ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V001T00A001. doi:10.1115/OMAE2016-NS1.

This online compilation of papers from the ASME 2016 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2016;():V001T01A001. doi:10.1115/OMAE2016-54088.

The structural integrity of subsea wellhead systems has to be maintained during the life cycle of a well and is of concern when frequent intervention and workover operations are performed to extend its service lifetime. Recent efforts have been made to improve and standardize methods for modeling wellhead fatigue-related loads and expected service life. Nevertheless, no comprehensive study of thermal loads and their implications on wellhead fatigue calculation has been published so far. This paper addresses the significance of thermal-related loads on wellhead fatigue damage assessment during the drilling phase of a subsea satellite well.

Subsea wells are exposed to dynamic loads during drilling operations that are transmitted to the wellhead through the marine riser, which may reduce their functionality over time. The wellhead housing is located on top of a shoulder in the conductor housing. As a result, forces on the wellhead datum are transferred between the wellhead housing, casing strings, the template and soil. The way in which forces are exerted on the different wellhead structural components is strongly dependent on factors such as wellhead design, soil support, cement level and bonding between conductor and surface casing.

To investigate the effects of thermal loading, numerical analyses have been performed based on the finite element method. First, the temperature profiles along the wellbore were estimated during the drilling operation, given the circulation of drilling fluids. Temperature data at selected moments were input to a 3-D structural local analysis of the wellhead. Then, its response to imposed mechanical and thermal loads was investigated. A beam proxy model of the wellhead was established and coupled to the global riser analysis, including the marine riser subject to environmental loading and the corresponding top-end vessel motion, which gave the load history on the wellhead according to the environmental loading. Local and global analysis results combined yielded the fatigue damage incurred in the wellhead during the drilling operation.

Four different top of cement configurations for the surface casing were considered in the study. It was found that the induced thermal variation on the wellhead may lead to significant variation in the numerical estimation of fatigue damage rates during a drilling operation. The results indicate that the impact of thermal loading is strongly dependent on the cement level.

Commentary by Dr. Valentin Fuster
2016;():V001T01A002. doi:10.1115/OMAE2016-54198.

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.

A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.

This paper summarizes the computational Finite Element Analysis (FEA) scope of work that provided the understanding and validation of the OPB mechanism through correlation with the test program results on chains. In addition, a multiaxial assessment of the fatigue stresses is studied and the main results are presented in this paper.

Commentary by Dr. Valentin Fuster
2016;():V001T01A003. doi:10.1115/OMAE2016-54199.

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.

A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the OPB mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.

This paper summarizes the full scale fatigue tests on chains and also the tests on small samples addressing the environmental influence on fatigue initiation and crack propagation stages. This paper also addresses the major step that was achieved: the implementation of a multiaxial fatigue criterion to address OPB hotspots as a standard practice in offshore industry. Moreover, the paper presents the first Industry OPB based S-N curves and its comparison to the existing industry fatigue S-N curve. Lastly, this paper provides a summary of the main steps in a framework for OPB fatigue calculation guidelines.

Commentary by Dr. Valentin Fuster
2016;():V001T01A004. doi:10.1115/OMAE2016-54300.

Hybrid risers systems used in recent offshore oil & gas projects to decouple large platform motions from main body of the risers can only support a single riser or a small number of risers. In the Sapinhoa Lula NE FPSO project, the challenge was how to design a riser system comprised of over forty risers and umbilical for each of the two FPSOs in 2,200 meter of deep water. The solution is large submerged Buoys with a unique hull form, each of them capable of supporting twenty plus risers and umbilical. The system is called Buoy Supported Riser system, or BSR system.

Comprehensive analyses have been performed during the design of the BSR system. In-depth study of the BRS’ hydrodynamic behavior has been performed to ensure that it meets all design requirements for a 30 year operation life and can be installed safely. Model tests including VIM tests have been conducted to verify hydrodynamic coefficients for the in-place and installation conditions.

The main objective of this paper is to present the hydrodynamic analysis methodology for the design of BSR system.

Commentary by Dr. Valentin Fuster
2016;():V001T01A005. doi:10.1115/OMAE2016-54399.

In South East Asia, Malaysia is one of the leading countries in the oil and gas industry. Today, Malaysia has expanded the explorations into the deeper water region. Before the installation of the Malaysia first deepwater platform, the Kikeh Spar, spar platforms can only be found in the Gulf of Mexico. Malaysian offshore regions are subjected to significant water current. From the literature review carried out, it was found that the current would change the behavior of the sea waves. This is contributing significantly to the environmental loading and affect the dynamic responses of the offshore structures. Hence, the study that focused on the effects of the current together with the wave on the structural dynamic response is necessary. In this study, the effect of the current coexisting with the wave on the dynamic responses of a truss spar model was experimentally investigated and quantified. The model tests were performed in the wave tank of the Offshore Laboratory in Universiti Teknologi PETRONAS with a scaling factor of 1:100. Two sets of environmental conditions were considered in the model tests i.e. wave only and wave-current condition. The dynamic responses of the truss spar model subjected to these conditions were measured. In order to quantify the effect of current, the measured results for the condition with and without current were compared among and presented here. From this investigation, it was found that the existence of the current in the water body has increased the truss spar motions, whereby the higher current velocity, give the higher response.

Commentary by Dr. Valentin Fuster
2016;():V001T01A006. doi:10.1115/OMAE2016-54426.

With a majority of the reported chain failures related to fatigue, this phenomenon is one of the main topics to be studied as part of Mooring Integrity Management. Present fatigue design is mainly based on fatigue curves for chains under tension-tension loads in seawater. However, the applicability of these curves for different loading modes and specific environments remains unclear.

This paper studies the fatigue behavior of the material used on chains as it builds the baseline for the performance of these mooring components. It includes uniaxial fatigue tests that were undertaken on R4 and R5 steel grades obtained from actual chains after all their manufacturing steps. Samples were not only tested in air and in synthetic seawater but different corrosion related parameters were also studied: frequency, temperature and cathodic protection.

From the results of these tests, separated SN curves were obtained. Subsequently, these curves were analyzed and compared against present recommended design curves for material. Fractographic examination was undertaken to assess the effect of corrosion and cathodic protection and comparison between material and component response was also addressed. Results showed the strong synergy between corrosion and fatigue. Also, the improvement from fatigue design curves to actual response of the materials was quantified.

Commentary by Dr. Valentin Fuster
2016;():V001T01A007. doi:10.1115/OMAE2016-54445.

The paper presents the modeling and simulation of the thermodynamics of the accumulator system of the heave compensation system. Heave compensators in offshore and subsea applications are used to reduce the relative vertical motion of the load caused by the vessel movement. The accumulator system is an integral part of the power system, which typically can be the hydraulic system of the crane cylinders or the winch motor. The accumulators are mainly used to reduce the power consumption during heave compensation through regenerating power. Hence, the working properties of the accumulator gas become important, and these properties are very dependent upon the temperature and pressure.

Modeling of the dynamics of the accumulator and the hydraulic system is complex due to the nonlinear characteristics of fluid dynamics and thermal processes. The derivation of the constitution formula for the accumulator dynamics is described, and the modeling is presented using the Bond Graph method. The implementation of the modeling and simulation is done in a software tool called 20-sim. The temperature and pressure variations of the accumulator during heave compensation were discussed considering the impacts of the heat transfer process and the pipe flow between the accumulator and the gas bank. Compared to the experimental testing data, the simulation results appear realistic and convincing. The simulation model provides useful information for system design and analysis.

Commentary by Dr. Valentin Fuster
2016;():V001T01A008. doi:10.1115/OMAE2016-54485.

Several recent benchmark studies have demonstrated that Computational Fluid Dynamics (CFD) is capable of capturing both nonlinear and viscous effects in offshore marine hydrodynamics and predicting well certain wave- and current-induced offshore platform motion. In order to apply CFD for practical global performance analysis of a complete hull-mooring-riser coupled floating system, we develop an advanced numerical wave basin that combines CFD, nonlinear irregular wave modeling, and finite-element mooring modeling. Specifically, CFD is used to simulate the violent free-surface flow with hull motions; nonlinear wave modeling is applied to generate a realistic wavefield and provide initial and far-field conditions to CFD for efficient long-duration simulation; and mooring modeling is two-way coupled with CFD to account for dynamic mooring response and its effects on hull motion. In this study, to demonstrate the capability of such tool, the global performance of a semi-submersible with 4 mooring lines in a 3-hour extreme sea state is simulated for both head and quartering sea. The simulation results are compared to model test data of hull motion, mooring line tension, and relative wave elevation around the hull for validation. It is shown with spectrum and statistics that the simulations predict well the platform’s global performance in all frequency ranges, including low frequency where the mooring lines have the greatest influence on the motion response. Compared to the predictions from a conventional global performance design tool that is based on diffraction analysis and empirical coefficients, the CFD results show significant improvements. The encouraging results from this study indicate that a CFD-based numerical wave basin, although still computationally expensive, is technically ready to be a complementary tool to physical wave basin for offshore platform global performance design.

Commentary by Dr. Valentin Fuster
2016;():V001T01A009. doi:10.1115/OMAE2016-54635.

Demand for renewable energy sources is rapidly increasing since they are able to replace depleting fossil fuels and their capacity to act as a carbon neutral energy source. A substantial amount of such clean, renewable and reliable energy potential exists in offshore winds. The major engineering challenge in establishing an offshore wind energy facility is the design of a reliable and financially viable offshore support for the wind turbine tower. An economically feasible support for an offshore wind turbine is a compliant platform since it moves with wave forces and offer less resistance to them. Amongst the several compliant type offshore structures, articulated type is an innovative one. It is flexibly linked to the seafloor and can move along with the waves and restoring is achieved by large buoyancy force. This study focuses on the experimental investigations on the dynamic response of a three-legged articulated structure supporting a 5MW wind turbine. The experimental investigations are done on a 1: 60 scaled model in a 4m wide wave flume at the Department of Ocean Engineering, Indian Institute of Technology, Madras. The tests were conducted for regular waves of various wave periods and wave heights and for various orientations of the platform. The dynamic responses are presented in the form of Response Amplitude Operators (RAO). The study results revealed that the proposed articulated structure is technically feasible in supporting an offshore wind turbine because the natural frequencies are away from ocean wave frequencies and the RAOs obtained are relatively small.

Commentary by Dr. Valentin Fuster
2016;():V001T01A010. doi:10.1115/OMAE2016-54709.

