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ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V01AT00A001. doi:10.1115/OMAE2014-NS1A.
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This online compilation of papers from the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Offshore Technology

2014;():V01AT01A001. doi:10.1115/OMAE2014-23006.

Analysis techniques and numerical formulations are available in a variety for mooring and riser designers. They are applied in the different stages of the design processes of floating production systems (FPS) by taking advantage of both the accuracy of results and the computational costs. In early design stages, the low computational cost is more valued with the aim of obtaining fast results and taking decisions. So in these stages it is common to use uncoupled analysis. On the other hand, in more advanced design stages, the accuracy of results is more valued, for which the use of coupled analysis is adequate. However, it can lead to excessive computing times.

To overcome such high computational costs, new formulations have been proposed with the aim of obtaining results similar to a coupled analysis, but with low computational costs. One of these formulations is referred as the semi-coupled scheme (S-C). Its main characteristic is that it combines the advantages of uncoupled and coupled analysis techniques. In this way, analyses can be performed with very fast execution times and results are superior to those obtained by the classical uncoupled analysis.

This work presents an evaluation of the S-C scheme. The evaluation is made by comparing their results with the results of coupled analyses. Both type of analysis were applied in a representative deep water platform. The results show that the S-C scheme have the potentially to provide results with appropriate precision with very low computational times. In this way, the S-C scheme represents an attractive procedure to be applied in early and intermediate stages of the design process of FPS.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A002. doi:10.1115/OMAE2014-23022.

Large relative wave motions along the side of a ship can lead to green water on the deck. With a simplified test setup of a thin plate under an angle with the wave direction (to separate non-linear wave run up from motion effects), the non-linear wave reflection along the side of ships is studied in the present paper. These pilot tests with regular and irregular waves gave new insight in the process of non-linear wave run up with plunging and spilling breakers close to the plate. The complex processes observed made clear that linear or second order models will not be able to predict this behavior accurately. Previously [1] it was concluded that CFD methods that allow wave breaking are necessary for a prediction of these important effects. In the present paper a first pilot study is presented with an improved Volume of Fluid (VoF) Method. It is concluded that the method is in principle able to present these relative wave motions, but that a finer gridding is necessary to study the detailed flows.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A003. doi:10.1115/OMAE2014-23040.

Normally, the DP capability of a vessel is calculated through the use of static force equilibrium programs in which the dynamic effects are either not taken into account or taken into account by empirical load amplification factors. However, competitive and safe S-laying of large diameter pipelines in deep waters lead to large and long pipe lay vessels for which DP requirements are demanding. The power/propulsion requirement of the vessel needs to be considered from an early stage especially when accounting for the pipe laying equipment demands. This imposes a need for detailed dynamic analysis of the lay vessel. This analysis needs to include the slow drift oscillations counteracted by DP and the analysis in entirety needs to ensure the pipe string integrity is maintained.

To this purpose Saipem developed in-house a time-domain simulator (FIPLA – Fully Integrated Pipe LAying) that employs all environmental forces (i.e. wind, wave, current) as well as the pipe tension on the vessel. It is used to assess the DP performances and laying capabilities of pipe lay vessels in harsh dynamic environments as well as critical areas for operations. The software can also be used to assess the performance of the vessel in case of failure of thrusters, power generators or bus bars, study DP Control System improvements, and assess the interaction between vessel, tensioner and pipe in deep and shallow waters.

This paper focuses on the advantages of using dynamic simulations as an enhancement to the static DP capability charts, to produce detailed information for the DP Operator (DPO) in terms of setting of the DP parameters and to analyze critical laying events. This information can be used together with the weather forecast and can help getting the best performance out of the DP system in harsh environments, reducing downtime, improving operability and ensuring a safe operation.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A004. doi:10.1115/OMAE2014-23054.

Model testing of deepwater offshore structures often requires the use of statically-equivalent deepwater mooring systems. The need for such equivalent systems arises due to the spatial limitations of wave basins in accommodating the dimensions of the direct-scaled mooring system. With the equivalent mooring system in place and connected to the model floater, the static global restoring forces and global stiffness of the prototype floating structure can be matched (to within some tolerance) by those of the model for specified offsets in the required degrees of freedom. A match in relevant static properties of the system provides the basis for comparisons of dynamic responses of the model and prototype floaters. Although some commercial programs are capable of designing equivalent mooring systems, the physics applied in these programs are protected by intellectual property, and their methodologies are generally inflexible. This paper illustrates a concise approach to the design of statically-equivalent deepwater mooring systems. With this approach, either manual or advanced optimization techniques can be applied as needed based on the complexity of the equivalent system to be designed. A simple iterative scheme is applied in solving the elastic catenary equations for the optimal static configuration of each mooring line. Discussions cover the approach as applied in developing a fit-for-purpose tool called STAMOORSYS, its validation, and its application to the design of an equivalent mooring system for a spar platform in deepwater. The spar model parameters are representative of a structure which could be tested in the Offshore Technology Research Center, College Station, Texas, USA. Results show that the method is capable of producing good design solutions using manual optimization and a genetic algorithm.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A005. doi:10.1115/OMAE2014-23068.

The dynamics of offshore heavy lift remains complex due to the interaction between the monohull construction vessel and the lifting object under the challenging environmental loads. This system becomes even more difficult to judge by common sense as the lifting weight offshore is usually above the motion reference point (the center of flotation of vessel), which makes the coupled system more unstable than the vessel without the lifting weight. This study proposes an analytical formula using the double pendulum, based on the Euler-Lagrangian equations, to explore the insight of the heavy lift dynamics. The effect of initial lifting condition is investigated to explain different possible outcomes for lifting similar weights. A range of vessel stiffness (GM), lifting heights and weights are considered in the parametric study for a better understanding of the coupled dynamic behaviour. While such analytical model is convenient for the understanding of the system mechanism, it is difficult to capture the effect from the wave loading. Therefore numerical models are used for this purpose. A comparison between the analytical model and the numerical model, performed in the frequency domain, evaluates the quality of ship motion response analysis performed by the numerical model. Results of these works will be useful in development of offshore crane curves for heavy lift offshore.

Apart from the theoretical formulation, several real time ship instrumentation records including the ship motion and the crane hook load have been collected and investigated in this paper. The offshore instrumentation records provide a valuable benchmark to calibrate the numerical model or to accumulate “experience” for future projects. Challenges in such process are depicted and possible solutions are discussed. To make the comparison a fair “apple to apple”, the major difficulty arises from the “unknown” environment itself, where wave is not always measured and operation relies heavily on weather forecast and “experience”. There are also limitations in offshore measurement itself. Possibility of using the ship motion as the indicator is explored for decision making (to lift or not to lift). The paper develops a practical approach for obtaining reasonable numerical results in the engineering office and investigates scientifically sound interpretation of the ship motion time history onboard towards a safe offshore heavy lift.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A006. doi:10.1115/OMAE2014-23074.

The German government has decided upon the changeover from fossil and nuclear based electrical power generation to renewable energies. Following from this offshore wind farms are erected in the exclusive economic zones of Germany. For the transportation and installation as well as the maintenance of the wind turbine generators very specialized vessels are needed. The capability of dynamic positioning even in very harsh weather conditions is one of the major design tasks for these vessels. For this reason it is important to know the external loads on the ships during station keeping already in the very early design stage. This paper focuses on the computation of wave drift forces in regular and irregular waves as well as in natural seaway. For validation the results of the introduced calculation procedure are compared to measured drift force data from sea-keeping tests of an Offshore Wind Farm Transport and Installation Vessel.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A007. doi:10.1115/OMAE2014-23110.

