ASME Conference Presenter Attendance Policy and Archival Proceedings

2015;():V001T00A001. doi:10.1115/OMAE2015-NS1.

This online compilation of papers from the ASME 2015 34th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2015) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Offshore Technology: Design and Analysis

2015;():V001T01A001. doi:10.1115/OMAE2015-41244.

This paper is dedicated to the simulation of fluttering (oscillatory) and tumbling (rotational) phenomenon that may occur during the flow induced rotation in the water or air current. Fluttering is the oscillation of body about an axis and the tumbling, better called here as autorotation, is a name given to the case when the body turns continuously around the axis. This work describes the simulation of these phenomena by a nonlinear time domain code on freely rotating plate about a fixed vertical axis. The dimensional analysis proves that the rotational motion induced by flow is governed essentially by the dimensionless moment of inertia (I*) and Reynolds number. For Reynolds number less than 15000, plate experiences small amplitude fluttering motion that is independent of I*. It is shown that by increasing I* the fluttering bifurcates to autorotation, with a transition point that is approximately independent of Reynolds number and is such that I*=0.083.

Commentary by Dr. Valentin Fuster
2015;():V001T01A002. doi:10.1115/OMAE2015-41249.

Requests for the accurate planning of the erection process using modelling and simulation techniques have recently increased in many engineering fields, including shipbuilding and the offshore industries. In this study, an efficient erection simulation framework is proposed based on three-dimensional (3-D) measurement data that can support the development of various simulation systems for modular construction planning in the offshore and shipbuilding industries. The proposed simulation framework can be used to predict the erection state to optimise any gap, weak point and/or overlap of the modular construction process on the basis of 3-D laser scanning measurement data. To evaluate the efficiency and applicability of the proposed simulation framework, the framework is applied to the drillship modular erection process. The results show that the proposed simulation framework provides a consistent, integrated developmental environment for an erection process in the offshore industries. In addition, it can be expected that the time costs and risks of on-site fatality associated with the erection process will be reduced.

Commentary by Dr. Valentin Fuster
2015;():V001T01A003. doi:10.1115/OMAE2015-41257.

In lack of simultaneous data of metocean parameters such as wind, waves and currents, Norwegian design regulations presently recommend a conservative combination of metocean parameters for estimation of characteristic metocean loads on offshore structures. A simplified parametric load model for a jacket, based on waves and currents, is assumed. Several approaches to load estimation are investigated and the following are considered; different averaging length of extreme currents, the effect of peak-over-threshold approach for estimation of extreme wave and currents compared to all-sea states approach and extreme load estimation directly from a load time series. When compared to the recommended approach, all other approaches yield a reduced estimated characteristic metocean load. The results are intended be illustrative and not suitable for use in design.

Topics: Stress , Waves , Currents
Commentary by Dr. Valentin Fuster
2015;():V001T01A004. doi:10.1115/OMAE2015-41323.

The authors propose a method to estimate full-scale propeller torque consisting of low-frequency and high-frequency components in waves using measured data of free-running model ship. The duct fan auxiliary thruster (DFAT) [1] and the rudder-effectiveness and speed correction (RSC) [2,3] ensure similar model ship motion to full-scale in external forces, where RSC controls the model ship propeller rate of revolution and the auxiliary thrust depending on measured model ship speed. Analyzing a fluctuating component of effective inflow velocity to propeller due to waves, the method estimates full-scale fluctuating propeller torque in waves. This method also makes it possible to adopt into free-running model ship tests any engine model simulating interaction between propeller torque and engine torque. Trial application of the method exemplifies the property of full-scale fluctuating propeller torque comparing with that of model ship.

Topics: Torque , Waves , Propellers , Ships
Commentary by Dr. Valentin Fuster
2015;():V001T01A005. doi:10.1115/OMAE2015-41392.

Further to the studies presented by Sayeed et al. (OMAE2014-23489), response surface models have been improved by including the effects of sinkage and trim to the vertical plane motions of planing hull in head sea. This was achieved by including longitudinal center of gravity (LCG) as an additional factor to the existing model and change in LCG eventually incorporated the effects of sinkage and trim. The validated non-linear mathematical model, Planing Hull Motion Program (PHMP) can predict the heave and pitch motions and bow and center of gravity accelerations with reasonable accuracy at planing and semi-planing speeds. This paper again illustrates an application of modern statistical design of experiment (DOE) methodology to develop response surface models to assess planing hull motions in a vertical plane in head seas. Responses were obtained from PHMP based on a multifactor uniform design scheme. Results showed that the simple one line regression models provided adequate fit to the generated responses and provided valuable insights into the behaviour of planing hull motions in a vertical plane. The new response surface models includes all possible influential factors that affect the vertical plane motions of planing hull in head sea. The simple surrogate models can be a quick and useful tool for the designers during the preliminary design stages.

Commentary by Dr. Valentin Fuster
2015;():V001T01A006. doi:10.1115/OMAE2015-41429.

This paper presents a new quantitative approach of defining current profiles for application to riser fatigue assessment using profile clustering methods.

The analysis presented here was undertaken using a one year long deepwater current profile dataset from the Gulf of Mexico. The data included near full water column measurements in 3250m water depth at one hour intervals, providing nearly 9000 individual profiles. Riser fatigue damage for each profile had been previously computed as part of the Worldwide Approximation of Current Profiles (WACUP) joint industry project.

The new assessment described in this paper applies clustering methods not considered in WACUP, including the K-Means Algorithm (KMA) and Maximum Dissimilarity Algorithm (MDA). These both demonstrate superior performance compared to a much simpler direct method of characterisation. Features of the KMA and MDA methods are contrasted, within the context of previously published application to ocean wave data.

Commentary by Dr. Valentin Fuster
2015;():V001T01A007. doi:10.1115/OMAE2015-41479.

In this paper, we present a system simulator of a marine vessel and power plant which contains the mechanical system with diesel engines, propellers, steering gear, and thrusters; the electrical system with generators, switchboards, breakers, and motors; and the plant level controllers with dynamic positioning controller, thrust control, and power management system. Interconnections are possible to simulate by using a multi domain simulator. This is important when evaluating system performance and fault handling. The simulator is implemented in Simulink and is modular, configurable and scalable. It can be extended to run on National Instruments’ cRIO embedded control and acquisition system, for real-time simulation.

Commentary by Dr. Valentin Fuster
2015;():V001T01A008. doi:10.1115/OMAE2015-41491.

Results from previous model test campaigns of various large-volume platforms indicate that wave impact loads on vertical platform columns can become high in extreme sea states. Moreover, column slamming is a highly non-linear and complex problem and reliable estimation of Ultimate Limit State (ULS) and Accidental Limit State (ALS) design loads is a challenge. A model test campaign dedicated to investigate column slamming has been performed on a large volume platform at Marintek. Special effort was put into designing a model and instrumentation package that could capture the complex phenomenon of slamming due to breaking or near breaking waves as accurately as possible.

As part of the validation of the instrumentation for this test, drop tests were performed on a circular section with 42 force transducers. In the model test, this section was mounted on one of the platform columns for measuring wave impacts. In the present drop tests, the same section was dropped in still water in a small basin. Different impact velocities and impact angles were investigated. High-speed video recordings were also used to document the tests.

This paper presents the setup used in the drop tests. The results from the drop tests are discussed and compared to theoretical solutions.

Topics: Stress , Transducers
Commentary by Dr. Valentin Fuster
2015;():V001T01A009. doi:10.1115/OMAE2015-41545.

The motion response of a mat-support jack-up during positioning is studied in this paper using numerical analysis software SESAM. In the process of jack-up positioning, the square bottom mat is gradually lowered and the floating jack-up, secured by anchor chains, moves in six degrees of freedom in response to the dynamic loading of wave, current and wind combined. Numerical simulations are carried out to solve motion responses of the floating structure with mat at different depths. The sensitivity of motion responses to wave periods and directions are analyzed. The maximum motion amplitudes under the design environmental conditions and the risk of the mat crashing with the seafloor are evaluated.

Commentary by Dr. Valentin Fuster
2015;():V001T01A010. doi:10.1115/OMAE2015-41616.

This paper presents a bond graph model of a maritime crane lifting system comprised of a 3DOFs crane with three revolute joints, a winch, a segment of wire, and a pendulum load. The multi-body model contains the dynamic properties of the system and 3D animation of the operational behaviors. Lagrange’s method was used to derive the dynamic equations of the multi-body crane. Lagrange’s equations provide a clean elegant form for implementation using a special type of bond graph called IC-field. The model based on the bond graph contains interfaces to other domain models, e.g. input devices, control systems, hydraulic actuators, and sensors. Maritime crane operations are challenging due to the impact of heavy lifting, system stiffness and load sway resulted from the unstable working platform. The industry increasingly demands an overall virtual environment for modeling and simulation of maritime operations. The accomplishment will highly increase the efficiency and effectiveness of product and system design, new component and control algorithm testing, and operator training. The multi-body dynamic model is the core building block for modeling and simulation of maritime crane operations.

Commentary by Dr. Valentin Fuster
2015;():V001T01A011. doi:10.1115/OMAE2015-41633.

To determine forces on fixed and flexible structures such as wind mills and oil platforms, experiments in wave tanks are useful to investigate the impacts in various types of environmental waves. In this paper we show that the use of an efficient simulation code can optimize the experiments by designing the influx such that waves will break at a predefined position of the structure. The consecutive actual measurements agree well with the numerical design of the experiments. Using the measured elevation close by the wave maker as input, the software recovers the experimental data in great detail, even for rather short (up to L/D=1) and very steep breaking waves with steepness parameter (ak) till 0.4.

The experiments were carried out in the TUD-wavetank and the simulation is done by HaWaSSI-AB, a spatial-spectral implementation of a Hamiltonian Boussinesq model with an eddy-viscosity breaking mechanism that is initiated by a kinematic breaking condition.

Topics: Simulation , Waves , Design
Commentary by Dr. Valentin Fuster
2015;():V001T01A012. doi:10.1115/OMAE2015-41634.

Workspace computation and visualisation is one of the most important criteria in offshore crane design in terms of geometry dimensioning, installation feasibility and operational performance evaluation. This paper presents a numerical method for the computation and visualisation of the workspace of offshore cranes. The Working Load Limit (WLL) and the Safe Working Load (SWL) can be automatically determined. A three-dimensional (3D) rectangular grid of voxels is used to describe the properties of the workspace. Firstly, a number of joint configurations are generated by using the Monte Carlo method, which are then mapped from joint to Cartesian space using forward kinematics (FK). The bounding box of the workspace is then derived from these points, and the voxels are distributed on planes inside the box. The method distinguishes voxels by whether they are reachable and if they are on the workspace boundary. The output of the method is an approximation of the workspace volume and point clouds depicting both the reachable space and the boundary of the workspace. Using a third-party software that can work with point clouds, such like MeshLab, a 3D mesh of the workspace can be obtained. A more in-depth description and the pseudo-code of the presented method are presented. As a case study, the workspace of a common type of offshore crane, with three rotational joints, is computed with the proposed method.

Commentary by Dr. Valentin Fuster
2015;():V001T01A013. doi:10.1115/OMAE2015-41703.

This paper describes the guidelines for preliminary workability analysis for offshore operation and planning using global wave persistence study. In offshore engineering, the weather information has to be studied to select the appropriate path for the project during the concept design, the detail design and the operation plan. The workability analysis is part of the weather study for various offshore operations and this study is used as a guide for the operation schedule, the selection of the operation method, the design of the equipment and the supporting structures. The detailed workability analysis is based on response based simulations which are only available after or during detail system design. On the other hand, preliminary workability can be calculated based on hindcast data with statistical methods before the detail design and this study can lead the project on the appropriate path at the beginning stages. The wave persistence analysis with thresholds on time and wave heights is used for the preliminary workability calculation and can also be used for route planning during offshore transportation. The objective of this paper is to provide global wave persistence analysis data to guide offshore operation and planning and makes the transition to workability with the required working wave period easier. For the global wave persistence analysis, the National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis Reforecast (CFSRR) system data was used which has 35-year numerical wave hindcast. Data resolution is 0.5°×0.5° grid size with a temporal resolution of 3 hours for the period from 1979 to 2013, and this database covers most offshore locations except the Caspian Sea, and the North and South Pole areas. The persistence analysis was calculated with 0.5m, 1.0m, 1.5m and 2m wave thresholds, and each wave threshold is categorized as 24 hours, 48 hours and 72 hours persistency.

