Abstract
Sand waves are large-scale bed forms commonly occurring on the continental shelf seabed and can result in free spans of submarine pipelines, which may have an influence on the stability of the pipelines. Existing span rectification procedures have primarily focused on local rectification methods for free spans caused by local scour or individual spans resulting from seabed unevenness. This paper aims to present a span rectification design applicable to the pipeline crossing sand wave region, and to offer practical guidance on sand wave intervention strategies. A large-scale approach is necessary for the rectification of multiple spans across the field, which may involve the use of either a mass flow excavator (MFE) or a remotely operated vehicle (ROV) jetting tool. A comparative analysis of the estimated durations for post-lay trenching using the MFE and ROV jetting tools is also provided. In instances where the large-scale method fails to achieve span lengths suitable for long-term operation, a localized approach is necessary to address individual spans. The desired trench depth can be attained through a combination of pre-lay and/or post-lay trenching techniques. The analysis of on-bottom roughness and free span has demonstrated that, given the natural seabed profile without trenching, there are no spans surpassing the ultimate limit state (ULS) or fatigue limit state (FLS) criteria for the temporary installation scenario. Consequently, pre-lay rectification is not necessary. However, the analysis indicates that post-lay rectification is essential to meet ULS and FLS criteria under operating conditions. All spans that exceed the ULS and FLS criteria can be effectively rectified by trenching to a depth of 1 m.
Published Version
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