Wave loads on a slender pile within a group of piles are studied by means of (i) large-scale laboratory tests carried out in the Large Wave Flume (GWK) of the Coastal Research Centre (FZK) in Hannover, and (ii) small scale experiments performed in 2 m-wide wave flume of Leichtweiss-Institute for Hydraulic Engineering and Water Resources (LWI), in Braunschweig, Germany. The small scale model tests (LWI) were scaled down (1:6.5) by Froude law from the large scale model tests (GWK). Scale and model effects are examined by comparing the results of small and large scale laboratory tests.

Topics: Stress , Waves , Electric load
Commentary by Dr. Valentin Fuster
2014;():V01AT01A008. doi:10.1115/OMAE2014-23144.

The paper respectively study the effect on hydrodynamic performances of FPSO under only swell and only wind-sea based on 3D potential theory and non-linear time domain coupled analysis method. The analysis results indicated that swell and wind-sea act as the same role in low frequency response. Swell is key factor in wave frequency response. Incident angle of swell evidently effects on mooring line tension. So it is necessary that we should consider swell and wind-sea at the same time in studying the hydrodynamic performances of FPSO in West Africa. The study offered an important reference on FPSO hydrodynamic performances analysis in West Africa.

Topics: FPSO , Mooring
Commentary by Dr. Valentin Fuster
2014;():V01AT01A009. doi:10.1115/OMAE2014-23161.

The Dynamic Positioning (DP) system is responsible for the stationkeeping and manoeuvrability of a vessel in offshore operations. The forces required by the DP system are distributed among the available thrusters of the vessel by a thrust allocation algorithm, which should be both accurate — i.e. the sum of the effective forces is close to the required forces — and efficient — i.e. the total consumed power is minimal.

Ideally, the thrust allocation algorithm accounts for the forbidden zones, a practical solution to avoid significant hydrodynamic interaction effects such as thruster-hull interaction, thruster-current interaction and thruster-thruster interaction. Moreover, the thrust allocation algorithm should also take into account the physical limitations of each thruster such as the maximum thrust (saturation), the maximum rate of turn (azimuth) and the maximum rate of change of RPM.

In order to include these complex requirements in the thrust allocation algorithm the hydrodynamic interaction effects are modelled as efficiency functions, which are incorporated in the (power) object function and in the (required forces) constraints.

With the purpose of solving the complete problem, including the physical limitations and the hydrodynamic interaction effects, several advanced optimization methods were investigated. The selected optimization algorithms were the Sequential Quadratic Programming (SQP) method and the Steepest Descent (SD) method.

In this paper a DP drill ship is considered, with 6 azimuthing thrusters. Results were obtained for environmental conditions of increasing strength, from benign sea states to higher sea states with large required forces, eventually leading to thruster saturation. As a final test case, the required forces are such that the thrust allocation algorithm forces the thrusters to cross large forbidden zones with significant thruster-hull interaction.

The results show that the enhanced thrust allocation algorithm can deal with both hydrodynamic interaction effects and physical limitations in a time-efficient and robust manner. Altogether, these improvements are expected to lead to higher accuracy and efficiency for DP operations.

Topics: Thrust , Algorithms
Commentary by Dr. Valentin Fuster
2014;():V01AT01A010. doi:10.1115/OMAE2014-23167.

Within the oil and gas industry, significant costs are often incurred by the operating company during the well-construction phase of drilling operations. Specifically, the operators cost to drill a well can cost tens or hundreds of millions of USD. One specific area where significant changes in drilling operations have occurred is in the offshore environment, specifically operations from mobile offshore drilling units (MODUs). With the ever-increasing demand for oil and gas, operators globally have increased drilling budgets in an effort to meet forecasted demand. However, the increased budgets are often eroded or offset by increasing drilling costs. Therefore, operators are continually in search of new technology, processes, or procedures to help improve drilling operations and overall operational efficiencies.

One Latin America operator identified a common operation as a possible area where operational cost could be easily reduced through the implementation of systems that allow the manipulation of valve manifolds remotely. Additionally, operating such valve manifolds remotely enhanced operational safety for personnel, which was an equally important consideration.

This paper details the evaluation of existing equipment and procedures and a process used to develop a new remote-control system using a machine logic control (MLC) that has been designed, built, tested, and deployed successfully on MODUs operating in Latin America.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A011. doi:10.1115/OMAE2014-23174.

For disconnectable turret-moored FPSOs, accurate prediction of turret buoy and FPSO motions during the buoy disconnection process is essential for safe operations. For deepwater high production rate systems, large size buoys are required to accommodate the large number of risers and heavy mooring legs. Analytical models of hydrodynamic forces on large size buoys must be verified before they are applied to motion predictions. To gain a better understanding of the transient hydrodynamic loads on the buoy and hydrodynamic interactions between the buoy and the hull during disconnection, we conducted a specially designed model test in a tow tank. In the model tests, both the buoy and the FPSO models were forced to oscillate by two independent actuators in calm water and in waves. Summary of test results, computed transient hydrodynamic forces from a simplified approach, a true time-domain transient hydrodynamic analysis based on instantaneous buoy positions, and computational fluid dynamics (CFD) results are presented in this paper.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A012. doi:10.1115/OMAE2014-23179.

In this study, in order to evaluate hydrodynamic behaviors of the hybrid floating body with cylinders, experimental studies were carried out. At first, two small-scale models of pontoon and hybrid with scale factor 1:75 were fabricated and tested under the wave loadings. Based on the measured data, hydrodynamic motions of the pitch, surge, drift force, and free decay motions were evaluated and compared with each other. As the result of small-scale test, it was found that the pitch motion is more sensitive to the wave period than the surge and drift motions. Whereas the pitch motion increased by increasing of the wave period, the surge and drift motion presented a small variance according to the increasing of the wave period for the both models of pontoon and hybrid. Also, it was found that the hybrid floating body significantly influenced on the hydrodynamic motions under the wave loadings. The pitch motion reduced significantly for the all wave period cases. The surge and drift motion reduced over the wave period of 0.982sec. Damping properties of hybrid model indicated more favorable in terms of logarithmic decrement and damping ratio because of the shape effects. Therefore, it was expected that the hybrid model of this study contribute to improve serviceability and safety of floating body as decreasing hydrodynamic motions.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A013. doi:10.1115/OMAE2014-23180.

New methods for estimation of extreme wave crest heights have resulted in an increase of the estimated 10,000 year crest height. At the Norwegian Continental Shelf this increase is typically 2 to 4 m, resulting in a crest height of 22 m to 24 m in the Central & Northern North Sea and the Haltenbanken area. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes.

For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights.

For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The wave calibration results were discussed in a previous paper, see [1].

For the wave-in-deck model tests, an existing North Sea jacket was built at scale 1:60 and instrumented in order to measure the global loads on the platform deck independently from the loads on the jacket itself. In this paper the model test setup as well as the measured wave-in-deck impact loads are discussed and compared to a simplified load prediction model. The presented results show that the simplified loading model, with wave properties based on Stokes 5th order wave theory, underestimates the measured horizontal deck loads.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A014. doi:10.1115/OMAE2014-23229.

The development of deep and ultra deep water fields, especially in marginal areas, always presents challenges related to profitability or associated flow assurance issues for subsea equipment.

Currently, Petrobras has a great number of production wells operating in deepwater, mainly in its mature fields (brown fields), therefore we are constantly motivated to improve production capacity and reduce operational costs (OPEX) in these scenarios.

In order to overcome this challenge (reduce OPEX and CAPEX) and keep focus in scenarios with high GOR, Petrobras has applied efforts in developing a high efficiency subsea gas-liquid separation system, particularly through PROCAP Technology Program - Future Vision [1]. The development of this technology is important to confirm whether is sound and reliable for use in large scale applications to increase oil production and, as well, improve both the flow assurance performance and recovery factor.

