PROJECT

Developing Better Design Guidance about role of Field Joints in Initiating Pipeline Scour for Trunklines

Developing better design guidance about role of field joints in initiating pipeline scour for trunklines 

Project Status: Complete 

Investigators

  • Hongwei An
    Senior Lecturer, Civil, Environmental and Mining Engineering, UWA

  • Terry Griffiths
    PhD Candidate, Oceans Graduate School, UWA

  • Weidong Yao
    Geotechnical Engineer, Fugro Australia Marine Pty Ltd

Scope

The STABLEpipe design guideline authored by UWA and published by DNV presents guidance on how to account for sediment mobility, scour and sedimentation in pipeline stability design. The methods presented within the guideline allow simulation of scour and sedimentation as a result of ambient and metocean conditions, enabling more representative estimates of soil resistance, hydrodynamic loads and pipeline stability.

 

The STABLEpipe guideline has been applied for pipeline engineering projects offshore the NWS of Australia in recent years, and is presently being used in design projects. A key learning from applying the guideline is that scour initiation points along the pipeline are critical for kick-starting the scour process, leading ultimately to pipeline lowering and improved stability. For concrete coated trunklines, field joints present natural locations for the onset of scour given the locally reduced diameter. Consequently the embedment conditions at field joints prior to extreme design storm conditions is of great importance to the behaviour of the pipeline during the design storm event. Understanding and being able to predict the as-laid embedment condition and subsequent changes under ambient / non-extreme conditions is therefore of great interest.

 

This project undertook experiments to assess the embedment at field joints resulting from scour episodes typical of ambient conditions; this understanding can then be used to assess the capacity of these joints to act as initiation points in subsequent storms. The work built on earlier research which has focused on assessing the scour initiation potential at field joints immediately following lay. 

 

 

Outcomes

The experiments were undertaken in the Small O-Tube facility at UWA. An existing set of model pipelines at UWA with joints that mimic those expected on concrete coated trunklines were used. The specific experiments will explored different (relevant) joint geometries and ambient flow conditions (typical of ambient conditions offshore the NWS of Australia). Each experiment involved placing a pipeline in a given current to simulate scour until equilibrium is reached. The pipeline was then lowered and the velocity sustained to see if the joint initiates further scour or backfills. This process will be continued until no additional backfill or scour is observed.

  

 

 

The research has shown that when the pipeline is lowered until the non-FJ sections touch down onto the seabed, the scour hole at the FJ remains open (as hypothesized). The confounding observation was that the same effect was observed for a plain pipe with no FJ feature. Even at significant far-field embedment, the presence of a small span was found to be self-cleaning, with winnowing of fine particles away from the scour hole leading to further stabilization of the coarse particles remaining under the scour hole.

Impact

The potential exists that this outcome helps to explain field-observed spans on mobile seabeds on deeply embedded pipes. It is also possible these effects reflect the limited width of the SOT and non-uniformity of flow across the test section.

It is therefore concluded that supporting large o-tube tests are required to confirm the observations, however it does appear that there is potential value for better characterizing and modelling the behaviour of large trunklines such as Scarborough which are anticipated to experience modest increases in far-field embedment due to ambient currents, and where FJs form an important remnant scour initiation feature to assist in self-lowering during the passage of the design storm event.