IBP3435_10 HYDRAULIC ASPECTS OF MARINE LNG RECEIVING TERMINAL DESIGN Hans Fabricius Hansen 1, Jens Kirkegaard 2, Henrik Kofoed-Hansen 3 Copyright 2010, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2010, held between September, 13-16, 2010, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute opinion, nor that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2010 Proceedings. Abstract Safe offloading of LNG at coastal facilities for re-gasification is a key issue for permanent delivery of gas to distribution networks. Receiving terminals can be located either in existing harbours or at open coasts. This presentation describes the hydraulic aspects of locating a terminal at sites where existing harbours or protected areas such as bays are not available. At such a site the import facility can be selected among the following types: Facilities placed on a gravity base structure (island) with unloading jetty for the shuttle tanker; Facilities placed on a floating platform (FSRU) permanently moored along a traditional tanker jetty and with the shuttle tanker moored alongside or at a separate tanker berth, Facilities placed on a point-moored FSRU with the shuttle tanker alongside. The selection of the optimal type of facility depends on a number of aspects such as local meteomarine conditions (waves, wind, etc.), soil and topographical conditions, time schedule and economy. FSRU s have the advantage that they can be reused at other locations, whereas island-based facilities have a permanent character. For planning and design of the facility, it is important to determine the conditions for berthing and mooring the shuttle tanker with regard to both operational and safety. Model investigations in a wave tank and by numerical models are the tools employed in this process. As part of the EC-supported Safe Offload project, numerical and physical modelling has been carried out to provide guidelines for use of these methods. Some results are described and interpreted in the context of the complex meteomarine conditions along the Brazilean coast. The paper also describes the possible need for additional protection of the berth by a breakwater and the potential benefits by a predictive web-based decision support system for weather and wave forecasts. 1 Research Engineer - DHI 2 Chief Engineer - DHI 3 Head of Department, Engineer - DHI
1. Introduction Transportation and unloading of LNG to a coastal facility involve a careful assessment of the exposure to wind, currents and waves and the potential response of the systems. The present paper describes the exposure of coastal terminals established on a gravity base fixed structure or as a floating storage and regasification unit (FSRU). The assessment relates specifically to the planning and design of such facilities; however, the tools used in these processes can be of great value to safe and efficient operation of the terminal once it is established. The use as the tools for forecasting purposes is described in the last section of the paper. When the local conditions are known, the optimization process includes prediction of the motions of the FSRU and the shuttle tanker when moored at a FSRU or a fixed berth. The mooring arrangements shall provide limited relative motions particularly at the manifold point and at the same time acceptable forces in moorings and fenders. These conditions are determined by use of model investigations, either experimentally in a wave tank, by numerical models or by a combination of these tools. Specific studies are recommended for final design of facilities. However, an initial selection among the alternative facility types can in most cases be conducted based on general information available from published experience supported by analytical assessments. A moored vessel exposed to quartering and/or directionally spread seas will present a complex motion pattern with interaction between the longitudinal and transverse modes of motion. The problem is further complicated by the non-linear response of the mooring system and sometimes also by reflected waves from various directions. Due to the complexity of the problem, maximum motion response cannot be determined for idealised conditions such as the longitudinal motions alone in head/stern seas and for the transverse motions alone in beam seas. In this paper, the full interaction between transverse and longitudinal motions of a vessel moored to a shallow water gravity based offshore structure has been assessed by a combination of physical model tests and numerical simulations. The numerical model adopts a hybrid approach, with phase-resolved wave fields from a two-dimensional Boussinesq wave model and hydrodynamic quantities from a radiation diffraction code, to solve the vessel equations of motion in the time domain. 