Members Of Offshore Structures Research Articles

Flexural response of underwater offshore structural members retrofitted with CFRP wraps and their performance after exposure to real marine conditions

Offshore structural members may require retrofitting due to various reasons as they age. This paper investigated strengthening and repairing of offshore steel tubular members of nominal diameter of 100 mm using Carbon Fibre Reinforced Polymers (CFRPs) subjected to four-point bending. Underwater retrofitting was compared with conventional in air retrofitting. The effect of the number of layers of retrofit was investigated parametrically for repairing and strengthening applications. Repairing corroded steel tubular members with CFRP was found to be very effective under bending. Strengthening of the steel tubular members also showed significant improvement in performance, but the improvement remained at the same level even when the number of CFRP layers increased. In the second part of the study, the durability of CFRP retrofitted tubular members was examined using ‘Real Corrosion’ test where the retrofitted members were fully submerged in a river (saline water) for a period of one year. While corrosion was observed in the non-retrofitted members, the CFRP layers prevented corrosion of retrofitted specimens. It was observed that the retrofit did not lose any structural integrity due to exposure to marine or underwater conditions for a prolonged period of one year. Lastly, a lower-bound interaction equation was developed for repaired steel tubular members under axial compression and bending.

Underwater strengthening and repairing of tubular offshore structural members using Carbon Fibre Reinforced Polymers with different consolidation methods

Offshore structural steel members will need strengthening or repairing due to various reasons. Strengthening is often required due to work over demands or increased environmental loads. Repairing becomes essential as the steel members are usually severely corroded due to the marine conditions. Conventional hot work strengthening and repairing solutions become very expensive and retrofitting using Fibre Reinforced Polymers (FRPs) is an economical alternative. This paper investigates strengthening and repairing of offshore steel tubular members using Carbon Fibre Reinforced Polymers (CFRPs) with the help of experimental testing and an analytical study. Two different specimen sizes which were representative of members in offshore structures namely leg and brace were examined in the study. Partial length strengthening of steel tubular members subject to combined loading of axial compression and bending was carried out using different curing environments (in air and underwater curing). Strengthening and repairing of leg and brace specimens with multiple layers of CFRP retrofitted in underwater conditions were also investigated. Two different consolidation methods (underwater vacuum consolidation and underwater stricture consolidation) were compared with the help of full length strengthening of leg specimens. Although partial length strengthening proved to be less effective, partial length repairing of corroded tubular members were successful in restoring their intact capacity. Full length strengthening for underwater leg specimens were experimentally tested and it was found to improve the structural behaviour in many aspects like ultimate strength, load displacement behaviour, elastic slope, energy absorption, and ductility. A novel technique of underwater vacuum consolidation of retrofit laminates was demonstrated through a set of full length strengthened leg specimens. Underwater vacuum consolidation proved to be advantageous compared to the normal underwater stricture consolidation. An analytical check to identify whether a retrofitted tubular member would reach its target section properties was also introduced. Overall, the paper demonstrates multiple successful applications of FRP composites for underwater strengthening and repairing of offshore steel tubular members. Studies like this expanding the current knowledge on how the FRP retrofitted steel tubular behave within a real offshore structure are essential to increase the uptake of this technology in the oil and gas industry.

Experimental and numerical investigation of underwater composite repair with fibre reinforced polymers in corroded tubular offshore structural members under concentric and eccentric axial loads

Repair of offshore steel structural members become essential due to the corrosive marine environment in which they exist. Structural repair using composite material has significant advantages in these cases than the conventional repair by welding or bolting steel sleeves. However, the composite repair technology with fibre reinforced polymers (FRP) is not very well established for their usage on load bearing offshore steel structural members either in underwater or above water conditions. The study had attempted to do a pilot experimental study on steel tubular members under the combination of axial compression and bending loads to address this gap in knowledge. The specimens were grouped into four categories: intact, corroded, repaired in air and repaired underwater. The repair scheme remained the same for in air and underwater repairs with one layer of glass and two layers of carbon fibre reinforced polymers, but with resin and repair methodology being different for both. The repaired specimens showed notable improvement in the ultimate strength from the corroded members to match the intact strength. Underwater repairs proved to be adequate to replicate the ultimate strength achieved by the conventional in air repairs. Investigations into parameters like load displacement behaviour, energy absorption, ductility and strain values also revealed that the underwater repair performed very similar to the conventional in air repair. A simplified Finite Element model simulating the repaired specimens was also presented in the paper which had predicted the experimental behaviour with great accuracy. In conclusion, the underwater and above water repair of structural steel tubular members using FRP composite materials seem a very viable solution for the corroded load bearing members in the offshore industry, however more detailed study on long term performance and cyclic behaviour of the composite repaired members is yet to be carried out.

