Current Induced Loads and Motions of Offshore Structures

Transcription

1 CURRENT AFFAIRS JIP, Project Plan V2.0 MARIN project Current Induced Loads and Motions of Offshore Structures Research Partners: Objective: 'To develop tools and guidelines to assist engineers in the assessment of current loads and effects in the different design stages.' MARIN Reference 21004, version 2.0, May 2007

2 CURRENT AFFAIRS project plan V2.0 Page 2 1. Executive Summary Objective The objective of the Current Affairs JIP is to develop tools and guidelines to assist engineers in the assessment of current loads and effects in the different design stages. Background The last few years the importance of current for the behaviour of offshore structures has become evident. The strong loop current in the Gulf of Mexico delayed installation projects and resulted in Vortex Induced Motions (VIM) of offshore platforms. It is important to note that different phases in the design require different levels of accuracy in the determination of the current induced loads and motions. In the initial design semiempirical methods (such as WINDOS) can be used, in further stages complex CFD and model tests are justified. The Current Affairs JIP wants to understand and quantify the possibilities and limitations of all these methods. The main focus of the project will be on the practical application of the available current assessment methods. DESIGN STAGE Conceptual Design Initial Design Final Design Scope of Work The scope of work for the Current Affairs JIP is divided in three work packages: WP1 Systematic model tests for mono hulls and multi column floaters WP2 Update of WINDOS semi-empirical code for current loads WP3 Benchmarking of CFD tools for current induced loads and motions of offshore structures Deliverables: A summary of the deliverables for the Current Affairs JIP is given below: Model test reports of systematic test series Model test dataset with statistics of column interaction in flow Model test dataset with current coefficients of 2 types of mono hulls Model test dataset with shielded coefficients for a side by side configuration Guidelines for model tests with multi column floaters VIM/Galloping design guidelines New version of WINDOS program (including current loads computation) Manual of the WINDOS program WINDOS Validation report for current CFD expert benchmark report CFD specialists will provide a benchmark report with: Blind CFD results and input files for a selected number of experiments An optimized result after comparison with the test results (including input files) CFD guidelines (turbulence models, grid size) based on this comparison CURRENT ASSESSMENT METHOD WINDOS (empirical shielding model) CFD (quasi-static) CFD (dynamic) Model Tests Participation fees Participation fees are foreseen of EURO 75,000 for oil companies and EURO 55,000 for others. The deadline for a Letter of Intent is 1 October The kick off is in October 2007 and the project duration is 2 years. Participants that contribute to the CFD work in WP3 will receive a EURO refund to their participation fee at the end of the project.

3 CURRENT AFFAIRS project plan V2.0 Page 3 2. Background The last few years the importance of current for the behaviour of offshore structures has been become evident. The strong loop current in the Gulf of Mexico delayed installation projects and resulted in Vortex Induced Motions (VIM) of offshore platforms. This focusses our attention to the following current induced effects: The constant current load on the offshore structures is one of the main components in the operability of DP and mooring operations [1]. Towing tests or windtunnel tests can be carried out to determine these loads, but the results of these are not available in the early phases of the design and fast calculation methods for current loads are still to be developed. The effect of VIM on single-column (Spar-type) floating structures in high currents has extensively been studied during the previous years [2-4]. This research has recently been extended to the effect of VIM on multi-column structures, such as deep draft semi-submersibles [5] and TLPs. This research confirmed that VIM should be taken into account in the design process of these structures. It is an important consideration for mooring and riser fatigue. Another observed current effect is the roll motion of semi submersibles in beam current. Lift effects on the floaters can induce galloping-type behaviour at the natural roll period. In the Offloading Operability JIP it was shown that for offloading operations (tandem and side-by-side) the interaction in the current loads is an important factor in the mooring behaviour of the combined vessels [6]. In the HAWAI JIP it was found that in shallow water bottom and basin wall effects require special attention. The complex interaction that occurs in offloading situations, also occurs for multi-column structures such as semis and TLPs: There is significant interaction between the flow around the different columns, whereas the motion of the different columns is coupled There is interaction with the flow around the floaters In addition to the Vortex Induced Motions around the columns, varying lift effects with different inflow angles relative to the floaters can have an additional (galloping) effect. More and more companies rely on Computational Fluid Dynamics (CFD) to obtain the current load coefficients for their designs. It should be noted that CFD can be a very cost efficient alternative to model tests. However, it also has its drawbacks, such as long computation times and the results can depend much on the experience of the user. On the other hand, model test results are more costly and also have their issues regarding scale effects. VIM on a semi, from [5]

