ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V01BT00A001. doi:10.1115/OMAE2014-NS1B.

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to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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;():V01BT01A001. doi:10.1115/OMAE2014-23786.

Tie-in between a pipeline and a riser is an important operation in the offshore oil and gas development. Normally, divers need to carry out the tie-in operation on the seabed by welding or using flanges in order to connect the pre-laid pipeline end to the jacket riser. However, working on the seabed can have leakage or cost problems. As one of solutions, the surface-tie-in method so called stalk-on method is used in the shallow water. In this method, the riser is welded to the pipeline which is lifted over the sea surface, and the welded pipeline-riser is lowered to the target position.

The main purpose of this paper is to demonstrate the safety evaluation procedure of the stalk-on operation and to share the experience learnt from actual field application. For the sake of the safety evaluation, the stalk-on operation is divided into 3 phases which are the phases of lifting a pipeline by davit cranes, holding a lifted-pipeline and lowering the weld-connected pipeline and the riser. Each phase is numerically simulated in time-domain analysis. The several lesson learnt from actual application are summarized including the consideration of discrepancy between time domain simulation result and the measured one during the installation.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A002. doi:10.1115/OMAE2014-23805.

The authors had proposed a method, rudder effectiveness correction, to realize full-scale equivalent manoeuvring motion using scale models with the auxiliary thruster. In the method they introduced the rudder effectiveness correction factor to make the effective inflow velocity to rudder defined in a steady straight course similar to a corresponding full-scale ship. The method assumes the factor constant during manoeuvring motion for a model ship running at a constant propeller rate of revolution. In this report the authors propose by a numerical approach a detailed method ensuring more precisely the similarity of manoeuvring motion than the former one. The detailed method takes into account the similarities both of the speed response and rudder effectiveness. The time-varying control both of the propeller rate of revolution and rudder effectiveness correction factor, based on the new method, enables scale model ships to realize the similar manoeuvring motion to full-scale ships more accurately. Simulation calculations using the modular mathematical model of manoeuvring motion clarifies how well the detailed method raise the precision of the full-scale equivalent manoeuvring motion in free-running model ship testing comparing with the former method.

Topics: Ships
Commentary by Dr. Valentin Fuster
2014;():V01BT01A003. doi:10.1115/OMAE2014-23808.

The past decade has shown a fast growing subsea production market tending to complete factories on the sea floor involving ever more complicated infrastructure and increasing size and weight components to be installed. The water depth of this market typically ranges between 1000 and 3000m. This new subsea market triggers a need for increased deep water lowering capacity to enable safe, fast and efficient installation of this infrastructure.

The authors’ company has gained extensive experience in the field of deep water installation over the past 15 years, ranging from complex mooring foundations to the installation of large subsea production units in up to 3000m water depth. This experience is used and further deployed in the specification of new and higher capacity deep water lowering systems for recent and upcoming investments.

The paper will share a number of the contractor’s considerations with respect to a state-of-the-art deep water lowering system meeting the needs of today’s subsea production market. The paper will address the challenges associated with the various stages of deep water installation, choices made in type of ropes (steel versus fiber), pros and cons of various floater types that can be used as an installation platform and the need for and effect of passive or active heave compensation.

Topics: Water
Commentary by Dr. Valentin Fuster
2014;():V01BT01A004. doi:10.1115/OMAE2014-23816.

Installation of subsea structures and equipment involves a lifting operation where the objects are exposed to large hydrodynamic forces when entering the oscillating sea-surface. During deployment, as the structure is lifted through the splash zone, snap forces due to slack or overload due to dynamic forces contribute to the maximum load experienced by the structure over the course of its design lifetime. This paper presents a method of how to verify the structural integrity of a subsea framework including determination of the maximum allowable sea-state in which the structure is safely installed. As well as describing an overall methodology for a subsea lifting analysis, hydrodynamic coefficients for cylinders in the splash zone is provided.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A005. doi:10.1115/OMAE2014-23833.

Among offshore floaters used to develop offshore resources, FPSO and FSO have a storage function whereas semi-submersible, Spar and TLP have only production function. The floaters with the storage function such as FPSO and FSO are designed as the typical ship type concept compared to the other floaters with small water plane area.

In order to design the floaters for offshore resource development, it is needed to estimate the seakeeping performance under operating condition and survival conditions and then carry out the structural design based on seakeeping performance results. The environment conditions of 1yr, 10yrs, 100yrs or 1,000 yrs return periods are used based on the metocean data of the installation field to evaluate the seakeeping performance under operating and survival conditions. In general, the wave conditions with the maximum wave heights for each return periods are selected on each wave contour lines in the wave scatter diagram. Then the seakeeping performance is evaluated from the seakeeping model test.

However, it was observed that the wave with the pitch forcing period, where the wave length is close to the ship length, is more important than the wave with the maximum wave height after several accidents caused by the green water in Northern North Sea and Norwegian Sea. Therefore, it became a common practice to include not only the wave conditions with maximum wave heights for each return period but also the wave conditions with the pitch forcing period to evaluate the seakeeping performance for offshore development floaters. Ship type floaters such as FPSO are more likely to experience higher impact force due to the large frontal area accompanied by large heave and pitch motions in head sea and bow quartering seas. Recently, it was reported that in an accident in North Sea of UK sector, the damage at the bow of the FPSO is caused due to the steep waves. Afterwards, studies on the steep waves have been made in several institutes such as UK HSE.

In this study, the effect of the impact load (so called slapping load) by the steep waves acting on the FPSO bow is investigated throughout the model test. For measurement of the pressure and impact force on the frontal area, a bow-shaped panel was fabricated with the pressure and force sensors, and installed on the bow starboard side of the model FPSO. During the model test campaign, the impact load was investigated using the steep waves with Hw/λ greater than 1/16 in addition to the general wave conditions with maximum wave heights.

Consequently, it is confirmed in the model test that the impact loads acting on the FPSO bow are significantly increased with the steep waves (Hw/λ > 1/16) compared to the general wave conditions. Therefore, it is necessary to consider whether the steep waves are additionally included in the wave conditions to estimate the seakeeping performance and how to apply the impact loads acting on the FPSO bow from the steep waves in structure design.

Topics: Stress , Waves , FPSO
Commentary by Dr. Valentin Fuster
2014;():V01BT01A006. doi:10.1115/OMAE2014-23843.

The dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. In this study the impacts of top-end heave on the riser undergoing vortex-induced vibration (VIV) are explored in terms of the parametric excitation and the consequent dynamic behaviors. By using finite element simulations based on a coupled hydrodynamic force approach, the dynamic responses of the integrated system including both a floating top-end and the riser experiencing VIV are examined.

Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion, and the impact of heave on riser VIV response gets larger as the modal order number dropping. Riser VIV amplitude becomes, almost linearly, more profound when the tension ratio, as one of critical parameters that influence the riser dynamic response, gets larger. Moreover, an interesting phenomenon called mode transition is observed, particularly at lower frequency, during modal dynamics response.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A007. doi:10.1115/OMAE2014-23915.

This paper investigates — based on model tests and potential flow calculations — several phenomena associated with the green water problem in severe sea states. The topics investigated are: the wave characteristics of the severe and steep sea states, the behavior of the vessel in these waves, the relative motions at the bow and the height of water on deck.

The green water problem is of interest since many floaters used for the oil and gas production by the offshore industry are permanently moored for typically 20 years or more, and they will experience severe storm conditions with large wave heights.

The investigations concern an unusual vessel type: SBM’s innovative midscale Twin-Hull FLNG concept [1]. The floater concept is to join two (standard) LNG carriers together to obtain sufficient storage volume and process deck space. The concept is under development and one aspect, from operability point of view, is the probability for green water. For this purpose a first model test campaign has been carried out.

The Twin-Hull FLNG concept has been tested in extreme sea states which are typical for offshore Brazil and East-Africa.

Linear potential flow calculations were applied to predict the freeboard exceedance based on relative motion amplitude operators. The correlation to the measured data is discussed.

