IN THIS ISSUE three significant papers look at aspects of what goes wrong when an on- or offshore pipeline is damaged. The opening paper, by Prof. Andrew Palmer and Jiexin Zheng from the National University of Singapore, and their co-authors from Subsea 7, re-examine the issue of what happens when a fishing trawl board is pulled over a pipeline.
Fishing trawls can damage pipelines on the seabed, and it is important to be able to predict the force on a pipeline when trawlgear is pulled over it. Analysis and comparison with full-scale measurements indicate that the conventional calculation is incomplete, but that it is usually conservative. The pull-over load has more than one component, and the components depend on the trawl velocity in different ways.
Trawlgear interaction with a pipeline is a complex sequence of processes that has more than one stage. The first impact stage occurs immediately after the trawl board or trawl beam makes contact with the pipeline, and the forward movement of the gear then pauses briefly. Shortly afterwards, the ‘pull-over’ stage begins, and a little later the gear climbs over the pipeline, sometimes after sliding along it (if the gear does not approach at a right angle). A number of research studies have been carried out into this issue, and have included analyses, model tests, and full-scale tests and, as a result of these, DNV published a Recommended Practice (DNV-RP-F11110 in 2010.
The authors of this paper conclude that Equations 4.2 and 4.3 in the DNV Recommended Practice are not fully consistent with observations in many tests, most of them at full scale. In particular, the observed velocity effect is weaker than it is in the equations. A more complete analysis recognizes that the pull-over force has more than one component, and that the different components depend on velocity in different ways; the pull-over force needs to take account of the weight of the trawlgear and its motion across the pipeline. That said, the analysis represented by the two DNV RP equations appears generally to give conservative results. Non-conservative results are found only when the equations are applied to very low velocities, too slow to occur in trawling practice, or to extremely long and flexible towing warps.
THE SECOND paper covering issues surrounding pipeline failure (on pages 49-62) has been prepared by Mohamed Dafea of GL Noble Denton (now DNV GL) and co-authors from Penspen, Trail, and Société du Pipeline Sud Européen. Its subject is an investigation into the failure of a 40-in diameter crude oil pipeline.
In August, 2009, there was a 2.5-m long rupture in the longitudinal seam weld of a crude oil pipeline in France, causing a spillage of approximately 2000 cum in a protected area. This rupture caused the authorities to withdraw the permit to operate a 260-km long section of this pipeline. The pipeline had had a similar failure in August, 1980 which was attributed to a fatigue crack initiating at the inner side of the longitudinal weld. There was evidence of ‘roof topping’ along this weld.
Penspen Ltd was contracted by the pipeline operator to carry out an independent investigation into the cause of the 2009 failure, review and confirm the actions needed for safe short-term operation to allow internal inspection, and determine a safe future life for the pipeline. The failed pipe specimen was not immediately available for inspection, and an initial analysis indicated that a purely analytical evaluation would not provide conclusive results, due to variability in material properties, geometry, and loading. It was decided that a better understanding of the behaviour of defects in the pipe, and the fatigue performance, was required.
A detailed laboratory programme of burst and fatigue testing on a section of the linepipe was recommended, and these tests were carried out on a combination of ‘defect-free’ ring specimens, and ring specimens with artificially machined slits to represent crack-like defects. These tests showed that the failure pressures of the burst tests could be predicted using a recognized industry model: in this case, roof topping, laminations and inclusions, and toughness variations were found to have no noticeable effect on the defect size at failure.
The corresponding fatigue-test results showed that the defect-free rings had a fatigue life one order of magnitude longer than those containing a machined slit, and that the fatigue life of a defect-free ring could be predicted using a standard S-N method. In addition, it was found that crack growth was conservatively predicted using standard fatigue fracture mechanics.
The fatigue-test results showed that the 1980 and the 2009 failures were caused by a combination of cyclic pressure loading, roof topping, and a pre-existing weld defect (probably present when the pipeline went into service in 1972). As a result of this failure investigation, a conservative methodology for predicting the remaining fatigue life of the pipeline has been developed and validated by testing, and the pipeline is now satisfactorily back in operation.
IN THE THIRD paper on the topic of pipeline failure in this issue, Dr Chris Alexander f Stress Engineering Services and co-authors from Williams Midstream, Saipem America, and GL Noble Denton International, evaluate anchor-impact damage to the subsea Canyon Chief pipeline using analysis and full-scale testing methods.
As the authors point out, this paper presents findings from a study conducted as part of a joint-industry effort to evaluate the severity of damage inflicted to Williams’ subsea 18-in x 0.875-in wall thickness, Grade X-60 Canyon Chief gas export pipeline due to an anchor impact at a water depth of 2,300 ft. The 158-km long natural gas gathering pipeline connects Williams’ Devils Tower platform and its Blind Faith natural gas lateral to MP 261A on the Gulf of Mexico’s continental shelf, where connection is made to Williams’ Transco pipeline. The natural gas from this system is primarily delivered to Williams’ Mobile Bay gas-processing plant in Coden, Alabama, and the system parallels the Mountaineer oil pipeline for around half its route.
The phases of work described in this paper included an initial assessment after the damage to the deepwater pipeline was detected, evaluating localized damage using finite-element analysis based on in-line inspection data, and full-scale destructive testing including burst tests. The final efforts included the design and evaluation of a subsea-deployed repair sleeve, and the study included modelling Saipem’s repair sleeve design accompanied by full-scale destructive testing. Strain gauges were used to measure strain in the reinforced dent beneath the sleeve: these were then compared to prior results for the unrepaired dent test results.
The work associated with this study represents one of the more comprehensive efforts conducted to date in evaluating damage to a subsea pipeline. The results of the analysis and testing work provided Williams with a solid understanding of the behaviour of the damage inflected to the pipeline and what level of performance can be expected from the repaired pipeline during future operation. After the engineering analysis and testing phases of this work were completed, the deepwater pipeline was repaired using a grouted subsea repair sleeve. In reviewing the associated body of work presented in this paper, the authors make the following observations:
A horizontally-split sleeve can be safely installed by an ROV on a live pipeline by use of a buoyancy module and pull-down winches.
Cement grout can be prepared on a surface vessel and then pumped down a long hose to fill a grout sleeve on the seafloor.
A purpose-built metrology tool with mechanical gauges and simple fabrication techniques can produce an accurate model of the shape of a damaged pipeline on the seafloor.
Using analysis and full-scale testing techniques prior to deployment of repair methods improves confidence in the design and ensures that stresses in the damaged region of the pipeline are reduced to acceptable levels.