Application of Reliability Analysis to Re-Qualification and Life Extension of Floating Production Unit Moorings

Life extension and asset integrity of Floating Production Unit (FPU) moorings are issues of increasing importance for operators due to changing production requirements, the requirement to extend service life, and circumstances where the met-ocean Basis of Design (BOD) has increased significantly over the life of the field. Reliability methods are gaining increasing acceptance as increased computing power allows large numbers of simulations to be undertaken using realistic fully coupled models that are validated against prior experiments. When applied to the re-qualification and life extension of FPU moorings, particularly with regard to re-qualification and life extension of in-place moorings, reliability analysis offers considerable advantages over conventional deterministic return period design. This paper details the application of a reliability approach to re-qualification and life extension of a turret-moored FPU that had design met-ocean conditions increased significantly over the life of the field. It explores key elements of reliability analysis including the probabilistic characterisation of met-ocean conditions, adequate representation of vessel dynamics and mooring loads in a Response Surface Model, and a selection of algorithms to solve for the system probability of failure. Discussion points include the advantages of the explicit identification of the most likely failure scenario versus uncertainty as to whether the worst design case has been identified, and the potential for rapid reassessment of reliability for specific design conditions (such as a degraded mooring system or a system for which degradation is ongoing). The results of this study demonstrate the significant advantages to the industry conferred by adopting reliability methods in the re-certification and life extension of existing FPU moorings. In particular, the study highlights that conventional mooring code deterministic design methods, whilst adequate for original design purposes, lack sufficient fidelity to address the multi-faceted issue of re-assessment of notionally marginal legacy systems. For a degraded existing mooring, an application of these methods can demonstrate that the level of reliability of the system is still acceptable, whereas a conventional approach may produce an over-conservative indication that the mooring is non-compliant. Applicable to a wide range of FPUs requiring re-qualification or life extension, the techniques discussed also provide pointers to more efficient and reliable mooring design for not just existing, but also for new FPUs.

Buckling/Ultimate Strength Evaluation for Continuous Stiffened Panel Under Combined Shear and Thrust

In this study, FE modeling method for the buckling/ultimate strength analysis of a continuous stiffened panel under combined shear and thrust is proposed. In order to validate the proposed method, shear buckling collapse tests of a stiffened panel and FEM analysis are carried out. As the result of these, it is confirmed that the buckling collapse behavior and the ultimate strength estimated by the proposed method are in good agreement with the test results.

Non-Stationary Estimation of Joint Design Criteria With a Multivariate Conditional Extremes Approach

Understanding the interaction of ocean environments with fixed and floating structures is critical to the design of offshore and coastal facilities. Structural response to environmental loading is typically the combined effect of multiple environmental parameters over a period of time. Knowledge of the tails of marginal and joint distributions of these parameters (e.g. storm peak significant wave height and associated current) as a function of covariates (e.g. dominant wave and current directions) is central to the estimation of extreme structural response, and hence of structural reliability and safety. In this paper, we present a framework for the joint estimation of multivariate extremal dependencies with multi-dimensional covariates. We demonstrate proof of principle with a synthetic bi-variate example with two covariates quantified by rigorous uncertainty analysis. We further substantiate it using two practical applications (associated current given significant wave height for northern North Sea and joint current profile for offshore Brazil locations). Further applications include the estimation of associated criteria for response-based design (e.g., TP given HS), extreme current profiles with depth for mooring and riser loading, weathervaning systems with non-stationary effects for the design of FLNG/FPSO installations, etc.

A Rubber Bag for Liquid Cargo to Improve Ship Collision Safety

Collisions and grounding accidents of ships, but also the failure of the hull-integrity, can lead to oil leakage. Examples are the Rena in 2011, the Hebei Spirit in 2007 and the much known accident of the Prestige in 2002. Consequently research regarding the enhancement of the structural design to increase the safety-level of ships in case of accidents is important. In this paper the use of a rubber bag as a second barrier is presented as an alternative concept to prevent oil leakage in case of accidents. The influence of the rubber bag is investigated using the example of a ship collision. A simplified tanker side structure as well as a box shaped rubber bag are analyzed with the finite element method. The material model for the rubber bag is calibrated with tensile tests to obtain the required material parameters. The reaction forces and the associated penetration depth are analyzed. The comparison is done between the structure with and without the rubber bag. For the latter, the general behavior is compared with large-scale experimental results. Furthermore an additional increase of the survivability of the ship due to the rubber bag without changing the common structural design is discussed.