Although the research is dedicated to the Twin-Hull FLNG vessel, the observed phenomena are considered applicable to any stationary vessel in general.

Topics: Water , Hull
Commentary by Dr. Valentin Fuster
2014;():V01BT01A008. doi:10.1115/OMAE2014-23939.

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.

Topics: Simulation , Mooring
Commentary by Dr. Valentin Fuster
2014;():V01BT01A009. doi:10.1115/OMAE2014-23976.

Since first installed at a water depth of 4,660 feet in 1997, polyester mooring systems have now been used on floating platforms in the Gulf of Mexico (GOM), Brazil and other regions. The Mad Dog Spar was the first floating production system (FPS) with permanent deepwater polyester moorings in the GOM. After the Mad Dog Spar, the deepest water depth is 7,800 feet in which a polyester mooring system was installed on the Perdido Spar. The polyester mooring systems have performed favorably, e.g., in the GOM, experiencing hurricanes without incident. The polyester rope in general is more advantageous than steel wire in deeper water due to reduced weight (and tension), high strength, durability (better fatigue and no corrosion), and improved floater global performances (less offset, etc.). Moreover, while a floating production platform is designed to support riser systems, fatigue damage of risers due to Vortex Induced Motions (VIM) of the platform are important design drivers particularly in the GOM. The polyester mooring system has a higher restoring force in horizontal (thus a higher lateral stiffness) in currents resulting in a significantly better fatigue performance (less current bins with VIM lock-in) than the steel mooring system does.

The paper herein presents a comparative study with two kinds of mooring systems (polyester ropes and steel wires) for the same platform. Differences between the polyester and steel mooring systems are evaluated for various aspects, such as the mooring system configuration and performance, installation risks, operations, and impact on hull and riser system design and performance. The results also indicate cost savings for the polyester mooring equipment and the overall production system.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A010. doi:10.1115/OMAE2014-24018.

Safety and dependability are major design objectives for offshore operations such as the construction of wind farms or oil and gas exploration. Today processes and related risks are typically described informally and process specification are neither reusable nor suitable for risk assessment. Here, we propose to use a specification language for processes. We integrate this specification language in a generic modeling approach in combination with an analysis tool and a tool to construct health, safety and environment (HSE) plans — a mandatory document for granting a construction/operation permit. Specifically, for each planned scenario a process is modeled, describing the detailed operation of the involved actors as well as the interaction with resources and environmental conditions. We enrich this process model with hazardous events which is facilitated by integration with an offshore operation generic hazard list, thereby giving access to expert knowledge for the specific situation to be planned. This in turn allows us to perform an automatic quantitative risk assessment using fault tree analysis. We exemplify our approach on a standard offshore operation of personnel transfer from an offshore building to another naval unit by modeling, annotating with hazards, performing the fault-tree analysis, and finally generating HSE plans.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A011. doi:10.1115/OMAE2014-24028.

This paper discusses thruster interaction effects for a DP shuttle tanker, equipped with two main propellers and rudders, as well as two bow tunnel thrusters. Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were taken of the wake flow behind the main propellers and rudders. Furthermore, PIV measurements were taken of the wake flow of one of the two bow tunnel thrusters. The flow velocities were measured in a large number of cross sections at different down-stream positions. The PIV measurements provide a detailed image of the velocities in the thruster wake, showing axial velocities, as well as transverse and vertical velocity components.

The results of the first set of measurements showed in detail the wake flow behind the main propeller of the DP shuttle tanker. The wake flow pattern was determined at rudder angles of 0 deg and 10 deg. Since the research is related to DP performance, bollard pull conditions (zero forward speed) were considered in the model tests.

The results of the second set of measurements showed in detail the wake flow of one of the bow tunnel thrusters. The wake flow pattern was investigated in zero speed conditions, as well as for the vessel at forward speed. The observed flow patterns helped to explain the reduced bow tunnel performance at forward speed.

The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are currently being performed.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A012. doi:10.1115/OMAE2014-24044.

For semi-submersible units, the magnitude of air gap or local wave impact in the survival condition is a key design driver. Linear analyses are widely used in the industry to predict survival air gap for semi-subs. Large relative motions, leading to large changes in shape of the submerged hull and large changes in water plane area make this approach questionable.

In this paper, the GG5000 [1], a twin pontoon four legged semi-sub is considered. Both linear analyses and model tests had been performed, but the results were diverging. It was decided to investigate further, using non-linear hydrodynamic analyses.

Initially, the model test setup is reproduced in the numerical model. The simulation model is verified for both response power spectra and extreme response distributions. In the non-linear simulations, the wetted surface of the hull is updated for each time step. Both excitation and restoring forces are based on the instantaneous wetted surface. This proves essential for the prediction of large motions. Later, the verified simulation model is run with realistic full scale setup including elastic catenary moorings with coupled cable dynamics, thruster assist, irregular waves and irregular wind.

Highly non-linear effects proven to be vital to accurate air gap prediction are investigated and their representation in the non-linear analyses is validated against model tests.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A013. doi:10.1115/OMAE2014-24115.

This paper presents the results of a numerical and experimental study concerning the hydrodynamic performance of the SBM midscale Twin-Hull FLNG concept. SBM’s innovative design consists of two LNG carriers joined together, and converted into a single FLNG facility. This option provides both sufficient deck space and storage capacity while having the benefit of a conversion project, being reduced delivery schedule and lower costs. This results in an unusually wide vessel for which only very limited number of precedents exist.

Important hydrodynamic performance indicators such as first order motions, green water and bottom slamming are investigated by means of sea keeping tests. In these tests the scale model of the FLNG is subjected to moderate and harsh environmental conditions from various headings. The results are used to verify the numerical predictions of the global motions, wave drift forces and internal loads and to assess green water and bottom slamming loads.

Current coefficients were calculated by computational fluid dynamics (CFD) and validated with wind tunnel experiments. All load components (wind, wave, current) are then used to evaluate the ability of the vessel to weathervane. This paper presents the investigation of the quality of numeric predictions by comparing model test data against numerical data and hence it summarizes the hydrodynamic performance of the Twin-Hull.

Topics: Hull
Commentary by Dr. Valentin Fuster
2014;():V01BT01A014. doi:10.1115/OMAE2014-24117.

An innovative and complicated strand jack lifting technology was applied to floatover installation of DPP integrated topsides in South China Sea. Due to initial failure to lower the four outer support legs, an unfortunate chain of events occurred thereafter, thus yielding a complete failure of the floatover installation never happened in history. A rescue operation was carried out in the two following days to successfully salvage both the topsides and the floatover barge stranded inside the jacket slot in extraneous circumstances. This paper presents the floatover systems and the failure events of the innovative low-deck floatover technology, as well as the successful rescue operations by floating on and floating off the topsides onto the eight-legged jacket. Big lessons learned are also addressed here.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A015. doi:10.1115/OMAE2014-24160.

Tank containers are widely used to transport a variety of liquid goods such as food products, oil, and different kinds of fuel including liquefied natural gas. Due to the unpredictable dynamic behavior of partially filled tank containers, regulations limit the containers to be either almost full (> 80%) or almost empty (< 20%), when handled by cranes. In order to provide arguments to ease these restrictions, the system is analyzed and control methods for assisting the crane operator are proposed. We deduce a very accurate and computationally favorable mathematical description of the coupled crane and fluid dynamics. The fluid is modeled by a potential flow approach resulting in a low dimensional approximation of the liquid dynamics. Coupling the fluid dynamic model with the load system model of a container crane leads to a nonlinear formulation of the overall system. The state estimation algorithm exclusively relies on the measured rope forces as well as the known motion parameters of the trolley and the rope winches. A nonlinear state feedback controller based on sliding modes for underactuated systems provides a stabilizing control signal for the system. Experimental results for validation of the model, the observer, and the control design are included.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A016. doi:10.1115/OMAE2014-24180.

