Risk Reduction for Floating Offshore Installations Through Barrier Management

Marine system accidents have a relatively high frequency worldwide. In Norwegian waters loss of installations has been avoided the last 20 years, but several near-misses and severe incidents confirm that the risk level is not insignificant. Barriers should prevent undesired events or reduce consequences should such events occur. The main purpose of barrier management is to establish and maintain the necessary barriers. It includes the processes, systems, solutions and measures needed to ensure implementation and follow-up of barriers. Petroleum Safety Authority has emphasized the need to develop barrier strategies during the last few years, and this has been particularly emphasized for topside systems and barrier functions to prevent and/or mitigate the consequences of hydrocarbon (HC) leaks. Insofar, the same focus on barrier management has not been put on marine systems and structural hazards. Risk associated with ballast systems, anchoring systems and dynamic positioning is at a level where further improvements should be made. Independent barrier elements and/or functions are required in order to provide substantial risk reduction. Some suggestions for independent barriers are discussed in this paper.

A Novel Solution to Compute Stress Time Series in Nonlinear Hydro-Structure Simulations

Ship transport is growing up rapidly, leading to ships size increase, and particularly for container ships. The last generation of Container Ship is now called Ultra Large Container Ship (ULCS). Due to their increasing sizes they are more flexible and more prone to wave induced vibrations of their hull girder: springing and whipping. The subsequent increase of the structure fatigue damage needs to be evaluated at the design stage, thus pushing the development of hydro-elastic simulation models. Spectral fatigue analysis including the first order springing can be done at a reasonable computational cost since the coupling between the sea-keeping and the Finite Element Method (FEM) structural analysis is performed in frequency domain. On the opposite, the simulation of non-linear phenomena (Non linear springing, whipping) has to be done in time domain, which dramatically increases the computation cost. In the context of ULCS, because of hull girder torsion and structural discontinuities, the hot spot stress time series that are required for fatigue analysis cannot be simply obtained from the hull girder loads in way of the detail. On the other hand, the computation cost to perform a FEM analysis at each time step is too high, so alternative solutions are necessary. In this paper a new solution is proposed, that is derived from a method for the efficient conversion of full scale strain measurements into internal loads. In this context, the process is reversed so that the stresses in the structural details are derived from the internal loads computed by the sea-keeping program. First, a base of distortion modes is built using a structural model of the ship. An original method to build this base using the structural response to wave loading is proposed. Then a conversion matrix is used to project the computed internal loads values on the distortion modes base, and the hot spot stresses are obtained by recombination of their modal values. The Moore-Penrose pseudo-inverse is used to minimize the error. In a first step, the conversion procedure is established and validated using the frequency domain hydro-structure model of a ULCS. Then the method is applied to a non-linear time domain simulation for which the structural response has actually been computed at each time step in order to have a reference stress signal, in order to prove its efficiency.

Sensitivity of Vessel Responses to Environmental Contours of Extreme Sea States

A comparative study of two methods for the generation of the environmental contours is presented investigating the sensitivity of the predicted extreme vessel responses to the type of the contour lines. Two approaches for the generation of environmental contours of the significant wave height and peak period are compared: the Inverse First Order Reliability Method (IFORM) and Constant Probability Density (CPD) approach. Case studies include several global responses of a ship-shaped weather-vaning vessel and a semisubmersible platform. The case studies reveal that the differences between the IFORM and CPD contours are more pronounced in the range of long wave periods. Vessel responses which are less sensitive to long wave periods exhibit less difference (less than 1.0%) in their maximum values between the two types of contours. In contrast, responses which are sensitive to long wave periods show significantly larger differences of up to 7.0%. Uncertainties also exist in the predicted extreme responses where the environmental contour and the response isoline behave tangentially. Differences between the extreme responses produced by the two contours generally decrease with an increase in return period; however exceptions exist due to the tangential behaviour. It is advised that these sensitivities should be taken into consideration when the environmental contours are used in the design.

