In the framework of the studies on the DeepStar Theme Structures for deepwater in the Gulf of Mexico, contractors and research institutes have carried out series of pre-model test computations. The DeepStar Theme Structures are a turret moored FPSO, a classic SPAR and a standard TLP. The Theme Structures were exposed to hurricane and loop-current condition. The scatter was significant in the pre-model test computational results. In order to compare the computed results with results of model tests, a series of benchmark model tests were carried out. Based on the model tests results post-model test validation analysis and guidelines for ultra deepwater model tests and coupled analysis for deepwater structures were carried out. In this paper, the sequence of the DeepStar Theme Structure developments will be given briefly and the philosophy behind and the performance of the benchmark model tests will be presented. Introduction In the framework of the DeepStar Theme Structures studies for deep water in the Gulf of Mexico, contractors and research institutes carried out series of pre-model test computations. The computations were carried out based on the technical specifications for the Theme Structures and weather conditions. The structures were exposed to hurricane and loopcurrent condition. The FPSO and Spar were installed in 3,000, 6,000 and 10,000 ft of water depth, while the TLP was installed in 3,000 and 6,000 ft. Comparing the pre-model test computation results of the same structures as carried out by different participants, the scatter was significant. In order to compare the computed results with results of model tests, a series of benchmark model tests were carried out. The motivation for the selected model scale and the description of the model basin facility is given. The description is given of the model manufacturing including mooring lines and risers. Further, the calibration of hurricane and loop-current environmental conditions is presented. A hurricane and a loop-current current profile had to be adjusted to match the specifications. In addition, the associated turbulences were measured in the basin. The results are presented. Concerning the waves, attention was given to not only the wave spectra but also to the wave group spectra. The Theme Structures were exposed to an API wind spectra. The procedure to adjust the wind spectra is described. The test set-up, instrumentation, data reduction and the program of the benchmark model tests will be presented. Finally some conclusions are drawn. Since the results of the benchmark model test program will be used for post-model test computations (model-the-model), the performance and the reporting of the model test set-up must be as accurate as possible. In the same framework of the DeepStar Theme Structures project, guidelines for model testing in ultra deepwater and guidelines for coupled analysis of deepwater floating structures are established. This paper documenting the benchmark model tests is the first in a series of papers covering the DeepStar Theme Structures project. The other papers deal with prediction of the responses of the Theme Structures (model test vs. analysis), and development of guidelines for model test in ultra deepwater and for coupled analysis of deepwater systems.

This paper summarizes one of the studies conducted by the ABS Joint Industry Project (JIP), “Polyester Rope Stiffness Modeling, Testing, and Analysis”. The objective of this study was to collect and investigate results of past studies, test procedures and data, code requirements, and fiber rope mooring analysis methods dealing with polyester rope stiffness. Based on the results of the study, conservative and practical stiffness models, test data analysis, and mooring analysis method were developed. The results from this study have been incorporated in the 2011 ABS Guidance Notes on The Application of Fiber Rope for Offshore Mooring [13]. Major conclusions of this study are as follows: 1. The 2-slope static-dynamic model is the best approximation for the complex fiber rope elongation behavior. It is based on rigorous research and allows efficient mooring analysis using typical commercial software. 2. Stiffness testing should be conducted to generate stiffness equations and design charts instead of design stiffness values. The design stiffness values can be obtained from these equations and design charts. A practical procedure for test data analysis is recommended. 3. Mooring analysis can be efficiently conducted in frequency or time domain with commercial mooring analysis software based on the recommended stiffness model.

