M.L. Kaminski

Validation of the corrected Dang Van multiaxial fatigue criterion applied to turret bearings of FPSO offloading buoys

ABSTRACT In engineering practice, multiaxial fatigue analyses are often avoided due to their complexity and computational intensity. However, damages have been encountered in turret bearings of Floating Production Storage and Offloading vessel (FPSO) offloading buoys which were likely caused by multiaxial fatigue. The Dang Van criterion has often been used to assess problems with multiaxial fatigue in rolling contacts. Therefore, this study set out to validate the application of the Dang Van criterion to turret bearings of FPSO offloading buoys. For this purpose, the criterion was corrected with a horizontal conservative locus for compressive hydrostatic stresses. Three load cases were identified based on the seakeeping analysis of an FPSO offloading buoy equipped with a wheel-rail turret bearing. For each load case, the surface pressure distribution and sub-surface stress states were determined analytically. Staircase tests were used to determine the characteristic parameters (α and β) of the Dang Van curve. Then, the Dang Van criterion was corrected and used to perform a multiaxial fatigue analysis in the critically stressed area of the wheel-rail contact. Finally, full-scale, long-duration fatigue tests were used to validate the results. The corrected Dang Van criterion shows agreement with the experimental results and is not rejected as multiaxial fatigue criterion for application to turret bearings in FPSO offloading buoys.

Comparative study of multiaxial fatigue methods applied to welded joints in marine structures

Marine structures are particularly prone to action of waves, winds and currents with stochastically varying composition, intensities and directions. Therefore, resultant stresses may cause multiaxial fatigue in specific welded structural details. For the assessment of multiaxial fatigue in welded joints, a wide variety of methods have been suggested. However, there is still no consensus on a method which can correctly account for non-proportional and variable amplitude loading. This paper beholds a comparative study of multiaxial fatigue methods applicable for design of marine structures. For the purpose of comparison several load cases were defined including non-proportional and variable amplitude loadings with different normal and shear stress amplitude ratios. Three types of methods are compared: those described by three different codes (i.e. Eurocode 3, IIW and DNV-GL), those described by three different multiaxial fatigue approaches from literature (i.e. Modified Carpinteri-Spagnoli Criterion, Modified Wohler Curve Method and Effective Equivalent Stress Hypothesis) and an approach based on Path-Dependent-Maximum-Range multiaxial cycle counting. From this study it has been concluded that non-proportional variable amplitude loading has a significant negative impact on the fatigue lifetime estimates, and that further research and experimental testing are essential to come to a consensus.

Full and Large Scale Wave Impact Tests for a Better Understanding of Sloshing: Results of the Sloshel Project

This paper outlines the progress made within the Sloshel User Group in the analysis of unidirectional breaking wave impacts on transverse walls in flume tanks at full (1:1) and large (1:6) scales. These tests, carried out during the Sloshel project, were intended to help understanding the physics of sloshing impacts in tanks of LNG carriers and Floating LNG terminals (FLNGs). Two test campaigns were performed at full scale involving respectively the NO96 and the MarkIII containment systems. The latter was performed recently (April 2010) and the analysis is in progress. This paper describes the physical phenomena observed during the impacts on the MarkIII containment system. This helps understanding the difficulties inherent to the scaling of sloshing pressures measured during model tests. The paper also shows how important local peak pressures are for the structural response of the MarkIII system (same kind of conclusion had already been demonstrated for NO96 in Brosset, Mravak, Kaminski, Collins and Finnigan, 2009). According to very preliminary analysis, reduction of impact pressures due to hydro-elasticity with the MarkIII containment system seems to be moderated if real.Copyright

Multiaxial fatigue assessment of welded joints in marine structures

Structural geometry and stochastic loads such as swell and wind seas can typically induce multiaxial stress states in welded details of marine structures. It is known that such complex time varying stress states determine the fatigue resistance of welded steel joints. Therefore, it is of importance to account for them in fatigue lifetime estimation. Over the past few decades a wide variety of design guidelines and methods have been developed for multiaxial fatigue assessment, but so far there does not exist a general hypothesis applicable to all possible load cases. This study provides an overview of the current state-ofthe-art in academia and engineering practice in terms of multiaxial fatigue assessment, and is focusing on the application to welded joints in marine structures. The progress of different approaches and methods is elaborated and commented upon, taking their hypothesis and (physical) basis into consideration. The insights that are provided in this paper form a valuable foundation for future investigations and emphasize the necessity of experimental proofs and model validation.