In the subsea gas-liquid separation technology one of the main challenges is related to variation in the design parameters and in the flowrate range expected during the life cycle of the project. The subsea separator has to be able to handle large flowrate variations since it can work with more than one production well connected.

The objective of this paper is to present the main results of the analysis carried out regarding the influence of the produced fluids properties and the flow behavior on the separation efficiency of a vertical annular subsea separator system (VASPS) [2]. The paper will present the subsea separator design and its performance when subjected to variations of fluid viscosity, fluid density, liquid and gas flowrates, bubble cut off diameter and water cut.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A015. doi:10.1115/OMAE2014-23265.

Since the last five years, an innovative technology for umbilical’s seam welded super duplex stainless steel tubes (grade 2507, UNS S32750) has been developed in order to meet new technical challenges. An extensive program of qualification, based on 4 dimensions covering the full manufacturing range, i.e. internal diameter and wall thickness, has been set. More than 11000 test results demonstrated the efficiency and the stability of the manufacturing process. The results were also confirming the expected properties of the tubes, in term of mechanical characteristics, corrosion resistance and manufacturing tolerances. At the end of this qualification phase, a Type Approval Certificate was delivered by an independent and official certifying authority demonstrating then, the suitability of the product for offshore static applications.

The next step was to confirm the fatigue performance of the product to meet the requirements of umbilicals for dynamic applications.

This paper describes how a dedicated fatigue test protocol has been jointly defined between a Major O&G player and the tube supplier. This protocol has been built in order to test the 3 dimensions already produced during the first step of the qualification, considering tube samples with and without orbital weld. Fatigue test programs are known to be very sensitive to the test parameters, i.e. temperature, test frequency and dimensional properties of the samples. As a consequence, special attention is paid to the description of the testing methodology, the selection of the acceptance criteria and the definition of the testing parameters. Finally, the results of this program are presented and discussed in order to assess the capability of the product with respect to the defined acceptance criteria.

Topics: Fatigue
Commentary by Dr. Valentin Fuster
2014;():V01AT01A016. doi:10.1115/OMAE2014-23277.

The prediction of the nonlinear rolling motions of a FPSO with extended bilge keels stills a challenging problem. Despite recent advantages in CFD computations, the use of model testing is considered the standard strategy for roll damping assessments. There are different ways to assess the roll damping via model testing and the three most common are the use of decay tests, tests in regular and irregular waves. The comparison among those different kinds of tests may present incoherent results, sometimes, introducing the question of which methodology is the most appropriated in those scenarios, with impact in model test specifications.

Decay tests are the easiest and the most economical way for roll assessments, but they are usually considered inaccurate compared to the other test types. Recent researches, however, have shown that a statistic approach which utilizes several decay data from the same model and loading condition improves the damping predictions. This paper provides comparisons of damping predictions from different test types based in extensive model test campaigns. It also addresses the issues and the lessons learned during the campaigns to obtain the FPSO damping database.

Topics: Damping , Testing , FPSO
Commentary by Dr. Valentin Fuster
2014;():V01AT01A017. doi:10.1115/OMAE2014-23278.

Monocolumn platforms are usually considered an alternative concept for drilling and offshore production unit designs. This concept could provide high stability, small motions and also storage capacity. More recently, it was considered for a logistic hub platform design. The use of this concept with an opening for boat docking seems to provide a feasible solution for the people transfer in harsh environments, especially in areas like pre-salt Brazilian offshore fields, which are also very far from the coast. Although the helicopter technology has been improved in recent years to attend transport requirements from shore to the platforms, the use of a hub platform at sea still shows some economical and operational advantages, related to the conventional operations.

Although the sheltered area provided by the interior of the monocolumn platform seems to solve the transfer problem in harsh environments, the hydrodynamic behavior of the moonpool became a problem to be solved during the feasibility studies of the concept, since the moonpool water column elevations tends to be resonant in special cases. To deal with this problem, some attempts were done to absorb the wave energy inside the moonpool and eliminate its resonance, achieving the desired level of absorption. This work shows the attempts and the results obtained, which has proven the feasibility of the concept.

Topics: Absorption , Waves
Commentary by Dr. Valentin Fuster
2014;():V01AT01A018. doi:10.1115/OMAE2014-23291.

In this article we present details of “ShallowFlow”, a computer program to model the hydrodynamic flow around ships in calm shallow water. The program is based on slender-body shallow-water theory. Outputs from the program include far-field hydrodynamic pressure and flow velocities; free surface drawdown; sinkage and trim. Varying transverse bathymetry including open water, dredged channels, and canals of arbitrary cross-section may be modelled. The method is best suited to displacement ships, including cargo ships, ferries, cruise ships, warships and superyachts.

Topics: Hydrodynamics , Ships , Water
Commentary by Dr. Valentin Fuster
2014;():V01AT01A019. doi:10.1115/OMAE2014-23297.

Sub-sea installation operations require a high level of accuracy and control in order to avoid misalignment and possible collisions between modules on the sea bed. To reduce costs, smaller and lighter construction vessels are now performing these operations. The most critical parts of the operation are lift-off from the deck, passing through the splash zone, and landing sensitive equipment on the sea bed. The hazards are: high dynamic loads, resonance effects, and slack line snap. Therefore, in this study, modeling and simulation are applied to optimize design parameters and develop operational procedures for each operation to reduce risk of failure. Further, the same models can be used in operator simulator training.

Modeling and simulation of interactive multi body systems is a rather complex task, involving the vessel as a moving platform, lifting equipment such as cranes and winches, guiding devices, lifting cables and payload behavior in air, all while partly to fully submerged. It is a multi-physics problem involving hydrodynamics, mechanics, hydraulics, electronics, and control systems. This paper describes an approach to link the different models to simulate the operations including interactions between the sub-systems. The paper focuses on the modeling approach used to connect the various dynamic systems into the complete operating system. The work is in its initial phase, and some of the sub-systems models are not complete. The models are described in this paper and will be included in future work. Some initial operational examples are included, to show how the models work together.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A020. doi:10.1115/OMAE2014-23343.

A new concept Spar FPSO (Floating Production Storage and Offloading) is proposed for South China Sea deepwater oil field development integrating the advantages of the deep-draft Spar concept whose excellent stability and global performance enables dry-tree drilling and production, and the conventional ship-shaped FPSO for oil storage and offloading.

This paper presents a study of the tandem offloading system for the new concept Spar FPSO. The paper will first give an overview of various offloading configurations and their characteristics for deepwater development; then a tandem offloading system similar to conventional ship-shaped FPSO is recommended for further investigation considering operational experience, safety and economical benefits. As the offloading operations are generally weather-limited, offloading environmental conditions for the study are selected based on API, ABS and South China Sea offshore operation practices. For a case study, hydrodynamic performance of the Spar FPSO in tandem offloading configuration with a shuttle tanker in 1500m water depth was investigated numerically and experimentally. Numerical simulations were conducted by means of fully coupled nonlinear time-domain analysis. The distance between the shuttle tanker and the Spar FPSO is optimized based on the relative motions of the two floaters and the hawser tension requirement of the shuttle tanker. Experimental results from model test are presented and compared with analytical time-domain simulation results. It is shown that the results of numerical simulations are consistent with the model test. The tandem offloading system demonstrates robust performance and cost-effectiveness for the proposed Spar FPSO concept.

Topics: FPSO , Spar platforms
Commentary by Dr. Valentin Fuster
2014;():V01AT01A021. doi:10.1115/OMAE2014-23352.

It is reputed in the Oil & Gas industry that marine growth is overestimated in the North Sea. Can we quantify this overestimation to better tackle the challenges marine growth incurs?