4 CURRENT AFFAIRS project plan V2.0 Page 4 Besides better understanding of the value of (and the balance between) model tests and CFD there is a need for an efficient and practical method to asses current loads and shielding effects in the early stages of the design. Based on the above observations MARIN, GustoMSC and Projemar are proposing the Current Affairs JIP. The Current Affairs JIP focuses on the development of a practical design tool and the applicability of CFD and model tests to obtain a best engineering practice for current loads assessment in the various design stages. This document describes the work plan for the Current Affairs JIP. The Current Affairs JIP will be performed in co-operation with GustoMSC and Projemar Engineering. GustoMSC will be responsible for the testing and validation of the semi-empirical building block method in the new WINDOS version based on the model tests. GustoMSC will also be a partner in the CFD studies of WP3. Projemar Engineering will be involved in the CFD study. 3. Current Affairs JIP objectives The objective of the Current Affairs JIP is defined as follows: To develop tools and guidelines to assist engineers in the assessment of current effects in the different design stages It is important to note that different phases in the design require different levels of accuracy in the determination of the current induced loads and motions. In the initial design semi-empirical methods (such as WINDOS) can be used, in further stages complex CFD and model tests are justified. The Current Affairs JIP wants to understand and quantify the possibilities and limitations of all these methods. The main focus of the project will be on the practical application of the current assessment methods. DESIGN STAGE Conceptual Design Initial Design Final Design CURRENT ASSESSMENT METHOD WINDOS (empirical shielding model) CFD (quasi-static) CFD (dynamic) Model Tests 4. Scope of Work The Current Affairs JIP has 3 main work packages: WP1 WP2 A systematic series of model tests on multi-column and ship-shaped structures (MARIN) Update of the semi-empirical WINDOS program for current loads (MARIN/GustoMSC) WP3 Benchmarking of CFD tools for the determination of current induced loads and motions on offshore structures (MARIN/PROJEMAR) Chapters 5 to 7 provide a summary of the work packages for the Current Affairs JIP. The detailed scope of work is presented in Appendix A to C of this project plan.

5 CURRENT AFFAIRS project plan V2.0 Page 5 5. WP1 Systematic model tests Based on MARIN s extensive experience in current loads and VIM testing a systematic series of tests is the basis of the research in this JIP. Two main model test programs are foreseen: WP1.1 - Current loads on mono-hulls WP1.2 Current loads on multi column floaters a Systematic column wake and shielding model tests b Model tests for 3 generic multi column floater designs A dedicated test series on ship-type hulls will be performed to investigate the current loads on a box shaped floater and a tanker. The tanker will be tested with and without bilge keels to investigate the effect on the current loads in current. A captive tow test will be carried out for 0 to 180 deg inflow angles. Flow F 6dof Current shielding effects in side by side mooring situations will also be addressed in these tests. This is tested using a side by side captive setup. F 6dof Flow F 6dof Besides the mono hull ships the Current Affairs JIP will address current shielding for multi column floaters. This will be tested in a 2D tow test setup, using 2 distances between the columns. Flow Various column shapes will be tested and Particle Image Velocimetry (PIV) measurements will be used to better understand the flow:

6 CURRENT AFFAIRS project plan V2.0 Page 6 Model tests will also be carried out for multi column floaters. The following aspects can be varied systematically for multi-column structures: Column shape (square, rounded corners, circular) Ratio between the draft (T) and the column diameter or width, as this affects the correlation length of the vortex shedding Column spacing Shape of the floaters (pontoons, rings, aspect ratios of length, height and beam) Free floating tests as well as captive and forced oscillation tests will be carried out and a lot of attention will be paid to flow visualization to better understand the underlying phenomena. Measurements of the motions will be carried out, but also the loads on the separate components (columns and floaters) to determine their part in the excitation and damping. This will give vital insight in the driving mechanisms for current loads and VIM and their effect on design parameters. A recent set of dedicated in house model tests at MARIN is a good basis for the model tests within the Current Affairs JIP. The figure below (right) shows the building block model used for these tests. The figure on the left shows an interesting comparison of the VIM of a deep draft semi, a conventional (shallow draft) semi and a TLP with the same draft as the deep draft semi (but with another mass ratio). It will be clear that the ratio column diameter/draft and the mass ratio are important parameters that require further analysis VIM Sway Response TLP Deep Draft Semi Conventional Semi t A/D [-] t Ur [-] A detailed scope of work for WP1 is presented in Appendix A of this project plan.

7 CURRENT AFFAIRS project plan V2.0 Page 7 6. WP2: Update of WINDOS semi-empirical code for current loads Beside insight in the complex VIM effect, the model tests in WP2 will also provide a database of current loads on (parts of) multicolumn structures and mono hulls. These can be used as part of a fast semi-empirical method to determine mean current loads, as extension to what is presently available for wind loads in the MARIN WINDOS program. This method allows the user to quickly compute the effects of several design variations. The building block method is based on semi-empirical drag coefficients for the wind loads on the components of a construction (the so-called Building Blocks ) and uses scaling rules, shielding models and interaction models to compute the total force on the construction. Flow Cd d As part of the Current Affairs JIP the WINDOS program will be extended for the calculation of current loads on multi-column structures. This method can be used as a fast tool in the early design of a structure, as wind tunnel tests are too expensive at that stage and complex 3-D Navier-Stokes CFD calculations take to much time as well. Current Affairs participants get a new Windows WINDOS version at the start of the project (for the calculation of wind loads) and at the end of the project a final version including current loads will be delivered. Furthermore, the current coefficients databases for mono hulls from WP1 will be implemented. The database implementation will be done such that it is possible to add your own (in house) coefficients. Also, a simplified shielding model based on projected areas will be implemented to take into account shielding of vessels that are moored side by side. A detailed scope of work for WP2 is presented in Appendix B of this project plan.