FPSOs: Design Considerations for the Structural Interface Hull and Topsides

This paper discusses the structural interface on FPSOs between the hull structure and the topsides modules. It identifies the most common topsides foundation concepts applied on FPSOs, and discusses the consequences of each configuration for the layout of the unit, the design of the hull structure and the topsides. The information needed by the hull designer and the topside designer is identified. Moreover, the differences between shipbuilding and offshore construction design practices are discussed, and it is identified where and how these fall short for FPSO purposes. Topics that are addressed are overall safety, operational aspects, such as tank entry and mechanical handling, and the design specifications for the hull and the topsides modules. In order to control the schedule and costs of FPSO projects, fabrication of the hull and topsides should be allowed without impractical or unduly strict specifications imposed on the shipyard or the topsides fabricator. At the same time the traditional design specifications for hull and topsides design may fall short to cover the functional needs for FPSO service. Introduction To date, FPSOs have been in operation for several decades. Initially this development concept was selected for marginal fields in remote and environmentally benign locations. With the further advancement of sub-sea completions, flexible risers and turret mooring systems, the FPSO made its way in the nineties to harsher environments and larger development schemes that were previously uneconomical. Comparatively little investment had to be made in productionand export facilities, whereas the investment still had a residual value after depletion of the field as it could be re-deployed * Formerly with Nevesbu B.V., The Netherlands elsewhere. Furthermore, contractors emerged that were offering lease schemes, lowering the up front investments even further. With the shift of new discoveries towards ever deeper water, the FPSO is becoming more and more the default development platform for deep water for the years to come. Traditionally, an FPSO consists of a converted tanker with the production facilities, or topsides, mounted on deck. After only a limited conversion the oil tanker will fulfil all functional demands for storage and offloading of the produced oil. The most common project strategy is to contract large blocks of work, such as hull conversion, topsides and mooring system, to independent specialized contractors parallel in time. This is possible since the concept of an FPSO is robust: space on a tanker deck is ample, and mono-hulls are relatively weight insensitive. The downside of this approach is that (contractual) interfaces are created that need to be carefully managed in order not to jeopardise the successful completion of the project. Over the years, two types of FPSO projects have evolved. Conversions the ‘classic’ approach is to convert a ‘vintage’ tanker. This comprises an extensive Repair and Life Extension (R&LE) program, after which a conversion will take place to accommodate the mooring system, production facilities, utility systems and offloading system. Key advantages of this concept are the low purchase cost of the hull and the short lead-time. A large number of conversion candidates is available, especially now that a major part of the world fleet is being phased out because of MARPOL 13G regulations [1]. Down side of this approach is the high uncertainty embedded in the conversion scope: only after the detailed inspections have taken place in the conversion yard, the exact extent of steel renewals and equipment overhauls / replacements can be determined. This makes such projects very susceptible to budget and, more important, schedule overruns. To overcome this disadvantage, some project teams have opted for converting a ‘new’ tanker. This can be either a unit that is just delivered, or is still under construction. An example of this approach is described in [2]. When a tanker contract can be secured before start of construction, limited possibilities may exist to tailor the tanker specification to the FPSO requirements, e.g. by increasing scantling or material grades in certain areas. The down sides of converting a ‘new’ OTC 13996 FPSOs: Design Considerations for the Structural Interface Hull and Topsides M.H. Krekel, Bluewater Offshore Production Systems (USA) Inc., M.L. Kaminski, Maritime Research Institute Netherlands (MARIN) 2 M. H. KREKEL, M. L. KAMINSKI OTC 13996 vessel are the high purchase costs, while the full conversion scope remains intact. New builds are the most logical approach to take when a tanker conversion can not meet the project requirements for, for instance sea keeping, strength, endurance or size. In theory, a purpose designed new build FPSO can be built by one contracting party, but to date most of these units were contracted out as a number of sub projects, leaving the interface problems intact. Time has proven that purpose designed FPSO units are considerably more expensive, and have longer schedules than conversions. This because their ‘non standard’ specification requires an extensive engineering effort by the shipyard, and because the owner’s requirements, regarding e.g. materials, fabrication and coating, interfere with normal ship production. An alternative to a purpose designed new build is to opt for a ‘standard specification’ new build. Such a unit would conform to a shipyard’s standard tanker design with only minimal modifications to its specification. Typically these would comprise the omission of the propulsionand associated auxiliary systems, specification of an improved coating system, increased scantlings and higher material grades in certain areas. Interfaces As a consequence of the split project execution, contractual interfaces arise. The control and resolution of these interfaces is beyond the scope of this paper but has been the subject of many other publications, for instance [3]. General consensus is to minimise the interfaces between the various parties by contracting systems either fully in the scope of the shipyard, or fully in the scope of the topsides’ fabricator. Moreover, the responsibility for a system should remain with one party and range from design up to (pre-) commissioning. ‘Hand-over’ work should be avoided as much as possible. Noteworthy exceptions to these rules are the safety systems and electric power distribution systems as these are integral to the whole unit. In this paper it is not so much the system interfaces we consider but the structural interface between the hull and topsides. The topsides are fitted using foundations at a certain elevation above the hull’s upper-deck as shown in Figure 1. For the foundations between the upper-deck and the underside of the topsides, the following functional requirements apply: support the topsides modules on the hull, provide space for all deck piping and hull equipment, provide space for safe (tank) access and mechanical handling operations on the hull’s upper-deck, allow for sufficient natural ventilation of the upper-deck in order to prevent build-up of explosive gaseous mixtures, create a fire division / barrier between the topsides and hull upper-deck, create a division in the hazardous area classification for electrical equipment selection. The elevation between the upper-deck and the topsides flooring depends on how much clear height is needed underneath the topsides to fulfil the above requirements. For an Aframax size hull, a typical value of 3750 mm at the centreline has proved sufficient but the elevation is dependent on the topside arrangement, i.e. the main girder height of the topsides deck. Topsides, Pre Assembled Unit

