Comparison Study of Pipeline Free span Mitigation

Abstract

Subsea pipelines are transportation infrastructure systems that are alternative to high cost FPSOs and oil tankers as the most fundamental way to transport oil and gas. However, uneven seabed surface cause free spans along the pipeline. This free spans mean the distance of un-supported by seabed with gap clearance between pipeline and subsea surface. However, even if pipeline is already installed, changes in seabed geography and the significant currents and waves influence could cause the longer free spans than allowable span length. Therefore, various methods to mitigate free spans have been studied and applied in actual field such as clamp on support and mattress or grout bags. For this, careful consideration should be given to excessive bending stresses due to self-loading of pipelines and fluid in addition to fatigue failure due to vortex induced vibration (VIV) caused by currents and waves. At this time, if the mitigation method is not properly analyzed and applied, it may cause unnecessary work or cause more problems in future operation. Especially, when applied conventional method such as grout bags, concrete mattresses, or mechanical supports could not be functioned properly due to several environmental limitations in the fields. So, in this thesis, static stress mitigation effects on helical strake and buoyancy module are compared as point of static stress mitigation and VIV fatigue lifetime analysis. It is also including grooved buoyancy module studied and developed as the latest technology. In order to effectively analyze and determine the aforementioned objectives, parametric studies are conducted based on the DNV regulated formulas and FEM analysis. Firstly, the buoyancy thickness was analyzed for selecting the most effective thickness as primary factor. Secondly, the coverage ratio analysis was performed to decide and confirm mitigation effect. Thirdly, static stress mitigation ratio was compared to define the effective environmental condition. Finally, analysis in section 3.13 in this thesis, VIV suppression effectiveness of helical strakes and grooved buoyancy modules had been analyzed with bare pipeline. In addition, the fatigue lifetime was calculated based on the method proposed by DNV as comparison value.