Measurement of Soil Suction and Soil-Water Characteristics of Bentonite-Sand Mixtures

The soil-water characteristic curve (SWCC) is a widely used experimental means for assessing fundamental properties of unsaturated soils for a wide range of soil suction values. The study of SWCC is helpful because some properties of unsaturated soils can be predicted from it. Nowadays, much attention has been paid to the behaviours of highly compacted bentonite-sand mixtures used in engineering barriers for high level radioactive nuclear waste disposal. It is very important to study the various performances of bentonite-sand mixtures in order to insure the safety of high-level radioactive waste (HLW) repository. After an introduction to vapor phase method and osmotic technique, a laboratory study has been carried out on compacted bentonite-sand mixtures. The SWCC of bentonite-sand mixtures has been obtained and analyzed. The results show that the vapor phase method and osmotic technique is suitable to the unsaturated soils with high and low suction.

Application of a Viscoelastic Model for Polyester Mooring

A new practical method to simulate time-dependent material properties of polyester mooring line is proposed. The time-dependent material properties of polyester rope are modeled with a standard linear solid (SLS) model, which is one of the simplest forms of a linear viscoelastic model. The viscoelastic model simulates most of the mechanical properties of polyester rope such as creep, strain-stress hysteresis and excitation period-dependent stiffness. The strain rate-stress relation of the SLS model has been re-formulated to a stretch-tension relation, which is more suitable for implementation into global performance and mooring analyses tools for floating platforms. The new model has been implemented to a time-domain global performance analysis software and applied to simulate motion of a spar platform with chain-polyester-chain mooring system. The new model provides accurate platform offset without any approximation on the mean environmental load and can simulate the transient effect due to the loss of a mooring line during storm conditions, which has not been possible to simulate using existing dual-stiffness models.

Experimental Study on Influences of Wave Height on Liquefaction Depth of Silty Soil Bed

Storm waves tend to cause seabed liquefaction by exerting strong cyclic loads on the seabed of the Yellow River Delta. In order to study influences of different wave heights on liquefaction depth of the soil bed, silty soil taken from the Yellow River Delta is used to prepare a soil bed for flume experiments and local parts of superficial soil layer were disturbed by hand. The weakened soil tended to liquefy and slide under wave actions and the liquefaction depth increased with the increasing of wave height. Based on the experimental results, an empirical relationship was proposed between liquefaction depth of silty soil bed and wave height under experimental conditions.

FourStar™ Dry-Tree Semisubmersible Development

This paper discusses the development of a dry-tree semisubmersible (DTS) platform concept appropriate for deployment in non-hurricane/non-cyclonic environments worldwide, and the verification of the concept through wave basin model tests. An example configuration is presented for an application in 2,100 m water depth offshore Brazil.

An Innovative Synthetic Mooring Solution for an Octagonal FPSO in Shallow Waters

An octagonal FPSO has been proposed for marginal oil and gas development in shallow waters. A shuttle tanker will be deployed near the FPSO during offloading operations. This new concept simplifies the design and manufacturing processes, yet maintains full production, storage, and offloading functions of a conventional ship-shaped FPSO. However, design of the mooring system for this floating unit imposes technical challenges due to: 1) high environmental loads expected on this unit, 2) large dynamic offsets of the unit in shallow waters, and 3) inadequate performance of catenary mooring systems in shallow waters. Thus, development of a viable station keeping solution becomes a key issue to the new concept FPSO design. In this paper, an innovative mooring system is designed to meet the challenges. The FPSO mooring system consists of pile anchors, bridle chains, anchorage buoys, and polyester ropes. Nine mooring lines are grouped into three bundles which evenly spread around the FPSO. The shuttle tanker is attached to the FPSO with a nylon rope hawser at the bow and secured to pre-installed anchorage buoys at the stern with two other nylon ropes. Analyses have been performed for the FPSO mooring system. It is concluded that the proposed mooring system is fully functional and effective.

Control of Unstable Ship Motions Using Anti-Rolling Tanks: Erosion of Safe Basins

The objective of the paper is to apply modern numerical techniques of nonlinear dynamics to the problem of control of the roll motion employing U-shaped anti rolling tanks (ART). Parametric rolling in head seas is the focus of the paper. A transom stern small vessel, well known for her tendency to develop strong parametric excitation is investigated. Nonlinear equations are employed to describe the liquid motion inside the tank, the forces and moments generated by the tank on the ship and the coupled ship motions (heave, roll and pitch). These are numerically solved for different initial conditions. An analysis of the dynamical behavior of the vessel with stabilization is presented in the form of numerical limits of stability, safe basins, integrity curves and integrity surfaces. Finally, curves of critical amplitude for different wave tunings are computed. A design procedure for quantitative assessment of the level of parametric rolling mitigation by means of ART’s is discussed.

