Wave and current forces on fixed offshore structures

1988 ◽  
Vol 15 (6) ◽  
pp. 937-947 ◽  
Author(s):  
Michael Isaacson
Keyword(s):  
Key Words ◽  
Recent Literature ◽  
Offshore Structures ◽  
Design Criteria ◽  
Wave Forces ◽  
Ocean Engineering ◽  
Detailed Survey ◽  
Ice Loads ◽  
Hydrodynamic Data ◽  
Design Construction

The Canadian Standards Association standard S471 "General requirements, design criteria, environment, and loads, Part 1 of the CSA code for the design, construction and installation of fixed offshore structures" contains an appendix "Wave and current loads." To compliment this appendix, the present paper provides a more detailed survey of this topic with a review of the recent literature and recommendations of hydrodynamic data needed in offshore design. In addition, hydrodynamic considerations in the calculation of earthquake and ice loads are mentioned. Key words: currents, current forces, hydrodynamics, ocean engineering, offshore structures, waves, wave forces.

Wave–current effects on large offshore structures

1989 ◽  
Vol 16 (4) ◽  
pp. 543-551 ◽  
Author(s):  
Michael Isaacson ◽  
John Baldwin
Keyword(s):  
Potential Flow ◽  
Offshore Structures ◽  
Wave Forces ◽  
Ocean Engineering ◽  
Viscous Effects ◽  
Offshore Structure ◽  
Time Stepping ◽  
Waves And Currents ◽  
Flow Effects ◽  
Interaction Problem

The various effects that influence loads acting on a large offshore structure due to the combination of waves and currents are reviewed. These may be broadly associated with potential flow effects and viscous effects. The potential flow effects are nonlinear and may generally be investigated by perturbation or time-stepping methods. Viscous effects include the onset of flow separation, which affects the validity of the assumed potential flow, as well as steady and oscillatory forces. The fluid mechanics of the complete wave–current–structure interaction problem are not yet well understood and areas in need of additional research are identified. Key words: currents, drag, drift forces, hydrodynamics, ocean engineering, offshore structures, waves, wave forces.

The Effect of Uncertainty in Wave Force Coefficients for Offshore Structures

1985 ◽  
Vol 25 (05) ◽  
pp. 757-764
Author(s):  
Kenneth G. Nolte
Keyword(s):  
Wave Force ◽  
Offshore Structures ◽  
Design Criteria ◽  
Wave Forces ◽  
Offshore Structure ◽  
Morison Equation ◽  
Force Coefficients ◽  
Wave Parameters ◽  
Random Occurrence ◽  
Wave Heights

Abstract A probability distribution, which incorporates the random occurrence of wave heights and the uncertainty in the force coefficients of the Morison equation, was derived for the forces on offshore structures. The random occurrence of wave heights was assumed to be described by a Weibull distribution, and the uncertainty in the force coefficients was assumed to be represented by a normal distribution. Wave force was assumed to be proportional to wave height raised to a power. The assumed distributions and force relationship may not describe exactly the actual problem within a general framework, but the assumptions are believed to be applicable to the range of wave heights and conditions occurring for the selection of static design criteria for the forces on offshore structures. The applicability of the assumptions is enhanced because the primary results are expressed as ratios, which require only relative accuracy and not quantitative accuracy. Introduction The wave forces on an offshore structure are determined by a wave theory (e.g., Stokes or stream function) that relates the water kinematics (velocity and acceleration) to the wave parameters (height and period) and a theory that relates the resulting pressures on the structure to the predicted water kinematics (e.g., the Morison equation or refraction theory). Generally, the Morison equation, which incorporates two force coefficients - the drag and inertia coefficients - is used. The wave parameters experienced by a structure during a storm are random. Also, inferred values of the force coefficients from field measurements indicate a random scatter from wave to wave caused by the random nature of the processes involved and imperfect wave and hydrodynamic theories. Therefore, the prediction of wave forces and, ultimately, the selection of design criteria for offshore structures involve both the random nature of the wave parameters (e.g., height) and the uncertainty in the force coefficients. Procedures for selecting wave heights for design criteria have received considerable attention and are well established; however, the problem of considering the uncertainty in the force coefficients has received little attention. Currently, there is no rational procedure to account generally for coefficient uncertainty except to use arbitrary, and potentially unrealistic, guidelines, such as the mean value plus a multiple of the standard deviation. The purpose of this paper is to provide a rational framework for dealing with the uncertainty in force coefficients. This framework is statistical and incorporates into the force statistics the uncertainty of the force coefficients and the random occurrence of the wave parameters. Background The wave force, Q, on an offshore structure is generally determined by the Morison equation,Equation 1 QD and QI are defined as the drag and inertia forces, respectively, per unit length acting normal to a structural element; CD and CI are the drag and inertia coefficients (i.e., the force coefficients); v and v are the water velocity and acceleration normal to the element; d is the element diameter; and ?w is the mass density of water.

