Offshore Jacket Structures Research Articles

A Tube with a Rectangular Cut-Out. Part 2: Subject to Axial Compression

Simply supported steel tubes with rectangular cut-outs of different sizes positioned at their mid-length were subject to axial compression to assess the effect of the cut-outs on the deformation behaviour. Experiments on 1000 mm long, 100 mm diameter, and 2.55 mm wall thickness DIN 2391 ST NBK supported steel tubes are described. The experimental tubes were relatively thick walled, with a thickness-to-diameter ratio typical of structural columns, rather than shell structures. The radial position of the cut-outs was such that the reduced cross-section was symmetric with respect to the direction of rotation of the pinned ends. Several permutations of the axial and circumferential dimensions of the cut-out were tested so that the effect of changing these dimensions could be assessed. Finite-element and semi-empirical analyses were carried out to predict the global load-deformation behaviour of the tubes. Both analyses gave good predictions of the experimental results for the majority of the cut-out parameters investigated. The motivation for the study was to investigate the feasibility of using cut-outs to initiate and control the toppling of offshore jacket structures.

A tube with a rectangular cut-out. Part 1: Subject to pure bending

The response to pure bending of tubes with rectangular cut-outs is considered. Experiments on 1000-mm long, 100-mm diameter, and 2.55-mm wall thickness DIN 2391 ST NBK supported steel tubes are described. Such a thickness-to-diameter ratio is typical of structural columns, rather than shell structures. Tubes containing cut-outs with an axial dimension of up to 30 mm and a circumferential size of up to 180° were tested. It was found that plastic hinge mechanisms dominated the response when the cut-out was on the compressive side, whereas fracture behaviour dominated the response when the cut-out was on the tensile side. Finite element and semi-empirical analyses were carried out to predict the global load-deformation behaviour of the tubes. All analyses gave reasonable predictions of the experimental results for the majority of the cut-out parameters investigated. The aim of this study is to investigate the feasibility of using cut-outs to initiate and control the toppling of offshore jacket structures.

Evaluation of damage to offshore platform structures due to collision of large barge

An offshore jacket platform in the South China Sea was impacted by a large derrick and lay barge during installation. This paper presents a non-linear dynamical analysis procedure for firstly determining the impact action based on the forensic evidence from the damaged components, and then evaluating the overall damage effects on the platform structure. The impact action of the barge is simulated with a triangle impulse load with different collision contact times. The curves relating the indentation deformations of the damaged member with different collision contact times are simulated using an estimated velocity of the impacting ship. On the basis of these curves and the actual detected dent damages, the contact time and the maximum impact load on the platform are determined. Taking into account the force–deflection relationship of the local indentation of the damaged cross-diagonal brace, the transmission of the impact load to the platform structure is simulated by a non-linear spring. The added mass coefficient with hydrodynamic effects and the pile–soil-structure interaction are considered in the computational model of the non-linear dynamic response of the platform structure. Subsequently, the dynamic response of the offshore jacket structure is computed and the critical stress and deformation of the tubular joints are obtained as indicators of the damage effects. The results are useful for choosing a feasible and reasonable repairing and strengthening scheme for the damaged platform. The procedure presented in this paper is generally applicable for the evaluation of typical offshore platform structures in the case of impact or collision.

Strength prediction for ring-stiffened DT-joints in offshore jacket structures

Internal stiffeners are commonly employed to enhance the strength of tubular joints found in offshore steel jacket structures. However, there is a lack of detailed design guidance on the determination of their strengths in major offshore codes and design guides. This paper first summarises the results of a finite element study on tubular DT-joints with internal ring stiffeners subjected to axial brace compression. The study generated a database of twenty-two stiffened joints, some of which included multiple stiffeners. Observations on the development of plastic zones in the stiffeners show that the failure mechanism is the formation of plastic hinges from bending. A theoretical model based on the observed failure mechanism in the stiffener is then presented. The model is based on the upper bound theorem, with a postulated six-hinge collapse mechanism. The proposed theoretical model was found to provide strength predictions to a very high degree of accuracy.

Investigation into the effect of circumferential through-wall slits on a cantilevered pipe subjected to a transverse end load

The main motive for carrying out the investigation is to explore the possibility of using slits to weaken the main legs of an offshore jacket structure as a means of creating a structural mechanism that facilitates safe and predictable toppling of offshore platforms. Cantilevered mild steel tubes containing through-wall circumferential slits are subjected to a transverse end load which causes the slit to open by hoop cracking. The effect of the slit geometry on the collapse characteristics of the tube is investigated experimentally and theoretically. A number of methods are used to predict the global load-deflection characteristics of the tube. These include a number of semiempirical methods based on the J integral and a numerical analysis utilizing the ABAQUS finite element code. The effective net section stress criterion is also used as a simple method of predicting the collapse load. This investigation has relevance to safety in chemical and nuclear plants.

