Stress Estimation of Offshore Structures

Abstract

Offshore structures are subjected to a harsh environment where the fluctuating waves continuously strain the structures and these forces cause the initiation and propagation of cracks in the structures. In other words, the structures accumulate fatigue damage, which eventually leads to structural failure. To avoid fatigue failure, the operational lifetime of a structure is limited to a design lifetime in which the structure is safe for operation. This design process is based on precautious stochastic assessments, norms, and industry standards that simplify the actual structure and environment in such a manner that it involves little risk of structural failure. As many structures in the North Sea approach the end of their design lifetime, the owners are faced with a dilemma: either abandon the field or replace the structures. Another option is the lifetime extension of the existing structures. This requires a reduction of the uncertainties in the design process - such as the stress history in fatigue-critical location. Unfortunately, these locations are often inaccessible or directly harmful to the sensors due to the hostile environment of the ocean. This thesis focuses on virtual sensing to estimate the stress/strain response of offshore structures by indirect measurements. The thesis addresses the state of the art and maps some essential issues within stress/strain estimation. In this thesis, stress/strain estimation is applied to different test specimens to address certain scientific issues. Parts of the thesis relate to the calibration of the system model for virtual sensing by operational modal analysis.

Analysis of a raise boring reamer in operation based on acceleration measurements

“…Analyzing a vibration signal in the frequency domain is a useful way to assess its properties and is therefore widely used for diagnostics of various systems, from gearboxes, motors, wind turbines etc 9 . to large civil engineering structures 10 and offshore installations 11 …”

“…9 to large civil engineering structures 10 and offshore installations. 11 An effective tool for frequency domain analysis of vibrations is the power spectral density (PSD). 12 It can be calculated from a signal using the Fourier-transform.…”

Data‐driven virtual sensing and dynamic strain estimation for fatigue analysis of offshore wind turbine using principal component analysis

“…Therefore, plenty of research has been conducted for stress/strain estimation for offshore structures. 2 In the literature of virtual sensing, there are several successful applications for stress/strain estimation, while most applications are conducted in laboratories under controlled conditions. On the other hand, the number of applications under the actual operating conditions is scarce.…”

“…On the other hand, the number of applications under the actual operating conditions is scarce. 2 Stress/strain estimation began in the 1950s with analytical relationships between response and strain for beams or plates [3][4][5] that enable virtual sensing limited to specific systems under the conditions and assumptions applied in the expressions. These analytical expressions dominated the field until transformation matrices were introduced in the 1990s that is not restricted to any specific linear system.…”

Towards offshore wind digital twins: Application to jacket substructures

“…The review is limited to the offshore wind application. For a comprehensive literature survey, the reader is referred to (Tarpø, 2020).…”