Supporting New Insight in Pipeline Hydrodynamics Using Stochastic Approaches on External Corrosion Damage

Several recent discoveries in the fluid-structure interactions between the external flows and circular cylinders placed close to the wall have added new values to the hydrodynamics of unburied marine pipelines on a seabed. The hydrodynamics of waves and/or currents introduced vortex flows surrounding the pipeline. External corrosions formed in marine pipelines were assumed to be partly contributed by such fluid-structure interactions. The spatial consequences of such interactions were of interest of this study. This paper summarized some experimental and numerical works carried out by previous researchers on these new discoveries. Actual field data were utilized in this study to support this hypothesis. The characteristics of corrosion orientations in the pipelines were studied comprehensively using stochastic approaches and results were discussed. Results adopted from the field data acknowledged well to the hypothesis from the reported literature. The updated knowledge from this fluid-structure interaction is hoped to be given more attention by the industry and perhaps to be incorporated into the current subsea pipeline designs.

Application of Multidisciplinary Design Optimization in the Truss Spar Concept Design

In the traditional design of a Truss Spar, designers usually choose different discipline as major concentration in different design phases. The coupling effect among disciplines can hardly be accounted for. Multidisciplinary design optimization has been proved to be an effective tool for the design of complex engineering systems, which takes all disciplines into account at the same time and exploit coupling effect among disciplines, thereby achieving the optimal system solution. In this paper, a multidisciplinary optimization scheme for a Truss Spar is firstly developed and the Truss Spar is decomposed into four modules: weight module, hydrodynamic module, structure module and stability module. Response surface method is used to replace the high-fidelity analysis to perform the approximate mathematical models of the objective function/constraints as a function of design variables. In order to enhance the accuracy of the predicted optimum, the response surface models are continuously updated using the information obtained from the numerical simulation of latest iterative results. Finally, an optimal design solution, which satisfies all the constraints, is obtained using collaborative optimization. The characteristics of the optimized design solution including hull weight, heave response, stability performance and strength of the bottom deck, are much improved comparing with traditional design.

System Identification of Jack-Up Platform by Spectral Analysis

For the purpose of structural health monitoring (SHM), it is beneficial to develop a robust and accurate numerical strategy so as to identify key parameters of offshore structures. In this regard, it is difficult to use time-domain methods as the time history of wave load is not available unless output-only methods can be developed. Alternatively, spectral analysis widely used in offshore engineering to predict structural responses due to random wave conditions can be used. Thus the power spectral density (PSD) of structural response may be more appropriate than time history of structural responses in defining the objective (fitness) function for system identification of offshore structures. By minimizing PSD differences between measurements and simulations, the proposed numerical strategy is completely carried out in frequency domain, which can avoid inherent problems rising from random phase angles and unknown initial conditions in time domain. A jack-up platform is studied in the numerical study. A search space reduction method (SSRM) incorporating the use of genetic algorithms (GA) as well as a substructure approach are adopted to improve the accuracy and efficiency of identification. As a result, the stiffness parameters of jack-up legs can be well identified even under fairly noisy conditions.

AC Ring Distribution: Architecture for Subsea Power Distribution

Subsea power transmission and distribution is an emerging technology that may enable the oil and gas industry to produce hydrocarbon reserves in deeper and more remote offshore waters. The longer tieback subsea operations will likely require pumps and compressors driven by electric motors to be located on the sea floor to pressure boost the oil and gas to surface and/or onshore platforms. Existing HVAC and HVDC technologies are efficient means for subsea power transmission and distribution. However, they are subject to a variety of limitations, for instance, the single-point failures that would impact production uptime. Furthermore, it is challenging to implement subsea bulk power transmission and distribution by using existing architectures due to the footprint, weight and electrical insulation requirements. This paper describes a subsea power distribution architecture — AC ring. It can be used to interface a high-voltage bulk power transmission network, either AC or DC, to a subsea multi-load AC system. The new topology uses series-connected, by-passable, open-winding transformers to provide better modular design flexibility. The system is expected to be more reliable than conventional “hub and spoke” architectures and more technically feasible regarding practical subsea equipment designs.