Daniel Testa

MOTION BEHAVIOUR OF A NEW OFFSHORE LNG TRANSFER SYSTEM AT HARSH OPERATIONAL CONDITIONS

Today, the demand of natural gas from offshore fields is on a high level and still increasing. Floating turret moored terminals receive gas directly from the field via risers and liquefaction is achieved by on-board processing plants. The LNG (liquefied natural gas) is transferred to periodically operating shuttle carriers for onshore supply. This paper presents an innovative offshore LNG transfer system, based on newly developed flexible cryogenic pipes of 16” inner diameter, which allow fast loading/offloading procedures in tandem configuration (see Fig. 1), even in harsh environmental conditions. The motion characteristics of the proposed concept are investigated in detail by the potential theory programmes WAMIT and ANSYS AQWA, respectively, with the focus on the dynamic behaviour of the multi-body system in waves. Each vessel is generating its own radiation and diffraction wave field affecting the motions of the adjacent vessels and vice versa. Results from calculations in the frequency and time domain are compared and show good agreement. Tolerable relative motions between terminal and carrier are limited by maximum torsion and bending of the flexible transfer pipe.

Critical Situations of Vessel Operations in Short Crested Seas—Forecast and Decision Support System

The encounter of extreme waves, extreme wave groups, or unfavorable wave sequences poses dangerous threats for ships and floating/stationary marine structures. The impact of extreme waves causes enormous forces, whereas an unfavorable wave sequence—not necessarily extreme waves—can arouse critical motions or even resonance, often leading to loss of cargo, ship, or crew. Thus, besides a well thought-out maritime design, a system detecting critical incoming wave sequences in advance can help avoiding those dangerous situations, increasing the safety of sea transport or offshore operations. During the last two years a new system for decision support onboard a ship or floating/fixed marine structure named CASH—Computer Aided Ship Handling—has been introduced. The preceding papers showed the step wise development of the main components of the program code—3d-wave forecast and 3d-ship motion forecast . These procedures provide a deterministic approach to predict the short crested seas state within radar range of the ship, as well as resulting ship motions in six degrees of freedom. Both methods have been enhanced with special focus on the speed of calculation to ensure a just-in-time forecast. A newly developed component is the adaptive 3d-pressure distribution . This method calculates the pressure distribution along the wetted surface of the ship hull using a newly developed stretching approach. With the end of the joint project Loads on Ships in Seaway (LaSSe), (funded by the German Government) the paper presents the CASH system, giving the possibility to detect critical situations in advance. Thus not only decision support onboard a cruising ship can be provided, but also time windows for offshore operations are identified well in advance.

Spatial Evolution of an Extreme Sea State With an Embedded Rogue Wave

In the last years the existence of freak waves has been affirmed by observations, registrations and severe accidents. Many publications investigated the occurrence of extreme waves, their characteristics and their impact on offshore structures, but their formation process is still under discussion. One of the famous real world registration is the so called “New Year Wave”, recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. Since there is only a single point registration available, it is not possible to draw conclusions on the spatial development in front of and behind the measurement point which would be indispensable for a complete understanding of this phenomenon. This paper presents a spatial development of the “New Year Wave” being generated in a model basin (L = 120 m, W = 8 m, d = 1 m, scale 1:70). To transfer the recorded “New Year Wave” into the wave tank, an optimization approach for the experimental generation of wave sequences with predefined characteristics is applied. The method is utilized to generate scenarios with a single high wave superimposed to irregular seas. At the end of this optimization process, a control signal for a deterministic wave sequence is obtained. The generated sea state with the embedded “New Year Wave” is measured at different locations in the tank, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position — altogether 520 registrations. The focus lies on a detailed description of a possible evolution of the “New Year Wave” over a large area and time interval. It is observed that the extreme wave at the target position develops mainly from a wave group of three smaller waves. In particular the group velocity, wave propagation and the energy flux of the wave group is analyzed. In addition, the WAVE FORECAST METHOD is applied. This method is based on linear wave propagation and provides a prediction of the wave train a few minutes in advance from a single surface elevation snap shot. The capability of the prediction of approaching extreme wave heights is shown.Copyright

Qualitative and Quantitative Validation of a Numerical Code for the Realistic Simulation of Various Ship Motion Scenarios