Tune and verification of control system algorithms for offshore installation operations involving complex and advanced machinery and difficult due to its safety factor. It may be also very costly or even impossible to establish certain test conditions in the physical process environment of the control system. To solve this problem, the Hardware-In-the-Loop-Simulation (HILS) can be regarded as an effective method for testing the control system prior to its final development. The sophisticated HILS is composed of a control system and a HIL simulator which is a simulation model of the offshore plant developed by software. In this study, we focus on the application of HILS for a heave compensator which is used to keep the position or the lowing speed of a lifting object. This study contains three components. Firstly, a physics-based analysis component is used to develop a simulation model of an offshore plant, that is, a HIL simulator. Secondly, the programmable logic controller (PLC) component, that is a control system, is used to regulate the offshore plant model, including a proportional-integral-derivative (PID) feedback controller which aims to control the position or lowering speed of the lifting object. Thirdly, an interface component is developed to communicate the data between the HIL simulator and the control system in real-time. To evaluate the applicability of HILS for a heave compensator, it was applied to an example of an offshore support vessel (OSV) crane. In order to verify the control system for the crane operation in case of heave stabilization of the lifting object, two simulation processes had been established with both a software PLC (software-in-the-loop) and a hardware PLC (hardware-in-the-loop). HILS makes it possible to test the heave compensator without building costly prototypes and without endangering natural environment.

Commentary by Dr. Valentin Fuster
2016;():V001T01A011. doi:10.1115/OMAE2016-54774.

Operators who want to im- or export bulk (coal, iron ore, scrap, etc.) by means of offshore bulk transshipment often want to determine the operability of their bulk transshipment configuration in an early stage to assess if offshore floating bulk transshipment is a feasible option for im- or export of the commodity. The downtime of such offshore (un)loading operations may be unacceptably large. The operational windows may become limited. This implies insufficient im- or export volumes, which may result in large and expensive storage capacity onshore.

In this paper a methodology is proposed to assess the operability of offshore floating bulk transshipment. Offshore bulk transshipment is a multi-phase operation. For offshore operations consisting of multiple phases, persistency of the environmental conditions has to be considered. Persistency analysis allows for multiple operational phases, individual phase duration and shift of sea states during the operation.

The proposed methodology shows that using persistency data of the waves (wind and current are not considered yet), the downtime of an offshore floating bulk transshipment operation can be estimated more accurately. Compared to persistency analysis, scatter analysis (using wave scatter diagrams) resulted in an optimistic estimate of the downtime. Persistency analysis is better in demonstrating the influence due to each consecutive step on the total operability. This methodology could be further extended to other multi-phase offshore operations.

Commentary by Dr. Valentin Fuster
2016;():V001T01A012. doi:10.1115/OMAE2016-54826.

This paper focuses on examining the response of a vessel moored to a Catenary Anchor Leg Mooring (CALM) buoy in squall conditions. This type of mooring arrangement is typically a temporary mooring used for loading and offloading product or a temporary arrangement used during construction and typically selected for shallow water locations, often in tropical environments when conditions are otherwise relatively benign. Squalls are mesoscale convective systems that cause rapid increases in wind speed and are often associated with large changes in wind direction and also occur mostly in tropical environments. Hence for some locations squall events are the design drivers for this mooring arrangement and are particularly important due to the imperfect squall forecasts available to the industry. To understand the risks in a squall environment the vessel-CALM buoy system is modelled for a range of both squall conditions and associated environmental conditions, covering typical associated wave and current conditions by season and direction. A response-based approach is used to determine the design parameters for the extreme loads, extrapolated using a peak over threshold (POT) approach and using a Generalized Pareto Distribution (GPD), for the vessel-CALM buoy system. The method for this approach is described in detail and contrasted with previous industry approaches.

Topics: Design , Mooring , Vessels , Buoys
Commentary by Dr. Valentin Fuster
2016;():V001T01A013. doi:10.1115/OMAE2016-54846.

The use of jacket structures to support offshore installations has for a long time been a popular choice in places with appropriate water depths. In recent years the use of jacket structures as offshore wind turbine foundations has also attracted increasing attention and is becoming a feasible alternative to traditional monopile foundations.

One of the key challenges in jacket design is optimizing tubular joints in terms of fatigue resistance. As it is not practically possible to include detailed FEM joint models in global jacket models designers are forced to look for alternative methods to obtain realistic joint representations. This is done by calculating influence factors (INF) and stress concentration factors (SCF) to be applied to simplified models of relevant tubular joints in global models in order to achieve a realistic force flow in the structure. One simple and widely used method is to apply parametric formulas, e.g. those suggested by Efthymiou. However, these approximating formulas have a fairly limited validity range. Therefore, on complex joint the most reliable way to determine INF’s is by setting up refined FEM models of relevant joints and then subsequently using the calculated factors in the global model. This strategy is computationally demanding and hence, very time consuming, as a new detailed FEM analysis of the tubular joint must be conducted for each step in the optimization process.

The present paper demonstrates how this time consuming procedure can be avoided by use of artificial neural networks (ANN) trained to estimate INF’s on tubular joints. The neural network is trained on a pre-generated library of detailed FEM joint models and is then able to predict INF’s on joints that are not part of the library — and thereby providing a significant reduction in calculation time during the jacket/joint optimization process. The analysis is conducted on a typical joint on a three legged jacket structure. The joint is located on a jacket leg and has two incoming braces. Such a joint has a finite number of free design variables, e.g. chord diameter/thickness, brace diameter/thickness, brace angle, gap etc. Each of these free variables can be considered as a dimension in the joint design space. Having a sufficient number of FEM joint models in the library the neural network can be trained to recognize and predict underlying patterns in this design space. The method is demonstrated on a limited number of design variables but should easily be extended to cover all variables as the joint library is expanded to include all dimensions.

Commentary by Dr. Valentin Fuster
2016;():V001T01A014. doi:10.1115/OMAE2016-54941.

As oil and gas (O&G) activities are moving north in the Arctic region new challenges for the offshore logistics have arisen. These challenges impose more stringent requirements for the logistics system setup, especially on the design and operation of vessels. We propose a methodology for quick evaluation of the feasibility and cost of the logistics system in the early stages of offshore supply planning. This methodology allows for testing the effects of using alternative ship designs and the overall supply fleet composition on system’s cost and performance while satisfying prospective campaign requirements. Safety standards and requirements for emergency preparedness and environmental performance are taken into account while cost effectiveness of the logistics system as a whole is the main quantifiable measure. The presented methodology is also very relevant for evaluation and planning of the logistics supply system for O&G activities in other areas around the world that are considered remote either due to physical distance or other characteristics that impact the complexity and cost of operations. The proposed methodology is tested on a real-life case for offshore supply planning of a drilling campaign in a remote area for one of the major international O&G operators.

Commentary by Dr. Valentin Fuster
2016;():V001T01A015. doi:10.1115/OMAE2016-54983.

The increasing demand for natural gas is encouraging the development of novel floating units’ designs, capable of processing large quantities of hydrocarbon. These units called FLNG (Floating Liquefied Natural Gas) are facilities that produce, process and store liquefied natural gas (LNG) offshore. Once the topside and tanks of a FLNG are larger and more complex than the regular FPSO vessels, a design process considering these particularities must be used.

Once just few FLNG units are under construction and under design and not yet in operation, the information on the design first stages is poor. It is difficult to obtain a first hull sizing without taking in account the complexity mentioned above.

Thus, a set-based approach that works with sets of possible solutions that are analyzed and compared using a merit function in order to select the best and feasible solutions was used.

However, to produce a sufficiently large family of solutions, which includes most of the solution space, either the solution descriptions or the models must be simplified. From the computational point of view, the analyses of a family of design solutions basically relies on an initial parameterization of the object and a set of mathematical models that, as a group, will be referred as synthesis model. Additionally, some restrains are also applied to eliminate unfeasible solutions. The output of the synthesis model is a set of performance quantities that will be used to rank the solutions.

This design approach is particularly useful to deal with project trade-offs and to optimize multiple characteristics. Optimal solutions belongs to a surface (or a hyper surface) called Pareto boundary. This paper aims to achieve a platform design capable of producing, storing and offloading liquefied natural gas. It must safely survive under environmental conditions of Santos Basin in São Paulo, Brazil. In the same way, the design should guarantee the shortest downtime as well as keep costs, of acquisition and operation, as low as possible. Each of these characteristics must be quantified to allow a ranking of the generated solutions through an objective function.

Capacities, production rates, equipments, load distribution, environmental actions, stability, sea keeping and structural design estimates are the major areas to consider and will be related to one or more mathematical models, constraint and objective functions. The work will present a general overview of each model separately and how they work together, as well as examples of solutions and analyses depending on the input values.

It must be clear that this approach is applicable just in the early stages of design to obtain the first hull sizing. After that it is necessary to fall back on the traditional iteration process to rely in a feasible design.

Topics: Design
Commentary by Dr. Valentin Fuster
2016;():V001T01A016. doi:10.1115/OMAE2016-55059.

To account for the viscous effects of damping devices, for instance, bilge keels or bilge boxes, on the motions of ships and offshore structures, Morison’s equation is often adopted as an empirical but practical approach in the design process. In order to combine the standard engineering panel method with the drag term in Morison’s equation, and remain in the frequency domain, the drag term has to be linearized based on, for instance, stochastic linearization. In this paper, the stochastic linearization scheme is implemented in an in-house code and verified through the comparison with the DNV GL software WADAM.

The model test results of a large cylindrical FPSO with bilge box are used to calibrate the drag coefficients in the Morison’s equation. When the linearized drag forces are included, heave motion RAOs correspond better to the model test results. However, the predicted natural periods of heave motions are seen to be smaller than those obtained from model tests. It is suspected that the viscous flow separation around the bilge box increases the added mass of the unit beyond what is predicted by potential flow alone. Discussions are made on the effect of viscous added mass on the heave natural period.

It is quite common to only include the damping effects in the motion analysis for large offshore structures and ignore the contribution of the viscous effects on the excitation force. For the considered cylindrical FPSO, this paper demonstrates that the viscous excitation force can be important in survival conditions.

Commentary by Dr. Valentin Fuster
2016;():V001T01A017. doi:10.1115/OMAE2016-55069.

This study is aimed to develop the dynamic analysis technique for a floating aquaculture in a shallow water region under the harsh sea condition. In case of the installation region to transform from a coastal area to the offshore area, the influence of sea bed with sea waves on the mooring lines was announced to be significant by other authors. In this study, the numerical tool was developed to solve dynamic behavior of the floating barge coupled with mooring lines in a shallow zone of the sea considering the influence of sea bed on the floating system.

Commentary by Dr. Valentin Fuster
2016;():V001T01A018. doi:10.1115/OMAE2016-55152.

This study describes a FLNG specifically designed to monetize Associated Gas (AG) of producing oil fields located within convenient distance of an existing LNG Plant or Port with LNG storage facility.

Limited production capacity combined with short range small capacity shuttles and limited LNG storage capacity, provide a cost effective means for LNG production.

This FLNG is designed to service an existing industry and does not require development of stranded gas discoveries.

Commentary by Dr. Valentin Fuster

Offshore Technology: FLNG Hydrodynamics

2016;():V001T01A019. doi:10.1115/OMAE2016-54065.

