Modified Thin Wall Pipe Formula for Deep Water Application

摘要

The effect of hydrostatic head on deep water pipelines is not negligible because the effect of external hydrostatic pressure increases as the water depth increases. Some pipe wall thickness determination formulas provided by federal regulations (DOI and DOT) do not account for the effect of the hydrostatic head. In some deep water cases, where the pipe is governed by the high internal pressure, these conservative formulas require unreasonably thick walls by neglecting the benefit of the external pressure. ASME B31.4/B31.8 and API 1111 consider the effect of the external pressure in the pipe wall thickness calculation. The formula uses the thin wall pipe theory subtracting the external pressure from the internal design pressure. However, as the external pressure increases, the hoop stresses from this equation become too high compared to the thick wall pipe theory which provides exact solutions. The erroneous results from the thin wall pipe formula for deep water application inspired the author to find a better solution which is as accurate as the exact formula (thick wall formula) and as easy to use as the thin wall pipe formula. In this paper, the thick wall pipe formula and the thin wall pipe formula are evaluated and compared in detail. Based on the evaluation, a reliable and easily usable formula is proposed in the end of the paper.