8 CURRENT AFFAIRS project plan V2.0 Page 8 7. WP3: Benchmarking of CFD tools for current induced loads and motions of offshore structures Experienced CFD specialists will be invited to participate in the project in order to define an adequate and accepted benchmarking and validation procedure for their (commercial or in house) codes. This will also give these specialists the opportunity to improve their CFD models on basis of the forthcoming experimental results. Also MARIN will perform these benchmarks with its internal FRESCO code and commercial CFX code. All CFD specialists (from participants) will provide a benchmark report with: 1. Blind test results of a selected number of experiments 2. An optimized result after comparison with the test results 3. CFD guidelines (turbulence models, grid size) based on this comparison 4. The CFD input files will be delivered as well to the participants, so that they can rerun the cases when they have the commercial code available themselves. The CFD project will be separated in 3 scopes of work: Column Wake prediction and Multi Column Interaction Current loads and Multi Column VIM of a generic Semi Submersible Current loads on a mono hull As indicated above Marin will perform the benchmark study with CFX and Fresco. Other JIP participants are invited to using their specific CFD software in the benchmark study. To avoid too many repeats of the same scope of work with the same program a maximum of 3 users per program is used: Table 7.1: Matrix with CFD scope of work CFD Software CFX Fluent Star CD Other code Mono hull Current Loads (WP3.1) GustoMSC Participant 4 Participant 7 Participant 10 Column Wake Interaction (WP3.2a) Participant 2 Participant 5 Participant 8 Participant 11 Semi Sub Current Loads & VIM (WP3.2b) Participant 3 Participant 6 Participant 9 Participant 12 Like MARIN will use its dedicated FRESCO code, universities can also join as participants with their own code A detailed scope of work for WP3 is presented in Appendix C of this project plan.

9 CURRENT AFFAIRS project plan V2.0 Page 9 8. Deliverables Below an overview is given of the deliverables of the Current Affairs JIP: Model test reports of systematic test series Model test dataset with statistics of column interaction in flow Model test dataset with current coefficients of 2 types of mono hulls (box shaped and tanker) Model test dataset with shielded coefficients for a side by side configuration Guidelines for model tests with multi column floaters VIM/Galloping design guidelines New version of WINDOS program (including current loads computation) Manual of the WINDOS program WINDOS Validation report for current CFD expert benchmark report CFD specialists will provide a benchmark report with: Blind CFD results and input files for a selected number of experiments An optimized result after comparison with the test results (including input files) CFD guidelines (turbulence models, grid size) based on this comparison 9. Cost Preliminary budgets General Management EURO 50,000. = WP1 Systematic Model Tests EURO 550,000.= WP2 Development and validation of WINDOS for current EURO 120,000.= WP3 CFD modeling and guidelines (excluding in kind contributions) EURO 200,000.= Total EURO 920,000.= In kind CFD work as presented in table 7.1 is not taken into account in the above budgets. The above cost for WP3 includes: CFD state of the art review CFD work carried out within MARIN (CFX and Fresco computations for WP3.1 and WP3.2, see table 7.1) CFD expert guidelines, benchmark report and management of WP3 Participation fees The participation fees for the Current Affairs JIP are as follows: Oil companies EURO 75,000.= Other companies EURO 55,000.= Participants that carry out in kind CFD work will receive a discount to the participation fee of Euro. 10. Contract The contract for the Current Affairs JIP will be similar to the HAWAI JIP. A draft Letter of Intent is attached at the end of this document.

10 CURRENT AFFAIRS project plan V2.0 Page Schedule The deadline for a Letter of Intent is 1 October The kick off is in October 2007 and the project duration is 2 years. The schedule of the three work packages is shown below Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Startup WP1 (Model Tests) Monohull Ships (WP1.1) Column Only 2D Model Tests (WP1.2a) Multi Column Floater Drag & VIM (WP1.2b) Development of Model Test Guidelines WP2 (WINDOS) Analysis of SBS Shielding (WP2.1a) Implementation of SBS Shielding Validation of WINDOS (SBS Shielding) Manual and validation report Analysis of Column Shielding (WP2.2a) Implementation of Column Shielding Validation of WINDOS (Column Shielding) Validation of WINDOS (Multi Column Floater, WP2.2b) WP3 (CFD) Blind Mono hull CFD computations (WP3.1) Best CFD Model Mono hull computations (WP3.1) Blind CFD 2D Column Only cases (WP3.2a) Best CFD Model 2D Column Only cases (WP3.2a) Blind CFD Multi Column Floaters (WP3.2b) Best CFD Model Multi Column Floaters (WP3.2b) Development of CFD Guidelines 12. Contact Information If you are interested in the Current Affairs JIP, please contact: Olaf Waals: o.waals@marin.nl, Bas Buchner: b.buchner@marin.nl, References [1] Hydrodynamic Research Topics for DP Semi Submersibles, J.L. Cozijn, B. Buchner, R.R.T. van Dijk, OTC 10955, OTC 1999 [2] "The Effect of Mooring System and Sheared Currents on Vortex Induced Motions of Truss Spars", R.R.T. van Dijk, A. Voogt, P. Fourchy and S. Mirza, OMAE , Cancun, Mexico. [3] Model Test Experience on Vortex Induced Vibrations of Truss Spars, Radboud van Dijk, Allan Magee, Technip Offshore, Inc., Steve Perryman, Joe Gebara, OTC 15242, 2003 [4] Truss Spar Vim In Waves And Currents, Tim Finnigan, Mehernosh Irani, Radboud van Dijk, OMAE , Halkidiki [5] "Vortex Induced Motions of Semi Submersible with Four Square Columns", O. Rijken, S. Leverette and K. Davies, DOT 2004, New Orleans, USA. [6] A New Method to Optimise the Operability of Deepwater Offloading Operations, Hielke Brugts, Jos van Doorn, Tim Bunnik and Bas Buchner, DOT 2005 [7] Flow Induced Motions of Multi Column Floaters", Olaf Waals, Amal Phadke, Stephen Bultema, OMAE 2007

