Lyle Finn

Model Testing for Vortex Induced Motions of Spar Platforms

The state-of-the art in model testing for Vortex Induced Vibrations (VIV) of Spars is presented. Important issues related to Spar VIV model testing are highlighted. The parameters that need to be modeled including hull geometry, strake configuration, mass and mooring properties and, considerations of test set-up and instrumentation are discussed. Results are presented from model tests of an as-built Spar deployed in the Gulf of Mexico. It is shown that the model test results compare well with the VIV responses measured in the field.Copyright

Improved Strake Design for Vortex Induced Motions of Spar Platforms

An extensive model test program was conducted to explore the effectiveness of alternate strake designs to reduce Truss Spar VIV response. Different strake configurations were tested to minimize VIV response. The paper presents results of the model tests. The model test set-up is described, important parameters that are modeled (including hull and truss geometry, strake configuration, mass and mooring properties) and considerations of instrumentation and test methodology are discussed. The paper also describes the analysis of the test results and shows the effectiveness of new strake design. The present results are compared with VIV response of existing Truss Spars with conventional strake design.© 2005 ASME

Time Domain FEA for Riser VIV Analysis

Risers are often subjected to non-linear boundary conditions, such as soil-SCR interaction and interaction between risers and the guides in the vessel, which often have gaps and non-linear stiffness. Also, current profiles may vary in direction through depth. In these cases, time domain FEA could represent the problems better than frequency domain analysis. Several computer codes are available for cross-flow VIV analysis using time domain FEA. This paper introduces SimVIV, a time domain computer code for cross-flow VIV and in-line VIV analysis. The algorithm used in SimVIV for cross-flow VIV analysis is consistent to the one used in the Highly Compliant Rigid Riser Joint Industry Project (HCR Riser JIP). The algorithm for in-line VIV is an added feature in this code, and for this initial attempt, this covers only in-line VIV that has about twice the frequency of the cross-flow VIV. This paper presents the results of SimVIV to predict cross-flow VIV and in-line VIV for model tests that have been done at the Delta Flume of Delft Hydraulics (May 2003). The results in general show good comparison to the measured data.© 2010 ASME

Float Over Installation Method—Comprehensive Comparison Between Numerical and Model Test Results

Installing a large deck onto a platform, such as a spar, using the floatover method is gaining popularity. This is because the operational cost is much lower than other methods of installation, such as modular lifts or a single piece installation by a heavy lift barge. Deck integration can be performed on land, at quay side and will not depend on a heavy lift barge. A new concept for a floatover vessel has been developed for operations in the Gulf of Mexico and West Africa. In this application sea state conditions are essential factors that must be considered in the Gulf of Mexico, especially for transportation. In West Africa, swell conditions will govern floatover deck (FOD) installation. Based on these two different environmental conditions, Technip Offshore, Inc. developed the FOD installation concept using semi-submersible barge type vessels. A significant amount of development work and model testing has been done on this method in recent years on spar floatover. These tests have validated our numerical methods. Another test was conducted to investigate the feasibility of a deck float-over operation onto a compliant tower for a West Africa project. The project consists of a compliant tower supporting a 25,401 metric ton (28,000 s. ton) integrated deck. This paper will describe comparisons between model test data and numerical predictions of the compliant tower floatover operation.

Full Scale Data Comparison for the Horn Mountain Spar Mooring Line Tensions During Hurricane Isidore

The Horn Mountain Production Spar was installed in 5,400 feet of water in June 2002. This was the deepest floating production unit at that time. A comprehensive instrumentation program was initiated to measure spar and riser responses (Edwards et al, DOT 2003), while motion comparisons were presented on previous publication (Halkyard et al, OMAE 2004). The present paper discusses the results of these measurements and compares with analytical predictions of spar mooring tension during hurricane Isidore in September 2002. Particular attention has been placed on the importance of Coulomb friction between wire chain and the fairlead bearing to the dynamic tension of mooring lines. Mooring tensions were measured at chain jack location (inboard tension), while analytical models computed those tensions at the fairlead location (outboard tension). Our conclusion is that there is excellent agreement between field measurements and computed tensions at the chain jacks when fairlead friction is included, and when the vessel motions are accurately predicted. Ignoring fairlead friction results in a slightly conservative estimate for the tension at the chain jack. This has been the standard practice in all spar designs to date.Copyright

