Sustained Load Cracking of Titanium Alloy Weldments

Failure of critical titanium parts, including some offshore components, has drawn attention to delayed cracking in Ti-6Al-4V alloys, but, given good design and alloy variant selection, such failures are avoidable. Delayed cracking, or sustained load cracking (SLC), can occur at low to moderate temperature (approximately: −50 to 200°C), depending on the titanium alloy and condition. Appropriate testing methods are required to generate stress intensity threshold values (KISLC) that can be incorporated into the design of titanium structures and recommendations are needed on the optimum chemistry and microstructure for greatest resistance. In the present work threshold stress intensity factor data (KISLC) were generated for Ti-6Al-4V alloy sheet, forgings, pipe and weldments using two different rising stress intensity factor test methods. It is concluded that material with a beta-annealed microstructure and low oxygen content (i.e. extra-low interstitial material such as ASTM Grades 23 and 29), has high resistance to SLC and that weld metal and transformed heat-affected zone also perform well, before and after postweld heat treatment, provided interstitial element pick-up during welding is prevented. Purchasing material in a general ‘mill annealed’ condition is not recommended without specifying acceptable microstructures. Further refinement of test method is also recommended for defining KISLC.

CFD Simulations of the Vortex-Induced Vibrations of Model Riser Pipes

The paper reports results from two strip theory CFD investigations of the Vortex-Induced Vibrations of model riser pipes. The first investigation is concerned with the vibrations of a vertical riser pipe that was subjected to a stepped current profile. An axial spatial resolution study was conducted to determine the number of simulation planes required to achieve tolerably converged numerical solutions. It was found that six to seven simulation planes are required per half-wavelength of pipe vibration in order to obtain convergence. The second investigation is concerned with the simultaneous in-plane and out-of-plane vibrations of a model Steel Catenary Riser that was subjected to a uniform current profile. The pipe’s simulated vibrations were found to agree very well with those determined experimentally. This result was achieved despite the questionable usage of simulation planes at high angles to the flow direction.

Laboratory Investigation of Environmentally Induced Cracking of API-X70 and X80 Pipeline Steels

Carbon steels, used in pipelines for the transport of oil and its derivatives, are frequently exposed to fluids. This can result in stress induced corrosion cracking (SCC) and/or hydrogen embrittlement (HE). The present paper evaluates the susceptibility of pipeline steels (API-X70 and API-X80) to SCC and HE, using a slow strain rate test (SSRT) based on the National Association of Corrosion Engineers’ (NACE) norm and a traditional standard NACE test. The (SSRT) method used, employed a sodium thiosulphate solution to evaluate susceptibility to HE, thereby offering a simpler experimental procedure than the standard NACE test. The results confirm the efficacy of the sodium thiosulphate as an H2S–SCC susceptibility test solution when utilised in SSRT testing. Though no secondary cracks were detected in the materials investigated, both steels were observed to suffer a ductility loss upon exposure to this solution. In NACE type tests, the test pieces were subjected to constant loading at 80% of σy. Fracture did not occur for these samples.