Mooring of offshore structures in very deep water has been made possible through the use of lines made of fibres of synthetic material. The mechanical behaviour of synthetic ropes is considerably more complex than that of steel wire rope and chain, due to the visco-elastic and visco-plastic properties of the synthetic material. In particular, the gradually developing permanent increase in rope length will affect the offset and motion of the moored structure and make its characteristics change from one storm to the next.

For design and analysis of offshore mooring systems incorporating synthetic ropes it is valuable to have good models that can be used for response simulation in the time domain. The paper describes the development of a time domain model for synthetic rope. The model structure and the values of the parameters are determined from experimental data using a system identification technique. The resulting model is implemented in an existing computer program for analysis of mooring and riser systems. In particular, the permanent elongation of the synthetic rope appears to be well represented.

Topics: Mooring , Ropes
Commentary by Dr. Valentin Fuster
2014;():V01BT01A017. doi:10.1115/OMAE2014-24185.

The offshore industry has spent the last several years developing semisubmersible platforms capable of supporting both drilling and production activities. The production trees are located on top of the top tensioned risers (TTRs) on a Dry Tree Semisubmersible. A key challenge in the design of these vessels is to reduce the heave motion as much as possible to enable the use of state-of-the-art riser tensioners. A model test campaign was executed as part of the developmental program. The primary objective of this campaign was to improve the accuracy of the numerical tools to be used in the design process.

Riser tensioners are typically hydro-pneumatic devices, with a nonlinear tension-stroke relationship. A riser tensioner was developed at model scale which had a similar nonlinear behavior to the prototype. Examining the effect of this tensioner on the global motions was an additional objective of this test campaign. The techniques used to model this nonlinear spring is described, and its effect on global motions investigated.

A key challenge in model testing platforms intended for ultra-deep water (e.g. greater than 7000 ft) is the modeling of the mooring and riser system. The premise for the design of the model mooring system is 1.) maintain as best as possible the force-offset relationship of the mooring lines and 2.) be able to describe the model test configuration in the numerical tools to be used for global design. The near taut behavior of the prototype mooring system is modeled using heavy chain and a high-catenary mooring line.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A018. doi:10.1115/OMAE2014-24218.

Towing as a specific type of sea transport is often used for installing objects for drilling and exploitation of underwater gas and oil wells. Also, towing proved to be a cost-effective solution for the installation of the offshore wind turbine electric generators at sea locations. Because of the mass of these objects the need for towing increases progressively. Time domain numerical model for the wave-induced motions of a towed ship and the towline tension in regular head seas is presented in this paper. For the sake of simplicity, one end of the towing line is attached to ship’s bow and another end has prescribed straight line motion. All considerations are done in the vertical plane so the ship is modeled as a rigid body with three degrees of freedom. Hydrodynamic loadings due to waves are taken into account along with added mass and damping. Dynamics of the towing line is described by finite elements. Due to the nonlinear properties of the problem calculations are done in time domain. Comparison of the obtained numerical results is made with previously published results.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A019. doi:10.1115/OMAE2014-24262.

The paper describes the development of a training simulator for boats used on Oil Spill response at sea. The simulator models the dynamics of tug boats pulling an oil boom under waves, wind and current. The boom is modeled as a flexible line connected by lumped masses and its flotation characteristics and loads are calculated by Morison equation. A simplified model is used to simulate the oil itself and its interface with the boom line. The verification process was applied and its outcomes are discussed. Also, some case study scenarios are presented and the results are used to evaluate the applicability of the simulator as a training system.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A020. doi:10.1115/OMAE2014-24273.

In this paper, we addressed the qualitative consequences on uncertainty for the execution of a inclining test of a Semi-submergible platform with mooring system and risers at production site and compared the results to the ones taken from typical inclining test procedures at sheltered waters, as defined by ASTM F1321. To accomplish that, we applied Uncertainty Analysis according to ISO procedures, by evaluating propagation of uncertainties from the measurements until the final calculations. We discussed the use of measurement devices for the variables of concern and performed some numerical simulations to address the mooring system restoring effects and its implications on uncertainty analysis. We used data from two semi-subs with different displacements and found small influence on final uncertainty from the addition of mooring system and risers. At end, we discussed how new technologies on data acquisition and filtering of signals can become an important tool to help in the safety of the offshore floating production units, by facilitating the verification of the updated center of gravity of them after important interventions.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A021. doi:10.1115/OMAE2014-24274.

A new conceptual design of a deepwater MONOBUOY, named DeepWater MultiColumn Buoy (DWMCB), patent PCT/BR2011/000133, was developed by PETROBRAS/CENPES. The DWMCB was designed to be part of an offloading system for a Spread Moored Floating Production Offloading Unit (FPSO). The offloading system principle consists of Oil being exported from the FPSO to a Shutle tanker passing through Offloading Oil Lines (OOLs) that are supported by the DWMCB. The system is designed to operate at a water depth of 2,200 meters, with expected in site life duration of 25 years. The geometry of DWMCB was defined after an optimization process in order to minimize its motions. This paper describes the development of this concept and discusses the results from some design verifications done with the help of a model tests campaign. An equivalent traditional shaped monobuoy was also tested for comparison purposes.

Topics: Water , Buoys
Commentary by Dr. Valentin Fuster
2014;():V01BT01A022. doi:10.1115/OMAE2014-24284.

This paper presents an investigation of the still water sink trajectories of an accidentally dropped Christmas Tree (XT). A 3D coupled nonlinear dynamics/hydrodynamics model of the XT is constructed and simulations executed from surface to seabed. XT initial water entry orientations are randomly varied in a Monte-Carlo simulation to predict the seabed impact zone. Still water simulation results are compared to simplified cone-angle type method predictions. The effect of XT flat panels on excursion mitigation is investigated. Finally, the influence of non-uniform current effects is studied.

Topics: Simulation , Water
Commentary by Dr. Valentin Fuster
2014;():V01BT01A023. doi:10.1115/OMAE2014-24286.

Ocean conditions have changed over the years, with severe and extreme wave generation occurring more frequently, resulting is larger loads and motions for offshore fixed and floating structures to contend with. It is not known exactly why ocean conditions are growing more severe, but we know that any structure being considered for offshore operations has to be designed to handle larger loads then in the past. Wave slamming, either directly or through run up is an area of growing concern with the offshore industry. During these slamming events the local forces can exceed the strength of the structural plating or the internal frames resulting in structural damage. It is often difficult to predict where these slamming events will occur, therefore large areas are required to be instrumented in order to capture these slamming events during model experimentation. Commercial pressure transducers are typically 5 mm in diameter, which corresponds to full-scale areas of approximately 0.05 m2 or less depended on the scale factor. Thus to cover a large area effectively, a large number of transducers are required. Alternatively a smaller number of larger devices such as force panels with areas of 8 m2 can be used. This area corresponds to the typical area that internal framing would experience slamming loading.

This paper describes the design and development iterations of Oceanic’s model-scale force panels and their applications on modeled offshore structures such as FPSO’s, FLNG’s, and GBS’s. A major advancement in construction of these force panels has been achieved through the use of in-house 3D printing technology to produce complex components.

The paper also presents results from controlled slamming experiments to validate the force panels with respect to piezoelectric pressure sensors and a theoretical slamming model based upon potential flow. Finally, the paper describes the transition from above water installation of the panels to submersion installation and the implementation of the inverse Duhamel analysis to remove the interference due to submerged panel dynamics from the measured response.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A024. doi:10.1115/OMAE2014-24300.

An overview of fiber optic sensors for temperature, pressure, strain, and fatigue of subsea structures is provided. Current progress details efforts to ensure proper installation and bonding to existing risers, flow-lines, mooring lines, trees, and other structures in actual subsea environments. Developments include clamp prototypes, bonding techniques, long-term fatigue analysis, sensor calibration, and temperature compensation.

