Abstract. We report on a field experiment held near Silver Lake playa in the Mojave Desert in February 1999 with the Marsokhod rover. The payload (Descent Imager, PanCam, Mini-TES, and Robotic Arm Camera), data volumes, and data transmission/receipt windows simulated those planned for the Mars Surveyor mission selected for 2001. A central mast with a pan and tilt platform at 150 cm height carried a high-resolution color stereo imager to simulate the PanCam and a visible/near-infrared fiberoptic spectrometer (operating range 0.35-2.5 /xm). Monochrome stereo navigation cameras were mounted on the mast and the front and rear of the rover near the wheels. A field portable infrared spectroradiometer (operating range 8-14 /xm) simulated the Mini-TES. A Robotic Arm Camera, capable of close-up color imaging at 23 /xm/pixel resolution, was used in conjunction with the excavation of a trench into the subsurface. The science team was also provided with simulated images from the Mars Descent Imager and orbital panchromatic and multispectral imaging of the site obtained with the French SPOT, airborne Thermal Infrared Mapping Spectrometer, and Landsat Thematic Mapper instruments. Commands sequences were programmed and sent daily to the rover, and data returned were limited to 40 Mbits per communication cycle. During the simulated mission, 12 commands were uplinked to the rover, it traversed •90 m, six sites were analyzed, 11 samples were collected for laboratory analysis, and over 5 Gbits of data were collected. Twenty-two scientists, unfamiliar with the location of the field site, participated in the science mission from a variety of locations, accessing data via the World Wide Web. Remote science interpretations were compared with ground truth from the field and laboratory analysis of collected samples. Using this payload and mission approach, the science team synergistically interpreted orbital imaging and infrared spectroscopy, descent imaging, rover-based imaging, infrared spectroscopy, and microscopic imaging to deduce a consistent and largely correct interpretation of the geology, mineralogy, stratigraphy, and exobiology of the site. Use of imaging combined with infrared spectroscopy allowed source outcrops to be identified for local rocks on an alluvial fan. Different lithologies were distinguished both near the rover and at distances of hundreds of meters or more. Subtle differences such as a contact between dolomite and calcite were identified at a distance of 0.5 km. A biomarker for endolithic microbiota, a plausible life form to be found on Mars, was successfully identified. Microscopic imaging of soils extracted from the surface and subsurface allowed the mineralogy and ñuvial history of the trench site to be deduced. The scientific productivity of this simulation shows that this payload and mission approach has high science value and would contribute substantially to achieving the goals of Mars exploration.
Current fatigue design criteria for offshore structures are based on SN-data and the Miner-Palmgren approach. The acceptable cumulative damage in some codes is made dependent on a vaguely defined feature of accessibility for inspection. This paper presents a reliability based fracture mechanics (FM) calibration of the allowable cumulative damage which depends on the quality of inspection in terms of probability of crack detection curves. Target safety levels are taken to correspond to a cumulative damage of 0.1 and 0.3 with no effect of inspection. INTRODUCTION Fatigue and fracture are important failure modes of welded offshore structures. Significant progress has recently been made in developing rational design methods to handle these failure modes, especially by using reliability methods, see e.g. [1]. Fatigue design criteria are commonly based on the SN-Miner- Palmgren approach where the allowable cumulative damage, D = ?n/N1, is made dependent upon the consequences of failure and access for inspection as specified e.g. by NPD [2]. However, existing criteria are qualitative. The purpose of the present paper is to quantity the effect of inspection depending upon its quality for a given inspection strategy. Welded joints in TLP tethers and tubular joints of jackets are considered for illustration purposes. This is done by determining the allowable cumulative damage in the SN approach as a function of the quality of inspection. The calibration of this simple design approach is based on a probabilistic fracture mechanics model of fatigue crack growth, CRACK GROWTH MODEL The crack growth is described by a generalization of the Paris-Erdogans equation,(Mathematical equation) (available in full paper) as introduced by Newman & Raju [3]. In this approach the surface crack is assumed to be semielliptical with semiaxes a and c at any stage during crack growth, where a is the crack depth, c is the half crack length, N is the number of stress cycles, ?K is the stress intensity factor range, and C and m are material parameters. Analysis of the distribution of the material parameter, C, for steels shows C to approximately follow a Iognormal distribution. The crack growth parameters may be reasonably modelled as: C Iognormally distributed with C(I.1 .10−13 [4],0.55), where (,) are the mean, with units N and mm, and the coefficient of variation, respectively. Instead of using C as a random variable, InC is used herein, see Tables 1 and 2. m is assumed fixed and equal to 3.1 [4]. The stress intensity factor range at any point of the crack front for surface cracks in plane plates subjected to tension and bending loads is expressed as [3] (Mathematical equation) (available in full paper) where S1 and Sb are tension and bending stress ranges, respectively. S is the total stress range, t is the plate thickness and W is the width of the plate.
The tension leg platform (TLP) concept is considered to be economically competitive with other offshore production concepts in deepwater locations. The critical part of this concept regarding structural failure is the tether system, in which fatigue is an important failure mode. The present paper presents a computational efficient and general probabilistic procedure for estimating the fatigue reliability of the TLP tether system. Two crack growth rate models of single crack sites are applied. Among uncertainties accounted are those in the long-term stochastic load process, the fatigue strength, and the initial crack sizes.
This paper addresses the feasibility of developing an ultra-deepwater gas field by producing directly from subsea wells into Compressed Natural Gas (CNG) Carrier ships. Production interruptions will be avoided as two Gas Production Storage Shuttle (GPSS) vessels storing CNG switch out roles between producing/storing via one of two Submerged Turret Production (STP) buoys and transport CNG to a remote offloading buoy. This paper considers the challenges associated with a CNG solution for an ultra-deepwater field development and the specific issues related to the risers. A Hybrid Riser Tower (HRT) concept design incorporating the lessons learned from the Girassol experience allows minimisation of the vertical load on the STP buoys. The production switchover system from one GPSS to the other is located at the top of the HRT. High-pressure flexible flowlines with buoyancy connect the flow path at the top of HRT to both STP buoys. System fabrication and installation issues, as well as specific met ocean conditions of the GOM, such as eddy currents, have been addressed. The HRT concept can be also used for tiebacks to floating LNG plants.
The Submerged Turret Loading Buoy System (STL TM ), which is well proven in the North Sea for Offshore Loading of Crude Oil, is now developed for Discharge of Gas and will be used as an Offshore LNG receiving Terminal. Presently two 138,000 m 3 LNG Carriers are being constructed at Daewoo Shipyard with onboard re-gasification and shipboard system for connection to the STL Buoy. The first Submerged Loading Buoy based Offshore LNG Receiving Terminal installation will be in Gulf of Mexico -West Cameron Block 603. The paper presents the technology and how the same loading technology system can be used for CNG Vessels; both for loading and offloading of Natural Gas.
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