In this paper, the kinematic flow field in geometric similar GTT membrane-type floating liquefied natural gas (FLNG) facilities is investigated. The experiments were conducted in three scaled prismatic tanks at scales of 1/20, 1/40 and 1/60 with sway excitation, and the wave elevations near the two vertical sidewalls were measured. The experimental results demonstrate that the kinematic free surface behavior during a single sway wave impact exhibit non-similarity in the three geometric similar tanks with the same experimental media. It is suggested to perform sloshing model test in the large size model tank in order to evaluate the liquid motion in the real LNG tank.

Commentary by Dr. Valentin Fuster
2016;():V001T01A020. doi:10.1115/OMAE2016-54141.

This paper addresses a study on the relative motions of the floating liquefied natural gas (FLNG) systems during side-by-side offloading operation through experimental method and numerical calculation method. Different filling levels and sea state conditions were adopted both in model test and numerical calculation. Model test results were used to validate the numerical models, which was then used to identify the influence of the filling levels and wave/wind directions on the relative motions of the vessels. It is indicated that the numerical calculation results show satisfactory agreement with the experimental results. More severe relative motions of vessels occur when the displacement of the LNGC reduces. When the wind and wave come from the side of the LNGC, the relative sway and heave of the two vessels become more severe while the relative surge is not affected by the shelter effect. The investigation will be beneficial to the further numeral research and FLNG side-by-side offloading operation.

Commentary by Dr. Valentin Fuster
2016;():V001T01A021. doi:10.1115/OMAE2016-54797.

Floating liquefied natural gas (FLNG) facilities are a new type of offshore structure, which have been developed as a game changer in offshore hydrocarbon development for unlocking stranded gas reserves. One of the key challenges associated is offloading from FLNG facilities to LNG carriers. Offloading may proceed with vessels in a side-by-side configuration, which allows offtake by un-modified vessels and minimizes requirements for new hardware or procedures (e.g. compared to a tandem operation). Significant challenges remain, however, and reliable offloading is critical for successful FLNG implementation. In this scenario, the two vessels are separated by a narrow 4 m wide gap. The resonant response of the sea surface in the gap has been predicted by numerical simulations [1] to be a few times that of the incident waves at particular frequencies. As a consequence, the gap resonant response may play a role in determining the operational window for side-by-side offloading operations, and thus has attracted a lot of attention recently. There have been studies on this topic both numerically and experimentally. However, many of these studies are in 2 dimensions (2D), for relatively large gaps and relatively shallow water depth, which may pose difficulties when extending the results to a real project. It is unclear what will happen for a gap resonance if the gap width gets narrower (say 4 m in full scale) and the water depth gets deeper (say 600 m in full scale). In this study, we conducted a series of model tests at a scale of 1:60 in a large wave basin, and focused on deep water and, crucially, narrow gaps, which are closer to a real project geometry. To facilitate future numerical simulations, we used two identical fixed bodies in the model tests and the vessels were simple barge-like shapes. Using white noise waves as the excitation, which covers a broad brand, the response of the fluid in the gap has been measured at several points. In these experiments, different modes of the gap resonance have been observed. Response amplitude operators (RAOs) of the gap resonance have been obtained through spectral analyses, which provide valuable information for the design of side-by-side operations and will benefit future numerical simulations. Test runs in white noise waves with different significant wave heights were also performed, to study the nonlinearities of the gap resonance phenomenon.

Topics: Resonance , Water
Commentary by Dr. Valentin Fuster
2016;():V001T01A022. doi:10.1115/OMAE2016-54811.

In the past decade, an innovative concept, the floating liquefied natural gas (FLNG) system has been developed as a more effective solution over conventional pipelines for exploiting offshore natural gas resources. Understanding the hydrodynamic behaviour of such a mega structure in a real seaway is essential for determining its performance as well as evaluating the operabilities of on-board facilities and safe offloading.

In this paper, experimental study on the hydrodynamic performance of a generic FLNG hull form has been presented. The 1:100 scale model was tested in the Australian Maritime College model test basin for head sea and oblique sea conditions at zero forward speed. The wave induced loads and motions were measured by load cells and linear variable differential transducers (LVDTs) respectively. Experimental uncertainties on each of the measured variables were studied by taking partial differentiations on the uncertainty sources. The time history measurements were decomposed by Fourier series for obtaining frequency domain force/moment and motion transfer functions. The results were compared with numerical solutions from potential flow and Reynolds-Averaged Navier-Stokes (RANS) solvers. A good correlation between the experimental and numerical results has been demonstrated.

Topics: Stress , Waves , Ocean waves
Commentary by Dr. Valentin Fuster

Offshore Technology: Hydrodynamics

2016;():V001T01A023. doi:10.1115/OMAE2016-54169.

The wave energy density spectrum provides useful information for both coastal engineering practice and applied sciences. However, it is not always available at desired location which can be far away from the observation stations, and should be achieved using approximated approaches. Nevertheless, the spectrum at the desired location may differ significantly from the approximated one due to nonlinear effects. In this paper, long-duration and large-scale evolutions of the wave spectrum in shallow water is investigated numerically. Direct simulations of random seas are carried out by using the weakly nonlinear KdV equation and the fully nonlinear Enhanced Spectral Boundary Integral (ESBI) model respectively. Due to nonlinear effects, the spectral shape is modified and the energy is redistributed after a long-duration (∼1000 peak periods) and large-scale (∼128 peak wave lengths) evolution. The results obtained by using fully nonlinear ESBI model here demonstrate that the flatness occurs only when both the conditions, i.e., large Ursell number and large wave steepness, are satisfied; It will not happen if the Ursell number is large but the steepness is small. This is different from the existing understanding in literature, i.e. spectra tended to become flat (or nearly uniformly distributed) in low frequency part as long as Ursell number is sufficiently large.

Topics: Waves , Water
Commentary by Dr. Valentin Fuster
2016;():V001T01A024. doi:10.1115/OMAE2016-54411.

The influence from a current on the relative motions and wave drift forces for moored floater can be quite significant. In this paper, a benchmark study is carried out for three programs, MULDIF, WADAM and WASIM, with the focus on their capability on handling the wave-current interaction problem. A semi-submersible model Troll B and a tanker model KVLCC2 are used for this study. The motions, free surface elevation at specified off-body points and mean drift forces are calculated by the programs in different current or forward speed conditions. Analysis results are compared and discussed, with the aim to evaluate the numerical performance of each programs and their validity range in terms of current speed.

Commentary by Dr. Valentin Fuster
2016;():V001T01A025. doi:10.1115/OMAE2016-54420.

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained two-dimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.

Topics: Damping , FPSO , Keel
Commentary by Dr. Valentin Fuster
2016;():V001T01A026. doi:10.1115/OMAE2016-54424.

A promising time domain model for calculation of cross-flow vortex induced vibrations (VIV) is under development at the Norwegian University of Science and Technology. Time domain, as oppose to frequency domain, makes it possible to include non-linearities in the structural model. Pipelines that rest on an irregular seabed will experience free spans. In these areas VIV is a concern with respect to the fatigue life. In this paper, a time domain model for calculation of VIV on free spanning pipelines is proposed. The model has non-linear interaction properties consisting of discrete soil dampers and soil springs turning on or off depending on the pipeline response. The non-linear model is compared to two linear models with linear stiffness and damping properties. One linear model is based on the promising time domain VIV model, while the other one is based on RIFLEX and VIVANA, which calculates VIV in frequency domain. Through four case studies the effect of seabed geometry, current velocity and varying soil damping and soil stiffness is investigated for a specific pipeline. The results show that there is good agreement between the results produced by VIVANA and the linear model. The non-linear model predicts smaller stresses at the pipe shoulders, which is positive for the life time estimations. Soil damping does not influence the response significantly.

Commentary by Dr. Valentin Fuster
2016;():V001T01A027. doi:10.1115/OMAE2016-54466.

ExxonMobil Canada Properties and its collaborators are building a gravity based structure (GBS) in Newfoundland and Labrador to be installed on the Hebron Field offshore Eastern Canada. The geometry of the Hebron GBS is driven by the design constraints of a long operating life placed on the seafloor in the harsh environments at the Hebron field location. Prior to installation, the GBS encounters a number of significant floating phases — deep-water construction, towage, and installation. In each of these stages the hydrodynamic character of the unique Hebron geometry must be quantified to predict dynamic motions, assess tow performance, and define installation procedures and limitations. For the floating GBS, both heave and roll natural periods are long, and resonant responses due to wind gusting and vertical wave drift take place. Assessing the hydrodynamics by industry standard empirical methods is insufficient as regards capturing the viscous damping generated by eddy formation off the sharp edges of the submerged GBS structure. This paper focuses on the utilization of Computational Fluid Dynamics (CFD) to better understand and quantify this viscous damping. The Hebron GBS work has challenged the traditional methods for model generation and use of CFD for damping estimation, primarily because these techniques are most frequently used for traditional hull shapes. For example, application of the overset mesh technique, as successfully applied for ship-shaped vessels, does not provide the required accuracy in this case. Alternative numerical models are tested and found to work well: morphing techniques and sliding meshes for heave and pitch, respectively. Based on convergence and sensitivity studies, the results are accepted. Ultimately, the CFD work allows the project to quantify the expected increases in hydrodynamic damping and progress in the process of improving predictions of GBS motions and thus optimizing the operational planning of towing and installation.

Topics: Gravity (Force)
Commentary by Dr. Valentin Fuster
2016;():V001T01A028. doi:10.1115/OMAE2016-54475.

A key challenge in long-duration modelling of ocean waves or wave-structure interactions in numerical wave tanks (NWT) is how to effectively absorb undesirable waves on the boundaries of the wave tanks. The self-adaptive wavemaker theory is one technique developed for this purpose. However, it was derived based on the linear wavemaker theory, in which the free surface elevation and the motion of the wavemaker are assumed to be approximately zero. Numerical investigations using the fully nonlinear potential theory based Quasi Arbitrary Lagrangian Eulerian Finite Element Method (QALE-FEM) suggested that its efficiency is relatively lower when dealing with nonlinear waves, especially for shallow water waves due to three typical issues associated with the wave nonlinearity including (1) significant wavemaker motion for extreme waves; (2) the mean wave elevation (i.e. the component corresponding to zero frequency), leading to a constant velocity component, thus a significant slow shift of the wavemaker; (3) the nonlinear components, especially high-order harmonics, may significantly influence the wavemaker transfer functions. The paper presents a new approach to numerically implement the existing self-adaptive wavemaker theory and focuses on its application on the open boundary, where all incident waves are expected to be fully absorbed. The approach is implemented by the NWT based on the QALE-FEM method. A systematic numerical investigation on uni-directional waves is carried out, following the corresponding validation through comparing the numerical prediction with experimental data for highly nonlinear shallow water waves.

Topics: Water waves , Waves
Commentary by Dr. Valentin Fuster
2016;():V001T01A029. doi:10.1115/OMAE2016-54493.