Fiber optic technology in subsea monitoring began over 20 years ago by migrating expertise from decommissioning of rocket motors. The first installations were on new installations of subsea pipelines, production risers, and drilling risers to measure strain and vibration for fatigue life monitoring. Of particular interest for these systems were detecting riser vortex induced vibration and strain throughout the touchdown zone. A prior limitation was that sensor installation was only performed top-side on new subsea equipment. This recent work demonstrates the capability to deploy on existing subsea equipment.

The novel contributions of this study are the developments that optimize the clamp design, bonding techniques, and factors that allow long-term service life. Button pull tests validate long term service life after the clamps are subjected to accelerated aging tests. Details on the subsea calibration also provide insight on the recent progress with post-installed sensors.

The purpose of reliable post-installed advanced sensors is not only to detect failures of subsea infrastructure but also to warn of signs of fatigue or hydrate formation that contribute to catastrophic failures. The calibration and testing mentioned in this paper are part of the Clear Gulf study, a collaboration formed in 2010 between the offshore energy industry and NASA. The study continues to make advances in highly sensitive monitoring systems that anticipate failures, catastrophic events, and flow assurance issues.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A025. doi:10.1115/OMAE2014-24303.

This paper analyzes the leg-seabed interaction due to motions of a 3rd generation jack-up vessel in irregular waves. The proposed model considers hydrodynamic forces on the hull as well as on the legs. The sea floor reaction forces are analyzed with a Coupled Eulerian-Lagrangian (CEL) method. The results are used to develop a mechanical rheological model for the sea floor. With computational efficiency in mind a state-space representation in time domain is derived. The excitation of the system is caused by irregular waves, which are described by the JONSWAP spectrum. The proposed method allows the efficient computation of short term (2–15 min) scenarios and events through sampling of realizations. It is used to compute the impact forces on the legs from the seabed induced by irregular vessel motions. The presented framework is suitable for the planning of jack-up operations in the offshore wind industry. It can also be applied for the design of dynamic positioning and jacking systems.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A026. doi:10.1115/OMAE2014-24305.

Blowout Preventers (BOP) are safety devices used to prevent uncontrolled flow of liquids and gases during a blowout. Blind Shear Rams (BSR) is one of the critical components of a BOP responsible for shearing the drill pipe and sealing the wellbore during a blowout scenario. Tests were conducted by shearing a drill pipe under non-flowing conditions to obtain the maximum shearing force, shape of sheared drill pipes, shearing time. A FE methodology has been developed to model the shearing process using Abaqus Explicit finite element solver. Simulations were performed to replicate the shop test and the results are compared with the shop test and found to be in good agreement. The current study uses a validated model for evaluating some of the challenges being faced by the BOP shear ram technology. These include drill pipe centralization in the well bore, shearing of a drill pipe subjected to axial tension, compression and buckling, and shearing in flowing well conditions. All these studies are performed and their effect on shearing process is discussed.

The effect of high velocity formation fluid through the drill pipe and annulus in the localized shearing region is also assessed separately by performing Computational Fluid Dynamics (CFD) simulations and it is found that the resistance offered by flowing fluid is not significant compared to the high pressure from accumulators required to shear the pipe. A shear ram design accommodating the results of the study is verified for increased efficiency of the shearing process. The study is conducted as part of a Technology Assessment Programs (TAP) for the Bureau of Safety and Environmental Enforcement (BSEE) in the areas of BOP stack sequencing, monitoring and kick detection.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A027. doi:10.1115/OMAE2014-24319.

Hull sizing system for tension leg platform (TLP) has been developed by using optimization algorithm. Hull sizing work is vital at the initial design stage of the offshore platforms. It is the driver towards a successful execution of a good design. In order to achieve the objective of producing an efficient and cost effective design, optimization of the hull configurations is very important. An optimized hull needs input from various criteria such as global performance, stability, strength, constructability, installation, etc. During the process of optimizing the design, it is inevitable that the hull configurations go through a number of iterations. Thus, the hull sizing program is developed by using optimization algorithm in order to solve multi-variable problem in the effective way in producing an efficient design in terms of performance, constructability and cost.

The principal dimension of the hull (column and pontoon size) and tendon size are the variables to be optimized. The objective function to be minimized is the sum of the of hull and tendon material weight. At each iteration of the optimization process, the program evaluates the global performance criteria, stability criteria, and simplified global strength criteria. The frequency-domain analysis method is used to perform the global performance analysis. The hydrodynamic coefficients are calculated by applying the boundary element method. Global strength analysis is carried out by using a beam model. Metocean data, water depth, riser information and topside information, such as weight, center of gravity and wind force coefficients, are input constant. A simulated annealing method is used for the optimization algorithm of the hull dimension and tendon size.

The program is being tested for a conventional TLP. The optimized hull shape provided in this program is in line with the existing TLP configurations. Henceforth, the program will be further studied and will be verified to improve its practical utility.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A028. doi:10.1115/OMAE2014-24336.

Although preventive maintenance is not a new concept when dealing with mooring systems, experience shows that the implementation is not always successful. As important as new technologies are in improving mooring integrity, it is equally important to ensure that well-established best practices remain in place. As the industry approaches a period where extending service life of floating structures is desirable, but the existing mooring system has reached the twilight of its service life, the need for properly maintaining the mooring system is more acute. With time on their side, the prudent operator can implement small regular steps to extend mooring system service life. This paper presents methodologies to maximize design life by highlighting design and operational best practices, maintenance and monitoring, and identifying common reasons why these best practices are not implemented.

Topics: Failure , Mooring
Commentary by Dr. Valentin Fuster
2014;():V01BT01A029. doi:10.1115/OMAE2014-24338.

This paper presents a methodology in order to perform a drift off calculation for drillships according to given parameters such as: environmental conditions and water depth. Drift off occurs when there is insufficient thruster force so that the vessel is drifted away from the target position by the environmental forces. For a safe operational drillship it is expected that the drifting off will be resumed in due time when blackout recovery system starts running and, therefore, enough thrust takes place. Water depth plays an important role when considering the default maximum release of Lower Flex Joint (LFJ) angle for physically disconnecting, which is 10 degrees in the majority of suppliers. This methodology is intended to be applied to drillship design, by comparing the time to stop drifting and the distance from the reference point after a total blackout occurs.

Electrical generators sets installed in drillships are designed to work with extreme environmental conditions. Since there is an excess of installed power for the majority of the operational time, drillships often operate with all high voltage busbars connected to each other improving engine efficiency, decreasing levels of pollution emissions and reducing maintenance. The use of this electrical power configuration is possible because there is no need to turn on all generators at the same time, but only the ones that are needed on that particular moment. However, when a single failure such as a short circuit occurs and the system is not prepared to disable and segregate the failure, all electrical system will crash, causing a total blackout and the drillship will start to drift off.

The drifting off time was obtained by numerical simulations conducted by modeling a standard drillship using time domain software. The model took into consideration the vessel hydrodynamics under environmental conditions (wind, current and wave), the drag force in marine drilling riser, and the thrusters in Dynamic Positioning (DP) operation.

The simulation is divided into three steps: First, the behavior of the DP system in full operation is simulated until system is stable. After that, all thrusters are turned off to simulate a total blackout. Finally, since the ship will not stop immediately because of its inertia, a time range for the ship’s inertia was also considered and this time is added to the pre-established blackout duration.

The conclusion of the study shows how parameters as water depth, environmental conditions, and blackout recovery time affect the necessary time to stop drifting off, so as to foresee that after a total blackout the standard drillship will remain in safe limits.

Topics: Drillships
Commentary by Dr. Valentin Fuster
2014;():V01BT01A030. doi:10.1115/OMAE2014-24354.

Dynamic responses of a heavy cycloidal object hanged by a large floating crane in the process of immersing into waves were studied experimentally and numerically in this paper. A floating crane with displacement of 14000t was selected as the reference. A model with the scale of 1:20 was conducted in a towing tank. The movement responses and forces of the heavy cycloidal object in the process of immersing in waves were researched.

