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Abstract The Ichthys Gas Field is located 220 kilometers offshore from the North West coast of Western Australia. The field is located within a cyclone-prone region and has two permanently moored floating production units in a water depth of approximately 250m. The floating systems consist of a semi-submersible Central Processing Facility (CPF) and a Floating Production Storage and Offloading (FPSO) facility. This paper presents an overview of the riser solutions developed for each facility while documenting challenges faced in design, cognisant of installation limitations and complex interface management within a large EPCI project. Both facility riser systems contain aspects that are first of their kind within the region and were developed with the added complexity of a 40-year riser design life and the requirement to withstand a 10,000 year return period survival cyclonic event. Within a single mooring sector to the North of the CPF, 25 risers are supported on a single riser support structure (RSS) in a fixed-S configuration. The RSS, which is the subject of another paper , has an arch length of 130 m to support the risers at a height of 110 m above the seabed. This paper explains the design aspects of the system including optimizing riser spacing and riser gutter geometry in close proximity with varying minimum bending radii. A robust design was required that allowed for future phase riser installation and riser replacement. A lazy-S riser configuration was developed to support 15 risers from the FPSO. The solution consisted of three mid depth buoys (MDBs), one in each mooring sector. The MDB design is the subject of another paper . Design challenges included significant dynamic response of the permanently moored FPSO in 10,000 year return period survival cyclonic conditions. MDB dynamic response was found to be significantly impacted by added mass, which provided challenges in maintaining positive tether tension and minimizing interface loads into the MDB structure and foundation. Diffraction analysis of the MDB structure was used to determine accurately the added mass coefficient in order to refine interface loads into the MDB and its foundation.
Abstract An innovative step riser configuration and its variations are presented for use in deep or ultra deepwater field developments. The configurations are based on existing proven technologies and free-hanging catenary configurations. The step riser configuration is characterized by the utilization of buoyancy tanks/modules array to generate over hundreds of tonnes net uplift buoyancy, directly exerting that buoyancy force to the riser midline connection rather than the riser body. This technology enables significant improvements in the stress utilization and fatigue performance of a flexible riser. In addition, the reduced tension loads greatly decrease installation vessel requirements. The large buoyancy system not only reduces the top tension but also decouples the motion passing from the upper section to the lower section. As a result, high compression and over-bending issues near the touchdown zone can be mitigated. This paper also investigates step riser configuration performance and its variations, step wave and tensioned step riser configuration, to address the specific issues associated with different deepwater applications. Parametric studies are performed using commercial software to predict and compare the behavior of the stepped riser system in various dimensions. Also discussed is a scaled step riser model test to validate the performance of the step riser configuration. The step riser model was towed horizontally at varying speeds to investigate the riser's tension, curvature and vortex induced vibration (VIV) responses. The study indicates that the step riser configuration and its variations are a practical cost effective solution for flexible riser systems in deepwater applications with severe metocean conditions.
The increasing use of reliable and flexible position reference systems for offshore operations is placing added emphasis upon the use of acoustic position-reference systems. Position measurement requirements exist for both navigation and precise station-keeping applications. To meet these requirements, several acoustic position reference configurations are available. In this paper, the various acoustic methods and position reference configurations are described and their merits and limitations are discussed.
For the offshore drilling operations the short-baseline configuration which measures the position of a single beacon or transponder in ship coordinates is a superior choice. The short-baseline systems are more flexible, more convenient to operate, less complex, and less expensive to operate than long-baseline systems. The latter establish the position of the vessel relative to a transponder grid mounted on the ocean floor. Without a precise sound-velocity-profile calibration and correction, long-baseline systems show no improvement in accuracy over the short-baseline systems.
For stat ion-keeping operations the Short-Baseline Configuration is recommended. This configuration can be implemented with a null accuracy, over the beacon, of 0.5 percent of water depth.
For a navigation application, the short-baseli.ne beacon configuration does not have the range capability. However, the short-baseline transponder configuration, which measures its position from a range-bearing method, can adequately provide for the navigation requirements. It is concluded from the standpoint of equipment complexity, operating convenience, and operating cost that the short-baseline transponder configuration provides a superior choice over the long-baseline configurations. For applications such as self-anchor laying or pipe-laying navigation, the short-baseline transponder system can adequately provide the need.
In the expanding offshore drilling operations the requirements for reliable position references are becoming exceedingly more important. With drilling operations moving to deeper waters, the submersible rigs (resting on the bottom) are unattractive and, therefore, floating rigs, semisubmersibles, barges and ships are used. These vehicles must be either moored (anchor net), actively positioned, or both. In each case, the necessity for an accurate position reference exists. Once the desired drilling location has been established (usually by radio navigation), it is necessary to maintain position reference relative to this location. Radio navigation, although sufficiently accurate for general navigational purposes, does not usually possess the accuracy and resolution necessary for precise station-keeping operation.
Taut-wire systems are also used as position reference sources. These, however, lack in flexibility, accuracy and operating convenience. Position reference systems based upon acoustic methods can adequately provide for the position reference needs in offshore operations. A number of methods are available, each displaying its own set of advantages and disadvantages. This paper provides a description of the basic principles of operations, the implementation and the performance capability of each candidate acoustic position reference .system. Several candidate systems are described and evaluated relative to the requirements in offshore operations, station keeping and limited range navigation.
In acoustics a source (projector) sends acoustic energy into the medium and the energy propagates through the medium as explained by the complex wave propagation equations.