John Halkyard

Benchmarking of Truss Spar Vortex Induced Motions Derived From CFD With Experiments

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.Copyright

Influence of Heave Plate Geometry on the Heave Response of Classic Spars

A production spar designed for West African (WA) offshore conditions must consider possible resonance with long period swell, which might result in large amplitude heave oscillations. Preliminary study of a classic spar with diameter of 39 m (128 ft) and draft 198 m (650 ft) for a WA application led the authors to believe that excessive heave response of 5.2 m (17 ft) may occur at the natural period of 28 seconds. This led the team to investigate the possibility of adding a heave plate (circular disk) at the base of the spar to control the response to within 3.1 m (10 ft), which is the limit set by a typical compensation system. Important design issues arose with regards to the geometry of the plate, i.e. diameter and thickness. Numerical simulations and model testing were used to identify the influence of a heave plate on the heave response of the spar. Heave response for various diameters and thickness were investigated. Comparison of added mass and damping values were found to be in reasonable agreement. Issues such as effect of a centerwell and moorings, plate cutouts for ease of transportation were also investigated. Discussion of the experimental results and comparison with numerical simulations are presented in this paper, and some recommendations are made on optimum heave plate geometry.© 2002 ASME

Truss Spar Vortex Induced Motions: Benchmarking of CFD and Model Tests

Spar production systems are subject to Vortex Induced Motions (VIM) which may impact mooring and riser design. Helical strakes are employed to mitigate VIM. Model tests are typically required to validate the performance of the strakes. This paper will report on the results of benchmarking studies that have been conducted over the past few years to compare model tests with computational fluid dynamics (CFD). The paper discusses comparisons of CFD with model tests, “best practices” for the use of CFD for these classes of problems and issues related to turbulence modeling and meshing of problems at large Reynold’s numbers. This work is ongoing.Copyright

CFD Simulation of Truss Spar Vortex-Induced Motion

Helical strakes are used to suppress the Vortex-Induced Motion of Truss Spars. Model experiments have demonstrated the efficiency of strakes in the Truss Spar design but also indicate that the VIM response is sensitive to the details of strake design and placement of appurtenances around the Spar hull. It is desirable to study these hydrodynamic effects using CFD. The following paper is a continuation of some of the earlier CFD simulations on this subject (see, J. Halkyard, et al., “Benchmarking of Truss Spar Vortex-Induced Motions Derived from CFD with Experiments”, Proceedings of OMAE’05). This paper in particular deals with the effect of holes in the strakes and appurtenances and their placement. All the simulations were done at model scale (1:40 scale model of an actual Truss Spar design) to compare the motions with experimental results. Mesh sensitivity and turbulence modeling issues are also discussed. Calculations were done using general purpose CFD code Acusolve™.Copyright

Comparison of Time and Frequency Domain Analysis With Full Scale Data for the Horn Mountain Spar During Hurricane Isidore

The Horn Mountain Spar is located in 1,654 m of water about 135 km from Venice, Louisiana in the Gulf of Mexico. The facility was instrumented extensively to measure key spar and riser response parameters (Edwards et. al. 2003). Halkyard et. al. (2004) and Tahar et. al. (2005) have compared measured spar responses such as motion and mooring line tensions with numerical predictions. This paper extends the work done on comparison of the full scale data during hurricane Isidore. All previous numerical simulations were based on a time domain analysis procedure. One concern related to this method is that it is computationally intensive and time consuming. In the initial stages of a project, a frequency domain solution may be an effective tool compared with a fully coupled time domain analysis. The present paper compares results of time domain and frequency domain simulations with field measurements. Particular attention has been placed on the importance of the phase relationship between motion and excitation force. In the time domain analysis, nonlinear drag forces are applied at the instantaneous position. Whereas in the frequency domain analysis, nonlinear drag forces are stochastically linearized and solutions are obtained by an iterative procedure. The time domain analysis has better agreement with the field data compared to the frequency domain. Overall, however, the frequency domain method is still promising for a quick and approximate estimation of relevant statistics. With advantages in terms of CPU time, the frequency domain method can be recommended as a tool in pre-front end engineering design or in a phase where an iterative nature of design of an offshore structure takes place.Copyright

