Subsea Pipeline Design Stages and Process
The design of subsea pipelines during subsea field development is usually performed in the following three stages:
- Conceptual engineering;
- Preliminary engineering or pre-engineering;
- Detail engineering.
The continuity of engineering design from conceptual engineering and preliminary engineering to the detailed design is essential to a successful project. The objective and scope of each design stage vary depending on the operator and the size of the project. However, the primary aims are discussed next.
The primary objectives of the conceptual engineering stage are normally:
- To establish technical feasibility and constraints on the system design and construction;
- To eliminate nonviable options;
- To identify the required information for the forthcoming design and construction;
- To allow basic cost and scheduling exercises to be performed;
- To identify interfaces with other systems planned or currently in existence.
The value of the early engineering work is that it reveals potential difficulties and areas where more effort may be required in the data collection and design areas.
Preliminary Engineering or Basic Engineering
The primary objectives of the preliminary engineering are normally:
- Perform pipeline design so that the system concept is fixed. This will include:
- Verifying the sizing of the pipeline;
- Determining the pipeline grade and wall thickness;
- Verifying the pipeline’s design and code requirements for installation, commissioning, and operation;
- Prepare authority applications;
- Perform a material take-off (MTO) sufficient to order the line pipe.
The level of engineering is sometimes specified as being sufficient to detail the design for inclusion into an engineering, procurement, construction and installation (EPCI) tender. The EPCI contractor should then be able to perform the detailed design with a minimum number of variations as detailed in its bid.
The detailed engineering phase is when the design is developed to a point where the technical input for all procurement and construction tendering can be defined in sufficient detail. The primary objectives of the detailed engineering stage can be summarized as follows:
- Optimize the route.
- Select the wall thickness and coating.
- Confirm code requirements on strength, VIVs, on-bottom stability, global buckling, and installation.
- Confirm the design and/or perform additional design as defined in the preliminary engineering stage.
- Development of the design and drawings in sufficient detail for the subsea scope. This may include pipelines, tie-ins, crossings, span corrections, risers, shore approaches, and subsea structures.
- Prepare detailed alignment sheets based on the most recent survey data.
- Prepare specifications, typically covering materials, cost applications, construction activities (i.e., pipe lay, survey, welding, riser installations, spool piece installation, subsea tie-ins, subsea structure installation), and commissioning (i.e., flooding, pigging, hydrotesting, cleaning, drying);
- Prepare MTO and compile necessary requisition information for the procurement of materials.
- Prepare design data and other information required for the certification authorities.
The object of the design process for a subsea pipeline is to determine, based on given operating parameters, the optimum pipeline size parameters. These parameters include:
- Pipeline internal diameter;
- Pipeline wall thickness;
- Grade of pipeline material;
- Type of corrosion coating and weight (if any);
- Thicknesses of coatings
Each stage in the design should be addressed whether it is a conceptual, preliminary, or detailed design. However, the level of analysis will vary depending on the required output. For instance, reviewing the objectives of the detailed design (Section 126.96.36.199), the design should be developed such that:
- Pipeline wall thickness, grade, coating, and length are specified so that the pipeline can be fabricated.
- Route is determined such that alignment sheets can be compiled.
- Pipeline stress analysis is performed to verify that the pipeline is within allowable stresses at all stages of installation, testing, and operation. The
results will also include pipeline allowable spans, tie-in details (including expansion spool pieces), allowable testing pressures, and other inputs into the design drawings and specifications.
- Pipeline installation analysis is performed to verify that stresses in the pipeline at all stages of installation are within allowable values. This analysis should specifically confirm if the proposed method of pipeline installation will not result in pipeline damage. The analysis will have input into the installation specifications.
- Analysis of global response:
- Expansion, effective force, and global buckling;
- Hydrodynamic response;
- Analysis of local strength:
- Bursting, local buckling, ratcheting;
- Corrosion defects dent.
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