A finite element study of stress fields and stress intensity factors in tubular joints

This paper reports a study on the determination of stress intensity factors in tubular joints in offshore jacket structures. Using finite elements, information on stress concentration factors and through-thickness stress distributions was first obtained from uncracked geometries. This was correlated with the stress intensity factors in joints containing semi-elliptical cracks which were modelled with line-spring elements. The validity of the numerical models was established, using a simple T-joint, by comparing the results with existing experimental data and results from three-dimensional finite element analyses. Various modelling assumptions used in previous numerical work were critically examined. The multi-planar effects in tubular joints were simulated by subjecting the out-of-plane braces to various loadings and restraints. It was found that a relationship exists between the stress concentration factor, the degree of bending and the stress intensity factor for the various loading and restraint cases considered, and that the stress intensity factors in multi-planar tubular joints can be estimated by suitably modifying an existing empirical equation for surface cracks in plain plates.

Offshore structural system reliability under changing load pattern

This paper considers structural system reliability analysis when the spatial force pattern changes as the load increases, specifically when the nodal forces on a steel offshore jacket structure change as the base shear increases under increasing wave height. This paper demonstrates the so-called “fragility approach” to systems analysis, wherein the reliability of the system is obtained for each of a set of increasing wave heights. These probabilities are conditional on the wave height. These results are multiplied by the (discrete) probability levels of these wave heights, and the products are summed to obtain the (total) system reliability. The method is applied to analyze a fixed offshore jacket in 140 feet of water and designed for a 100-year (63 foot) wave. The change of the relative importance of different member-failure sequences with wave loads corresponding to different wave heights is studied. In this case study, the inclusion of wave slamming forces on the deck at higher wave heights led to an increase of the failure probability by an order of magnitude. The use of an improved fixed wave-load pattern treatment is explored. A comparison of the member-level and system-level failure probabilities is presented in terms of factors measuring different system effects.

An expert system for the preliminary design of offshore topside superstructures

In this article, we discuss a knowledge-based expert system, IPDOTS, implemented for the preliminary design of offshore topside superstructures. This study is an extension of the study, described in our earlier article [1], into using an expert system for the preliminary design of offshore jacket structures. As we have already introduced the concept of knowledge–based expert systems and their application to the design of offshore structures in our earlier article, this article concentrates on the architecture of IPDOTS and how it can assist in the design of offshore topside superstructures.

A knowledge-based approach to the design of offshore jacket structures

A discussion is given on an alternative approach, using a knowledge-based expert system, to the design of offshore structures. The implemented expert system can assist engineers in the preliminary design of shallow-water offshore jacket structures. The system is capable of configuring and automatically generating the basic structural model of a fixed template-type steel jacket structure for analysis and design. In addition, the system allows the user to alter any of the inferred recommendations. The system is called IPDOJS (interactive preliminary design of offshore jacket structures). >

Using meta-level knowledge in the design of offshore structures

The design of offshore structures is a complex iterative process involving a very diversified range and depth of knowledge. A major portion of this knowledge is factual and algorithmic in nature. Yet, as in other engineering design, the creativity and success of design is possible only through the extensive experience of the designers, utilizing this factual and algorithmic knowledge. This generally heuristic and non-algorithmic experience forms a relatively small proportion of the knowledge used in design. A usable knowledge-based engineering system for the design of offshore structures must be able to operate quickly enough, without any deterioration in the quality of the solutions. The ‘number crunching’ involved in the factual and algorithmic knowledge quickly leads to a combinatorial explosion. We need to use the heuristic knowledge to rapidly reduce the many possibilities in design. In this way, a usable system which uses multi-level reasoning about a huge volume of diversified knowledge can be constructed. In this paper, we examine the feasibility of using meta-level knowledge to swiftly narrow the search for quality designs. We describe a system, IPDOJS ( Interactive Preliminary Design of Offshore Jacket Structures), to illustrate the use of such metal-level knowledge.

Example of Intelligent Structural Design System

This paper describes the framework of the proposed intelligent structural design system, ISTRUDS, currently under development, and an implemented example, IPDOJS, intended for the preliminary design of fixed‐steel offshore jacket structures. The framework comprises of a knowledge‐based system development tool, a conventional structural analysis and design package, and an interfacing automatic data generator cum interactive graphic editor. Function of the latter is essentially to couple the first two tools into an integrated system so that the expertise knowledge in the implemented expert system can be used to guide the numerical procedures embedded in the analysis and design package. An interactive graphic editor has also been provided to facilitate visual modification of the pre‐ and postprocessed structural model. The objective of the example is to illustrate an approach to the development of an intelligent structural design system for the design of offshore and/or onshore structures.