In this paper, the current drag of a barge-shaped floating liquefied natural gas (FLNG) vessel was studied. Three model tests were performed — a wind tunnel model test, a submerged double-body tow test and a surface tow test. Computational fluid dynamics (CFD) simulations were carried out to gain further insights into the test results. During testing, the tow speed was kept low to avoid surface waves. When the current heading was around the beam current direction, the transverse drag coefficient measured from the wind tunnel test was significantly lower than those of the submerged tow and surface tow tests. The submerged tow and the surface tow provided similar drag coefficients. Results presented in this paper indicated that the difference between the wind tunnel test and the tow tests was caused by the wind tunnel boundary layer effect on the incoming wind profile and formation of a recirculation zone on the upstream side of the model, with a possible additional contribution from the wind tunnel floor constraint on the flow in the wake. Such effects are not accounted for with the simple corrections based on flow velocity reduction in the wind tunnel boundary layer. When conducting future wind tunnel model tests for barge-shaped FLNG hulls, one should consider the potential under-measurement of the transverse drag. In this paper, details of the FLNG model, test setup, test quality assurance (QA), measurement and CFD simulation results are presented, as well as discussions and recommendations for model testing.

Commentary by Dr. Valentin Fuster
2016;():V001T01A030. doi:10.1115/OMAE2016-54542.

Throughout the world many Platform Supply Vessel designs have been proposed as the optimal form for their given operating environment, but evaluating these claims has been difficult due to a poor understanding of the relationships between hull form shapes and performance for these vessels. This paper presents the results of analysis aimed at determining these relationships.

Results of CFD calculations to determine the Effective Horsepower/tonne for a series of PSV designs were presented in the paper A step towards an optimum PSV Hull form. This paper presents results for 16 separate hull forms, which were designed as each possible combination of four two-level hull form parameters. The hull form features considered were bow shape (vertical stem or bulbous), flat of bottom (flat or deadrise), length of parallel mid body (short or long), and stern shape (convention or integrated); resistance was calculated at two typical operating speeds (10 and 14 knots). This set of results was favourable for analysis using the statistical design of experiments technique: analysis of variance, which was used to determine the relationship between the hull and resistance performance.

The same hull form series was used to study the effects of the hull form parameters on motions in head waves. A 2 level factorial experiment was designed based on the hull parameters with the heave and pitch response calculated using the potential flow ship motion prediction code Shipmo3D, for each of two representative wave conditions (summer light seas and winter heavy seas) at the zero speed and 10 knot operating speed. Analysis of variance was used to analyze the heave and pitch responses measured, and was used to determine the relationship between each hull parameter and each response.

In both cases a 5% F-test was used to determine the significance of each parameter studied, and the significant effects were analyzed to determine their contributions to the overall model of the data.

The results have found the relationships between the hull design parameters and the Effective Horespower/tonne, heave, and pitch response of the vessel, indicating which factors provide the largest contribution to minimizing each response. The interaction effects between factors were also examined to allow for a generalized understanding of the resulting effect of selecting one hull parameter over another. A numerical model combining all significant factors was fitted to the data, allowing for multiple objective optimization to determine which hull forms provide the most desirable performance for each response.

Commentary by Dr. Valentin Fuster
2016;():V001T01A031. doi:10.1115/OMAE2016-54549.

To investigate the dynamics of gravity cage, the finite element program Aqua-FE™, was applied to gravity cages with two different meshes, square-mesh net and diamond-mesh net. Two case studies are chosen to compare the dynamics of cages based on the numerical modeling techniques. The numerical models were developed in Aqua-FE™ to simulate the effects of waves and current. They were validated by comparison with water tank results. The comparison showed good agreement. In both case studies, we consider several loading conditions consisting of different uniform currents and monochromatic waves. Assuming that the system can be modeled as a linear system, we investigated the motion response (heave, surge and pitch) characteristics of the fish cages with square-mesh net and diamond-mesh net, their deformation and the resultant tension in mooring lines. For different wave conditions, we study the storm response of the two cages based on the response amplitude operators (RAOs). In particular, the length of the grid line has significantly effect on the motion of the cage and the tension in mooring lines in wave. In additional, the effect of the mesh shape on the motion of the cage and tension in mooring lines is also analyzed. The comparison results show that the surge motion of the fish cage with square-mesh net is significantly larger than that with diamond-mesh net.

Commentary by Dr. Valentin Fuster
2016;():V001T01A032. doi:10.1115/OMAE2016-54627.

Very Large Floating Structures (VLFS) are highly specialized floating structures with variety of applications ranging from airport strips to floating motels offshore ports etc. Their economic design is based on their hydro-elastic behavior due to wave environmental forces. VLFS are extra large in size and mostly extra long in span and for that reason they are mostly modularized into several smaller structures and integrated. VLFSs may be classified into two broad categories, namely the semi-submersible type and the pontoon-type. The former type of VLFSs having their platform raised above the sea level and supported by columns resting on submerged pontoons and can minimize the effects of wave actions. In open sea, where the wave heights are relatively large, the semi-submersible VLFSs are preferred. On the other hand, the pontoon-type VLFS is a simple flat box structure floating on the sea surface. It is very flexible compared to other kinds of offshore structures, and so its elastic deformations are more important than their rigid body motions. The critical problem is the longitudinal bending moment of the long floating vessel in waves/current environment. Most of the present available VLFS designs are not economical for applications in hostile ocean. This paper presents hydrodynamic analysis carried out on an innovative VLFS called truss pontoon Mobile Offshore Base (MOB) platform concept proposed by Srinivasan [1]. The concept uses a strong deck with strong longitudinal beams to take care of the needed bending moment of the vessel for the survival, standby and operational conditions of the wave. At the submerged bottom just above the keel-tank top, a simple open-frame truss-structure is used instead of a heavy shell type pontoon. Thus the truss-pontoon provides the necessary flow transparency for the reduction of the wave exciting forces and consequently the heave motions and the vertical acceleration. Numerical analysis of truss pontoon MOB platform is carried out using HYDroelastic Response ANalysis (HYDRAN). Responses of the isolated scaled module in waves are obtained from these numerical tools and compared with published literature. Unconnected two modules and three modules are analysed using HYDRAN and the responses are compared with the isolated module. The proposed concept yielded lesser responses as compared to semisubmersible conventional MOB platform.

Commentary by Dr. Valentin Fuster
2016;():V001T01A033. doi:10.1115/OMAE2016-54651.

When a permanently moored FPSO in deep draft condition finds itself in harsh weather conditions it most likely will experience freeboard exceedance at the bow and consequently green water on deck. Knowledge about how and how much water will come onto the deck, is relevant for both a turret moored and spread-moored FPSO since both are expected to experience the most severe design conditions in head to bow quartering waves.

In this paper we focus on green water phenomena observed on a Suezmax FPSO in severe head seas in the model test basin. Using an on-board video in combination with deck-mounted wave probes, green water events are recorded in detail. This provides unique insights in how the water floods onto the deck. Very different flooding phenomena are observed between different events and they can strongly deviate from a dam-breaking kind of event.

Through a detailed description of three typical flooding events, the present paper improves the understanding of how green water flows onto the deck. It highlights the benefit of detailed model tests and emphasizes that new and more detailed modelling is required since the dam-breaking theory has limited value for the present problem.

Topics: FPSO , Water
Commentary by Dr. Valentin Fuster
2016;():V001T01A034. doi:10.1115/OMAE2016-54716.

A Tender Assisted Drilling (TAD) is typically a support vessel that serves support of a drilling rig. The TAD acts as a platform for supplies and is stationed alongside the drilling rig from which the rig will work off to reduce the loads on the actual rig. While operating, the TAD needs to be structurally coupled with the floating production platform. The coupling makes sure that the Tender does not drift away from the platform and provides stiffness to the relative motions between the TAD and the production platform. Current practice is to couple the TAD and the platform by using nylon hawser ropes. The hawsers provide adequate elasticity to accommodate the low frequency motions and do not allow the TAD to drift away from the production platform. In this study the individual free floating TAD and Tension Leg Platform (TLP) system is model first using the commercial software Hydrostar. After that the technique of developing equivalent stiffness matrix to represent the TLP tendons are applied in order to simulate the influence of the tendons on TLP motions. The response of these individual models are then calculated numerically and validated against existing data. After validation is performed, coupled interactions of this two body is simulated while both the bodies are freely floating side by side. Several possible side by side orientations of the TAD and TLP are tested and the results are compared for this first phase of the study. Later, the freely floating bodies are connected to each other by means of springs. The connection between the two floating bodies is modeled using equivalent stiffness matrix for connections.

The results obtained from this first phase frequency domain analysis will help to clearly understand the coupled behavior of such system under various orientations. In later phases, this model will be further developed in order to perform the time domain analysis and several innovative types of connections other than simple hawsers will be tested in order to couple this TAD and TLP system.

Commentary by Dr. Valentin Fuster
2016;():V001T01A035. doi:10.1115/OMAE2016-54724.

Nowadays, more and more nearshore LNG terminals are being built as it offers easy access to vessels coming from deep water and mitigates the risk by isolating regasification units from the cities. However, designing these terminals can be challenging in shallow water, as it is exposed to low-frequency waves which can excite the moored vessels at their natural periods. By lack of knowledge and adequate numerical simulation techniques, the effect of these low-frequency waves on the motions of moored vessels are unfortunately often ignored in the design. This is likely to result in an underestimation of the vessel motions and terminal downtime. In this paper, a methodology for the design of terminals in a nearshore wave climate is presented. The methodology consists of six steps which guide the engineer from the definition of the deep-water sea states to the calculation of the vessel motions and terminal downtime. In an initial stage, computational efficient tools are used, with the limitation that several approximations need to be made. In a later stage, more detailed but expensive methods are applied. The objective of this paper is to show how the developed methodology can give insight in the expected downtime due to the low-frequency waves in any nearshore mooring location. As an example, the methodology is applied on a fictive but realistic case, for which the motion response of a LNG carrier moored to a jetty on a sloping bottom is calculated. From seven years of deep-water sea states, the terminal downtime is estimated. The application of the methodology to the design case confirms that the terminal downtime can be significantly underestimated if shallow water effects are not taken into account. So the influence of the water depth, bathymetry, wave directionality and low-frequency waves on the vessel motions should be investigated with care. However, the results obtained in the design case also show that the spectral shape of the low-frequency waves predicted by the wave models are sensitive to the tuning of numerical parameters. Tuning the wave models against model tests or full scale data is therefore highly recommended, because the motion response of a low-damped moored vessel can be dominated by the amount of low-frequency free wave energy at its natural periods.

Commentary by Dr. Valentin Fuster
2016;():V001T01A036. doi:10.1115/OMAE2016-54732.

Current generation systems consisting of pumps or impellers are conventionally used in deepwater model test facilities for producing realistic full-depth steady current flows. A typical current inlet design involves the diversion of current into a mixing chamber which is then channeled into the basin through an array of flow-conditioning filters (screens) installed along the width of the basin. The screens are used to reduce turbulence and create an overburden pressure within the mixing chamber to enhance inflow uniformity. However, one undesirable consequence of this setup is that it increases energy loss to the system which leads to a higher operating cost.