Validation of Wave Propagation in Numerical Wave Tanks

During the last few years there has been a strong growth in the availability and capabilities of numerical wave tanks. In order to assess the accuracy of such methods, a validation study was carried out. The study focuses on two types of numerical wave tanks: 1. A numerical wave tank based a non-linear potential flow algorithm. 2. A numerical wave tank based on a Volume of Fluid algorithm. The first algorithm uses a structured grid with triangular elements and a surface tracking technique. The second algorithm uses a structured, Cartesian grid and a surface capturing technique. Validation material is available by means of waves measured at multiple locations in two different model test basins. The first method is capable of generating waves up to the break limit. Wave absorption is therefore modeled by means of a numerical beach and not by mean of the parabolic beach that is used in the model basin. The second method is capable of modeling wave breaking. Therefore, the parabolic beach in the model test basin can be modeled and has also been included. Energy dissipation therefore takes place according to physics which are more related to the situation in the model test basin. Three types of waves are generated in the model test basin and in the numerical wave tanks. All these waves are generated on basin scale. The following waves are considered: 1. A scaled 100-year North-Sea wave (Hs = 0.24 meters, Tp = 2.0 seconds) in deep water (5 meters). 2. A scaled operational wave (Hs = 0.086 meters, Tp = 1.69 seconds) at intermediate water depth (0.86 meters) generated by a flap-type wave generator. 3. A scaled operational wave (Hs = 0.046 meters, Tp = 1.2 seconds) in shallow water (0.35 meters) generated by a piston-type wave generator. The waves are generated by means of a flap or piston-type wave generator. The motions of the wave generator in the simulations (either rotational or translational) are identical to the motions in the model test basin. Furthermore, in the simulations with intermediate water depth, the non-flat contour of the basin bottom (ramp) is accurately modeled. A comparison is made between the measured and computed wave elevation at several locations in the basin. The comparison focuses on: 1. Reflection characteristics of the model test basin and the numerical wave tanks. 2. The accuracy in the prediction of steep waves. 3. Second order effects like set-down in intermediate and shallow water depth. Furthermore, a convergence study is presented to check the grid independence of the wave tank predictions.

Integrity Assessment and On-Site Repair of a Floating Production Platform

Following the warning of a flooded bow horizontal brace of a semi-submersible production platform, an inspection diving team was mobilized and cracks were found at both bow and aft K-joints. Analysis of the service life of the platform, together with the results of structural analysis and local strain measurements, concluded that cracking was caused by fatigue initiated at high stress concentration points on the gusset plates inserted in the tubular joints. As a consequence of the fractured plates other cracks were nucleated close to the intersection lines of the braces that compose the K-joints. Based on this analysis different repair possibilities were proposed. To comply with the production goals of the Business Unit it was decided to repair the platform on-site and in production in agreement with the Classification Society. The proposed repair contemplated the installation of two flanges on the gusset plates between the diagonal braces by underwater wet (UWW) welding. Cracks at the gusset plates were also removed by grinding and wet welding. Defects located at the braces are being monitored and repaired by the installation of backing bars, by wet welding, followed by grinding and welding from the inside. To carry out the job two weld procedures and ten welder-divers were qualified.

Benchmarking of Truss Spar Vortex Induced Motions Derived From CFD With Experiments

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.

Influence of Pressure in Pipeline Design: Effective Axial Force

This paper discusses use of the effective axial force concept in offshore pipeline design in general and in DNV codes in particular. The concept of effective axial force or effective tension has been known and used in the pipeline and riser industry for some decades. However, recently a discussion about this was initiated and doubt on how to treat the internal pressure raised. Hopefully this paper will contribute to explain the use of this concept and remove the doubts in the industry, if it exists at all. The concept of effective axial force allows calculation of the global behaviour without considering the effects of internal and/or external pressure in detail. In particular, global buckling, so-called Euler buckling, can be calculated as in air by applying the concept of effective axial force. The effective axial force is also used in the DNV-RP-F105 “Free spanning pipelines” to adjust the natural frequencies of free spans due to the change in geometrical stiffness caused by the axial force and pressure effects. A recent paper claimed, however, that the effect was the opposite of the one given in the DNV-RP-F105 and may cause confusion about what is the appropriate way of handling the pressure effects. It is generally accepted that global buckling of pipelines is governed by the effective axial force. However, in the DNV Pipeline Standard DNV-OS-F101, also the local buckling criterion is expressed by use of the effective axial force concept which easily could be misunderstood. Local buckling is, of course, governed by the local stresses, the true stresses, in the pipe steel wall. Thus, it seems unreasonable to include the effective axial force and not the true axial force as used in the former DNV Pipeline Standard DNV’96. The reason for this is explained in detail in this paper. This paper gives an introduction to the concept of effective axial force. Further it explains how this concept is applied in modern offshore pipeline design. Finally the background for using the effective axial force in some of the DNV pipeline codes is given.

