Global Buckling and Walking in Subsea Pipelines: Consequences and Mitigation Measures

Pipeline Global Buckling

Pipelines like other slender strcutures with compressive forces, can buckle globally if the axial compresssion goes beyond a certain level. Buried pipeline normally tend to buckle in upheaval direction (upheaval buckling) and exposed pipelines normally tend to buckle laterally (lateral buckling).

In most cases, evaluations relevant to the global buckling threat will already start taking place in e.g. feasibility studies carried out during the concept phase. With regard to global buckling, the system risk review and strategy development activity should be initiated by participating in such early studies [1].

The most relevant failure modes of global buckling are as follows [1]:

• Local buckling, which is normally the governing failure mode resulting from excessive utilization. Local buckling appears as wrinkling or as a local buckle on the compressive side of the cross section. Local buckling can lead to excessive ovalisation and reduced cross-section area. This means reduced production, or even full production stop if e.g. a pig should get stuck. A locally buckled pipeline cannot stand an increased bending moment in the pipeline. This could lead to pipeline collapse and full production stop.
• Loss of containment, as a result of:

Fracture, is failure on the tensile side of the cross section also resulting from excessive utilization. Fracture leads to leakage or full bore rupture, meaning reduced production, or even full production stop.

Low cycle fatigue, which can occur for limited load cycles in case each cycle gives strains in the plastic region; i.e. the utilization is excessive in periods. Low cycle fatigue may lead to leakage or rupture, meaning reduced production, or full production stop.

Hydrogen induces stress cracking (HISC), can occur in martensitic steels ("13%Cr) and ferritic-austenitic steels (duplex and super-duplex). Blisters of free hydrogen can create cracks in steel or weld at a CP/anode location when the steel is exposed to seawater and stresses from the buckle. The pipeline utilization does not have to necessarily be excessive. HISC leads to leakage or full bore rupture, meaning reduced production, or full production stop.

True Stories

• In January 2000, a 17km 16-Inch pipeline in Guanabara Bay, Brazil, suddenly buckled 4m laterally and ruptured, leading to a damaging release of about 10,000 barrels of oil, and to great embarrassment to the operator. Field observation showed that as a result of temperature increase, the pipeline displaced laterally, when failure took place. Operating pressure and temperature of the pipeline were 400bar and 95°C, respectively. The soil beneath the pipeline was very soft clay with about 2kPa undrained shear strength at seabed [2].
• In December 2003, side-scan sonar survey of a 10km pipeline transported wet gas in South East Asia, identified six lateral buckles along the pipeline length. The original pipeline design did not consider lateral buckling as a design issue; consequently, the effect of lateral buckling on the pipeline integrity was not clear. Results of a detailed lateral buckling study showed that the pipeline should be replaced within few years. Design Methodologies

Pipeline design against lateral buckling involves three main Levels:

LEVEL 1: SCREENING

In this level, an analytical approach (e.g. Hobbs [4]) will be used to check if the pipeline is susceptible to lateral buckling. The results of this level answer to the following questions:

• Is pipeline susceptible to lateral buckling?
• Which areas of the pipeline are susceptible to lateral buckling?
• Can we avoid lateral buckling by changing the concrete coating thickness of the pipeline?

LEVEL 2: LATERAL BUCKLING ANALYSIS OF THE PIPELINE

In this level, a detailed finite element analysis will be performed on the areas of the pipeline, which found to be susceptible to lateral buckling in Level 1 analysis. The results of this level answer to a main question: are limit state conditions acceptable in areas of the pipeline with unplanned buckle?

LEVEL 3: MITIGATION

If the answer to Level 2 question is no, this level will be commenced. In this level, a mitigation measure will be selected based on project and client requirements. The most well known mitigation methods are as follows:

• Increasing the concrete coating thickness in selected regions of the pipeline. One example of this approach is Reshadat 16-Inch oil pipeline in Persian Gulf, the concrete weight coating thickness of the first 5km of the pipeline was increased from 45mm to 65mm.
• Laying of the pipeline in zig-zag shape (snake lay). This method was successfully utilized in South Pars Phase 6 and 7, Jade pipeline in North Sea, Penguins flow line in North Sea
• Laying of the pipeline on pre-installed sleepers (vertical upset method). This method was successfully utilized in PC4-B11 pipeline in Malaysia, King flow line in gulf of Mexico.

Other less popular methods such as adding either expansion spool or buoyancy modules at selected intervals, and rock dumping also may be used.

Sumber: http://www.advancepipeliner.com/site/index.php/downloads/61-global-buckling-and-walking-in-subsea-pipelines-consequences-and-mitigation-measures.html (11 Februari 2014)