Qianjin Yue

Life-Cycle Cost-Effective Optimum Design of Ice-Resistant Offshore Platforms

In China, the oil and natural gas resources in Bohai Bay are mainly marginal oil fields, which freeze in the winter. It is necessary to build both ice-resistant and economical offshore platforms. However, risk is involved in the design, construction, utilization, and maintenance of offshore platforms as uncertain events may occur within the life-cycle of a platform. In this paper, the optimum design model of the expected life-cycle cost for ice-resistant platforms based on the cost-effectiveness criterion is proposed. Multiple performance demands of the structure, facilities and crew members, associated with the failure assessment criteria and evaluation functions of costs of construction, consequences of structural failure modes including damage, revenue loss, death, and injury, as well as discounting cost over time are considered. Different reliability analysis approaches involved in life-cycle cost evaluation, such as the global reliability under the extreme ice load, the dynamic reliability, and fatigue life induced by ice vibration, are studied. The proposed life-cycle optimum design formulas are applied to a typical ice-resistant platform in Bohai Bay, and the results demonstrate that the life-cycle cost-effective optimum design model is more rational compared with the conventional static design and the optimum dynamic design.

A Spectral Model for Forces Due to Ice Crushing

This paper presents a model of dynamic ice forces on vertical offshore structures. The model concerns a loading scenario where a competent ice sheet is drifting and crushing against the structure. Full scale data obtained on two offshore structures were used in the derivation of a method that applies both to narrow and wide structures. A large amount of events with directly measured local forces was used to derive formulas for spectral density functions of the local ice forces. A nondimensional formula that was derived for the autospectral density is independent of ice thickness. Coherence functions were used to define cross-spectral density functions of the local ice forces. The two kind of spectral density functions were used to obtain the spectral density of the total ice force. The method takes into account both the spatial and time correlation between the local forces. Accordingly, the model provides a tool to consider the nonsimultaneous characteristics of the local ice pressures while assessing the total ice force.

Results of Field Monitoring on Ice Actions on Conical Structures

Ice-structure interaction plays a central part in determining ice loads and ice-induced vibrations. This is a controversial research issue, and many factors make the problem more complicated. The authors have been monitoring several ice resistant structures in the Bohai Sea for 20 years and have measured ice forces and simultaneously observed ice-structure interaction processes. This paper describes typical physical ice sheet–conical structure interaction processes, field data, and theoretical explanations for different ice conditions and structure dimensions. The conclusions are more widely applicable, and we relate them to field work on ice resistant conical structures in other ice-covered regions. Further work will quantify ice loads on conical structures once the interaction process is understood.

Prototype Data Analysis on LH11-1 Semisubmersible Platform in South China Sea

There are still uncertainties and challenges by using floating platforms as the main oil exploitation mode due to lake of actual drilling experience in South China Sea. In order to ensure the safety of platform operation, large amounts of prototype data are needed for environment and structure analysis. From 2011 till now, a prototype measurement project is carried out on a semisubmersible platform “NanHaiTiaoZhan” FPS. Large amounts of data have been collected including parameters of ocean environmental loading, motions of floater and underwater responses of mooring and risers.In this paper, statistical analysis of floater responses together with marine environmental conditions are carried out for deep understanding of the characteristics of semisubmersible floating platform. In order to study the motion behaviors of floater under harsh storm, the data recorded in a typical typhoon “Nock-ten” are chosen to perform the deep analysis. The measured data, wind, wave, current and floater motions, are analyzed in time-domain and frequency-domain separately. The comparison study of theoretical prediction and prototype measurement result in typhoon weather has been carried out. The data recorded by standalone sensors are processed by program MoorForce. The result of frequency-domain analysis of measured mooring force proves that the significant wave frequency dynamic behavior of mooring line during typhoon weather should be considered in design.Copyright

Model Test of Ice-Structure Interaction

Different scales of model tests for investigating ice-structure interaction due to ice crushing have been conducted, and successful experiences and conclusions obtained from previous tests are summarized. On the other hand, the uncertainties within current methodology of ice-structure interaction model tests are discussed. Considering the nature of dynamic ice loads, several main aspects which should be taken into account during design of model tests are analyzed, including layout of the test system, application of scaling laws, and techniques of making model ice sheet, etc. Decision can be made for each of these aspects according to objective of the test, and based on this viewpoint, a small scale model test system was designed and constructed, a series of model tests were conducted and the ice-structure interaction processes which are the same as prototype phenomenon were observed.Copyright