11 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 11 WP1 Systematic model tests (MARIN) Objective The objective of WP1 is to obtain a consistent data set for current loads on mono hull and multi column offshore floaters. The dataset will be used to better understand the involved physics of the flow and the results will be used in WP3 as validation material for CFD studies and in WP2 for the development of an empirical model to predict current loads in the early stages of the design. Methodology Based on MARIN s extensive experience in current loads and VIM testing a systematic series of tests is the basis of the research in this JIP. Two main model test programs are foreseen: WP1.1 - Current loads on mono-hulls WP1.2 Current loads on multi column floaters a Systematic column wake and shielding model tests b Model tests for multi column floater designs The main purpose of these tests is to obtain reliable data for CFD validation and benchmarking. The model tests will be used to obtain more insight in the involved physics using state of the art measurement techniques such as Particle Image Velocimetry (PIV) measurements. The sections below describe the scope of each model test series in more detail.

12 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 12 WP1.1 - Current loads on mono-hulls Objective The overall objective of WP1.1 is to better understand current loads and shielding for mono-hull floaters. In this work package the current loads on two separate ship type floaters will be investigated using model tests. Furthermore, shielding between the two ships will be tested. This is done for a typical side by side offloading situation in a captive towing test. The results will be used to validate an empirical shielding model (WP2) and CFD studies with various commercial codes (WP3). Background For many offshore projects current loads and interaction of current effects between ship type floaters are important for the workability. An example of this is the current shielding between an FPSO and shuttle tanker in offloading situations. For side by side offloading situations the total current loads (moments) on the combination of a turret moored FPSO/FSRU and shuttle tanker are important. This is because, in combination with wind and waves, the current loads determine the mean heading in the environment during offloading. At present, simplified shielding models based on shielded area and incoming current direction are used to take into account the shielding in side by side offloading situations. These models need verification to ensure reliable workability analysis results. Methodology The current loads on two mono hulls will be measured separately in a captive setup. The models will be mounted to the carriage with a 6 component force measurement frame. From this setup the current loads will be obtained by towing the model through the basin. The current loads will be measured for 0 (stern) to 180 deg (bow) incoming current angle. 1. Captive Tow test for box shaped floater (0 to 180 deg, 10 deg steps). The first mono hull that will be tested is a box shaped floater (FSO/FSRU). Since this vessel has a sharp cornered blunt shape the wake behind it will be significant. Therefore the shielding effects for a tanker in close proximity will be evident. Flow F 6dof

13 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page Captive Tow test for tanker (0 to 180 deg, 10 deg steps). The second mono hull that will be tested for current loads is a tanker. The shape of this vessel is more streamlined than the first hull shape, which makes the prediction of current loads for small angles from the bow using CFD more complicated because the separation point of the flow is not well defined and needs to be calculated correctly (see WP3). Two drafts will be tested. Flow F 6dof For 1 draft, the tests with the tanker will be carried out with and without bilge keels. The bilge keel affects the location of the separation point of the flow and thereby the total drag load in beam on conditions. 3. Captive Tow test for Side by Side configuration (0 to 360 deg, 15 deg steps). When the current loads on both mono hulls are known separately the shielding effects can be measured by towing the two vessels together in side by side configuration. The 6 d.o.f. forces will be measured on each vessel separately, such that changes in the current coefficients due to the presence of a second vessel can be detected. For this configuration 2relative drafts will be tested. F 6dof Flow F 6dof 4. For the side by side tests a second series is foreseen with relatively larger size of the box shaped floater (0 to 360 deg, 15 deg steps, 1 draft assumed). F 6dof Flow F 6dof

14 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 14 Test Matrix The following test matrix is foreseen for WP1.1: Mono hull type Configuration Angle [deg] Purpose Box shape captive 0 and 90 Reynolds Check Box shape captive (10 deg steps) Current coeff. Tanker captive 0 and 90 Reynolds Check Tanker (w.o. bilge keel) captive (10 deg steps) Current coeff. Tanker (w.o. bilge keel) captive (2 nd draft) (10 deg steps) Current coeff. Tanker (with bilge keels) captive (10 deg steps) Current coeff. Box+ Tanker side by side (captive) (15 deg steps) Shielding effects Box+ Tanker side by side (2 nd draft) (15 deg steps) Shielding effects Box (large)+ Tanker side by side (captive) (15 deg steps) Shielding effects Deliverables WP1.1 Below an overview is given of the deliverables in WP1.2b of the Current Affairs JIP: Model test report and data DVD (MARIN) Current coefficients for the FPSO/FSRU alone (MARIN) Current coefficients for the tanker alone (MARIN) Shielded current coefficients for the side by side configuration (MARIN) The head on current coefficients will be delivered with and without correction for scale effects Options The presented scope is preliminary. Options such as described below can be discussed at the informative meeting. More bilge keel configurations could be tested (e.g. 2 bilge keel lengths and/or heights) More relative positions between the FPSO/FSRU and the tanker could be tested. (distance, relative x position). The side by side configuration could be tested at various water depths. (shallow water)

