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Article # 0039

General Overview Of The Design Of An Offshore Production Platform.

 by Kevin Foran, P.E.

This article is a general overview of the design considerations for an offshore production platform. It is not intended to be a design guideline or specification for the system or components. The design of such equipment should only be undertaken by an experienced team of engineers. This article is based on a shallow water surface wellhead production well and does not consider deep water or subsea production issues.

Design Parameters

 

A good start point for the design of an offshore production platform is the design and operating parameters. Be sure to consider potential future development of the property.

These parameters include

 

Well shut in tubing pressure

Well flowing pressure

Well flow temperature

Gas flow rate

Oil flow rate

Water flow rate

Sediment.

Number of wells

Number of departing pipelines

Number of interconnecting platforms or pipelines

Pipeline pressure rating

Anticipated useful life of platform

Codes and Standards

 

Another consideration is the codes and standards to be used. For piping, the design standard is API RP 14E. API  RP 14E guidelines reference ASME B31.3 for more detailed aspects of the piping system. For example table 2.5 from API RP 14E lists the maximum allowable working pressures for A106 GrB seamless piping, however ASME B31.3 is the source for information on stress values and other materials and thickness not included in table 2.5.

 

Piping standards also need to be considered at the specification breaks that will occur at the wellhead and at the pipeline. The wellhead will fall under API 6A, while the pipeline can fall under ASME B31.8 or B31.4. These breaks in specification may change the flange rating or wall thickness.

 

Another important standard to consult early and often is API RP 14C. This standard includes minimum recommendations on surface safety systems to prevent undesirable events that may release hydrocarbons and to minimize the affects of any potential releases.  Documenting the design meets the recommendations of API RP14C is a valid means of hazard analysis and may eliminate the need for any further hazard analysis.

 

The owner may have their own standards and guidelines that exceed industry codes and standards.

Wellhead and Flowlines

 

The wellhead equipment, including the choke valves or orifices, are specified by the drilling engineer and are not normally in the scope of the platform production piping and equipment. The platform piping and equipment scope interfaces with the wellhead via the wellhead control panel (WCP) and ancillary items. These items may include sampling lines, injection lines, gas lift lines, valve actuator pressure signals, sump level switches and ESD stations.

Manifolds

 

The production platform will include one or more manifolds or headers. Each wellhead may be piped to each of the following headers.

 

High Pressure Production Manifold

Low Pressure Production Manifold

High Pressure Test Manifold

Low Pressure Test Manifold

Atmospheric Manifold

 

The manifold is a common pipe connection between the wellheads and production equipment. The manifold pipe is sized to limit the maximum flow velocity under all potential flow conditions. The manifold ends are typically blind flanged to provide fluid cushion and minimize erosion. All changes in flow direction upstream of the manifold are typically flow tees or sand tees.

 

Each header / wellhead combination will have the valves and instruments as required by codes and operating parameters. This piping can become expansive. On offshore platforms space and weight are always important considerations. Special compact valves and manifolds should be considered at higher operating pressures.

 

Separators and Treatment

 

Three phase separators are the most common production platform equipment. The three phase separator breaks the flowline into produced gas, produced oil, and produced water. Each flowline from the separator is likely to have a system of equipment downstream to further treat the fluid.  There may be a flowline heat exchanger upstream of the separator but it is more likely to be downstream on the produced gas flowline.  The flowline heat exchanger is designed to to prevent hydrate formation in the gas pipeline. The produced water line may include cyclones and flotation cells. The specific equipment depends on the well analysis and final destination of the produced fluids.

 

A production platform may not have a separator. The flowline can be piped directly from the manifold to the pipeline for separation and treatment at a host platform or onshore facility.

Pig Launchers and Receivers

 

Pipelines need to be cleaned on a regular basis. The frequency of this cleaning depends on the risk of corrosion, scale build up, and the ability to detect corrosion or scale. Cleaning of the pipeline is accomplished by pigging. Pigs are plug like devices inserted into the pipeline and flow the length of the pipeline driven by differential pressure.

 

In addition to cleaning, pigging is also used for startup, decommissioning, separating products, and dewatering. Utility pigs and smart pigs are the two basic types. Smart pigs can provide detailed data on the pipeline and its contents. Utility pigs provide no data.

 

Pigs are injected into the pipeline through launchers and receivers located at pipeline termination points such as platforms. The minimum required pipe bend radius can vary with pig manufacturer and size. 3:1 is normally acceptable but it can be 5:1 or higher. Barred tees are required to make sure the pig flows in the right direction. Pipe inner diameter needs to closely match the chosen pig.

 

This is a brief overview of the design of platform piping systems. Many details have not been covered including corrosion, coatings, and utility systems. The intent is to cover these topics and provide more detail in future articles.

Reference Codes and Standards:

 

American Petroleum Institute

 

API 14C Recommended Practice - Basic Surface Safety Systems for Offshore Production Platforms

API 14E Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems

API 14J Recommended Practice for Design and Hazards Analysis for Offshore Production Facilities

API 520 Part 1 and 2 Sizing, Selection and Installation of Pressure Relief Devices

API 521 Guide for Pressure Relieving and Depressuring Systems

 

American Society of Mechanical Engineers

ASME B31.3 Process Piping

ASME B31.4 Pipeline Transportation Systems for Liquid Hydrocarbons and other Liquids

ASME B31.8 Gas Transmission and Distribution Piping Systems

About the Author

 

Kevin Foran, P.E. is a consulting engineer in Houston, TX. He has over 20 years experience in the process and offshore industries. He has a Bachelors of Science Degree in Mechanical Engineering from the University of Illinois at Urbana-Champaign.


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