Mitigation of pressure containment capacity reduction caused by compressive true force: Preshot

Joint Industry Project

We are currently seeking participants for phase 2.

DNV-ST-F101 states that a reduction of pressure containment capacity due to true compressive force (load controlled) should be considered. A document containing frequently asked questions (‘FAQ’) has previously been issued by DNV as a tentative guidance on how to perform this assessment. This has attracted close attentions from industry, especially for Pipe-in-Pipe (PiP) systems and buried HPHT pipelines.

DNV launched a Joint Industry Project (JIP) on this topic at end of 2022 and finalized in 2023 Q4. The objective of the JIP was to form basis of developing design approach on how to take high true compressive axial force into account for the pressure containment resistance. The objective was further to provide less conservative pressure containment reduction than proposed in the present FAQ.

DNV is now launching phase 2 of the JIP with the objective to further mature the guideline and expand on the applicability.

Objective

The objective of the phase 2 of the JIP is to further mature and improve the basis and assessment methods provided in the guideline from phase 1. It aims to provide less conservative pressure containment reduction currently available and to verify that the stress and strain state at the ‘design point’ is safe with respect to the relevant limit states.

Approach

The project will be run as a Joint Industry Project led by DNV and typical participants would be Operators, EPCI contractors and engineering consultants working with PiP and HPHT pipeline.

The proposed work packages for phase 2 will cover:

Establishing basis for distributions of wall thickness and material strength to be used for assessing the system effect
Re-assessment of level 2 methodology (FE based) as proposed in phase 1 taking into account the system effect for typical variations along a pipeline
Establish strain limits which may be used to verify that the stress and strain state at the ‘design point’ is safe with respect to the relevant limit states
Improve level 1 methodology from phase 1 with the aim to predict strain levels without the need for advanced finite element analyses (i.e. a stress or strain concentration factor which may be combined with a nominal stress/strain level)
Update of the Guideline
Clad/lined pipes and pipes with corrosion allowance will be explored.

Solution

Based on the work tasks performed in phase 1, a two-level approach is proposed to address the combination of compressive axial force and pressure containment. Phase 2 will further improve the guideline:

Level 1 presents a calculation method that may be performed by hand. The approach will potentially be improved by considering the system effect, i.e. potentially allowing a higher utilization than the current level 1 approach
Level 2 presents a detailed finite element assessment methodology that may be applied to reduce potential conservatism in the level 1 approach. The methodology will be improved to take into account the system effect and also provide a strain limit at the “design point”.