The Effect of Different Methods of Simulating Water Particle Kinematics on the 100-Year Responses

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). A number of empirical techniques have been suggested to provide a more realistic representation of near surface wave kinematics. The empirical techniques popular in the offshore industry include Wheeler stretching, linear extrapolation, delta stretching, and vertical stretching. Each of these methods is intended to calculate sensible kinematics above the MWL, yet they have been found to differ from one another in the results yielded. In this paper, two new methods of simulating water particle kinematics are introduced. In this study, the values of 100-year responses derived from different methods of simulating wave kinematics are compared.

Estimation of Force Reduction on Ocean Structures With Perforated Members

While retrofitting and rehabilitation are usually related to strengthening of members, the presented concept is a novel attempt as it addresses decrease in the encountered forces on the members. Presence of perforated members in ocean structures reduces the wave-structure interaction significantly; breakwaters with perforated members are classical examples of such kind. This concept of encompassing the perforated outer cylinder with inner existing structure is found to be most feasible rehabilitation concept as it does not demand replacement of any damaged members. The presence of outer perforated cover alters the water particle kinematics significantly and eventually this remains the reason for the force reduction mechanism. In this study, the variation of water particle kinematics along the depth of the cylinder is estimated on the cylindrical structure with and without perforated outer cover. Forces on the inner cylinder are quantified numerically and experimentally; experimental results show a close agreement with that of the numerical ones. Velocity variations along the water depth are quantified in the form of design charts, which shall be helpful for the practicing professionals while attempting for retrofitting or re-design. Force variations derived through numerical analyses, which are functions of the water particle kinematics along the depth shall be useful in the design offices for cylindrical members encompassed with perforated outer cover. Introduction of perforated member over the existing cylindrical structure showed a significant force reduction around 60% on an average for all the wave steepness indexes considered for the study, when compared to the force on the member without perforated cover.

Variations of Hydrodynamic Characteristics With the Perforated Cylinder

Presence of the perforated outer cover on the existing column members of offshore platforms reduces the direct wave impact on these members. Such applications are common in the coastal structures where perforated covers are provided on the seaside to dissipate the wave energy and to reduce the pressure on the members. Detailed studies on the variations of the hydrodynamic characteristics on the inner cylinder, encompassed by a perforated outer cover are scarce in the literature. Present study is focused the development of numerical model to investigate the variations in the water particle kinematics on the inner cylinder encompassed by perforated outer cover. Hydrodynamic characteristics are examined along the water depth through computation fluid dynamics (CFD) for perforation ratios (p%) varying in the range of 10 to 15%. Velocity profiles for different wave steepness are developed along with the design charts for the chosen perforation ratios. These design charts can be readily used for estimating the water particle kinematics for perforated members along the water depth.

Comparison of Extreme Offshore Structural Response from Two Alterna-tive Stretching Techniques

Linear random wave theory (LRWT) has successfully explained most properties of real sea waves with the ex-ception of some nonlinear effects for surface elevation and water particle kinematics. Due to its simplicity, it is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record; however, predicted water particle kinematics from LRWT suffer from unrealistically large high-frequency compo-nents in the vicinity of mean water level (MWL). To overcome this deficiency, a common industry practice for evaluation of wave kinematics in the free surface zone consists of using linear random wave theory in conjunction with empirical techniques (such as Wheeler and vertical stretching methods) to provide a more realistic representation of near-surface wave kinematics. It is well known that the predicted kinematics from these methods are different; however, no systematic study has been conducted to investigate the effect of this on the magnitude of extreme responses of an offshore structure. In this paper, probability distributions of extreme responses of an offshore structure from Wheeler and vertical stretching methods are compared. It is shown that the difference is significant; consequently, further research is required to deter-mine which method is more reliable.

Comparison of Extreme Responses From Wheeler and Vertical Stretching Methods

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques such as Wheeler and vertical stretching methods are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. In this paper, the Monte Carlo time simulation technique is used to investigate the effect of these two different methods of simulating water particle kinematics on the probability distribution of extreme responses. It is shown that the difference could be significant leading to uncertainty as to which method should be used.

Splash Zone Lifting Analysis of Subsea Structures

The lifting analysis of a subsea structure determines the maximum allowable design sea state in which the structure can be installed safely. Normally, such analysis on the structure at the splash zone governs the expected largest forces in the hoisting system and in turn the allowable sea state since the water particle kinematics is larger in the splash zone. In this paper, the DNV Recommended Practice for Modelling and Analysis of Marine Operation (DNV-RP-H103, April 2009) is discussed with emphasis on the hydrodynamic coefficients and analysis methodology for the splash zone lifting analysis. An approach is suggested here to take into account the free surface proximity effect on added mass of flat surfaces in the absence of test results. Discussions on the following points are also included: • For structures which show restricted sea state due to large double pendulum motion and consequently high dynamic tension in the crane wire, a solution could be obtained by lowering the sling angles. • For inertia dominated structures, the drag coefficients should be chosen with caution unless experimental results are available since the drag may induce unrealistic damping in the system. • For the structural design of large subsea structures, the design DAF for submerged condition should be chosen from a preliminary lifting analysis result. The current industrial practice of using DAF = 2 with respect to the static submerged weight could be increased following the analysis result to optimise the use of the crane capacity by achieving a higher design sea state. • For lifting analysis of structures with large added mass / submerged weight, modelling of winch speed may represent a worse loading case as compared to the case with zero winch speed in the splash zone. • For the splash zone analysis, correct modelling of the stiffness of the crane structure along with the wire is important. The assumption that the crane structure is rigid may lead to unrealistic analysis results. Experimental programmes to obtain further information on the amplitude dependent characters of the hydrodynamic coefficients, the stiffness and the damping of the Crane, the wires etc are furthermore recommended.

Derivation of Water Particle Kinematics in the Near Surface Zone by the Effective Water Depth Procedure

Linear Random Wave Theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques (such as Wheeler & vertical stretching methods) are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. In this paper, a modified version of LRWT, based on the derivation of an effective water depth, is introduced. The proposed technique leads to predicted kinematics (in the near surface zone) which lie between corresponding values from the Wheeler and the vertical stretching methods. Furthermore, it does not suffer from exaggerated high-frequency components in the near surface zone.

Simulation of Water Particle Kinematics in the Near Surface Zone

Linear Random Wave Theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. It is, however, well known that wave kinematics calculated from LRWT suffer from unrealistically large high-frequency components in the vicinity of mean water level. To overcome this deficiency, a common industry practice consists of using linear wave theory in conjunction with empirical techniques, such as the Wheeler or the vertical stretching methods, to provide a more realistic representation of the near-surface water particle kinematics. In this paper, a modified version of LRWT is introduced, which, unlike the standard LRWT, does not lead to unrealistically large high-frequency components in the vicinity of mean water level. The proposed method leads to predicted kinematics in the near surface zone which lie between corresponding values from the Wheeler and the vertical stretching methods, respectively.