Ocean engineering construction is always affected significantly by waves, so the selection of a shield to attenuate incoming waves is an important aspect for ocean engineering construction. The sea condition in our project is 3-state sea condition; it’s not an acceptable condition for real construction considering safety of crews and the ability of equipments. So an effective floating breakwater validated experimentally in a towing tank in our project was proposed as effective shields. The steel cylinder lifted by a floating crane in the process of water-entering was experimented, the movements of the steel cylinder and the subjected forces of the hoist were recorded; the effects of different shields on the responses of steel cylinder with different depth in water were studied. Numerical simulations of the motion of steel cylinder subjected to waves in the process of immersing were conducted to illustrate the effects of headings. The basis of model design, model-making and measurement methods are also introduced in this paper. The results are translated into full scale results which will be the basis for safety construction of ocean engineering and measurement proposed for accurate positioning and installation of the steel cylinder.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A031. doi:10.1115/OMAE2014-24371.

The disconnectable buoy turret mooring (BTM) for an FPSO has been a proven technology for areas where hurricane and seasonal cyclones are predominant, or are subject to ice features which need to be avoided. With the requirements of large buoy payload from deep water moorings and heavy risers, the disconnectable buoy size can be substantial. Recent projects are considering buoy net displacements well above 5000Te. The increased buoy size imposes large inertia loads on the supporting structure and mechanical connectors when connected, and possibly more so on the structural integrity and winching system during disconnect and reconnect operations. In order to provide structure design loads as well as to estimate safe operational windows and procedures, it is desirable to understand the physical properties of 2-body motions when the buoy and ship are close enough to have significant interactions.

This paper focuses on the hydrodynamic and structural (Winch, Mooring/riser, Fender/bumper etc.) interactions with the buoy operating in close proximity to the turret. Since the readily available offshore engineering tools have limitations to deal with close body hydro-interaction, particularly with the presence of moon-pool resonances, rational approximations are derived for hydrodynamic forces during pull-in and disconnection of the buoy. To analyze fully coupled motions and loads in combined environmental conditions, a typical diffraction/convolution based numerical tool (AQWA) is extended by a custom defined external force function. This analysis allows simulating the real time motions during disconnect, reconnect, or at suspended elevations. Various findings from dedicated model tests and analyses are presented and discussed with implications for configuring and operating the disconnectable buoy system.

Topics: FPSO , Buoys
Commentary by Dr. Valentin Fuster
2014;():V01BT01A032. doi:10.1115/OMAE2014-24378.

In the case of a floating offshore plant such as FPSO (Floating, Production, Storage, and Off-loading unit), many equipment should be installed in the limited space, as compared with an onshore plant. At this time, special conditions, such as the movement due to external force by wind and wave, salt content, and so on, should be also considered because the floating offshore plant should be operated in the special environment of ocean. The requirement for an optimal layout method of the plant has been raised due to much considerations for layout design. Thus, a layout method of the floating offshore plant was proposed in this study. For this, an optimization problem for layout was mathematically formulated, and then an optimization algorithm was implemented based on the genetic algorithm or mixed integer linear programming in order to solve it. To evaluate the applicability of the proposed method, it was applied to an example of FPSO and LNG FPSO topsides. As a result, it was shown that the proposed method can be applied to layout design of the floating offshore plant.

Topics: Optimization , FPSO , Storage
Commentary by Dr. Valentin Fuster
2014;():V01BT01A033. doi:10.1115/OMAE2014-24379.

The weight information of a floating offshore plant, such as an FPSO, is one of the important data to estimate the amount of production material and to determine the production method for its construction. In addition, the weight information is a key factor which affects in the building cost and production period of the offshore plant. Although the importance of the weight has long been recognized, the weight has been roughly estimated by using the existing design and production data, and designer’s experience. To improve this task, a simplified model for the weight estimation of the offshore plant using the statistical method was developed in this study. To do this, various past records to estimate the weight of the offshore plant were collected through the literature survey, and then the correlation analysis and the multiple regression analysis were performed to develop the simplified model for the weight estimation. Finally, to evaluate the applicability of the developed model, it was applied to some examples of the weight estimation of topsides of the offshore plant. The results showed that the developed model can be applied the weight estimation process of the offshore plant at the early design stage.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A034. doi:10.1115/OMAE2014-24395.

Escape, evacuation, and rescue (EER) analysis is one of vital roles to protect and save personnel in the event of a major accident on offshore oil and gas production platforms. However, most of EER analysis method for evaluating EER strategies of offshore plants relied on deterministic or probabilistic methods including several risk assessment tools. This study focused that all partial processes of EER analysis are similar to the things of passenger ship escape and evacuation: “Means of Escape”, “Simplified Evacuation Analysis”, and “Advanced Evacuation Analysis”. Since various methodologies and some authorized rules for EER plans and resources were already studied in the domain of passenger ship, this study tried to adopt them to offshore plant fields with their minor revisions. In the evacuation process of EER analysis, especially, a human behavior model was adopted to simulate the evacuation sequence in more detail. It was supposed that the human behavior model is based on the velocity-based model, and consists of individual, crowd, and emergency behaviors. As an example, the method is applied to some offshore plant projects and compared with the result of conventional EER analysis.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A035. doi:10.1115/OMAE2014-24401.

Over the recent years, the need for active monitoring of offshore production facilities has increased. Due to significant amount of mooring line failures in the recent past, permanent mooring systems for Floating Production Units such as FPSO’s, FSO’s and future FLNG’s are typically required to have active monitoring systems to provide feedback of the mooring system’s status, performance and integrity.

This paper will address some of the offered solutions with regards to active mooring system monitoring. Some pros and cons are given for several techniques within the industry currently available, and then a simple but elegant alternative is proposed. With the use of position monitoring, for instance with GPS signals, qualitative and quantitative feedback can be provided on the status of the permanent mooring system. A detailed explanation is given backed up by a few real life examples in which such a system was used to quickly assess the status of the mooring system.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A036. doi:10.1115/OMAE2014-24414.

This paper presents the mathematical model of the real-time ship simulator for low-speed maneuvering developed by the University of São Paulo, Transpetro and Petrobras, with the technical collaboration of Brazilian Pilots Association (CONAPRA). The software is based on the TPN (Numerical Offshore Tank) numerical code which had several modifications, in order to perform real-time simulations.

After the complete description of the mathematical model, some illustrative results of simulations executed with pilots are exposed.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A037. doi:10.1115/OMAE2014-24418.

The recent discoveries and development of the Pre-salt reservoir in Brazilian coast require a new logistical model for crew transportation and transhipment to the drilling and oil rigs due to the large distance from coast, harsh environment conditions and large amount of workers to be transported against the actual model adopted considering only transportation by helicopters in order to reduce overall costs. The adoption of a logistic model with maritime transportation in these scenarios could provide several advantages, however there are several challenges from the technical point of view in transhipment between ship-shaped vessels, that could represent a great limitation in terms of operational window. Previous works showed the feasibility of monocolumn platforms with an internal moonpool as a Logistic HUB [1], allowing the boat docking in sheltered conditions. This work shows an overview of the model testing of a semi-submersible with an internal dock and the comparison of the free-surface elevation and RAOs (Response Amplitude Operators) between experimental results and potential flow computations. The tests were performed for 5 headings considering 10 regular, 5 irregular and 1 transient waves under a single draft and 5 different devices to reduce wave energy in the interior region.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A038. doi:10.1115/OMAE2014-24431.

