Fluid Loads on Pipelines: Sheltered or Sliding

ABSTRACT The results of three test programmes investigating the hydrodynamic loading on marine pipelines are presented. These test programmes are extensions to a comprehensive test programme with a pipeline on a flat seabed. The individual test series cover partially buried pipelines, pipelines in open trenches, and pipelines sliding on the seabed. Each of these cases represents a variation from the fully exposed, fixed pipeline, and for all cases reductions in the hydrodynamic loading appear. The paper presents these reductions and the associated force coefficients as functions of the dimensionless, governing parameters. INTRODUCTION Since 1983 a series of model tests with pipelines have been carried out by the Danish Hydraulic Institute on behalf of the Pipeline Research Committee of the American Gas Association. The basic test programme with on-bottom pipelines has been described extensively in /1/, /2/, and /3/. In these references the testing technique and testing facility have been described in detail. The pipeline fixed on the seabed constitutes the worst case with respect to hydrodynamic loading. Any deviation from this case results in reduced fluid loading, as is demonstrated in this paper. Partial burial of a pipeline may result from sediment transport. However, the main reason for performing the tests presented here was the fact that several geotechnical test programmes, see refs. /4/, /5/, /6/, had shown that if pipelines undergo oscillatory movements (e.g. resulting from excessive wave loading) they tend to dig themselves into the seabed. For small oscillatory movements the pipe-seabed geometry is as shown in Fig. 1a In addition to the increased soil resistance, the hydrodynamic loads are decreased for this case, and this effect has been quantified in the tests for burial degrees up to 40 per cent. As a consequence of regulatory requirements or as a means for obtaining static stability, pipelines may be laid in open trenches. The trench geometry depends on the soil characteristics and the trenching methodology. Four simplified and shallow trench profiles have been used in the tests to determine slightly conservative estimates of the force reductions due to the sheltering effects of the open trenches. As discussed in /7/, /8/, and /9/ the traditional static stability philosophy is being transformed into dynamic and quasi-static stability methods that allow pipeline movements during severe environmental conditions. These movements occur as a result of excessive fluid loading. Giving the simplified example of a pipeline moving synchronously with the surrounding water, where the resulting hydrodynamic load is zero, it is obvious that the fluid forces are reduced for a moving pipeline. Traditionally, the relative movement effects have been treated by simply introducing the relative kinematics into the standard force expressions. However, in reality the forces are related to the near pipe flow field which even for a fixed pipeline is affected significantly by the presence of the pipeline itself. It is thus not a straightforward task to evaluate the force reduction based solely on analytical procedures.