Probabilistic Fatigue Reliability of Large Diameter Steel Catenary Risers (SCR) for Ultra-Deepwater Operations

For deepwater development in the Gulf of Mexico, steel catenary risers (SCRs) supported from both SPAR and semi-submersible platforms have proven to be successful solutions for in-field flowlines, tie-backs, and export systems. It is envisaged that this will continue to be a promising solution in ultra deep-water applications, up to and beyond 10,000 ft. The study, commissioned by the Mineral Management Service (MMS), investigated the reliability of large-diameter SCRs in ultra-deepwater operations. The primary damage mode considered is fatigue failure. A probabilistic methodology for fatigue reliability is developed, which utilizes deterministic cumulative fatigue damage indicators, namely the stress levels and cycles associated with the various sea states and the fatigue strength of the members. Uncertainties in structural load and material properties are accounted for by assigning probability distributions and standard deviations to the deterministic stress levels. Furthermore, fatigue strength parameters, Miner’s indices, and capacities are modeled as random variables. First order reliability method (FORM) is employed for estimating fatigue reliability. The methodology is applied to three deterministic case studies presented by Intec Engineering (2006a, 2006b). The case studies involved either a SPAR or a semi-submersible platform. For the sake of brevity, a case study involving only a SPAR platform is presented in this paper. The effect of uncertainties in parameters on fatigue reliabilities is investigated. It is observed that the fatigue reliability estimates followed similar trends as the deterministic cumulative damage results, and hence can be used to complement deterministic estimates. Additional benefit and insight gained from the probabilistic study, which can be used for design decisions, include information regarding probabilistic importance and probabilistic sensitivity analysis. For case study presented here, it is seen that in general, uncertainty in the fatigue strength exponent (m) has the highest impact on fatigue reliability of SCRs. The second most important random variable is the stress range (S), which captures uncertainties in parameters such as loads and material properties. Parametric sensitivity studies on the fatigue strength parameters indicate that SCR reliability is sensitive to both the standard deviation and probability distribution of the parameters, thus highlighting the need for accurate probabilistic calibration of the random variables.

Very Large Floating Structures

Very Large Floating Structure (VLFS) is a unique concept of ocean structures primary because of their unprecedented length, displacement cost and associated hydroelastic response. International Ship and Offshore Structures Congress (ISSC) had paid attention to the emerging novel technology and launched Special Task Committee to investigate the state of the art in the technology. This paper summarizes the activities of the committee. A brief overview of VLFS is given first for readers new to the subject. History, application and uniqueness with regard to engineering implication are presented. The Mobile Offshore Base (MOB) and Mega-Float, which are typical VLFS projects that have been investigated in detail and are aimed to be realized in the near future, are introduced. Uniqueness of VLFS, such as differences in behavior of VLFS from conventional ships and offshore structures, are described. The engineering challenges associated with behavior, design procedure, environment, and the structural analysis of VLFS are introduced. A comparative study of hydroelastic analysis tools that were independently developed for MOB and Mega-Float is made in terms of accuracy of global behavior. The effect of structural modeling on the accuracy of stress analysis is also discussed. VLFS entails innovative design methods and procedure. Development of design criteria and design procedures are described and application of reliability-based approaches are documented and discussed.

The Influence of Different Scenarios of Supply Ship Collision on the Dynamic Response of a North-Sea Jacket-Pile-Soil System

In the present study, the influence of various scenarios of supply ship collisions, namely, bow, stern and also broad-side impacts on a jacket-pile-soil system is investigated. In the previous study of ship impact on an 8-leg North-Sea Jacket Platform by Amdahl et al. [2] and also other authors, the effect of jacket-pile-soil interaction was not considered. The collision points on the jacket structure are also taken as joints and mid-span of leg, horizontal and vertical braces, namely, hard and soft impact points. The speed and the weight of the colliding vessel are also varied for typical supply vessels. Several supply ship collision analyses are carried out for bow, stern and broad-side impact scenarios on an 8-leg North-sea Jacket platform It is observed that by taking into account the jacket-pile-soil interaction effects, in particular in softer clay soils the amplitude of displacement response after supply ship impact at the deck level is increased due to yield in the upper soil layers. Contrary to this finding, less linear dynamic effects can be seen in the studied jacket-pile-soil system subjected to the supply ship impact. It can also be concluded that for soft impact scenario, the dynamic effects in the global response of the platform located in the mainly OC clayey soil may be much less than those for hard impact scenario on the same platform. For instance, for a brace impact at its mid-span, a less significant dynamic effect has been observed than for a leg impact. The duration of impact in such cases is shown to play an important role in determining the dynamic influence of the platform response. The relative energy absorption of the platform is shown to be more for broad-side loading. It is shown that the global response of the jacket platform during the collision with a supply vessel might depend largely on the scenario of the impact and to some extent on the pile-soil behavior. It is found that for the bow and stern impact scenarios, the energy contribution of the local member dent or buckling might be more significant than for the broad-side loading for which the global frame energy contribution and the overall inertia effect of the platform might be a dominant factor.

Extreme Response Predictions for Jack-Up Units in Second Order Stochastic Waves by FORM

The aim of the present paper is to advocate for a very effective stochastic procedure, based on the First Order Reliability Method (FORM), for extreme value predictions related to wave induced loads. All kinds of non-linearities can be included, as the procedure makes use of short time-domain simulations of the response in question. The procedure will be illustrated with a jack-up rig where second order stochastic waves are included in the analysis. The result is the probability of overturning as function of sea state and operational time.