Hull Deformation Effect on Membrane-Type LNG Containment Systems

The objective of this study is to investigate the relationship between the maximum allowable hull deformation, which includes global elongation and local deflection, and the capacity of the CCS in membrane-type LNG vessels. The LNG CCS mainly consists of the primary barrier (e.g. a corrugated membrane for GTT MK III system and an invar membrane for GTT NO 96 system) and the insulation panel which is attached to the inner hull through mastics or couplers. The excessive hull elongation due to dynamic wave loads may cause fatigue damage of the primary barrier. Thus, the maximum allowable hull elongation (global deformation) can be determined based on the fatigue strength of the primary barrier. On the other hand, the excessive hull deflection due to cargo or ballast water pressure may cause failure of the insulation panel and the mastic. Therefore, the maximum allowable hull deflection (local deformation) in the hull design can be determined based on the strength of the insulation panel and the mastic. In the present paper, the determination of fatigue life vs. strain curves of materials has been summarized for the primary barrier. Fatigue curves based on either structural fatigue tests or standard specimen tests can be applied in fatigue assessment of a primary barrier. As an example, the finite element (FE) analysis has been conducted on the MK III CCS with the hull structure under pressure loads. Two different load cases including full load and ballast load conditions have been considered to evaluate the structural integrity of the insulation system in numerical simulations. FE results show that the mechanical behavior of the insulation system and the mastic under the maximum allowable hull deflection has been examined based on the yielding strength of each individual component. Finally, the complete procedure to determine the maximum allowable hull elongation and the maximum allowable hull deflection has been developed for meeting the requirements of containment system design for membrane-type LNG carriers.

Extreme Response Prediction for Fixed Offshore Structures by Monte Carlo Time Simulation Technique

For an offshore structure, wind, wave, current, tide, ice and gravitational forces are all important sources of loading which exhibit a high degree of statistical uncertainty. The capability to predict the probability distribution of the response extreme values during the service life of the structure is essential for safe and economical design of these structures. Many different techniques have been introduced for evaluation of statistical properties of response. In each case, sea-states are characterised by an appropriate water surface elevation spectrum, covering a wide range of frequencies. In reality, the most versatile and reliable technique for predicting the statistical properties of the response of an offshore structure to random wave loading is the time domain simulation technique. To this end, conventional time simulation (CTS) procedure or commonly called Monte Carlo time simulation method is the best known technique for predicting the short-term and long-term statistical properties of the response of an offshore structure to random wave loading due to its capability of accounting for various nonlinearities. However, this technique requires very long simulations in order to reduce the sampling variability to acceptable levels. In this paper, the effect of sampling variability of a Monte Carlo technique is investigated.

Optimization of Flexible Pipes Dynamic Analysis Using Artificial Neural Networks

Irregular wave dynamic analysis is an extremely computational expensive process on flexible pipes design. One emerging method that aims to reduce these computational costs is the hybrid methodology that combines Finite Element Analyses (FEA) and Artificial Neural Network (ANN). The proposed hybrid methodology aims to predict flexible pipe tension and curvatures in the bend stiffener region. Firstly using short FEA simulations to train the ANN, and then using only the ANN and the prescribed floater motions to get the rest of the response histories. Two approaches are developed with respect to the training data. One uses an ANN for each sea state in the wave scatter diagram and the other develops an ANN for each wave incidence direction. In order to evaluate the accuracy of the proposed approaches, a local analysis is applied, based on the predicted tension and curvatures, to calculate stresses in tension armour wires and the corresponding flexible pipe fatigue lifes. The results are compared to those from full nonlinear FEM simulation.

