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.
Desfgn 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 baais 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.
Following a recent inspection, approximately 60 km of a 138 km methanol pipeline was found to be under-protected due to the premature consumption of the original zinc bracelet sacrificial anodes. The cathodic protection system had been designed to last 40 years accommodating uniform coating holidays of up to 3% of the total surface area. However, substantial coating damage (in excess of the design allowance) in localised areas has resulted in accelerated depletion of the system in less than 12 years. A retrofit, sled anode, cathodic protection system was designed to furnish the current required to protect the pipeline for the next 30 years. This system is composed of a 330 kg, Al-Zn-In alloy, stand-off anode mounted upon a low profile sled. The design can provide a 3 amp current for 30 years, which will protectapproximately 200 metres of uncoated pipeline at a current density of 50 mA/m2 During the autumn of 1983, 50 of these sled anodes were installed on the main methanol line. The result was a 200 mV cathodic potential shift within 24 hours. INTRODUCTION Following a recent inspection of all pipelines associated with the Viking Gas Field, all the gas pipelines were found to be in an acceptable condition, but a substantial proportion of the methanol pipelines were found to be in need of remedial action. This paper deals only with the methanol lines, however results of the gas line survey are given for comparison. It addresses the possible causes of observed damage and the remedial work carried out on this polyethylene coated 3 methanol pipeline. The work will be of interest to other operators with lines of similar age and coating type. FIELD DESCRIPTION The Viking Gas Field is located 86 miles due East of the Lincolnshire coast in the southern North Sea (Figure 1). The field consists of two main complexes (A and B) and five unmanned satellite platforms (Figure 2).The satellite platforms, the B Complex and the A Complex are connected by infield feeder pipelines. The main gas export pipeline is a 28" line of 138 km in length between the A Complex and the shore terminal. Current winter production figures for the field are:Dry gas present peak production: 750 MMSCFD Condensate average well production:4 bb1s/MMSCF Connate water average well production:less than 1 bbl/MMSCF To prevent the formation of hydrates, methanol is injected into the gas stream at various places. The methanol is pumped from shore along a 31/2 pipeline piggyback to the 28" gas line, and distributed throughout the field by piggyback pipelines. The produced gas, condensate, water and methanol are separated at the shore terminal. After storage and make-up, the methanol is pumped offshore as required.
TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract
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