Jan's advert

My own bibliography on ocean engineering, structural reliability, ship rolling etc.:

biblio.txt (size 456 KB), last updated:2013.01.01

This is a simple databse in ASCII format (plus Norwegian characters: , , ). Blank lines separate the items. A 2-character key identifies each attibute, as follows: %A=Author, %T=Title, %J=Journal, %V=Volume, %N=Number,%P=Pages, %I=Issuer, %D=Date, %C=City, ...

Paper - Prediction of Mooring Line Tensions for Hurricane Conditions in the Gulf of Mexico

OMAE2014-23446_v7.pdf (size 1.2 MB), from 33rd International Conference on Ocean, Offshore and Arctic Engineering, San Francisco.
ABSTRACT - A probabilistic metocean model for hurricane conditions is briefly described. The model is based on site-specific, hindcast data and defines the time variation of the metocean conditions during the realisation of a hurricane at the site. The annual extreme value distribution of mooring line tension for a large, semi-submersible, mobile drilling unit is computed. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the short-term distribution parameters in order to describe the tension response during the varying conditions associated with the passage of a hurricane. The hurricane duration is split into a sequence of 15-minute intervals such that the conditions can be assumed stationary during each such short interval. The tension distribution, conditional on the realisation of a hurricane, is accumulated across the sequence of short intervals. The distribution of hurricanes is taken into account to obtain the tension distribution in a random hurricane. Finally, the frequency of hurricanes is taken into account to give the annual extreme distribution of line tension. The characteristic tension computed using 10-year return conditions and the ISO 19901-7 design standard is found to correspond to a return period of 29 years in the test case. The effects of various assumptions in the design analysis are investigated. Sensitivities to simplifications of the metocean model are considered. The effects of uncertainties in the response calculation and in the distribution of peak significant wave height during hurricanes are quantified and included in the response analysis.

Paper - A Probabilistic Metocean Model for Mooring Response in Gulf of Mexico Hurricanes

omae2013-10735.pdf (size 1254 KB), from 32nd International Conference on Ocean, Offshore and Arctic Engineering, Nantes.
ABSTRACT - Hindcast data for a specific location is utilised to develop a joint probability function for the metocean variables that are expected to have a significant effect on mooring line tensions for a floating platform moored at that location. The main random variables comprise: peak significant wave height, peak wind speed, peak surface current speed, peak wave direction, peak wind direction and peak current direction, where "peak" indicates the maximum intensity of the metocean effect during a random hurricane. The time lead of peak wind relative to peak waves and the time lag of peak current after peak wind are included as random variables. It is also necessary to describe the time variation around the peak events. Simple models are assumed based on inspection of the time variations during severe hurricanes. Only the part of the hurricane during which the significant wave height exceeds 80% of the peak value is taken into account. The duration of this interval is included. Linear variation is assumed for the directions, hence the rates of change of the 3 directions are included. A linear (triangular) plus parabolic model is assumed for the time variation of the intensities of the 3 metocean effects around their respective peaks. A single parameter is required to define the proportion of linear and parabolic models for each effect and the values of this parameter for each of the 3 metocean effects are also included as random variables. A random hurricane can be drawn from this metocean model, such that the time variation of the metocean actions is deterministic once the values of the random variables have been selected. The overall duration of the hurricane is split into short intervals, each of 15 minutes duration, such that stationary response may be assumed during each short interval. The extreme value distribution of line tension during each short interval is obtained. These distributions are combined to obtain the extreme distribution of line tension during the hurricane. Second order reliability methods are applied to integrate over the distribution of the metocean variables and obtain the distribution of extreme tension during a random hurricane. The annual frequency of hurricanes is used to derive the annual extreme value distribution of line tension. The model is intended for the reliability analysis of the ultimate limit state of mooring lines, but may also be applicable to other response variables. The present paper is primarily concerned with the metocean model, but it is intended to include sample results for the extreme line tension.

Paper - Hull Girder Vibration Effects in Bending Stresses Measured under Harsh, Stationary Conditions:

Hull_vibration_effects_under_harsh_conditions_v8.pdf (size 425 KB), from RINA/SSNAME Int. Symp. on Ship Design & Construction, Shanghai, 2007.
ABSTRACT - Wave-induced, hull girder vibration is known to have a significant effect on fatigue in some types of ships. Extensive measurements on ore and bulk carriers in service have been used to investigate and quantify these effects. A pilot study has been carried out in an attempt to use some of the same data to determine if there is also a significant effect on extreme wave loads. A few time series of hull bending stresses from two bulk carriers, operating in the North Atlantic iron ore trade, have been analysed. Harsh conditions due to a combination of waves, heading, speed and draught have been sought in order to approach extreme conditions. Somewhat milder conditions have also been included to investigate trends. Long, continuous records, which imply greater confidence in response statistics than is available from shorter records, have been obtained from harsh, stationary conditions. Low pass filtering has been applied to separate the ordinary, non-vibratory, wave-induced stresses from the total wave-induced stresses. Stress maxima and minima have been extracted from the time series and distribution functions have been fitted to these data. The results show a significant contribution to the total stress from the vibratory component, even under the harshest conditions that were available. This indicates that hull girder vibrations may need to be taken into account in the prediction of the extreme stresses in certain ship types.