There is an ongoing discussion on safety guidelines to be considering more recent developments in ship design. Numerical simulations of ship motions are considered as powerful tool for the safety evaluation of a given design. However, the consequent use of numerical codes calls for their thorough validation which has to be performed both qualitatively and quantitatively. This paper focuses on a code used and further developed by the Flensburg Shipyard. For its validation, the capsizing scenario in steep wave sequences is realized in the wave tank first. The dedicated computer controlled experimental technique ensures the exact phase correlation of wave excitation and resultant ship motions. Thus, the registered wave and the track of the ship model in the model test serve as input to the numerical simulation which results in the specific motion time traces. These are now directly compared to the motion registrations from the model tests. First results of the validation by direct comparison of time series have been presented in earlier publications, still with the restriction that only a few cases have been investigated. In this paper, the promising method is applied to another scenario in a long-crested sea state including steep wave combinations. Different aspects are discussed which results in the conclusion that the method is feasible for free running ships in stern and stern quartering seas.Copyright

Gap Effects at Side-by-Side LNG-Transfer Operations

The current demand of liquefied natural gas (LNG) from remote marine locations pushes the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred between shuttle carrier (LNGC) and terminal. Even if the tandem configuration is the primary choice for LNG transfer at rough offshore locations, side-by-side configurations would be the preferred option because of existing midship coupling manifolds on the present carrier fleet (no need for manifold modifications) as well as standard mooring systems and transfer-process-chains similar to oil-transfer. Therefore, the operation conditions at rough seas have to be improved to allow side-by-side LNG-transfer and to reduce offloading downtime.Within the SOTLL-project, side-by-side LNG transfer up to HS = 3 m is reached as a transfer limit using a new flexible pipe design, the advantages of sheltered areas at the leeside of the terminal barge and an optimized ship transfer position due to a flexible longitudinal offloading position. In addition to the evaluation of the hydrodynamic characteristics of this multibody system, one key aspect is the analysis of the exciting forces and motions due to wave amplification between the ships. In the gap between the hulls, the incoming wave field is amplified and changes dramatically. Depending on gap width, longitudinal offset, wave heading and length, large wave amplifications, standing waves and other resonance phenomena are observed which may result in high relative motions and increased forces of the entire mooring system. In this paper, the gap effects are investigated in detail with numerical approaches in frequency domain, validated by model tests at TU Berlin. A typical offloading scenario with barge and carrier is investigated for different gap sizes to identify suitable transfer configurations and ensure safe LNG offshore transfer up to HS = 3 m.Copyright

Forecast of Critical Wave Groups From Surface Elevation Snapshots

Reports on damages of ships, cargo and structures during heavy seas have been increasing within the last years. The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of crew, ship and cargo. Dangerous situations can be predicted by a forecast of encountering wave trains and the identification of critical wave groups. The paper presents a method to calculate the wave train a ship will encounter from surface elevation snapshots of the surrounding sea, taken by the ship radar. The time-dependent surface elevation snapshot far ahead of the ship is transferred into frequency domain by the use of Fast Fourier Transformation (FFT). The resulting complex Fourier spectrum given over the inverse wave length 1/L is converted into an amplitude spectrum and a phase spectrum. By shifting the phase spectrum to the position of the cruising ship the encountering waves can in turn be calculated in advance — depending on speed. The permanent processing of incoming snapshots delivers a continuous prediction of the water surface elevation at the position of the cruising ship. Based on these data the expected ship motion behaviour can be calculated continuously in time domain. In addition the response spectra, resulting from the wave spectrum and the relevant RAOs, are also evaluated. As wave data far ahead of the ship are used, it allows a forward glance, and dangerous situations, particularly resonance and parametric resonance are detectable before the ship is encountering this wave train. Consequently, the procedure can be used by the master as an assistance support system.Copyright