In order to address this problem, the present work is aimed at establishing an effective and energy-efficient inlet design and screen configuration to improve spatial uniformity of inflow with minimal energy loss to the system. A wind tunnel study is carried out to determine an optimal tapered culvert angle and honeycomb screen configuration to achieve the aforementioned objectives. Uniformly-distributed wind flow of up to 20 m/s is channeled into a Perspex chamber and allowed to escape sideways through plastic honeycomb screens. Pressure drops are measured with an array of transducers at locations upstream and downstream from the honeycomb screen, while streamwise velocity distribution across the width of the chamber is captured with a pitot-static tube mounted on a traversing mechanism. Three cases of flow resistance are investigated, i.e. no screen, thin honeycomb screen (thickness = 12mm), and thick honeycomb screen (thickness = 50mm). For each case, the culvert angle is varied between 7°, 13° and 21°. A CFD model is developed with OpenFoam and compared with the present experimental data. The validated model is expected to be used to investigate more complex inlet configurations and assess the performance of realistic ocean basin geometry.

Topics: Design , Oceans
Commentary by Dr. Valentin Fuster
2016;():V001T01A037. doi:10.1115/OMAE2016-54829.

The EXWAVE Joint Industry Project (JIP) was initiated with the aim of improving methods and standard industry practice in design prediction of nonlinear wave loadings on Mobile Offshore Drilling Units (MODUs) induced by severe and steep seastates. The focus is on slowly varying wave excitation, including wave-current interaction effects. The present paper describes the technical background which led to the EXWAVE JIP, the project objectives, the planned work program and initial results.

An important part of the work program consists on performing model tests with representative floating structures. Two floating structures have been selected for investigation, namely a semi-submersible drilling rig and a floating production storage and offloading vessel (FPSO). Comparisons between experimental results and standard numerical calculations are presented for the semi-submersible. Computational fluid dynamics (CFD) is applied for detailed analysis and interpretation of complex physical phenomena.

Commentary by Dr. Valentin Fuster
2016;():V001T01A038. doi:10.1115/OMAE2016-54844.

The prediction of ship motions in extreme seastates is very complex as it involves strong nonlinearities. It deals with high motions of the ship and implies strong mooring system loads. These seastates are usually modeled in tank tests but an alternative in the near future could be CFD computations.

In this article, all required steps to setup and verify the hydrodynamic and numerical model are performed. The setup of the hydrodynamic and numerical model enable us to show that CFD computations of motion RAOS and pitch decay tests provide results in agreement with diffraction-radiation results.

Wave only simulations enable us to verify that irregular waves are accurately modelled in the CFD domain. Since the wavemaker motion used in tank tests to generate irregular waves is not available, a process of linear back propagation is set up from the wave elevation on a wave probe in tank tests. High Order Spectral (HOS) simulations are performed to reproduce the seastate measured in tank tests.

Finally, a test was performed to model the ship motions in irregular extreme waves with ICARE solver coupled to the computed HOS wave field through Spectral Wave Explicit Navier Stokes Equations (SWENSE).

Topics: Waves , Mooring , Ships
Commentary by Dr. Valentin Fuster
2016;():V001T01A039. doi:10.1115/OMAE2016-54865.

The Energy Efficiency Design Index (EEDI), introduced by the IMO [1] is applicable for various types of new-built ships since January 2013. Despite the release of an interim guideline [2], concerns regarding the sufficiency of propulsion power and steering devices to maintain manoeuvrability of ships in adverse conditions were raised. This was the motivation for the EU research project SHOPERA (Energy Efficient Safe SHip OPERAtion, 2013–2016 [3–6]). The aim of the project is the development of suitable methods, tools and guidelines to effectively address these concerns and to enable safe and green shipping. Within the framework of SHOPERA, a comprehensive test program consisting of more than 1,300 different model tests for three ship hulls of different geometry and hydrodynamic characteristics has been conducted by four of the leading European maritime experimental research institutes: MARINTEK, CEHIPAR, Flanders Hydraulics Research and Technische Universität Berlin. The hull types encompass two public domain designs, namely the KVLCC2 tanker (KRISO VLCC, developed by KRISO) and the DTC container ship (Duisburg Test Case, developed by Universität Duisburg-Essen) as well as a RoPax ferry design, which is a proprietary hull design of a member of the SHOPERA consortium. The tests have been distributed among the four research institutes to benefit from the unique possibilities of each facility and to gain added value by establishing data sets for the same hull model and test type at different under keel clearances (ukc). This publication presents the scope of the SHOPERA model test program for the two public domain hull models — the KVLCC2 and the DTC. The main particulars and loading conditions for the two vessels as well as the experimental setup is provided to support the interpretation of the examples of experimental data that are discussed. The focus lies on added resistance at moderate speed and drift force tests in high and steep regular head, following and oblique waves. These climates have been selected to check the applicability of numerical models in adverse wave conditions and to cover possible non-linear effects. The obtained test results with the KVLCC2 model in deep water at CEHIPAR are discussed and compared against the results obtained in shallow water at Flanders Hydraulics Research. The DTC model has been tested at MARINTEK in deep water and at Technische Universität Berlin and Flanders Hydraulics Research in intermediate/shallow water in different set-ups. Added resistance and drift force measurements from these facilities are discussed and compared. Examples of experimental data is also presented for manoeuvring in waves. At MARINTEK, turning circle and zig-zag tests have been performed with the DTC in regular waves. Parameters of variation are the initial heading, the wave period and height.

Topics: Databases , Ships
Commentary by Dr. Valentin Fuster
2016;():V001T01A040. doi:10.1115/OMAE2016-54868.

Wave-current interaction effects may significantly influence the mean wave drift forces on a structure as well as the motion responses and wave elevation around the structure. Additionally, the drift force may be used to estimate the wave drift damping of a moored structure.

A new numerical potential theory code for industry applications (MULDIF) has been recently developed, where the hydrodynamic interaction between waves and current of arbitrary direction with large volume structures is consistently included. The code also handles multiple bodies and finite water depth including wave-current interaction effects. The aim has been to create a robust and easy-to-use practical tool.

Initial validation studies against model tests have been conducted. The numerical results show a strong heave-pitch coupling due to the presence of the current. Preliminary results for a semi-submersible show good agreement for the motions provided that the mooring used in the model tests are accounted for. The free surface elevation around the semi-submersible is presented in contour plots.

Commentary by Dr. Valentin Fuster

Offshore Technology: Offshore Platforms

2016;():V001T01A041. doi:10.1115/OMAE2016-54029.

Rack Phase Difference or RPD may occur during the installation phase of a jack-up rig when it is jacking up at a location. Instances of failure of leg members of a jack-up have been reported in the past. Therefore, this is an important design issue. Among the many causes of high RPD, important ones are when one of the jack-up spudcans becomes eccentrically supported on the bottom or has lateral offset due to preexisting soft spots in the soil during installation. The resulting moment and shear on the leg is carried essentially by the horizontal guide reactions that may induce high stresses leading to failure of a diagonal brace in extreme cases.

In a previous OMAE paper written by the authors [1] the importance of a 3D model of the jack-up that uses non-linear analysis methods by including large deformations and rotations was highlighted. In this paper it was also emphasized that the meaning of RPD has to be understood properly in relationship with moment or shear carrying capacity at the spudcan for a given jack-up.

The present paper includes material plasticity effects in addition to the other nonlinearities in the 3D analysis to investigate the failure mechanism of a critical brace. Both applied moments due to the vertical load eccentricity at the spudcan and lateral displacement effects of it are considered. The behavior of a trussed leg jack-up depends heavily on the bracing pattern, chord and bay spacing as well as the size of the braces. For a well-designed jack-up the reserve strength beyond the initial exceedance of the allowable limits of member stress following the codes to the actual physical failure is considered to be important. This reserve strength could prevent actual damages to the rig. This paper discusses these aspects using one particular jack-up as example. The particular jack-up has shown very high RPD and reserve strength when a brace fails due to a large spudcan moment being applied or base deflection. The failure of the brace, however, does not indicate that the ultimate strength of the rig has been reached which is expected to be much higher. Although the analysis is for one particular jack-up, it discusses the parameters that in general could make the reserve strength large indicative of good design. These observations and the overall methodology of analysis used here could be beneficial to other rigs.

Commentary by Dr. Valentin Fuster
2016;():V001T01A042. doi:10.1115/OMAE2016-54166.

The horizontal wave drift force acting on a vertical floating column, without then with a heave plate, is considered. Computations are performed with a diffraction-radiation code and through the Morison and Rainey equations. Focus is on wave frequencies around the heave resonance where the drift force may be significant, even though the scattered wave-field being weak. It is found that the Morison equation overpredicts the drift force while Rainey equations perform rather well.

Commentary by Dr. Valentin Fuster
2016;():V001T01A043. doi:10.1115/OMAE2016-54303.

The long term analysis is performed to predict the hull global motion and mooring strength of a Spar platform and the results from long term analysis are compared with the predictions from the short term analysis. The long term motions are also used to investigate the long term riser strength response in Ref. [5]. The results includes the lateral offset, heel angle, heave motion and the mooring line tension at the fairleads. In the short term analysis, the environment events with various return periods and various realizations are considered for each environment load. The Gumbel fitting is used to predict the extreme response. In the long term analysis methodology, the prediction of the global performance and the mooring tension are based on 56-year hindcast wind and wave data with 3-hour intervals. Weibull fitting is used to predict the extreme response for various return periods. The comparison between the long term and the short term predictions indicate that the short term predictions are generally conservative compared to the more accurate, but computationally expensive long term analysis method. The long term methodology is not widely adopted currently due to the computation inefficiency. However, it is expected that this long term methodology could provide a better option in the future with the consideration of its accuracy and the application of high speed computer.

Topics: Mooring
Commentary by Dr. Valentin Fuster
2016;():V001T01A044. doi:10.1115/OMAE2016-54609.

Vortex-Induced-Motions (VIM) is a cyclic rigid body motion that is induced by vortex shedding of a large sized floating structure (Spar, Semi-Submersible and Tension-Leg Platform) in deep water. Since the potential impact of VIM on fatigue life of mooring and riser systems can be critical, the study of the VIM phenomenon has drawn considerable attention among offshore engineering community. Despite the effort, there is still lack of understanding the complex fluid-structure interaction phenomenon. To date, it is very much relying on the simplified empirical approach in practical design.

Most of the Semi-Submersibles consist of four rounded-cornered rectangular columns. To investigate the flow interaction between each columns can reveal some physics behind the VIM. In this study, flow past four stationary rounded-corner rectangular columns are tested in the circulating channel and numerically simulated by the Star-CCM+ CFD package. Forces on the columns and the flow characteristics behind the structures are present in this study. A 2-D particle imaging velocimetry (PIV) technic has been adopted in present study to obtain the flow characteristics. The aim of this study is to investigate the interactions between four square columns.