Assessment of Residual Ultimate Hull Girder Strength of Damaged Ships

The risk of ship collision and grounding has increased significantly in recent years as a result of the growing size and number of ships at sea. The potentially costly consequences of collision and grounding in the form of fatalities, property, and cargo, as well as environmental pollution in the form of oil spills, etc., are the main motivations for research on collision and grounding. From a structural evaluation standpoint, there is a great deal of uncertainty related to the residual strength of damaged ships considering various influential parameters, such as damage size, geometry and location, internal structural arrangement, material property, loading case, and sea weather. Therefore, it is important to clarify the residual hull girder strength of damaged ships by collision or grounding in order to ensure their safety. The present study undertook a deliberate finite element analysis to investigate the residual ultimate strength of damaged ship hull, where two damage models were assumed and compared. One model simulated actual damage resulting from an accident in the form of hole with adjacent plastic deformation, while the other applied simplified damage, considering unavailable measurement of the damage by removing the damaged part from the original ship hull. The comparison showed that the assessment of residual ultimate strength of a damaged ship based on the simplified damage model could produce a sufficiently accurate result and stay slightly safer, provided that a reasonable criterion of simplification was defined first. The studies showed that it is possible to accurately estimate the residual ultimate strength of a damaged ship without detailed measurement of the damage, and consequently facilitate decision-making regarding the ship salvage under emergency.Copyright

Dynamic Response of Marine Structures

The paper presents conclusions, recommendations and offshore relevant elements of the report of the Committee II.2 -Dynamic Response as presented and discussed by the authors at the 16th International Ship and Offshore Structures Congress (ISSC 2006) in Southampton, UK, 20–25 August 2006. This includes wave-induced response, fluid impacts, noise and vibrations, explosion and shock, damping, structural monitoring, countermeasures, uncertainties, random response and a benchmark study.Copyright