Nonlinear wave forces on a circular cylinder

1989 ◽  
Vol 16 (2) ◽  
pp. 182-187 ◽  
Author(s):  
Michael Isaacson ◽  
Qi-Hua Zuo
Keyword(s):  
Circular Cylinder ◽  
Nonlinear Wave ◽  
Perturbation Methods ◽  
Wave Force ◽  
Offshore Structures ◽  
Wave Forces ◽  
Ocean Engineering ◽  
Time Stepping ◽  
Wave Effects ◽  
Accuracy And Stability

Nonlinear wave forces on a surface-piercing vertical circular cylinder are considered using a time-stepping method previously developed which is based on Green's theorem. Possible improvements in the efficiency, accuracy, and stability of the method are considered. Results based on this method are compared with those obtained previously using perturbation methods as well as with experimental results. It is found that the time-stepping method adopted here is quite reasonable. Wave force coefficients are given as functions of the governing parameters of the problem and the importance of nonlinear wave effects on the forces is assessed. Key words: hydrodynamics, ocean engineering, offshore structures, waves, wave forces.

Development of a net pen system for aquaculture farming

1993 ◽  
Vol 20 (2) ◽  
pp. 189-200 ◽  
Author(s):  
Michael Isaacson ◽  
Norman Allyn ◽  
Gary Loverich
Keyword(s):  
Offshore Structures ◽  
Mooring Line ◽  
Wave Forces ◽  
Prototype System ◽  
Ocean Engineering ◽  
Fish Farms ◽  
Juan De Fuca ◽  
Prototype Structure ◽  
Structure Strength ◽  
Juan De Fuca Strait

The present paper describes a study carried out to verify a new net pen system which has been developed for aquaculture farming at exposed coastal sites. The structure is based on the use of spar buoys rather than rigid floats to support the net. A prototype structure has been deployed in Juan de Fuca Strait, and a verification of the system is described. This has involved an assessment of the environmental conditions and hydrodynamic loading for the structure, strength and fatigue analyses, and a calibration of wave conditions and mooring line forces with respect to prototype measurements. Overall, the system has been found to perform very well. Recommendations are made for monitoring the prototype system and for carrying out a refined verification of the system based on additional data. Key words: aquaculture, fish farms, hydrodynamics, ocean engineering, offshore structures, waves, wave forces.

Correction factors for nonlinear runup and wave forces on a large cylinder

1994 ◽  
Vol 21 (5) ◽  
pp. 762-769 ◽  
Author(s):  
Michael Isaacson ◽  
Kwok Fai Cheung
Keyword(s):  
Experimental Studies ◽  
Diffraction Theory ◽  
Wave Force ◽  
Offshore Structures ◽  
Wave Forces ◽  
Correction Factors ◽  
Wave Loads ◽  
Wave Runup ◽  
Ocean Engineering ◽  
Simplified Approach

A recently developed numerical method for second-order wave diffraction is summarized and is used to develop a simplified approach to predicting nonlinear runup and maximum wave loads for large coastal and offshore structures subjected to regular waves. The perturbation method on which the method is based is extended to provide correction factors for the runup and maximum loads. These correction factors apply directly to the predictions of linear diffraction theory, and are independent of the wave height. The correction factors for runup, maximum force and maximum overturning moment are provided for a range of geometric parameters relating to the case of a large circular cylinder extending from the seabed to the free surface. Nonlinear runup and load maxima calculated by the correction factors are compared with the results of previous experimental studies; in general, favourable agreement is obtained. An example application of the proposed procedure is provided, the importance of nonlinear effects in the evaluation of runup and wave loads is discussed, and the limitations of the results are indicated. Key words: coastal structures, diffraction, hydrodynamics, ocean engineering, offshore structures, wave runup, wave force, waves.

Combined wave – iceberg loading on offshore structures

1996 ◽  
Vol 23 (5) ◽  
pp. 1099-1110 ◽  
Author(s):  
Ricardo Foschi ◽  
Michael Isaacson ◽  
Norman Allyn ◽  
Steven Yee
Keyword(s):  
Numerical Analysis ◽  
Offshore Structures ◽  
Wave Forces ◽  
Combined Effects ◽  
Ocean Engineering ◽  
First Order ◽  
Load Combination ◽  
Wave Parameters ◽  
Annual Probability ◽  
Verification Process

The Canadian Standards Association has developed and published a code for the design and construction of fixed offshore structures. This code has been subjected to a comprehensive verification process which has identified several issues warranting further study. One of these relates to the combined effects of wave and iceberg collision loading. At present, this combination is treated by the use of a load combination factor specified in the Code. The present paper describes a recent study which was undertaken to determine the appropriateness of the recommended value of the load combination factor. The study involves a numerical analysis in which loads due to waves alone, an iceberg alone, and an iceberg and waves in combination have been calculated for a range of iceberg and wave parameters. These results have been applied to a first-order reliability analysis in order to study the force levels corresponding to an annual probability of 10−4 or to the onset of global sliding with an annual probability of 10−4. The paper thereby makes recommendations for load combination factors applicable to combined wave–iceberg loading. Key words: hydrodynamics, icebergs, ocean engineering, offshore structures, wave forces, waves.

