This paper presents a discussion of the effect of including mooring line dynamics and riser friction on Spar response. Data from the Neptune Spar revealed that the heave motion of the vessel was considerably less than predicted by an uncoupled analysis1. It was believed that the primary cause of the reduced heave was damping forces such as friction between the risers and the supporting guides and mooring line dynamic drag that were unaccounted for. A new analysis capability was subsequently developed to simultaneously predict the dynamic response of the vessel, mooring lines, and risers. Results of the coupled analysis reveal that mooring line dynamics and riser friction have significant effect on the Spar heave response. In this paper, comparison of coupled analysis results to Neptune Spar motions during Hurricane Georges will be presented. As a result of reduction in heave response, the draft of the Spar can be reduced. A coupled analysis of a shorter draft Spar is presented. Results for matthieu's instability problem, effect of coupling in different water depths and comparison of coupled response of classic versus truss Spar configurations are also presented. Coupled response of Spar and a second vessel moored together with chains is presented to demostrate the multiple vessel simulation capability of the coupled analysis program. Introduction A characteristic feature of moored offshore structures such as a Spar or a semi-submersible platform is their slow oscillatory motion that occurs at resonant frequencies, well beyond the frequency range of the wave spectrum. Since the damping of such structures is low at resonant periods, correct estimation of damping is important in predicting the motions, maximum offsets and extreme mooring loads. Generally, response of Spar platforms is predicted conservatively by excluding the damping from mooring lines and risers. Damping from risers on the Spar platform occurs from Coulomb friction at the riser guides and keel as well as from the hydrodynamic viscous effects. The risers exert a normal force at the keel guide and other air can guide locations. As the Spar pitches or offsets laterally, the riser induced normal reaction increases. As the Spar heaves vertically, a friction force is developed on the guides, which is proportional to normal reaction from the risers and depends on the coefficient of friction. If the Spar vertical motion is small enough, the static friction will prevent the Spar from moving further. When the Spar motions are larger, the kinetic friction opposes the motion and thus produces damping. In addition to the damping, coupling forces between the riser and the Spar arise in both surge/sway motions as well as pitch/roll motions. The buoyancy force of the riser air cans provide additional restoring moments that affect the pitch/roll motions. The riser lateral reaction at the keel and other guide locations affects the surge/sway motions. Current drag on risers, if significant, produces additional lateral reaction at the keel which can affect both surge/sway and pitch/roll motions.
A time domain analysis procedure is used to calculate maximum design tensions for the mooring system of a truss spar. Fairlead motions are used as input to calculate time histories of dynamic mooring line tensions. Several random simulations are used to derive design values of maximum tension. The variability from simulation to simulation in maximum tension and in the dynamic portion of the response is summarized. Alternative methods for estimating the maximum value are compared and the effect of the number of simulations on the uncertainty in the maximum is demonstrated. The dynamic characteristics of the mooring line tensions are examined by looking at the high frequency and low-frequency portions of the response. A comparison of mooring line tensions for a truss spar and a classic spar is also provided. In addition, maximum tensions from the time domain analysis are compared with results from a frequency domain analysis.
Oil and gas development in certain harsh environments, such as extreme storm prone areas or arctic regions, may require the floating production platform to be designed to enable it to be released from its risers and moorings and moved out of the way of the approaching threat. Such floating platforms generally employ an underwater disconnectable buoy to support the moorings and risers after the main platform is moved away. For a deep draft floating structure, the risers can be released from their support near the top of the platform and lowered through the hull to a disconnectable buoy. In such a case, the risers can be routed through I-tubes and lowered in a controlled manner using rigging during a normal release operation. However, an emergency disconnection may require lowering of the risers without guidance of rigging. To avoid damage to the risers and the buoy during the emergency disconnection, risers can be fitted with passive damping devices to limit the lowering riser speed. This paper presents the numerical efforts to define the emergency riser release and lowering procedure. CFD simulations were performed to evaluate the hydrodynamic behavior of a disconnected riser in a flooded I-tube with the controlling devices attached to the risers. Applying the CFD results, riser lowering performance was computed using finite element analysis method. Primary parameters that affect flexible riser behavior, including stress level and curvature, are identified and sensitivity study results are presented. This paper concludes that a safe and controlled riser release procedure and system is achievable.
TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract The development of the cell spar Platform for the Kerr-McGee Red Hawk Field in the Gulf of Mexico represents the first application of this deepwater concept. The Red Hawk cell spar Platform was installed in about 5300 feet of water at Garden Banks Block 876 in early 2004 and achieved full production within 15 days of start-up.This paper describes the design of the integrated spar hull and topside, with particular emphasis on the hull and related hull systems, due to their novelty. It provides a general description of the hull structure as well as the various hull and marine systems, riser support systems, and main structural appurtenances, including strakes.The operation of the ballast system using compressed air is compared to a conventional ballast pump design. The inspection philosophy for the ballast tanks is described along with regulatory issues related to the ballast system. The operation of the mooring line winch and the handling of the chain at the top of the spar is also described. Vortex Induced Motion (VIM) performance, the upending of the spar, and initial impressions of the spar motion by field personnel are also discussed.Red Hawk is the first application of the cell spar hull form. Building on several years of development, this new technology offers a more cost-effective solution than previous spar designs for small topsides payloads and low riser counts. The engineering of the cell spar demanded solutions to several challenging new problems. These solutions are now ready for implementation on future projects.
This paper will address the advantages and challenges involved in designing a truss spar for wet tree application. The spar has traditionally been used as a dry tree concept with top tensioned production risers. Chevron's Tahiti project in the Gulf of Mexico will be the first wet tree truss spar. For Tahiti, the favorable motions characteristics of the spar enabled successful fatigue design of the sour service HPHT steel catenary risers without use of corrosion resistance alloys. Use of polyester mooring significantly improved hull vortex induced motions response to loop currents relative to traditional chain-wire-chain mooring and enabled SCR departure angles to be reduced. HPHT production riser were hung off using pull tubes, which simplified riser installation and eliminated uncertainties related to performance of flex joints under HPHT conditions. Challenges for a wet tree truss spar design include increased hull density and float-off requirements due to smaller moonpool dimensions than traditional spars with top tensioned risers. In addition, the large number of pull tubes created challenging VIV fatigue designs and as a result, stringent welding and NDE requirements were imposed for the pull tubes in the shipyard.
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