Laboratory Measurements of Breaking Wave Impact Pressures on a Slender Cylindrical Member

Slender cylindrical members form the major components of many of the coastal and offshore structures. These members are frequently subjected to breaking wave impact which often resulted in damages and failure of structures. In order to overcome this intricacy, it is essential to understand the physics of the breaking wave impact on offshore structural members and the resulting induced critical stresses. An experimental investigation has been carried out to measure the effect of breaking wave impact on a slender vertical cylinder. Simultaneous qualitative visual observations and quantitative pressure measurements were made to appreciate the impact induced effect. The induced impact pressure on the cylinder varies with the intensity of wave breaking and the relative location of the cylinder. The impact pressure is maximum when the wave profile reaches its maximum steepness just before the crest destabilization. Impact pressure observed due to a severe plunging wave is about nine times higher than due to spilling. The pressure rise time is found to be an important parameter in dictating the nature of impact.

Dent 손상을 갖는 원통부재의 최종강도에 관한 연구(제1보) -축 하중을 받는 경우-

Loads on offshore structures are largely transferred to the bracing members in the form of axial forces. The detrimental effects of imperfections on compressive strength are well recognized. Damage in the members of offshore structures would significantly affect the compressive behavior of the members. As a result, such damages may also affect the ability of the structure to withstand the functional and environmental loads. It is important to be able to assess the residual strength of damaged members quickly and accurately. This will help operators to make the decision whether the member has to be repaired or not. In this study, a series of calculation is performed to study the effects of different parameters on the behavior of such damaged members under axial load. And the results of analysis are compared with those of experiment.

Editorial

Editorial

Prediction of the initial normal stress in piles and anchors constructed using expansive cements

Uses for expansive cements and additives have extended well beyond off-setting the shrinkage characteristics of grout and concrete to include enhancement of rock anchor and pile performance, providing an alternative form of connection for tubular members in off-shore structures and as an excavation tool in open-pit mines. In each case, the design rules governing the quantity of expansive additive to be used are based on guesswork or empiricism. This paper presents analytical solutions for estimating the degree of expansion and the level of normal stress developed for a range of different boundary conditions and expansive additive contents. The expansion process is modelled as a thermal expansion and is governed by one parameter that depends on the type of expansive additive and its dosage. Simple laboratory procedures for determining this property are outlined. Predictions from the analytical solutions are compared with laboratory experiments.

Dynamic behaviour of axially pre-loaded tubular steel members of offshore structures subjected to impact damage

The behaviour of an axially pre-loaded cylindrical member of an offshore structure, hit by a supply ship, has been investigated. The effects of axial pre-loading on the dynamic properties of members, the extent of damage and propagation of dynamic instability in the tubular members have also been investigated. Numerical models have been validated using available experimental data from the literature although these are mostly static with no pre-loading. Axial pre-loading was found to change the dynamic characteristics of some cylindrical members quite dramatically while it had no significant effect on others. The study examined whether there was any change in the behaviour between local and global modes when the loading was dynamic rather than static. The effect of damping on the dynamic instability of axially pre-loaded tubes under lateral dynamic loads was also studied. It was observed that for some geometries the quasi-static response can be used to define the boundary between bounded and unbounded dynamic responses. The main contribution of this study, compared with previous investigations reported in the literature, is that both pre-loading and dynamic effects have been included.

Effect of impact damage on the capacity of tubular steel members of offshore structures

The effect of axial pre-loading on the behaviour and instability of a cylindrical member of an offshore structure struck by a supply vessel has been investigated. Numerical models have been used to analyse a tubular member with and without axial pre-loading. It has been found that axial pre-loading has a marked effect on the lateral collapse load of the member and more dramatically on the level of energy that the member can absorb prior to its collapse. The effect of different end conditions on the behaviour of the tubular members has been examined and four distinct modes of deformation for a cylindrical member under quasi-static concentrated lateral loads have been defined. The finite element model was validated using available test results from the literature. These were mostly static tests, carried out on cylindrical members with no axial loading. Good agreement was obtained between the finite element model and the test results. Significant differences have been found between the results presented in this study and previous results by the other workers who represented the impact damage as an imperfection effect.

Finite element analysis of tube stability in deep water

In this paper, computational techniques for the analysis of tube stability in deep water are outlined and applications to pipelines and tubular members of offshore structures are discussed. Results are presented on buckle propagation and arrest in pipelines. Also, the role of external pressure on the axial and bending capacities of tubular members is examined.

