Experimental Investigation of Power Umbilical Damping

Ocean currents may cause vortex induced vibrations (VIV) of deep-water umbilicals and cables. Since the VIV response may give significant contributions to the total fatigue damage it is important to know the structural damping for relevant curvature levels. A laboratory test has been performed on a 12.5 m long test specimen to determine the damping for a range of curvature levels that are in the vicinity of the stick-slip transition region. The energy input to maintain steady state oscillations with curvature amplitudes in the range 0.0002–0.001 m−1 was measured. The steady state energy input is consistent with damping ratios obtained using the free decay method. The structural damping depends on construction temperature and curvature and is less for typically low seawater temperature and low curvatures. The transition between the stick- and the slip regime is seen for typical seawater temperature.

Subsea Production Layout: Design and Cost

The development of a subsea field implies a complex design procedure with very high costs involved. The combination of low oil price, harsh environment, very deep waters and high drilling costs has emphasized the need for new ideas to reduce both CAPEX and OPEX. The paper considers different subsea layout scenarios of a typical deep water field. Each scenario is optimized considering pipeline length, equipment cost and flow assurance. The software QUE$TOR is then applied to estimate the cost of each optimized scenario. Two of them are selected for further studies, FPSO connected to six slots subsea manifolds, and subsea separator connected to eight slots cluster manifolds with tie-back to an existing platform. The latter representing a hybrid solution based on the subsea-to-shore concept. Cost and technical feasibility are considered in order to evaluate the two scenarios.

Modelling of Complex DEH Systems

Direct Electric Heating (DEH) is an increasingly attractive method for flow assurance that has been in use for more than 15 years. All current systems in operation consist of a single flowline or have subsea architecture such that interaction between systems as well as components in the proximity is limited. DEH is selected as flow assurance for two ongoing field developments where the layout is such that there will be interaction with other subsea architecture. One system has numerous flowline systems in close interaction and the other has unfavorable crossing. This paper presents some of the design aspects which do arise for fields where the flowlines are in close proximity to each other. The paper also discusses how to handle interference with existing infrastructure when installing DEH in a complex field layout. The aspects studied are effect on power rating, ac corrosion and thermal rating of cables and flowlines.

Effect of Weld Geometry on the Fatigue Behaviour of Umbilical Super Duplex Stainless Steel Tubes

The fatigue behavior of welded components is complicated by many factors intrinsic to the nature of welded joints. The mechanical properties of the material, the welding process and position, the type and geometry of the weld and the residual stress distribution across the weld are a few factors affecting fatigue behavior. Published studies [1, 2] have shown that weld geometry is significantly important in determining the fatigue strength of the weld. For a given weld geometry, the fatigue strength is determined by the severity of the stress concentration at the weld toe or at weld defects and by the soundness of the weld metal. The effect of external weld geometry profile on the fatigue behavior of welded small bore super duplex umbilical steel tubes is investigated. Root cause analysis consisting of fractography, metallography and weld profile measurement is carried out on pairs of fatigue failure samples which were tested at the same stress range but failed at significantly different number of cycles. The samples are selected from Technip Umbilicals Ltd (TU) fatigue database. Following the failure analysis, weld geometric profile measurements are performed on fatigue test samples that were prepared for testing. The weld profile was measured in terms of the external weld cap height, weld width and external linear misalignment. Axial fatigue tests are carried out on these samples which are pre-strained before test to simulate the plastic bending cycles typically experienced during the manufacturing and installation processes prior to operational service. The fatigue tests results are interrogated together with the measured geometric data to identify trends and anomalies. Key weld geometric fatigue performance criteria are subsequently identified. For the welded super duplex stainless steel (SDSS) tubes studied, the height of the weld and the weld toe angle provided the best correlation with fatigue life — shorter lives were obtained from specimens with the highest weld aspect ratio (weld height to width) and lowest weld toe angle.

The Effect of Stick Stiffness of Friction Models on the Bending Behavior in Non-Bonded Flexible Risers

This paper investigates the effect of stick stiffness on the bending behavior in non-bonded flexible risers. The stick stiffness was normally implemented in the friction model for calculating the friction stress between layers in such structures. As the stick stiffness may be too small to achieve the plane-surfaces-remain-plane assumption under low contact pressure in some friction models [1], a new friction model was proposed for maintaining the constant stick stiffness in the present work. Less stick stiffness than that obtained by the plane-surfaces-remain-plane assumption was observed in test data. It was assumed that the stick stiffness reduction is caused by shear deformation of plastic layers. A numerical study on stick stiffness by including the shear deformation effect was carried out and verified against full scale tests with respect to the bending moment-curvature relationship.

Deep Water XLPE Cable With Aluminum Conductor: Risk of Stress Induced Electrochemical Degradation (SIED)

One of the challenges with cables in deep waters is the high topside tension caused by the weight of the cable and the risk of overloading the conductor. Keeping the cable weight as low as possible is therefore a key parameter to reach deep waters. One way to reduce the cable weight is to replace the copper conductor in the cable with an aluminium conductor. When using aluminium conductor in a wet design XLPE cable (no water barrier) there is a risk of local corrosion of aluminium that forms porous structures in the inner semiconductor, leading to accelerated ageing of the XLPE insulation. This phenomenon is known as Stress-Induced Electrochemical Degradation (SIED). SIED has so far only been seen in service-aged cables, which had been exposed to external mechanical damage causing water penetration in the conductor. It has also been found that the deterioration of the inner semicon is more severe when the cable is mechanically stressed. In order to learn more about the risk of SIED in a wet XLPE cable an extensive ageing test program has been carried out. This paper summarizes the results from this testing.

