Thermal Expansion/Global Buckling Mitigation of HPHT Deepwater Pipelines, Sleeper Or Buoyancy?

There are many design challenges we have to face as offshore industry marches towards deepwater. Safely transporting the high pressure and high temperature (HPHT) product, oil and gas, is one of them. Industry is now commonly using advanced analysis tools such as FEA to assess the structural response of the flowline system that operates over a wide temperature and pressure ranges between the non-producing condition and the maximum production flow. Due to soil restraint, the pressure and thermal expansion can generate a significant level of compression that can cause flowline to buckling globally like a steel bar. It is more realized by the industry to initiate such global flowline buckling in a controlled manner so to release the compressive load and to avoid the excessive expansion and pipe bending at other locations if any. At the controlled buckle locations, pipeline sections (generally in 100m to 300m long) are welded in a lower flaw size and higher fatigue acceptance criterion, so to have prudent safety against the bending load and fatigue damage from the global buckling. The typical buckle initiators are sleepers (vertical upsets) and buoyancy sections (flowline section with much reduced submerged weight). Both devices can reduce the soil lateral resistance and lead to global buckling to occur at the designated locations. This study intends to focus on the utilization of sleepers and buoyancy sections. A series of analysis are performed aiming to provide supports for the industry in using these devices. Sensitivity analyses are conducted towards the key design parameters, such as pipeline-sleeper friction and sleeper side slope, and buoyancy section length and buoyant force. Recommendations are given in term of use of sleepers and buoyancy sections for different pipeline design/installation conditions.