15 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 15 WP1.2a Systematic wake and shielding model tests for multiple columns Objective The objective of these tests is to better understand the shielding between columns that are placed behind each other. A tow test setup will be used where a cylinder is towed horizontally through the basin. First, the wake behind a single column will be measured at 3 positions in the wake using PIV measurements. Then a second column will be tested at 2 positions behind the first column. The forces will be measured on each column individually. Background For present CFD applications it is a challenge to compute the wake behind a column and the interaction with a downstream column. These interactions will affect the total current load and VIM behavior of the floater. The proposed tests are required to gain better insight in the flow behavior. Furthermore, the test results will be used in WP2 to validate an empirical shielding model and in WP3 to understand the possibilities (and limitations) of various CFD methods. Methodology Four column types will be tested in a 2D flow the towing setup in the high speed basin: D1 D2 D3 For each of the above column types, the flow velocity distribution will be measured behind a single column at 3 positions: <1D 3D 4D Flow The first measurement position is chosen such that the flow separation from the cylinder can be observed (<1D). The second and third positions are chosen at a location where a second column may be present (3D, 4D behind the first column). Then, a second column will be tested at two positions behind the first column: Flow 3D Since the wake measurement and the positions of the second column will coincide, the effect of the flow disturbance of the first column on the second column can be investigated. This flow shielding effect is important for the total load on a multi column floater. For this test configuration the PIV measurement will be done on the downstream cylinder. 1D

16 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 16 In addition, the squared and rounded corner columns will also be tested for a second orientation as shown in the figure below. Flow 3D 2D The rounded corner columns will also be tested for two offsets in transverse direction: 3D 2D Flow The next page shows a summary table of the tests that will be carried out in WP1.2a

17 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 17 Test Matrix The table below shows the test matrix that is foreseen for WP1.2a: Column shape Wake measurements Position of downstream cylinder Orientation Cylindrical 1D - - 3D - - 4D - - downstream cylinder 3D - downstream cylinder 4D - Square 1D - 45 deg 3D - 45 deg 4D - 45 deg downstream cylinder 3D 45 deg downstream cylinder 4D 45 deg 1D - 0 deg 3D - 0 deg 4D - 0 deg downstream cylinder 3D 0 deg downstream cylinder 4D 0 deg Rounded Corners (RC) 1D - 45 deg 3D - 45 deg 4D - 45 deg downstream cylinder 3D 45 deg downstream cylinder 4D 45 deg 1D - 0 deg 3D - 0 deg 4D - 0 deg downstream cylinder 3D 0 deg downstream cylinder 4D 0 deg Transverse offset 1 1D - 0 deg 3D - 0 deg 4D - 0 deg downstream cylinder 3D 0 deg downstream cylinder 4D 0 deg Transverse offset 2 1D - 0 deg 3D - 0 deg 4D - 0 deg downstream cylinder 3D 0 deg downstream cylinder 4D 0 deg The above test matrix will be carried out for 3 tow speeds. As shown in the table below the Reynolds number of a D=15 m cylinder will vary between 0 and 3.1E7 for 0 to 4 kn current speed. To obtain information at lower Reynolds numbers (for which model tests and CFD computations are generally carried out), the following velocities will be tested for the above test matrix: Prototype Model Vtow Vtow Re Vtow Re kn m/s m/s 1 0,51 7,72E+06 0,07 2,18E+04

18 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page ,03 1,54E+07 0,15 4,37E ,06 3,09E+07 0,29 8,73E+04 From previous studies it is known that the drag loads and wake of cylinders and rounded columns depend on the Reynolds number. The Reynolds number for the prototype column is a factor 100 to 1000 higher than at model scale for the above tests. Therefore the cylindrical and rounded corner columns (except for the transverse offset tests) will also be tested at a higher model scale Reynolds number. Practical limitations of the test setup allow a maximum Reynolds number of about 1.5E6. Although this is still a factor 10 smaller than the prototype Reynolds number at 2 kn, it is widely accepted that the Reynolds dependency of the drag is limited above Re 1.5E6. The graph below also shows this: Deliverables WP1.2b Below an overview is given of the deliverables in WP1.2b of the Current Affairs JIP: - Current loads on 4 types of columns - Shielding factors for 2 distances - PIV measurements showing the velocity field at 3 positions behind the column - Model test report and data DVD Options Optionally, the test program can be carried out for other column shapes (e.g. rectangular column): <1D 3D 4D Flow

19 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 19 WP1.2b Model test series for 3 generic multi column floater designs Objective The objective of the model tests with 3 generic multi column floater designs is to obtain realistic design data that can be used for validation of the empirical load model that is developed in WP2. The prediction of VIM response using CFD will be addressed in WP3. The focus of the test program will be on: Total floater drag Contribution of separate columns and pontoons to total drag Flow induced motions (VIM, galloping) Changes in behavior due to variations in geometry Background Deep draft multi column floaters show Vortex Induced Motion (VIM) and galloping type of behavior [7]. Recent model tests for a sharp cornered study model show a significant difference in response for varying draft and mass ratio. The amount of response may also depend on the sharpness of the column corners and the column distance. In WP1.2b the VIM behavior of various multi column shapes is investigated. The model test results will be used to obtain guidelines for multi column design regarding VIM and galloping. The measured drag loads will be used to validate the WINDOS tool for current that is developed in WP2. The presented scope of work is preliminary and will be discussed at the informative meeting April 27 th Methodology Free floating tests as well as captive and forced oscillation tests will be carried out. A typical test program is described below: 1 Captive tow tests To obtain the total floater drag a captive test will be used. The model of the multi column floater will be towed and the forces between the carriage and the model will be measured at 19 angles (from 0 to 180 deg, 10 deg steps). Two of the columns will be instrumented to measure the individual loads on these columns. From these measurements the contribution to the total drag of each column can be derived. Typical configurations that can be tested are summarized below: 3D Configuration 1: Sharp cornered, 3D column distance 3D Configuration 2: Rounded cornered, 3D column distance