A fuzzy model for load combination in structural analysis

A linear programming model for load combinations in structural analysis is presented by means of a generalized load module. The established model is a process of maximizing internal forces in the concerned sections assuming that the load factors of the load cases are unknown. Obviously, the fuzziness of structural responses to internal forces should be expressed by introducing a quantitative analysis of loading fuzziness in advance. Therefore, a model for treating fuzzy bounds of load uncertainty is presented in this paper with engineering applications, so that the fuzzy model of load combinations can be realized on the basis of a definite mathematical programming process. A single frame and an offshore jacket structure are analyzed in detail for the purpose of illustration.

Mudmat design considerations of offshore jacket structures on soft clay : Boon, P J; Tan, C G; Tan, C K Proc 8th Southest Asian Geotechnical Coference, Kuala Lumpur, Malaysia, 11–15 March 1985 V1, P1/49–54. Publ Kuala Lumpur: Organising Committee, 1986

Mudmat design considerations of offshore jacket structures on soft clay : Boon, P J; Tan, C G; Tan, C K Proc 8th Southest Asian Geotechnical Coference, Kuala Lumpur, Malaysia, 11–15 March 1985 V1, P1/49–54. Publ Kuala Lumpur: Organising Committee, 1986

Load spectra for slender offshore structures in waves and currents

AbstractThe influence of currents on the wave induced dynamic response of offshore structures with slender members is assessed by deriving appropriate load spectra. The analysis permits the implications of the modified Morison equation to be examined, and suggests that methods of equivalent linearization commonly employed may yield unconservative results. An alternative formulation is proposed, in which the equations of motion are linear in the structural kinematics but the non‐linear dependence of drag force on fluid particle motions is retained. This leads to a load spectral density matrix expressed as a sum of terms, involving convolutions of the wave kinematics spectra. Results are obtained for a plane frame idealization of an offshore jacket structure. These demonstrate the strong influence of current, and the importance of retaining in the loading terms up to the third power of wave particle velocity.

Research into composite tubular construction for offshore jacket structures

Abstract This paper describes recent developments in the use of steel and concrete composite construction for offshore structures. The developments concern filling steel tubular members with cementitious materials to improve static and fatigue strength of memebers and joints. Applications are described for jacket to pile connections, nodal joints and repair situations.

SOAP — a computer program package for the static analysis of offshore structures

A new computer program package, SOAP (Structural Offshore Analysis Package), developed by Keith, Feibusch Associates, Engineers, for the static analysis and code compliance evaluation of offshore jacket structures subjected to wave and current forces is described. The program calculates the hydrodynamic forces due to wave and current using an appropriate linear or non-linear wave theory and the Morrison's equation, and performs the structural analysis by the finite element method. It has the capability to analyse both fixed base structures and pile-supported structures. While the superstructure is assumed to behave linearly, the non-linear flexural behaviour of the piles, as well as the non-linear response of the soil may be included in the analysis. In addition to calculating the joint displacements and member stresses, the program also compares the calculated stresses against API Code allowables, and identifies those members that do not satisfy the code criteria.

Rosenhain Centenary Conference - 3. Materials development present and future 3.4 Development of steels for offshore structures

The steel for offshore jacket structures, Fe52Nb, is a silicon-aluminium killed steel, microalloyed with niobium. As the steel, in fabrication, has to be hot-formed, sometimes normalized or stress-relieved, the steelmaker cannot make use of low temperature treatments, such as controlled rolling, quenching plus tempering, etc. Therefore, the steel is delivered in the normalized condition and its strength and toughness are attained by promoting a fine ferrite grain through the action of grain growth inhibitors, such as Al-nitrides and Nb-carbonitrides. Further increase of strength and toughness, if necessary for future structures, should be pursued by the application of new, as yet unknown or not yet applied, kinds of grain growth inhibitors, in order to further refine the microstructure. These inhibitors should, if possible, be stable at almost steel melting temperatures in order to provide as an additional benefit the suppression of grain growth at the fusion line in high input welding. Maybe also a normalizing treatment at an intermediate thickness during the rolling process could be beneficial for further grain refinement of the finished plate, but most probably, not for grain-growth resistance at fusion line temperatures. Plates used in the construction of nodes are subjected to loading in the thickness direction and, therefore, are liable to lamellar tearing. In order to avoid this, the steel should have sufficient ductility in the thickness direction, the criterion being reduction of area in a tensile test, the axis of which is perpendicular to the plate surface. Sufficient resistance, for the application at present-day nodes is obtained by reducing the sulphur level of the steel to about 0.006 % and by careful deoxidation of the steel. Various ways of making this steel are put forward for discussion