Stergios Liapis

Free‐surface flow over a semi‐circular obstruction

The fully non-linear free-surface flow over a semi-circular bottom obstruction was studied numerically in two dimensions using a mixed Eulerian–Lagrangian formulation. The problem was solved in the time domain that allows the prediction of a number of transient phenomena, such as the generation of upstream advancing solitary waves, as well as the simulation of wave breaking. A parametric study was performed for a range of values of the depth-based Froude number up to 2.5 and non-dimensional obstacle heights, α up to 0.9. When wave breaking does not occur, three distinct flow regimes were identified: subcritical, transcritical and supercritical. When breaking occurs it may be of any type: spilling, plunging or surging. In addition, for values of the Froude number close to 1, the upstream solitary waves break. A systematic study was undertaken to define the boundaries of each type of breaking and non-breaking pattern and to determine the drag and lift coefficients, free-surface profile characteristics and transient behavior. Copyright

A time-domain method to evaluate the use of moving weights to reduce the roll motion of a ship

Abstract A method to evaluate the use of actively controlled moving weights on board ships to reduce roll motion is developed. The weights can simulate in principle anti-roll-tank systems, or they can be considered a possible anti-roll device in their own right. The ship, the moving weight, and the control device are considered components of a single dynamic system. The full eight-degree-of-freedom set of coupled governing equations for the complete dynamic system is derived. And a three-degree-of-freedom non-linear approximation for the roll motion only (MOTSIM) is derived from these eight equations. The reduced set of equations is used to determine the influence of various parameters and to evaluate control strategies. A PID controller is developed to command the position of the weight and a servomechanism model is used to predict its actual position. Then, the moving-weight system is incorporated into LAMP (Large–Amplitude–Motion Program), a computer code that integrates the governing equations of the sea and the motion of the ship interactively and simultaneously and predicts the motion of the ship in the time domain. A comparison of the results from the two simulations shows that there is fairly good correlation between the simple and complex models, but the simple model is a little optimistic in predicting the effectiveness of the moving-weight system. The results predict that the moving-weight system can be an excellent roll-suppressing device when the moving weight is as small as 1% of the displacement of the ship and the maximum distance the weight moves is as small as 15% of the half-beam.

Three-dimensional analysis of submerged, moored, horizontal, rigid cylinders used as breakwaters

Wave attenuation by moored cylinders is considered. The cylinders are submerged with their axes horizontal. Linear potential theory is applied. Three-dimensional motions of the cylinders subjected to normal and oblique monochromatic waves are determined using potential theory and a boundary integral method. Each cylinder has length 9.1 m (30 ft) and radius 1.5 m (5 ft), with its top 1.5 m (5 ft) below the still water line and its bottom 3.0 m (10 ft) above the seabed. Free-surface elevations are obtained for a single cylinder and for two cylinders in series. These configurations are effective wave barriers for a range of wave frequencies and incident angles.

Subsea Wellhead and Riser Fatigue Monitoring in a Strong Surface and Submerged Current Environment

A drilling campaign was recently undertaken by Shell Oil Company in a region with high surface and submerged currents. The water depth ranged from 5500-7000 ft at the various well sites in the region. Strong surface currents with maximum speeds of 4.5-5.0 knots were measured using an Acoustic Doppler Current Profiler (ADCP). In addition, submerged currents with maximum speed of around 1.5 knots were recorded. High fidelity Subsea Vibration Data Loggers (SVDLs) were used to monitor the in-situ riser and BOP stack vibrations due to the arduous current environment, as well as wave and vessel-driven motions. A semi-analytical method was developed to estimate wellhead fatigue damage directly using the measured BOP stack motion data. High quality vibration data from the SVDLs were used in conjunction with analytical transfer functions to directly compute stress time histories and S-N fatigue damage at any location of interest in the conductor/wellhead/BOP system. The method was utilized in a larger fatigue reconstruction scheme that was applied to subsea wellhead and riser fatigue monitoring activities during drilling operations in the region. ADCP data was correlated to the SVDL data to determine the source of vibrations at low and high frequencies. Simultaneous ADCP and SVDL data were also used to calibrate SHEAR7 v4.2 parameters. In between SVDL deployments, wellhead and riser stress and fatigue values were determined using the calibrated SHEAR7 models, driven by the measured current profiles. Wellhead motions were tabulated from ROV video and used to validate vibration reconstruction from the SVDL data and predictions from SHEAR7 simulation. Using these methods, stress and fatigue life consumption estimates are robust to unavailability of ADCP data and/or ROV video and/or data from one or more SVDLs. Normalized vibration, stress and fatigue consumption are presented over the riser deployment period. It was found that moderate speed submerged currents, which extend over a broad range below typical fairing depths, lead to significantly higher wellhead stress and fatigue life consumption rate compared higher speed surface currents. The sensitivity of a typical wellhead and BOP stack to lower-frequency vibrations was examined. It is shown that because the submerged currents are of a lower speed, they excite modes that are closer in frequency to the “flagpole” mode of the casing/wellhead/BOP subsystem, leading to higher wellhead motion and stress. The methods introduced herein provide rapid turn-around of raw data to fatigue consumption, enabling informed decisions to be made in adverse conditions. The methodology is easily extendable to real-time fatigue monitoring using a cabled system or acoustic modem to transmit data to the surface. In addition, the significance of regional submerged currents for wellhead stress and fatigue is highlighted, as well as considerations for vibration mitigation.

