Subsidence monitoring of offshore platforms

AbstractThe normal subsidence monitoring technologies, used in civil engineering, are hard to apply in ocean engineering. Because it is hard to find a fixed reference for subsidence monitoring. A new method, which is suitable for subsidence monitoring of offshore platforms, is proposed in this paper. Firstly, the compression characteristic of the soil was analyzed and the harms of subsidence are discussed. Based on the analysis, the subsidence monitoring method was given. Finally, an real application is shown. Some advanced measurement technologies, such as the FBG strain measurement techniques and so on, were used in this application. The real application indicates that the new method is suitable for the subsidence monitoring of offshore platforms

OMAE 2009-79849 STRUCTURAL SAFETY ASSESSMENT OF EXISTING ICE-RESISTANT JACKET PLATFORMS IN ICE ZONE

ABSTRACT Until now dozens of offshore structures have been deployed in Bohai bay since the first drilling platform was erected in 1965. The oil and natural gas resources of Bohai Bay are mainly marginal oil fields. It is necessary to build both ice-resistant and economical offshore platforms. 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 assessment and life extension become key problems. In this paper, failure modes of ice resist jacket platforms, the related failure evaluation criteria, and risk grade are treated. Combined with monitoring data of ice loads, assessment strategy is presented. Lastly, as an application example, safety assessments of a practical platform in Bohai Bay are carried on

Flexible Riser Configuration Design for Extremely Shallow Water With Surrogate-Model- Based Optimization

The aim of this paper is to study the optimization design of a steep wave configuration based on a surrogate model for an extremely shallow water application of a flexible riser. As the traditional technique of riser configuration design is rather time-consuming and exhaustive due to the nonlinear time domain analysis and large quantities of load cases, it will be challenging when engineers address an extreme design, such as the configuration design in the case of extremely shallow water. To avoid expensive simulations, surrogate models are constructed in this paper with the Kriging model and radial basis function (RBF) networks by using the samples obtained by optimal Latin hypercubic sampling (LHS) and time domain analysis in a specified design space. The RBF model is found to be easier to construct and to show better accuracy compared with the Kriging model according to the numerical simulations in this work. On the basis of the RBF model, a hybrid optimization is performed to find the minimum curvature design with corresponding engineering constraints. In addition, an optimized design is found to meet all of the design criteria with high accuracy and efficiency, even though all of the samples associated with construction of the surrogate model fail to meet the curvature criterion. Thus, the technique developed in this paper provides a novel method for riser configuration design under extreme conditions

Failure Analysis of Topside Facilities on Oil/Gas Platforms in the Bohai Sea

The jacket platform in the Bohai Sea oilfield is an important engineering development, offering design alternatives in this economically important region. However, ice-induced vibration in cold areas threatens the safety and operation of these platforms. On two occasions, intense ice-induced vibrations triggered the rupture of a well’s blow down pipeline, loosening the flanges on the Bohai platform, and leading to the ejection of high-pressure natural gas. Subsequent mechanical analysis of the failed parts helped define the mechanism of failure and identified the failure criteria, based on a prototype structure monitoring system. The analysis revealed that the deck’s inertial force, which resulted in ice-induced steady-state vibration, was the major cause of the accident. Three fixed cones were thus installed on the platform the following winter, effectively reducing the vibrations

Study on the dynamic characteristics of a soft yoke mooring system for floating production storage and offloading based on field measurements

The position and inclination of the mooring legs and yoke of a soft yoke mooring system for floating production storage and offloading in Bohai Bay were measured by full-scale monitoring technique. A novel multi-body dynamic analytical method of the horizontal restoring force for the soft yoke mooring system was established and investigated based on the monitoring data. The result predicted by the dynamic method agrees well with that obtained by fibre Bragg grating strain sensors. The peak value determined by the dynamic analysis is approximately 25% higher than that of the static results in harsh environments, which indicates the high reliability of the proposed method. After significant monitoring time, the advantages of the stability of the measurement by the proposed method are also manifested

Study on the Dynamic Ice Load of Offshore Wind Turbines with Installed Ice-Breaking Cones in Cold Regions

The dynamic ice load of conical offshore wind turbines (OWTs) in cold regions is unclear. The ice force period is the key parameter used to establish an ice force model for conical structures. To obtain ice load data, a field monitoring system was installed on an OWT in a cold region in China. Based on the monitoring data, a new formula for calculating the ice force period of conical structures was established. By comparing the period calculated with this formula and the measured ice force period, it was found that the calculated data generally agreed with the measured data. Then, a random dynamic ice force model for conical OWTs can be established. Based on this ice force model, the ice-induced vibration of an OWT was analyzed with the ANSYS finite element software. The results are in good agreement with the measured data obtained from the OWT in the time and frequency domains. Therefore, the random dynamic ice force model established in this paper can be used to evaluate the ice resistance performance of conical OWTs in cold regions

Structural Performance Analysis of Simple Offshore Platforms under Ice-Vibrations in Bohai Sea

 In China, the oil and natural gas resources of Bohai Sea are mainly marginal oil fields. It is necessary to build both ice-resistant and economical offshore platforms. So, several simple offshore oil platforms with a jacket sub-structure or with a single pillar have been deployed. There are very good economic benefits in the manufacture, installation, removal, and other aspects. These platforms were primarily designed to withstand extreme static ice forces. However, sea ice motion can induce significant vibrations for the platforms in the region. The structural ice-resistant performances have not been well developed. In this paper, combined with the field monitored data of some simple platforms in Bohai Sea, ice-induced vibrations are analyzed. The results show that even though these structures may effectively resist extreme static ice forces, the ice-induced acceleration is more significant. Then, spectral-based method is provided to analyze the fatigue life of a real simple platform. Lastly, the ice-resistant strategy is presented, which provides a basis for the design of these platforms in ice zone