Experimental Research on Damages Detection of Pipeline Structure by Using PZT-Based Waves

Pipeline structure may generate damages during its service life due to the influence of environment or accidental loading. The damages need to be detected and repaired if they are severe enough to influence the transportation work. Non-destructive detection using smart materials combined with suitable diagonal algorithms are widely used in the field of structural health monitoring (SHM). Piezoelectric ceramics (such as Lead Zirconate Titanate, PZT) is one of the smart materials to be applied in the SHM due to the piezoelectric effect. So far, the PZT-based wave method is widely used for damage detection of structures, in particular, pipeline structures. A series of piezoelectric patches are bonded on the surface of the pipeline structure to monitor the damages such as local crack or effective area reduction due to corrosion by using diagonal waves. The damage of the pipeline structure can be detected by analysis of the received diagonal waves which peak value, phase, and arriving time can be deferent from the health ones. The response of the diagonal wave is not only correlated to the damage location through estimation of the arrival time of the wave peak, but also associated with the peak value of the wave for the reduction of wave energy as the guided wave passing through the damages. Therefore, the presence of damages in the pipeline structure can be detected by investigating the parameter change of the guided waves. The change of the wave parameters represents the attenuation, deflection and mode conversion of the waves due to the damages. In addition, the guided wave has the ability of quick detecting the damage of the pipeline structure and the simplicity of generating and receiving detection waves by using PZT patches. To verify the proposed method, an experiment is designed and tested by using a steel pipe bonded the PZT patches on the surface of it. The PZT patches consist of an array to estimate the location and level of the damage which is simulated by an artificial notch on the surface of the structure. The several locations and deep heights of the notches are considered during the test. A pair of the PZT patches are used at the same time as one is used as an actuator and the other as a sensor, respectively. A tone burst of 5 cycles of wave shape is used during the experiment. A wave generator is applied to create the proposed waves, and the waves are amplified by an amplifier to actuate the PZT patch to emit the diagonal waves with appropriately enough energy. Meanwhile, the other PZT patch is used as a sensor to receive the diagonal signals which contain the information of the damages for processing. For data processing, an index of root mean square deviation (RMSD) of the received data is used to estimate the damage level by compare of the data between the damaged and the health peak valves of the received signals. The time reversal method which aimed at increasing the efficiency of the detection is also used to detect the damage location by estimating the arrival time of the reflected wave passing with a certain velocity. The proposed method experimentally validates that it is effective for application in damage detection of pipeline structure.

Mechanical Qualification of Dynamic Deep Water Power Cable

There is an increasing demand for subsea electrical power transmission in the oil- and gas industry. Electrical power is mainly required for subsea pumps, compressors and for direct electrical heating of pipelines. The majority of subsea processing equipment is installed at water depths less than 1000 meters. However, projects located offshore Africa, Brazil and in the Gulf of Mexico are reported to be in water depths down to 3000 meters. Hence, Nexans initiated a development programme to qualify a dynamic deep water power cable. The qualification programme was based on DNV-RP-A203. An overall project plan, consisting of feasibility study, concept selection and pre-engineering was outlined as defined in DNV-OSS-401. An armoured three-phase power cable concept assumed suspended from a semi-submersible vessel at 3000 m water depth was selected as qualification basis. As proven cable technology was selected, the overall qualification scope is classified as class 2 according to DNV-RP-A203. Presumed high conductor stress at 3000 m water depth made basis for the identified failure modes. An optimised prototype cable, with the aim of reducing the failure mode risks, was designed based on extensive testing and analyses of various test cables. Analyses confirmed that the prototype cable will withstand the extreme loads and fatigue damage during a service life of 30 years with good margins. The system integrity, consisting of prototype cable and end terminations, was verified by means of tension tests. The electrical integrity was intact after tensioning to 2040 kN, which corresponds to 13 000 m static water depth. A full scale flex test of the prototype cable verified the extreme and fatigue analyses. Hence, the prototype cable is qualified for 3000 m water depth.

COOL™ Hose Qualification Process of the First EN1474-2 LNG Floating Hose

In 2005, SBM Offshore recognized that in future offshore Floating LNG Production Units, key enabling technologies would be required to ensure the safe and reliable tandem offloading to LNG Carriers. The foundation was thus laid down for the development of the COOL™ Hose: A cryogenic marine floating hose that would enable the tandem offloading between two vessels offshore. This paper presents, after an introduction to the COOL™ Hose design (Hose in Hose concept-HiH), the different steps in the qualification process following the EN1474-2 [1] guidelines and recommendations for design and testing of LNG transfer hoses. This qualification process has been endorsed by two Classification Societies ABS and DNV and has resulted in a Certificate of Fitness for Service from DNV and a Product Design Assessment Certificate from ABS making the SBM COOL™ Hose, the first EN1474-2 qualified LNG floating hose.