Commentary by Dr. Valentin Fuster
2016;():V001T01A045. doi:10.1115/OMAE2016-54666.

SET (Samsung Enhanced hull for Tendon) TLP (Tension Leg Platform) has been designed as an innovative TLP hull form with optimized number of tendons compared to conventional TLP design. SHI (Samsung Heavy Industries) designed the SET TLP to have the minimized hull weight with narrow and thin pontoon which results in the less number of tendons without any outboard extension of column or pontoon.

The SET TLP has 8 tendons which are evenly attached along with the octagonal shaped-ring pontoon. The unique shape of octagonal ring pontoon distributes the wave load and concentrated tendon loads in larger areas, which can minimize the structural reinforcement.

To verify the feasibility of the proposed hull concept with regard to the hydrostatic/hydrodynamic characteristics and tendon design, the numerical analyses for the hydrostatic stability and global performance are carried out. Hydrostatic stability is investigated for all the possible loading cases such as float-off, wet transit, tendon installation and operational conditions, and the proper tank compartments are achieved for all the scenarios without using any temporary stability module.

The global performance is validated for all the possible combinations of wave, swell, wind, current and/or squall for a site in Western Africa. Through the frequency-domain analysis and nonlinear time-domain analysis as well, the essential items such as the maximum offset/set-down/top tendon tension, minimum bottom tendon tension are examined and confirms that the certain design criteria of TLP operation are satisfied.

Commentary by Dr. Valentin Fuster
2016;():V001T01A046. doi:10.1115/OMAE2016-54669.

As many deep-sea oil and gas fields are being developed, floating platforms becomes larger with more complicated topsides. However, the construction time is demanded to be shorter to meet the overall project schedule. The use of very large topside modules is one of the effective ways to reduce the integration time and decrease the possibility of fabrication defects.

The Ichthys Project’s CPF (Central Process Facility) is currently being constructed at Samsung Heavy Industries (SHI). The CPF will be the world’s largest production semi-submersible platform. The hull is constructed on the Offshore Floating Dock and most of the topside modules are prepared at workshops and integrated by floating cranes on the hull.

To maximize production efficiency in terms of reduced integration time and reworks, several topside modules are assembled into a larger module weighing up to 7,400 tonne spanning 150m long. Such large module cannot be handled by a single 8,000 tonne F/C, the largest crane that SHI owns. This fact initiated the idea of dual-lifting with a combination of the two F/Cs (8,000 tonne and 3,600 tonne) which enables lifting, transportation and integration of the mega-modules properly and safely without building a larger capacity F/C. Using different sized F/Cs increases the flexibility of the operation of the F/Cs.

To ensure safety during the dual-lifting, the two F/Cs are synchronized to be controlled as a single crane unit. During hoisting, all the measured data such as loads and positions of the hooks, and rotation of the module, are monitored in real time basis and used to hoist the module automatically. All the systems are designed to be redundant. Additional engineering works are performed to check the safety such as 1) structural analysis to investigate the structural safety with out-of-phased motions at boom-tip 2) time-domain analysis and 3) model test in ocean basin with the operation scenarios in real environments to obtain the dynamic load factors and the guidance on the operation limits in terms of wave heights. The two crane barges are moored side-by-side using fenders and mooring lines, and tug operation conditions are planned not only to move but also to push the two barges from sideways to be moored tightly, which minimizes the possibility of relative motions between the two barges.

The developed system had been applied to the integration of all the modules successfully weighing from 4600 to 7400 tonne. It is expected to be applied to many other offshore projects to keep the construction schedule on time. This paper will address and share the technical experiences obtained during the dual lifting of the mega-modules for the Ichthys Project’s CPF.

Commentary by Dr. Valentin Fuster
2016;():V001T01A047. doi:10.1115/OMAE2016-54699.

Gantry crane is used frequently in the dock. How to reduce the number of lifting and improve construction efficiency are given more attention by shipyard. According to site and equipment conditions of a shipyard, a tooling which is used for the large blocks lifted is designed to improve construction efficiency. Moreover, the processes of lifting the large blocks are described. Tooling structure strength is checked by FEM. And the influence of different rails arrangement for tooling structure is analyzed. The successful tooling design plan is of great significance for improving construction efficiency, reducing building period and saving construction cost.

Topics: Design , Tooling
Commentary by Dr. Valentin Fuster
2016;():V001T01A048. doi:10.1115/OMAE2016-54764.

The vortex-induced motions (VIM) phenomenon of semi-submersibles has drawn increasing attention with the development (mainly increase of column size) of new semi-submersibles. Due to the elongated submerged columns and the enlarged projected area to current, deep-draft semi-submersible platforms are susceptible to higher in-line drag forces and transverse vortex-induced lift forces, resulting in considerable horizontal motions in a current environment. In order to check the influence of draft conditions on VIM of the semi-submersible platform with four square columns, experimental investigations with five draft ratios varying from 0.87 to 1.90 were carried out in a towing tank.

The 6-degree-of-freedom (6-DOF) motions of the model were recorded by the motion acquisition system, in synchronisation with restoring forces provided by four load cells, one for each horizontal mooring spring. This paper discusses the dynamic behavior of a semi-submersible platform in five different draft conditions, including coupled motions at the water surface plane, drag and lift forces, and spectral analysis. It is shown that the largest transverse amplitudes are around 75% of the column width in the range of 6.0 ≤ Ur ≤ 8.0 for the deep-draft semi-submersible (H/L = 1.90). With 50% of the immerged column height of the deep-draft model, a 30% decrease in the transverse motion amplitude can be seen. Furthermore, the effects of the draft condition on yaw responses and current loads are also addressed.

Commentary by Dr. Valentin Fuster
2016;():V001T01A049. doi:10.1115/OMAE2016-54779.

Spar platforms have been used for oil and gas exploration in deep water for the past two decades. Spar experience low heave and pitch motions in operating conditions with its deep draft and large inertia. The heave motions can be large when encountered by long period swells. These resonant response leads to unstable motions due to heave-pitch coupling in spar platforms when the heave/pitch natural period ratio is 0.5, 1.0, 1.5 and 2.0, referred to as Mathieu-type instability. This instability can be avoided by changing heave or pitch natural periods, so that the heave-pitch coupling can be avoided.

The buoy form Spar proposed in this study is a cylindrical hull with curved surface near the water plane. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 m and 20 m diameter at the water plane area have been considered. The moon pool diameter of 12.5 m and the displacement of 63000 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. Numerical simulations have been carried out using panel method.

The classic Spar experiences Mathieu-type instability, since the heave/pitch natural period ratio is 0.5. The heave natural period of the buoy form Spar is higher than the classic Spar by 24% and 72%. The heave/pitch natural period ratio of the first buoy form Spar with 25 m diameter at the water plane area is 0.667; hence the heave-pitch coupling is avoided. The second buoy form Spar with 20 m diameter at the water plane area does not experience Mathieu-type instability, even though the heave/pitch natural period ratio is 1.0. Also the heave natural period of the second buoy form Spar is 36s (3.6 s in scale model) which is much above the design wave period. The possibility of Mathieu-type instability is avoided in the Spar by varying the hull shape near the water plane.

Commentary by Dr. Valentin Fuster
2016;():V001T01A050. doi:10.1115/OMAE2016-54839.

In order to provide qualitative and quantitative information on the hydrodynamics loads during green water events on a module on the deck of a TLP in 10000-year conditions, MARINTEK has carried out CFD simulations. This paper presents extreme wave events and corresponding hydrodynamics loads on the module which can be directly linked to the extreme events observed in model tests. This means that the simulated extreme events can be related to a probability of occurrence, found from the model test.

A prerequisite for the structural design is that reliable estimates of hydrodynamic loads during a green water event can be made. Measured time series of waves from existing model test data are compared with CFD generated synthetic numerical waves. The selection of steep wave events are based on two physical parameters: the wave crest height and the rise velocity (time derivative of the free surface elevation at a given location). These parameters are relevant for green water and corresponding loads. The comparison of the measured free surface elevation of the calibrated waves with the time series of the numerical waves, as well as the measured and simulated relative wave probes time series shows that the applied numerical wave events have similar physical conditions as those observed in the model test.

In a new procedure developed by MARINTEK one identifies observed steep wave events, which are similar to existing numerical wave events, instead of trying to reproduce measured events. This procedure reduces the computational time, as well as computational costs, to an industrially acceptable level.

Traditional load estimation is not able to provide such reliable detailed local load history for structural design purpose at areas exposed to wave impacts. Therefore, topside modules are currently not installed in such areas. This new procedure, where CFD simulates realistic breaking waves with coupling to structural analysis tools, offers new possibilities for the design of structures subject to risk of green water loading.

Commentary by Dr. Valentin Fuster
2016;():V001T01A051. doi:10.1115/OMAE2016-55140.

Floating Production Storage and Offloading (FPSO) floaters have the advantages of providing the required storage in the hull and direct offloading to tankers of opportunity in deep and ultra-deep water in areas lacking infra-structure.

Steel Catenary Risers (SCRs) are the preferred solution in wet-tree applications due to their simplicity, robustness and low Capital costs (CAPEX) and Operational costs (OPEX) compared to other riser options. However, due to its relatively high dynamic motions, FPSO is not a feasible host for SCRs in most environments and especially so in the North Sea very harsh environment. Also, for efficient production from rich reservoirs, large diameter and number of risers are typically required. This makes it more challenging to find a robust and commercially attractive riser solution.

In this paper a novel design for an FPSO with the ability to host SCRs in the North Sea very harsh environment is presented and evaluated. The novel design, namely, Low Motion FPSO (LM-FPSO), has a hull form with a generally rectangular cross-section. The platform is moored in-place using a conventional mooring system. The LM-FPSO performance is enhanced with the robust low-tech feature, namely, free-hanging solid ballast tank (SBT). The SBT is located certain distance below hull keel and connected to the hull through four groups of short tendons. All tendon components are the same as those used in conventional TLPs. Through the mass and added mass of the SBT, the LM-FPSO provides significantly improved heave, roll and pitch responses.

The paper presents detailed description of the novel North Sea LM-FPSO design and its in-service performance. The SCR’s feasibility is discussed. The identified risks and associated mitigations for the new design compared to the conventional FPSO are investigated and reported. The paper concludes with discussions on the project execution plan and cost benefit when developing fields using this novel design.

Topics: Design , FPSO , North Sea
Commentary by Dr. Valentin Fuster

Offshore Technology: Station Keeping

2016;():V001T01A052. doi:10.1115/OMAE2016-54195.

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.

A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.

This paper summarizes the quasi static OPB stiffness measurement campaign and the post processing work to derive the OPB interlink stiffness.

Topics: Fatigue , Chain , Stiffness
Commentary by Dr. Valentin Fuster
2016;():V001T01A053. doi:10.1115/OMAE2016-54260.