A compliant tower (CT) is modeled as a partially dry, partially tapered, damped Timoshenko beam with the superstructure modeled as an eccentric tip mass, and a non-classical damped boundary at the base. The foundation is modeled as a combination of a linear spring and a torsional spring, along with linear and torsional dampers. The mean empty space factor due to the truss type structure of the tower is included. The effect of shear deformation and rotary inertia are included in the vibration analysis; with the non-uniform beam mode-shapes being a weighted sum of the uniform beam mode-shapes. The weights are evaluated by the Rayleigh-Ritz method, using the first ten modes and verified using Finite Element Method (FEM). The superstructure adds to the kinetic energy without affecting the stiffness of the beam, thereby reducing the natural frequencies. The weight of the superstructure acts as an axial compressive load on the beam, reducing its frequencies further. Kelvin-Voigt model of structural damping is included. A part of the structure being underwater, the virtual added inertia is included to calculate the wet natural frequencies. The CT is first subjected to steady current loads of a given velocity profile. The static deflection and overturning moment is estimated for current loads. The CT is then studied for wave excitation at various seas states. Morrison’s equation and Pierson-Moskowitz Spectrum are used to derive the forces for different sea states. The forced vibration analysis of the structure is done via Rayleigh-Ritz method and verified using FEM. The maximum horizontal deflection and shear stress of the base of the superstructure, and the normal/shear stresses at the foundation are analyzed. Finally, the CT is subjected to earthquake excitation, modeled as an arbitrary horizontal impact excitation at the base. The above forced vibration analysis is repeated.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A039. doi:10.1115/OMAE2014-24458.

Commercial vessels under 5000 dwt, specifically sea-river vessels, constitute a substantial part of the world’s merchant fleet. These vessels are as a rule running between sea, estuary and river ports.

Mostly these vessels are restricted in operation by areas, seasons, distance from port of refuge, wind and wave conditions. In this connection the operational safety of such vessels, which are mainly engaged in carriage of oil products, is a critical issue.

This paper addresses integrated studies on seakeeping of sea-river vessels starting from the early design stage with preliminary CFD estimates and model experiments in wave basin up to the operational sea trials. The wave conditions for analytical and experimental studies are chosen to be as close as possible to the specific ship profile and expected area of operation.

Some conclusions regarding the effect of block coefficient on seakeeping performance of sea-river vessels are made, estimations and experimental data are compared for a ship with extremely large block coefficient and wider operation area (beyond coastal waters), as well as seakeeping performance data recorded at sea during operational voyage of the vessel on the Black sea and Mediterranean sea are given.

Topics: Ships , Seas , Rivers , Hull
Commentary by Dr. Valentin Fuster
2014;():V01BT01A040. doi:10.1115/OMAE2014-24482.

Fatigue of subsea wellhead systems due to wave-induced loads from riser and rig motions has been subjected to increased attention in recent years. Major accidents due to fatigue failure has not been experienced up to now, but with increasing size of drilling rigs and BOPs in combination with longer drilling campaigns on wells, proper documentation of adequate fatigue capacity for a planned operation is becoming increasingly important. Achieving this has turned out to be a challenging issue for some wells, where application of existing fatigue analyses methodology provides rather short fatigue lives. It is expected that such calculated fatigue life results are somewhat conservative due to conservatism in the assumptions commonly applied as both input parameters and methodology are associated with some uncertainty. However, it is difficult to quantify the degree of conservatism in such analytical results. One area of conservatism relates to wellhead fatigue analyses typically being performed under the assumption that the environmental actions and response is unidirectional (in one plane only) while assuming the most unfavorable direction. This is clearly not the case in real life, and one would like to take some benefit of distributing the loading more accurately around the circumference of the pipe section. For offshore structures subjected to continuous loading over many years, one may use directional scatter diagrams for wave height and wave period based on average over years of data. However, for wellhead fatigue, the duration of a drilling or well intervention operation may be relatively short, perhaps weeks or a few months, and it is more likely that there one prevailing environmental direction may dominate such short term exposures. The environmental conditions will vary from one year to another, i.e. the environmental conditions in one particular year may be less or more severe than the average represented by the scatter diagram. Both these variation effects are studied in the present paper by doing fatigue analysis for different durations; 3 days, 10 days, 1 month, 3 months, 6 months and one year. The same period is taken from each of the 56 years of environmental data, and individual results for that period within each year are calculated. Statistics quantifying the directional effects is determined. The directional effect is here defined as the ratio of the fatigue life where the directional information is taken into consideration and the fatigue life using an omnidirectional scatter diagram considering one direction only. Directional effect with reference to both head sea and beam sea are considered. Similarly, statistics of the fatigue life variation between years are obtained. Statistics are estimated separately for the six different durations. Finally, the statistics derived are applied in a structural reliability analysis (SRA). The SRA results are then used to propose reasonable factors that can be applied in conventional wellhead fatigue analysis to account for directional effects and for variation in environmental conditions from one year to another, depending on duration of the planned operation.

Topics: Fatigue , Uncertainty
Commentary by Dr. Valentin Fuster
2014;():V01BT01A041. doi:10.1115/OMAE2014-24486.

Recently, an experimental campaign was carried out to assess the feasibility of the launching operation of two jack-up units using a barge as the launching platform. This experimental study was divided in four stages. In stages 1 to 3, a series of preliminary model tests were performed in order to provide scientific understanding of the mechanics of the operation, and investigate systematically the influence of launching parameters. The experimental approach developed for testing this operation and the results of the preliminary launching tests have been discussed in detail in a previous paper [1]. Based on the analyses of the experimental results of stages 1 to 3 and, the results of numerical simulation tools, in stage 4, a final launching condition was designed and a new set of model tests were specified to check the safety of the operation. This paper presents the results and analyses of the experimental tests in stage 4. The conditions tested in this stage covered the expected real launching condition and possible deviations in some launch parameters. The tests results include the 6-DOF motions and trajectories of the launched jack-up and the launch barge, and the reaction forces on the barge rocker arms. Later on, the success of the launching operations of P-59 and P-60 jack-up units confirmed the experimental investigation results and the feasibility of this novel launching procedure. Furthermore, it can be concluded that the experimental approach efficiently served as a tool for the assessment of high risk operations.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A042. doi:10.1115/OMAE2014-24490.

Owing to friction in the weathervaning system and in the swivel stack of a turret-moored FPSO, the turret can be locked to the FPSO in day-to-day environments meaning that no relative yaw motions occur in such conditions. When the FPSO yaws sufficiently, the mooring yaw restoring moment builds up to a point where the friction torque is overcome freeing within seconds the turret from the FPSO which then yaws back towards its neutral yaw position. This phenomenon is referred to as a turret release event. Evidence of the existence of such a phenomenon has been confirmed for one North Sea FPSO. In this paper, the basic physics of turret release are investigated from first principles. Incorporation of a friction module in a readily available time domain simulation software is described. Application of this enhanced tool towards a better understanding of the FPSO/Turret relative yaw motions & velocities during the FPSO service life is then discussed in the context of existing SBM Offshore turret systems.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A043. doi:10.1115/OMAE2014-24531.

The maximum displacement of turret moored floating production and storage units (FPSO, FLNG) has been steadily increasing over the past decades and is likely to increase even further in the future. Furthermore, some of these turret moored units are designed to keep position in even the harshest of environments (e.g. 10,000-year storms) and very deep water (e.g. more than 2000m depth). As a consequence, the demands on the capacity of mooring systems are also increasing and larger turret sizes with relatively smaller chaintable openings result. The turret cylinder is neither completely open (due to strength considerations) nor completely closed (due to installation and inspection requirements). Therefore, the mass of water inside the turret will to a degree be forced to move with the turret, but also to a degree be able to enter and exit the turret. As a result, a combination of inertial, hydrostatic, and piston mode effects similar to those found in open moonpools can occur. An additional complicating factor is that the turret is surrounded by an open annular space which influences the flow of water around the turret bottom. Both piston mode and inertia of water inside the turret will impose additional loads on the turret and its support system. It is therefore imperative that these loads be taken into account in the design phase. A number of tools, both experimental and computational may be used to obtain estimates of loads. However, whichever method is chosen, a necessary step is to identify the natural periods of the piston mode taking place in the turret. Investigations, whether numerical or experimental, must obviously focus on these periods. In this paper, analytical, computational (CFD) and experimental methods to obtain natural piston mode periods of turret moonpools are described. The methods are applied to simplified, yet realistic, turret models. Results from these different methods are compared. Respective figures of merit are discussed.

Topics: Design , Pistons
Commentary by Dr. Valentin Fuster
2014;():V01BT01A044. doi:10.1115/OMAE2014-24534.

