater oil pipelines
Deepwater pipeline systems are designed to operate in harsh subsea environments — deep below the ocean’s surface on the cold ocean floor made up of soil and sand and teeming with marine life. In these turbulent waters, pipelines are exposed to changes in pressure and temperature. As the oil and gas industry moves further offshore and into increasingly hostile and more remote places, modern field developers are requiring flowline designs that are longer with larger diameters, lighter and run at higher temperatures.
Dynamic subsea environments
Traditionally, operators spend a great deal of resources designing, manufacturing and installing pipeline systems to ensure they are fit for service. Care is taken to verify their long-term integrity and that operational loads do not exceed extreme design parameters for the life of a project.
One challenge that operators are taking into consideration in the design phase is the increasingly widespread buildup of axial forces along the pipeline due to temperature and pressure differences. Coupled with the influence of soil on the seabed, which can restrict free movement of the pipeline, a phenomenon called lateral buckling occurs.
Figure 2. Traditional nonrotating cylindrical buoyancy module
Damaging effects of buckling
Buckling typically occurs during startup and shutdown sequences as thermal fluctuations cause pipelines to expand and contract, potentially resulting in downtime and unexpected costly shutdowns. Left unchecked, buckling can cause local axial strains severe enough for welds to fracture, the entire pipeline to collapse or even full-bore rupture.
High-temperature subsea pipelines require special mitigations to prevent unpredictable lateral buckling fatigue cycles during operation and shutdown events. Almost all deepwater subsea flowlines use mitigation devices to ensure safe lateral buckling, and one of the commonly used methods is attaching fixed buoyancy modules to sections of the pipeline.
In certain offshore conditions, lateral movement of traditional fixed buoyancy modules can cause soil compaction, increased lateral soil resistance and the formation of berms. Post-production subsea surveys and engineering evaluations show these berms may actually further restrict lateral movement, causing secondary regions of high stress. This further increases the risk of lateral pipeline buckling in other sections of the pipeline and affect fatigue life.
Figure 3. Side view of rotating buoyancy modules engineered to roll on the seafloor, reducing lateral friction and berm creation
Predictable deepwater oil pipeline behavior
Deepwater pipeline systems are often required to be held in specific geometric configurations subsea to prevent overuse of the system. Traditional nonrotating cylindrical buoyancy modules are installed to reduce weight and friction while promoting controlled bending on pipelines. As previously mentioned, traditional fixed buoyancy modules, in certain conditions, displace seabed material, creating ridges or berms of earth on the seafloor, which can start to restrict lateral movement of the pipeline and the buoyancy modules, leading to buckling.
Rotating buoyancy technology can solve this problem while increasing the robustness and safety of the system. It represents a step change in the way risks associated with buckling of high-temperature pipelines are managed and effectively mitigated. The advanced solution is engineered to reduce lateral friction and berm creation, thereby generating repeatable and predictable pipeline behavior. In addition, the rotating buoyancy design reduces the number of modules needed along the pipeline, significantly lowering overall project costs for operators.
Testing information
The ability of advanced software analysis to produce high-performance, robust and dependable solutions is revolutionizing deepwater drilling and production. By increasing the performance of products before they even enter the water, software analysis is able to provide the solutions needed to combat the increasing harsh environments presented by the offshore world, virtually.
For rotating buoyancy technology, both small-scale and large-scale tests can be performed using prototype fixed and rotating buoyancy modules. By testing both options, operators can compare the two mitigation methods and validate performance with mathematical modeling techniques. The testing simulates each system’s in-service conditions, including pipeline weight, soil properties, soil interaction and pipeline travel.
Figure 4. Subsea view of rotating buoyancy modules
Conclusion
Operating in today’s globalized markets requires a long-term approach with solutions that protect people, the environment and a customer’s investment. Forward-thinking companies are committed to improving the functionality of their products and solutions to provide wide-ranging benefits to meet and exceed customer needs.
As the search for new oil deposits goes further offshore and deeper in the ocean, new challenges will emerge and new solutions will materialize. Uncertainty exists everywhere, and the unpredictable nature of lateral buckling is an unstable phenomenon, yet it can be managed with innovative solutions like rotating buoyancy modules.
Steven Bray is the oil and gas business manager with Trelleborg’s offshore operation.
