Plem Design and Analysis
The objective of this section is to summarize the typical method used for PLEM design and analysis, but it is not a cookbook approach, because PLEMs have different design criteria and requirements, and the configurations of PLEM are based on each project’s requirements.
As oil/gas field developments move further away from existing subsea structures, it becomes advantageous to consider subsea tie-ins of their export systems with existing deepwater pipeline systems that offer spare transport capacity. This necessitates incorporating pipeline end manifolds (PLEMs) at both pipeline ends to tie in the system. A PLEM is a subsea structure (a simple manifold) set at the end of pipeline that is used to connect a rigid pipeline with other subsea structures, such as manifolds or trees, through a jumper. It is also called a pipeline end termination (PLET), especially when serving as a support for one pipeline valve and one vertical connector. The in-line structure is a simple manifold set at the middle of the pipeline, which is connected in line with the pipeline and used as a tee connector to divide or combine pipelines.
The deepwater PLEM with its mudmat and up-looking hub was designed as an economical and reliable method for terminating pipelines of all sizes. The mudmat is favored for economy and position compliance. The PLEM must be remotely installable and designed to support ROV execution. PLEMs are installed with the pipeline end from the installation barge and are lowered into their final position. A PLEM can also be a platform for a range of optional components such as valves, taps, and instrumentation. After installation, the PLEM can be accessed for repair or maintenance by removing the pipeline connection jumper and recovering the PLEM to the surface.
Design Codes and Regulations
The design codes and regulations used for PLEM design should be approved by clients depending on the project’s requirements. The following codes and regulations are often used for the design and structural analysis of PLEM in the deepwater of GOM (Note: The latest edition of the codes should be used):
- AISC, Steel Construction Manual, Allowable Stress Design (ASD);
- API RP 2A-WSD, Recommended Practice for Planning, Designing and Construction: Fixed Offshore PlatformsdWorking Stress Design (WSD);
- API 2RD, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs);
- API 5L, Specification for Line Pipe;
- ASME B31.8, Gas Transmission and Distribution Piping Systems.
The following specifications are used for the reference:
- DNV RP B401, Cathodic Protection Design;
- Classification Notes No. 30.4, Foundations, for the design of the mudmat;
- NACE RP 0176, Corrosion Control of Steel Fixed Platforms Associated with Petroleum Production;
- NACE TM 0190, Impressed Current Test Method for Laboratory Testing of Aluminum Anodes;
- NACE No.2 ISSSP-SP10, Near-White Metal Blast Cleaning;
- NACE RP0387, Metallurgical and Inspection Requirements for Cast Sacrificial Anodes for Offshore Applications;
- NACE 7L198, Design of Galvanic Anode Cathodic Protection System for Offshore Structures;
- API 1104, Pipeline and Pipe Welding, 18th ed.;
- AWS D1.1-02, Structural Welding CodedSteel;
- ISP 8501-1, Preparation of Steel Substrates before Application of Paints and
The design procedure for a PLEM may be divided into the following four steps:
1. Architecture design, which includes geometry and piping configuration, foundation selection (pile or mudmat), installation requirement (yoke, hinges), and fabrication and installation limitations;
2. Initial sizing, which includes determining the principal load path, configuring the primary structural components (forging, yoke, etc.) and the layout of structure frames and supports to support valves and pressure/temperature components that facilitate installation and maintenance, and sizing the mudmat based on loads, structure dead weight, and soil data;
3. Drafting a model for analysis, which includes developing a 3D PLEM system with AutoCAD 3D software and saving the 3D model in a SAT format file for finite element detailed analysis; meanwhile a SACS/StruCAD model is developed to determine the structure steel frame member’s sizes;
4. Stress analysis, which includes importing the SAT file into ABAQUS/ANSYS finite element software, meshing the 3D model, applying the loads in the operating and installation conditions, then applying a stress analysis and checking stress to satisfy the design criteria.
The following data are required for a PLEM design:
- Geotechnical data;
- Specifics of the equipment, including weight and center of gravity (CG)of each item;
- Thermal loads and maximum anticipated excursion due to expansion;
- Jumper installation and operation loads;
- Jumper impact velocity;
- ROV access requirements;
- Vessel limitations for handling;
- Installation methodology;
- Maximum tension during installation;
- Maximum installation sea conditions;
- Specifics of hubs, connectors, and miscellaneous tools;
- Specifics on ROV-activated valves where applicable.
 M. Faulk, FMC ManTIS (Manifolds & Tie-in Systems), SUT Subsea Awareness Course, Houston, 2008.
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 American Petroleum Institute, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms–Working Stress Design, API RP 2A-WSD (2007).
 American Institute of Steel Construction, Manual of Steel Construction: Allowable Stress Design, nineth ed., AISC, Chicago, 2002.
 DET NORSKE VERITAS, Cathodic Protection Design, DNV RP B401 (1993).
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 DET NORSKE VERITAS, Foundations, DNV, Classification Notes No. 30.4 (1992).
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 R.T. Gilchrist, Deepwater Pipeline End Manifold Design, Oil & Gas Journal, special issue (1998, November 2).
 D. Wolbers, R. Hovinga, Installation of Deepwater Pipelines with Sled Assemblies Using the New J-Lay System of the DCV Balder, OTC 15336, Offshore Technology Conference, Houston, Texas, 2003.