2. Metocean Conditions A large amount of wave studies have been conducted for sites along the coast of Brazil. The most frequent sea state is waves from the north-eastern sector which vary during the day due to local sea-land breeze. This gives rise to intensification of short-period waves from noon. The occurrence of cold fronts progressing from the south along the coast creates waves of longer period from the south-easterly sector. Those waves are typically with periods of 12 14s. Extreme waves above H s =4m are very rare and annual maxima are on the average in the order of 3m. There is a seasonal variation of extreme sea states as the cold fronts are stronger in the winter season from June to September. For operational assessment of terminal facilities, it is important to pay attention to the typical daily sea states which seldom have significant wave heights lower than 1m. 2.1. Current Conditions At coastal sites on the east coast of Brazil, the currents generated by the astronomical tide are relatively weak because of the small phase shift of the tidal wave along the coast. The dominant currents are thus related to variations of the meteorological conditions particularly related to the system of cold fronts. These give rise to north-going currents, whereas the more persistent north-easterlies create south-going current. The currents typically do not exceed 0.5m/s. Local coastal features give rise to minor variations of this general pattern. 3. Physical Model Tests The SAFE OFFLOAD project was carried out with financing from the European Commission. A main topic of the project was to verify the validity of multi-body numerical models for description of relative motions between floating bodies. As reference for these numerical models, a series of physical model tests were conducted in DHI in Denmark. In the following are presented the results of two series, tests with an offloading concept from a shuttle tanker to a gravity base structure (GBS) and a series with a tanker moored alongside a large (400m long) storage and production unit (FSRU). In both cases the shuttle tanker was a 150,000m 3 vessel with an overall length of 290m. Tests were carried out at a model scale of 1:100. The waves were generated as multi-directional waves ranging from wind sea of T p = 6 10s and swell of 10 15s period. In some cases, the models were exposed to bi-spectral waves (simultaneous sea and swell) from different directions. 2
3.1 Tanker moored alongside a Floating Receiving Unit The FSRU is moored by a bow turret and thereby free to weathervane according to the predominant combination of environmental forces. Although the tests were conducted in a deep water facility (300m of water depth) the relative motions will be descriptive of conditions in lower water as well. The LNG tanker was moored by 16 lines (6-2-2-6), each with a stiffness of about 250kN/m, against 4m diameter pneumatic fenders. Figure 1. Models of shuttle tanker and turret moored FSRU As the system will weathervane, the longitudinal motions will be dominating. However, in natural sea states the system will experience yawing motions. At extreme yaw, which is typically at 10 degrees to the direction of the main wave field, roll motion of the vessels will be introduced. 3.2 Tanker moored against a Gravity Base Structure The GBS was located in about 30m water depth and exposed to currents along the berth and waves at a small angle or perpendicular to the moored vessel. Wind was generated broadside to the vessel. The mooring system between the vessels was similar to the one used for the FSRU tests, but here the fendering was provided by SUC2500 fenders. Figure 2. Moored tanker model along a GBS in shallow water This set-up exemplifies a coastal facility, where operations can be restricted to relatively low sea states because the waves are almost broadside to the moored vessel. As the natural roll period is about 14s, swell waves give rise to rather large motions in sway and roll. 3.3 Test Results, Tanker at FSRU An extract of the results is shown below. Figure 3 show the examples of the relative motions for the Tanker- FSRU system exposed to H s =2.5m and T p =12s. The relative motions are expressed as the variation of relative distances in three components, along the vessels, transverse and vertical, between the manifold points at midship. It is seen that the longitudinal motion has a long (10 min) oscillation, which is caused by variations of the heading of the system. The overall relative motions of the manifold point are less than 1.5m in all directions. This is of course strongly influenced by the mooring system between the two floating bodies. In the tanker-fsru tests, the forces on the turret were measured as well. These will depend very much on the water depth will need to be tested in the exact water depth, where a floating unloading terminal would be located. 3
Relative Longitidinal Excursion (m) Relative Transverse Excursion (m) Relative Roll Excursion (deg) Rio Oil & Gas Expo and Conference 2010 Figure 3. Example of relative motions derived for tanker-to-fsru manifold points. The two upper plots to the left show the motion envelopes in the horizontal plane and in the vertical plane. The third plot is a time series of relative roll. The lower plot is the time series of the relative longitudinal motion. Swell-0 Swell-25 Sea Swell-0, C+W SEA+swell, C+W SWELL+sea, C+W 8.00 7.00 6.00 5.00 3.50 2.50 1.50 0.50 Figure 4. Tanker moored along FSRU. Extreme relative motions (peak-peak) as measured during tests of 3-hour duration. Results from all tests of the FSRU system are summarized in Figure 4. The results presented are the relative excursions between the vessels as described at the manifold points. The different symbols of the legend indicate tests in sea and swell, with and without current and wind (C+W). For bi-spectral sea states, the dominant spectrum is given with capital letters. It is seen that the swell impact from 25 degrees results in increased longitudinal and transverse motions. This is an effect of the vessels being less in contact through the fenders at oblique wave impact. The introduction of currents results in significant yawing of the system which leads to oblique wave impact and hence large variations of results in apparently identical conditions. 3.4 Test Results, Tanker at GBS The results for tanker at the GBS are shown in Figure 5 in a similar manner as the results above. In this case the orientation of the vessel is kept more stable and the impact of the low wind and current is of minor importance. The important differences are between sea and swell impact. Swell introduces much larger motions because the roll period of the vessel is close to the period of the swell. This is an important aspect when considering the orientation of a GBS in bi-spectral wave environment with predominant swell from a narrow direction sector. 1 9.00 8.00 7.00 6.00 5.00 4