Commentary by Dr. Valentin Fuster
2015;():V001T01A014. doi:10.1115/OMAE2015-41866.

Large scale model tests have been conducted in a towing tank facility for the determination of the hydrodynamic coefficients of subsea blowout preventers. A subsea blowout preventer (BOP) is a large, complex device 10–15 [m] tall, weighing 200–450 [ton]. The BOP stack consists of two assemblies, the ‘lower marine riser package’ (LMRP) connected to the riser string and the BOP itself, connected to the wellhead. Together they represent a large lumped mass, which directly influences the natural frequencies and vibration modes of the riser system, particularly those of the BOP-wellhead-casing assembly.

Large uncertainties in the estimates of the hydrodynamic coefficients (added mass, lift and drag or damping) result in large uncertainties in the fatigue damage predictions of the riser and wellhead system. The trend toward larger and heavier BOPs, which could place BOP-wellhead-casing oscillation frequencies in the range of wave frequencies, has motivated Statoil and BP to start a new research project on this subject. The project involves a large scale model test for experimental determination of hydrodynamic coefficients.

Two different BOP designs were tested in a towing tank at model scale 1:12. The models weighed about 50 [kg] in air and were about 1.2–1.5 [m] tall. A six-degree-of-freedom oscillator was mounted under the carriage of the towing tank for oscillation of the models in different directions. Static tow tests and forced oscillation tests with and in the absence of steady current were carried out. Keulegan-Carpenter (KC) numbers ranged between 0.2 and 2.0, while the Sarpkaya frequency parameter β was in the range from 4,000 to 50,000. The Reynolds numbers of the static tow tests ranged between 50,000 and 150,000. This paper focuses particularly on tests in the surge direction with and in the absence of a steady current. Results indicate that the hydrodynamic coefficients for BOP stacks are quite different from those of simpler geometries like a circular cylinder. In addition, they provide new insight for analytical modeling of global hydrodynamic forces on BOPs in many configurations and scenarios.

Commentary by Dr. Valentin Fuster
2015;():V001T01A015. doi:10.1115/OMAE2015-41879.

Aibel is developing an offshore power converter platform concept. Its foundation is gravity-based and consists of four columns interconnected by a ring-shaped pontoon at the seabed. The platform is intended for water depths in the order of 20 to 40m. In these waters, breaking waves typically cause large wave loads on the foundation that need to be accounted for in the design. The slamming loads, pressures and air gap at the platform were investigated with a combined approach of physical and numerical modeling. This paper summarizes the set-up, test program, measurement techniques, results and analysis of the physical model tests.

The tests showed that reflection and diffraction patterns caused a significant steepening of the waves between the columns, reducing the air gap and increasing the slamming frequency and magnitude on the downstream columns and underside of the deck. Excitation of resonant wave modes was identified for certain wave frequencies. Although the global wave loads were primarily governed by inertia, largest loads occurred under slamming impacts on the upstream columns, in phase with the inertial force.

Commentary by Dr. Valentin Fuster
2015;():V001T01A016. doi:10.1115/OMAE2015-41891.

Nowadays the offshore wind energy market is clearly oriented to be extended around the world. Bottom fixed solutions for supporting offshore wind turbines are useful in shallow waters which are available in a limited extent unless a continental shelf exists. Considering the Oil & Gas background knowledge, move from bottom fixed solutions to floating solutions is not a technical challenge, but the cost of each structure in terms of industry profit is currently the main issue for its commercial implementation. That point has induced huge research efforts on the topic.

Recently, a new concept consisting of a monolithic concrete SPAR platform was experimentally and numerically studied in the framework of the AFOSP KIC-InnoEnergy project (Alternative Floating Platform Designs for Offshore Wind Towers using Low Cost Materials) [1] [2]. The studies comprised a set of hydrodynamic tests performed in the CIEM wave flume facility at UPC, with a 1:100 scaled model assuming Froude similitude.

The whole test campaign includes free decay tests, RAO’s determination, regular and irregular waves with and without wind mean force. For the determination of the platform RAO’s, a set of 21 regular waves trains with periods ranging from 0.8s up to 4.8s were applied. The 6 DOF motions of the platform were measured with an infrared stereoscopic vision system.

In this paper, a summary of pitch and heave RAO’s tests will be presented with the main objective to calibrate and validate the accuracy of the Morison-based numerical model for floating wind turbine platforms developed at the Universitat Politècnica de Catalunya.

Because the wave flume spatial constraints, both Airy and Stokes wave theories are necessary to reproduce the correct wave kinematics. The numerical model includes both theories and a comparison between them has been done, checking the validity range of each one.

The simulations revealed a reasonable good agreement with the experimental results, as well with the computed RAO’s in commercial software.

Commentary by Dr. Valentin Fuster
2015;():V001T01A017. doi:10.1115/OMAE2015-42047.

Real-Time Ship Maneuvering Simulation Models is becoming more common and necessary in some feasibility analysis of ports and horizontal design. Due to the complexity of the hydrodynamic effects, a fairly realistic modeling is difficult to obtain, so that this type of simulation becomes limited in some cases. Large ships face difficulties to access ports once the shallow water and bank effects become significant. Since these factors are essential in some maneuverability studies, they must be modeled to Real-Time Simulations. In order to increase the application range of this kind of simulation, this paper presents simplified models to estimate additional hydrodynamic forces related to ship-to-ship and ship-to-bank interactions. Based on some physical input data which can be easily obtained during a Real-Time simulation, such as vessel speed and relative distances between a ship and another solid body, applying a set of measured points from the vessel to check the respective environment geometric shapes, identifying the bank conditions and other nearby vessels at a given instant. Thus, we are able to determine a realistic hydrodynamic effect. Due to the difficulty to create an accurate model, the prediction of the hydrodynamic forces was obtained from experimentally validated numerical methods such as the Boundary Elements Method (BEM), using the Rankine Panel Method. This validation consists in a comparative study among other works in this area to ensure a reliable response. The model calibration was performed using dimensionless coefficients and the BEM results are applied to the Real-Time simulation for a vessel type studied. In this study, a modeling of ship-to-ship will be presented modeled to interact in a Real-Time simulator, as a way to improve the real-time simulation and it will show that this method provides more realistic results to studies related to ship-to-ship interaction.

Topics: Ships
Commentary by Dr. Valentin Fuster
2015;():V001T01A018. doi:10.1115/OMAE2015-42162.

As oil and gas exploration moves to deeper waters the need for methods to conduct reliable model experiments increases. It is difficult obtain useful data by putting a scaled model of an entire mooring line systems into an ocean basin test facility. A way to conduct more realistic experiments is by active truncated models. In these models only the very top part of the system is represented by a physical model whereas the behavior of the part below the truncation is calculated by numerical models and accounted for in the physical model by active actuators applying relevant forces to the physical model. Hence, in principal it is possible to achieve reliable experimental data for much larger water depths than what the actual depth of the test basin would suggest. However, since the computations must be faster than real time, as the numerical simulations and the physical experiment run simultaneously, this method is very demanding in terms of numerical efficiency and computational power. Therefore, this method has not yet proved to be feasible. It has recently been shown how a hybridmethod combining classical numerical models and artificial neural networks (ANN) can provide a dramatic reduction in computational effort when performing time domain simulation of mooring lines. The hybrid method uses a classical numerical model to generate simulation data, which are then subsequently used to train the ANN. After successful training the ANN is able to take over the simulation at a speed two orders of magnitude faster than conventional numerical methods. The AAN ability to learn and predict the nonlinear relation between a given input and the corresponding output makes the hybrid method tailor made for the active actuators used in the truncated experiments. All the ANN training can be done prior to the experiment and with a properly trained ANN it is no problem to obtain accurate simulations much faster than real time — without any need for large computational capacity. The present study demonstrates how this hybrid method can be applied to the active truncated experiments yielding a system where the demand for numerical efficiency and computational power is no longer an issue.

Commentary by Dr. Valentin Fuster
2015;():V001T01A019. doi:10.1115/OMAE2015-42274.

The evaluation of the principal dimensions of the Floating Production, Storage and Offloading (FPSO) system is one of the most critical tasks at the initial design stage of the vessel. It is therefore important to get this right from the onset. This paper presents a simple method of determining the optimal principal dimensions of FPSO vessels of any specified oil storage capacity. An interactive programme, the Principal Dimensions Programme (PDP) has therefore been designed to accurately evaluate them based on the required cubic number (L × B × D) and the needed oil storage capacity (as the modern segregated vessels are volume-limited). The prediction of these dimensions has been given to ensure a safe operation and optimal performance of the vessels with regards to their motion responses in deep sea waves.

Topics: Dimensions , FPSO , Vessels
Commentary by Dr. Valentin Fuster

Offshore Technology: Fixed and Floating Platforms

2015;():V001T01A020. doi:10.1115/OMAE2015-41174.

The configuration of MOSES TLP is quite different from conventional types. A compact central columns group and extended tendon support structure provides better deck support and hydrodynamic efficiency. A fully-coupled time domain analysis was adopted to evaluate effects of first and second order wave forces, motions and tendon tensions in hurricane conditions. The nonlinear responses due to the environmental load and interaction between the hull and tendons that includes large surge-heave motion in the low frequency and resonant heave/pitch responses with the spring loads in the high frequency are focused on in the paper. Results from numerical simulation are compared with data in full scale measurement.

Commentary by Dr. Valentin Fuster
2015;():V001T01A021. doi:10.1115/OMAE2015-41188.

The mitigation of Vortex Induced Motion (VIM) of the HVS (Heave and VIM Suppressed) semisubmersible is investigated through extensive comparisons between CFD analysis and VIM model test results. It is shown that the lower VIM response of the HVS semisubmersible results from the break in coherence of vortex shedding along the length of column due to the column step. The present CFD application was carried out on the basis of in-house best practices for VIM analysis of multi-column floaters. The analysis results show excellent comparison with the model test results. The present findings and methodology can be applied to optimize semisubmersible hull designs for suppressed VIM response.

Commentary by Dr. Valentin Fuster
2015;():V001T01A022. doi:10.1115/OMAE2015-41195.

This paper presents wave slamming loads on the Aasta Hansteen Spar from model test data, and it discusses analysis methodologies for extreme loads in order to derive slamming design pressures for the local and global design of the Spar hull.

A 3×3 array of slamming load transducer panels was employed to measure the horizontal wave impact loads for the 100-year and 10,000-year storms at the Aasta Hansteen field in the Norwegian Sea. The wave height and period for these storms were varied to investigate wave steepness effects on slamming loads. Three-hour simulations and as many as 20 realizations per sea state were used to capture the statistics for the slamming loads.

Gumbel distribution was used to derive extreme pressures for the local and global design at the measured direction by using various panel combinations. The short term target percentiles used in the Gumbel distribution were determined by long term analysis. The detailed long term analysis results are the subject of another paper [1].

The main objective of this study is to establish the extreme pressure distribution along the length of the Spar above the mean water level (MWL). The linear correlation was found between the slamming pressure coefficient and incident wave steepness and it was used to obtain the extreme pressure profile for other wave directions around the Spar hull.

The methodology presented in this paper can also be applied to slamming pressure of other platforms/floaters.

Commentary by Dr. Valentin Fuster
2015;():V001T01A023. doi:10.1115/OMAE2015-41227.

A long term analysis was performed to determine extreme wave slamming loads on the Aasta Hansteen Spar, the first production and storage Spar to be installed in the Norwegian Sea. The Spar will experience high slamming pressures on the hull due to harsh environments in the field. Extensive model tests were performed to measure the wave slamming pressure which is one of challenging design parameters.

The slamming loads were measured with a 3×3 array of force transducer panels attached to the Spar hull. The extreme slamming loads were estimated from 3-hour simulations of the 100-yr and 10000-yr wave environments at the Aasta Hansteen field in the Norwegian Sea. The wave simulations included fourteen sea states, and each sea state was represented by as many as 20 realizations.

Based on model test data, short term analysis of 3-hour extreme pressure at each tested sea state was performed using the Gumbel distribution. Due to high variability of 3-hour maximum pressures, a long term analysis was required to investigate the proper percentile level to be used in the design.