Visual Assessment of Corroded Condition of Plates With Pitting Corrosion Taking Into Account Residual Strength: In the Case of Webs of Hold Frames of Bulk Carriers

Corrosion pits with a conical shape are typically observed in hold frames in way of cargo holds of bulk carriers which carry coal and iron ore. The ratio of the diameter to the depth of the typical corrosion pits is in the range between 8–1 and 10–1 and its diameter might become up to 50mm. The evaluation of residual strength of members with large uneven pitting corrosion is difficult compared with that of members with general corrosion. Therefore, it is of crucial importance to develop a method for the evaluation of residual strength of pitted members. The purpose of the present study is to investigate the effect of pitting corrosion on the ultimate strength of steel plates under various loading conditions and explore a method for the evaluation of residual thickness of pitted plates. In the present study, a series of non-linear FE-analyses has been conducted with steel plates with a variety of random pit distributions under various loading conditions such as uni-axial compression, bi-axial compression, shear and combination of these. In these analyses, random pit distributions were calculated by the previously developed corrosion model. It has been shown that equivalent thickness loss, which is defined as thickness loss of uniformly corroded plates with the same ultimate strength as the randomly pitted plates, is smaller than or equal to 1.25 times the average thickness loss. It has been also revealed that the equivalent thickness loss for the ultimate strength under the above-mentioned loading conditions is smaller than average thickness loss at the minimum cross section, where the average thickness loss at the minimum cross section almost corresponds to the equivalent thickness loss for the tensile strength. Based on these findings, a method for the estimation of equivalent thickness loss of pitted plates has been discussed using the thickness diminution-DOP relationship, where DOP (Degree of Pitting Intensity) is defined as the ratio of the pitted surface area to the total surface area.

Comparison of Bivariate Models of the Distribution of Significant Wave Height and Peak Wave Period

This paper presents the results of a comparison of the fit of three bivariate models to a set of 14 years of significant wave height and peak wave period data from the North Sea. One of the methods defines the joint distribution from a marginal distribution of significant wave height and a set of distributions of peak period conditional on significant wave height. Other method applies the Plackett model to the data and the third one applies the Box-Cox transformation to the data in order to make it approximately normal and then fits a bivariate normal distribution to the transformed data set. It is shown that all methods provide a good fit but each one have its own strengths and weaknesses, being the choice dependent on the data available and applications in mind.

Quantification of Uncertainty in Reserves Estimation From Decline Curve Analysis of Production Data for Unconventional Reservoirs

Decline curve analysis is the most commonly used technique to estimate reserves from historical production data for evaluation of unconventional resources. Quantifying uncertainty of reserve estimates is an important issue in decline curve analysis, particularly for unconventional resources since forecasting future performance is particularly difficult in analysis of unconventional oil or gas wells. Probabilistic approaches are sometimes used to provide a distribution of reserve estimates with three confidence levels (P10, P50 and P90) and a corresponding 80% confidence interval to quantify uncertainties. Our investigation indicates that uncertainty is commonly underestimated in practice when using traditional statistical analyses. The challenge in probabilistic reserves estimation is not only how to appropriately characterize probabilistic properties of complex production data sets, but also how to determine and then improve the reliability of the uncertainty quantifications. In this paper, we present an advanced technique for probabilistic quantification of reserve estimates using decline curve analysis. We examine the reliability of uncertainty quantification of reserve estimates by analyzing actual oil and gas wells that have produced to near-abandonment conditions, and also show how uncertainty in reserves estimates changes with time as more data become available. We demonstrate that our method provides more reliable probabilistic reserves estimation than other methods proposed in the literature. These results have important impacts on economic risk analysis and on reservoir management.

Loads on Ships and Offshore Structures

The Loads Committee of the International Ship and Offshore Structures Congress (ISSC) critically reviews the state of the art of environmental and operational loads. Amongst these, elements more relevant to the offshore industry will be presented in this paper. These comprise wave-induced loads, including linear and nonlinear methods, multi-body interactions, slamming, green water, sloshing and rogue waves, cables and risers, vortex-induced vibrations, ice loads, fatigue loading and, verification and validation.

Implementation of a Bottom-Hole Assembly Program

A state of the art windows graphical user interface (GUI) program has been developed to predict and design the bottom-hole assembly (BHA) performance for drilling. The techniques and algorithms developed in the program are based upon those developed by Lubinski and Williamson. The BHA program facilitates in conducting parametric studies, and in making field decisions for optimal performance. The input parameters may include: formation class, dip angle, hole size, drill collar size, number of stabilizers, stabilizer spacing. The program takes into consideration bit-formation characteristics and interaction, drill collar sizes, square collars, shock absorbers, MWD tools, reamer tools, directional tools, rotary steerable systems etc. The output may consist of hole curvature (build up or drop rate), hole angle, weight on bit and is presented in drilling semantics. Additionally, the program can perform mechanical analyses and solve for the bending moments and reactions forces. Moreover, the program has the capability to predict the wellpath using a drill ahead algorithm. The program consists of a mathematical model which makes assumptions of 2-D, static, constant hole curvature resulting in a robust computationally efficient tool that produces rapid reliable results in the field.

Reliability Assessment and Management Lessons From Hurricane Katrina

Hurricane Katrina disclosed a wide variety of flaws and defects that were embedded in the flood protection system for the greater New Orleans area (NOFPS). One of the primary reasons for this catastrophe was failure in risk assessment and management (RAM). Based on results from several extensive forensic studies, this paper details how RAM failed. Critical limitations in current RAM methodologies and results intended to help rehabilitate and further develop the NOFPS are highlighted. Recommendations are advanced to address these limitations.