Structural Integrity Evaluation of Jackets Submitted to Through Cracks

The study of the impact of through cracks on structural integrity of jacket platforms still a challenge. The detection of such cracks is of great importance and a miss, or a spurious indication can lead to maintenance costs overrun. In the context of risk-based inspection and monitoring of such structures, a global methodology is proposed. The detection of large cracks is first addressed. A probabilistic model is proposed, taking into account the in situ inspections performances and the probability of crack presence. This is achieved by the use of the detection theory. Second, a finite element that is able to represent the structural behaviour of through cracked tubular nodes is proposed and a global structural integrity measure is suggested. Finally, inspection results are introduced in order to compute the expected platform structural integrity. It is illustrated by considering FMD inspections results of a tripod structure. Effects of false alarms can then be underlined.

A Comparison of Self-Rectifying Turbines for OWC Based Wave Energy Power Extracting Device Using Numerical Simulation

This paper presents a comparison of self-rectifying turbines for the Oscillating Water Column (OWC) based Wave Energy power extracting device using numerical simulation. The two most commonly used turbines for OWC based devices, the Impulse and the Wells turbines were evaluated under real sea simulated conditions. Assuming the quasi-steady condition, experimental data for both 0.6m turbines with 0.6 hub to tip ratio was used to predict their behavior under real sea conditions. The real sea water surface elevation time history data was used to simulate the flow conditions using standard numerical simulation techniques. A simple geometry of the OWC was considered for the simulation. The results show that the overall mean performance of an Impulse turbine is better than the Wells turbine under unsteady, irregular real sea conditions. The Impulse turbine was observed to be more stable over a wide range of flow conditions. This paper reports the comparison of performance characteristics of both these turbines under simulated real sea conditions.

Reliability Analysis of Jack-Up Platforms Based on Fatigue Degradation

Generally, jack-up structures are used for production drilling and exploration of hydrocarbons. The combination of mobility and the behavior as a fixed structure in operational conditions has made it an important structure in the offshore industry over the last 40 years. When a jack-up structure has been in operation for a great part of its original design-life and intention is there to extend the usage of this structure at a specific location, an investigation on fatigue degradation of the structure is an essential factor that has to be carried out before taking any decision. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In this paper, the remaining fatigue capacity of the jack-up structure is considered as an indicator for adequate use of the structure. It can be specified based on the difference between design-fatigue and fatigue experienced by the structure. The design-fatigue can be determined based on fluctuation of loads during the lifetime of the structure and experienced fatigue is specified by the load conditions, which the structure has experienced during its service time. When the information on the load conditions which the structure has experienced in its service time is available or known precisely, determination of the remaining fatigue capacity could be carried out by using the Palmgren–Miner’s rule. In practice, uncertainties are present in loads and characteristics of material. Hence it will be reasonable to determine the remaining fatigue reliability of the structure by the reliability methods. In this paper, based on a crack propagation approach and achieved information from inspection, it is shown that the remaining fatigue reliability of jack-up structures could be determined and updated by using a Bayesian procedure in the duration of the service time.

Reliability Based Design Optimisation of a “No Burst” High Pressure Pipeline

The design philosophy of a pressure-protected subsea pipeline is intimately linked to the reliability of the Pressure Protection System (PPS) and to the probability of burst of the pipeline if it is exposed to full wellhead shut-in pressure. A reliability based design approach is presented that allows the pipeline wall thickness (and cost) to be reduced under the philosophy that the pipeline will “not burst” in the event of PPS failure. This paper describes how uncertainties in the pipeline design parameters may be initially modelled statistically to allow structural reliability techniques to be adopted at the design stage (before the pipe is manufactured). It further addresses how correlation of these parameters can be included and their extreme value distributions developed, which is particularly relevant as the length of the tieback increases. A method to incorporate inspection inaccuracy is also presented. The initial estimates of the design parameters necessarily err on the conservative side. These can be later updated when manufacturing data is available.

Robust Reliability Assessment for Offshore Minimum Structures Under Dynamic Loads

The offshore minimum structures are widely applied in the development of margin oilfield because of their simplicity in fabrication, low initial investment and fast-track schedule. However, they would suffer large dynamic response under exciting loads such as seismic and ice loads, which might lower their service safety. The paper is focused on robust reliability assessment for the offshore minimum structures under dynamic loads by considering first-exceeding failure mode, fatigue failure mode. The robust reliability of offshore structure is a measure of its resistance to the uncertainties, and it suits very much to the condition that the information and data are scarce. The multi-level fortification for ice-resistant offshore minimum structures is presented in this paper, that is, the structure should not be defective under normal ice condition, repairable under heavy ice condition and not collapse under the heaviest ice condition. A numeric calculation method of robust reliability and several robust reliability dynamic models for offshore minimum structures are put forward in this paper, in which plastic collapse, fatigue, and fracture are dealt with. The interaction between the different failure modes under these loads is also considered in the paper. Based on research work mentioned above, the comprehensive safety assessment for the offshore minimum structures would be more easily realized under dynamic loads such as ice loads, seismic loads. Finally, the example is given.