The paper presents a long term statistical methodology for extreme wave slamming loads that is used to calculate long term slamming pressures corresponding to a specified annual exceedance probability of q (e.g., q = 10−2 and q = 10−4). The paper also derives the appropriate non-exceedance probability for a short term wave environment that reproduces the long term pressures of a specified annual exceedance probability, q.

Various sensitivity analyses (e.g., on the two Gumbel parameters, number of realizations, etc.) were performed to validate the short term target percentiles and associated extreme pressures derived from this approach.

Details of the model tests and methodology to define the design pressure profile above mean water level (MWL) are presented in a companion paper of this Conference.

Commentary by Dr. Valentin Fuster
2015;():V001T01A024. doi:10.1115/OMAE2015-41236.

Some offshore platforms need modifications or addition of new modules that require re-assessment of their design. An economical way of allowing for changes without major strengthening of the platform is by reducing the safety margin used in the original design by use of measured data during the operation. This paper presents application of this concept to Troll A platform in which the measured changes in the platform’s natural periods due to the nonlinear soil behavior under different storm conditions were used to calibrate the soil-foundation spring adopted in the design.

Commentary by Dr. Valentin Fuster
2015;():V001T01A025. doi:10.1115/OMAE2015-41349.

At the present time design values of ice loads on fixed offshore structures are rather conservative. Conservatism of design ice loads consists in assuming the most unfavorable ice action direction and the worst ice drift speed; the most unfavorable combination of the consolidated layer thickness, ridge keel depth and ice strength; as well as supposing the ice ultimate strength value constant along the whole ice–structure contact area perimeter.

With accumulation of the knowledge on ice formation failure under interaction with ice-resistant fixed platforms, the requirements contained in Rules of classification societies are reduced. For example, for the last forty years the lowering of requirements to design ice load values was equal to about four times [1].

For the last time specialists of Krylov State Research Centre have performed design and experimental studies where further tendency to decreasing design values of ice loads is traced. Ice monitoring is one of the main elements for justification of design ice load values’ decrease.

Modern monitoring systems permit to warn about occurrence of a state close to a limit one, as well as to record actual ice loads. Ice load monitoring is a necessary part of accident prevention during ice-resistant structures operation. Monitoring of ice loads is a necessary part for providing safe operation of ice-resistance structures, and systematic accumulation of monitoring data for several years gives a positive effect in the form of justified decrease of static and dynamic design ice loads.

Commentary by Dr. Valentin Fuster
2015;():V001T01A026. doi:10.1115/OMAE2015-41362.

As a typical floating platform, TLP (Tension leg platform) is widely used around the world of the water depth from 140m to 1400m due to its significant advantages over other deep-water production systems. This paper describes a conventional TLP type application for relatively shallow water depth of 410m, it was studied for a potential development plan in China south sea. Considering the safety and efficiency in the construction, installation and operation, a comprehensive sensitivity study of the simulation results against various analysis/environment parameters (including column form, wind coefficients, tendon pretension, etc.) was carried out and the role of each critical parameter was analyzed deeply. The numerical simulations were conducted for 1000-yr Hurricane condition with collinear wind, wave, and current directions. The results based on this study show that the change of the column pattern (from round column to square column) does not have much difference on the TLP hydrodynamic performance without other conditions changed; the variation of pre-tension has limited influence to the TLP hydrodynamic performance as this relatively shallow water.

Commentary by Dr. Valentin Fuster
2015;():V001T01A027. doi:10.1115/OMAE2015-41442.

A deepwater Spar Drilling Production Storage Offloading (SDPSO) floating system or Spar FPSO that integrates Spar dry-tree production, oil storage and offloading has been proposed for offshore oil exploitation. Benefits of the deep drafted first generation classic Spar, such as excellent stability and hydrodynamic performances that allow dry tree drilling and production, large capacity of the mid hull section for oil storage, cost efficiency for construction and save in-service operations, are combined to provide a competitive solution from moderate deep water of 300m to ultra-deep water of 3,000m.

The present Spar FPSO is proposed for the potential deepwater oil field development in South China Sea. It uses the hull mid-section with large capacity for wet oil storage, utilizing the density difference between the oil and the sea water for wet storage and oil-water displacement in the storage tank. To demonstrate the feasibility and to investigate its hydrodynamic performances of the Spar FPSO, comprehensive experimental investigations, including the in-place model tests in the deepwater offshore basin and the VIM model tests in the towing tank, have been undertaken. For the in-place model tests, both the stand alone and the tandem offloading conditions were considered. The classic Spar hull, the taut mooring system and the top tensioned risers (TTRs) were all included to represent the coupling effects of the whole system. Various metocean environments, such as the 100-year storm, the 1-year storm and the offloading condition, were considered to give a complete assessment. Different mooring configurations were also studied to adapt to the deep and moderate water depths correspondingly. The global responses, such as the hull six degree of freedom motions, top tensions of mooring lines and risers, possible green water occurrences, were monitored and recorded in real time. Additionally, a preliminary VIM experiment was also carried out in the towing tank since the vortex induced motions (VIM) is one of the most concerned issues for the deep drafted Spar. The VIM characteristics with different heading angles and current velocities were obtained.

It is demonstrated that the Spar FPSO has satisfactory hydrodynamic and VIM performances in metocean environmental conditions in South China Sea. It gives a feasible and competitive alternative with integrated dry-tree drilling, production, oil storage and offloading functionalities for offshore oil field development in deep water depth.

Commentary by Dr. Valentin Fuster
2015;():V001T01A028. doi:10.1115/OMAE2015-41461.

In the initial design stage of an offshore platform, some conservative assumptions might be used for the platform hull and Topsides weights, wind area and Metocean criteria of environments. After the platform is installed, actual results from weight surveys and real-time snapshots of the as-built operating platform typically provide more accurate information for the purpose of assessing real operational conditions and future changes of these conditions. For example, in field operations, it is particularly useful to quantify how much weight can be added to the as-built operating facility, while assuring that the platform still meets the overall design criteria and regulatory requirements.

This paper presents an effective method for evaluation of an in-field operating Spar platform, to determine the maximum allowable envelops for topsides weight change verse the topsides VCG, based on the Spar global performances and riser operating limits. The case study is performed for a Spar in the Gulf of Mexico (GOM), based on the Spar as-built data and actual in-field configurations of hull/mooring and risers, and using the updated Metocean criteria (after the Hurricane Katrina in the GOM).

The analysis results of the allowable topsides weight change and topsides VCG limits, accounting for the Spar in operating, extreme and survival conditions, are based on three governing factors: (1) the Spar motions meet the original design criteria for global performances, and are within the safety range of riser operations; (2) the maximum loads at critical connections between the Spar hard tank and the topsides structures are within the design loads; and (3) the reserve variable ballast is sufficient to balance the Spar at an even keel position at the design draft for all required operating conditions.

Topics: Spar platforms
Commentary by Dr. Valentin Fuster
2015;():V001T01A029. doi:10.1115/OMAE2015-41658.

The response based analysis of a floating system aims at the prediction of extreme N-year return period responses by analysing their statistics in site-specific metocean conditions. After the extreme responses have been determined the inverse problem is solved: the most probable design metocean conditions (DMC) should be determined which cause these responses. The most probable DMC associated with a given response can be determined by using the joint probability density function (PDF) of metocean parameters. Mathematically, the most probable DMC is equivalent to a “design point”, at which the joint metocean PDF has a conditional maximum. This paper presents a method for finding the most probable DMC by the extremum search. Several numerical examples are presented to demonstrate finding the most probable DMCs for typical responses of an FPSO in North Sea environment, including global motions, wave loads, turret motions and relative wave motions. The most probable DMCs are found by the search process implemented in the space of physical metocean variables, which allows for easy interpretation of the results and exploring the response sensitivity to variability of metocean parameters.

Topics: Design , FPSO
Commentary by Dr. Valentin Fuster
2015;():V001T01A030. doi:10.1115/OMAE2015-41764.

Deep draft semisubmersible (DDS) concepts have been developed recently in order to improve the vertical motion characteristics of the platform, due to the smaller wave exciting forces on the pontoons than a conventional semisubmersible. However, the DDS may experience critical vortex-induced motions (VIM) stemming from the fluctuating forces in a strong current environment. Aiming to investigate the excitation loads and the mechanism of VIM, Computational Fluid Dynamics (CFD) analyses are performed to study the flow around the DDS in a cross-flow. Special attentions are paid to the effect of the pontoon and the heading angle. Good agreement between CFD simulations and model test results for the current loads of a DDS is observed. Detailed computational results including hydrodynamic loads and flow patterns are presented.

Commentary by Dr. Valentin Fuster
2015;():V001T01A031. doi:10.1115/OMAE2015-41792.

In order to increase the gross generation of wind turbines, the size of a tower and a rotor-nacelle becomes larger. In other words, the substructure for offshore wind turbines is strongly influenced by the effect of wave forces as the size of substructure increases. In addition, since a large offshore wind turbine has a heavy dead load, the reaction forces on the substructure become severe, thus very firm foundations should be required. Therefore, the dynamic soil-structure interaction has to be fully considered and the wave acting on substructure accurately calculated. In the present study ANSYS AQWA is used to evaluate the wave forces. The wave forces and wave run up on the substructure are presented for various wave conditions. Moreover, the substructure method is applied to evaluate the effect of soil-structure interaction. Using the wave forces and stiffness and damping matrices obtained from this study, the structural analysis of the gravity substructure is carried out through ANSYS mechanical. The structural behaviors of the strength and deformation are evaluated to investigate an ultimate structural safety and serviceability of gravity substructure for various soil conditions. Also, the modal analysis is carried out to investigate the resonance between the wind turbine and the gravity substructure.

Commentary by Dr. Valentin Fuster
2015;():V001T01A032. doi:10.1115/OMAE2015-41805.

Linear solvers for the flow exterior to the hull may be used to solve for the fluid dynamics also in the interior of a tank, as discussed in Newman (2005) and Ludvigsen et al. (2013). This introduces extra, erroneous terms in the radiation part of the pressure in the tank, but due to cancellation in restoring and radiation terms, the total representation of the pressure and the global response is correctly obtained.

For some kinds of analysis, specific knowledge is needed of the radiation and restoring parts separately. The cancellation of the extra terms can then not be utilized.

Examples of this are stability analysis and eigenvalue analysis. In stability analysis we need to know the actual real global restoring coefficients. In eigenvalue analysis, we should have the separately correct representations of the added mass and restoring coefficients, respectively, to be able to conveniently use them as input to standard eigenvalue solvers.

Here, we develop the expressions for the corrected, actual terms of the total added mass and restoring coefficients for tanks. This is used in our computer program for performing eigenvalue analysis. Results for peak global response and natural periods of the structure with the influence of tank dynamics are presented. Comparisons are made with results obtained by a quasi-static method for an FPSO and a ship with more largely extensive tanks.

For a completely filled tank, the boundary value problem (BVP) for the velocity potential is reduced to Laplace equation in the fluid domain, subject to a Neuman condition on the fixed boundary and it is not closed. The extra condition of having zero pressure at some point in the tank is then added.

Direct re-use of the BVP solver for the external flow, gives an undetermined set of linear equations for the velocity potential in the tank fluid. A typical solver for sets of linear equations may still return a solution, but this will contain a random undetermined constant. After imposing zero pressure in the top of the tank, this solution is still unstable, contaminated by numerical noise.

An improved method is introduced by imposing algebraically, in the equation system, the constraint of zero pressure in the top of the tank. This gives a non-singular equation system with a stable solution holding zero pressure in some selected point in the tank.

Commentary by Dr. Valentin Fuster
2015;():V001T01A033. doi:10.1115/OMAE2015-41931.

Different codes are used in the industry for the fatigue design of offshore structures. Because the fatigue phenomenon is very sensitive to different design conditions, standards select those with higher impact and try to cover all other unpredictable factors making use of safety factors. To the authors’ knowledge there is not any comparison of fatigue codes with focus on analyzing the strength and limitations of the design approaches and safety factors included in each one available. This paper has the aim of comparing a number of recognized fatigue design codes for this purpose. Based on this review BS (British Standard) presents the most complete information for offshore design applying the fracture mechanics approach while DNV (Det Norske Veritas) offers the most updated and complete information for fatigue design applying the endurance approach. The safety factors contemplated in the codes are not always comparable and consequently it is not possible to confirm whether one standard is more or less conservative than the others.