Thirteenth Canadian Geotechnical Colloquium: Ice design criteria for wide arctic structures

1990 ◽  
Vol 27 (6) ◽  
pp. 701-725 ◽  
Author(s):  
Denis Blanchet
Keyword(s):  
Key Words ◽  
Shear Zone ◽  
Design Criteria ◽  
Correction Factors ◽  
Return Periods ◽  
Ice Load ◽  
Risk Probability ◽  
Arctic Exploration ◽  
Ice Conditions ◽  
Ice Loads

The derivation of design global ice loads for wide vertical caisson-type arctic exploration structures is presented. Correction factors are applied to global ice-load data measured during ice–structure interactions to create a set of design criteria for two typical exploration structures: circular (100 m diameter) and rectangular (160 × 50 m). Return periods for the proposed deterministic design ice loads for these two structures are then determined for typical ice conditions encountered in the Harrison Bay shear zone area in 20–30 m of water. Key words: exploration structures, global design ice loads, risk, probability of exceedence, return periods.

Experimental Study of Combined Wave and Ice Loads on a Fixed Offshore Structure

2021 ◽  
Author(s):  
Shafiul Mintu ◽  
David Molyneux
Keyword(s):  
Offshore Structures ◽  
Numerical Code ◽  
Ocean Engineering ◽  
Offshore Structure ◽  
Engineering Research ◽  
Combined Loads ◽  
Individual Wave ◽  
Ice Loads ◽  
Ice Floes ◽  
Memorial University Of Newfoundland

Abstract Ice floes in the marginal ice zone (MIZ) are exposed to wind, wave, and current forces which greatly influence the dynamics of the ice floes. ISO 19906 recommends considering combined wave and ice actions while designing offshore structures for arctic and cold regions. Few studies have focused on ice-structure interactions in waves. There are not many tools available to estimate these combined loads on structures. A numerical tool “SAMICE” has been developed to simulate the hydrodynamics of wave-ice interactions, but there exists a lack of data for a realistic MIZ under wave actions for validation studies of the numerical code. To address this gap and to investigate the hydrodynamics of ice floes under waves, a set of experiments was conducted at the wave tank of Ocean Engineering Research Center (OERC) of Memorial University of Newfoundland. A six-component dynamometer was used to measure the loads on a model scale aluminum cylindrical gravity-based offshore structure. Loads were measured for five regular waves of various steepness in combination with three current speeds. Two ice concentrations with various floe sizes of random shapes were prepared from polypropylene sheets to represent the MIZ. Most of the tests were repeated three times and a statistical approach was used to analyze the loads. The preliminary analysis shows that the average wave-ice loads may be determined by ISO guidelines, but the predictions of impulse loads from individual wave-driven ice floes are very uncertain.

Ice action on fixed offshore structures: a state-of-the-art review

1987 ◽  
Vol 14 (3) ◽  
pp. 381-407 ◽  
Author(s):  
M. A. Nessim ◽  
M. S. Cheung ◽  
I. J. Jordaan
Keyword(s):  
Interaction Design ◽  
Size Effects ◽  
Offshore Structures ◽  
Probabilistic Methods ◽  
Background Information ◽  
Design Criteria ◽  
Load Calculation ◽  
Ice Conditions ◽  
Ice Loads ◽  
Probabilistic Load

The methods and approaches for calculating global ice loads on fixed offshore structures are critically reviewed. The review covers both the mechanical approaches used in load calculation and the probabilistic methods used to deal with the uncertainties involved. The paper gives a brief review of background information related to ice conditions, ice-load characterization, and ice mechanics. Load calculation methods for different types of ice behaviour are reviewed for vertical structures, as well as sloping-sided structures. Size effects are discussed, and the effects of rubble piles on ice loads are briefly reviewed. The emphasis of the review is on summarizing the different models and approaches reported in the literature, and making evaluations, comments, and suggestions for further research from the viewpoint of choosing design criteria. Key words: ice-structure interaction, design criteria, ice indentation, probabilistic load assessment, Arctic structures, size effects.

Factored axial compressive resistance of schifflerized angles

1991 ◽  
Vol 18 (6) ◽  
pp. 926-932 ◽  
Author(s):  
Seshu Madhava Rao Adluri ◽  
Murty K. S. Madugula
Keyword(s):  
Key Words ◽  
Design Method ◽  
Design Criteria ◽  
Design Practice ◽  
Buckling Mode ◽  
Flexural Buckling ◽  
Mode Of Failure ◽  
Current Design ◽  
Similarities And Differences ◽  
Compressive Resistance

The concept of schifflerization of 90° equal-leg angle is presented and its application in triangular-base latticed steel towers is explained. The similarities and differences between schifflerized angles and regular 90° angles are discussed. The current design practice for schifflerized angles is reviewed and its limitation is highlighted. A design method which includes the effect of the torsional-flexural buckling mode of failure is proposed. For ready use of designers, the factored axial compressive resistances of schifflerized angles are tabulated for both the present and proposed design methods. Key words: buckling, compressive resistance, design criteria, schifflerized angles, stability, standards, steel, struts, towers, guyed towers.

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