This paper provides insight into the intrinsic and diverse nature of marine growth. It documents the biofouling development and the factors that affect it globally. Focus is made on fixed steel platforms in the North Sea approaching the end of their economic lives. A methodology to quantify the weight of marine growth is proposed and illustrated. 28 years after its installation, the weight of marine growth accumulated on Valhall 2/4G jacket located in Southern North Sea is evaluated based on the proposed method. It is confirmed that the current regulations overestimate the weight of marine growth in the North Sea.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A022. doi:10.1115/OMAE2014-23364.

Extreme value estimation of significant wave height is essential for designing robust and economically efficient ocean structures. But in most cases, the duration of observational wave data is not efficient to make a precise estimation of the extreme value for the desired period. When we focus on hurricane dominated oceans, the situation gets worse. The uncertainty of the extreme value estimation is the main topic of this paper. We use Likelihood-Weighted Method (LWM), a method that can quantify the uncertainty of extreme value estimation in terms of aleatory and epistemic uncertainty. We considered the extreme values of hurricane-dominated regions such as Japan and Gulf of Mexico. Though observational data is available for more than 30 years in Gulf of Mexico, the epistemic uncertainty for 100-year return period value is notably large. Extreme value estimation from 10-year duration of observational data, which is a typical case in Japan, gave a Coefficient of Variance of 43%. This may have impact on the design rules of ocean structures. Also, the consideration of epistemic uncertainty gives rational explanation for the past extreme events, which were considered as abnormal. Expected Extreme Value distribution (EEV), which is the posterior predictive distribution, defined better extreme values considering the epistemic uncertainty.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A023. doi:10.1115/OMAE2014-23380.

An accurate prediction of the global response of a floating production and storage offloading (FPSO) system under harsh environmental conditions is of great importance in order to achieve the reliability and safety operation of the whole system. FPSOs may be subjected to significant resonant oscillations in the horizontal plane due to low frequency (LF) wave effects and wind excitation forces. These characteristics may contribute to the increase in surge due to the low level of viscous hull damping. Additionally, it has been observed that when the water depth increases, the coupled effects (damping, inertia and restoring force) contributions from mooring lines and risers increases.

This paper investigates the LF response behavior of a deepwater FPSO unit in the Gulf of Mexico by carrying out a coupled analysis based on a nonlinear time domain analysis.

A 3D model based on boundary integrated element method is used to investigate the hydrodynamic behaviour of the floater as well as a 3D finite element model for each of the slender elements representing the mooring lines and risers. The LF motions of a FPSO with a typical arrangement of catenary mooring lines and steel catenary risers is studied for surge, sway and yaw mainly. The hydrodynamic characteristics of the FPSO are studied through both Newman’s approach and the full Quadratic transfer function.

The coupling effect of the floater and mooring/riser systems is examined by comparing the tensions in mooring lines/risers and the global responses of the system in six degree of freedom. The nonlinearity of the hydrodynamics of wave-vessel interaction and the dynamic contribution of mooring lines and risers are investigated with storm and hurricane events for a particular location in deep water Gulf of Mexico (GOM).

Commentary by Dr. Valentin Fuster
2014;():V01AT01A024. doi:10.1115/OMAE2014-23389.

Seakeeping behavior of a multibody system in side-by-side configuration in head sea condition is discussed in this paper. The system, which can be assimilated to a FLNG and LNG carrier during an offloading operation is composed of a barge and a prismatic geosim with two gap values. Seakeeping tests in regular waves have been performed in the model basin of CEHINAV-Technical University of Madrid (UPM). The movements for the geosim were restricted to the surge, heave and pitch motions (on the vertical plane), whereas the barge was kept fixed. In this way the gap remained constant during the tests. Numerical modeling has been undertaken using WAMIT and an in-house time-domain Rankine Panel Method (TDRPM). Response amplitude operators in terms of movements and wave amplitude in the gap obtained from seakeeping test and numerical models are documented in the paper, illustrating the limitation of the numerical codes regarding the modeling of this hydrodynamic problem. Numerical results indicate a resonant behavior of the waves in the gap for a range of frequencies, with amplitudes much higher than those observed during the tests. Due to the small distances considered in the experiments, these resonant waves are related to longitudinal wave modes in the gap. In order to overcome this problem, a procedure for introducing an external damping factor that attenuates the wave amplitude along the gap in the time-domain RPM is evaluated based on the experimental data.

Topics: Resonance
Commentary by Dr. Valentin Fuster
2014;():V01AT01A025. doi:10.1115/OMAE2014-23395.

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system?

This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system.

The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

Topics: Mooring , North Sea
Commentary by Dr. Valentin Fuster
2014;():V01AT01A026. doi:10.1115/OMAE2014-23400.

The roll damping coefficient is a crucial parameter for several design and operational aspects of FPSOs. The accurate prediction of the coefficient is not a trivial task and generally performed experimentally. A polynomial linearization of the decay test data has been widely applied in the offshore industry. However, research has indicated that for FPSOs with rectangular cross section and attached bilge keels, this methodology may lead to inaccurate damping coefficients.

This paper presents a study on the experimental determination of roll damping coefficients for FPSOs, obtained by free decay tests. For this purpose model tests are executed in the towing tank of the Marine Hydrodynamic Laboratory at Newcastle University. The model is based on the design of a purposely build FPSO, as typically applied in the central North Sea sector. The cross section of the FPSO is boxed shaped with a characteristic knuckle shaped bilge. The tests are conducted using three different bilge keel arrangements. The parametric change in bilge keel size results in the variation of the flow characteristics around the bilge knuckle. The damping coefficients are then established from the decay test data using a polynomial approach, a bi-linear approach and a hyperbolic approach.

A comparison between the damping evolutions obtained with the different methodologies is performed for each bilge keel configuration. Further, a numerical model of the FPSO is created using DNVs Sesam software. With the established damping coefficients, damping matrices are manually defined as an input to Sesam and roll transfer functions are numerically established. The computational determined transfer functions are then compared against the RAOs obtained from the model tests in regular waves to determine the most appropriate methodology.

The damping coefficient for the bare hull is well established by all three proposed methodologies. However, with the attached bilge keels the bi-linear and the hyperbolic methodologies produce damping coefficients reflecting the experimental results more accurately than the polynomial approach, indicating that the recently developed hyperbolic method is a valid alternative, and in certain cases, is more suitable to determine the roll damping coefficient. The experimental measurements could serve as a benchmark for further research and contribute to the practical application of FPSO roll damping determination.

Topics: Damping , FPSO
Commentary by Dr. Valentin Fuster
2014;():V01AT01A027. doi:10.1115/OMAE2014-23422.

Fixed platform have been widely employed in the offshore oil and gas reservoirs development. Pile foundation reliability is critical for these platforms where drilling, production and other functions are integrated. The lifting operation for the long pile, being a key step in the jacket installation, has been considered for further developments. With deep water developments, the sizes and weights of long piles are reasonably bigger. The corresponding process and equipment employed are subsequently altered, which brings challenges to developing a cost-effective, easy-operable approach for lifting operation. In this paper, the technology for the offshore long pile upending lifting operation including pile feature, installation methodology, lifting rigging and analysis model, covering water depths ranging from shallow to near deep water zone (60–300 m water depth) has been suggested. In addition, the applicability of the adoptable novel approaches has been discussed considering the practical project experience.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A028. doi:10.1115/OMAE2014-23430.