Commentary by Dr. Valentin Fuster

Offshore Technology: Hydrodynamics

2015;():V001T01A034. doi:10.1115/OMAE2015-41290.

The accurate definition of the extreme wave loads which act on offshore structures represents a significant challenge for design engineers and even with decades of empirical data to base designs upon there are still failures attributed to wave loading. The environmental conditions which cause these loads are infrequent and highly non-linear which means that they are not well understood or simple to describe. If the structure is large enough to affect the incident wave significantly further non-linear effects can influence the loading. Moreover if the structure is floating and excited by the wave field then its responses, which are also likely to be highly non-linear, must be included in the analysis. This makes the description of the loading on such a structure difficult to determine and the design codes will often suggest employing various tools including small scale experiments, numerical and analytical methods, as well as empirical data if available.

Wave Energy Converters (WECs) are a new class of offshore structure which pose new design challenges, lacking the design codes and empirical data found in other industries. These machines are located in highly exposed and energetic sites, designed to be excited by the waves and will be expected to withstand extreme conditions over their 25 year design life. One such WEC is being developed by Aquamarine Power Ltd and is called Oyster. Oyster is a buoyant flap which is hinged close to the seabed, in water depths of 10 to 15m, piercing the water surface. The flap is driven back and forth by the action of the waves and this mechanical energy is then converted to electricity.

It has been identified in previous experiments that Oyster is not only subject to wave impacts but it occasionally slams into the water surface with high angular velocity. This slamming effect has been identified as an extreme load case and work is ongoing to describe it in terms of the pressure exerted on the outer skin and the transfer of this short duration impulsive load through various parts of the structure.

This paper describes a series of 40th scale experiments undertaken to investigate the pressure on the face of the flap during the slamming event. A vertical array of pressure sensors is used to measure the pressure exerted on the flap. Characteristics of the slam pressure such as the rise time, magnitude, spatial distribution and temporal evolution are revealed. Similarities are drawn between this slamming phenomenon and the classical water entry problems, such as ship hull slamming. With this similitude identified, common analytical tools are used to predict the slam pressure which is compared to that measured in the experiment.

Topics: Pressure , Waves , Surges
Commentary by Dr. Valentin Fuster
2015;():V001T01A035. doi:10.1115/OMAE2015-41373.

The numerical prediction of green water loads on super-structures is challenging due to the high number of required calculations to identify the critical operational conditions in the seaway which lead to overcoming seawater on deck. Further, the simulation of the non-linear behaviour of water on the deck and the prediction of impact loads require high computational effort. This paper presents an efficient three-step approach to simulate green water loads. The application of the developed procedure will be demonstrated on a mega yacht geometry.

Topics: Simulation , Stress , Water
Commentary by Dr. Valentin Fuster
2015;():V001T01A036. doi:10.1115/OMAE2015-41443.

Riser wake interference analysis is conducted based on analytical / semi-empirical models such as Blevins’ and Huse’s models. These models are used for modeling the reduction in particle flow velocity due to the presence of a cylindrical object upstream in the flow path. However, these models are often too conservative and accurate only for circular cylinders. Many top tensioned risers (TTRs) use vortex induced vibration (VIV) suppression devices such as strakes or fairings. There is a need for alternate methods to obtain drag and lift coefficient datasets for circular cylinders with strakes and fairings. Two such approaches are to obtain data from Computational Fluid Dynamics (CFD) simulations or from experimental large-scale model test data. Interpolation and/or extrapolation methods are needed to obtain additional data points for global riser finite element analysis.

This paper presents a methodology to obtain hydrodynamic coefficients for TTRs with VIV suppression devices. The proposed methodology uses a combination of empirical formulas based on Blevins’ model and numerical interpolation techniques along with experimental tow tank test data and CFD analysis. The resulting data is then input as user-defined drag/lift coefficients into a global riser finite element analysis to obtain a more realistic riser system response.

Commentary by Dr. Valentin Fuster
2015;():V001T01A037. doi:10.1115/OMAE2015-41675.

This research paper presents the active heave compensation control design using a novel hybrid riser tensioning system by integrating an electrically powered riser tensioning system into existing hydro-pneumatic tensioners. This is a further control design development paper following the previous framework introduction paper of riser hybrid tensioning system. The realization of the active heave compensation operation mode using this novel hybrid tensioning system is analyzed. A mathematical model for describing this operation mode is built integrating electrical and hydro-pneumatic tensioning systems. As a multi-input multi-output system, one Linear Quadratic Gaussian control design is proposed and implemented in both Matlab simulation and hardware realization. The simulation results and the experimental study have confirmed that the riser hybrid tensioning system is a feasible concept with better control accuracy and improves riser reliability compared to the traditional hydro-pneumatic system. The main benefit of the electrical tensioning system is its ability to deliver dynamically changed tension within milliseconds. The actualization of this principle control mode gives us confidence to carry on for further riser dynamic control using this riser hybrid tensioning system.

Commentary by Dr. Valentin Fuster
2015;():V001T01A038. doi:10.1115/OMAE2015-41933.

The forced heave motion of a dummy ship model with moonpool, including a fixed box-shaped object, was realized experimentally in the Towing tank at MARINTEK. The blockage effect caused by a large object was investigated. Regular and irregular forced heave motions were imposed. In the regular motion tests, four forcing amplitudes, and 11 forcing periods near the piston-mode resonance period were tested. PVC3D (Potential Viscous Code) was used to study the regular heave motion problem numerically. PVC3D is a code developed at MARINTEK, in collaboration with Statoil RDI, which couples a Naviér-Stokes solver with the linear potential flow theory for the free-surface waves. PVC3D has in previous studies proven to be fast, robust and accurate for marine resonance problems. It has not previously been validated for object in moonpool. Here, a validation study is presented. The moonpool response is well predicted by PVC3D both for the case of empty moonpool and moonpool with object. The studied object has a non-negligible blockage effect in resonant condition.

Commentary by Dr. Valentin Fuster
2015;():V001T01A039. doi:10.1115/OMAE2015-42048.

Ships at sea almost invariably carry liquids onboard, and liquids are contained in appropriate tanks. Being able to take into account the effects of liquids onboard when predicting ship motions is, therefore, of utmost importance for the safe operation of a vessel. In certain conditions, such predictions also require taking into account nonlinearities in both ship motions and in the internal flow, and linear approaches are not sufficient. Within this context, the present paper describes a simulation approach where a blended 6-DOF nonlinear ship motions prediction solver handling the external fluid-ship interaction, is coupled with a Smoothed-Particle-Hydrodynamics (SPH) solver for simulating the internal flow tank dynamics. The solvers are described and an example application is reported.

Topics: Ships , Sloshing
Commentary by Dr. Valentin Fuster
2015;():V001T01A040. doi:10.1115/OMAE2015-42234.

This paper presents a higher order time domain boundary elements method based on the Rankine sources for the computation of both linear and weakly non-linear effects for both fixed and free floating bodies. The geometry is described based on surfaces in a standard iges file, considering a NURBS (Non Uniform Rational Basis-Spline) description. The potential function, velocity, free-surface elevation and other quantities are defined using b-splines of arbitrary degree and the floating body interaction is solved using the potential acceleration approach on a Runge-Kutta scheme for time evolution. The integral equation is obtained and solved considering several possibilities for the collocation points, leading to an over-determined system. The integration over the panels is performed using a mixed desingularized-numerical method over Gaussian points. The results comparison are performed with WAMIT solution for a floating sphere concerning wave runup, body motions, velocity field, mean drift components in time domain.

Topics: Computation
Commentary by Dr. Valentin Fuster
2015;():V001T01A041. doi:10.1115/OMAE2015-42237.

This paper presents a time domain boundary elements method that accounts for relative displacements between two bodies subjected to incoming waves. The numerical method solves the boundary value problem together with a re-meshing scheme that defines new free surface panel meshes as the bodies displace from their original positions and a higher order interpolation algorithm used to determine the wave elevation and the velocity potential distribution on new free surface collocation points. Numerical solutions of exciting forces and wave elevations are compared to data obtained in a fundamental experimental text carried out with two identical circular section cylinders, in which one was attached to a load cell and the other was forced to move horizontally with a large amplitude oscillatory motion under different velocities. The comparison of numerical and experimental result presents a good agreement.

Commentary by Dr. Valentin Fuster

Offshore Technology: Offshore Topics

2015;():V001T01A042. doi:10.1115/OMAE2015-41040.

The aim of this study is to investigate methods of assessing the turbulence effect for the helideck availability study. Due to the limited space on offshore platforms, a helideck is normally positioned on cramped areas and this makes turbulence flows around the helideck. CAP 437, the representative standard for the offshore helideck design suggests various criteria for the helideck availability assessment and recommends a Standard Deviation of Vertical airflow Velocity (SDVV) value to be used for the turbulence effect assessment. Although there is a specific value of SDVV recommended in CAP 437, different interpretations are possible in the calculation of the value resulting in totally different assessment outcomes even under the same analysis condition. In this study, two different approaches are investigated and their results are compared. One approach is based on the spatial variation of the mean vertical velocity while the other utilizes the Turbulence Kinetic Energy (TKE) value from the Computational Fluid Dynamics (CFD) simulation. With a CFD tool, Kamelon FireEX (KFX), a couple of 3-dimensional simulations is performed and turbulence flows around an offshore semi-rig are obtained. SDVV values are calculated using both approaches and compared each other as well as with criteria recommended in CAP 437. It is hoped that the result of this study is helpful to engineers for understanding evaluation methods of turbulence effects in the helideck availability assessment.

Commentary by Dr. Valentin Fuster
2015;():V001T01A043. doi:10.1115/OMAE2015-41055.

Lifting activity has been usually employed to deploy subsea equipment, especially for huge structures. In one target field of China South Sea, there was one 3D “M” shaped, long and massive jumper being lowered to 200 meter water depth. The jumper was nearly 90 meters long, 22 inches in diameter, deployed by using a spreader frame. The total weight of jumper and frame has been up to nearly 400 Te in air. The whole lifting system mainly includes spreader frame, jumper, two connectors, wire rope and serial slings, etc. Two connectors were welded on both ends of the jumper, and jumper was lifted under spreader by several slings. The deployment operation was proposed by deepwater pipelay crane vessel Hai Yang Shi You (HYSY) 201. To ensure a successful installation, COTEC Offshore Solutions, together with its mother company, China Offshore Oil Engineering Company, have developed an advanced analysis by considering practical offshore field procedure and accurate modeling technology.

In this paper, firstly, wet lift capacity of wire rope and natural period of the hosting system have been obtained for certain circumstances including axial resonance caused by hostile environment. In addition, lowering though the splash zone operation procedure was simulated by OrcaFlex 9.8a. Allowable Max crane tip tension force, Dynamic Amplification Factor (DAF), Min wire rope force, Max Cranemaster® stroke motion and heave velocity at jumper were obtained during the numerical analysis. Also, allowable values of these key factors are further studied.

According to the above analysis, COTEC provides a series of environmental parameters such as the optimum wave direction and period for the offshore lifting operation.

Commentary by Dr. Valentin Fuster
2015;():V001T01A044. doi:10.1115/OMAE2015-41159.

The container sector has been growing fast since its dawn, and it has almost reached a climax in terms of handling mega containerships at existing ports. In that respect the offshore port system presented here can offer operational advantages. Furthermore, it can bring improvements to the utilisation of ultra large containerships and the enhanced energy efficiency they introduce. In this framework, this paper presents a cost analysis of the possible offshore port systems including their investment cost. This research is based on data from relevant projects and studies coupled with a number of assumptions to be presented. The results suggest that the offshore container port system can play a remarkable role to reduce energy consumption in the container shipping.

Commentary by Dr. Valentin Fuster
2015;():V001T01A045. doi:10.1115/OMAE2015-41352.

