A coupled temperature-displacement model for predicting the long-term performance of offshore pipeline insulation systems

Insulation systems for offshore oil and gas pipelines operate in aggressive environments over lifetimes of about 20 years. The effectiveness of the insulation over the entire lifetime is essential to provide flow assurance. Fluid temperatures must be kept high enough to ensure that waxes or hydrates do not form and result in pipeline blockages. The operating environment for offshore pipelines will become more severe with planned developments in 2000 m waters having well temperatures of up to 160°C. Insulation systems proposed for such deepwater wells are typically multilayered with foamed polymers providing most of the thermal resistance. The high hydrostatic pressures and well temperatures associated with deepwater wells can lead to a degradation of the thermal performance of foamed materials due to creep. The prediction of the long-term thermal behaviour is complex. The material characteristics of foams are dependent on the density, temperature and stress level, all of which change with time as the material creeps. This leads to a non-linear coupled thermal-structural response. This paper describes analytical models, which predict the system behaviour over time. These models have been implemented in a user-friendly design tool, named DISDANS. Good agreement is found between the model results and equivalent finite element models.