An Experimental Investigation of Roll Motions of an FPSO

FPSO roll motions can be major contributor to riser fatigue. This is especially true in regions where wind, waves and currents are non-collinear. Roll motions as high as 23 degrees have been reported in the Campos Basin. The most common roll mitigation strategy consists of adding bilge keels to the FPSO. Motivation for this work came from a need to develop a better understanding of roll motions as a function of bilge keel width. In addition to roll motions, the hydrodynamic forces on the bilge keels were measured. A series of tests were conducted at the LabOceano offshore basin. This new facility has a length of 4 0 m, a width of 30 m, a depth of 15 m and is equipped with a multi-flap wave generator on one side. A ship-shaped FPSO design with sponsons for a deepwater offshore development in Brazil was tested. It has a length of 316 m, a breadth of 57.2 m and a draft of 28.3 m. A 1:70 scale model was constructed. A horizontal soft mooring system consisting of four lines with springs was used. Regular waves of different amplitudes as well as random waves were generated in the basin. Two different loading conditions, ballast (draft = 6.7 m) and loaded (draft = 21.7 m), as well as three wave headings, beam seas (90°), and quartering seas (22.5°, 45°) were considered. Tests were undertaken for four bilge keel configurations, corresponding to a case without bilge keels, as well as bilge keels of 3 different widths (1 m, 2 m and 3 m). In all cases, the bilge keels had a length of 200 m. An optical system was used to measure ship motions in all six degrees of freedom. The hydrodynamic loads on the bilge keels were measured using strain gages.© 2008 ASME

“Bigfoot” DVA Semi-Submersible Model Tests and Comparison With Numerical Predictions

The Bigfoot direct vertical access (DVA) semisubmersible is a novel floating drilling and production host that provides an attractive alternative to the spar. This concept utilizes heave plates (big feet) that improve the motion characteristics of a semisubmersible in all mild environments (S.E. Asia, W. Africa and Brazil). Bigfoot offers direct-vertical access (DVA) which is often a project requirement. This floater works in all water depths, in particular ultra-deepwater (5000+ ft) where a tension leg platform (TLP) is not an option, supports top tensioned risers and enables drilling and workover operations. The Bigfoot has several advantages over a spar. These include: 1) Quayside topsides integration. This eliminates offshore topsides integration, a significant issue for all spar projects in terms of cost, safety and schedule. 2) A more open deck layout compared to a spar, 3) No fabrication location restrictions as it can be built by many yards worldwide potentially offering local content to a project.Model tests were undertaken at the Shanghai Jiao Tong University (SJTU) Offshore Basin to assess the dynamic response of the Bigfoot in waves, swell, wind and current. Five mild (non-Gulf of Mexico) environments were considered. In all cases, the floater motions are an order of magnitude smaller than those of a conventional semisubmersible for similar deck payload thus enabling drilling operations and top-tensioned production risers. In a parallel effort, a COSMOS numerical model of the Bigfoot was developed for coupled motion analysis. The experimental results and the COSMOS numerical predictions are in close agreement. In addition to measuring global motions, two heave plates were instrumented with load cells to measure forces and moments. The force measurements from the model tests are in good agreement with numerical predictions using computational fluid dynamics (CFD).Copyright