Float Over Installation Method: Numerical and Model Test Data

Installing a large deck onto a platform, such as a spar, using the floatover method is gaining popularity. This is because the operational cost is much lower than other methods of installation, such as modular lifts or a single piece installation by a heavy lift barge. Deck integration can be performed on land, at quay side and will not depend on a heavy lift barge. A new concept for a floatover vessel has been developed for operations in the Gulf of Mexico and West Africa. In this application sea state conditions are essential factors that must be considered in the Gulf of Mexico, especially for transportation. In West Africa, swell conditions will govern floatover deck (FOD) installation. Based on these two different environmental conditions, Technip Offshore Engineering developed the FOD installation concept using semi-submersible barge type vessels. A significant amount of development work and model testing has been done on this method in recent years on spar floatover. These tests have validated our numerical methods. Another test was conducted to investigate the feasibility of a deck float-over operation onto a compliant tower for the Benguela Belize (BBT) project. The BBT project consists of a compliant tower supporting a 25,401 metric ton (28,000 s. ton) integrated deck. This paper will describe comparisons between model test data and numerical predictions of the compliant tower floatover operation.Copyright

Efficient Dynamic Analysis of a Combined Spar System via a Frequency Domain Approach

The response of a combined Spar/ risers/mooring lines system is conventionally determined by conducting nonlinear time domain analysis. The system nonlinearity is introduced by the mooring nonlinear force, the friction between the buoyancy-can and the preloaded compliant guide, and the quadratic model of the fluid related damping. Obviously, during the design process, it is important to understand the sensitivity of the Spar responses to various parameters. To a great extent, these objectives cannot be readily achieved by using time domain analysis since, in this context, elements with frequency dependent representation such as the added masses and supplementary damping must be incorporated in the analysis; this may require the use of elaborate convolution techniques. This attribute of the time domain solution combined with the necessity of running a significant number of simulations makes it desirable to develop alternative methods of analysis. In the present paper, a frequency domain approach based on the method of the statistical linearization is used for conducting readily a parametric study of the combined Spar system. This method allows one to account by an equivalent linear damping and an equivalent linear stiffness for the mooring nonlinearity, friction nonlinearity, and the damping nonlinearity of the system. Further, frequency dependent inertia and radiation damping terms in the equations of motion are accommodated. This formulation leads to a mathematical model for the combined system, which involves five-by-five mass, damping and stiffness matrices. In the solution procedure, the equivalent parameters of the linear system are refined in an iterative manner, and by relying on an optimization criterion. This procedure is used to assess the sensitivity of representative Spar system responses to various design parameters. Further, the effect of various design parameters on the combined system response is examined. The environmental loadings considered are of the JONSWAP format of a 100-yr hurricane in the Gulf of Mexico.Copyright

Inplace Model Test Result Correlation of Multi Column Floater (MCF): Drilling and Production Unit

This paper presents the comparison of model tests and numerical results of Multi Column Floater (MCF). MCF is a new drilling and production deep-draft semisubmersible platform developed by Horton Wison Deepwater in Houston. A vertically restrained well deck or Multi Riser Buoyancy Can (MRBC) configured in the center of the MCF is technology enabler for dry tree semisubmersible. The MRBC independently supports and tensions the production and drilling risers. The production risers may be a combination of dry tree risers and subsea tree tubing risers. Inplace model tests were carried out at the FORCE Technology basin during the period September-October, 2007 for the internal study. The test emphasis was to understand the physical behavior of MCF at model scale so that a thorough correlation study could be carried out. MCF mooring stiffness, riser stiffness, natural periods and mass properties of the model were closely matched with target full-scale properties. Guide loads and relative motions between MRBC and hull were measured to obtain model-scale behavior. A wide range of sea states and towing speeds were covered in the test programs. Detailed comparison summarizing the statistical quantities of interests as well as time series and spectral plots are included in this paper. RAO and free decay comparisons are also described.Copyright