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

Full Scale Data Comparison for the Horn Mountain Spar

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). The present paper discusses the results of these measurements and comparison with analytical predictions of spar behavior during two selected events, hurricane Isidore in September 2002 and a summer storm in August 2003. Particular attention has been placed on the slowly varying surge and pitch motions and the importance of coupling with risers and mooring on hull motions. Our conclusion is that uncoupled analytical models for spar behavior predict accurately the wave frequency responses, however riser coupling has an influence on the slowly varying responses. This conclusion is consistent with earlier measurements of classic spar behavior (Gupta et al, OTC 2000, Prislin et al, OTC 1999).Copyright

Kikeh Development: Spar Topside Floatover Installation

Deck installation is always a major challenge for floating structures, particularly deep draft floaters like the Spar which must be installed in relatively deep water. Derrick barges have been used for Spar deck installations until now. Murphy’s Kikeh Spar, the 1st outside of the Gulf of Mexico, is the 1st Spar to use topside floatover installation technology and represents the 1st catamaran floatover installation of a topside onto a floating platform in open water. The successful execution of the Kikeh 4000Te topside floatover installation has established this method as a viable and cost effective alternative to lift installation. This paper presents an overview of the topside floatover installation for the Murphy Kikeh Spar. The paper describes all aspects of the floatover installation including topside loadout and transportation using a single barge, transfer from the transportation barge to the catamaran barge configuration, catamaran open water tow and floatover to the Spar at the Kikeh location. This paper focuses on the naval architectural, structural and operational tasks that were performed in support of these operations. INTRODUCTION New offshore developments may include several Spar type platforms with varying deck sizes ranging from 16,000 mt to 35,000 mt dry weights. Topsides for all previous Spar platforms were installed by deck lifts ranging from about 3,000 mt (Oryx single lift) to over 10,000 mt multiple lifts. The largest deck installed this way on a Spar was the Diana Deck with a dry weight of about 20,000 mt. This deck installation required five separate lifts [1]. There are potentially large advantages, particularly for the large decks, if an integrated deck could be installed using floatover methods. Some advantages include: • Schedule and cost advantages for the integration and commissioning of modules on land rather than at sea, • Uncoupling the deck fabrication schedules from the availability of heavy lift vessels There is a long history of successful floatover deck operations for floating Gravity Based Structures (GBS) and other floaters in protected waters ranging from the Beryl A Mobil facility in the UK North Sea, 1975, 14000mt deck weight to the Hibernia HMDC facility offshore Newfoundland Canada, 1997, 46000mt deck weight. Until recently, however, only one (1) floatover has been performed on a floating structure in open waters which was the 24,000 ton Auger TLP Deck in 1993 [3]. In 2006, the first floatover deck was installed on a Spar platform: the Kikeh Spar. This installation was performed in 1320 m water depths in the South China Sea, offshore East Malaysia. The deck weight was 4000 mt and the swell at the time of installation was Hs of 0.7m at periods of 7 8 seconds. This was also the first catamaran type floatover performed in open waters. The 46,000 mt Hibernia deck was set using a catamaran configuration in protected waters of Bull Arm in Newfoundland. There are some significant differences between installing decks on a fixed platform versus a floating platform, and of course between sheltered and open water installations. Some of these differences are listed below. The most important difference is the length of time to perform the load transfer between the transportation barges and the floating structure. For a fixed platform installation, jacks may be used to transfer the majority of the deck load within a matter

Floatover Deck Installation on Spars

Deck installation is always a major installation challenge for floating structures, particularly deep draft floaters like the spar which must be installed in relatively deep water. Derrick barges have been used for spar deck installations until now. The 4000 mt deck for the Kikeh Spar was successfully installed using the floatover method in November 2006, off the coast of Sabah in the South China Sea. This demonstrates the feasibility of this concept and opens the door for more floatover decks on spars in the future. This paper will review the technical challenges associated with this type of installation. In particular, the authors will review past studies, which included analysis and model testing of similar deck floatovers for decks up to 30,000 t, and the analysis methods use to validate the procedures and equipment which was successfully used on the Kikeh project. The requirements for application of this technology in the Gulf of Mexico will be highlighted.Copyright