+2 votes

In API 1111, the installation safety factor f1 is 3.33 for buckling check. When can a lower safety factor be used ?

asked May 9, 2016 in Installation by AshwiniS (171 points)
In API 1111 to calculate the buckling check unity factor, an installation safety factor f1 of 3.33 is used. But since for the pipeline parameters under consideration the unity check is more than 1, we would like to take advantage of using a lower safety factor (2.0 as mentioned in the code). Under what conditions can a lower safety factor be used ?
commented May 11, 2016 by odadmin (21,030 points)
Thanks @pasmith and @gagonza for answering @AshwiniS question.

@AshwiniS do you have any further comments or clarification to ask?

2 Answers

+2 votes
answered May 9, 2016 by pasmith (3,320 points)

Per my understanding of API-1111, it suggests to use 2.0 for both safety factors f1 and f2 where f1 is bending safety factor for installation bending plus external pressure and f2 is bending safety factor for in-place bending plus external pressure. 

Safety factor f1 can be larger than 2.0 for cases where installation bending strain could increase significantly due to off-normal conditions, or smaller then 2.0 for cases where bending strains are well defined e.g. reeling.

+3 votes
answered May 10, 2016 by gagonza (3,020 points)
The issue is the control of the installation vessel position and actual response of the tensioner active control system.  Some issues that are significant is that the actual tension control is upstream on the vessel with considerable frictional resistance prior to leaving a stinger (inertia of pipe, inertia and friction of rollers and tracks).

The use of .15% unit strain was initially devised by Shell for their projects which entered into new depths and diameters without a corresponding increase in lay vessel capability.  While buckling for most pipes will be above the 0.15% unit strain, the idea is to avoid the buckling.

Reducing the factor of safety could be done but I would think that a specific non-linear FEA of the pipeline buckling and residual curvature would have to be performed along with the vessel capabilities and the environmental sea-state that can surge the vessel and change the tension.

Vessel motions affect sag-bend and stinger tip strains more in shallow water than deep water.  In deepwater, where thrusters are also in the tension control equation there could be a case for a reduced factor of safety‚Ķbut not without a thorough analysis to meet the intent of 1111.

My 2 cents.