SLOSHING: FROM THEORY TO OFFSHORE OPERATIONS

The current demand in liquefied natural gas (LNG) from remote marine locations drives the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred to shuttle carriers (LNGC). During the loading procedure, which takes about 18-24 hours for a standard sized LNGC, free fluid surfaces and varying filling levels occur inside the internal cargo tanks. This condition is critical since the seakeeping behavior of the LNGC — especially the roll motion — is strongly influenced and varying. In order to estimate and forecast the LNGC motions, numerical methods based on potential theory are the most efficient and appropriate method. The selected approach is validated by model tests at 30% water filling height inside four prismatic tanks. In-depth analyses, including force and moment measurements between tanks and hull, revealed a discrepancy between the analytical natural modes of a prismatic tank and the resonance frequencies for four prismatic tanks mounted to a LNGC hull. This effect is caused by the ratio of rigid to added mass of the system as well as the fact that the tanks are mounted to a standard hull shape featuring a longitudinal bow-stern asymmetry. In order to investigate this phenomenon

Hydrodynamic Considerations for FLNG Concepts

The current demand in liquefied natural gas (LNG) encouraged the design of various concepts for floating LNG (FLNG) liquefaction or regasification facilities. With increasing transport distance, e.g. from remote marine locations to the onshore gas supply net, gas pipelines become uneconomic compared to shuttle carriers for LNG (LNGC). Due to its high energy density, offshore transfer from processing terminals to carriers and from carriers back to receiving terminals has to be analyzed in detail. During the transfer period, free fluid surfaces occurring in the cargo tanks of the LNGC are leading to a significant decrease of the initial intact stability and altered motion behavior. This paper focusses on the influence of resonant tank sloshing on the LNGC’s roll and surge motions. Analyses of transverse and longitudinal sloshing yield a surprising phenomenon: the frequency shift Dw between the theoretical natural frequency of the tank alone and the respective motion peak for a vessel with four tanks mounted to the hull. Force measurements between tank and hull reveal a peak at the tank’s natural frequency that causes strong liquid motions with related forces and moments on the hull but no increased vessel motions. Additional investigations comprise the offloading situation with a multi-body arrangement of LNGC

Evaluation of Critical Conditions in Offshore Vessel Operation by Response Based Optimization Procedures

During the design process of floating structures, different boundary conditions have to be taken into account. Besides the basic determination of the type of vessel, the range of application and the main dimensions at the initial stage, the reliability and the warranty of economical efficiency are an inevitable integral part of the design process. Model tests to evaluate the characteristics and the performance of the floating structure are an important milestone within this process. Therefore it is necessary to determine an adequate test procedure which covers all essential areas of interest. The focus lies on the limiting criteria of the design such as maximum global loads, maximum relative motions between two or more vessels or maximum accelerations, at which the floating structure has to operate or to survive. These criteria are typically combined with a limiting characteristic sea state (Hs , Tp ) or a rogue wave. However, the important question remains: What is the worst case scenario for each design parameter — the highest rogue wave or a wave group of certain frequency? And which sea states have to be taken into account for the experimental evaluation of the limiting criteria? As an approach to these challenges, a response based wave generation tool for critical wave sequence detection is introduced. By means of this procedure, model tests can be conducted more efficiently. Besides the theoretical background of the response based wave generation tool, an exemplary practical application for a multi-body system is shown with maximum relative motions as the limiting criterion.Copyright

Forecast of Critical Situations in Short-Crested Seas

The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of ship, cargo and crew. One approach to avoid dangerous situations is to adjust heading and cruise speed. To identify critical situations well in advance the forecast of the incoming wave train is essential. Concerning the method to predict the wave train a ship will encounter within the near future — some minutes ahead — the so far unidirectional WAVE FORECAST method, pre-calculating an encountering wave train from surface elevation snapshots of the surrounding sea — taken by radar — has been improved. This paper presents a method to predict the entire sea state within the surrounding area of the vessel considering multidirectional waves. Thus the evolution of critical waves coming from various directions can be predicted. In addition the SHIP MOTION FORECAST method — pre-calculating the vessel response — has also been enhanced. Taking into account the encounter angle of the incoming wave components, depending on time and course angle of the vessel, the ship-fixed compass rose is divided into a number of sectors. The corresponding encountering wave train for every sector is derived by superimposing all wave components coming from certain directions. With a set of directional R esponse A mplitude O perators (RAOs) for the six degrees of freedom the sector-wise vessel responses can be calculated as well. The response spectra are derived in frequency domain and transferred into time domain by the use of I nverse F ast F ourier T ransformation (IFFT). Thus the overall vessel response is obtained by superimposing the time domain responses for every sector and degree of freedom, delivering a comprehensive data base for the analysis of critical situations in advance.Copyright