In this paper, a thrust allocation algorithm is proposed to minimize the fuel consumption and the gas emission of the offshore platform dynamic positioning system. The thrust allocation algorithm generates thruster commands that keep the position of offshore platform while physical limitation. Generally, the offshore platform control system is an over-actuated system. Thus, a thrust allocation problem of the offshore platform can be determined as an optimization problem. In this research, a thrust allocation problem is designed to minimize the fuel-consumption and the gas emission. Fuel-optimal thrust allocation was newly formulated and solved based on penalty-method based optimization. Developed thrust allocation method was evaluated by comparing to conventional pseudo-inverse based thrust allocation. The proposed thrust allocation method was validated with comparison with an offshore support vessel static allocation cases. A fully coupled dynamics of hull, mooring, riser, and dynamic positioning system were simulated in time domain. The proposed thrust allocation method that uses penalty-method achieved a 3% accumulated fuel consumption reduction compared to the conventional pseudo-inverse method based thrust allocation algorithm in GOM 1-yr storm condition.

Commentary by Dr. Valentin Fuster
2016;():V001T01A054. doi:10.1115/OMAE2016-54307.

A cylindrical floating structure can basically experience Vortex-induced Motion (VIM) in strong current. Since the VIM on the structure with long term low-frequency motion causes fatigue damage of the structure’s mooring lines and risers, precise VIM assessment is needed for the safety evaluation of them. In the standard of the International Organization for Standardization ISO19901-7, ‘Specific requirements on stationkeeping systems for floating offshore structures and mobile offshore units’, for instance, a concrete method of assessing VIM displacement is not represented in the standard document, though the requirement on the VIM demands to do the assessment on the basis of proper ways.

Then in this paper, a VIM simulation method on a floating structure with circular cylinder form, that is, for example a Spar, a MPSO (Mono-column type floating Production Storage and Offloading) and so on, is shown using the wake-oscillator model. Transverse VIM is only treated since it is dominant factor on the fatigue damage of the mooring lines and risers. The assessment quality of the simulation method on the transverse VIM of floaters in current is confirmed by model test data.

Commentary by Dr. Valentin Fuster
2016;():V001T01A055. doi:10.1115/OMAE2016-54308.

Semi-submersible type offshore floating structures are expected to be used in the Japanese coastal area and at sea off Japan for promoting resource exploitation and development in near future. As a moored offshore floating structure may be suffered from current, Vortex-induced Motion (VIM) effect should be assessed in an appropriate manner since the VIM may cause fatigue damage of the floating structure’s mooring lines. VIM phenomenon on semi-submersible type floating structures, however, is not clear, and its comprehension is insufficient since there are only small number of open research papers with lack specifications of the structures. Moreover, a standardized assessment method for the VIM on them has not ever been proposed.

At first in this paper, the results of VIM measurement test using some large semi-submersible floating structure models in relatively high Reynolds number flow are presented, and afterwards simplified estimation method, that is useful in the design stage for a semi-submersible type offshore floating structure, is introduced as one trial.

Commentary by Dr. Valentin Fuster
2016;():V001T01A056. doi:10.1115/OMAE2016-54505.

The emphasis in this paper is to investigate the potential impact on offshore operation caused by the sudden change of wind condition. First, a questionnaire survey was conducted on experienced operation workers in Japan. The results show 74% respondents have experienced the danger of sudden change of wind condition in offshore operation. It is found that both the operation on board and the motion of ship can be affected by sudden change of wind condition.

One-dimensional time-domain simulations are done to understand the performance of dynamic positioning system (DPS) when suffering from sudden change of wind condition. Squall models are determined on the basis of real cases on the sea near Japan. The results show an operation ship with PD control cannot keep its positioning in the allowable operation zone if the peak of wind speed exceeds more than a certain degree of the designed value. Wind feedforward can decrease the ship’s maximum displacement only with a short measurement interval. Adding integral control to the DPS can also achieve the ship staying in its allowable zone but bring severe destabilization. The jointed effects of unbefitting wind feedforward and PID control may make the ship more difficult in dealing with the squall in a short time. By using the Davenport spectrum, squall models are turned into fluctuating wind form. The gust with higher speed than the mean wind speed appears. However, it does not affect the performance of the DPS. It also illustrates that squall, a sudden change of wind condition in a relatively long time, can bring effects on DPS with motion outside its control. This will result in negative effects on offshore operation.

Commentary by Dr. Valentin Fuster
2016;():V001T01A057. doi:10.1115/OMAE2016-54713.

Station keeping system for arctic floaters such as mooring cables requires an innovative engineering design coupled with a detailed investigation on its resistance against the impact loading. The ice-induced vibration, together with the wave and current actions, imposes a strong demand on the mooring cable design. However, in present industry practice, the mathematical models/software used in designing the mooring cables for offshore structures use linear strain theory. In this paper, an analytical modal based procedure for underwater submerged cable considering its geometric nonlinearity is presented. Introducing geometric nonlinearity into the modal procedure enables coupling between different modes, which is not included in the standard the linear analysis of integrated mooring system. In the present analysis, the second in-plane and first two out-of-plane modes are considered to highlight the effect of geometric nonlinearity near the 2:1 internal resonance phenomena of underwater mooring cables. The differential equation for cable is solved using a modal decomposition method considering second-order terms of the finite strain tensor. A simply supported boundary condition is assumed at both ends of the cable. A unidirectional wave loading is considered and thereby, the floater will have two translational motions, i.e., surge and heave. The floater motions will cause a support excitation at the pinned connection between the floater and mooring. This phenomenon is modelled as a base excitation at the top support point of the mooring cable. The support excitation frequency is chosen to be close to the natural frequency of the second in-plane mode. Therefore, the in-plane mode is excited directly. Ice load is applied at the support from an out-of-plane direction as a pulse load which may come from the ice impact and/or breaking. So, the out-of-plane mode is excited parametrically. It is observed that the out-of-plane mode responses show instability under certain base excitation amplitude, i.e., the responses due to the pulse load from the ice impact never decay. This instability in the responses may lead to the fatigue failure of the mooring cables. It is observed that this instability in the responses arises from the modal interaction between the different modes, i.e., autoparametric excitation, which the linear analysis is unable to capture. Numerous simulations are carried out to determine the stability boundary of different out-of-plane modes for various amplitude and excitation frequency. The stability boundaries are also determined using the harmonic balance method to verify the results obtained from the modal analysis. It can be concluded from this analysis that the nonlinear coupling terms play a significant role, close to the 2:1 resonance region which can lead to an unstable response of the mooring cables in the presence of ice loads.

Topics: Cables , Stress , Ice , Mooring
Commentary by Dr. Valentin Fuster
2016;():V001T01A058. doi:10.1115/OMAE2016-54965.

Moored offshore floating units may operate in regions where the bathymetry changes significantly over the mooring spread. Traditional mooring analysis methods make the assumption that the seabed slope is constant along the azimuth direction of each mooring line. This assumption, while reasonable for a seabed with nearly constant slope, can lead to significant errors with respect to the line tensions and vessel offsets in the mooring design assessment when the seabed slope is variable.

This paper demonstrates the impact of bathymetry changes on the mooring design with the help of numerical analysis examples. The floating vessel considered is a semi-submersible moored by an eight point all chain catenary mooring system.

Two methods of analysis are compared. In the first method, a true representation of the seabed surface that accounts for all variations in the bathymetry is used. In the second method, the anchor depths and the seabed slopes at the anchor locations estimated in the previous method are used, however, with the assumption of constant seabed slopes along the line azimuth directions. The dynamic analysis program Orcaflex is used for performing the numerical analyses in the time domain for both the methods. Differences in the performance of the mooring system are demonstrated by comparing the static and the dynamic line tensions as well as the vessel offsets in different environmental conditions. The paper also discusses how maximum offsets and line tensions are estimated.

Commentary by Dr. Valentin Fuster
2016;():V001T01A059. doi:10.1115/OMAE2016-54968.

This paper reviews the response of a hawser moored vessel to squalls and addresses a novel method for obtaining statistically reliable design loads. Industry paradigms related to squall selection for analysis input are reviewed and renewed.

A benchmark database consisting of more than 15,000 unique squall-wave-current induced extreme values enables the validation of a range of less computationally demanding analysis and squall selection methods.

Extreme values are extrapolated to a design value using a Peak Over Threshold (POT) method to fit a Generalized Pareto Distribution (GPD). The influence of associated metocean conditions and squall characteristics on the vessel response is presented. By means of bootstrapping a satisfactory population size for design purposes is studied. The findings challenge common design practices currently employed throughout the industry.

Topics: Mooring , Vessels , Buoys
Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Anchors and Pile Foundations

2016;():V001T10A001. doi:10.1115/OMAE2016-54417.

Anchors are widely used offshore as foundation systems to moor floating platforms and renewable energy devices to provide uplift resistance in vertical or resistance in inclined direction. Most of the available research work has concentrated on in-plane loading. However, any failure of a single mooring line in a storm can cause out-of-plane loading on the remaining anchoring system. This research targets how this out-of-plane loading can affect the trajectory and ultimate holding capacity of the anchors. This paper presents results of a numerical investigation of the undrained bearing capacity of deeply buried square plate anchor in clay under six degrees-of-freedom loading. Finite element analyses are conducted to define the combined loading yield surface (or yield surface for the anchor in its current position) and an analytical expression of the yield surface is proposed and written directly in the six degrees-of-freedom loads. This analytical expression allows implementation of the yield surface in a plasticity approach, where the load-displacement relationship of the anchor can be described. The method to achieve this and also an illustration of how this framework on a simple plate anchor can be extended to predict the performance of more complex anchor geometries is described at the end of the paper in a description of future research.

Commentary by Dr. Valentin Fuster
2016;():V001T10A002. doi:10.1115/OMAE2016-54513.

This paper presents an alternative numerical method in addition to the traditional ‘probe test’ to investigate the combined loading failure envelopes of foundations in soil. In the ‘probe test’ method, the foundation is displaced with a specified displacement path and eventually the soil resistance force reaches a stabilised point sitting on the failure envelope in the load space. While the displacement paths are arbitrarily or empirically set, the positions of the stabilised loads on the failure envelope can not be predetermined or planned. This paper’s new method, however, can specify the load paths, which directly shoot onto the failure envelope. This allows the investigation of the failure envelope can be better achieved with planned load paths. In addition, this new method is advantageous in checking the plastic flow conditions (i.e. normality of the failure envelope) as the load path directions are predetermined.

Commentary by Dr. Valentin Fuster
2016;():V001T10A003. doi:10.1115/OMAE2016-54934.