Two-dimensional liquid sloshing in rectangular tank of FLNG system is investigated both numerically and experimentally. In numerical simulation, a time-domain scheme has been developed based on potential flow theory in boundary element method. Tank movement is defined by wall boundary condition to produce a reciprocating oscillation. Nonlinear free surface condition is adopted to capture free surface elevation. Energy dissipation caused by viscous effects is considered by applying artificial damping term to the dynamic free surface condition, which is also vital to achieve a steady-state solution. For comparison, experiments of a rectangular tank filled with water subjected to specified oscillation are carried out. As coupling effects between sloshing and tank motion is not included in this research, the testing apparatus is required to produce consistent oscillation movement and not affected by the change of filling condition and sloshing load. Liquid surface elevations in several typical places of the tank were measured. Sloshing related parameters including oscillation amplitude, frequency and filling level are analyzed systematically. It’s found that numerical simulation results have good agreement with phenomenon observed under small amplitude excitation, and this nonlinear analysis method is proved to be effective in capturing liquid surface elevation. It is found that sloshing in tank is sensitive to filling level as well as excitation frequency, especially in the crucial combination cases of them. For given filling level, sloshing tends to be violent near corresponding natural frequencies, and viscous damping has limited contribution to sloshing amplitude when resonance occurs. This fundamental investigation also paves path for the study of more complicated sloshing problems.

Topics: Sloshing
Commentary by Dr. Valentin Fuster
2015;():V001T01A046. doi:10.1115/OMAE2015-41401.

This paper presents a simple method to study the installation process of a flexible jumper lowered into deep water by use of cables in a 3D space. Based on the catenary theory, the initial configuration of the installation system can be obtained easily. Then an iterative procedure, which uses force equilibrium and compatibility requirements as convergence criteria, is adopted to establish the final configuration considering the environmental loads. The internal force, bend radius and displacement of the flexible jumper are also obtained by finite-element discretization. The acquired results are compared with those derived from OrcaFlex finite-element model, in order to verify the accuracy and reliability of the proposed method. The influence on the minimum bend radius and maximum axial tension of the flexible jumper due to the difference between the lowering cable lengths, is also studied. The approach presented throughout this paper can offer some suggestions in regards to the installation of a flexible jumper in practical engineering.

Commentary by Dr. Valentin Fuster
2015;():V001T01A047. doi:10.1115/OMAE2015-41415.

This paper examines historical Arctic marine accidents from 1995–2004. It was seen during this time period that sinking and grounding of (fishing) vessels was the most common type of Arctic marine accident. A comprehensive accident model is presented to describe Arctic shipping accidents and their causation factors. The accident model is based on epidemiological concepts which explain how non-sequential factors result in an unwanted outcome, analogous to disease spreading through a human body. The causation factors are non-sequential and non-linearly dependent. The applicability of the model is demonstrated through examination of two past accidents: the Kolskaya and the Kulluk. Detailed description of how the accident model could be used for predictive accident modelling and risk analysis of Arctic shipping scenarios is also presented.

Commentary by Dr. Valentin Fuster
2015;():V001T01A048. doi:10.1115/OMAE2015-41428.

This paper addresses the severity of consequences of ineffectively performing evacuation operations of offshore installations in harsh environments. Hazards, particularly fire, smoke, heat, and extreme weather conditions, can harm personnel both directly and indirectly and prevent personnel from performing an evacuation operation successfully. To demonstrate the dependence of consequences on the hazards, this paper uses an event tree (ET) analysis. The event tree analysis maps all possible sequences of events leading to an accident and thus, reflects the level of knowledge about the evacuation operation. The paper uses ET analysis to develop a logical approach to hazards and consequences with the presence of safety functions. Applications of an ET analysis are discussed for two emergency response actions: a) detecting the emergency alarm, and b) moving along the escape route. In a situation where the offshore installation manager (OIM) delays activating the emergency alarm or the alarm system is damaged, personnel may not receive or hear the message. Availability of both primary and alternative escape routes enable personnel to move to a safe area in an escalating event, such as a series of fires and explosions. The paper discusses the ET analysis of hazards and consequences in specifically a qualitative manner. Information from the analysis can be used in a risk assessment of evacuation operations.

Topics: Evacuations
Commentary by Dr. Valentin Fuster
2015;():V001T01A049. doi:10.1115/OMAE2015-41519.

The identification of potential failure locations in ageing topside piping equipment of offshore production and process facilities (P&PFs) requires rigorous analyses. The risk based inspection (RBI) analysis methods defined in industrial standards classify the equipment according to the risk levels. The RBI analysis carried out on the plant equipment of P&PFs identifies the systems and sub-systems according to the different risk levels. However, the sub-systems (i.e. corrosion loops) are mainly defined according to the degradation behavior of the equipment. The equipment hierarchy for P&PFs is designed based on the RBI analysis outcome.

The major part of risk level evaluation is based on equipment’s probability of failure, which is related to degradation. The risk level analysis of equipment and thickness measurement locations (TMLs) within a sub-system is based on the expert engineering judgments of field experts. This analysis varies depending on the expertise of the evaluator in identifying and classifying critical TMLs in a sub-system. This manuscript illustrates an expert system using fuzzy inference systems (FISs) to identify the potential failure of TMLs in a sub-system suppressing the variation of analysis results given by different field experts. The method is illustrated by considering two degradation mechanisms (corrosion and erosion) to identify the probability of potential failure and integrating the production and process data to the FIS based expert system to generate a dynamic output.

Commentary by Dr. Valentin Fuster
2015;():V001T01A050. doi:10.1115/OMAE2015-41871.

This paper describes how the operation of deep, subsea oil wells can be analyzed and optimized using artificial lift systems. A modest explanation was offered about an enhanced Hubbert model for determining production targets at pre-feed phase of project. In addition, the impact of artificial lifts on the economics of subsea wells facing hyperbolic production decline was illustrated. The principle of Nodal analysis was highlighted and applied to optimize a proposed subsea oil production case. Configurations of a nominally rated rod pump, a multiphase pump and an electrical submersible pump were modelled in a steady-state flow using Pipesim software and the simulated results which were functions of liquid flow rate and pressure distribution across the production system exposed the behavior of the system.

The results showed that over 100% volumetric efficiency was achieved using a combination of electrical submersible pump at the bottom hole and a multiphase pump at riser base. A guide is presented on how to predict, analyze and enhance the recovery curve of subsea oil production using artificial lifts and nodal-system analysis. The benefit of this work is an enabling cost-effective approach for ensuring production assurance in deep water oil and gas production.

Commentary by Dr. Valentin Fuster
2015;():V001T01A051. doi:10.1115/OMAE2015-41940.

The prediction of wave-induced loads on Remotely Operated Vehicles (ROVs) during deployment through the splash zone is an important requirement to the design of a launch and recovery system. This paper presents the results of numerical and experimental hydrodynamic analysis of the motion response and wave-induced loads on a subsea trencher ROV during its deployment through the splash zone. The main focus of the study is to determine the maximum wave-induced loads and also to establish the maximum range of sea conditions in which the ROV can be operated safely. The numerical measurement of the hydrodynamic responses is completed using a 3D potential theory–based solver. The results obtained are compared with the predicted experimental responses measured using a 1/12 scale model of the ROV in a wave tank. A further comparison of the numerical responses with a box-shaped model of approximately similar overall dimensions to the ROV is performed in order to establish the validity of using a simplified shape to represent the actual ROV in various modelling scenarios. The result of this comparison shows that using a box-shaped model grossly over-predicts the responses and can lead to overly conservative load prediction.

Topics: Waves
Commentary by Dr. Valentin Fuster
2015;():V001T01A052. doi:10.1115/OMAE2015-41944.

Subsea wellhead systems are exposed to wave induced cyclic loading when a drilling unit connects to the well with a marine riser and a BOP. When connected, access is provided to the well and reservoir, and allows for operations such as further drilling, side tracking or workover. Once the operation is completed, the BOP is disconnected from the well, and the wellhead system is not exposed to cyclic loading any longer. Over the lifetime of a well, a number of such operations take place. A wellhead system is perhaps exposed to a total duration of fatigue loading of up to a year, which comprises a sequence of operations of different durations in different seasons. Fatigue predictions for offshore structures are typically based on statistical average of environmental conditions over a large number of years. This is appropriate for permanent installations exposed to continuous wave loading over the lifetime which is often 20 years or more, since variations in the environmental conditions from one year to another is equally represented in the statistics and experienced by the structure. However, for an operation of short duration, the uncertainty in the environmental conditions for that particular period in that particular year needs to be addressed. The weather during October this year is unlikely to be the same as in October last year, and can also be significantly different from average October weather. Although there exists no standard way of doing wellhead fatigue analysis, a commonly applied approach is to do the analysis in a single plane. This is obviously conservative since the wave direction will vary over time, and the fatigue loading will be distributed more around the circumference of the pipe sections in the wellhead system. Furthermore, the environmental conditions are typically based on statistical average for the month or season when the operation is to be executed, sometimes with some conservatism of including the adjacent more severe month or using annual data. Long crested waves are often assumed. This paper address the effect of the uncertainty in the environmental conditions on the accumulated fatigue damage for single and sequences of operations of different durations at different times of the year. A drilling rig in the North Sea has been analyzed using 56 years of hind cast data of significant wave height, peak wave period and main wave direction. Statistics of the fatigue damage rates are calculated and used in a structural reliability analysis in order to estimate reasonably but not overly conservative factors that are to be multiplied with the fatigue damage estimated in a conventional design analyses. Results based on long crested and short crested sea are calculated. An annual variation factor is proposed to account for the variability from one year to another. Secondly, a directional effect factor is proposed to account for the directional variations and its uncertainty on fatigue. Both factors are first estimated considering a single operation only, where the duration is varied between 3 days and up to a year. Thereafter, a sequence of operations of different durations at different times of the year is analyzed, and it is proposed how to consider the accumulated duration of such sequences compared to a single continuous operation. The expected result is an annual variation factor which is greater or equal to unity and a directional effect factor which is less than unity, both with lower values the longer the duration. The product of the two is a quantification of the degree of conservatism associated with a deterministic design analysis using long crested head sea and statistical average omnidirectional weather for the planned drilling operations.

Topics: Fatigue , Drilling
Commentary by Dr. Valentin Fuster
2015;():V001T01A053. doi:10.1115/OMAE2015-42088.

In the last years hydrodynamic interaction between two vessels in side-by-side configuration is one of the hot issues in offshore floating body dynamics. The paper investigates the hydrodynamical aspects of a floating two body system. The topic is geared towards analysing the influence of the vessel’s draft in side-by-side configuration and in head sea condition. The need to solve this problem arises when one wants to study the hydrodynamic variation for the various stages of a offloading process with a defined operational gap. The system is composed of a barge and a prismatic geosim with a fixed gap value and with two barge’s draft values. Regular wave tests have been performed in the model basin of CEHINAV-Technical University of Madrid (UPM). The motion for the geosim was restricted to the surge, heave and pitch motions (just motions on the vertical plane), whereas the barge was kept fixed. The costant gap value is guaranteed during the tests. A numerical model has been created with WAMIT and with an in-house time-domain Rankine Panel Method (TDRPM). In each case the numerical and experimental response amplitude operators (RAOs) are obtained and compared, researching the limitation of the numerical codes for the gap flow modeling. In the past the gap effects on the numerical results have been studied varying the gap value finding resonant behavior in terms of motion and wave amplitude RAOs. Now the draft value contribution on the hydrodynamic effects is investigated. Also in this case the numerical results indicate a resonant behavior in determined frequencies in motion as well as in wave in the gap, that is not found in the tests. In order to overcome this problem, a procedure for introducing an external damping factor that attenuates the wave amplitude along the gap in the time-domain RPM is evaluated based on the experimental data.

Topics: Resonance
Commentary by Dr. Valentin Fuster
2015;():V001T01A054. doi:10.1115/OMAE2015-42191.

In August 2014 Heerema Marine Contractors (HMC) have successfully installed the Solan Subsea Oil Storage Tank (SOST), using its Semi-Submersible Crane Vessel (SSCV) Thialf. The SOST installation is regarded as unique due to its dimensions, complex hydrodynamics and the utilization of nearly the full crane capacity.

The Solan SOST is a Subsea Oil Storage Tank with a future storage capacity of 48,000 m3 of oil. The SOST has a dimension of 45m × 45m × 25m, a dry weight of 9,500mT and a total submerged mass of 60,000mT including ballast water. It was installed offshore west of the Shetland Islands at a depth of 135m. Prior to installation the SOST was wet-towed to the field. At location the SOST was installed by a dual crane lift sequence, lowering the SOST from 2m freeboard to the seabed. The lowering sequence is characterized by the usage of a compressible air pocket to reduce hookloads during the most critical stage of the installation, the lowering of the SOST through the splash zone.

