Wave Impact on FPSO's and Floating Platforms

WaveLand JIP

Reliable methods and robust tools for design and verification of floating production units against wave impact.

Phase II: Run-up and extreme waves on floating platforms and large-volume structures; semis, TLPs, SPARs and gravity based structures.

Critical situations caused by wave impact in platform decks are receiving growing attention. Such situations may be caused by extreme wave situations or by local diffraction and run-up around platform columns. Subsidence of gravity-based structures require renewed attention to air-gap, run-up and water impact loads. And increased loading or tie-in of new satellites prompts similar considerations for floating platforms. There is a continuous development of advanced methods on different parts of the problem, e.g. nonlinear and random waves, complex vessel motions, wave diffraction etc. To date, however, a consistent design tool combining the complete chain of events in an engineering format does not seem to be available.

To address these needs, MARINTEK has launched a JIP project to develop reliable methods and practical design tools for engineering use. The first phase of the project focused on bow slam and water-on-deck loading on ships and FPSOs. This phase is now completed. Phase II of the project will focus on run-up and wave impact on floating platforms and large-volume structures, i.e. semis, TLPs, SPARs and gravity-based structures.

A semi-empirical approach is taken, in which analytical formulations are calibrated against model test data. The objective of the development is to establish reliable tools for the prediction of:

• probability of run-up and wave impact on floating platforms
• design loads for water impact loads
• structural integrity
• effect on global motions of the platform

Sponsorship

Phase I of the project was sponsored by Norsk Hydro, Statoil, Petroleum Geo-Services (PGS), Advanced Production & Loading (APL), NAVION, Rolls Royce Marine and the Norwegian Petroleum Directorate (NPD). Phase II was launched this winter, and was very well received by the offshore industry. Initial work will start late 2002, with the main thrust starting early 2003.

Incoming waves are simulated by a second-order random wave model, which describes water elevation as well as kinematics. Vessel motions are calculated with slow-drift motions taken into account. Second-order diffraction is used to determine the diffracted wave field.

Relative motions are estimated by combining the second-order vessel motions, second-order incoming waves and second-order diffraction. The relative motions are used to assess the probability of run-up or water impact. Initial conditions for the water flow up platform columns are estimated from the relative wave kinematics; in-flow conditions are calculated at selected points as continuous time series of hi(t) and vi(t). Systematic model test observations are used to check and calibrate the models. Empirical corrections can be applied to the calculated in-flow conditions.

Water propagation and local loads

The highly non-linear vertical water flow up the columns are simulated by a shallow water approach, extended from the one implemented for Green Sea flow in Phase I of the project. The formulation is reformulated in polar coordinates on the vertical surface of the columns. The effect of gravity is included.

Time-varying boundary conditions for the local water flow are estimated from the relative wave kinematics described above, given in terms of time-variable vertical fluid velocity and time-variable flow thickness at the boundary.
Water impact loads are calculated with a similarity solution whereby the impacting water is described as a wedge with an angle a that travels with velocity n just before the impact. The two parameters a and m are predicted by the shallow-water formulation. The pressure distribution as a function of time and space is then calculated as input to the structural integrity assessment.

Loads on secondary structures in the path of the water flow are calculated using Morrison’s equation with the calculated water particle velocities.

Structural integrity assessment

Calculated impact loads are mapped onto existing FE models (NISA, SESAM, ASAS) for simulation of the structural response for each specific impact scenario. A special module based on Biggs’ and Baker’s methods is implemented for screening of admissible water impact scenarios.

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(Article in MARINTEK Review No 1-2002)