Chris Timms

DETERMINATION OF THE COLLAPSE AND PROPAGATION PRESSURE OF ULTRA-DEEPWATER PIPELINES

In the design of ultra-deepwater steel pipelines, it is important to be able to determine the pipe behaviour while subjected to external pressure and bending. In many cases, the ultra-deepwater lay process, where these high loads exist, governs the structural design of the pipeline. Much work has been performed in this area, and it is generally recognized that there is a lack of test data on full-scale samples of line pipe from which analyses can be accurately benchmarked. This paper presents the results of a nil-scale test program and finite element analyses performed on seamless steel line pipe samples intended for ultra-deepwater applications. The work involved obtaining full-scale test data and further enhancing existing finite element analysis models to accurately predict the collapse and post-collapse response of ultra-deepwater pipelines. The work and results represent a continuing effort aimed at understanding the behaviour of pipes subjected to external pressure and bending, accounting for the numerous variables influencing pipeline collapse, and predicting collapse and post-collapse behaviour with increasing confidence. The test program was performed at C-FER Technologies (C-FER), Canada, with the analyses undertaken by the Center for Industrial Research (CINI), Argentina. The results of this work have demonstrated very good agreement between the finite element predictions and the laboratory observations. This allows increased confidence in using the finite element models to predict collapse and post-collapse behaviour of pipelines subject to external pressure and bending.Copyright

Modeling the UOE Pipe Manufacturing Process

It has been demonstrated in previous work that, for deepwater applications, the cold forming processes involved in UOE pipe manufacturing significantly reduces pipe collapse strength. To improve the understanding of these manufacturing effects, Tenaris has embarked on a program to model the phases of the UOE manufacturing process using finite element analysis simulations. Phase 1 of this work, presented previously in the literature [1], formulated the basis for the model development and described the 2D approach taken to model the various steps of manufacture. This paper presents the results of the Phase 2 work, and includes a description of the enhancements made to the modeling approach, a summary of the full-scale collapse testing performed at C-FER, and a comparison of the model predictions to the test results. Variations are made to the simulated manufacturing process in order to evaluate the sensitivity of collapse strength to key parameters. Based on the modeling approach taken, the findings of the Phase 2 work have shown that the deterioration of the collapse pressure diminishes with increasing O-press compression. The residual stress value is the most sensitive parameter when the strain hardening varies. It increases with the compression ratio and with the strain hardening value. In addition, given the assumed compression ratio of the test pipes, predictive behavior of the test results was found to be acceptable.Copyright

Effects of a Thermal Coating Process on X100 UOE Line Pipe

The increased demand for high strength linepipe for onshore and offshore pipeline systems has been well documented over the past few years. The economic benefits have been demonstrated, and solutions have been developed to address the technical issues facing high strength linepipe use. However, there are still a few unanswered questions, one of which is addressed in this paper: what is the effect of thermal treatment during the pipeline coating process on the material behaviour of high strength linepipe? This paper presents the results of a thermal coupon study investigating the effects of low temperature heat treatment on the tensile and compressive stress strain curves of samples taken from X100 linepipe. Thirty axial test coupons and thirty circumferential test coupons were machined from a 52 inch diameter, 21 mm wall thickness UOE X100 linepipe. Some of the coupons were maintained in the as-received condition (no heat treatment) while others were heat-treated in a manner that simulates a coating plant induction heat treatment process. All coupons were subsequently tested in tension or compression, either at room temperature or at −18°C. This study has provided a number of interesting results. In regards to material strength, the heat treatment increased the tensile and compressive yield strengths in the longitudinal and circumferential coupons. Axial tensile, axial compressive and circumferential tensile yield strength increases ranged from 5 to 10%. Circumferential compressive yield strength increases ranged from 14 to 24%. A Y/T ratio increase of approximately 7% was observed for all heat-treated tensile coupons. The coupon tests conducted at −18°C were only slightly different than their room temperature counterparts; with an average yield strength increase of 4% in all directions and orientations and a slight reduction in Y/T ratio.Copyright

The Influence of the UOE-SAWL Forming Process on the Collapse Resistance of Deepwater Linepipe

The UOE-SAWL pipe manufacturing process introduces considerable plastic deformations and residual stresses to feedstock plate material. Previous experimental and analytical studies have demonstrated that the effects of this process, predominantly in its final expansion stage, significantly reduce the collapse resistance of deepwater linepipe. Finite element analyses, sensitivity analyses and full-scale tests were conducted by Tenaris and C-FER Technologies (C-FER) over the last several years to better comprehend the impact of cold forming on collapse resistance. This paper presents the findings of the latest segment of this ongoing study, the objective of which was to optimize the collapse resistance of UOE-SAWL linepipe by varying three key thermal ageing parameters: time, temperature and number of thermal cycles. Six X70M and four X80M UOE pipe samples were manufactured and thermally treated with varied parameters. Full-scale collapse and buckle propagation tests were then carried out in an experimental chamber that simulates deepwater conditions. These experimental results were evaluated with respect to collapse predictions from API RP 1111 and DNV OS-F101. Material and ring splitting tests were also performed on samples obtained from these pipes to better assess the extent of the UOE pipe collapse resistance recovery. The outcomes of this study will be employed to further optimize the collapse resistance of subsea linepipe in order to reduce material and offshore installation costs.Copyright