Integrity Assessment of a Free-Fall Lifeboat Launched From a FPSO

The paper addresses the structural integrity assessment of lifeboat launched from floating production, storage and offloading (FPSO) vessels. The study is based on long-term drop lifeboat simulations accounting for more than 50 years of hindcast data of metocean conditions and corresponding FPSO motions. Selection of the load cases and strength analyses with high computational time is a challenge. The load cases analyzed are those corresponding to the 99th percentile of long term distribution of indicators for large slamming loads (CARXZ) or large submergence (Imaxsub). For six selected cases, the time-varying pressure distribution on the lifeboat hull during and after water impact is calculated by CFD simulations using StarCCM+. The finite element model (FEM) of the composite structure of the lifeboat is modelled by ABAQUS. Quasi-static finite element (FE) analyses are performed for the selected load cases. The structural integrity is assessed by the maximum stress and Tsai-Wu failure measure. In the present study, the load and resistance factors are combined and applied to the response. A sensitivity study is performed to investigate the non-linear load/response effects when the load factor is applied to the load. In addition, dynamic analysis is performed with the time-varying pressure distribution for selected case and the dynamic effect is investigated.

FEA Analysis of Subsea Mooring Suction Anchor Under Operation Condition

The analysis of a mooring suction anchor involves both geotechnical and structural engineering. The design starts with the geotechnical analysis of a mooring suction anchor where the design loads are used to determine the size of the mooring suction anchor. Typically, a conservative estimate would be made for the soil strength and analysis would involve several layers of soil with different properties. The mooring suction anchor is then designed using the relevant soil parameters for various limit states under the combined vertical, lateral, torsional, and moment loading. Soil pressures or reactions acting on a rigid steel mooring suction anchor for each limit state are calculated. The calculation results are then provided to structural engineers to perform strength analysis to verify the integrity of the anchor. Therefore, it is important to understand how the soil reactions interact with the suction anchor in the structural model. The current analysis used the soil reaction data developed for an in-place loading condition for a mooring suction anchor. The structure of the mooring suction anchor was modelled using a 3D finite element method. Two studies were performed. The first study performed the regular mapping of the soil pressure to the suction anchor. The second study assumed that all the loads would be applied on the mooring padeye and the bottom of the suction would be fixed. It was presumed that the second study would yield a conservative result. However, the analysis results showed that the second study did not provide a conservative result. Therefore, it is recommended that the care should be taken when making such assumptions in future studies.

Simulation on the Dynamic Behavior of the Propulsion System Subjected by Hull Deformation for Large Vessels

This paper aims to investigate the dynamic behavior of the large ship propulsion system subjected by hull deformation. Evident tendency of development of large scale ships was shown that the interaction between the propulsion shaft and ship hull becomes much severer than before. The excited forces caused by severe sea waves have considerable effects on the hull deformation which could have further impact on the shaft propulsion system. On the contrary, the operation quality of ships and the durability of machines are threatened by the malfunctions of shaft propulsion system. As a result the reliability of the vessels has been put in an important position by the companies and the governments all over the world. For scientists, investigating the dynamic behavior of the propulsion system subjected by the hull deformation is a meaningful research to avoid malfunction of machine in navigation. Numerical analysis is now an effective method to analyze some key components on large vessels. Taking the 8530TEU container as an example, a numerical model of the large ship propulsion-hull coupling system is presented in this paper to analyze the dynamic behavior of the ship propulsion system subjected by hull deformation. The hull deformations are obtained under different sea conditions as the exciting forces which are used on the coupling system. Then the dynamical responds of the ship shaft are obtained. Based on the results, suggestions are proposed to ensure the normal operation of the propulsion system in different sea conditions.

Calculation of Shell Element Failure Based on the State of Stress Inside of a Neck

Simulation of failure in thin-walled structures is critical for the correct determination of crash performance of ships and offshore structures. Typically, shell elements are used, but these elements are not able to adequately capture local failure, especially inside of a neck. This paper addresses these gaps by adapting the Bridgman (1952) model of a neck inside of a plate by making it three-dimensional and offering an estimate of the relationship between state parameters of a shell element and the geometry inside of a neck. Finally, recommendations are also made about how to interface this information with the Modified Mohr-Coulomb failure locus to create a practical algorithm for assessing failure in shell elements.