20 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 20 4D 4D Configuration 3: Sharp cornered, 4D column distance Configuration 4: Rounded cornered, 4D column distance 4D 4D Configuration 5: Sharp cornered, 4D column distance, ring shaped Configuration 6: Rounded cornered, 4D column distance, ring shaped 4D 4D Configuration 7: Sharp cornered, 4D column distance, truncated columns only Configuration 8: Rounded cornered, 4D column distance, truncated columns only

21 CURRENT AFFAIRS project plan V1.0 APPENDIX A: WP1 Page 21 2 VIM tow tests (free floating) For the same configurations as in the captive tow tests, VIM tow tests can be carried out for 3 angles (0, 45 and 90 deg tow direction) and 5 Urs. For this test the multi column floater will be moored in a soft spring mooring system. Measurements of the motions will be carried out, but also the loads on the separate components (columns and floaters) to determine their part in the excitation and damping. This will give vital insight in the driving mechanisms for current loads and VIM and their effect on design parameters. For the VIM tests it is important to determine the sensitivity to certain design parameters. Draft and mass ratio may be varied systematically to investigate the effect on VIM response. From these test results design guidelines for VIM behavior can be derived. 3 Forced oscillation tow tests Forces oscillation tow tests will be carried out for the same 3 angles (0, 45 and 90 deg tow direction) to investigate the added mass, damping and excitation for the same 5 Urs. These tests will be carried out for 3 amplitudes. Together with the free floating VIM tow test this will deliver a unique dataset. The data from the forced oscillation tests may be used to predict the VIM amplitude using a method similar to an approach that is used for riser VIV. The motion amplitudes that are found from this method can be validated against the free floating VIM tow tests. Test Matrix The following test matrix is foreseen for WP1.2b: Test Type Config Angle [deg] A/D range Ur rang e Purpose Captive tow TBD (10 deg steps) - - Current loads Captive tow TBD (10 deg steps) - - Current loads Captive tow TBD (10 deg steps) - - Current loads Captive tow TBD (10 deg steps) - - Current loads Captive tow TBD (10 deg steps) - - Current loads Soft spring tow TBD 0, 45 and 90 deg VIM tests Soft spring tow TBD 0, 45 and 90 deg VIM tests Soft spring tow TBD 0, 45 and 90 deg VIM tests Soft spring tow TBD 0, 45 and 90 deg VIM tests Soft spring tow TBD 0, 45 and 90 deg VIM tests Forced Oscillation TBD 0, 45 and 90 deg Determine added mass, damping and flow excitation Forced Oscillation TBD 0, 45 and 90 deg Determine added mass, damping and flow excitation Forced Oscillation TBD 0, 45 and 90 deg Determine added mass, damping and flow excitation Forced Oscillation TBD 0, 45 and 90 deg Determine added mass, damping and flow excitation Forced Oscillation TBD 0, 45 and 90 deg Determine added mass, damping and flow excitation Deliverables WP1.2b Below an overview is given of the deliverables in WP1.2b of the Current Affairs JIP: Model test report and data DVD (MARIN) Model test dataset with statistics of column interaction in flow (MARIN) Guidelines for model tests with multi column floaters (MARIN) VIM/Galloping design guideline (MARIN) Options The variations in geometry that may be of interest are as follows: Ratio between the draft (T) and the column diameter or width 2 nd Roughness of the columns Shape of the floaters (pontoons, rings, aspect ratios of length, height and beam)

22 CURRENT AFFAIRS project plan V1.0 APPENDIX B: WP2 Page 22 WP2 Update of WINDOS semi-empirical code for current loads (MARIN/GUSTO MSC) Objective The objective of WP2 is to develop and validate a tool that computes the mean current loads on offshore floaters. The tool will be based on the 'building block' program WINDOS that is currently used for computing wind loads. The main deliverable of WP2 is a program that uses a fast semi-empirical method to determine mean current loads in the early design stages. Background As part of the Current Affairs JIP the WINDOS program will be extended for the calculation of current loads on multi-column structures. This method can be used as a fast tool in the early design of a structure, as wind tunnel tests are too expensive at that stage and complex 3-D CFD calculations take to much time as well. Current Affairs participants get a new Windows WINDOS version at the start of the project (for the calculation of wind loads) and at the end of the project a final version including current loads will be delivered. Methodology In WP2 we will develop an empirical shielding model for current loads, as extension to what is presently available for wind loads in the MARIN WINDOS program. This method allows the user to quickly compute the effects of several design variations. The building block method is based on semi-empirical drag coefficients for the wind loads on the components of a construction (the so-called Building Blocks ) and uses scaling rules based on Reynolds number. Flow Cd d The shielding between two columns is calculated based on the size and Cd of the upstream column and the distance (d) to the downstream column. The tool will be developed in cooperation between MARIN and GustoMSC to ensure a user friendly program that can be used in daily design practice. The ship database will be implemented such that it can be expanded with new (in house) model test data. The main four tasks for this scope of work are described below: 1. Investigation for 'public domain' mono hull coefficients (MARIN) As a first task in this scope of work it will be investigated if there are current coefficients available (from literature) to include in the program. Participants will be asked if they have data that they can share 2. Analysis and Implementation of column shielding (MARIN) The model test results for column shielding in WP1.2a will be used to implement a current shielding model similar as used for wind loads in the present WINDOS version.

