Design And Operational Considerations For Unsupported Offshore Pipeline Spans (1986)

ABSTRACT Design and operational considerations have been examined in determining the significance of unsupported offshore pipeline spans that may develop during pipeline installation or during field operation, Allowable unsupported span lengths determined during the design phase are generally based on the strict code compliance and the design basis encompassing the worst possibilities of the environmental and operational loads. During operation unsupported spans, however, do develop beyond the allowable limits, perhaps due to various unforeseen local conditions. Applying original design criteria is likely to result in cost prohibitive repair predictions, while lack of action may result in loss of production or other concerns. Without violating the design code requirements of an existing or a proposed pipeline system, realistically safe design can be approached by evaluating the sensitivity of the key design parameters. Remedial repair priorities of the unsupported spans can be established for the pipelines in service while recognizing operating constrains, budget and schedule. Based on periodic survey, properly formatted data of the survey and reanalysis of the design parameters, pipeline span repair cost estimates and schedule can be appreciably reduced. Similar principles can be applied to a pipeline in planning and design stages in order to ensure a safe operating design and to eliminate an uneconomically conservative design. These design and operational considerations are illustrated through a case study of work recently carried out. INTRODUCTION One of the factors to be dealt with in designing an offshore pipeline is the effect of static and dynamic loads on free span and assessing the risk of free span failure due to the fatigue caused by vortex shedding phenomena. After a successful design effort, pipeline free spans may develop during pipeline installation and testing or at any time during operation. Operators are often faced with the situation that requires partial or total remedial action to alleviate the problem of free spanning of an unburied offshore pipeline. Time and budget constraints limit remedial field activities once the pipeline is in operation. Considering such circumstances, what are the design and operational factors that should be considered in minimizing or totally avoiding the free spans in an unburied offshore pipeline? This paper discusses the effect of unsupported offshore pipeline spans, and the factors to be considered during the design and operational phases. The design consideration includes a review of the work published by several researchers in recent years. The sensitivity of design parameter, that enter into the analysis of assessing the fatigue life and equivalently the allowable unsupported span, is examined. The operational considerations include remedial actions to be considered, preventive maintenance and planned inspection of pipeline during operation from the viewpoint of the budget and time constraints. A case study is presented discussing the recent engineering and repair activities in the offshore pipeline system of the Dubai Petroleum Company.