A New Formulation to Calculate Effective Tension in Large Pipeline Free Spans

Since the 70’s, when offshore oil exploitation advanced, submarine pipeline design has been the subject of many researcher studies. Pipeline free spans are common and frequently assessed. In spite of this, up to this day, there is no simple and specific formula to calculate the effective tension (Seff), which takes into account the pipeline sagging in moderate or large span lengths. Therefore, engineers normally apply finite element analyses, even for preliminary evaluations. On the other hand, there are simple formulae to calculate other parameters such as the deflection, the moment and the natural frequency, which require previous knowledge of the Seff value. In the present contribution, an approximate formula is proposed, in order to solve the beam differential equation for both pinned and fixed end conditions and make it possible to calculate the Seff value using a straightforward and relatively simple equation, which takes into account the pipeline sagging. The values calculated using the proposed method were compared to numerical results obtained for many different cases, varying the span length and the effective tension. The analysis results showed that the proposed equation is highly accurate for practical conditions and, therefore, may become a useful tool for the assessment of the pipeline free spans.