As one of the most promising equipment for deep far sea gas field development, FLNG has been widespread concern in the engineering sector. Due to the characteristic of LNG, the sloshing of liquid tank in FLNG is inevitable. Therefore, the sloshing load of liquid tank is one of the hotspots in engineering and academic research. In this paper, in order to simulate the sloshing load of liquid tank in FLNG, a secondary development software is proposed based on the commercial software FLUENT. The simulation results agree well with previous experimental results. This shows the accuracy of the simulation of the software. In addition, the longitudinal and transverse section of LNG tank is analyzed. The results shows that different excitation frequencies and amplitudes great influence on the sloshing load of liquid tanks, different filling levels also have a certain influence. This will be used to estimate the size of sloshing load for the FLNG equipment which may encounter in the real sea condition, and have certain significance in practical engineering.

Topics: Stress , Sloshing
Commentary by Dr. Valentin Fuster
2014;():V01AT01A029. doi:10.1115/OMAE2014-23434.

The paper presents an industrial application of CFD for calculation of Wave-In-Deck load due to an extreme wave.

Particular attention is given to flow kinematics initialization that is necessary to start up a CFD simulation. The applied CFD code, ComFLOW, is a Navier-Stokes equation solver with an improved Volume of Fluid (iVOF) method employed to displace and re-construct fluids free surface.

For incoming waves high enough for a negative air-gap and therefore with Wave-In-Deck loads, a jacket platform was tested in model basin, for both regular and irregular wave cases. One of goals of these model tests was verification of CFD codes. The experimental and computational models of the structure are exactly the same. In the paper, the measured Wave-In-Deck forces are compared with CFD results.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A030. doi:10.1115/OMAE2014-23439.

Possible expansion of the LNG sea transportation region to the Russian Arctic makes topical the issue on feasibility of using various types of cargo containment systems for large ice LNG carriers of higher ice class notations.

When analyzing safety of LNG transportation in the Arctic conditions in case of using cargo containment systems of various types, both fatigue issues adjusted for ice effects (this is especially topically for membrane containment systems) and consequences of emergency situations shall be studied.

To evaluate fatigue of membrane cargo containment systems, a special finite element analysis was performed; at that ice effects on the hull were considered together with vibration caused by screws and engines for the purpose of determining maximum possible excitations occurring during the ice navigation. Static, dynamic and life tests, that permitted to estimate experimentally the fatigue margin of membrane containment system elements’ attachment to the vessel hull, were conducted as well.. The study results permit to conclude that the fatigue of CCS NO96 membrane cargo containment system is quite sufficient for resisting vibration loads, which occur during 40-year service life of large Arctic LNG carriers in severe ice conditions.

Another important aspect of LNG transportation safety in Arctic conditions consists in accidental load problems. The following types of emergency situations were considered:

• accident collision of vessels (LNG carrier ramming by another vessel);

• landing on rock (bottom collision with rock in sea conditions);

• hull side impact against an iceberg fragment during turning.

Consequences of vessels’ collision and impacts against an iceberg were evaluated with the use of software LS-DYNA. It was ascertained based on the performed analysis that the feasibility of LNG carriers with membrane containment systems is minimal; the feasibility of LNG carriers with SPB containment systems is slightly higher, and the feasibility of LNG carriers with MOSS containment systems is essentially higher than that of LNG carriers with membrane containment systems. It could be explained physically by the fact that MOSS cargo containers approach the outer shell plating near the centerline plane (for the bottom) and in the compartment middle (for the side and bottom) only; when moving off from the centerline plane and the compartment middle the distance between cargo containers and the outer shell plating essentially grows.

Nevertheless, development of large Arctic LNG carriers having sufficient accidental load levels is possible on application of any containment system type.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A031. doi:10.1115/OMAE2014-23450.

Offshore wind farms are not planned in sheltered and shallow waters any longer. Especially in the North Sea there exist many approved offshore wind farm projects at water depth between 30 and 50 meters. In particular the installation process of these projects is strongly influenced by weather conditions and the sea-keeping capabilities of the installation vessels. For reliable planning of the entire project, not only the weather statistics, but also the vessel’s sea-keeping capabilities need to be known accurately. For this purpose different kinds of sea-keeping analyses can be conducted. In this paper a sea-keeping analysis is presented, where the focus is upon the jack-up process. For the numerical computation the sea-keeping code E4ROLLS is applied. The results of this sea-keeping analysis are operational limitations for the jack-up process, caused by two different criteria derived from jack-up classification requirements.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A032. doi:10.1115/OMAE2014-23456.

This paper presents a fast numerical method to analyze heavy-lift operations of ships in short crested waves. For this purpose, a sea-keeping simulation method for the coupled motions of a heavy-lift vessel and a freely suspended load is developed. The method considers the motions of the ship in six degrees-of-freedom and the suspended load as a point mass. The coupling of the multi-rigid-body system of the ship and the suspended load is considered by solving the equation of roll motion together with the Euler-Lagrange equations of the load. This approach allows the simulation of several hours of real time motion in short crested waves within only a few seconds. Consequently, the method is particularly suitable when very long or numerous sea-keeping simulations or statistical results are required. Moreover, the method is applied to evaluate the sea-keeping capabilities of a heavy-lift vessel during a lifting operation conducted offshore in 2013.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A033. doi:10.1115/OMAE2014-23472.

Offshore wind energy development has gained considerable momentum around the world as wind is stronger and steadier offshore compared to land. This has led to a significant increase in production in recent years, especially offshore wind turbine embedded in shallow waters, such as the recent large scale offshore wind farms in the Northern Europe region. Being at the offshore waters, the wind turbines are subjected to harsh environment. The pile supporting the wind turbine must be reliable and able to withstand such sea condition. It is an important part of the design to study the structural behaviour of the piles under the wave loads.

Due to the significant capital cost associated with the fabrication of the large circular cylinders, a new recommended innovative design to overcome such problem is to substitute the circular cylinder with a vertical monopile of octagonal cross-sectional shape. This paper describes the development of an efficient numerical model for structural analysis of wave interaction with octagonal pile using a modified semi analytical Scaled Boundary Finite Element Method (SBFEM). In contrast to the existing solutions obtained using the traditional methods such as the Finite Element Method (FEM) which typically suffer from high computational cost and the Boundary Element Method (BEM) which faces limitation from fundamental equations and problems with singularities. The most prominent advantage that SBFEM has over the FEM is in terms of the number of elements used for calculation and hence a reduction in computational time. When compared with BEM, the SBFEM does not suffer from computational stability problems.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A034. doi:10.1115/OMAE2014-23489.

A non-linear mathematical model, Planing Hull Motion Program (PHMP) has been developed based on strip theory to predict the heave and pitch motions of planing hull at high speed in head seas. PHMP has been validated against published model test data. For various combinations of design parameters, PHMP can predict the heave and pitch motions and bow and center of gravity accelerations with reasonable accuracy at planing and semi-planing speeds. This paper illustrates an application of modern statistical design of experiment (DOE) methodology to develop simple surrogate models to assess planing hull motions in a vertical plane (surge, heave and pitch) in calm water and in head seas. Responses for running attitude (sinkage and trim) in calm water, and for heave and pitch motions and bow and center of gravity accelerations in head seas were obtained from PHMP based on a multifactor uniform design scheme. Regression surrogate models were developed for both calm water and in head seas for each of the relevant responses. Results showed that the simple one line regression models provided adequate fit to the generated responses and provided valuable insights into the behaviour of planing hull motions in a vertical plane. The simple surrogate models can be a quick and useful tool for the designers during the preliminary design stages.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A035. doi:10.1115/OMAE2014-23491.

With more than 50.000 tons in service to date, the Oil&Gas Industry has the need to understand the tension fatigue performance of grade R5 chains in straight tension, and corroborate the validity of the existing design methods.

