During the course of a flow assurance analysis of most deepwater field developments, low temperature excursions within the flowline/pipeline system is an area of concern that sometimes receives too little attention or is frequently not considered early enough in the design process. If this issue is not adequately addressed, it may limit the operational flexibility of the development, compromise material selection or certification and result in potential operational hazards being overlooked.
This paper will attempt to address these concerns while discussing the following aspects of this important design issue:
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Description of operational scenarios where low temperature excursions are likely to occur.
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Considerations in selecting applicable design cases while cautioning against selection of design cases that result in overly restrictive design criteria.
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Review of relevant design guidelines and codes currently applicable to low temperature design issues.
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Overview of design procedures for effectively evaluating low temperature excursions.
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Modeling considerations when evaluating low temperature excursions using a variety of simulation tools.
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The possible impact on project interfaces and the resulting effects on project schedule and challenges to equipment certification.
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Overview of remedial measures if low temperature excursions present operational problems.
In addition, this paper will include two design examples:
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Low temperature on start up across a subsea choke valve.
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Low temperature excursion due to rapid depressurization of a small pipeline volume, e.g. riser only blowdown.
These design examples will be used to examine the issues stated above while providing a useful heuristic to assess any low temperature design issues within a flowline/pipeline system.
The focus of this paper will be on low temperature excursions that occur within the subsea domain of an offshore system. This paper will not consider the low temperatures experienced in a topside flare system downstream of a blowdown valve, which is often a valid concern but is typically examined in detail during the topsides design.
The two main scenarios that the paper will consider are:
Low temperature across a subsea choke during the cold restart of a well: If the system downstream of the choke has been depressurized to a low pressure prior to restarting the well, there will be a large differential pressure across the valve due to potentially high shut in tubing pressures. If the tubing is filled with a gas column, very low temperatures can be experienced downstream of the subsea choke due to Joule Thomson cooling during restart. The magnitude of the temperature drop will be impacted by the fluid composition and the initial pressure differential.
Blowdown of a gas filled riser: When a small volume of gas is depressurized, the gas will undergo a rapid expansion, which can result in low temperatures upstream of a topsides blowdown valve.