Closed-Form Expressions Of Wave Induced Forces On Members Of Offshore Structures In Deep Water

The paper deals with the estimation of equivalent nodal forces and moments which can be useful in the analysis of beam element based Finite Element (FE) models of offshore structures. Ocean wave forces do not follow any standard pattern, thus, if the global X-Y reference plane is horizontal, the X and Y co-ordinates will not vary along the length of a vertical beam element. Taking advantage of such a situation, a number of closed form expressions for equivalent nodal loads can be derived. The analytical diffraction force from the MacCamy and Fuchs' theory as well as the Morison equation based inertia and drag forces are considered here as wave loading. A step-by-step calculation procedure is also proposed which transfers complex member loads to the nodes of a FE model with beam elements, arbitrarily oriented in space.

Hydrodynamic interactions between two vertical circular cylinders in linear oscillatory flow

Linear potential theory is used to investigate the hydrodynamic interactions between two vertical cylinders in harmonic flow. The behaviour of the added mass and damping is studied for various cylinder diameters, lengths of immersion and spacing between the cylinders. In practice, the results have relevance to the fluid loading on offshore structural members, such as risers and platform legs, and pipeline bundles. On a more theoretical level, comparisons are possible between the results of a three-dimensional diffraction program and various asymptotic expressions to establish the range of validity of the latter and the need, where appropriate, of the former.

A general method for calculating hydrodynamic forces

This paper presents the derivation of a general method for calculating wave forces on the cylindrical members of offshore structures. By means of the proposed method one can calculate the wave loading on cylindrical members of fixed or floating offshore structures orientated randomly in waves. This method of calculating wave forces is based on the linear Airy wave theory. Calculation procedure of wave force components is presented in great detail on the basis of wave particle kinematic properties obtained from the linear Airy wave theory. In the procedure of calculating wave forces presented, definitions of the wave reference system for propagating wave, the structure reference system for the platform and the member reference system for the tubular members of the structure are first established, and then the calculation of wave forces is given in terms of its components, which are pressure, acceleration and velocity forces, including current forces. At the end of the paper, expressions of total heave, sway and surge forces and total roll, pitch and yaw moments acting on the platform are given as a sum of these forces acting on each member of the platform. The calculation procedure derived in this paper provides a very efficient means of calculating wave forces and moments during the time-domain simulations of a floating platform experiencing large amplitude motion in intact, progressive flooding and damaged conditions. Comparisons of the predictions with the measurements which will be presented elsewhere reveal that the calculation procedure developed can predict large amplitude oscillatory and steady motion characteristics of an intact and damaged platform in waves with an acceptable degree of accuracy.

A Computing Method for the Analysis of Water Impact of Arbitrary Shaped Bodies (2nd Report)

Traditionally, circular cylinders have been adopted as structural members of offshore structures due to structural simplicity, strength, cost and facility of construction, etc. However, investigations on the slamming of the circular cylinders indicate that they suffer very high impact loads. Furthermore, despite the impact loads are strongly influenced by their geometry, almost no alternative cross sectional shape has been systematically studied.This paper describes an optimization of the cross sectional shape of the horizontal members of offshore structures in the view point of slamming. Investigations are performed by using a numerical method for the analysis of water impact of arbitrary shaped bodies that the authors have presented in the first report.First, we numerically study water impact on a circular cylinder to clarify the mechanism of slamming on the existing structural members. Next, alternative cross sectional shapes for the horizontal members are obtained by an analysis based on the momentum theory associated with the concept of added mass. The results are checked by the numerical simulations. The analyses show that cylinders with parabolic section minimize total impact force. Finally, the acceptable range of angle of the parabolic section to the water surface is determined. The cross sectional shapes we obtained can be reasonable alternatives for the horizontal members of offshore structures.

Longitudinally-stiffened large diameter fabricated steel tubes

Fabricated steel cylinders are often used as conveyor galleries, stacks or masts, or as members in offshore structures. The cylinders are fabricated by first cold rolling steel plates to form short cans and then joining these together by circumferential girth welds to yield long spans. Circumferential stiffeners are almost always present, in order that the circular shape of the tube is maintained and as an assistance when the cylinder is being handled. Longitudinal stiffeners may be used as well. If they are present, they will be welded to the cylinder surface, thereby improving the behaviour under axial compression or beam bending. The paper addresses the design of the type of cylinder commonly used as a conveyor gallery. In this application, diameters of from 2·5 to 4·0 m are typical and the radius to thickness ration, R/t, ranges from about 100 to 400. Spans can reach 60 m. The results of four large-scale flexural tests on large diameter fabricated steel cylinders that had longitudinal stiffeners and a parametric study of hypothetical cylinders of this configuration are used as the basis of the study. The paper reviews current design standards in the light of those test results, presents the results of the parametric study and makes recommendations for the design of longitudinally-stiffened fabricated steel cylinders.

