ABS 249-2017 GUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES.pdf

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1、Guidance Notes on Air Gap Analysis for Semi-Submersibles GUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES MARCH 2017 American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 2017 American Bureau of Shipping. All rights reserved. ABS Plaza 16855 Northchase

2、Drive Houston, TX 77060 USA Foreword Foreword These Guidance Notes supplement the Rules and Guides that ABS has issued for the classification of semi-submersibles, which refer to column-stabilized drilling units, column-stabilized units and column-stabilized installations. Unless the upper structure

3、 and deckhouses are satisfactorily designed for wave impact, ABS classification requirements mandate a reasonable air gap between the deck structures and the wave crests for all afloat modes of operation, taking into account the predicted motion of the semi-submersible relative to the surface of the

4、 sea. Calculations, model test results, field measurements or the combinations of these methods may be used to determine the appropriate air gap. These Guidance Notes address the air gap analysis methods and design considerations. These Guidance Notes provide detailed procedures for the air gap anal

5、ysis for semi-submersibles. The suggested method is based on direct analysis of vessel motions and wave surface elevations using the linear diffraction-radiation analysis or the second order analysis. Additional considerations may be needed for a particular case, especially when a novel design or ap

6、plication is being assessed. These Guidance Notes should not be considered mandatory, and in no case are these Guidance Notes to be considered a substitute for the professional judgment of the designer or analyst. In case of any doubt about the application of these Guidance Notes, ABS should be cons

7、ulted. These Guidance Notes become effective on the first day of the month of publication. Users are advised to check periodically on the ABS website www.eagle.org to verify that this version of these Guidance Notes is the most current. We welcome your feedback. Comments or suggestions can be sent e

8、lectronically by email to rsdeagle.org. Terms of Use The information presented herein is intended solely to assist the reader in the methodologies and/or techniques discussed. These Guidance Notes do not and cannot replace the analysis and/or advice of a qualified professional. It is the responsibil

9、ity of the reader to perform their own assessment and obtain professional advice. Information contained herein is considered to be pertinent at the time of publication, but may be invalidated as a result of subsequent legislations, regulations, standards, methods, and/or more updated information and

10、 the reader assumes full responsibility for compliance. This publication may not be copied or redistributed in part or in whole without prior written consent from ABS. ii ABSGUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES .2017 Table of Contents GUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-S

11、UBMERSIBLES CONTENTS SECTION 1 Introduction 1 1 General . 1 3 Definition of Air Gap 1 5 Air Gap Design Considerations 2 7 Air Gap Analysis Methods. 2 7.1 Linear Air Gap Analysis . 2 7.3 Second Order Time-Domain Air Gap Analysis 3 7.5 CFD-Based Air Gap Analysis 3 7.7 Model Test . 3 9 Field Points for

12、 Air Gap Analysis 3 11 Scope and Overview of These Guidance Notes . 4 13 Symbols 4 FIGURE 1 Air Gap Definition 1 FIGURE 2 Field Points in Grid Pattern . 3 FIGURE 3 Overall Procedure for Air Gap Assessment 7 SECTION 2 Motion Responses of Semi-submersibles . 8 1 Definition of Motions . 8 3 Diffraction

13、-Radiation Analysis . 9 3.1 Panel Model Development 9 3.3 First Order Hydrodynamic Load 9 3.5 Second Order Hydrodynamic Load . 10 5 First Order Wave Frequency Motion Analysis in Frequency Domain 11 5.1 Structural Mass Matrix . 11 5.3 Hydrostatic Stiffness Matrix . 11 5.5 Natural Periods 11 5.7 Visco

14、us Damping 12 5.9 Motion RAO . 13 7 Second Order Low Frequency Motion Analysis in Frequency Domain 14 9 Motion Analysis in Time Domain 15 11 Motion Analysis Using the Combined Time Domain and Frequency Domain Approach . 15 13 Vertical Motions at Given Points . 15 ABSGUIDANCE NOTES ON AIR GAP ANALYSI

15、S FOR SEMI-SUBMERSIBLES .2017 iii 15 Motion Characteristics of Semi-submersibles . 16 TABLE 1 Typical Natural Periods for Deep Water Semi-submersibles 12 FIGURE 1 Definition of Motions 8 FIGURE 2 Typical Added Mass and Radiation Damping in Heave Mode . 9 FIGURE 3 Second Order Wave Force Components 1

16、0 FIGURE 4 Typical Heave RAO (Amplitude) under Different Levels of Heave Viscous Damping . 13 FIGURE 5 RAO Components . 14 SECTION 3 Free Surface Elevation . 17 1 Linear Free Surface Elevation 17 3 Wave Asymmetry Factor . 17 5 Quadratic Transfer Function of Free Surface Elevation . 18 7 Wave Run-up

