One of the most important activities during the design of a pipeline is the engineering and analyzes required for its installation. It is normally performed by the installation contractors engineering department who have access to commercially sensitive data about the performance, capabilities and weather sensitivity of their equipment.
Generally, following analyses should be performed for pipeline installation:
Pipelay Analysis
The static pipeline installation analysis essentially forms a check on the pipe laying vessel and equipment capabilities. It is used to set the stinger departure angle, roller box heights , and initial tension of the pipeline as well as to initially check the stresses and strains within the pipeline.
Following a static analysis it is a dynamic pipeline analysis. Dynamic analysis is a more sophisticated version of the static analysis which accounts for vessel dynamics, wave and current action on the suspended pipeline. It can be time consuming and expensive to perform and is normally only used for deeper water or where the static analysis shows the vessel or pipeline will be close to its limit. An additional useful piece of information supplied by a dynamic analysis is the limiting sea state that the pipeline can be safely installed in. Without this data the engineer will be forced to rely on the judgment of the vessel superintendent, who works for the contractor and therefore may have reasons other that pipeline stresses affecting his judgment on whether or not to lay down.
Abandonment and Recovery Analysis
Start-up, abandonment and recovery analyses, used to determine the correct vessel positions and tension requirements for these operations.
Installation Fatigue Analysis
Start-up, abandonment and recovery analyses, used to determine the correct vessel positions and tension requirements for these operations.
Review of Design
The pipe lay contractor will normally review those aspects of the original consultant's design where he may be able to save costs through optimization. Typically decisions on pre-sweeping and trench depth requirements are the first to come under scrutiny as minimizing these activities can save considerable costs. A check will also be made that the bend radii the pipeline must follow will not be pulled straight by the installation tensions set as part of the static and dynamic pipe lay analyses. It is interesting to note that many pipeline consultants have never been offshore, let alone seen a pipeline installed. Similarly many contractors have no idea of the complexities of pipeline design.
Tie-Ins
Above water tie-ins. This is when a pipeline end is retrieved from the seabed for connection to another pipeline or tie-in of an end termination. As with the abandonment and recovery analyses the vessel positions and tensions must be calculated to prevent any possibility of over stressing the pipeline.
Start-Up Method
When the seabed is already crowded with other pipelines, for instance at a platform location, pipe lay start-up may employ a piled sheave. The pile will be loaded primarily horizontally, with a peak load of double the maximum start up tension. Start-up piles are therefore normally quite short with a large diameter. By a strange quirk of engineering a single 40' joint of the pipeline is often ideal for the pile. If the pipe lay method has not been decided at the time of Survey it may be prudent to assume a pile will be used and sample accordingly. Luckily as most pile start-ups occur near platforms, the deep soil data taken for the original platform construction should be available.
Anchor Patterns
Anchor patterns will be required for the pipe layer in the vicinity of platforms, subsea stuctures and other vessels. These are pre-drawn to show the locations the anchors must be placed to prevent clashes with other vessels and structures as the pipe layer passes them. Obviously for a dynamically positioned pipe layer anchor patterns are not required.
