The objective of this paper is to share lessons learned with the industry from a recent accident and provide a recommended procedure for evaluating airgap and possible horizontal wave impact loads for column-stabilized units operating in harsh environments. On 30 December 2015, when operating on the Troll field, a column-stabilized drilling unit, COSL Innovator, was hit by a steep wave that impacted directly on the forward port side of the deck box. This resulted in several damaged windows, one fatality, four injuries and structural damages to the front of the deck box. Following the accident, there is increased attention and concerns raised by some regulatory bodies for safe operation of semi-submersibles in harsh environment. In order to evaluate the adequacy of Class rule requirements and pertinent industrial design practices, DNV GL has investigated and analyzed the circumstances surrounding the accident. The conclusion from this work is that there is a need for improved guidelines for calculating horizontal wave impact loads on deck box structure if a column-stabilized unit is designed to operate with a negative airgap, together with improved methods for calculating the airgap with a prescribed annual probability of occurrence. DNV GL has developed "Offshore Technical Guidelines" (OTGs) that can be used to document compliance with classification rules stipulated in DNVGL-OS-C103 [1]. Development of these guidelines has brought a deeper understanding with respect to airgap and associated horizontal wave impact loads on column-stabilized units. This paper discusses the technical challenges related to airgap prediction and wave impact load calculation, provides the technical background and insight of recently released Offshore Technology Guidance OTG-13 [2] and OTG-14 [3], and suggests a way forward.
The paper presents a proposed methodology to evaluate wind-induced fatigue of onshore slender frame structures with lattice-type design. The methodology for fatigue evaluation is based on a similar offshore industry design practice and has been repurposed for large onshore structures fabricated from tubular pipes, square tubes, and angle bars exposed to wind loading, including wind buffeting loads and wind vortex shedding. Multiple cracks resulting from wind-induced fatigue have been experienced on offshore installed flare booms and flare towers. Consequently, design codes were introduced, and it has been a standard design practice to evaluate potential fatigue damage of slender frame structures due to vibrations induced by vortex shedding, as well as buffeting loads due to gusting. This is, however, not yet standard design practice for land-based slender frame structures despite obvious similarities in terms of structural configuration, size, wind loads, and consequences of failure. Some operators have internal requirements to assess these operational risks. In this study, wind-induced fatigue design and analysis methodologies have been developed and calibrated based on offshore design practices. The proposed methodology uses a spectral analysis method that considers wind data, and includes simplified design checks as well as advanced analysis approaches. Further comparisons are presented for structural configuration and various end connection types with relevant examples. Both welded and gusset-type bolted connection of tubular members are investigated. This paper provides designers with methodology guidance to calculate structural fatigue damage due to both wind buffeting loads and vortex shedding vibrations. Key design considerations for wind fatigue analysis, typical design solutions and recommendations for improvements of fatigue life, and practical ways for the global fatigue screening are also provided.
This paper presents a concept of a mooring digital twin frameworkand a standardized inspection datatemplate to enable digital twin. The mooring digital twin framework supports real-time and/or on-demand decision making in mooring integrity management, which minimizes the failure risk while reducing operation and maintenance cost by efficient inspection, monitoring, repair, and strengthening. An industry survey conducted through the DeepStar project 18403 identified a standard template for recording inspection data as a high priority item to enable application of the digital twins for integrity management. Further, mooring chain was selected as a critical mooring component for which a standard inspection template was needed. The characteristics of damage/performance prediction with the proposed mooring digital twin framework are (i) to utilize surrogates and/or reduced-order models trained by high-fidelity physics simulation models, (ii) to combine all available lifecycle data about the mooring system, (iii) to evaluate current and future asset conditions in a systematic way based on the concept of uncertainty quantification (UQ). The general and mooring-specific digital twin development workflows are described with the identified essential data, physics models, and several UQ methodologies such as surrogate modeling, local and global sensitivity analyses, Bayesian prediction etc. Also, the proposed digital twin system architecture is summarized to illustrate the dataflow in digital twin development andutilization. The prototype of mooring digital twin dashboard, web-based risk visualization and advisory system, is developed to demonstrate the capability to visualize the system health diagnosis and prognosis and suggest possible measures/solutions for the high-risk components as a digital twin's insight.
By 2050, global installed floating wind capacity is expected to reach 264 GW. Project sanction will require levelized cost of energy (LCOE) being significantly below current levels, but also to demonstrate that the life, property, and the environment are safeguarded. Most class societies were established in late 1800’s as independent organizations addressing technical risks in shipping industry. In 1980’s classification model has been successfully extended to the offshore oil and gas, resulting in significant safety improvements and cost savings. Most recently, the same model has been successfully applied to offshore fish farms. During the last decades classification has been transforming towards fully risk and data driven solutions. By using fleet-wide knowledge, significant cost savings are generated and passed to the asset owner and operator. This makes classification ideally suited to floating wind. The paper discusses areas of class society involvement in certification of floating wind foundations, including mooring system and wind turbine interface. Risk based integrity management in harsh environments the floating wind platforms are expected to operate are addressed. Potential impact is illustrated using examples from the past where such benefits were realized, including using fleet data to improve efficiencies and reduce costs. Required adaptation of classification concepts to specifics of the floating wind are also discussed. The paper also shares examples of class society role and activities beyond certification and assurance. We hope this will help appreciate the overall value of classification provides and encourage the industry to tap into benefits from its broad knowledge and experience.
This paper summarizes a current outcome of DeepStar project titled, "21164 Standardization of Inspection to Enable Digital Twin (Phase 2)", which develops a digital twin framework for mooring chains with physics models, standardized inspection data template, and various analytical tools, including automated tools for data conversion, anomaly detection, etc., to enable real-time integrity monitoring and failure prediction. Four activities are included in this study: (1) literature study and industry workshop which collects industry feedback to prioritize the development of case studies for tools that aid digital twins; (2) development of automatic data converting tool to collect digitized data in standardized form from existing inspection reports; (3) development of automated anomaly/feature detection tool that detects anomaly, i.e., marine growth, and tracks link ID from existing ROV footage data; and (4) development of digital twin process which incorporates mooring analysis, failure analysis and integrity management. The failure analysis process incorporates standardized inspection data into the physics model while accounting uncertainty from each information source. Integrity management is achieved through a web-based condition visualization system, called digital twin dashboard.
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