A novel underwater measurement method for mooring system using self-contained technique

This article introduces a prototype measurement method for investigating deep-water mooring systems. Measurement information and specifications are determined according to mooring system analysis. Mooring line information is difficult to measure underwater, particularly for aging platforms. To solve this problem, a novel mooring line tension measurement strategy based on lumped mass method is proposed. A type of self-contained sensor is designed and manufactured to realize dynamic response measurement of mooring systems underwater. The sensor is described in detail, including its design requirement, hardware design, and installation mode. A comprehensive field test system was constructed on a semi-submersible platform in South China Sea. Prototype data, including the data measured during typhoons, are introduced and discussed to improve the accuracy, feasibility, and stability of the proposed measurement method.

Residual Stress Evaluation and Buckling Analysis of Carcass Layers in Flexible Pipes Using 3D Finite Element Model

A carcass layer plays an important role in unbonded flexible pipes to resist the high level of external pressure without buckling collapse. The layer is made of an interlocked helically wound metal wire of profiled section that cold-deformed by a series of rollers. In typical design and analysis process of the carcass layer, the initial stress is always assumed to be zero. During the carcass layer manufacturing (a cold forming process), the metal strap is however subjected to varies of squeezing and bending and deformations to take “S” shape, and then the profiled wire to a cycling sequence of bending, squeezing and twisting deformations, which take it beyond its material elastic limit. The residual stress is therefore introduced and could has effect on the critical pressure of the carcass layer.This paper presents a 3D finite element model to investigate the detailed residual stress distribution and variation during the forming process of the carcass layer. A study case using a 3D ring model is presented to systemically study the influence of imperfections, especially the residual stress, on the critical pressure of carcass layers.Copyright

Design of Cross-Sectional Layout of Steel Tube Umbilical

A design method is presented, which aims at the optimal cross-sectional layout of the multilayer steel tube umbilical. The merit of the method is to improve the tension stiffness and the fatigue life of umbilical. An optimization model is developed for arranging each component layer of the layout by considering the symmetry of mechanical performances of components. Finite element analysis (FEA) is carried out to check the contact force and the radial deformation of different layout types and to confirm the optimal layout design.

The Effects on Facilities and Crew Members of the Ice-Resistant Platforms Induced by Ice Vibrations in Bohai Sea

The current design and safety assessment of ice-resistance platforms have usually been considered the maximum bearing capacity under the extreme static ice loads. However, the effects on the facilities and crew members induced by the dynamic ice loads are not been considered, and the risks are greater than the structure security. So it is necessary to analyze the effect on the facilities and crew members by ice-induced vibrations. Based on the data monitored on the platforms in the Bohai Sea and the ISO standard about human body in vibration environment, human fatigue on the platforms is evaluated. The typical history curve of the deck acceleration is obtained and used to analyze the excitation of pipelines. A mechanical model of structural vibration of pipeline system is developed. With finite element modeling, the dynamic responses of the pipeline systems induced by deck vibration are calculated. The results show ice-induced vibration may have major effect on the crew members and the facilities, and this failure mode by strongly ice-induced accelerations should be considered in safety producing of the existing platforms and optimization design of the new platforms.Copyright

Structural Safety Ensuring System of Offshore Platforms in Bohai

Nowadays, many existing offshore platforms which exceed or approach their design life are still carrying on with producing in the ice zone, so the safety operation is the key issues. In winter, sea ice is the controlling load. Full scale measurement for many years shows the design of some ice-resistant platforms is not so sophisticated, the most significant is that ice induced vibration is the main which has caused harmful accidents in Liaodong bay of Bohai. In order to ensure security operation, structural safety ensuring in winter becomes key problems. In this paper, sea ice management system oriented structural safety ensuring of oil or gas ice-resistant platforms in ice zone is presented. By combining structural monitoring, safety assessment and risk forecast, the system belays and incorporates high safety, cost-benefit, alternative emergency measures.Copyright