Progressive Collapse Approach Compared to the Traditional Temperature Screening on Structural Assessment of Offshore Structures Against Fire

The present study compares the progressive collapse approach with the traditional temperature screening method on determination of PFP requirements at topside offshore structures. The advantage to evaluate the consequences of fire scenarios on the global integrity and stability of topside modules can be revealed by a substantial reduction of the required amount of PFP, and consequently significant cost savings for operators, when compared to the traditional approach. In the case study presented in this paper, there is a reduction of 79% in PFP allocation.

Reliability Analysis of Redundant Dynamic Positioning Control System With Human Factor Involved

Deepwater oil and gas explorations bring more safety and reliability problems for the dynamically positioned vessels. With the demands for the safety of vessel crew and onboard device increasing, the single control architecture of dynamic positioning (DP) system can not guarantee the long-time faultless operation for deeper waters, which calls for much more reliable control architectures, such as the Class 2 and Class 3 system, which can tolerate a single failure of system according to International Maritime Organization’s (IMO) DP classification. The reliability analysis of the main control station of DP Class 3 system is proposed from a general technical prospective. The fault transitions of the triple-redundant DP control system are modeled by Markov process. The effects of variation in component failure rates on the system reliability are investigated. Considering the DP operation involved a human-machine system, the DP operator factors are taken into account, and the human operation error failures together with technical failures are incorporated to the Markov process to predict the reliability of the DP control system.

Structural Fire Integrity Testing of Lightweight Structures

To reduce environmental impact and to manage weight in shipping and offshore, lightweight structures are becoming increasingly important. A critical issue for loadbearing structures is their structural fire integrity. It is generally evaluated by loaded furnace fire resistance tests based on ISO 834. As part of the EU project BESST, a series of such tests were performed with typical lightweight fiber reinforced polymer (FRP) composite sandwich structures. The purpose was to determine whether structural fire integrity is sensitive to the design load, design method and safety factor against buckling. In particular was examined whether the temperature at the interface between the exposed laminate and the core is critical for structural integrity and how it depends on the applied loading. Independence of the applied load would make performance solely a matter of heat transfer, which would significantly reduce necessary testing. The tests were carried out with starting point in an insulated sandwich panel system, certified as a 60 minute Fire Resisting Division (FRD-60) for high-speed craft in accordance with the Fire Test Procedures (FTP) Code. The structure consisted of 1.3 mm glass fiber reinforced polyester laminates surrounding a cross linked PVC foam core called Divinycell H80 (80 kg/m3). It was constructed for a 7 kN/m design load, which is the loading applied in the FTP Code furnace test for high-speed craft. Hence, with a conventional safety factor against buckling of 2.5 it was designed to resist a critical load of 17.4 kN/m. With basis in this design, tests were performed with structures where the thickness of the laminates or core had been altered and with adjusted safety factor against the applied loading. In addition, a test was performed with a stiffened panel. Firstly it was noted that 60 minutes of fire resistance was not achieved in most of the tests, which was a consequence of an alteration in the FTP Code test procedures. The FRD-60 structure used as starting point was certified before the 2010 edition of the FTP Code was ratified. This harmonized the test procedure between laboratories and gave a slightly tougher temperature development than when the structure was certified. However, the test results are still valid and show a small variation in the time to failure in the tests with unstiffened sandwich structures, ranging between 51 and 58.5 minutes. Changing the safety factor from 2.5 to 1.5 resulted in a relatively small decrease in time to failure of 3 minutes. The stiffened test showed that structural resistance is better achieved by use of stiffeners than by thick laminates. Furthermore, applying this as a design principle and using a safety factor of 2.5 leaves a test variation between 55 and 58.5 minutes. The temperature at the exposed laminate-core interface was quite similar in the tests at the time of failure. This excludes the test when the laminate thickness was increased as a measure for structural improvement. In conclusion, the test series shows that fire resistance bulkhead testing of insulated FRP composite panels can be simplified and does not have to be performed with varying design loads. To achieve conservative evaluation, a design concept should be evaluated by testing the panel designed for the highest applicable load level, not by testing a weak panel at 7 kN/m loading. This applies to non-stiffened solutions.