23 CURRENT AFFAIRS project plan V1.0 APPENDIX B: WP2 Page Validation of current loads (GustoMSC) When the column shielding model is implemented the mean current loads from the model tests will be used to validate the Windos program. Other data (such as from the literature or CFD results) may also be used to compare with Windos. An accuracy level of approximately 15% is foreseen in the comparison. 4. Implementation of total current loads and shielding model on 2 mono hulls (GustoMSC) The shielding between to tankers can be computed using a simplified method based on projected shielding area. This method will be implemented in WINDOS and validated against the model test data of WP1.1. Deliverables WP2 Below an overview is given of the deliverables in WP2 of the Current Affairs JIP: Literature study towards available current coefficients for mono hulls (MARIN) New version of WINDOS program including current loads (MARIN) Manual and guidelines for the WINDOS program (GustoMSC) Validation report for (shielded) current loads (GustoMSC)

24 CURRENT AFFAIRS project plan V1.0 APPENDIX C: WP3 Page 24 WP3 Benchmarking of CFD tools for current induced loads and motions of offshore structures (MARIN/PROJEMAR) Objective The objective of WP3 is to provide the participants with state of the art knowledge for the application of CFD for offshore floaters. The possibilities (and limits) of commercial CFD codes will be investigated for the model test cases carried out in WP1. Special attention will be paid to practical issues such as for example computation times and effort it takes to generation grids. It should be noted here that the development of new CFD codes is not part of the Current Affairs scope. The project focuses on validating and benchmarking of already existing and commercially available codes. The purpose of these benchmarking studies is to assist engineers in their work if they use CFD for their designs. The outcome will also help to define a scope of work for CFD in commercial projects. The results and the input files of the CFD expert studies will be delivered to the participants, so that they can rerun the cases when they have the commercial code available themselves. Guidelines for the use of CFD in offshore designs will be provided. Background For present CFD applications it is a challenge to compute the wake behind a column and the interaction with a downstream column. The results of CFD depend on number of parameters: CFD type (VOF, FEM, DNS) Grid size Turbulence model Numerical solver Boundary conditions These parameters can strongly influence the outcome of CFD. It is important to quantify the sensitivities of CFD to the input and provide transparent benchmark cases that are validated against model tests. Methodology At the start of this work package a state of the art literature review will be produced. This is to make sure that the work in WP3 is relevant to the industry and will make use of knowledge that is already available. Experienced CFD specialists will be invited to participate in the project in order to define an adequate and accepted benchmarking and validation procedure for their (commercial or in house) codes. This will also give these specialists the opportunity to improve their CFD models on basis of the forthcoming experimental results. Also MARIN will perform these benchmarks with its internal FRESCO code and commercial CFX code. CFD Software CFX Fluent Star CD Other code Mono hull Current Loads (WP3.1) GustoMSC Participant 4 Participant 7 Participant 10 Column Wake Interaction (WP3.2a) Participant 2 Participant 5 Participant 8 Participant 11 Semi Sub Current Loads & VIM (WP3.2b) Participant 3 Participant 6 Participant 9 Participant 12

25 CURRENT AFFAIRS project plan V1.0 APPENDIX C: WP3 Page 25 Three tasks are foreseen for each part of the CFD scope: 1. 'Blind' CFD computations model test cases The model tests that are carried out in WP1 will first be computed before the tests results are known. This 'blind' approach provides the participants with the uncertainty level that may be expected from CFD results. 2. Optimized CFD computations model test cases Then, after the model test results are known the CFD results will be optimized. The changes to the original input in step 1 will be documented, such that guidelines for future computations will be available. The computations in step 1 and 2 will be done for the model scale Reynolds numbers. For the column shielding tests (WP1.2a) the CFD computations will be carried out for both the low and high Reynolds number to investigate the differences in flow behavior. 3. Sensitivity analysis to input parameters: Generally the solution from CFD and effort it takes to obtain an answer is sensitive to a number of input parameters: Type of CFD (RANS, DES, DNS) Grid size Turbulence model Numerical solver These parameters will be documented and if required sensitivity checks will be carried out. A typical example is grid refinement study for drag on a circular column. The focus of the CFD study will be on reproducing the best possible result with present knowledge and CFD models. Most important is a thorough documentation of the modeling process. The choices that are made will be documented and reported with the input files, such that a guideline for best CFD modeling practice will be the main deliverable of this work package. To get the required documentation information from the CFD experts a questionnaire is being prepared that provides a minimum documentation level for the work. The required work for each of the CFD participants is presented below. Note: the final required flow angles an Reynolds numbers will be discussed and decided in the participant meeting. Deliverables WP3 Below an overview is given of the deliverables in WP3: State of the art CFD methods review (MARIN/PROJEMAR) CFD expert benchmark report including guidelines for CFD use (MARIN) All CFD specialists will provide a benchmark report with: Blind CFD results of a selected number of experiments (including input files) An optimized result after comparison with the test results (including input files) CFD guidelines (turbulence models, grid size) based on this comparison The minimum scope in each of the CFD tasks that will be carried out in WP3 is described below