Validation of a Software Tool (DROPSIM) to Predict the ‘Drop and Getaway’ Behaviour of a Lifeboat

The purpose of a free fall lifeboat is to evacuate people from platforms in case of emergency, and when other, normal means of evacuation, are not possible. For instance, when the weather is too rough, and evacuation cannot be performed by helicopters, the lifeboats are the last way of escape. It is thus essential to be able to properly assess the operability of a lifeboat and the safety of its occupants upon evacuation. Over the past four years, methods to quantify the operability limits of a lifeboat were analysed in a research project carried out for Statoil. As part of this project, a prototype software (denominated DROPSIM) was developed to predict the ‘drop and sailaway behaviour’ of a lifeboat. DROPSIM is a simplified method based on strip theory, with the objective to obtain predictions that are consistent with the relevant statistical behaviour of the lifeboat, and for the same target level of probability. Particularly because DROPSIM is a simplified tool, it is vital to verify that the software is adequate for simulating thousands of random lifeboat drops, yielding robust statistical predictions with sufficient accuracy. In order to show the performance of the simulation tool, an extensive validation procedure was established, based on a large amount of model test and data from other simulation tools. The following topics were considered in the validation: A. Verification: basic checks, e.g. related to buoyancy without comparison to model tests B. Consistency of simulated and measured response for basic test cases, including free-falling wedge tests and a variety of impact tests with a bullet shaped model C. Prediction of the sailaway behaviour of a lifeboat in comparison to model tests D. Comparison with integrated drop and sailaway model tests in normal and off-design (extreme) conditions in calm water and in waves. In this paper the results of the validation of DROPSIM are presented and discussed. Another dedicated paper gives insight into the mathematical model of DROPSIM ([1]).

On Prediction for Structural Performance of Ship Side Structures in Head-On Collision Scenario

The crashworthiness of ship side structures should be taken into consideration during the structural design phase. When a ship is struck by a head-on collision, the main participants of the side structures include outer plating, longitudinal girders, transverse frames and the stiffeners attached on them. This paper is centered on establishing an integrated deformation mechanism program by identifying the theoretical deformation modes of the side structures, including the plating and stiffeners. The primary failure models of plating structures are the crushing, stretching and tearing modes, and a new crushing model of side plating structures subject to an ellipsoid-shaped indenter is proposed. As for the stiffeners on outer plating, the smeared thickness method is often used, but the role of the stiffeners cannot be traced clearly during the deformation process and the structural performance predictive accuracy may not be guaranteed. Therefore, a theoretical model of stiffeners is established in this paper, on purpose of providing deep insight of the deformation mechanism with reasonable accuracy. The expressions of resistance force of the side structures are also derived based on a study of the progressive deformation process of numerical simulation results and the plastic analytical methods. The accuracy of the analytical method is verified by numerical simulations using code LS_DYNA. The proposed analytical method can be used for quick assessment of the performances of ship side structures during ship collision.

Estimation of Most Probable Maximum From Short-Duration or Undersampled Time-Series Data

The most probable maximum (MPM) is the extreme value statistic commonly used in the offshore industry. The extreme value of vessel motions, structural response, and environment are often expressed using the MPM. For a Gaussian process, the MPM is a function of the root-mean square and the zero-crossing rate of the process. Accurate estimates of the MPM may be obtained in frequency domain from spectral moments of the known power spectral density. If the MPM is to be estimated from the time-series of a random process, either from measurements or from simulations, the time series data should be of long enough duration, sampled at an adequate rate, and have an ensemble of multiple realizations. This is not the case when measured data is recorded for an insufficient duration, or one wants to make decisions (requiring an estimate of the MPM) in real-time based on observing the data only for a short duration. Sometimes, the instrumentation system may not be properly designed to measure the dynamic vessel motions with a fine sampling rate, or it may be a legacy instrumentation system. The question then becomes whether the short-duration and/or the undersampled data is useful at all, or if some useful information (i.e., an estimate of MPM) can be extracted, and if yes, what is the accuracy and uncertainty of such estimates. In this paper, a procedure for estimation of the MPM from the short-time maxima, i.e., the maximum value from a time series of short duration (say, 10 or 30 minutes), is presented. For this purpose pitch data is simulated from the vessel RAOs (response amplitude operators). Factors to convert the short-time maxima to the MPM are computed for various non-exceedance levels. It is shown that the factors estimated from simulation can also be obtained from the theory of extremes of a Gaussian process. Afterwards, estimation of the MPM from the short-time maxima is explored for an undersampled process; however, undersampled data must not be used and only the adequately sampled data should be utilized. It is found that the undersampled data can be somewhat useful and factors to convert the short-time maxima to the MPM can be derived for an associated non-exceedance level. However, compared to the adequately sampled data, the factors for the undersampled data are less useful since they depend on more variables and have more uncertainty. While the vessel pitch data was the focus of this paper, the results and conclusions are valid for any adequately sampled narrow-banded Gaussian process.