Scott T. Slocum

Numerical Analysis of Experimental Data of Subsea Jumper Vortex Induced Vibrations

Vortex Induced Vibrations (VIV) may cause fatigue damage to subsea jumpers that are exposed to bottom currents. ExxonMobil Upstream Research Company (URC) has been conducting research on VIV of subsea jumpers since 2011. Model tests conducted on an “M” shaped jumper in 2012 showed that VIV can occur in subsea jumpers over a wide range of bottom current speeds and directions [1]. Presently, there is no well-established industry practice for assessing subsea jumper VIV and determining the need for its suppression. In this paper, we present two different methods for characterizing jumper VIV response based on the model test data described in [1]. Specifically, these methods consist of spectral analysis of local response and modal scalar analysis of global response. These methods are used to analyze measured response over a wide range of towing speeds and towing directions. A brief summary of the findings are provided and some general conclusions are drawn.Copyright

Mooring Analysis of a Turret Moored FPSO in a Squall Environment

The transient dynamic response of a FPSO in a squall environment is dependent on several input parameters. Because the response’s dependence on these input parameters is unclear prior to performing the analysis, a large number of parameter combinations need to be considered to find the combination that gives a worst-case load or response as required by reference [1]. Because the required time-domain simulations are computationally intensive, there is often a practical need to limit the number of simulations that are performed, raising questions about how many are necessary to meet the analysis objectives.This study investigates the effect of different squall scenarios on a turret moored FPSO in the West African offshore environment. A large number of cases with selected vessel headings, squall types, squall approach directions and vessel drafts are studied and parameters affecting the critical mooring loads and turret positions are identified. Possible reductions in the load case matrix along with a sensitivity study of a few parameters affecting the results are also discussed.Copyright

Determination of Wave Impact Loads for the Hebron Gravity Based Structure (GBS)

ExxonMobil Canada Properties and its co-venturers are building a gravity based structure (GBS) in Newfoundland and Labrador to be installed on the Hebron Field offshore Eastern Canada. This area is characterized by harsh storms with large waves and high winds. The geometry of the Hebron GBS has an effect on the behavior of the incident waves with regards to their likelihood of breaking onto the shaft. Model tests of the structure in storm waves were executed to provide local wave impact load data on the shaft of the GBS. These tests required significant planning and design of the model, environment, and instrumentation in order to properly satisfy the test objectives. The results of the test showed that the measured wave impact loads on the structure were highly variable, requiring a long-term, response based method to quantify the design loads on an annual exceedance basis. In this paper, we discuss the salient aspects of the model testing effort and the long-term analysis approach which was utilized to define the wave impact loads that were incorporated into the Hebron GBS structural design.Copyright

Cryogenic Structural Performance of Corrugated Pipe

One alternative to developing offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG using tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a cryogenic hose, whose main body is a vacuum insulated, pipe-in-pipe hose made of corrugated stainless steel pipe (c-pipe) and flanged terminations. Given the novelty of the transfer system, ExxonMobil conducted an experimental program to understand the structural performance of the basic c-pipe under static and cyclic loading at room and cryogenic temperatures. This paper discusses overall qualification issues and presents the experimental methodology and results of structural performance tests of the full-scale c-pipe at both ambient and cryogenic temperatures. Fourteen full-scale, c-pipe static tests are reported, including tension, compression, bending, torsion, and internal pressure. In addition, 11 axial and three pressure fatigue tests are presented. One key result is that, overall, cryogenic temperature improves structural performance for the limit states tested, indicating that future qualification at room temperature would be sufficient. Moreover, the fatigue performance at both ambient and cryogenic temperatures surpassed the design curve reported in the literature for c-pipe.Copyright