The chain fatigue design curves in API and DNV are based on fatigue tests obtained in the nineties and early two thousands. However the tests were performed on lower grades such as ORQ, R3 and R4, and small chains, 76 mm diameter being the largest studless chain tested.

The industry has moved towards the use of large studless chains, especially in permanent units, where chain diameters above 150 mm are not unusual.

This paper gathers information from a full scale fatigue test program on grade R4 and R5 studless chains, performed in seawater and with diameters between 70 mm and 171 mm. The chains being tested are actual production chains supplied for different drilling units and large permanently moored production floating units.

The paper analyses the data and determines tension-tension fatigue curves based on API and DNV methods for computation of cumulative fatigue damage, regardless of other damaging mechanisms. Improved fatigue capacity is obtained with respect to the above recommended design methods.

Topics: Fatigue , Chain , Mooring , Seawater
Commentary by Dr. Valentin Fuster
2014;():V01AT01A036. doi:10.1115/OMAE2014-23499.

For the ship design system E4 a universal crane model was developed. With the help of this model the design of crane vessels and cranes can be harmonised better and lifting operations can be analysed more precisely. The model is able to handle any type of cranes which are used in the shipping and offshore industry and provides the input for every simulations method within E4 which needs to consider crane operations. Special focus was given on keeping a common user interface both for the designer of a crane vessel and the planning engineer for lifting operations independently of the type and functionalities of the crane. Such a universal crane model has not been implemented in any other ship design software until now.

In this article the motivation for the development of the model is described together with the requirements for a universal crane model. The principle of the model explained and its functionality is demonstrated in several examples.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A037. doi:10.1115/OMAE2014-23503.

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

Commentary by Dr. Valentin Fuster
2014;():V01AT01A038. doi:10.1115/OMAE2014-23504.

A novel frequency domain identification (FDI) strategy for the identification of radiation force models from frequency domain hydrodynamic data is proposed. First, a subspace identification method is augmented with a convex constraint that guarantees a stable solution. Then, in a second convex optimization problem, constraints on low- and high frequency asymptotic behavior and passivity are enforced. This novel method, constrained frequency domain subspace identification (CFDSI), is validated by comparing both SISO and MIMO CFDSI results with the state-of-the-art FDI toolbox, which is part of the Marine Systems Simulator MATLAB toolbox. In two test cases, it is shown that the novel algorithm can successfully identify a model with either a SISO or MIMO structure, where stability, passivity and the desired low- and high-frequency asymptotic behavior are guaranteed. For the two test cases presented, the quality of the CFDSI models matches the quality of the state-of-the-art FDI models.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A039. doi:10.1115/OMAE2014-23511.

Rack Phase Difference or RPD may occur during jacking up at a location when the jack-up spudcan is eccentrically supported on the bottom or has lateral offset and the resulting moment and shear is carried essentially by the horizontal guide reactions. The RPD is the difference in vertical positions of rack teeth of different chords of one leg. This is a result of vertical shear deformation of the adjacent chords of one leg of a trussed jack-up. Among the causes of RPD the following are common: uneven or sloping seabed, preexisting spudcan holes, leg offset, rapid penetration, uneven jacking force on different chords of a leg, scour etc. While jacking up at a location, operating procedures normally dictate that the RPD be measured. When there is significant RPD, it means the leg chords are not centrally placed within the legwell area but one side could be touching the upper guide while the opposite side is in contact with the lower guides. There is a safe limit of the RPD that is normally specified for a given jack-up that has to be kept in mind while jacking up at a location.

For a jack-up fitted with rack chocks, once at the elevated condition, the jack-up is designed to take up the moment produced at the legs due to the environmental loads by the vertical couple at the chocks and pinions rather than the horizontal couple at the guides. As a result, the brace size requirement is normally lower than units that do not have the chocks. For these units RPD becomes all the more important as it could be built-in unless suitable mitigating measures were taken before the chocks are engaged. The built-in RPD could affect the strength for design environmental loads.

Unfortunately, the most commonly used guidelines for jack-up design such as SNAME T&R 5-5A, ISO 19905-1 or DNV OS-C104 do not have any detailed discussion on the topic. HSE Research Report 289 prepared by MSL Engineering has a good technical description of this phenomenon and some general qualitative guidelines. There are some studies published in the public domain that have details of RPD analysis. However, most of these papers deal with very simplified modeling of only one leg of the jack-up and idealized behavior.

The present paper discusses complete and more realistic analysis of a 3D model of the jack-up that uses non-linear analysis methods for including large deformations and rotations. The nonlinearities come about due to physical reasons, such as pinion-rack contact is compression only and guide contact is also compression only with a gap. In addition, one may like to include nonlinearities due to geometric stiffness for large deformations and plasticity effects. Geometric stiffness allows it to include so-called P-delta effects and simulate buckling behavior of beam elements.

The results of such 3D non-linear analysis for various levels of assumptions are compared with a more traditional single leg analysis. The paper brings out the important differences in behavior of these two types of models and discusses the importance of those in the analysis results. The authors think 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 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. The paper discusses these aspects also. Finally, although the analysis is for one particular jack-up, it recommends the most appropriate modeling for RPD analysis for similar jack-up rigs that could be beneficial to other rigs.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A040. doi:10.1115/OMAE2014-23524.

Marine installation operations are becoming ever longer, larger and deeper. With campaigns regularly being carried out in the 2,000–3,000m water depth range and tieback lengths on the increase; installation, replacement or recovery works for this specification of product requires specialist equipment that can accommodate the increasing bulk of the flexible products being installed, without compromising the performance of the vessels which carry them.

In order to fulfil this technical challenge, new and innovative carousel designs are being developed. Placing the ‘reel’ horizontally allows maximum product capacity whilst minimising the impact on the vessel, thus maintaining operational efficiency in both shallow and deep waters and for projects such as subsea tiebacks and connections to processing platforms and offshore installations.

This paper explores recent developments in technology, where product loads of up to 1,500Te (Tonnes) are frequently required and ever shorter schedules are placed on contractors for initial spooling and subsequent installation. Technical innovations, operational solutions and commercial advantages will be assessed and the ability to open up new markets with the technology on offer will be scrutinised.

Alongside this, the benefits of modularity are evaluated to quantify the advantages of being able to disassemble and transport this type of hardware worldwide and the vessel utilisation improvements that can be made.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A041. doi:10.1115/OMAE2014-23533.

The Tension Leg Semisubmersible (TLS) floater is the second generation of the Free-Hanging Solid Ballast Semisubmersible (FHSB Semi) design. The FHSB Semi is the state-of-the-art design that maintains the advantages of the conventional Semisubmersibles and provides the motions of Spars. The FHSB Semi design is a conventional semisubmersible with an added feature of a Free-Hanging Solid Ballast tank (SBT) located deep below the hull keel level and connected through four groups of chains (on its four corners) to the semisubmersible hull. During the pre-service conditions, the chains are used to retract the SBT and maintain it at a close proximity to the hull keel to facilitate the transportation and installation operations. Once the semisubmersible is moored in-place, the chain system is operated to lower the SBT to its desired elevation below the hull keel and maintain it permanently connected to the semisubmersible hull during the in-service condition. The SBT provides substantial in-water weight and added mass that significantly reduce the platform response and maintains the chains in tension.

In this paper, detailed description of the Tension Leg Semisubmersible (TLS) design as the second generation of the FHSB Semi is presented. The paper also includes discussions on the proposed project execution plan for the TLS covering the fabrication, transportation and installation as well as the added risks and risk mitigations associated with the new semisubmersible design and execution.

A case study is introduced where a TLS is used as a dry tree host for a field located at 6,000 ft of water in the central Gulf of Mexico (GoM) harsh environment.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A042. doi:10.1115/OMAE2014-23541.