Prior to the installation ballast and dynamic lift models were generated to understand both the hydrostatics and hydrodynamics of the SOST - SSCV Thialf system throughout the installation. Numerical simulations of the flooding operation were performed to predict all relevant parameters for various scenarios. The lift dynamics were analysed with frequency domain models. Most resonance modes between SSCV Thialf and SOST were found not to be excited due to the differences in natural periods of the hydrodynamic systems and the occurring wave periods.

During installation vital parameters such as ballast volume, compartment fill rates, differential pressure and hookloads were continuously monitored with dedicated measurement systems. The values were compared with the results from the engineering models. As some of the parameters were directly related to each other, the values could be back-calculated and cross-checked, thereby increasing the reliability of the measurements.

Both the understanding of the principle hydrostatics and hydrodynamics of the system as well as the close monitoring of all vital parameters have resulted in a safe and controlled installation of the Solan SOST by the SSCV Thialf; the installation of a 60,000mT dual crane pendulum. This paper describes the hydromechanic engineering work performed by HMC and the SOST offshore installation.

Commentary by Dr. Valentin Fuster
2015;():V001T01A055. doi:10.1115/OMAE2015-42245.

As the cost of drilling and completion of offshore well is soaring, efforts are required for better well planning. Safety is to be given the highest priority over all other aspects of well planning. Among different element of drilling, well control is one of the most critical components for the safety of the operation, employees and the environment. Primary well control is ensured by keeping the hydrostatic pressure of the mud above the pore pressure across an open hole section. A loss of well control implies an influx of formation fluid into the wellbore which can culminate to a blowout if uncontrollable. Among the factors that contribute to a blowout are: stuck pipe, casing failure, swabbing, cementing, equipment failure and drilling into other well. Swabbing often occurs during tripping out of an open hole. In this study, investigations of the effects of tripping operation on primary well control are conducted. Failure scenarios of tripping operations in conventional overbalanced drilling and managed pressure drilling are studied using fault tree analysis. These scenarios are subsequently mapped into Bayesian Networks to overcome fault tree modelling limitations such s dependability assessment and common cause failure. The analysis of the BN models identified RCD failure, BHP reduction due to insufficient mud density and lost circulation, DAPC integrated control system, DAPC choke manifold, DAPC back pressure pump, and human error as critical elements in the loss of well control through tripping out operation.

Commentary by Dr. Valentin Fuster
2015;():V001T01A056. doi:10.1115/OMAE2015-42270.

Lifting operations with offshore cranes are fundamental for proper functioning of a platform. Despite the great technological development, offshore cranes load charts only consider the significant wave height as parameter of environmental load, neglecting wave period, which may lead to unsafe or overestimated lifting operations. This paper aims to develop a method to design offshore crane operational limit diagrams for lifting of personnel and usual loads, in function of significant wave height and wave peak period, using time domain dynamic analysis, for a crane installed on a floating unit. The lifting of personnel with crane to transfer between a floating unit and a support vessel is a very used option in offshore operations, and this is in many cases, the only alternative beyond the helicopter. Due to recent fatal accidents with lifting operations in offshore platforms, it is essential the study about this subject, contributing to the increase of safety. The sea states for analysis were chosen covering usual significant wave heights and peak periods limits for lifting operations. The methodology used the SITUA / Prosim software to obtain the dynamic responses of the personnel transfer basket lifting and container loads on a typical FPSO. Through program developed by the author, it was implemented the automatic generation of diagrams as a function of operational limits. It is concluded that using this methodology, it is possible to achieve greater efficiency in the design and execution of personnel and routine load lifting, increasing safety and a wider weather window available.

Commentary by Dr. Valentin Fuster

Offshore Technology: Station Keeping

2015;():V001T01A057. doi:10.1115/OMAE2015-41120.

Dynamic positioning capability (DPCap) analysis is a traditional method to verify a DP vessel’s ability to resist the environmental conditions for given headings. However, DPCap analysis is inherently quasi-static, the environmental forces are statically balanced by the thrust forces provided by the thrust system. Dynamic station-keeping capability (DynCap) analysis can provide a more detailed study of a vessel’s station keeping capability in realistic dynamic conditions. DynCap analysis is based on a DP simulator, which is absolutely dynamic and relatively realistic. However, since the numerical simulation based on the simulator is more time-consuming compared to the static analysis, the selection of the environmental condition sequence becomes critically important to the efficiency of the DynCap analysis. This paper proposes application of the bisection method to achieve the environmental condition limit and a controller gain database method to tune the control parameters in the DP simulator. The efficiency of the DynCap analysis is expected to be improved. Numerical analyses for a semi-submersible were conducted to investigate the performance of a program developed based on the present study. The feasibility of the program and the effectiveness and efficiency of the methods were both demonstrated.

Commentary by Dr. Valentin Fuster
2015;():V001T01A058. doi:10.1115/OMAE2015-41390.

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.

Topics: FPSO , Mooring
Commentary by Dr. Valentin Fuster
2015;():V001T01A059. doi:10.1115/OMAE2015-41404.

By monitoring the variation of weights of floating production units (FPUs), the sum of total weight computed by load calculators on board very often does not match the actual displacement based on the current drafts. Differences can also be observed in the trim and heel of FPUs, which present values different from zero degree in the calculations, but in fact they are frequently kept near zero by ballast control.

The mooring lines and risers tensions are one of the most uncertain weight items in loading conditions reported by the crew on board, therefore, this paper aims to assess the influence and behavior of these systems to a variety of situations in which FPUs operate.

Analyses were performed for semi-submersibles and FPSOs considering two configurations of mooring system: catenary and taut-leg. The purpose is to evaluate how the magnitude of the resulting force varies — and hence how the trim and heel change — for a range of offsets caused by environmental conditions.

The effect of mooring lines and risers is also discussed regarding the water depth by means of case studies considering a range of water depths. Actual lines properties and seabed bathymetry from mooring system models of platforms located offshore Brazil have been taken as reference.

In short, the mooring lines and risers loads will be calculated for different types of floating production units, mooring system configurations and water depths in order to evaluate their influence on the trim, heel and displacement of FPUs.

Commentary by Dr. Valentin Fuster
2015;():V001T01A060. doi:10.1115/OMAE2015-41514.

The scaled model testing of a FPSO with its mooring lines and risers for deepwater and ultra-deepwater installation sites is considered to be the most reliable methodology to study the complex hydrodynamic behaviour of the complete system, since it can represent the nearest real environmental conditions and the comprehensive hydrodynamic interactions of the waves, current and wind with the total floating production system. The best technical option, at the present time, is a combination of an appropriate scale model of the FPSO and a suitable level of truncation obtained using a hybrid passive truncated experimental methodology for the mooring lines and risers. This is in order to minimize the various uncertainties in model behaviour and to study the hydrodynamic behaviour of the complete floating system and thus to validate numerical design of prototype systems for installation in deepwater and ultra-deepwater locations. This paper investigates the global response of a specific FPSO to prevailing environmental conditions, based on a hybrid passive truncated experimental methodology for the mooring lines and risers in a specific deepwater location with a water depth of 1000m in the GOM. The main objective of the experiment is to examine the nonlinear effects of the FPSO vessel and its mooring lines and to provide reliable experimental data for subsequent extrapolation to a full scale prototype design. Several case studies were carried out. The FPSO global responses for Full load and Ballast Load conditions with static and dynamic load tension components of the truncated mooring lines were studied for both collinear and non-collinear extreme storm environmental conditions. The experiments revealed that the main horizontal plane motion response of the FPSO (surge) under non-collinear loading condition is almost two-times that of the collinear loading condition.

Commentary by Dr. Valentin Fuster
2015;():V001T01A061. doi:10.1115/OMAE2015-41897.

Vessels equipped with a Dynamic Positioning (DP) system for station keeping have become quite common on the offshore market. The propulsion system of such ships has the capability to compensate the counteracting environmental forces caused by wind, waves and current. Since the DP capability is an important part of the specification, it is necessary to consider this aspect in the early ship design stage. For this purpose a procedure of a fast calculation method is developed by the authors to predict the limiting environmental conditions and the maximum deviation of the position and the course angle for a pre-established propulsion system.

Commentary by Dr. Valentin Fuster
2015;():V001T01A062. doi:10.1115/OMAE2015-41948.

The principle of dynamic positioning with thrusters is described and the problem of thrust allocation discussed. To allocate the demand for surge and sway forces and yaw moment from the DP controller to a greater number of thruster variables requires some extra mathematical conditions to make the problem uniquely solvable. Often the extra conditions are obtained by formulating a criterion for optimal thruster usage, typically an allocation that is power-minimal. Such problems usually cannot be solved without resorting to numerical search methods, which can be unattractive when the available time is scarce, which is the case when DP with allocation is used in model testing or when a large number of long stochastic simulations need to be carried out. For such cases, simple weighted least squares allocation with linear equality constraints is an attractive alternative. This method is very close to being power minimal provided the weights are chosen correctly. To handle cases of thruster saturation a method is proposed, which uses bias thrust and a certain matrix of nullspace vectors. A similar principle is proposed for presetting of favourable angles for azimuth thrusters to reduce the effect of the slow azimuth response.

Topics: Thrust
Commentary by Dr. Valentin Fuster
2015;():V001T01A063. doi:10.1115/OMAE2015-42022.

A new riser anti-recoil control design is presented in this research paper using the novel riser hybrid tensioning system by combining a fast acting electrically powered riser tensioning system with existing conventional hydro-pneumatic tensioners. This work is the further development of a control system for the previous work presented in the framework proposal of riser hybrid tensioning system.

This innovation that we bring forward here is all designed to increase operating envelope of the vessel. A system model integrating both electrical and hydro-pneumatic tensioners and the riser string is built for anti-recoil control purpose. The main goal of this control design is a safer and more predictable position control of the riser string. A LQG control design technique is applied here. A feedback controller with Kalman estimator of the system state space variables is designed and implemented. The Matlab simulation helped to demonstrate the concept feasibility of this anti-recoil control strategy and to further verify that a more robust and accurate control performance could be achieved by this riser hybrid tensioning system.

This control technique increases the testability of the riser anti-recoil system; reduce potential damage and increase operator’s confidence. The new proposed data logging system improves the detectability of the anti-recoil system. And the position control, when the riser string “soft hang-off” on the tensioners, opens the possibility to extend the operability to other applications.

Commentary by Dr. Valentin Fuster
2015;():V001T01A064. doi:10.1115/OMAE2015-42127.

This paper presents a CFD simulation to study the ventilation phenomenon of tunnel thrusters in dynamic positioning (DP) mode of a typical North Sea shuttle tanker consisting of two main propellers, two rudders, and two bow tunnel thrusters. Measurements of blade thrust and moment for the propeller shaft (corresponding to propeller torque) with ventilated propellers at different submersion positions are presented and discussed. Additionally, in order to obtain insight into the effect of waves on ventilation which further affects propeller loading and dynamic fluctuations, simulations of a tunnel thruster are performed at different immersion ratios.

This paper also presents and discusses the factors in the evaluation of thruster performance, such as, the extent of present knowledge for tunnel thrusters as related to ahead ship speed, and interaction between thruster jet flow and the mainstream with various drift angles. Moreover, thrust degradation of tunnel thrusters is considered in the thrust allocation algorithms for the DP capability calculation.

The objective of this study is to understand the dynamics of thruster forces. In addition, results of the study provide knowledge for a robust thrust allocation algorithm for dynamic positioning capability assessment.

Topics: Ventilation , Tunnels
Commentary by Dr. Valentin Fuster
2015;():V001T01A065. doi:10.1115/OMAE2015-42397.

As offshore activities are growing, the marine operations are becoming more complex involving the presence of few or several vessels in proximity to each other which increases the risk associated to those operations. Shuttle tankers, PSVs, floatels are often equipped with DP systems for maintaining position. The capability of these systems is defined during design phase by the DP manufacturer based on the assumption of standalone operation and considering environmental load cases prescribed in Industry standards (ex. wind, wave and current all aligned). During a realistic operational condition, however, the presence of other unities may significantly alter the loads acting on the DP vessel which will affect somehow its station keeping capacity. Furthermore, in some areas of the world, the misalignment between the environmental loads and the presence of several wave trains from different directions (ex. off-shore Brazil) shall be considered in the sake of safety of the operation. In order to provide the clients means to simulate these complex operations (including moored vessels), a DP module has been integrated to Bureau Veritas multi-body mooring software, ARIANE. In this paper, the case of a DP floatel vessel operating close to a turret moored FPSO in Brazilian waters is analyzed and the differences in the DP capacity under realistic conditions with respect to the original DP capability are presented and discussed.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Anchors and Shallow Foundations

2015;():V001T10A001. doi:10.1115/OMAE2015-42214.