The Effect of Coverage Length and Density on the Performance of Fairings

While the application of fairings can both increase a tubular’s fatigue life and decrease a tubular’s drag, it is not always possible or economical to fully cover a tubular with fairings. Aside from the desire to minimize the cost of VIV suppression by only covering the necessary portion of a tubular’s span, the presence of connectors, anodes, and other appurtenances can restrict the ability to install fairings over the entire tubular’s length. The result is that it is critical to understand how much of a tubular’s span should be covered with fairings and the effect of partial coverage on the overall suppression performance, and drag reduction, of the fairings system.This paper utilizes data from a two similar test programs to examine the effects of coverage length and density on the performance of fairings. The results show that the location of the fairings has a substantial effect on both the local and global response of the tubular. In practical terms, having fairings present in the high current region can suppress VIV in both the high current and low current regions of the tubular.Copyright

The Effect of Coverage Length and Density on the Performance of Helical Strakes

When utilizing helical strakes, a critical decision is how much of the tubular to cover with helical strakes. While minimizing the cost of the helical strakes is one objective, often it is not possible to fully cover a tubular due to connectors, anodes, and other appurtenances. There are also important questions around ascertaining the performance of helical strakes when only a portion of the tubular is covered, for example when the top portion of a tubular is straked and the remainder is left bare.This paper combines results from two testing programs on long cylinders towed at high Reynolds numbers to assess the performance of helical strakes with differing conditions along the cylinder length. The results show that the coverage length, density, and location of the helical strakes have a substantial effect on both the local and global response of the tubular.Copyright

On the Occurrence of Higher Harmonics in the VIV Response

Vortex Induced Vibrations (VIV) can lead to fast accumulation of fatigue damage and increased drag loads on slender marine structures. A cylinder subjected to VIV can vibrate in both in-line (IL) and cross-flow (CF) directions. The CF response is dominated by the primary shedding frequency and the IL response frequency is often two times of the primary CF frequency. In addition, higher harmonics can also be present. The third order harmonics are more pronounced when the motion orbit of the cylinder is close to “figure 8″ shape and cylinder is moving against the flow at its largest transverse motion. Recent studies with flexible beam VIV tests have shown that higher harmonics can have significant contribution to the fatigue damage in addition to the loads at the primary shedding frequency. However, there is a lack of understanding of when and where higher harmonic loads occur. The fatigue damage due to the higher harmonics is not considered in the present VIV prediction tools.In the present paper, the test data of selected cases subjected to linearly sheared flow profile from two test programs, the Shell high mode VIV test[11] and the Hanoytangen test[5] have been studied. The factors that may influence the occurrence of the higher harmonics, such as the bending stiffness, reduced velocity and orbits stability, have been studied. The importance of higher harmonics in VIV fatigue has also been investigated. Finally, a method to include higher harmonics in the fatigue calculation is presented.Copyright

Full Scale Fairing Qualification Tests

Production risers as well as drilling risers are often subjected to Vortex-induced vibrations (VIV) when exposed to ocean currents. VIV have been observed in the field and can cause fatigue failure and excessive drag on the riser. In order to suppress VIV and reduce drag, fairings are often used. This paper presents hydrodynamic qualification tests for two types of fairings: the short crab claw (SCC) and a tapered dual fin design. The short crab claw fairing design is a novel design that was developed by the Norwegian Deepwater Programme (NDP). As will be detailed in this paper, the SCC design offers very low drag, completely suppresses VIV and reduces riser interference.In 2012, a model test campaign was undertaken to understand and qualify the hydrodynamic performance of fairings at prototype conditions. The program consisted of testing the three fairing geometries and a strake to understand the stand-alone performance in VIV and the performance in interference. This was accomplished by utilizing a single pipe setup for the standalone test and a two-pipe setup for the interference tests. The paper reports the results of the program and draws conclusions on the hydrodynamic performance of the VIV suppression devices tested.Overall, all VIV suppression devices tested were able to suppress VIV with the SCC fairing being the most effective. In all cases tested, the downstream fairings / strakes were very effective in suppressing VIV in an interference scenario where a fairing was placed upstream. Contrary to the well-documented case of two strakes in tandem, in this case the upstream fairings did not reduce the effectiveness of the downstream fairings/strakes.Copyright