In this work, a specific experimental protocol is developed to study the bubble sweep-down phenomenon on oceanographic research vessel. The combination of the utilization of a wave and current flume tank and an hexapod allows the simulation of conditions under which the phenomenon can be observed at full scale. Visualizations and PIV measurement results both obtained in the bow vicinity of the Ifremer oceanographic vessel Pourquoi pas? are presented in this paper.

Topics: Bubbles , Vessels
Commentary by Dr. Valentin Fuster
2014;():V01AT01A043. doi:10.1115/OMAE2014-23564.

A hydropneumatic heave compensation system and a semi-active control are proposed according to the requirements drilling offshore of 6 km deep. The goal of this semi-active control is to maintain an acceptable performance of the system when the drill string mass changes, which also changes the performance of the hydropneumatic system. The control action is executed just by a servo valve, which modifies the system damping to optimize the compensator performance. This servo valve is the only moving part of the control system and therefore, this system is more robust regarding mechanical/ electrical failures and it has low consumption energy. The results show a satisfactory frequency response of semi-active control when the drill string mass is changed.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A044. doi:10.1115/OMAE2014-23565.

For oil/gas production/processing platforms, multiple liquid layers can exist and their respective sloshing motions can also affect platform performance. To numerically simulate those problems, a new multi-liquid MPS (Moving Particle Simulation) method is developed. In particular, to better simulate the relevant physics, robust self-buoyancy model, interface searching model, and surface-tension model are developed. The developed multi-liquid MPS method is validated by comparisons against Molin et al’s (2012) three-liquid-sloshing experiment and the corresponding linear potential theory. The verified multi-liquid MPS program is subsequently coupled with a vessel-motion program in time domain to investigate their dynamic-coupling effects. In case of multiple liquid layers, there exist more than one sloshing natural frequencies, so the relevant physics can be much more complicated compared with the single-liquid-tank case. The numerical simulations also show that liquid cargo can function as a beneficial anti-rolling device.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A045. doi:10.1115/OMAE2014-23589.

In the last few decades, the hydrodynamic performance of offshore structures has been widely studied to ensure their safety as well as to achieve an economical design. However, an increasing number of reported accidents due to rough ocean waves call for in-depth investigations on the loads and motions of offshore structures, particularly the effect of freak waves.

The aim of this paper is to determine the sea conditions that may cause the maximum motion responses of offshore structures, which have a significant effect on the loads of mooring systems because of their tight relationship. As a preliminary step, the response amplitude operators (RAOs) of a semisubmersible platform of 500 meters operating depth are obtained with the frequency-domain analysis method. Subsequently, a series of predetermined extreme wave sequences with different wave group characteristics, such as the maximum crest amplitude and the time lag between successive high waves, are adopted to calculate the hydrodynamic performance of the semisubmersible with mooring systems in time-domain. The paper shows that the maximum motion responses not only depend on the largest wave crest amplitude but also the time lags between successive giant waves. This paper will provide an important reference for future designs which could consider the most dangerous wave environment.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A046. doi:10.1115/OMAE2014-23601.

In order to extend the operational window of marine vessels, the high-level control of the dynamic positioning (DP) system is revised. Major contributions of this paper include the modeling of a hybrid controller for a DP vessel in a varying sea state using the hybrid dynamical systems framework proposed by [1], and establishing global asymptotic stability of the closed-loop hybrid system. Simulations in a sea state varying from calm to extreme are conducted with the hybrid controller, and a single controller with adaptive wave filtering for comparison. The single controller becomes unstable in extreme seas whereas the hybrid controller shows good performance. Switching is based on spectral analysis of the vessel wave frequency motions.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A047. doi:10.1115/OMAE2014-23623.

This paper presents the development and validation of a customizable Dynamic Positioning (DP) System for a real time full bridge simulator. The Maritime Waterway Simulator (SMH) was developed based on the code of the Numerical Offshore Tank (TPN) simulator. It is able to perform study of maneuvers feasibility and crew training. Many simulated operations such as oil offloading, pipe-laying, support to platforms or station keeping, require a DP system. In order to meet the demand for such a system, a complete system was developed with an important requirement in mind: The DP system of a full bridge simulator must be easily customizable for different types of vessels. In order to validate the developed DP system, a commercial DP system is used for comparison. DP operations with the vessel is conducted with both systems: the in house developed DP and the commercial one. The results obtained with the developed DP system are compared with the one obtained with the commercial system. This comparison demonstrates that the in-house DP system can indeed be used for simulating different types of DP vessels.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A048. doi:10.1115/OMAE2014-23629.

The problem of wave-in-deck loading on offshore structures involves complex physical mechanisms which require close study. In this paper, the wave-in-deck forces generated on the bottom plate of a rigidly mounted, box-shaped structure subjected to unidirectional regular waves are quantified by means of two approaches. The first is an analytical momentum approach recommended by classification societies and the second is a computational fluid dynamics (CFD) approach based on the volume of fluid (VOF) method implemented in the commercial code FLUENT. The change in force due to very small variations in wave steepness and air gap is investigated and discussed.

Several numerical trials are conducted to optimise the computational domain and model discretisation suggestions are made. The numerical results are compared with physical model tests recently carried out at the Australian Maritime College (AMC). The results of the successive wave impacts are analysed using a discrete wavelet tool to ensure that the temporal information of slamming events is not lost in signal analysis and filtering. By comparing the theoretical and experimental results it was found that in many cases the momentum method underestimates the magnitude of the horizontal and upward directed wave-in-deck forces. Although the three-dimensional CFD cases tested are noticeably time-consuming, these simulations were found to be in good agreement with the experimental measurements.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A049. doi:10.1115/OMAE2014-23653.

Validating modern oceanographic theories using models produced through stereo computer vision principles has recently emerged. Space-time (4-D) models of the ocean surface may be generated by stacking a series of 3-D reconstructions independently generated for each time instant or, in a more robust manner, by simultaneously processing several snapshots coherently in a true “4-D reconstruction.” However, the accuracy of these computer-vision-generated models is subject to the estimations of camera parameters, which may be corrupted under the influence of natural factors such as wind and vibrations. Therefore, removing the unpredictable errors of the camera parameters is necessary for an accurate reconstruction. In this paper, we propose a novel algorithm that can jointly perform a 4-D reconstruction as well as correct the camera parameter errors introduced by external factors. The technique is founded upon variational optimization methods to benefit from their numerous advantages: continuity of the estimated surface in space and time, robustness, and accuracy. The performance of the proposed algorithm is tested using synthetic data produced through computer graphics techniques, based on which the errors of the camera parameters arising from natural factors can be simulated.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A050. doi:10.1115/OMAE2014-23686.

The improvement of the seakeeping capabilities of Floating, Production, Storage and Offloading (FPSO) vessels increases safety and allows its operation on severe weather conditions. It also increases the fatigue life of the risers. Hence, any improvement on the FPSO motion is mostly welcome. Guimarães [1], following similar efforts by Silva [2], studied the reduction of pitch motions of FPSO vessels with the use of the OWCs (Oscillating Water Columns) passive system. However, both experimental and numerical results were inconclusive due to green water effects during experiments and panel issues with the panel code WAMIT [3], respectively. The objective of the present work is to report a series of new tests that prove the feasibility of an “L-shaped” moon pool concept and estimates and tests the ideal length of such concept that maximizes the restoring moment and minimizes pitch the most. The tests were conducted in the Laboratório de Ondas e Correntes (Laboratory of Waves and Currents) of the Federal University of Rio de Janeiro (COPPE/UFRJ).

Commentary by Dr. Valentin Fuster
2014;():V01AT01A051. doi:10.1115/OMAE2014-23687.