Flexible Risers Lifetime Extension: Riser In-Service Monitoring and Advanced Analysis Techniques

The demand for the lifetime extension of flexible pipes is increasing due to the need to extend the lifetime of the existing production fields. There have been many challenges with the lifetime extension of flexible pipes after the end of the initial design service life due to the inherent conservatism with the common analysis approach, safety factors and operation beyond the design limits. A lifetime assessment should be performed on flexible risers for re-qualification during the original design life if the design envelope is exceeded or there is a need for lifetime extension. Hence, a systematic approach for lifetime assessment execution is established to determine the integrity level of the flexible risers and define the recommended actions required, such as mitigations, repairs or monitoring to maintain an acceptable risk for the required extended service life based on consistent methodology. The primary objective of this paper is to present a riser integrity management field-proven technology to monitor the riser’s behaviour in-service in addition to the advanced analyses guidelines to form a basis for the lifetime extension of flexible risers. The primary objective for the integrity management is to manage and control the risk of failure by detecting failure at an earlier stage when preventive action can be taken to avoid failure propagation. In addition, it is demonstrated that the primary hot-spots for the dynamic behaviour and fatigue life assessments of the flexible risers are primarily in bend stiffener regions and the touchdown zone (TDZ) due to large tension fluctuations caused by vessel motions and cyclic movement in the TDZ. Therefore, analysis techniques have been developed in two primary areas: advanced bend stiffener modelling using pipe-in-pipe (PIP) to model the sliding friction and the bend stiffener/flexible pipe’s annular space and flexible pipe–seabed interaction modelling using a non-linear seabed model. Therefore, the flexible riser’s lifetime extension assessment will be based on more reliable models that reflect the realistic and dynamic behaviour of the flexible risers. Consequently, these advanced analysis techniques can be used for new designs or lifetime extension of flexible pipes.

On the Feasibility of Using Underwater Acoustic Data Transmission for Subsea Equipment Monitoring

The present work uses the BELLHOP ray tracing model to simulate an acoustic propagation channel in a deep water environment in order to analyze its viability to provide data transmission for monitoring submarine equipment. The simulated scenario is located in the Campos Basin, Rio de Janeiro, on the Brazilian coast, responsible for more than 80% of Brazilian oil and gas production. Temperature and salinity data from five stations were used to calculate the sound speed profiles required to the transmission loss simulations of the acoustic propagation channel. In order to estimate the signal detection capacity according to the medium characteristics, a characterization of the parameters that influence the physical propagation channel was performed. The parameters of three modem models with different operation frequencies were selected and analyzed in order to obtain the Signal to Noise Ratio (SNR) of the transmission signal.

An Experimental and Numerical Investigation of the Effect of Axial Thermal Gradients in Flexible Pipes

This paper discusses the structural challenges associated with high axial temperature gradients and the corresponding internal cross section forces. A representative flexible pipe section designed for high operational temperature has been subject to full scale testing with temperature profiles obtained by external heating and cooling. The test is providing detailed insight in onset and magnitude of relative layer movements and layer forces. As part of the full-scale testing, novel methods for temperature gradient testing of unbonded flexible pipes have been developed, along with layer force- and deflection-measurement techniques. The full-scale test set-up has been subject to numerous temperature cycles of various magnitudes, gradients, absolute temperatures, as well as tension cycling to investigate possible couplings to dynamics. Extensive use of finite element analysis has efficiently supported test planning, instrumentation and execution, as well as enabling increased understanding of the structural interaction within the unbonded flexible pipe cross section. When exploiting the problem by finite element analysis, key inputs will be correct material models for the polymeric layers, and as-built dimensions/thicknesses. Finding the balance between reasonable simplification and model complexity is also a challenge, where access to high quality full-scale tests and dissected pipes coming back from operation provides good support for these decisions. Considering the extensive full scale testing, supported by advanced finite element analysis, it is evident that increased attention will be needed to document reliable operation in the most demanding high temperature flexible pipe applications.

Consistent Free Span VIV Fatigue Analysis of Flexibles

The fatigue life assessment of free spanning flexible products, such as subsea cables and umbilicals, due VIV requires special attention to the structural properties due to the stick/slip behaviour of helix elements in bending. Essential parameters for assessment of stick/slip effects in free span VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behaviour in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behaviour in bending is essential for fatigue life predictions. The consistent evaluation of stick/slip behaviour requires more sophisticated calculation procedures due to the non-linearity it introduces. Hence, industry practice has been to consider simplified, linear calculation procedures. However, future flexibles utilization may be much benefitted by a consistent stick/slip treatment in free spanning VIV fatigue assessments, as it may allow for longer allowable free span lengths or longer fatigue life. The overall objective of the paper is to establish a consistent free span VIV analysis methodology for flexibles in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’ and the overall philosophy of DNV-RP-F105 ‘Free Spanning Pipelines’. A consistent fatigue analysis scheme for VIV in free spans is outlined using commercially available state-of-the-art computer programs for free span VIV response analysis (FatFree) and cross section stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies in a complex long-term current loading environment. It is shown that consistent treatment of the mechanical properties of flexibles is essential for VIV fatigue life assessments of free spans.