Pipeline Free Spans: Influence of Internal Pressure

The trend the last decades has been to develop the offshore fields by use of Subsea templates and flowline tie-backs rather than traditional platforms and topside processing. The exploitation of new offshore fields has moved towards deep waters, rougher seabed and reservoirs with high temperatures and high pressures. As flowlines are not necessarily trenched and buried, an uneven seabed may cause significant free spans. Then, if the current and wave induced flow velocities are sufficient, the free spans may be exposed to cyclic loading and associated fatigue failure. One of the governing parameters to ensure the structural integrity of free spans is the natural frequency of the span. As such it is in many cases of vital importance to estimate a realistic value of this frequency. The natural frequency is mainly determined by the bending stiffness and the span length but also operational parameters like the internal pressure and the fluid temperature. Recently some papers have been published that claim to prove that the effect of the internal pressure is the opposite of what has been the understanding in the industry and given by various codes. This paper tries to give an overview of the effect of the internal pressure and hopefully clarify what is believed to be misinterpretations of some experimental data.