In design of offshore wind turbines, extreme wave conditions are of interest. Usually, the design wave condition is taken as the sea state corresponding to an annual exceedance probability of 2·10−2, i.e. a return period of 50 years. A possible location for a future wind farm, consisting of bottom fixed wind turbines, is the Doggerbank area, see Figure 1. The water depth in this area varies from about 60m in the north to about 20m in the south.

The hindcast database NORA10 provides sea state characteristics from 1957 to present over a domain covering Doggerbank. Regarding the deeper areas just north of Doggerbank, this hindcast model is found to be of good quality. Larger uncertainties are associated with the hindcast results as we approach shallower water further south. The purpose of the present study is to compare sea state evolution over Doggerbank as reflected by NORA10 with the results of commonly used shallow water hindcast model SWAN.

The adequacy of the default parameters of SWAN for reflecting changes in wave conditions over a sloping bottom is investigated by comparison with model test results.

Extreme wave conditions for two locations 102.5km apart in a north–south direction are established using NORA10. This is done using both an all sea states approach and a peak over threshold approach. Assuming the extremes for the northern position to represent good estimates, the wave evolution southwards is analyzed using SWAN. The extreme condition obtained from NORA10 in the northern position is used as input to SWAN and the results from the two hindcast models are compared in the southern position. SWAN seems to suggest a somewhat faster decay over Doggerbank compared to NORA10.

Topics: Waves
Commentary by Dr. Valentin Fuster
2014;():V01AT01A052. doi:10.1115/OMAE2014-23702.

VIM (Vortex Induced Motion) is one of the important issues in the safety evaluation for cylindrical floating offshore structures. The VIM is basically placed as a phenomenon to occur in strong current, but that also appears in current and waves in the sea where offshore structures are installed.

The authors have recognized the phenomenon that the motion amplitude of a cylindrical floater in current and irregular waves together is larger than the sum of the motion amplitude in current and in irregular waves respectively in a VIM experiment. This VIM amplification phenomenon in the current and waves is remarkable when wave height is relatively low that has high occurrence frequency in the sea. It is, therefore, expected that the amplification phenomenon has large influence on the accumulative fatigue damages of the offshore structure’s mooring system.

In order to make clear this VIM amplification phenomenon, the authors have conducted detailed VIM experiment in waves using a circular cross sectional mono-column floater model. The results of the VIM experiment in current and waves are described in detail in this paper. The results of the experiment in irregular waves show different characteristics for VIM amplitude in current. The results in regular waves show the effect of wave height and wave period on VIM amplitude in waves. Using those results, the mechanisms of the VIM amplification in waves are investigated.

Topics: Waves
Commentary by Dr. Valentin Fuster
2014;():V01AT01A053. doi:10.1115/OMAE2014-23730.

The aim of this study was to develop a new method for determining nominal values for sloshing loads in the design of storage tanks in LNG FPSO (liquid natural gas, floating production, storage and offloading units). Details of the procedure are presented in a flow chart showing the key sub-tasks. The applicability of the method is demonstrated using an example of a hypothetical LNG FPSO operating in a natural gas site off a hypothetical oceanic region. It is concluded that the developed method is useful for determining the design sloshing loads of storage tanks in ship-shaped offshore LNG installations in combination with virtual metocean data and operational conditions.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A054. doi:10.1115/OMAE2014-23734.

The proposed paper is going to address the development of single point mooring FPSO (Floating Production, Storage and Offloading) monitoring and forecast system design. With 17 FPSOs deployed in both Bohai Bay and South China Sea, CNOOC owns one of the largest FPSO fleet in the world. Most of those FPSOs have been or will be moored to the seabed for decades. The extreme response during storm conditions could cause serious environmental problem, asset loss, personnel safety etc. In order to timely understand the tanker operation conditions and avoid potential risk of system failure when experiencing hurricanes, a monitoring and forecast system is developed for FPSO to monitor the environment conditions, tanker motions, green water, mooring tensions, FPSO heading and to predict the extreme mooring tensions and global motions before typhoon coming. The forecast system could further suggest the optimum loading condition for minimizing the extreme mooring tension and tanker motions to enhance the safe operation. In this paper, we take the Internal Turret Mooring FPSO 111 and the Submerged Soft Yoke Mooring FPSO 112 as the examples to introduce the design technology of the system. Through the integrated onboard interface information, the personnel could proactively take actions to mitigate the tensions on mooring lines and vessel motions. Furthermore, the measured mooring line tension, motion and environment history could assist the numerical studies of global performance. The details of designing or selecting the measuring and monitoring equipment, theory background of forecast system and the integrated onboard interface will be described.

Topics: Design , FPSO , Mooring
Commentary by Dr. Valentin Fuster
2014;():V01AT01A055. doi:10.1115/OMAE2014-23739.

This paper mainly introduced the research and design procedure, method and conclusion for internal turret mooring system of FPSO in South China Sea. According to the environmental condition characteristic and operating experience of FPSO in South China Sea and other reasons, the permanent internal turret mooring system with disconnected function is recommended as the final scenario after comparison with the other internal turret systems including disconnected system in typhoon condition and permanent system without disconnected function. For the gradually bad environmental conditions such as wind, wave and current in South China Sea, 500-year return typhoon condition is required as the extreme design condition of mooring system. According to the environmental condition distribution and classification requirement, the design standards, rules and corresponding design parameters for 500-year return condition are determined as the basis of mooring system design. The mooring system is very difficult to design because of very shallow water for catenary system and 500-year return typhoon environmental condition in South China Sea. For the problems appeared because of 500-year return extreme design condition and other parameters, a reliable solution was proposed from the aspect of wave characteristics research and got approval from classification society. The new design criteria for mooring system of FPSO in South China Sea is updated and the mooring system design is completed based on the updated design criteria. The research and design procedure and method for internal turret mooring system of FPSO in South China Sea are summarized too. This paper can be helpful to the similar projects in South China Sea.

Topics: Design , China , FPSO , Mooring , Seas
Commentary by Dr. Valentin Fuster
2014;():V01AT01A056. doi:10.1115/OMAE2014-23769.

Depletion of shallow-water hydrocarbons is increasingly forcing the oil and gas industry to explore in deeper water. Dynamically installed anchors (i.e. torpedo anchors and deep penetrating anchor) are increasingly used as a cost-effective solution for floating offshore structures in deep water environments because their installation cost is largely independent of water depth. In addition, dynamically installed anchors can be deployed accurately, and their performance is less dependent on accurate assessment of the soil shear strength since lower seabed strengths permit greater penetration depths. Despite of the economic advantages afforded by dynamically installed anchors, there remain significant uncertainties in the prediction of the embedment depth and verticality, which is likely to affect their long-term holding capacity. Currently, the holding capacity of the dynamically installed anchors is assessed using conventional pile capacity techniques, which neglect discrepancies in the rate of installation and failure mechanism between them.

This paper presents a series of model tests simulating dynamic installation and monotonic pull-out of dynamically installed anchors in normally consolidated clay. The model tests are carried out in a beam centrifuge at 100g, with varying penetration angles, extraction angles and model masses. A special designed apparatus allows model anchors to be penetrated and extracted with different penetration angles. The test results show that for models without fins, no matter by which angle the model penetrated the soil, the smallest value of holding capacity is obtained when the pullout and penetration directions are the same. Results also indicate that the penetration depth linearly increases with the anchor mass. This study also reported the results from finite element (FE) analyses. The Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit is carried out to simulate dynamic anchor installation.

The findings of this study points to a method of assessing the minimum holding capacity of the anchor and its depth of penetration. Further study is now on-going to study the behavior of finned anchors.

Commentary by Dr. Valentin Fuster

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