J-tube Pull-In: Whether to Land on Seabed?

Question:

The question I often received is whether to land the line on the seabed during J-tube pull-in?

Answer:

I have carried out the sensitivity study by performing the static analysis for J-tube pull-in. The results show that:

• If the pipeline is rigid and the clearance between the bell mouth and seabed is small, the most favorable way of J-tube pull-in is to firstly land the rigid riser onto the seabed and followed by J-tube pulling;
• However, if the pipeline is flexible and the clearance between the bell mouth is bigger (e.g. 2 m to 3 m), the most favorable way of J-tube pull-in is to pull the flexible riser directly into the J-tube without touching the seabed.

Notes:

Please note that the answer given above is only based on the sensitivity study I have carried out and may not be applicable to all cases.

J-tube Pull-in Design

The introduction of J-tube can be found in the following link:

pipelineencyclopedia.blogspot.com/search/label/J-tube

The following assessments are required as part of the J-tube pull-in design:

1) Pull-in Force Required:

The pull-in force refers to the load requires to pull the PIP initiated from the lay vessel through the J-tube up to the topside termination.  Generally, the pull-in force depends on the following parameters:

• PIP bending stiffness;
• Coefficient of friction between PIP and J-tube;
• J-tube geometry/radius.

When calculating the pull-in force for winch selection, it is important to take into account of capstan effect during J-tube pull-in.

2) PIP integrity check:

If the PIP is flexible line such as umbilical/cable, it is important to ensure that:

• J-tube pull-in load is within flexible line allowable tension (described above);
• J-tube bend radius is larger than the flexible line MBR.

If the PIP is the rigid riser, the following criteria have to be checked:

• The pull head shackle is within the allowable pull-in load;
• Riser within the J-tube is checked against the local buckling displacement controlled criteria (allowable strain criteria);
• Riser outside the J-tube is assessed against the local buckling local controlled criteria (allowable stress criteria).

Recommendation:

The possible PIP jamming in the J-tube during the pull-in operation shall be examined during the detailed design.