Offshore structures – why all offshore facilities should have a demanning requirement

Most offshore structure design codes focus on setting appropriate safety factors to achieve an acceptable annual level of risk. Recent work by Atkins SNC-Lavalin, together with a large number of operators in Australian waters and the North Sea, has discovered that a large number of aging assets are implementing a demanning requirement to limit the risk of platform collapse to personnel, due to changes in loading or degradation of the structure. This work has shown there are two risk scenarios that should drive this requirement. The first scenario which is intended by the codes in limiting the overall annual risk. The second is to limit the collapse risk associated with a known forecast storm, as the level of risk from helicopter demanning is much lower. For all the older offshore fixed and permanently mooring floating structures assessed for a risk level considered acceptable for a forecast storm, this risk level would govern the sea-state demanning criteria. For recently installed facilities that are compliant with current standards, the findings were the same: that all facilities should have a demanning requirement. The level of this demanning sea-state limit has been shown to be lower than expected and is likely to occur only once in the asset’s life; therefore, the cost implications of implementing demanning procedures are minor. This paper presents the basis and range of findings for calculating the risks associated with an annual occurrence and an ‘in a forecast storm’ risk. Further, this paper proposes acceptable demanning limits for facilities designed to current and historical design codes.

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Improved Model for Ice Force on Conical Structure

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 2 ◽

10.1115/omae2005-67300 ◽

2005 ◽

Author(s):

W. Feng ◽

Z. M. Shi ◽

L. M. Liu

Keyword(s):

Ice Sheet ◽

Bending Moment ◽

Offshore Structures ◽

Structure Design ◽

Offshore Structure ◽

Conical Structure ◽

Offshore Structure Design ◽

Improved Model ◽

Ice Force ◽

Bending Failure

Ice force is an important factor to be taken into account for offshore structures in cold region, and the calculation method of the ice force is meaningful for the offshore structure design. Cone is now used as optimal ice-resistant structure because it can cause bending failure of the ice sheet. The interaction between ice sheet and conical structure is studied in this paper and Croasdale’s model is modified based on the field observations. The newly built model separates the ice sheet into emersed part and floating part, and the equilibrium analyses are carried out respectively. The bending moment distribution of the ice sheet is analyzed to determine the position of bending failure, which serves as a supplementary restriction. Analytic solution of ice force on conical structure is got and it is verified by the experimental data of previous researches.

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Offshore structure design and development

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1982.0110 ◽

1982 ◽

Vol 307 (1499) ◽

pp. 263-277

Keyword(s):

Deep Water ◽

Oil And Gas ◽

Offshore Structures ◽

Structure Design ◽

Offshore Structure ◽

Oil And Gas Exploration ◽

Exploration And Production ◽

Offshore Oil And Gas ◽

Offshore Structure Design ◽

Offshore Oil

The kinds of technology currently being applied to the design, construction, installation and operation of offshore structures for oil and gas exploration and production are quite sophisticated and include many examples of innovative configurations and approaches. The decade of the 1990s should see further evolution, reinterpretation and improvements of concepts that are already in service or being readied for service. The importance of offshore oil and gas may be judged by the projection that over half of overall exploration investments will go to offshore prospects in future years. This paper surveys some expected evolutions, with particular emphasis on the challenging area of deep-water applications. Some features of a tension leg platform design are discussed as an example of a deep-water oil production system. An attempt is made to recognize the problems of applying advanced engineering and analytical capabilities, when many specialists must interact, to producing a thoroughly engineered design, which is also balanced and economical, for such innovative systems.

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Reliability-Based Design and Assessment for Lifetime Extension of Ageing Offshore Structures

Volume 3: Structures, Safety and Reliability ◽

10.1115/omae2016-54206 ◽

2016 ◽

Cited By ~ 1

Author(s):

E. Mat Soom ◽

M. K. Abu Husain ◽

N. I. Mohd Zaki ◽

N. U. Azman ◽

G. Najafian

Keyword(s):

Service Life ◽

Offshore Structures ◽

Risk Level ◽

Type I ◽

Offshore Structure ◽

Design Assessment ◽

Reliability Based Design ◽

Remaining Service Life ◽

The North ◽

Cost Efficient

The methodology for Reliability-Based Design and Assessment (RBDA) of an ageing fixed steel offshore structure was established to support detailed re-assessment applied to the management of the structure’s safety, integrity analysis and reliability by evaluating the loading acting on the structure. It is a tool for the high-end analysis of the structure for risk-based design assessment and has been succesfully implemented in the North Sea under Shell operating company. The main purposes of RBDA are to manage a structure’s risk level over its remaining service life and to initiate the cost-efficient inspection or mitigation actions (if required). This method consists of Type I and II uncertainties used to determine the probability of failure for the structure over its remaining service life. However, limited work has been done so far on its application at many different regions, particularly in the South East Asia. Therefore, this paper investigates the robustness of the RBDA methodology applied to fixed offshore structures at shallow waters of Malaysia by considering the native environmental criteria, local authorities’ obligation and company requirements. It is shown that this procedure can efficiently assist in understanding the structure’s failure mechanism and correctly define the relevant type of mitigations required.