Three-Dimensional Effects on Wave Forces on Horizontal Members of a Semisubmersible

It is known that wave-induced forces on horizontal cylinders are significantly decreased due to a circulating flow generated around the cylinder at low Keulegan-Carpenter numbers. An estimation method of wave forces on a two-dimensional horizontal circular cylinder using a simple vortex shedding model has been proposed by the authors. For estimating the wave forces on horizontal members of offshore structures like lowerhulls of a semisubmersible, however, it is necessary to clarify three-dimensional effects, such as the end effect or the juncture flow effect.In this paper, the end effect and the effect of columns on wave forces on a lowerhull submerged in regular waves with its axis parallel to the wave crests were experimentally investigated. A two-dimensional lowerhull cylinder and finite-length lowerhull models with and without columns are used in force measurements, velocity measurements and flow visualizations. The results of the experiments showed that the wave force reduction decreases in the case of the finite-length lowerhull model with columns due to the disappearance of the circulating flow at the end and the junctions with columns. The results also demonstrated that the vertical wave forces on the finite-length lowerhull model with columns are asymmetric because of the flow accelerated between the columns.

Hydrodynamic force on a moving circular cylinder submerged in a general fluid flow

This paper develops an expression for the fluid loading on a circular cylinder due to classical inviscid, irrotational and incompressible flow. The cylinder has an arbitrary rigid body movement, and the fluid, and in the absence of the cylinder, has an arbitrary flow pattern, subject only to the requirement that the length scale over which the flow changes is sufficiently large compared with the dimensions of the cylinder cross-section that only velocities and velocity gradients need be considered. The result is intended to be used as the ‘inertia’ force component of wave loading on the cylindrical members of offshore structures in the conventional Morison’s Equation formulation, and as such it differs significantly from that currently in use.

Extreme values of Morison-type processes

Nonlinear probability distributions for Morison-type wave loading can be used to derive statistical properties (measures) of the extreme responses of quasi-statically responding members of offshore structures. The extreme value statistics can be described, for example, in terms of quantiles (i.e. values with a constant probability of exceedance), mode or mean value. For Morison's equation, the departure from linearity can be quantified in terms of a drag-inertia parameter, which is a function of both the member hydrodynamic properties and the sea state. In general, exact evaluation of the extreme value statistics can only be accomplished numerically. However, by investigating the asymptotic behaviour of extreme value distributions, simple formulae can be derived for estimating the statistical properties of the extreme response as a function of the drag-inertia parameter, the number of response peaks and the probability of exceedance.

Influeuce of Bead Toe Shapes on Fatigue Strength of Fillet Welds (2nd Report)

Experimental studies were carried out to examine the influence of undercut depth at the weld toe on the fatigue strength. Non-load carrying cruciform and T fillet welded joints were tested and quantitative life reductions due to undercut were obtained. Test results and life estimation procedure were reported in the previous paper.In this report, the fatigue strengths for 2 × 106 cycles were studied and were compared with each other. Adding to 16 test series joints concerning to undercuts, 40 test joints having various fillet shapes were also analysed. The equivalent fatigue strengths for 2 × 106 cycles were calculated from each failure life of test data, referring to the estimated S·N relations of these joints. The joint strengths were evaluated through the mean and the standard deviation of these equivalent strengths. The influence of the undercut depth and the bead toe shapes could quantitatively be discussed considering the undercut dependence of estimated strength as well as the scatter of equivalent strength data. These results led to the draft of the acceptance level that the allowable maximum undercut depth is 0.3 mm for the special members of offshore structures.

Resonant and non-resonant behaviour of a flexibly mounted cylinder in waves

Previous research has indicated that the fluid-structure interactions of slender members in offshore structures are extremely sensitive to the ratio of wave frequency to the natural frequency of the structure, ƒw/ƒNw. This paper presents experimental measurements of forces and response displacements from a spring-mounted vertical cylinder in regular waves for a constant surface Keulegan-Carpenter number, SKC, equal to 8 and frequency ratios in the range 0·3≤ ƒw/ƒNw≤0·8. Peak values of force coefficients and response amplitudes are observed to occur when the frequency ratio f. If,. is an integer submultiple. Further peaks may also occur for ƒw/ƒNw =1/[(SKC/2π) + 1] and ƒw/ƒNw =1/[(SKC/2π) + 2] due to coupling between the in-line force and transverse response. Reasonable agreement is found between the experimental resonant force and response displacement loci with a simulation based on a finite-difference time-stepping solution of the non-linear equations of motion.