Relative Longitudinal Excursion (m) Relative Transverse Excursion (m) Relative Roll Excursion (deg) Rio Oil & Gas Expo and Conference 2010 Swell-0 Swell-25 Swell-25, C+W Swell-25 SEA C+W 3.50 6.00 1 2.50 1.50 0.50 0.50 1.50 5.00 0.50 1.50 1 8.00 6.00 0.50 1.50 Figure 5. Tanker at GBS. Extreme relative motions (peak-peak) as measured during tests of 3-hour duration 4. Numerical Studies Numerical studies provide a fast and cost-effective way of assessing motions of moored vessels for different offloading concepts. DHI has used its in-house multi-body vessel motion analysis package, WAMSIM, to evaluate the response to incident wave forcing of a vessel moored to a gravity based LNG facility. WAMSIM relies on the industry-standard code WAMIT to provide the frequency-domain hydrodynamic characteristics (the frequency-response functions or FRF's) of the body. WAMSIM takes a Fourier transform of the FRF's to get the body's impulse-response functions (or IRF's), which are then combined with incident wave, hydrostatic, mooring system, wind, current and viscous damping forces to solve the equations of motion for the body in six degrees of freedom as described by Bingham, 2000. Validation of WAMSIM against physical model tests is reported in Christensen et al., 2008 and Hansen et al., 2009. The wave forcing is provided by a phase-resolving MIKE 21 Boussinesq wave model that includes all important physical aspects of waves in confined, shallow waters. The present model set up with a LNG tanker moored at a gravity base LNG facility is compared to results of the physical model tests conducted under the Safe Offload project. Close resemblance with the laboratory conditions is obtained by using the forcing signal from the laboratory wavemaker to generate waves in a numerical MIKE 21 Boussinesq wave model. Surface elevations at a certain point in time are shown in Figure 6, with waves propagating from the bottom of the plot. Although some diffracted wave energy does reach the lee side of the gravity base structure, its sheltering effect is clearly seen from this figure (longer distance between isolines in the lee of the structure). Figure 6. Snapshot of simulated wave field around gravity base structure. Possible vessel mooring positions marked 5
A comparison of simulated and measured motion spectra is shown in Figure 7 for a LNG tanker moored on the exposed side of the gravity base structure. The significant wave height of the incident wave field is 1.0m and the peak period 10s. The numerical model is capable of capturing well the resonance frequencies of the mooring system in sway and roll modes, with a sway eigenperiod around 28s and a roll period exactly half of this. Motions are relatively large, with peak-to-peak sway motions of up to 3m and roll motions of up to around 7. The beneficial effect of moving the vessel to the lee side of the GBS is clearly seen from Figure 8. Sway and roll motions are significantly reduced compared to those of the vessel moored on the exposed side. Maximum peak-topeak motions in sway and roll are now around 20cm and 0.5 respectively. Corresponding time series of sway and roll motion are shown in Figures 9 and 10. The response related to both sway and roll eigenperiods is clearly seen in the second half of the sway signal in Figure 9. As a preliminary assessment, it is found that a GBS-based LNG receiving terminal at the East coast of Brazil will require protection against waves, either in the form of a breakwater or by locating the berth on the landward side of the GBS. Figure 7. Comparison of vessel motion spectra from physical model tests and numerical model. Exposed side of GBS. Figure 8. Comparison of vessel motion spectra from numerical model with vessel moored on exposed and lee side of GBS, respectively. Figure 9. Time series of simulated sway motion for vessel moored on exposed and lee side of GBS. 6
Figure 10. Time series of simulated roll motion for vessel moored on exposed and lee side of GBS. 5. Operational Conditions making the right Decisions in due Time As discussed above, industry standard numerical models are excellent and accurate tools for analysis of terminal plans and design. They link meteorological and oceanographic information to impacts on moored vessels and thereby provide the metocean conditions along the berths. By use of relevant motion and safety criteria for moored vessels, numerical modelling studies can assist the designer and the terminal owner to determine the optimal protection for the operations. During the construction phase of a receiving terminal, detailed information of metocean weather windows is required for safe, efficient and economical construction. Operational metoecan forecast systems are nowadays applied for weather-sensitive