Paper - Calibration of a Fatigue Limit State for Mooring Lines:

moorfls_v8.pdf (size 234 KB), from Int. Conf. on Computational Methods in Marine Engineering MARINE 2005.
ABSTRACT - A structural reliability method is applied to calibrate the safety factor of a fatigue design method for the mooring lines of floating offshore platforms. Both methods are based on the Miner Palmgren hypothesis for the accumulation of fatigue damage. The reliability analysis takes explicit account of the various uncertainties in the fatigue capacity, the damage accumulation and the calculation of mooring line tensions. These uncertainties are all intended to be covered by the safety factor in the design analysis. The uncertainty in the fatigue capacity is developed for chain and steel wire rope. A test set of six floating platforms in various water depths is considered. The effect of varying numbers of mooring line components is taken into account. A target probability of failure is established. The safety factor of the design method is calibrated to yield designs close to the target probability. This calibration is effected by minimising an objective function based on the difference between target probability and calculated probability of failure for the mooring lines in the test set. Under-design is penalised more heavily than over-design.

Paper - Probabilistic Modelling for Reliability Analysis of Jackets:

OMAE2004-51227.pdf (size 315 KB), from Offshore Mechanics and Arctic Engineering Conf., 2004.
ABSTRACT - Experience from recent reliability analyses of jacket platforms is used to discuss selected aspects of probabilistic modelling in more detail. These modelling details can have a significant effect on the computed reliabilities. An overview of basic considerations and failure modes in jacket reliability analysis is included to set the various details into context. Ultimate limit states for jackets in relatively shallow water are emphasised; i.e. quasi-static structural response is applicable. The following topics are considered: (a) Failure modes and some requirements to load and resistance analysis. (b) Directionality in loading and resistance. (c) Random periods of individual extreme waves. (d) Foundations axial and lateral capacity modelling for multiple piles and model uncertainty for pile capacity.

Paper - Reliability Reassessment of a Jacket Platform with Gas Seepage in the South China Sea:

OMAE2003-37472.pdf (size 695 KB), from Offshore Mechanics and Arctic Engineering Conf., 2003.
ABSTRACT - A structural reliability analysis is carried out on a jacket platform in 75 m water depth, in the South China Sea. A platform collapse failure mode is considered, with emphasis on uncertain soil conditions around the pile foundations, due to gas seepage while the platform has been in service. Random environmental conditions due to wind, waves and current are taken into account, based on observed data. Allowance for the short duration of environmental measurements is included and has a marked effect on the results. Two response surfaces are applied in the reliability calculation, to model the loads and the system capacity.

Paper - Risk-Based Inspection Planning For Mooring Chain:

OMAE2002-28409.pdf (size 123 KB), from Offshore Mechanics and Arctic Engineering Conf., 2002.
ABSTRACT - The use of risk-based inspection planning for offshore structural components is becoming quite familiar. This paper describes an application of this technique to mooring chain. In many cases, the technique is based on probabilistic modelling of fatigue crack growth in the structural components, and updating of the failure probability on the basis of inspections. The extension of this basis from a single component to very many components is necessary to tackle series systems, such as mooring chain, where a fatigue fracture can arise in any chain link. The theoretical basis for the analysis is described, including details of the model for stochastic dependency between the chain links. Results are shown that compare failure probabilities for a single chain link and a chain segment. The effects of various levels of inspection coverage are illustrated. An example of a cost optimal inspection plan is developed for mooring chain on an FPSO in the northern North Sea.

Paper - Risk-Based Inspection Planning for Permanent Mooring Lines:

NIF_RBI_Moor.pdf (size 86 KB), from conf. on Dynamic Positioning and Mooring of Floating Offshore Structures, organised by the Norwegian Society of Chartered Engineers, 2001.
ABSTRACT - The present work describes a first attempt to apply risk-based inspection planning to mooring chain. The main challenges in the analysis are to provide a realistic model for probabilistic crack-growth in a single chain link, and to take a large number of chain links into account, with a realistic correlation between links. The result of the analysis is a plan stating when it is cost-optimal to inspect the chain. Some results are shown for chain from a turret-moored ship in Norwegian waters. The uppermost chain segment in the most heavily loaded mooring line is considered. This chain has been conservatively designed, with a fatigue safety factor of 10. The present example is intended to illustrate the method without providing full details of the case study.

