Considerations of Pipeline Crossing Design

Pipeline crossings are an integral part of brown field developments and their design for shallow water conditions poses a number of challenges which require special considerations. Hydrodynamic stability and Vortex Induced Vibration (VIV) are typical examples. For both of these design aspects, there is no specific design criteria and methodologies dealing with crossings as is the case for pipelines resting of the seabed which, among other codes and standards, are covered by DNVGL RP F109 and DNVGL RP F105 for stability and VIV respectively. In the absence of specific design criteria, the industry have relied on interpretations of existing codes leading to crossing designs with varying failure probabilities and safety levels for different projects and different operators. This paper discusses the main aspects of pipeline crossing design in shallow water conditions as is the case for the Arabian Gulf Region. In this region, the water depths could be close to the limits of applicability of the conventional design codes which were established based on experiences and considerations for other geographical regions. As a result, the design of crossings poses an engineering challenge as the calculated allowable span lengths tends to be short and the number of supports greater than what can be reasonably constructed for the pipeline to sit on all supports. The viability of using design criteria adapted from these codes for VIV is assessed using full dynamic simulations taking into account the compressive axial loads in the crossing. It is shown that for the short crossing spans, which are typical in the shallow water of the gulf region, the onset of any VIV will result in phenomenon which is called here "crosswalking". This crosswalking will result in a lateral displacement of the pipeline over the crossing sleepers resulting in an increase in the first natural frequency due to the reduction in axial compressive loads and the change in behavior from beam to shallow arch. Implementing the analysis procedure discussed in this paper and taking into account the potential for crosswalking will help increase the length of the crossing spans. This increase can be to a level where the hyperstatic conditions can be eliminated and the crossings can be designed to be construction friendly. This will have direct cost and schedule impact on the execution of brown field developments where crossings constitute a major element of the project. It is hoped that further work will be done on studying the VIV of pipeline spans and crossings under axial compression as this area of pipeline engineering has not been fully explored and the focus of VIV research has been directed to risers which are always under axial tension.