Evolution of Pipe Properties During Reel-Lay Process: Experimental Characterisation and Finite Element Modelling

The development of finite element analysis, in terms of simulation power and theoretical model accuracy, enables one to understand and simulate industrial processes more precisely, especially those involving non linear behaviour and analysis. Reeled pipe technology is one of these, and has a lot to gain from this increasing efficiency. In the reel-lay process the pipe is first reeled onto a drum on a vessel for transportation. During offshore installation the pipe is unreeled, straightened and deployed into the sea. During the process, the pipe is fully and cyclically plastified. Plastification modifies the pipe properties, which is not by itself detrimental but should be understood by the designer. Pipe properties are affected in three ways: geometrical shape – reeling and straightening induce some residual ovalisation; mechanical properties – yield stress, hardening slope, isotropy are modified; and fatigue properties. Technip and IFP have studied these property evolutions for many years, both from an experimental and a numerical point of view. The present paper discusses the first two points. A wide experimental programme has been performed. Full scale pipes were reeled and straightened on a bending rig device especially built for that purpose. Pipe ovalisation was monitored through the whole process. Pipe mechanical properties were also fully characterised in the pipe axial, hoop and thickness directions, both in tension and compression, before and after reeling process. Extruded and UOE pipes were tested and characterised. Pipe initial properties are dependent on the manufacturing process but they are modified by the reeling process. Reeling induces some anisotropy that cannot be properly accounted for by usual plasticity models. Finite element simulations with Abaqus software, using the material behaviour of unreeled pipe, underestimate stiffness evolution in the hoop direction and overestimate ovalisation induced by the reeling process. Anisotropy has indeed a great effect on ovalisation that results from an interaction between axial and hoop loading. Hardening is also a key parameter. A new plasticity model has been written in an Abaqus User Material Model, known as UMAT. The new model is based on an anisotropic Hill criterion and special attention is paid to the hardening. This new model reduces by more than two the error on ovality estimation, and gives a realistic prediction of material anisotropy evolution through the process. Although, the tuning of the model coefficients is more complex than for usual models, its use is quite straightforward and does not increase computation time.

Experimental and Numerical Study of Heave-Induced Lateral Motion (HILM)

Experiments were carried out at IFREMER Brest to obtain data on the displacement of Steel Catenary Risers (SCR) in the Touch-Down Zone (TDZ) induced by top motion. Measurements were conducted both in the section on the ground (2D motion) and in the section above the touch-down point (3D motion) with an optical tracking system. In the model test, the bottom 1/10 of the riser was represented at a scale of about 1/10. The present paper is focusing on Heave-Induced Lateral Motion (HILM) in the bottom part of the riser. The model riser is based on a full scale case which is first described. Based on Froude’s similitude, the geometrical characteristics of the model are derived. The tracking system and the various instrumentation are then detailed. In the last section, experimental results on one specific case are presented. Numerical calculations with a coupled fluid/structure solver are performed. A comparison for the amplitude and frequencies of HILM is presented. Comments are made on the analogy between the present experiments and some simpler experiments on a one-degree-of-freedom cylinder/spring system sinusoidally excited. Such an analogy should prove itself very fruitful in understanding and quantifying HILM.

Change of Mechanical Properties of High Strength Line Pipe by Thermal Coating Treatment

In strain-based design, the overmatch condition in the girth weld portion primarily must be maintained. The pipes may also be required to have a low yield to tensile (Y/T) ratio and a high uniform elongation (U.EL) in the longitudinal direction to achieve a high compressive buckling strain. However, change in the mechanical properties by heating during coating treatment has not been paid attention so much. Furthermore, how much the mechanical properties change is affected by production conditions is unclear. This study aims to clarify firstly the relation between the mechanical properties (Y/T ratio, U.EL etc.) and the microstructure and secondly the change in mechanical properties by thermal coating treatment. The Y/T ratio and U.EL are affected by the volume fraction of ferrite and the secondary phase, which are changed by thermomechanical control processing (TMCP) conditions. For example, use of dual phase microstructure is very effective for decreasing the Y/T ratio and increasing the U.EL as the pipe. On the other hand, yield strength (YS) rises and the U.EL does not change after coating. The increase in the YS after coating is influenced by the microstructure and TMCP conditions. Resultantly, dependence of the Y/T ratio on the microstructure and TMCP conditions is reduced for line pipes after thermal coating treatment.