Paper - Inspection Interval for Tendon Joints of Jolliet TLP:

omae99_6010.pdf (size 139 KB), from Offshore Mechanics and Arctic Engineering Conf., 1999.
ABSTRACT - A reliability analysis is developed for the welded joints of the tendons on the Jolliet TLP. Two potential failure modes are considered: (a) fatigue failure in terms of growth of a fatigue crack through the tendon wall thickness, and (b) rupture due to brittle fracture or plastic collapse in the presence of a fatigue crack. Linear fracture mechanics are used to model the growth of a fatigue crack. Final rupture is modelled using a failure assessment diagram approach defined in PD6493 (from BSI, 1997). Material data from the production testing of the tendons are applied. Initial crack sizes are based on measurements made during the initial in-service inspection carried out in 1989/90. Extreme load distributions are based on the original design analyses of the TLP, while fatigue loads are based on in-service measurements. Axial tension and transverse bending moments at the tendon joint are included in both cases. The annual probability of failure is computed as a function of the time elapsed since the inspection. Both failure modes and the combination of the two failure modes are considered. Comparison of these results with a suitable target reliability level is used to evaluate when the next inspection of the tendon joint welds should be performed.

Paper - Calibration of an Ultimate Limit State for Mooring Lines

OMAE98-1457_Mooring_ULS.pdf (size 1.4 MB), from 17th Int. Conf. Offshore Mechanics and Arctic Engineering, 1998, Lisbon.
ABSTRACT - Structural reliability analysis has been used to calibrate a design equation for mooring lines in their ultimate limit state. The calibration is based on six test cases, for mooring systems in water depths ranging from 70m to 2000m. Three of the cases apply to a turret-positioned ship and three to a semisubmersible. Conventional catenary mooring systems with chain and/or wire components have been studied, whereas taut moorings with tibre rope are not yet included. Environmental conditions from the Norwegian continental shelf and from the Gulf of Mexico have been considered. A design equation format involving two partial safety factors, applied to two tension components is recommended. The two components are: (i) the static tension due to pretension and due to tension induced at the offset position corresponding to the mean environmental forces in an environmental state, and (ii) the dynamic tension component due to time-varying loads; i.e. in this paper defined as the sum of time- varying low-frequency and wave-frequency tensions in the environmental state. The recipes for characteristic values of the tension components and the line capacity are specified, and partial safety factors are given.

Paper - Calibration of a Progressive Collapse Limit State for Mooring Lines

OMAE98-1458_Mooring_PLS.pdf (size 998 KB), from 17th Int. Conf. Offshore Mechanics and Arctic Engineering, 1998, Lisbon.
ABSTRACT - A structural reliability analysis is formulated for a progressive collapse limit state of offshore mooring systems. The failure of two adjacent mooring lines is considered in some detail, while incorporating the empirical frequency of line failures in the modelling of the initial line failure. Reliability results are presented for the mooring system of a semisubmersible platform, and a turret-positioned ship. A simplified design rule for the progressive collapse limit state is discussed and the reliability results are utilised in a preliminary calibration of the design rule. Additional test cases are required to finalise the calibration.

Paper - Modifications to DNV Mooring Code (POSMOOR) and their Consequences

OMAE98-1460_Mooring_Code.pdf (size 704 KB), from 17th Int. Conf. Offshore Mechanics and Arctic Engineering, 1998, Lisbon.
ABSTRACT - DNV's mooring code, given in the Rules for Classification of Mobile Offshore Units and known as POSMOOR, will be modified in accordance with the findings of the joint industry project "Reliability-Based, Cost-Effective, Design Methods for Deep Water Mooring Systems - DEEPMOOR". Other international mooring codes and DNV's code are currently based on formats with a single safety factor or utilization factor. This format will be replaced by a limit state format, which is already widely used in the oil offshore industry. The code will be reformulated in terms of three limit state functions: ULS (Ultimate Limit State), PLS (Progressive collapse Limit State) and FLS (fatigue limit state). The partial safety factors involved in the limit state functions will be based on calibration against structural reliability analyses. An alternative formulation of the characteristic strength of mooring line components will be introduced, based on component test data. If such data is not available, then the characteristic strength can still be derived from the conventional minimum breaking strength.

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