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Evaluation of pack ice pressure approaches and engineering implications for offshore structure design

Cold Regions Science and Technology ◽

10.1016/j.coldregions.2018.01.020 ◽

2018 ◽

Vol 149 ◽

pp. 71-82

Author(s):

Lawrence Charlebois ◽

Robert Frederking ◽

G.W. Timco ◽

David Watson ◽

Martin Richard

Keyword(s):

Structure Design ◽

Offshore Structure ◽

Pack Ice ◽

Offshore Structure Design ◽

Ice Pressure

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Wave current interaction: Effect on force prediction for fixed offshore structures

MATEC Web of Conferences ◽

10.1051/matecconf/201820301011 ◽

2018 ◽

Vol 203 ◽

pp. 01011

Author(s):

Mohamed Latheef ◽

Nasir Abdulla ◽

Mohd Faieez Mohd Jupri

Keyword(s):

Vertical Structure ◽

Offshore Structures ◽

Base Shear ◽

Sea State ◽

Clear Trend ◽

The North ◽

Relative Water ◽

The North Sea ◽

Current Interaction ◽

Typical Design

MetOcean conditions in the South China Sea (SCS) indicates that unlike other locations such as the North Sea, the magnitude of the currents can be relatively large. In addition, these currents are strongly sheared. The present study focused on the typical design problem of calculating the ultimate base shear and overturning moments for slender fixed structureswiththe inclusion of the interaction between the currents and the wave field. It has been found that the loads on average can be around 15% larger when this interaction is accounted for in the calculation of the loads, highlighting the importance. In addition, the level of these amplifications were found to be dependent on the sea state steepness and the relative water depth. While no clear trend was found (changed case by case) in the present work, incorporating the vertical structure of the current was found to change the pattern of the amplification of the loads.

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Offshore Structure Design Under Ice Loads

Encyclopedia of Ocean Engineering ◽

10.1007/978-981-10-6963-5_126-1 ◽

2020 ◽

pp. 1-7

Author(s):

Zhenhui Liu

Keyword(s):

Structure Design ◽

Offshore Structure ◽

Ice Loads ◽

Offshore Structure Design

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On Efficient Long-Term Extreme Response Estimation for a Moored Floating Structure

Volume 3: Structures, Safety, and Reliability ◽

10.1115/omae2018-78763 ◽

2018 ◽

Cited By ~ 1

Author(s):

HyeongUk Lim ◽

Lance Manuel ◽

Ying Min Low

Keyword(s):

Wave Train ◽

Offshore Structures ◽

Offshore Structure ◽

Floating Structure ◽

Sea State ◽

Peak Period ◽

Frequency Wave ◽

Extreme Response ◽

Term Response

This study investigates the use of efficient surrogate model development with the help of polynomial chaos expansion (PCE) for the prediction of the long-term extreme surge motion of a simple moored offshore structure. The structure is subjected to first-order and second-order (difference-frequency) wave loading. Uncertainty in the long-term response results from the contrasting sea state conditions, characterized by significant wave height, Hs, and spectral peak period, Tp, and their relative likelihood of occurrence; these two variables are explicitly included in the PCE-based uncertainty quantification (UQ). In a given sea state, however, response simulations must be run for any sampled Hs and Tp; in such simulations, typically, a set of random phases (and deterministic amplitudes) define a wave train consistent with the defined sea state. These random phases for all the frequency components in the wave train introduce additional uncertainty in the simulated waves and in the response. The UQ framework treats these two sources of uncertainty — from Hs and Tp on the one hand, and the phase vector on the other — in a nested manner that is shown to efficiently yield long-term surge motion extreme predictions consistent with more expensive Monte Carlo simulations, which serve as the truth system. Success with the method suggests that similar inexpensive surrogate models may be developed for assessing the long-term response of various offshore structures.

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The New Year Wave: Spatial Evolution of an Extreme Sea State

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.3160533 ◽

2009 ◽

Vol 131 (4) ◽

Cited By ~ 7

Author(s):

Günther F. Clauss ◽

Marco Klein

Keyword(s):

Single Point ◽

Target Position ◽

Spatial Development ◽

Offshore Structures ◽

Time Interval ◽

Optimization Approach ◽

Wave Group ◽

Large Area ◽

Sea State ◽

The North

In the past years the existence of freak waves has been affirmed by observations, registrations, and severe accidents. Many publications investigated the occurrence of extreme waves, their characteristics and their impact on offshore structures, but their formation process is still under discussion. One of the famous real world registrations is the so called “New Year wave,” recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. Since there is only a single point registration available, it is not possible to draw conclusions on the spatial development in front of and behind the point of registration, which is indispensable for a complete understanding of this phenomenon. This paper presents the spatial development of the New Year wave generated in a model basin. To transfer the recorded New Year wave into the wave tank, an optimization approach for the experimental generation of wave sequences with predefined characteristics is applied. The extreme sea state obtained with this method is measured at different locations in the tank, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position—520 registrations altogether. The focus lies on the detailed description of a possible evolution of the New Year wave over a large area and time interval. It is observed that the extreme wave at the target position develops mainly from a wave group of three smaller waves. The group velocity, wave propagation, and the energy flux of this wave group are analyzed, in particular.

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