26 CURRENT AFFAIRS project plan V1.0 APPENDIX C: WP3 Page 26 WP3.1 CFD scope mono hull floaters In WP3.1 three mono hull configurations will be studied, which are the same as in the model test WP1.1: Barge Tanker Side by Side The minimum required scope of work for each of the configurations is discussed below. The results and computation methods that are used will be documented in a report template provided by MARIN. The CFD participants will apply CFD using their best knowledge and/or common practice. The modeling choices (e.g. turbulence model, wall functions, free surface boundary condition, time to prepare the grid, computation time etc.) will be documented in the report. The CFD computations will be carried out before the model test result is known. After comparison the results with the model test the runs may be optimized when needed. Changes to the input and improved results after comparison with the model test will be described in a separate chapter of the CFD report. Tanker (10 cases total) For the tanker at least the following computations will be carried out: -4 grids for 1 angle (to show grid-independent results) -5 angles (different grids when needed) -2 Reynolds numbers (Model scale and Full scale for 1 case) -1 case with and without bilge keels (beam current) Barge (3 cases total) For the barge at least the following computations will be carried out: -1 Reynolds number (model scale) -3 flow angles (e.g. 0, 30 and 45 deg) Side by Side (1 case, exploratory) For the side by side configuration at least the following computations will be carried out: -1 angle (e.g. 180 deg) Reporting and deliverables -Reply questionnaire with modeling choices -All input files needed to repeat a run: -grids -input files -Output: - Forces (time series if computation is unsteady) - Velocity field (several time steps if computation is unsteady) - Vorticity field (several time steps if computation is unsteady) - Graph with Y+ distribution - Plot showing iterative convergence - Grid pictures Note: the total output will be limited to relevant data to avoid too much data in the deliverables. -All the above will be documented in a standard report template, which will be proposed and discussed in the participant meeting prior to the start of the CFD work.

27 CURRENT AFFAIRS project plan V1.0 APPENDIX C: WP3 Page 27 WP3.2a CFD study 2D column flow In WP3.2a the 2D cylinder flow will be studied with CFD. The cross sections of the cylinders that are used are the same as in the model test WP1.2a. The minimum required scope of work for each of the configurations is discussed below. The results and computation methods that are used will be documented in a report template provided by MARIN. The CFD participants will apply CFD using their best knowledge and/or common practice. The modeling choices (e.g. turbulence model, wall functions, free surface boundary condition, time to prepare the grid, computation time etc.) will be documented in the report. The CFD computations will be carried out before the model test result is known. After comparison the results with the model test the runs may be optimized when needed. The comparison and changes to the input and improved results after comparison with the model test will be described in a separate chapter of the CFD report. Single cylinder - 2D flow - Unsteady flow - At least one grid variation study (4 grids assumed) - Compare velocity field near and behind cylinder with PIV -The following shapes will be computed: Round (2 Reynolds numbers) Square (0 deg, 1 Reynolds numbers) Square (45 deg, 1 Reynolds numbers) Rounded corner (0 deg, 2 Reynolds numbers) Rounded corner (45 deg, 2 Reynolds numbers) Two cylinders (shielded flow) -2D flow -Unsteady flow -Compare velocity field near downstream cylinder with PIV -1 grid (finest possible) -1 (most realistic) shape will be used for the shielded flow Reporting and deliverables -Reply questionnaire with modeling choices -All input files needed to repeat a run, e.g: -grids -input files -Output: - Forces (time series if computation is unsteady) - Velocity field (several time steps if computation is unsteady) - Vorticity field (several time steps if computation is unsteady) - Graph with Y+ distribution - Plot showing iterative convergence - Grid pictures Note: the total output will be limited to relevant data to avoid too much data in the deliverables. -All the above will be documented in a standard report template, which will be proposed and discussed in the participant meeting prior to the start of the CFD work.

28 CURRENT AFFAIRS project plan V1.0 APPENDIX C: WP3 Page 28 WP3.2b CFD study 3D multi column and VIM In WP3.2b the 3D multi column flow will be studied with CFD. This will be done both captive, in forced oscillation and for the VIM tests that are carried out in WP1.2b The minimum required scope of work for each of the configurations is discussed below. The results and computation methods that are used will be documented in a report template provided by MARIN. The CFD participants will apply CFD using their best knowledge and/or common practice. The modeling choices (e.g. turbulence model, wall functions, free surface boundary condition, time to prepare the grid, computation time etc.) will be documented in the report. The CFD computations will be carried out before the model test result is known. After comparison the results with the model test the runs may be optimized when needed. The comparison and changes to the input and improved results after comparison with the model test will be described in a separate chapter of the CFD report. Captive(5 cases) - 3D flow (1 Reynolds number) - 2 column configurations distances (3D and 4D distances between the columns, see WP1.2b) - Square versus rounded corner columns - 2 nd Reynolds number for 1 case Forced oscillation (5 cases, 1 config: e.g. rounded columns) - 3 motion amplitudes (at 1 period) - 3 periods (at 1 amplitude) - 1 headings (e.g. 45 deg) Free oscillation (5 cases, 1 config: e.g. rounded columns) - 1 mooring stiffness - 5 current speeds (1 heading e.g. 45 deg) - 1 columns only case Reporting and deliverables -Reply questionnaire with modeling choices -All input files needed to repeat a run, e.g: -grids -input files -Output: - Forces (time series if computation is unsteady) - Motions (for VIM tests) - Velocity field (several time steps if computation is unsteady) - Vorticity field (several time steps if computation is unsteady) - Graph with Y+ distribution - Plot showing iterative convergence - Grid pictures Note: the total output will be limited to relevant data to avoid too much data in the deliverables. -All the above will be documented in a standard report template, which will be proposed and discussed in the participant meeting prior to the start of the CFD work.