operations, typically covering a two to ten days horizon. The forecast may either be based on coarse spatial resolution global or regional atmospheric and oceanographic models or high-resolution local models in areas with complex hydrography. The latter is typically the case in shallow water environments. However, also during the operation of the constructed terminal, metocean forecast systems is of increasing interest for planning purposes within the next few days. Operators and users of receiving terminals will from time to time experience downtime due to adverse weather conditions, in particularly for facilities located at exposed locations. Downtime can be associated with strong waves and currents or with difficulties encountered for pilot assistance. It can also be due to excessive motions of the vessel resulting in suspension of loading/unloading LNG. If the operator and the user receive early information on conditions for a terminal and are able to interpret this information into expected conditions for arriving vessels and loading/unloading processes, they will be able to make prudent decisions on optimal and safe operation of the terminal. Figure 11. Example of a terminal with a vessel pair attacked by incoming waves During the last 10 years, DHI has developed and operated global, regional and local metocean forecast and decision support systems (DSS) and provided operational services to a large variety of customers. Common for all these applications is the use of MIKE by DHI hydrodynamic modelling technologies, which is developed, maintained and supported by DHI. The meteorological forcing is obtained from either global or local providers. The hydrodynamic models consist typically of 2D and 3D oceanographic models for prediction of water level, current speed, water temperature, salinity and water quality parameters. For wave prediction, phase-averaged spectral models are applied. Also operational forecast of erosion/sedimentation risk has been provided. The spatial resolution of such models is in the order of 10 2 m for operational systems in shallow water. For detailed prediction of wave conditions and vessel 7
movements in ports and terminals, a much higher resolution is required, typically in order or 10 1 m. Further phaseresolving models are required in order to accurately simulate phase-depending processes such as wave diffraction and wave reflection as illustrated in Figure 11. The methodology described in Section 4 using a combination of the phase-resolving MIKE 21 BW model and WAMIT (= DHI s WAMSIM model) can in principle be executed also in operational mode and provides not only detailed information of the wave agitation but also predicts expected future ship motions, mooring forces, etc. Such information is extremely useful to incorporate in decision support systems to help the users in making even better decisions relating to operation of the ports and terminals. The typical wave forecast modelling sequence is sketched in Figure 12 for a sample application; see Kirkegaard et al (2010). Detailed wave conditions inside the port (could also be an offshore terminal complex) are computed by the time-domain Boussinesq wave model, Christensen et al (2008). This model computes the time history of waves in the entire computational domain. Ideally this computation should be done concurrently, but the computational demands make this impossible in most cases. Consequently, we adopt another strategy by using a library of conditions which link wave conditions at the port entrance to wave conditions inside the port. The details of this library the wave agitation database are such that for a specific wave condition at the port entrance (wave height, period and direction), the condition at any point inside the port can be extracted. To build the database, a large number of scenarios have to be calculated. For each of these scenarios, the corresponding movements of vessels (with predefined mooring/fender systems) can be calculated, which results in a comprehensive look-up table. This table is the engine behind the decision support system. The decision support system applies web technologies for data communication and includes event managers which send out alerts to users and can manage information about occurred events and good/bad experiences entered by users. Figure 12. Metocean forecast modelling sequence for usage in ports and terminals. From Kirkegaard et al (2010) 6. Acknowledgements Part of the work was carried out under the SAFE OFFLOAD project. The authors would like to acknowledge the support to this project under the 6 th framework programme of the European Commission (EC), Project Reference: 12560. 8
7. References BINGHAM, H.B, (2000): A hybrid Boussinesq-panel method for predicting the motion of a moored ship, Coastal Engineering Vol. 40, pp 21-38, 2000. CHRISTENSEN, E.D., JENSEN, B., MORTENSEN, S., HANSEN, H.F., KIRKEGAARD, J. Numerical Simulation of Ship Motion in Offshore and Harbour Areas, OMAE2008-57206, Proc. ASME 27 th Int. Conf. on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, 2008. HANSEN, H.F., CARSTENSEN, S., CHRISTENSEN, E.D., KIRKEGAARD, J. Multi Vessel Interaction in Shallow Water, OMAE2009-79161, Proc. ASME 28 th Int. Conf. on Offshore Mechanics and Arctic Engineering, Honolulu, Hawaii, USA, 2009. KIRKEGAARD, J., KOFOED-HANSEN, H., SLOTH, P., TACHER, E. Metocean forecasting for ports and terminals, Port Infrastructure Seminar, 22-23 June 2010, Delft, The Netherlands, 2010. 9