An Integrated Design Approach to the Use of Sleepers as Vertical Upsets for Thermal Buckle Management
Authors: Wang James , Jukes Paul 
Published: July 2009

Pipe-in-Pipe (PIP) arrangements for offshore pipelines have become a viable approach to handling High Pressure and High Temperature (HPHT) conditions in deepwater. However, using sleepers to control the buckle location and stresses (thermal buckle management) in this type of pipeline is facing challenges regarding free spanning and sleeper embedment. A sleeper design should ensure adequate vertical upset of the pipeline, thus helping buckling of the pipeline as part of the thermal management plan. However, this approach generates free spans in the pipeline, which could become susceptible to Vortex Induced Vibration (VIV) if these free spans prove excessive. Further, PIP pipelines are usually heavy and may raise additional challenges in very soft soils, especially given the great uncertainty in predicted penetrations provided by currently available models. This paper presents an integrated approach to designing sleepers and the approach is applicable to both PIP and single pipes. It takes into account the interaction between pipeline structural integrity and sleeper embedment, thus determining the required sleeper general sizing and the possibility of the need for mudmats or mattresses. Finite element analysis of both the pipeline and sleepers is used in the presented approach. During the FEA modeling, importance is addressed for the model length, element size, concrete induced Stress Concentration Factor (SCF) at the field joints for single pipes, etc. In addition, the analysis scenarios are addressed to ensure the results from all the necessary cases are accurately identified. The sleeper design in the integrated approach details the appropriate selection of sleeper locations to release excessive axial loads as well as to ensure buckling stability. During the selection, some factors contributing to the buckling analysis results are discussed and these factors include route bends, pipe ovality, residual stress/strain, and rogue buckles. Different sleeper sizes are assessed with respect to the pipeline structural integrity (e.g., stresses and strains due to vertical bending, lateral buckling and VIV), coupled with an assessment of lost height due to sleeper penetration in the soil. Results indicate that the sleeper size should be maintained within a certain range to ensure proper function of the sleeper inducing lateral buckling of the pipeline, while reducing the possibility of excessive VIV. In some cases, this may require the help of mudmats or mattresses to support the sleeper. Results also show that the sleeper width should be selected such that after buckling, the pipeline would not fall off either end of the sleeper. The ULS check and fatigue assessment due to VIV/direct wave loading are also discussed for wave/current data and wave load application to interacting spans. To ensure that conservative estimates of the fatigue life, sensitivity studies are performed to account for the uncertainty due to soil properties and concrete conditions (intact or damaged). The tolerance for each item varies from case to case, thereby varying the inputs. This integrated design approach combines pipeline lateral buckling and span analyses together with the analysis of sleeper penetration in the soil. The proposed integrated analysis would ensure that the designed sleeper would not cause excessive VIV/direct wave load to the pipeline and that thermal stresses and buckling of the pipeline are properly managed.