Underwater gliders, a type of highly efficient underwater vehicle which uses gravity and buoyancy for propulsion, has been studied for a long time during the last 3 decades. This paper describes the development of the principle dynamic models of a general underwater glider, including hydrodynamic forces and buoyancy effects. The numerical analysis model was developed for the underwater glider motion. Dynamic forces equations including the model’s buoyancy, gravity, and hydrodynamic forces in gliding are derived. Gliding velocities with changes of the net buoyancy are compared. The numerical method was used to calculate the hydrodynamics coefficient of the glider. Dynamic characteristics of the Seaglider and SLOCUM have been used as validation objects for the numerical method. The glide angle is merely the function of the lift/drag ratio and has no relation to the net buoyancy. The gliding velocity would increase when the buoyancy increases.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A045. doi:10.1115/OMAE2014-24565.

This paper summarizes the hydrodynamic work that was performed on the Robinson R66 helicopter’s water emergency landings as part of the roadmap to obtain FAA approval. The emergency system consists of two floats that are rapidly inflated as soon as the helicopter touches the water using gas from a high pressure cylinder. This type of design is common in the helicopter industry and is known as a “pop out” float system. The floats have already been shown to provide enough buoyancy to keep the helicopter afloat in calm water.

Recognizing that once the helicopter is in the water, it is subjected to wave forces and behaves as a small water craft, a numerical study was performed using OrcaFlex. Over 500 numerical simulations, each lasting 10 minutes, of the helicopter floating in different wave environments were performed. The helicopter’s 6 degree-of-freedom (DOF) motions were monitored throughout. At the end of the run, if the helicopter had not capsized, the run would report: “no capsizing”. Sensitivity studies were performed by varying parameters individually. This led to an understanding of each parameter’s effects on the overall helicopter floating performance. These parameters included: wind, wave period, initial helicopter relative heading against the waves, wave height, and simulation seed (different random wave sets with the same spectral characteristics). The FAA expects floatation characteristics to be evaluated in “reasonably probable” water conditions and has issued guidance that World Meteorological Organization (WMO) sea state 4 is one acceptable definition of reasonably probable. A wind speed of up to 21 knots is also associated with sea state 4.

Topics: Water
Commentary by Dr. Valentin Fuster
2014;():V01BT01A046. doi:10.1115/OMAE2014-24569.

The wave diffraction and upwelling around vertical columns such as platform legs and offshore wind turbine foundations is studied. Nonlinear contributions in steep wave conditions are particularly addressed. Data and analyses from a previous experiment with fixed cylinders in deep water regular and random waves are reviewed, and essential findings are highlighted. Measured extreme crests are clearly higher than linear predictions, connected with an increase in basic harmonics as well as with second-order components. Improved results are obtained with fully second-order modeling, while this is still incomplete, and such analyses can be computationally demanding as well as non-robust.

An alternative, simplified empirical nonlinear correction approach based on these findings, previously suggested for engineering design prediction of extreme crests in random waves, is validated against a re-analysis of the fixed column data. In addition, validation is also made against model test data for two floating production semis. The method agrees well with the data, taking into account the simplicity of the method and the complexity of the problem, and is a tool for early design phases. Detailed studies will still require model tests and CFD development.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A047. doi:10.1115/OMAE2014-24572.

This paper is concerned with calculations of the two-dimensional nonlinear vertical and horizontal forces and overturning moment due to the unsteady flow of an inviscid, incompressible fluid over a fully-submerged horizontal, fixed box. The problem is approached on the basis of the Level I Green-Naghdi (GN) theory of shallow-water waves. The main objective of this paper is to present a comparison of the solitary and cnoidal wave loads calculated by use of the GN equations, with those computed by Euler’s equations and the recent laboratory measurements, and also with a linear solution of the problem for small-amplitude waves. The results show a remarkable similarity between the GN and Euler’s models and the laboratory measurements. In particular, the calculations predict that the thickness of the box has no effect on the vertical forces and only a slight influence on the two-dimensional horizontal positive force. The calculations also predict that viscosity of the fluid has a small effect on these loads. The results have applications to various physical problems such as wave forces on submerged coastal bridges and submerged breakwaters.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A048. doi:10.1115/OMAE2014-24585.

This paper describes an experimental and numerical study on the effect of bilge keels on a specific offshore vessel. Forced roll and free decay experiments were carried out at MARINTEK in a joint project between VARD, DNV GL and MARINTEK during Nov. 2012. The 1:25 ship model was tested both with and without bilge keels. 19 forced roll periods were tested at 4 roll angles (2, 4, 6 and 10deg). Free decay tests were performed for initial roll angles 2, 4, 6, 10 and 15deg. Damping was extracted and compared with numerical simulations from a potential flow strip theory program (VERES) and a commercial CFD tool (StarCCM+).

Both qualitative and quantitative agreement between the experimentally and numerically obtained damping parameters was documented. Discrepancies were identified and discussed. The three different methods of investigation gave together a basis for discussing the effect of bilge keels. From the present study, it is recommended that both experimental and numerical work in combination, for both forced roll and free decay tests, should be performed to assess the damping efficiency of bilge keels.

Topics: Damping , Ships , Keel
Commentary by Dr. Valentin Fuster
2014;():V01BT01A049. doi:10.1115/OMAE2014-24586.

A time-domain Higher-Order Boundary Element Method (HOBEM) based on cubic shape functions for second-order wave-current-body interaction developed by Shao & Faltinsen [1] is further refined by investigating the feasibility of adopting the unstructured meshes on the free surface and body surfaces from an open source mesh generator [2]. When the steady local flow effect is considered in the time-domain boundary-value-problem formulation, the advection terms in the free surface are part of the sources of numerical instability. In this paper, the advection terms are taken care of in an implicit way in a 4th order Runge-Kutta scheme with much better stability. Some numerical examples extensively studied in the literature are studied in order to validate the present numerical model.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A050. doi:10.1115/OMAE2014-24600.

This paper presents numerical results of the monodirectional second order forces acting on a rudimentory FPSO hull shape. The exact calculation of second order loads through potential theory is computationally challenging and requires the evaluation of a slowly-convergent free surface integral. In practice, this integral is often neglected as it is widely assumed that it does not contribute significantly to the overall load and few commercial seakeeping software propose this option anyway. An even rougher and widely used approximation consists in expressing the second order loads occuring at a non-zero difference frequency from the mean drift loads. It is called the Newman’s approximation; it enables serious CPU time gains, but at the cost of a worse accuracy. The object of this paper is to investigate on a simple case the accuracy of each of these two approximations (without the free surface integral and Newman’s approximation) and the influence of parameters such as the water depth and the sea state wave length. Depending on the resonant period of the considered mooring system, it may give some insight on a good compromise between CPU time and accuracy when choosing the way of determing the second order loads.

Topics: Approximation
Commentary by Dr. Valentin Fuster
2014;():V01BT01A051. doi:10.1115/OMAE2014-24605.

The conceptual design of floating ship shape platform for well maintenance, survey and workover in prospective fields of Russian Arctic and Sakhalin Island shelf (further The platform) was developed by Krylov State Research Centre specialists with the assistance of CDB “Korall”. The platform under design intended for operation in the Arctic conditions and designed to resist the ice formations such as continuous thin first-year ice and broken ice being the first one in the world practice. The form and chosen hull ice strengthening class allow ensuring safe operation in the mentioned ice conditions.

The main purpose for development of the platform was to minimize the offshore wells downtime providing the cost effective way for maintenance of subsea production systems comparing with floating drilling rigs and drilling vessels that takes a significant investment. The economic assessments have shown that operation of such platform will cost two to three times less than renting of a rig or drillship. Having the working deck space of about 1,000 m2 and the ability to download a variety of equipment the platform can perform a variety of testing and wells repairing procedures both for underwater wellhead and for wells operated on fixed platforms.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A052. doi:10.1115/OMAE2014-24609.