Offshore platforms for oil and gas production in seismic regions around the world are often required to be designed for seismic hazards according to International Standards (e.g., ISO 19901-2 [1] and ISO 19902 [2]). This paper discusses three important aspects of the nonlinear dynamic time history analysis commonly used to design for Abnormal Level Earthquakes (ALE) in light of findings from recent centrifuge modeling and numerical simulation of the response of offshore structures under earthquake excitations. First, greater-than-expected ground motion de-amplification has been observed in a recent seismic soil-structure interaction centrifuge program for typical “soft” marine clays with undrained shear strength up to 100 kPa per API RP 2GEO [3]. Second, the current industry practice of using uniform down-pile ground motions in the time history analysis tends to underestimate pile bending moments. Use of depth-varying ground motions is strongly recommended to better characterize pile bending moments. Alternatively, a simplified design approach is proposed to account for the higher bending moments from the use of more realistic depth-varying ground motions. This approach is illustrated with a design example. Lastly, hysteretic and radiation damping in soil-structure interaction is discussed. Modeling of hysteretic damping is achieved using nonlinear elasto-plastic soil springs with unload-reload behavior following Masing’s rule, whereas modeling of radiation damping is achieved using viscous dashpots in a parallel or series arrangement with the axial and lateral soil springs and with dashpot coefficients based on O’Rourke and Dobry [4]. The centrifuge data show that proper modeling of radiation damping is important to accurately predict pile load and settlement.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Bucket Foundations and Suction Caissons

2016;():V001T10A004. doi:10.1115/OMAE2016-54362.

Suction Bucket Jackets (SBJ) are found as a suitable alternative to driven piles for the support of jacket or tripod foundations for offshore wind energy converters. Offshore wind energy turbines are characterized by a small self weight and they can be subjected to different load combinations. The work presented here aims to show the numerical investigation on the behavior of suction bucket foundations under different kind of loads as well as load combinations. In order to do so, a suitable numerical model is much needed. The theoretical basis of the model lies on the Swansea formulation of Biots equations of dynamic poroelasticity combined with a constitutive model that reproduces key aspects of cyclic soil behavior in the frame of the theory of generalized plasticity. An adequate FE formulation, the representation of appropriate soil-structure interfaces and the computational efficiency are key aspects in order to successfully model such complex systems. The 3D numerical simulation allows a special insight into the fundamental behavior of the founding of Suction Bucket Jackets such as the evolution of the pore water pressure or the occurrence of the so called soil liquefaction.

Commentary by Dr. Valentin Fuster
2016;():V001T10A005. doi:10.1115/OMAE2016-54739.

The influence of skirt to the seismic behavior of the bucket foundation supporting offshore wind turbine was analyzed by performing dynamic centrifuge tests. Three types of bucket foundation with aspect ratios (length of the skirt/diameter of the foundation) of 0.50, 0.75 and 1.00 were tested. Kobe and El Centro earthquakes were used as excitation patterns. Clean sand was used to simulate the soil deposit. Peak acceleration, settlement and tilting of foundations on drained sand condition were analyzed and compared. The bucket foundation with the longest skirt accelerated the most at the platform level and experienced the least degree of tilting and normalized settlement. Obtained results were expected to contribute for a better understanding on the performance of bucket foundation during an earthquake.

Commentary by Dr. Valentin Fuster
2016;():V001T10A006. doi:10.1115/OMAE2016-55075.

Suction anchors are used within mooring systems for a range of deep water floating structures. The number of suction anchor systems has increased rapidly over the last two decades due to ease of installation and a well-defined design approach. As with other offshore installations, anchor systems may be subjected to re-assessment during their operational lifetime, due to changes in loading, codes or other issues. Such assessment was performed recently for a floating storage unit (FSU) presently in service in the Norwegian Sea, where the geotechnical design of the anchors was re-evaluated.

The re-assessment of this storage unit allowed for a number of changes compared to the original design, including updated loads and load histories as well as changes to the safety factors. The geotechnical aspects of the design were also updated, including more refined analyses and a detailed review of the design soil profiles based on back-analysis of installation data.

This paper presents the work done related to the geotechnical holding capacity of the anchors during the assessment project. The work performed highlights the benefit of efficient FE analysis and a thorough evaluation of the soil properties and engineering behaviour.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Fluid-Soil-Structure Interaction

2016;():V001T10A007. doi:10.1115/OMAE2016-54377.

Offshore Gravity Base Foundations (GBFs) are often designed with non-uniform cylindrical geometries. Such structures interact with the local hydrodynamics which amplify the adverse dynamic pressure gradient, which is responsible for all flow and scour phenomena including the bed shear stress amplification. In this study a method for predicting the effect non-uniform cylindrical structure geometries have on local scour around offshore structures under the forcing of a unidirectional current is presented. The interaction of the flow field with the sediment around these complex structures is described in terms of non-dimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights in the influence a form of the Euler number has on the equilibrium scour around uniform and non-uniform cylindrical structures. Here the Euler number is defined as the depth averaged pressure gradient (calculated using potential flow theory) divided by the product of the square of mean flow velocity and the fluid density. The insights are confirmed through a series of experiments where the equilibrium scour was monitored for different types of structures and flow conditions.

The results of this study show that the Euler number is a more appropriate parameter for describing the scour potential of a structure compared to using the equivalent pile diameter. The experimental data show that an increasing Euler number yields an increase in the non-dimensional equilibrium scour. The results of this study also suggest that an increase in the water depth yields a decrease in the equilibrium scour depth for the conical, cylindrical base structures and truncated cylinders and an increase in the equilibrium scour depth for the uniform cylinders which can also be explained in terms of changes in the Euler number. Finally, the Buckingham π theorem in conjunction with the experimental data was used to derive a simple shape correction factor that could be used to determine the scour depth of a non-uniform cylindrical structure based on the equilibrium scour produced for the same flow conditions by a uniform cylinder.

Commentary by Dr. Valentin Fuster
2016;():V001T10A008. doi:10.1115/OMAE2016-54431.

This paper presents a comparison between simplified engineering and FEM (Finite Element Method) methods for on-bottom stability analysis of a subsea pipeline. The simplified engineering method is first used to assess the absolute on-bottom stability of empty and filled pipelines under different scenarios. The calculations of the hydrodynamic loads for three scenarios, i.e. steady current alone, regular waves alone and combined regular waves and current, are implemented in MATLAB code. The drag and lift coefficients are determined based on Keulegan-Carpenter number, Reynolds number and surface roughness of the pipelines. Only the friction force is considered in the simplified methods. In order to achieve the absolute stability, the vertical (lift force/submerged weight<1) and horizontal (in-line force/friction force<1) criteria need to be fulfilled at same time. Time-domain dynamic on-bottom stability analysis is performed by PONDUS for the same cases. The results of water particle velocity, hydrodynamic force, lift force and soil resistance force are compared between the simplified engineering and advanced FEM methods. Their results are in good agreement for the cases, which fulfills the absolute on-bottom stability criterion. For the cases which the pipelines will move under the combined wave and current loadings, the soil resistance force predicted by the simplified engineering method is different from that of the FEM method. The study shows that for engineering purpose the simplified engineering method could be used to check the absolute on-bottom stability of the pipeline, whereas the more advanced FEM method needs be performed when the pipeline is allowed to move within a limited distance.

Commentary by Dr. Valentin Fuster
2016;():V001T10A009. doi:10.1115/OMAE2016-54873.

Energy transition towards sustainable power generation affects the offshore wind energy sector greatly. Due to extensive research work and technological developments, the number of foundation types for offshore wind energy plants has increased significantly. Independent of foundation type, each structure influences the ecological and hydrodynamic regime surrounding the structure. As a consequence, local flow turbulences may cause scours at the seabed and can lead to a reduction of structure stability. Geotextile sand containers (GSC) are an approved method for scour protection. During installation of scour protection systems, the sinking behaviour of GSC is affected by translational and rotational movement, which impedes an accurate positioning of GSC. Physical model tests have been conducted to analyse the influence of container shape and material properties of GSC. This paper presents the results of these model tests.

Commentary by Dr. Valentin Fuster

Offshore Geotechnics: Spudcans and Pipelines

2016;():V001T10A010. doi:10.1115/OMAE2016-54639.

Aiming at simulating the preloading process of jack-up rig at drilling locations with typical stratum combination near the Yellow River estuary, FE models of interaction between spudcan and sub-soil are set up based on a large deformation finite element method known as Remeshing and Interpolation Technique with Small Strain (RITSS). The ABAQUS finite element package is used to calculate small strain solutions following each step. The seabed can be simplified as a three-layer deposits with soft clay layer, sand layer and soft clay layer from surface to bottom according to geological data. The backfill phenomenon during preloading is described. The results show that the surface soft clay falls onto the upper surface of spudcan very soon after the preloading process begins, afterwards the backfill tendency occurs in the middle sand layer. SNAME recommends two sets of formulas for calculating penetration resistance respectively for backfill penetration and penetration with no backfill. In this study the model with no backfill is realized by condition control in program codes and the penetration resistance is compared with the results of practical backfill model. The variance proves the shortcomings of the method given by SNAME.

Topics: Sands , Soil
Commentary by Dr. Valentin Fuster
2016;():V001T10A011. doi:10.1115/OMAE2016-54795.

The paper presents results of experimental works on intact marine clay samples in Malay and Sarawak Basins, which is located in Malaysia. Recent development in economic and search of new energy has brought to the usage of areas with plenty of marine clay inshore and offshore. For offshore area, the seabed foundation support or subsurface installation often create several complexities in geotechnical design due to high compression behavior and low bearing capacity. Knowledge of seabed soils and rocks is very important in designing any structures onshore or nearshore. However, the seabed data properties in Malaysia is less and not many researches have been done to contribute to this database. Series of oedometer tests performed intact clay samples from different depths showed inconsistency patterns of compression behavior. The preconsolidation pressure for all samples ranged from 200 to 500 kPa, which can be compared with the presence literature for the compression behavior towards denser soil. The compression index, Cc ranged from 0.177 to 0.797 and Swelling index, Cs ranged from 0.133 to 0.066.

Topics: Compression
Commentary by Dr. Valentin Fuster
2016;():V001T10A012. doi:10.1115/OMAE2016-55003.

The interaction between a spudcan and an existing footprint is one of the major concerns during jack-up rig installation, especially in the surface clay layer. No guidance was provided in the recently finalised version of ISO guidelines 19905-1 [1] in regards to mitigating spudcan-footprint interaction issues except some proposed considerations due to the lack of detailed investigations. This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of a novel spudan shape tested through 3D large deformation finite element (LDFE) analyses. The LDFE analyses using the Coupled Eulerian-Lagrangian (CEL) approach in the commercial finite element package ABAQUS. The soil conditions tested simulate soft seabed strength profiles close to the mudline. A critical reinstallation locations of 0.55D (D = spudcan diameter) and existing footprint depth (of 0.33D) were investigated. The main aim was to investigate if the spudcan shape itself can be used to mitigate potential footprint interaction when compared to a conventional spudcan. The results from this study indicated that the novel spudcan had the potential to ease spudcan-footprint interaction issues.

Commentary by Dr. Valentin Fuster

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