Hybrid subsea foundations (HSF) are combined foundation systems of mudmats and piles. The primary motivation of combining these two foundation types is to provide greater resistance to large horizontal loads in addition to vertical loads, for which use of mudmats alone will require it to be of impractically large size. The contribution from the piles in the lateral capacity helps to limit the size of the mudmat, which is critical in subsea environment. In a brownfield situation, this is sometimes a hard limit with only limited space available to place a new mudmat in the existing field layout. Also, in some cases, the HSF may prove to be a more economical option for resisting large horizontal loads compared to, for example, to suction piles. While the authors are aware of some scattered project-specific design and use of subsea mudmat-pile hybrid foundations by individual contractors and operators, there is no industry-wide publicly known best practice currently available. These designs of HSF appear to be generally based on simplified analytical approach that require superimposition of conventional shallow and deep foundation capacity calculation methods, hence violates the static and kinematic compatibility requirements fundamental for a sound and robust prediction procedure.

This paper attempts to provide some insight into the behavior of mudmat-pile foundations as a hybrid integrated system numerically using finite element modeling and analysis (FEA). The interactions between the mudmat and the piles in an HSF are complex and hence a FEA-based approach is considered most suitable. The FEA model in this study included the mudmat, the corner piles, the pile-mudmat connections and the seabed soil. Sensitivity of the HSF capacity to the size of the piles (length and diameter), the connection type of the piles to the mudmat, and the number of piles are selectively investigated and the results presented. Based on these results some pertinent observations relevant to design of HSFs are also given.

While the study is of limited scope, it offers important insights into the effects of the primary design variables on HSF’s capacities. Therefore, the authors hope the information herein will be of benefit to practicing subsea engineers who might have to face choices to consider mudmat-pile hybrid foundations as a real option for their projects.

Commentary by Dr. Valentin Fuster
2015;():V001T10A002. doi:10.1115/OMAE2015-42340.

An increasing number of well-related and pipeline-related subsea structures (e.g. PLETs, ILTs, buckle initiators, etc.) are being placed on the seabed. Many of these structures are founded on unskirted and skirted mudmat foundations as a cost-effective, low risk solution.

Much attention has been placed in the literature on the geotechnical capacity of subsea mudmat foundations, but less attention on how these types of foundations are installed. In some soil conditions this can be a critical aspect of design: the requirement to add skirts to ensure sufficient foundation capacity comes with the penalty of increasing the necessary foundation weight to ensure that these skirts can be reliably installed. The addition of weight to a foundation increases its installation and fabrication cost and so requires careful treatment in design. As an example of this design optimization process, attention is given to dealing with uncertainty associated with variable soil conditions. It is shown how probabilistic geotechnical analysis can give a clearer perspective of cost and risk and how carefully targeted site investigation can also be used in this context.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Fluid-Soil-Structure Interaction

2015;():V001T10A003. doi:10.1115/OMAE2015-41474.

The pile foundations of offshore structures are vulnerable to local ground loss by scouring. The vortex flow affected by the flow conditions, ground conditions, and the shape of the pile is the main cause of scouring. The decrease of an embedded depth of foundation pile by scouring may lead to an excessive displacement of the structure and a decrease of the bearing power, leading to a collapse of the offshore structure. In this work, a three-dimensional commercial code named STAR-CCM+ has been employed to predict the final depth of the scouring-hole around the foundation of offshore structures. Assuming the bed as a fluid having a high level of dynamic viscosity and density, the prediction has been accomplished by using the multi-phase flow model. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations, and standard k-ε turbulence model to estimate the depth of the scouring-hole. To guarantee the reliability of the model, the results of the numerical model have been compared with available laboratory measurements. A reasonable agreement has been observed.

Commentary by Dr. Valentin Fuster
2015;():V001T10A004. doi:10.1115/OMAE2015-42141.

Designing the cover height of buried pipelines to prevent them from buckling requires a method that can thoroughly and realistically model the phenomenon. This paper introduces a new technique to assess the risk of upheaval buckling (UHB) by using backfill soil springs (BFSS) to represent the uplift resistance provided by the backfill soil on top of a buried pipeline. This paper investigates the pre-buckling pipeline behavior related to UHB and highlights some of the key parameters governing the analysis. UHB assessment based on a case study was carried out and the results were then compared with those obtained from force-equilibrium methods generally used in the industry. The comparison shows that UHB assessment can be performed more rigorous using BFSS than using force-equilibrium methods. Therefore, using BFSS for UHB assessment improve the reliability in cover height design.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Pile Foundations

2015;():V001T10A005. doi:10.1115/OMAE2015-41088.

Installation of driven piles and suction caissons in clayey soils generates excess pore pressures that temporarily reduce load capacity due to side resistance. Time dependent dissipation of these excess pore pressures leads to recovery of side resistance, a process known as ‘setup’. Since many facilities cannot be put into operation until sufficient pile load capacity has been mobilized, realistic predictions of setup time can be important. This study consists on the analysis of setup time following open ended pile and caisson installation. Initial excess pore pressures due to installation disturbance are predicted based on a strain path analysis based on a ring source moving at constant velocity in an incompressible medium. It is assumed that the setup occurs primarily due to dissipation of excess pore pressures generated during the installation process; thixotropic effects are neglected. The analysis employs an elastic perfectly plastic model of soil behavior and an uncoupled analysis of consolidation to simulate conditions on the pile shaft outside of the influence of tip effects. A parametric study shows that wall thickness and soil rigidity index can exert order of magnitude differences on setup time. Strain path solutions show reasonable agreement to laboratory and field measurements of pore pressure dissipation around thin-walled piles typical of suction caissons. Strain path solutions tend to underestimate setup time for driven piles, likely due to partial plugging during pile driving.

Topics: Caissons
Commentary by Dr. Valentin Fuster
2015;():V001T10A006. doi:10.1115/OMAE2015-41146.

The influence of wave propagation through the soil on hydro sound immission due to offshore pile driving bears a lot of uncertainties. In acoustic analyses of impact pile driving, the sound source description is essential for wave propagation models. The excited open-ended steel pile and the soil interact with each other. In numerical analyses the choice of the used material law for the soil should be well suited, because of the effect on the pile vibration. Prognoses often use the linear elastic approach for non-cohesive granular materials which does not represent the inelastic material behavior of the soil. On the basis of experimental data, a numerical model to simulate one blow of impact pile driving is validated. A parameter study of the wall friction angle between steel and sand is conducted to investigate the influence on the radiated hydro sound level under drained conditions.

Commentary by Dr. Valentin Fuster
2015;():V001T10A007. doi:10.1115/OMAE2015-41332.

Offshore wind power has gained momentum as a means to diversify the world’s energy infrastructure; however, little is still known of the global stiffness behavior of the large diameter low aspect ratio monopiles which have become the foundation of choice for offshore wind towers. Traditionally, offshore foundations have been associated with gravity structures for the oil and gas industry, which in general need to resist large vertical loads with limited lateral and moment loading. However, wind towers are purposely designed to be subjected to large lateral and moment loads from the wind and waves in order to maximize power generation. Geotechnical centrifuge tests were conducted and numerical models are being developed to examine the behavior of low aspect ratio piles in clayey soils. Monopiles with aspect ratio of two are being tested in the the 150g-ton centrifuge at Rensselaer Polytechnic Institute. Initial results include momenttheta and force-displacement for various loading conditions. Numerical studies consist of finite element (FE) simulations in order to predict capacities and permanent deformations. The comparisons are to be performed in terms of the total resistance that is exerted by the soil on the caisson. FE studies allow to model capacity for different displacement fields and also to compute interactions between different loading modes. This paper outlines our progress to date including both numerical and experimental results.

Commentary by Dr. Valentin Fuster
2015;():V001T10A008. doi:10.1115/OMAE2015-41735.

Offshore anchor piles are usually loaded at a padeye on pile surface. The padeye depth can be at the seabed or below it. Using a padeye below the seabed is widely used in case of suction caissons. However, anchor piles are more flexible and the mode of failure will be different from that for suction caissons. In the current parametric study, the effect of padeye depth on the behavior of offshore anchor pile subjected to mooring forces in dense sand was studied. Finite Element Model (FEM) had been established. The model had been calibrated based on the centrifuge tests that were carried out by the authors. Three piles of different soil-pile rigidity covering a wide range of pile flexibility were used in the study. The piles were pulled out at an angle of 15° to horizontal. In all cases the padeye depth was changed from at the ground surface to a depth of four times the pile diameter. From this parametric study, it was found that pulling out an offshore anchor pile at a level below the seabed has some advantages of increasing the ultimate capacity of the pile, decreasing pile deflection, and decreasing bending moment. An optimum depth of padeye was recommended.

Commentary by Dr. Valentin Fuster
2015;():V001T10A009. doi:10.1115/OMAE2015-42218.

With increasing demand of energy, attention to the alternative sources of sustainable energy is getting priority over the last decades. Offshore wind turbine is one of them. The most widely used foundation system for the wind turbine is the monopile, which is a large diameter single pile. In the present study, three-dimensional finite element (FE) analyses are performed to evaluate the capacity of large diameter monopiles in dense sand using the Arbitrary Lagrangian-Eulerian (ALE) approach available in Abaqus/Explicit FE software. The behavior of sand is modeled using the Mohr-Coulomb (MC) and a modified Mohr-Coulomb (MMC) model where the pre-peak hardening, post-peak softening and the effects of mean effective stress and relative density on stress-strain behavior of dense sand are considered. Comparison with physical model test results shows that the MMC model can simulate better the load-displacement response than that with the MC model. The mechanisms involved in soil deformation are also explained using FE results.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Pipeline Geotechnics

2015;():V001T10A010. doi:10.1115/OMAE2015-42076.

Deepwater surface laid pipelines generally penetrate a fraction of their diameter into the seabed. The near surface penetration behaviour of steel catenary risers (SCRs) is equally important in offshore oil and gas developments. Theoretical, physical and numerical investigations have been performed to understand pipeline–soil interaction during vertical penetration. The large deformation finite element (LDFE) modeling is a recent and advanced tool among different numerical modeling techniques. The authors of this study simulated the penetration of pipeline using Abaqus CEL Finite Element (FE) software [1]. They also developed a numerical modeling technique based on finite volume approach using ANSYS CFX [2] and showed some of its advantages. However, in that study an ideal soil (i.e. no softening or strain rate effects on undrained shear strength) was used. Strain rate and softening have significant effects on penetration behaviour and therefore in this study a numerical technique has been developed to incorporate these effects in ANSYS CFX. Comparison of the results shows that ANSYS CFX can also model the penetration behaviour. Moreover, ANSYS CFX has some advantages including low computational time, modeling of suction and pipeline–soil–water interaction. A parametric study is also presented to provide more insights into the pipeline–soil–water interaction.

Commentary by Dr. Valentin Fuster
2015;():V001T10A011. doi:10.1115/OMAE2015-42138.

Buried pipelines are extensively used in onshore and offshore for transportation of hydrocarbons. The response of pipeline due to lateral and upward relative displacements is one of the major concerns in pipeline design. Both physical modeling and numerical analyses have been performed in the past to understand pipeline-soil interaction mechanisms. The numerical analyses are generally performed using finite element (FE) modeling techniques. For the pipelines buried in sand, a large number of analyses available in the literature have been performed using the Mohr-Coulomb model assigning constant values of angle of internal friction (ϕ′) and dilation (ψ). However, dense sand shows post-peak softening behavior and the behavior of sand also depends on mode of shearing, such as triaxial (TX), direct shear (DS) or direct simple shear (DSS) conditions. In the present study, FE analysis of buried pipelines in dense sand is presented. The first set of analyses are performed using the built-in Mohr-Coulomb model in Abaqus FE software with constant angles of internal friction and dilation, as typically used in previous FE analysis of pipeline-soil interaction. The second set of analyses are performed using a modified Mohr-Coulomb model where pre-peak hardening, post-peak softening, density and confining pressure dependent friction and dilation angles are considered. The FE analyses are performed using the Arbitrary Lagrangian-Eulerian (ALE) approach available in Abaqus/Explicit FE software. The modified Mohr-Coulomb model is implemented in Abaqus FE software using a user defined subroutine. Shear band formation due to strain localization and failure patterns for both lateral and upward pipeline-soil interactions are discussed from the simulations with MC and MMC models. FE results show that the MMC model can simulate the load-displacement behavior and failure pattern better than the simulations with the MC model.