Establishing statistical distributions of the response extremes is of particular importance for the design of FPSO mooring systems and the related riser design. Long term time domain simulation is the most accurate design approach to determine the extreme responses. It involves, however, coupled dynamic analysis of FPSO mooring system for a large number of sea states and consequently the task is often prohibitively time consuming. To solve this problem, an approach for the long term extreme analysis based on a metamodel in conjunction with the design of experiment methodology is proposed in the paper. In this approach, Latin Hypercube Sampling (LHS) based on the design of experiment method, is performed to select a sub-set of sea states from all sea states. Short term distributions for this sub-set of sea states are simulated and estimated. Kriging metamodel, which can map the relations between the sea states characteristics and the short term distribution parameters, is then applied. The accuracy of the metamodel is investigated. The long term response distribution of moored FPSO systems for all sea states can be predicted based on the metamodel. This approach for the long term extreme analysis of FPSO mooring systems avoids the response analysis over all sea states and can greatly improve the computational efficiency of the long term extreme analysis of FPSO mooring systems.

Topics: FPSO , Mooring
Commentary by Dr. Valentin Fuster
2014;():V01BT01A053. doi:10.1115/OMAE2014-24610.

There are still uncertainties and challenges by using floating platforms as the main oil exploitation mode due to lake of actual drilling experience in South China Sea. In order to ensure the safety of platform operation, large amounts of prototype data are needed for environment and structure analysis. From 2011 till now, a prototype measurement project is carried out on a semisubmersible platform “NanHaiTiaoZhan” FPS. Large amounts of data have been collected including parameters of ocean environmental loading, motions of floater and underwater responses of mooring and risers.

In this paper, statistical analysis of floater responses together with marine environmental conditions are carried out for deep understanding of the characteristics of semisubmersible floating platform. In order to study the motion behaviors of floater under harsh storm, the data recorded in a typical typhoon “Nock-ten” are chosen to perform the deep analysis. The measured data, wind, wave, current and floater motions, are analyzed in time-domain and frequency-domain separately. The comparison study of theoretical prediction and prototype measurement result in typhoon weather has been carried out. The data recorded by standalone sensors are processed by program MoorForce. The result of frequency-domain analysis of measured mooring force proves that the significant wave frequency dynamic behavior of mooring line during typhoon weather should be considered in design.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A054. doi:10.1115/OMAE2014-24649.

Many measurements of hurricane waves have been made from deep water production facilities in the Gulf of Mexico. Measurements made on different sides of the platforms differ from one another and the incident wave field because the platforms diffract and radiate waves. For many purposes, we would like to know the incident wave field. Forristall and Aubault (OMAE2013-10860) used WAMIT diffraction calculations to successfully invert wave spectra measured under a TLP model in the Marin offshore basin. We have now used similar techniques to invert spectra measured at offshore platforms during Hurricanes Gustav and Ike. We do not have any measurements of the undisturbed wave spectra for testing the results. The tests were made by checking whether inverse calculations on all the gauges deployed at different locations on the platforms could produce the same undisturbed wave field.

Wave directions are needed for the diffraction calculations. Information from the wave gauges can be used to find the directions by optimizing the agreement among the inverted power spectra. To perform the optimization, we varied both the mean direction and spreading at each spectral frequency. The rms difference between the inverted probe spectral densities was minimized at each spectral frequency.

When spectra from four gauges on a platform are inverted, they agree reasonably well with each other. The average of the inverted significant wave heights is slightly lower than the average of the measured significant wave heights. But when spectra from pairs of the four probes are inverted, the results differ depending on which pair is used. This result implies that our inversion method cannot be used on data from platforms with two probes, and casts doubt on the accuracy of four probe inversions.

Topics: Diffraction , Waves
Commentary by Dr. Valentin Fuster
2014;():V01BT01A055. doi:10.1115/OMAE2014-24657.

Response Based Analysis (RBA) aims at prediction of the long term distributions of critical responses such as motions, accelerations, wave loads, which have significant impact on the design of a floating system. As compared with the conventional analysis, which predicts the responses of the facility to the N-year return period metocean conditions, RBA provides directly the N-year return period responses by analysing the statistics of their long term time histories. Another outcome of RBA is the Design Metocean Conditions (DMCs) which are combinations of sea state, wind and current causing the corresponding N-year response. The knowledge of the DMCs enables the more detail time domain analysis and model tests to be performed for a set of critical combinations of metocean parameters. It also enables all the associated responses to be determined.

The RBA framework is generally well addressed in the literature, but the DMC identification methods are not necessarily clearly established. The objective of this paper is to present the theoretical background, numerical method and an example for the determination of the N-year return period responses and the associated DMCs for a floating facility. The method includes prediction of the N-year response, identification of the metocean combinations within the available time history which produce this response, determination of the required percentile of the short term response to match the N-year response, and the search for the most probable DMC within the joint probability density of the metocean parameters. Features of the method are discussed and results are presented for several critical responses of a weather vaning vessel.

Topics: Design
Commentary by Dr. Valentin Fuster
2014;():V01BT01A056. doi:10.1115/OMAE2014-24694.

Traditionally, it has often been assumed that the flow conditions in a moonpool are only moderately altered when an object is introduced therein. Moreover, the hydrodynamic forces acting on the object has typically been estimated by Morison’s equation for small volume structures, using the fluid kinematics of the empty moonpool as a basis and applying correction factors for the confined flow conditions, as for an object in a tube or a channel.

To investigate the validity of the traditional approach, an experimental study on the forces acting on objects in a moonpool was performed at NTNU/MARINTEK in Trondheim, Norway in 2013. The experiments were done using a simplified 2-dimensional moonpool model which was given a forced heave motion. Two objects, both with square cross sections but of different sizes, were put inside the moonpool one at the time. The resulting wave elevations inside the moonpool and the forces acting on the objects were recorded and analyzed. To get a deeper understanding of the flow characteristics in the moonpool, PIV measurements were used to obtain the fluid velocity fields.

The experiments revealed that even moderately sized objects (relative to the size of the moonpool) change the fluid motions in the moonpool to a large extent; the overall wave elevation amplitude is strongly reduced and the resonance period is altered. A consequence of this is that there is a large discrepancy between the hydrodynamic forces acting on the objects measured in the experiments and the forces calculated using the traditional approach. The PIV results showed the formation of vortices at the inlet of the moonpool and at the edges of the objects, which is the main source of non-linear damping of the wave elevation inside a moonpool.

Commentary by Dr. Valentin Fuster
2014;():V01BT01A057. doi:10.1115/OMAE2014-24714.

A Blowout Preventer (BOP) consists of several sets of shear rams stacked together out of which Blind Shear Rams (BSR) is a very critical component to achieve complete shearing of drill pipe. The current study focusses on developing a methodology to model the shearing process and validate it with the shop test. Shop tests were conducted to shear a drill pipe using a surface BOP to obtain the maximum shearing force, shape of sheared drill pipes, shearing time etc. The shear rams used in the test are laser scanned and used in FEA simulations. The FEA model is developed such that it replicates the shop test and simulations are performed using Abaqus Explicit finite element solver. The output from simulations and shop test are compared and found to be in good agreement. The developed methodology is further verified by applying it for shearing different drill pipe sizes and the output is compared with the available data and found to be comparative which proves that the methodology can be implemented for various shearing studies.

Original Equipment Manufacturer (OEM) formulas are currently being used in industry to compute the maximum shearing force required to shear the drill pipe under various scenarios. The OEM shearing force is computed for the model under consideration and compared with the shop test and simulation and is found to be conservative. The validated model can be used for conducting studies that provide information on the governing parameters in terms of loading or positioning of the drill pipe to be considered in shear ram designs. This study provides a tool for optimizing the preliminary design of new shear rams and can contribute to more reliable and efficient shear ram design. The study is conducted as part of a Technology Assessment Programs (TAP) for the Bureau of Safety and Environmental Enforcement (BSEE) in the areas of BOP stack sequencing, monitoring and kick detection.

Commentary by Dr. Valentin Fuster

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