Prediction of Relative Motion and Probability of Contact Between FPSO and Shuttle Tanker in Tandem Offloading Operation

Tandem offloading safety between FPSO and shuttle tanker is under concern. A few collisions between the two vessels have happened in the North Sea in recent years. In these incidents, excessive relative motions (termed as surging and yawing in this paper) between FPSO and tanker are identified as “failure prone situations” which have contributed to the initiation of most collision incidents. To quantitatively assess the probability of surging and yawing events, and more importantly, to effectively reduce their occurrence in tandem offloading operation, we present a simulation-based approach in this paper, which is carried out by a state-of-the-art time-domain simulation code SIMO. The SIMO simulation models are setup and calibrated for a typical North Sea purpose-built FPSO and a DP shuttle tanker. This 2-vessel system motion in tandem offloading is simulated. The simulated relative distance and relative heading between FPSO and tanker are analyzed by fitting their extreme values into statistical models. This gives out probabilities of surging and yawing events. Sensitivity studies are performed to analyze contributions from various technical and operational factors. Measures to minimize the occurrence of surging and yawing from design and operational point of view are proposed.

Probabilistic Computations of Downtime for Inland Shipping in the Second Maasvlakte

This paper deals with the accessibility for inland shipping to several alternatives for the extension of the Maasvlakte in the Port of Rotterdam (Maasvlakte 2). The accessibility is governed by the risks associated with high wind and wave conditions, which can cause structural damage to the ship, excessive intake of water and high loads in the coupling cables of push-barge units. This paper discusses the computational methods for the following aspects: wave conditions in the port area; risk of failure in several failure modes and total downtime. The use of interviews with the skippers of the inland ships is discussed. A summary of the results of the study is also presented.

Intercomparison of Seastate and Zerocrossing Parameters From the WACSIS Field Experiment and Interpretation Using Video Evidence

During the WACSIS field experiment, wave elevation time series data were collected over the period December 1997 to May 1998 on and near the Meetpost Nordwijk platform off the coast of the Netherlands from an EMI laser, a Saab radar, a Baylor Wave Staff, a Vlissingen step gauge, a Marex radar and a Directional Waverider. This paper reports and interprets, with the help of simultaneous dual video recordings of the ocean surface, an intercomparison of both single wave and sea state wave parameters.

Statistics of Wave Crests From Models vs. Measurements

The analysis phase of the Wave Crest Sensor Intercomparison Study (WACSIS) focussed on the interpretation of the wave data collected by the project during the winter of 1997–98. Many aspects of wave statistics have been studied, but the main emphasis has been on crest height distributions, and recommendations for crest heights to be used in air gap calculations. In this paper we first describe comparisons of the crest height distributions derived from the sensors (radars, wave staffs, laser) and from simulations based on 3D second order irregular wave models. These comparisons permit us to make conclusions on the quality of these models and to qualify the ability of some sensors to measure the crest heights accurately. In the second part two new parametric models of the crest height distributions are discussed and their superiority to standard parametric models is demonstrated.

A Simplified Method for Response Behavior and Reliability Analysis of Long Submerged Structures With Tension Legs in Irregular Waves

This paper deals with the elastic response behavior of marine tunnel structures with tension legs in regular and irregular waves. Firstly, a simplified estimation method for dynamic responses under regular wave conditions is analytically presemed using a simple beam on an elastic foundation. Then, in order to demonstrate the validity of above results, experimental studies are carried out for a marine tunnel structure model with tension legs under wave-induced loads. Next, a simplified estimation method is presented for the elastic response behavior under irregular wave conditions by using above analytical results and combining irregular sea wave spectra. Then, the limit state failure mode of the main structure is presented for estimating the reliability level for cracking failure under extreme wave loads. Finally, the applicability of the methods is investigated through numerical examples carried out for a 1,000m-class marine tunnel structure with tension legs under some irregular sea state conditions. And characteristics of the short-term responses and reliability levels for the cracking failure are numerically shown.