Commentary by Dr. Valentin Fuster
2015;():V001T10A012. doi:10.1115/OMAE2015-42216.

It is increasingly recognized that the state of the seabed surrounding an on-bottom pipeline may change during the operating life of the pipeline. For seabed sediments that are soft and fine-grained, the strength may vary through episodes of pipeline movement due to consolidation effects. For seabed sediments that are mobile due to waves and currents, the burial state and the adjacent seabed topography may vary due to sediment transport and scour.

These changes in the strength and topography of the surrounding seabed alter the exposure of the pipeline to hydrodynamic loads and ambient cooling, as well as the level of geotechnical support and insulation provided by the seabed.

The design relevance of these changes in seabed condition is amplified by modern design approaches in which the pipeline itself can be tolerably mobile — for example in a dynamic onbottom stability approach or through engineered schemes of global buckling and axial walking.

This paper illustrates the interactions between the geotechnical and sediment transport processes and the resulting global pipeline behaviour. Two interactions are considered: the long-term axial walking behaviour on soft soil, and the long-term insulation and temperature profile on a mobile seabed.

The examples highlight the potential for over or underestimation of various inputs to a pipeline design when these temporal changes in pipe-seabed condition are overlooked. Emerging analysis methods for pipeline-seabed interaction that incorporate these temporal effects can lead to more reliable and cost-effective design.

Topics: Pipelines , Seabed
Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Seabed Processes

2015;():V001T10A013. doi:10.1115/OMAE2015-41025.

As the unexpected wave-induced seabed instability may cause foundation failure, the evaluation of wave-induced pore pressure and effective stress in seabed plays an important role in the design of the foundation of marine structures. In this study, a two-dimensional integrated mathematical model, based on COBRAS wave model and SWANDYNE seabed model is developed to numerically investigate the mechanism of wave-induced seabed response in the vicinity of a permeable submerged breakwaters. Numerical results indicate that this model has a great ability in predicting the dynamic response of the pore pressure and effective stress around the breakwater. Both the pore fluid pressure and effective stress in seabed largely changes with an increasing water depth. It is also found that the responses of the pore pressure and effective stress of different locations to the dynamic wave loading are significantly different in the cases with variable top width of the breakwater.

Commentary by Dr. Valentin Fuster
2015;():V001T10A014. doi:10.1115/OMAE2015-41827.

Shallow foundation structures in marine environments can rarely be placed on top of the sea floor. Weak soils usually need to be excavated to place the structure on more stable ground. Steep but stable slopes of the resulting pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. This paper focuses changes of geometry of submarine slopes in non-cohesive soils (erosion, sedimentation, breach failure, liquefaction failure) due to surface waves.

After Terzaghi the angle between slope and the horizontal of the ground surface of cohesionless soil is at most equal to the critical state friction angle, as obviously true for dry soil. However, it can be observed that natural submarine slopes of sandy soils are always mildly sloped. During the construction of artificial submarine pits under offshore conditions it should be considered that the long-term slope-inclination is less than onshore due to hydrodynamic actions (e. g. flow, waves, earthquakes). Large surface waves cause excess pore water pressures within the soil body, leading to a reduction of effective stresses and in case of submarine slopes to changes of the slope geometry depending on wave length L, wave height H, water depth h and soil properties (permeability k, relative density Dr).

During our preliminary work we investigated such processes based on the coupling of linear wave theory and linear quasistatic consolidation theory (e.g. [1]). With the help of numerical modelling we solved corresponding equations considering also materially nonlinear consolidation. However, deformations were always limited by used Lagrangian-FEM. Recent developments at our Institute enable the use of an Eulerian-FEM approach with an u-p-Formulation for fully saturated soil [2]. This allows larger deformations of the subaqueous slope to be numerically investigated.

Topics: Stability , Stress , Waves , Soil
Commentary by Dr. Valentin Fuster
2015;():V001T10A015. doi:10.1115/OMAE2015-41829.

This paper investigates the stability of a submarine gassy slope triggered by tidal variations. Under tidal variations on an unsaturated slope, failure may occur under specific combinations of increasing degree of saturation and soil permeability, and decreasing tidal period. A novel physical model test in a geotechnical centrifuge was undertaken of a submarine slope containing gassy sediments. The model preparation techniques, measurement systems and preliminary results are presented. The response observed in the model test is discussed and further developments proposed. Existing numerical simulations may provide a basis for verification and validation of future physical model test results.

Commentary by Dr. Valentin Fuster
2015;():V001T10A016. doi:10.1115/OMAE2015-42241.

Human activities such as construction loading in upslope areas could be a potential triggering factor for many offshore landslides such as the 1979 Nice landslide. Post-slide investigations show that the existence of marine sensitive clay layers might be one of the potential causes of many large-scale submarine landslides. In this paper, a finite element (FE) modeling technique is developed to analyze the failure of a slope in undrained condition. Nonlinear strain softening behaviour of undrained shear strength of marine sensitive clays is incorporated in the FE analysis. Strain localization in narrow zones (i.e. shear bands) could be successfully simulated. The formation of shear bands and their propagation could explain some potential failure mechanisms. The FE results show that large-scale catastrophic failure of submarine slopes might have occurred due to shear band propagation through strain softening clay layers, which cannot be explained using the traditional limit equilibrium methods for slope stability analysis. Effects of different factors, such as thickness of the marine clay layer and its sensitivity, on stability of submarine slope are also examined.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Seabed Properties

2015;():V001T10A017. doi:10.1115/OMAE2015-41583.

Herschel Island, Yukon, Canada, is made of ice-rich permafrost and is affected by high rates of coastal erosion, likely to increase with decreasing summer sea ice extent. During an interdisciplinary expedition to Herschel Island in July 2014, geotechnical investigations were carried out in shallow water environments of up to 20 m water depth and at different beaches. The free-fall penetrometer BlueDrop was deployed at 299 positions. Apart from obtaining vertical profiles of sediment strength and the pore pressure response upon impact, the pore pressure evolution over a period of one hour after deployment was investigated. The focus area for these tests was Pauline Cove, located at the south-eastern side of the island, being sheltered by a spit from the open Beaufort Sea and affected by a number of old and young retrogressive thaw slumps, delivering large amounts of mud. The sediment resistance profiles revealed up to three distinct layers of sediment strength, expressing different consolidation states, or possibly changes in sediment composition. This stratification was supported by the pore pressure results, including pore pressure evolution “on-the-flight” during penetrometer penetration as well as pore pressure evolution at maximum penetration depth with the penetrometer being at rest. The sediment surface layer 1 was characterized by a thickness of 5–20 cm depending on the respective location, low sediment resistance and predominantly hydrostatic pressure. It most likely has frequently been reworked by wave action, and exhibited similar geotechnical signatures as fluid mud. Layer 2 reached sediment depths of 30–60 cm, showed an increase in sediment resistance and distinct subhydrostatic pore pressures during penetration, while pore pressures increased in an asymptotic manner to suprahydrostatic (160–180% of hydrostatic pressure) over an observation period of 30–50 minutes. Based on comparison to other examples from the literature, it was hypothesized that layer 2 was composed of overconsolidated mud. Layer 3 featured a significant increase in sediment resistance as well as pore pressure during penetration. As soon as the probe came to rest, the pressure decreased significantly to subhydrostatic conditions, before swinging back to being suprahydrostatic and then slowly dissipating. A similar behavior has been associated to silty sands and high bulk densities. Here, it may suggest a change in sediment composition, likely influenced by coarser nearshore and beach sediments, representing also a denser sediment matrix. The pore pressure results will complement the geological and geotechnical characterization of the coastal zone of Hershel Island, and contribute to the investigation of erosion and deposition processes.

Commentary by Dr. Valentin Fuster
2015;():V001T10A018. doi:10.1115/OMAE2015-41750.

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) operates the scientific deep-sea drilling vessel Chikyu, which undertook the drilling project under Japan Trench Fast Drilling Program (JFAST) to determine the mechanism of the catastrophic Tohoku Earthquake and the resulting huge tsunami. As targeted drilling sites were located about 1,000 m below the seafloor, at water depths of 7,000 m, JFAST was a challenging deep riserless drilling.

Drilling operation can also provide information on the properties of the drilling layer. This is important as a main aim of the scientific drilling is to obtain core samples from sediment layers under the seabed for evaluating sediment properties. Even a rough estimate of sediment properties could potentially provide an important notice for conducting coring operations.

Thus, we attempted to estimate properties of sediment layers using surface drilling data, such as drilling torque acquired from JFAST, and compare with properties of the core samples. As shear stress of sediment is a key parameter, it was estimated from surface drilling data. It was then compared with shear stress data determined from laboratory tests using core samples, and data from temperature sensors set in borehole.

Commentary by Dr. Valentin Fuster
2015;():V001T10A019. doi:10.1115/OMAE2015-41877.

The stick-slip is one of the critical problems for the scientific drilling, because it causes a crushing of the sampled layer. The present study investigates the characteristics of stick-slip phenomena of the drill pipe with the model experiments and numerical methods. The model experiments are carried out using a 1m length drill pipe model made with the Teflon. The angular velocity at the top and the bottom of the pipe are measured with the gyro sensor on some conditions of rotating speed at the pipe top and the weight on bit (load at the pipe bottom). The numerical simulations are also carried out to reproduce the stick-slip phenomena of the model experiments. The stick-slip is a kind of torsional vibration which is governed by the convection equation. By considering the boundary condition at the top and bottom of the pipe, we can obtain a neutral delayed differential equation (NDDE). The solutions of the NDDE is depend on not the initial value but the initial history of the solution, because NDDE contains a delayed function term. Therefore, it should be solved carefully to avoid the numerical error. The NDDE is solved with the 4th order Runge-Kutta scheme with very small time increment until the truncation error could be neglected. And also, we have found out that the effect of the initial history on the solution become to be very small after a certain period of time. The experimental results are compared with the numerical results under the same rotating condition. The experimental results of the stick-slip suggest that the period of the slip is mainly depend on the rotation speed at the pipe top and the magnitude of the slip is mainly depend on the weight on bit. Those characteristics of the stick-slip such as the period or the magnitude of slip are also obtained with the numerical calculations. However, in order to obtain an acceptable numerical results of NDDE, we have to adjust the frictional torque acting on the drill bit. Though, the frictional torque model was determined by reference to the measured torque at the top of the drill pipe model in the present study, it is desired to be improved. Therefore, the physical model of the frictional torque on the drill bit should be evaluated much carefully for the precise estimation of the stick slip in the future.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Suction Foundations

2015;():V001T10A020. doi:10.1115/OMAE2015-41808.

This paper was developed in cooperation between the TUHH and Overdick GmbH & Co. KG. The goal of the presented work is gaining further knowledge about the installation and bearing behavior of suction bucket foundations for fixed offshore platforms based on sand.

Buckets are usually made of steel and consist of a cylinder and a lid at the top. They are installed into the sea floor by pumping water out of the buckets to create suction, which drives the bucket into the soil. Suction buckets do not require heavy hammer-equipment for construction like common piles. Thus the installation procedure is much faster and protects the environment significantly by avoiding noise emissions. Therefore, suction buckets are to be considered as a serious foundation alternative compared to steel piles.

For this paper numerical investigations are performed with the finite analyses software ABAQUS. A total of five finite element models — three for the bearing and two for the installation — were created to carry out parametric studies, while using a hypoplastic constitutive model to describe the soil conditions. Therefore, the buckets diameter, embedded depth and the pore-ratio are to be investigated. In addition three different load conditions are applied in the bearing capacity tests: the maximum vertical load, the maximum horizontal load and the minimum vertical load. During the simulation of the installation procedure different pore ratios are tested and it is attempted to simulate an installation by water-extraction.

Based on these numerical investigations it is possible to investigate known and currently more or less unknown phenomena of the bearing and the installation of suction buckets. Thus, a more detailed knowledge about the function of this kind of foundation is to be gained. In addition, the numerical studies are compared to the design-procedure according to API RP-2A-WSD and the DNV CN-30.4.

Commentary by Dr. Valentin Fuster

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