17、Factor . 18 SECTION 4 Linear Air Gap Analysis . 19 1 Relative Surface Elevation RAO . 19 3 Environmental Conditions . 20 3.1 Wave Spectrum . 20 3.3 Sea State . 20 3.5 Wave Scatter Diagram and Rosette 20 3.7 Wave Spreading 20 3.9 Wind and Current. 20 5 Extreme Relative Surface Elevation . 20 5.1 Shor

18、t-term Extreme Value Analysis . 21 5.3 Long-term Extreme Value Analysis 21 5.5 Consideration of Mean and Low Frequency Vertical Motions 21 7 Summary of Linear Air Gap Analysis Procedures 22 7.1 General 22 7.3 Key Steps for Linear Air Gap Analysis . 22 FIGURE 1 Relative Surface Elevation Time Histori

19、es 19 FIGURE 2 Flowchart of Linear Air Gap Analysis 24 SECTION 5 Second Order Time Domain Air Gap Analysis . 25 1 Applications . 25 3 Responses to Irregular Waves 25 5 Time Histories of Relative Surface Elevation 27 5.1 Time Histories of Vertical Motions at Field Points 28 5.3 Time Histories of Free

20、 Surface Elevation at Field Points . 28 7 Prediction of Extreme Relative Surface Elevation in Time Domain 29 iv ABSGUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES .2017 FIGURE 1 Illustrating the Decomposition of Wave Spectrum 26 FIGURE 2 Wave Profile Time Histories Generated by Different Ra

21、ndom Seeds . 27 APPENDIX 1 References 30 ABSGUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES .2017 v This Page Intentionally Left Blank Section 1: Introduction SECTION 1 Introduction 1 General The term Semi-Submersibles used in these Guidance Notes refers to: Column-Stabilized Drilling Units

22、 defined in the ABS Rules for Building and Clasing Mobile Offshore Drilling Units (MODU Rules) Column-Stabilized Units defined in the ABS Guide for Building and Clasing Mobile Offshore Units (MOU Guide) Column-Stabilized Installations in the ABS Rules for Building and Clasing Floating Production Ins

23、tallations (FPI Rules). A semi-submersible is a column-stabilized floating offshore structure, which consists of a deck structure with large diameter support columns attached to submerged pontoons. In the design and operation of semi-submersibles, the air gap as defined in Subsection 1/3, also known

24、 as the deck clearance, is an important design parameter. A low air gap may lead to wave impact that the structures and equipment of a semi-submersible are not designed for and could potentially cause serious damage. Conversely, an excessive air gap could increase the cost, decrease the stability, r

25、educe payload capacity and impair global performance. These Guidance Notes provide procedures for the air gap analysis based on either the linear or the second order potential theories, address nonlinear effects on the air gap analysis, and identify industry common design practice. The theories and

26、equations referenced in these Guidance Notes are based on the infinite water depth assumption. However, the procedures described in these Guidance Notes are in general also applicable to finite water depth condition. 3 Definition of Air Gap FIGURE 1 Air Gap Definition An air gap or a deck clearance

27、represents the distance between the water surface and the lowest deck of a semi-submersible. Section 1, Figure 1 gives a schematic view of the air gap in the absence of waves and in the presence of waves. In still water, the air gap at a particular location (x, y) on the lowest deck is denoted as a0

28、(x,y), which is determined by the hydrostatic analysis. In the presence of waves, (x,y,t) denotes the free surface elevation at a particular location on the lowest deck at time instant t, and (x,y,t) is the corresponding vertical motion of the point on the platform at time instant t. In these Guidan

29、ce Notes, a location on the structure selected for air gap calculation is referred to as a field point. The instantaneous air gap at a field point is: ABSGUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES .2017 1 Section 1 Introduction a(x,y,t) = a0(x,y) (x,y,t) (x,y,t) . (1.1) The total chang

30、e of air gap is a linear combination of two sources: free surface elevation, (x,y,t) and corresponding vertical vessel motion, (x,y,t), at a field point. In this document herein, the total change of air gap is referred to as the relative wave elevation, r(x,y,t). r(x,y,t) = (x,y,t) (x,y,t) . (1.2) D

31、eck impact occurs if relative wave elevation, r(x,y,t), exceeds the still water air gap, a0(x,y). The air gap analysis in these Guidance Notes is to determine the minimum air gap corresponding to the extreme relative wave elevation in storm conditions. 5 Air Gap Design Considerations The air gap sho