Quick Connection Couplings in the Permeated Gas Relief Systems of Flexible Riser End Fittings

The objective of the paper is to present a new conception for the permeated gas relief system of flexible riser end fittings. The new conception is based on a multi-function gas relief system, depending on the requirements established by the operators. The main function of the system is to relief the accumulated gas in the riser annulus. Due to the interconnection of the annulus to the external environment through the top end-fitting, other functions of this system may be required such as to allow fluid injection into the annulus or vacuum testing, for example. The presented conception of the permeated gas relief system is based on the usage of quick connection and disconnection couplings. It was firstly developed focusing the particular cases of submerged top end fittings but it can be applicable in both situations where the riser top end fitting is located either at an emerged or a submerged position. In the course of its development, the conception has led to different patterns of the system, depending on the different required functions. The gas relief system was conceived in order to allow — in an easy and quick manner, without the risk of flooding the annulus with seawater — the execution of typical operations like those required for installing a monitoring and control system, for injecting fluids into the annulus, for vacuum testing or, additionally, for removing any damaged relief valve from the end fitting in order to substitute it for a new one. The paper describes the components of the gas relief system for each required function. The idea is to have many interchangeable items of standardized sets that meet specific requirements, case by case. The performance of the permeated gas relief system is an important issue for flexible riser integrity.

Rayleigh Damping Effects on Global Analysis of Umbilicals

Despite global analysis of umbilicals is a well-known area in the offshore systems design, some topics are still opened for discussions. One of these topics refers to the structural damping. Obviously, the viscous damping caused by hydrodynamic drag forces is the major source of damping to the whole system. However, in some severe load cases, the host vessel dynamics may induce high snatch loads to the umbilical top end and these loads are more related to structural damping, specifically in tension–elongation hysteresis, than to viscous damping. The snatch loads must be taken into account in the whole design process, which leads to an umbilical designed to resist to higher tension loads and implies also, in most cases, in over-dimensioned accessories, such as the bending limiters. Actually, due to the high level of friction between layers, the umbilical presents some level of structural damping which is, in fact, related to hysteretic moment-curvature and tension-elongation relations. This intrinsic structural damping may in fact contribute to the reduction of the snatch loads and considering it may reduce the level of conservatism in the design. However, due to the complexity and diversity of umbilical designs, it is not straightforward to come up with general-use hysteretic curves. A simplification then is to apply classic Rayleigh damping. Typically, damping levels of 5% are accepted in the offshore industry when using stiffness-proportional Rayleigh damping (the 5% damping is a percentage of the critical damping and is accounted for at the regular wave period or irregular wave spectral peak period). The problem here is that stiffness-proportional Rayleigh damping increases linearly with the frequency and the damping level at 1Hz, for example, may get to 60%. This fact indicates that the high-frequency part of the response may be simply discarded from the results, which in turn may lead to an incorrect, over-damped analysis. The present work aims tackling the Rayleigh damping issue, evaluating its effects on tension levels and spectral density of the tension time history. A recommendation of how to apply Rayleigh damping is proposed.

Pipeline On-Bottom Stability Analysis Based on FEM Model

Pipeline on-bottom stability is one of the sophisticated problems in subsea pipeline design procedure. Due to the uncertainty of the pipe-soil interaction and environment loads, including wave, current, or earthquake, etc., it is classified as the typical nonlinear problem. The Finite Element Method is introduced into pipeline engineering several years ago. More and more special engineering software such as AGA, PONDUS are available in market. However, when doing a project, some abnormal data was found when compared the DnV calculation results and AGA. In order to know the behavior of pipeline on seabed under wave and current load, finite element method – ABAQUS is introduced to do this analysis. The ABAQUS/explicit is used to simulate 600s pipeline dynamic response. The pipeline is supposed to be exposed on seabed and the selected seabed model is large enough to avoid the edge effect. ABAQUS calculation results are compared with the requirements in DnV rules to verify the validity of finite element model.