The Prediction and Enhancement of UOE-DSAW Collapse Resistance for Deepwater Linepipe

With the increasing development of oil and gas reserves in water depths greater than 1500 m, linepipe used for deepwater and ultra-deepwater applications will require enhanced resistance to hydrostatic collapse. To support this need, Corus Tubes has been investigating methods by which increases in UOE linepipe collapse strength can be achieved. In particular, it has been theorised that modifications to the UOE manufacturing process can provide the necessary collapse strength enhancements. Pipe production trials were conducted focusing on the effect of processing parameters during UOE linepipe production, and in addition low temperature heat treatment was used to assess its effect. Full-scale collapse tests were then performed on the resulting linepipe specimens to validate the increase in collapse strength. The results of this work have demonstrated the beneficial effect of a modified UOE manufacturing approach on linepipe collapse resistance. This paper summarizes the work performed, quantifies the increase in collapse strength, and compares the test results to collapse equations found in offshore pipeline standards. It is also demonstrated that the UOE fabrication factor of 0.85 in the DNV offshore pipeline code (DNV OS-F101) may be considered to be over conservative, when linepipe is manufactured using the modified approach summarized herein.Copyright

THE INFLUENCE OF THE UOE FORMING PROCESS ON MATERIAL PROPERTIES AND COLLAPSE OF DEEPWATER LINEPIPE

It has been demonstrated in previous work that, for deepwater applications, the cold forming process involved in UOE pipe manufacturing significantly reduces pipe collapse strength. To improve the understanding of these effects, Tenaris has embarked on a program to model the stages of the UOE manufacturing process using finite element methods. Previous phases of this work formulated the basis for model development and described the 2D approach taken to model the various stages of manufacture. More recent developments included some modeling enhancements, sensitivity analyses, and comparison of predictions to the results of full-scale collapse testing performed at C-FER. This work has shown correlations between manufacturing parameters and collapse pressure predictions. The results of the latest phase of the research program are presented in this paper. This work consists of full-scale collapse testing and extensive coupon testing on samples collected from various stages of the UOE pipe manufacturing process including plate, UO, UOE, and thermally-aged UOE. Four UOE pipe samples manufactured with varying forming parameters were provided by Tenaris for this test program along with associated plate and UO samples. Full-scale collapse and buckle propagation tests were conducted on a sample from each of the four UOE pipes including one that was thermally aged. Additional coupon-scale work included measurement of the through-thickness variation of material properties and a thermal ageing study aimed at better understanding UOE pipe strength recovery. The results of these tests will provide the basis for further refinement of the finite element model as the program proceeds into the next phase.Copyright

Thermal Ageing Effects on Thickwalled Line Pipe

Line pipe is often coated prior to installation in order to achieve some protection against the environment. Many of the coatings used today require the pipe to not only be cleaned and degreased, but also to be preheated to a temperature of 200–240°C during application of the coating material. A typical coating thermal cycle involves rapid heating of the pipe using induction coils, application of the coating, and quenching to cool the pipe for handling purposes. It is generally understood that this thermal treatment on UOE line pipe, which can last from a couple to as many as ten minutes, has an effect on the pipe yield and, to a lesser extent, tensile strength. For ultra-deepwater offshore applications, where collapse is often the controlling design case, the increase in hoop-compressive yield strength is viewed as desirable because of the corresponding increase in collapse pressure. For onshore applications, however, bending due to differential ground movements can be the primary design consideration. In this case, an increase in tensile yield strength in the longitudinal direction may occur, and may result in a higher Y/T ratio. This increased Y/T can reduce the critical buckling strain of a pipeline designed to this limit state. In this paper, the effects of a coating thermal treatment on X70 grade UOE line pipe material properties are presented with particular attention being paid to the effect on the hoop-compressive and axial tensile yield as well as Y/T ratio. Both coating mill and coupon scale thermal treatment were investigated. Comparison was made between full thickness and round bar axial tensile samples. In addition, the influence of over-ageing or extended duration heating is investigated with respect to its impact on strength and ductility as well as the Charpy impact properties. Results of this study indicate increases between 13 and 16% for hoop-compressive yields while increases in the axial direction were approximately half that magnitude. Y/T ratio increases of around 5% were seen in the axial direction. The over ageing study did not demonstrate any detrimental effect of extended duration thermal treatment on the tested material properties.Copyright