Design and Operational Considerations for Unsupported Offshore Pipeline Spans

Abstract

Summary Design and operational considerations have been examined in determining the significance of unsupported offshore pipeline spans that may develop during pipeline installation or field operation. Allowable unsupported span lengths determined during design are generally based on strict code compliance and a design foundation encompassing the worst possible environmental and operational loads. During operation, however, possible environmental and operational loads. During operation, however, unsupported spans develop beyond the allowable limits, perhaps as a result of 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. Without the design code requirements of an existing or a proposed pipeline system being violate realistically safe design can be approached through evaluation of the sensitivity of the key design parameters. Priorities for remedial repair of the unsupported spans can be established for the pipelines in service while operating constraints, budget, and schedule are recognized. On the basis of periodic surveys, properly formatted survey data, and reanalysis of the design parameters, pipeline span repair cost estimates and schedules can be appreciably reduced. Similar principles can be applied to a pipeline in planning and design stages to ensure a safe operating design pipeline in planning and design stages to ensure a safe operating design and to eliminate an uneconomically conservative design. These design and operational considerations are illustrated through a case study. Introduction The design of an offshore pipeline must consider the effect of static and dynamic loads on free span and the risk of free-span failure resulting from the fatigue caused by the vortex-shedding phenomenon. After a successful design effort, pipeline free spans may phenomenon. 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 a situation that requires partial or total remedial action to alleviate the problem of free 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. Therefore, what are the design and operational factors that should be considered in minimizing or avoiding free spans in an unburied offshore pipeline? This paper discusses the effect of unsupported offshore pipeline spans, the factors to he considered during design and operation, and a review of the work published recently. The sensitivity of design parameters that enter into the assessment of the fatigue life and equivalently the allowable unsupported span is examined. The operational considerations include remedial action, preventive maintenance, and planned inspection of the pipeline during operation view of budget and time constraints. The case study discusses recent engineering and repair activities in the offshore pipeline system of the Dubai Petroleum Co.

Calculation of maximum allowable free span length and safety assessment of the DF1-1 submarine pipeline

“…Method The static analysis method is used to calculate the MAFSL through the maximum bending moment M MAX , which can be described by the following equation (MMS, 1997;Shah et al, 1986 (1) where L is the length of free span; w is the submerged weight of the pipeline per meter, which can be expressed as…”

“…where C is the free span end fixity constant, which is generally taken as 1.54 for simply supported ends and 3.50 for fully fixed ends, but usually the free span ends will be partially fixed, so an average value of 2.52 is suggested for the calculation (Shah et al, 1986); E is the Young's Modulus; M is the pipeline dynamic mass; I is the pipe moment of inertia which can be calculated as …”

Performance of a Covering Pipe for the Protection of Underwater Cable Subjected to on-Site Impact Loadings

“…Environmental hazards could be from regular ones such as the water pressure, possible corrosion, erosion and abrasion from sand and debris action under sea or from major hazards like huge storms, earthquakes and tsunamis et al [1,2,3,4]. Most cable systems are able to resist the regular threatening, but for the major hazards [5,6], special design based on the degree of the natural hazard must be applied. As for the threatening of human being activities such as the possible damage from ship anchoring or fishing operation that may induce impact loading or dragging exerting on the cables, it will be dangerous to cause serious damages.…”