32、uld normally be determined by an appropriate model test. Alternatively, the air gap may also be calculated using numerical approaches that account for relative motions between the semi-submersible and waves. The following items should be considered in the determination of the air gap: i) Environment

33、al condition headings relative to the semi-submersible; ii) Motions of semi-submersible in six degrees of freedom; iii) Wave crest elevation, including wave asymmetry; iv) Wave/structure interaction effects (e.g., wave enhancement, run-up); v) Effects of interacting systems (e.g., mooring and riser

34、systems); vi) Local wave crest effects as described in APR RP 2FPS (2011), where applicable. For semi-submersibles operated as floating production installations, the ABS FPI Rules require a reasonable clearance between the lowest point of the topside deck and the wave crest. A commonly referenced mi

35、nimum deck clearance is 1.5 m (5 ft), see API RP 2FPS (2011), in the case where the semi-submersible is subject to the 100-year return period environmental conditions. Consideration is also given to performing the robustness check, in which a rarer, but still possible event (e.g., 1,000-year wave cr

36、est in the Gulf of Mexico or 10,000-year return period wave crest in the North Sea) is applied and the topside deck elevations should be such that no negative air gap takes place. If the air gap criterion is not satisfied, the anticipated local and global wave forces (including slamming) in both ver

37、tical and horizontal directions, where applicable, should be suitably considered in the design of the structural strength. For semi-submersibles operated as mobile offshore units, the ABS MODU Rules require a reasonable clearance between the lowest point of the topside deck and the wave crest, unles

38、s the upper structure and deckhouses are satisfactorily designed for wave impact. For mobile offshore units with a DP system, larger vertical motions due to rolling or pitching induced by overturning moment generated by the force from the thrusters, especially at field points near the columns and at

39、 the extreme of ends of the unit, may lead to a smaller air gap than what is predicted from numerical analysis. A static trim due to the unfavorable combination of thrust forces may be applied in the absence of a more detailed analysis. For semi-submersibles used as a mobile offshore unit but which

40、are intended to be deployed at one site for no less than 5 years without dry docking, the air gap analysis should in general follow the requirements for floating production installations. 7 Air Gap Analysis Methods The following four methods may be used for the air gap analysis. It is possible, howe

41、ver, that other approaches may also exist. 7.1 Linear Air Gap Analysis Linear air gap analysis relies on the linear three-dimensional diffraction-radiation theory to calculate the first order wave frequency vessel motions and free surface elevations at field points in frequency domain, and then uses

42、 a stochastic method to determine the extreme relative wave elevation. Corrections are applied to approximately account for the nonlinear effects of wave surface elevations and vessel motions. 2 ABSGUIDANCE NOTES ON AIR GAP ANALYSIS FOR SEMI-SUBMERSIBLES .2017 Section 1 Introduction This approach is

43、 commonly used in practical application due to its simplicity and ease of use. Empirical corrections are applied through a wave asymmetry factor and a wave run-up factor to consider the nonlinearity of the free surface elevation and the local run-up, respectively. Without these corrections, experime

44、ntal observations indicate that the linear analysis may significantly under-predict the crest elevation in steep waves and, in particular, at locations very close to columns where local run-up may appear. In addition, the mean and low frequency vessel motions should be considered for the air gap des

45、ign of semi-submersibles. Current industry practice, however, often ignores the mean and low frequency vessel motions for the air gap design of semi-submersibles operated as mobile offshore units. The mean and low frequency motions can be calculated by the global motion analysis in either frequency

46、domain or time domain. 7.3 Second Order Time-Domain Air Gap Analysis The second order air gap analysis attempts to model the nonlinear effects of vessel motions in time domain. It usually can improve the prediction accuracy and result in a better agreement with model test results. The free surface e

47、levation can be determined through the linear diffraction-radiation theory and adjusted using empirical corrections as described above in 1/7.1. Alternatively, a second order method based on the second-order diffraction theory may be applied to model the asymmetry in the free surface elevation provi

48、ded the analysis results of the free surface elevation and the correction to the wave run-up at near-column locations are validated by model tests. 7.5 CFD-Based Air Gap Analysis Computational Fluid Dynamics (CFD) analysis can solve complex nonlinear flows around an offshore structure. This may be t

49、he most promising numerical approach to model the wave run-up effects. However, the computational cost is high and the analysis method should still be validated by the comparison with model tests. 7.7 Model Test Model test is the most direct method to predict air gap. However, model test results are usually not available in the early design stage. Even in the final design stage, model test is often conducted under limited environmental conditions. In general, model test and numerical analyses are not to replace, but are rather to complement each other. Model test data