Challenges Faced in the Design of SLWR Configuration for the Pre-Salt Area

With the recent discoveries of the pre-salt reservoir, new areas of the Brazilian coast rose to prominence, especially for the Santos Basin. This area is adjacent to the Campos Basin, which now accounts for around 80% of Brazilian production. In this new area, in addition to the difficulties of drilling in salt rock, the deployment of subsea production systems have also to overcome new challenges, since environmental conditions are more severe than those in the Campos Basin. Other important issues are: the water depth of about 2200 meters; the high pressure for gas injection riser; and the high CO2 content, requiring special attention to the materials that will be in contact with the production fluid. At this new production frontier, priority was given to the use of floating units with storage capacity like VLCC hulls, in order to export oil production through shuttle tankers, as no pipeline grid is available. Depending on the motions level of these VLCC vessels, the selection of a viable configuration of riser becomes crucial. Thus, some alternatives have been studied and the Steel Lazy Wave Riser (SLWR) configuration was one of the options considered to be used for production and gas injection riser functions, besides being possibly used for risers with large diameters. As this area of the Santos Basin presents more severe conditions, the free-hanging configuration (SCR) was not feasible, even with the use of VLCCs with optimized motions. In this case, the SLWR configuration was better suited to overcome the problems faced by free-hanging configuration. This paper aims to present a set of variables and its right combination involved in SLWR configuration to make it feasible, considering some key points in the design of SLWRs, for example: motions level of the floating unit, thermal insulation required for the flow assurance of production risers; difficulties faced during the installation process and the need of using clad pipes or lined pipes due to the high level of corrosion imposed by CO2 fluid content.

Inline and Vertical Wave Force Variation due to Burial of Submarine Pipeline in Random Wave Fields

Marine pipelines encounter significant dynamic forces due to the action of waves. In order to reduce such forces, they are buried below the seabed. The wave force on the pipeline at any depth of burial for the given hydrodynamic condition depends on the properties of the sea bed soil. Physical model is used for assessing the hydrodynamic force on the pipeline for a wide range of random wave conditions, for different burial depths and in four types of soils. It is found that for all the four soil types, the horizontal force reduces with increase in depth of burial, whereas the vertical force generally increases up to certain depth of burial, mainly due to the significant change in the magnitude as well as the phase lag between the pore water pressures in the vertical direction. Among the soils, well graded soil is good for half burial of pipeline, since the least vertical force occurs for this soil. On the other hand, uniformly graded and low hydraulic conductivity soil attracts the maximum vertical force for half burial. On the other hand, such soil is good for full burial or further increase of burial, since it attracts less vertical force when compared to the other soils. The results of this study will help the submarine pipeline design engineers to select the minimum safe burial depth in a range of cohesion-less soil.

Development of Method/Apparatus for Close-Visual Inspection of Underwater Structures (Especially Pipelines) in Muddy and Unclear Water Condition

Ordinarily, the Remotely Operated Vehicles and underwater divers, even with modern illuminating lamps, would be unable to observe objects clearly in muddy or unclear underwater condition. Efforts have therefore been made to demonstrate that in such underwater condition, it is possible to perform visual inspections and observations adequately and reliably for underwater leaking structures using novel equipment. The novel equipment works by simply supplying a clear laminar flow of water which flows over the surface of the structure to be observed. A camera eye is then placed to observe through the steady flowing clear water. Different configurations of the equipment were checked and it was found that the equipment with fitted valves installed in the flooding box in-line with flowing clear water produced the best result. The volume of water required for the observations appears constant and independent of the depth of water except during the first initial stage of flooding. On the other hand, the period of time required for clear observations increases with increase in water depth. The performance of the equipment was found independent of the nature of underwater visibility. The benefits of this work ranges from leaking structures’ close-visual inspection including leaking pipelines, to subsea pipeline field joint wrap damage inspection for beach pulls in cofferdams. This technique is considered cheap, robust and flexible.

On the Structural Response of a Flexible Pipe With Damaged Tensile Armor Wires

This paper studies the structural response of a 6.0″ flexible pipe under pure tension considering two different situations: the pipe is intact or has five wires broken in its outer tensile armor. A three-dimensional nonlinear finite element model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each wire of the tensile armors and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests are also presented in order to validate the theoretical estimations. The theoretical and experimental results indicate that the imposed damage reduced the axial stiffness of the pipe. High stress concentrations in the wires near the damaged ones were also observed and, furthermore, the stresses in the inner carcass and the pressure armor are affected by the imposed damage, but, on the other hand, the normal stresses in the wires of the inner tensile armor are not.