The subsea Coulomb field is a two-well gas/condensate development in 7500 fsw, tied back to the NaKika platform in the Gulf of Mexico via a single 27-mile, 8" bare flowline. First production was in June 2004. The design basis employed continuous injection of monoethylene glycol for hydrate inhibition along with a "wax-in-place strategy"; this assumed a low rate of wax deposition, controlled through the use of inhibitor, such that solids accumulation during field life would not constrain production. Fluids from one well are significantly waxier than the well used in the design basis, but this was not known until April 2004. Prior to startup, predictions of the most severe deposition rate in the absence of inhibitor suggested a steady increase in flowline ?P to 3500 psi after 20 years of production. However, in reality the Coulomb flowline experienced a rapid increase in pressure drop to ~4000 psi within the first month of production from the waxier of the two wells, ultimately resulting in the temporary shut-in of that well. Correlation of field observations, transient multiphase modeling and laboratory experiments eventually determined that the deterioration in flowline performance was most likely due to accumulation of a highly viscous material, either a wax/glycol/ condensate emulsion or a wax slurry, stable at ambient seafloor emperature. Further work strongly suggested that burying the flowline in order to improve heat retention by the produced fluids might mitigate this phenomenon. This paper discusses the evolution of the technical justification to bury the flowline and the ultimate impact of the burial with respect to re-establishing (and ultimately increasing) production from the two wells. Introduction The "Coulomb" subsea gas/condensate field is located in the deepwater Gulf of Mexico, at a water depth of approximately 7500 feet. Production from two subsea completions flows 27 miles along the seabed through a single, 8-inch subsea flowline, tied to the host platform by a steel catenary riser (Figure 1). The two wells, referred to as C-2 and C-3, produce fluids with significantly different condensate/gas ratios (CGRs); production from C-2 is rather leaner (65 bbl/MMscf) than that from C-3 (200 bbl/MMscf). Monoethylene glycol (MEG) is injected at the tubing head of each well to provide continuous hydrate inhibition throughout the subsea system. Shortly before production was started up, analysis identified that the fluids from the C-3 well contained a higher wax percentage than those from the C-2 well, which had been used to develop the design and operational philosophy for the field. Shortly after startup, a steady increase in pressure drop was observed in the flowline/riser system; the pressure gradient was approximately twice that predicted by steady-state multiphase flow models. The high ?P in the subsea flowline was attributed at first to wax deposition; the backpressure imposed on the flowing wells caused a reduction of some 30% in the total production capacity of the system. Attempts were made to identify an effective paraffin inhibitor that might reverse the phenomenon, however none was found.

TXThis paper presents preliminary results of the DeepStar polyester taut 'leg mooring (TLM) project. The 3100 ft (945 m) deep single-leg TLM contained short and long sections of three different polyester rope designs.It was installed in the Gulf of Mexico in 1996 and retrieved 27 months later, after exposure to two hurricanes. The retrieved polyester rope sections were then examined, tested, and analyzed in order to assess any indications of degradation.

The growth in deepwater application of synthetic fiber rope moorings has created a need for an industry design code for such mooring systems. American Petroleum Institute (API) and the DeepStar consortium took the initiative by having the American Bureau of Shipping (ABS) consolidate existing design guides, joint industry research results, and industry operation experience into a new API Recommended Practice (RP) -- 2SM [Ref. 1]. This paper presents an overview on the development of this RP. Synthetic fiber ropes have many advantages for deepwater moorings, but their unique properties and influence on the mooring system performance must be properly accounted for in the design. Furthermore, rope manufacturing, testing, and deployment require special considerations. This RP is therefore developed to address the unique features associated with mooring design and analysis; rope design, testing and manufacturing; rope handling and installation; and rope inspection and maintenance. It is hoped that use of the RP can help achieve synthetic fiber rope mooring systems that have similar reliability as their steel counterparts, and can improve efficiencies in deepwater field developments. The paper outlines the development basis and principles, the participation of an Ad-Hoc API Work Group consisting of operators, designers, regulatory and government agents, rope manufacturers, and installation contractors. It also discusses the key technical issues identified and resolved, which include design concerns and factors of safety for overloading, fatigue, stiffness, creep rupture, minimum tension, rope damage evaluation, and discard criteria. Introduction and Background DeepStar has been actively involved in the study and testing of alternative deepwater mooring components since its inception in 1992 (Fig. 1), most notably polyester moorings. The DeepStar funded polyester taut leg mooring (TLM) test system was installed by Aker Marine Contractors in 1996 and retrieved in 1998 after a 27-month service in the Gulf of Mexico. The intent of the test program was to gain installation and handling experience, to determine extent and rate of creep of rope samples and bedding-in requirements, and to study other effects on the rope due to exposure to a realistic environment. In January 1998, a workshop on synthetic fiber mooring rope was co-sponsored by the Minerals Management Service (MMS) and the Composites Engineering and Application Center (CEAC) of the University of Houston [Ref. 2]. During that meeting, participants acknowledged that use of synthetic fiber ropes is probably the only practical solution for ultradeep water moorings. MMS also pointed out the need for an industry standard for such mooring systems, and that an API/ISO code is preferred over other design guidelines. As a fitting way to culminate several years of development in synthetic mooring technology, and to take advantage of the collective learning and expertise of DeepStar members and the industry as a whole, DeepStar volunteered to undertake this task. In September 1998, API and DeepStar initiated the present work by having ABS consolidate existing design guides, joint industry research results, and industry operation experience into the new API RP 2SM.