Line Best Practice
1.The order of the line stack-up is important and is dependent on how the line is connected in the Model component. Remember that the line can start at either the vessel or on the seabed depending on whether the selected start point is a vessel connection or seabed connection. This note is particularly relevant to A&R and start-up operations where there is a winch cable in the line stack-up and this cable needs to be attached to a particular location.
2.The overall length of a line should be balanced, meaning there should be enough line on the seabed to prevent complete lift off (or stretching) during dynamics, but not too much either such that there is a significant seabed length that is not undergoing any substantial loading and is unnecessarily increasing the mesh size and run time.
3.The recommended ratio of line length to water depth is as follows (larger ratios for smaller depths). Note that a default line length option will be provided in a future version.
a.Deep water (above 200m) line length : 1.5 to 3.5 times the water depth
b.Shallow water (below 200m) line length : 15 to 35 times the water depth
4.The duplicate function on the line stack-up can be useful for quickly building multi-section lines. This enables pattern replication, for example field joint creation between pipe joints of standard lengths.
5.The meshing of Line components with FE nodes and elements is done automatically by the PipeLay UI when you run an analysis. The meshing process is controlled by certain allowable element length inputs on the Constants dialog of the Project component. Default values are provided for these inputs, which are sufficient for most scenarios except:
a.In shallow water you may wish to reduce Maximum Element Length from 10m to a smaller value – e.g. 10% to 20% of the water depth
b.Where a very accurate touchdown prediction is needed you may wish to include a Touchdown Region mesh by setting Minimum Length of Touchdown Region to a non-zero value – e.g. 50m to 200m
6.Secondary transfer, attachment or lift cables should not be included in the line stack-up (e.g. Davit lift cables or cables attaching buoyancy to the main line). Instead, these cables should be added directly to the Model component by attaching them to connections along the line, as defined in the Line Connections dialog at the bottom of the stack-up tab.
7.If you wish to actively/continuously change the line length during a single restart analysis stage, rather than the standard approach of assessing separate snapshots of length in separate non-restart analysis stages, then use the Active Length specification at the bottom of the stack-up tab. When doing this you must remember to place the region of active length change in an area of no contact and little curvature (e.g. just below the stinger/tower/ramp) as length changes in such areas can adversely affect analysis predictions, particularly when friction is included. Also, keep in mind the target length change when specifying the associated initial/final element length inputs.
a.If the length change is an increase then the initial element length should be small and then allowed to increase up to a suitably large maximum length.
b.If the length change is a decrease then the initial element length should be large and then allowed to decrease down to a suitably small minimum length.
It is also important to remember to offset/move the vessel suitably as well when actively changing the line length during a restart analysis; otherwise the catenary shape will become unrealistic and unstable after a certain point in the analysis.
8.For pipe-in-pipe configurations pay attention to the bulkhead spacing. If bulkheads are omitted then the inner pipe is free to slide down inside the outer pipe, leading to convergence issues for static analysis. The spacing for centralisers is also important as this dictates how much the inner pipe can bend relative to the outer pipe. Offsets to the first bulkhead/centraliser should be specified with respect to the start of the line in the model, so it is worth keeping in mind where the line start will be. For reference the OTC 2008 paper, “Advancements in Response Prediction Methods for Deep Water Pipe-in-Pipe Flowline Installation”, outlines a case study of pipe-in-pipe modelling in PipeLay.
9.For pipe-on-pipe configurations the connection spacing is again important to ensure proper coupling between the two lines. Also, unless there is significant sliding of the piggy back expected (e.g. tens of metres) use the default Fixed connection type rather the more complex Interchangeable type which increases the run time of the analysis.