Prediction of VIV for Risers With Effective Strakes

Accurate prediction of straked riser response can be important for determining the minimum strake coverage needed to ensure adequate riser fatigue life in demanding high-speed current environments. Previously published results from ExxonMobil’s 2003 vortex-induced vibration (VIV) model tests on a long, flexible, straked cylinder showed that strakes can greatly reduce, but may not entirely eliminate flow-induced vibration. The data also showed that the low-amplitude vibrations that persist when the cylinders are straked are different in character from VIV on bare cylinders, suggesting that the strakes are effective in disrupting the alternating vortex shedding mechanism that excites bare-cylinder VIV and that other flow mechanisms may drive the straked-cylinder vibration. This further implies that classic VIV models that have been developed to predict bare-pipe response due to alternating vortex shedding may not be accurate for straked cylinders even if the excitation model (“lift curve”) is adjusted to represent lower excitation levels on strakes.Classic VIV prediction methods for bare risers rely on special-purpose theoretical models to account for the strongly nonlinear interactions between bare cylinder motion and hydrodynamic excitation. However, for the low amplitudes of vibration observed in tests with straked cylinders, the influence of the cylinder motion on the flow may be less important. In an effort to more accurately predict the performance of strakes, ExxonMobil has recently explored a new approach for predicting flow-induced vibrations for straked risers that neglects any influence of riser motion on excitation, and instead assumes that hydrodynamic excitation can be approximated to useful accuracy by excitation measured on a fixed straked cylinder. New experiments show that this excitation is stochastic, with broad band-width, and that it conforms to relatively simple scaling laws.The measured stochastic excitation data has been used with a conventional linear random vibration model to predict the response data acquired from the 2003 model tests on the long, flexible, straked cylinder. Comparison shows a dramatic improvement in prediction scatter compared to predictions using a classic bare-pipe VIV formulation with a “lift curve” model of the strake hydrodynamic excitation.Copyright

Stress Analysis of a Cryogenic Corrugated Pipe

One method to develop offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG via tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a flexible cryogenic hose whose main body is a pipe-in-pipe hose made of two concentric corrugated 316L stainless steel pipes (C-pipe) with flanged terminations. Thermal insulation is achieved by maintaining vacuum between the inner and outer corrugated stainless steel pipes. In addition, the hose assembly contains two outer layers of helical armor wires to sustain the axial load. Given the complexity and novelty of the transfer system, a finite element study was performed on the inner C-pipe — the critical fluid containment layer. The effects of strain hardening of corrugations due to cold forming and temperature were modeled. Finite element (FE) analyses of the C-pipe under axial, bending, and internal pressure loading were carried out to evaluate global load-deformation and local stress responses. Comparisons of full-scale tests at room and cryogenic temperatures to simulation predictions including the novel material model showed good agreement. However, fatigue life predictions for the C-pipe that were based on local stresses and sheet metal fatigue S-N curves did not agree with the full-scale fatigue test results. The results indicated that the spatial variation in strain hardening due to corrugation forming and biaxial local stresses during pipe deformation could play important roles in the fatigue response of the C-pipe.Copyright

Determination of Viscous Damping for Low Frequency Motions of Floating Structures

ExxonMobil Upstream Research Company developed an advanced model test method to determine reliable damping values for predicting low frequency motions of an FLNG barge and an LNG carrier [1]. An inertial compensation system was introduced in the test to confidently isolate the relatively very small viscous damping force from the total measured forces in the forced oscillation tests. In the system, the spring stiffness in the restoring mechanism was tuned such that the test was done near resonance. This method has been successfully applied to ExxonMobil forced oscillation tests to measure damping of deeply submerged, double body models. Three types of motions were generated in the tests: sinusoidal motions, decay motions and motions with multiple frequencies. In this paper, the authors attempt to correlate the damping obtained from decay tests and from tests with motions of multiple frequency components. Findings from this work help determine damping for predictions of full scale motion in irregular waves.Copyright

An Advanced Test Method to Determine Viscous Damping of Floating Structures

ExxonMobil Upstream Research Company developed an advanced model test method to determine reliable damping values for predicting low frequency motions of an FLNG barge and an LNG carrier. Since viscous damping forces are a very small portion of the total force on the model, how to separate the viscous forces from the total forces is the key technical challenge. To better isolate viscous damping forces, an inertial compensation system consisting of springs was employed in the test. The spring stiffness was designed such that the restoring force cancelled the large inertial loads at the oscillation frequency. Furthermore, double-body models were built and were deeply submerged to minimize surface wave damping. With such an experimental setup, the total force measured was mainly the viscous damping force. Viscous damping was derived from the measured force and motion time histories.© 2010 ASME