Daniel Karunakaran

Nonlinear dynamic behaviour of jack-up platforms

Abstract Jack-up platforms may, under extreme wave loading conditions, exhibit significant nonlinear behaviour. This must be accounted for in the design of such platforms, in order to ensure satisfactory structural safety. In this paper an overview of the different sources of nonlinear platform behaviour is presented. Furthermore, it is outlined how those nonlinear effects may be modelled and handled in numerical simulation studies of structural response, with a special focus on the estimation of extreme response and dynamic amplification factors. Finally, it is discussed how nonlinear dynamic response may be accounted for in the design of jack-up platforms. The discussions and outlines are illustrated by examples from numerical simulation studies of jack-up behaviour.

Analysis Guidelines and Application of a Riser-Soil Interaction Model Including Trench Effects

A general overview of design aspects related to metal catenary risers is first given. Response characteristics of these types of riser configurations are considered. It is subsequently focused on models for soil-pipe interaction, which have a strong influence on computed riser stresses in the touch-down region. The so-called CARISIMA models for riser-soil interaction in relation to horizontal and vertical motion (i.e.horizontal resistance and suction) are presented. Inherent limitations of the models are summarised. Basic input parameters are described, considerations relevant for establishment of initial trench profile are given, and examples of riser response analysis are presented. Typical behaviour of the numerical models which is observed in the example analyses is also reported.Copyright

Full-scale measurements from a large deepwater jack-up platform

Abstract Analysis of full-scale measurements obtained from the instrumented Smedvig West Epsilon jack-up platform operating in the STATOIL Sleipner Vest field in the North Sea is described. This jack-up platform has a skirted spud-can foundation. The following topics are discussed in the paper: • Comparison between measured and calculated natural frequencies and modes of the platform based on modal analysis. • Comparison of measured foundation stiffness with the design stiffness. • Comparison between measured and simulated platform response by application of a nonlinear time domain analysis. Implications with respect to procedures and assumptions employed for design of the platform are addressed. The comparison between measured and simulated response is performed for a moderate sea state with a significant wave height of 9.3 m.

Site-dependent reliability of a mobile jack-up platform

Abstract The site-dependent response characteristics of a jack-up platform operating at two different locations in the North Sea are investigated. Both the water depth and wave environment differ at the two sites. The effect from uncertainty in boundary conditions at the platform base on the natural frequencies and dynamic amplification factors is highlighted. Characteristic load-effects in the jack-up legs are considered, and corresponding reliability measures are compared for the two sites. Implications for design are briefly discussed.

VERIFICATION OF METHODS FOR SIMULATION OF NONLINEAR DYNAMIC RESPONSE OF JACK-UP PLATFORMS

The validity of mathematical and numerical models commonly applied for simulation of the nonlinear stochastic response of jack-up platforms is discussed. The basis for the discussions is the comparison of simulated response to model test data. The model test data refer to an experiment, which in full scale refers to a typical harsh environment truss leg jack-up. The test data include wave elevation, wave particle kinematics and static and dynamic response. The verification study emphasises wave kinematics, hydrodynamic loading, damping and dynamic response.

Estimation of fatigue damage and extreme response for a jack-up platform

Abstract Results from four different methods for stochastic dynamic response analysis of a proposed jack-up platform are compared. This structure exhibits both significant dynamic amplification and non-linear transfer of sea elevation into load effects. Both estimation of extreme response and fatigue damage are considered. The most complex procedure based on time-domain simulation and step-by-step integration is employed as a benchmark for assessment of three simplified methods. The simplifications consist of various types of linearization in conjunction with transfer function approximations. Applicability of the methods to structures with increasingly non-linear behaviour and dynamic amplification is briefly discussed.

Effects of non-Gaussian waves to the dynamic response of jack-up platforms

Abstract The paper discusses the nonlinear response effects to jack-up platforms caused by non-Gaussian waves. The basis for the paper is field and model test observations of wave characteristics and simulation of wave processes and dynamic response applying different wave and wave kinematics models. Both normal design sea states and extremely steep sea states are investigated. In the design sea states, effects to the steady state dynamic response are evaluated. For the steep wave situations transient response effects—so called ringing response— are focused. Consequences and implication for design calculations are considered in both cases.

Application of Nonlinear Stochastic Mechanics in Offshore Engineering

Nonlinear stochastic mechanics are widely used in the field of offshore engineering. In this paper we survey four applications: nonlinear collapse and fatigue analysis of jacket structures, dynamic analysis of jack-up structures, springing and slow-drift response of a tension leg platform, and hysteretic foundation behavior of a concrete gravity platform.

Review of Steel Lazy Wave Riser Concepts for North Sea

The Steel Catenary Riser (SCR) is considered a favorable solution for deepwater development. However, the application of SCRs in harsh environments is challenging, mainly due to fatigue at the touch down zone. One of the solutions to improve SCRs’ fatigue performance is the application of buoyancy modules in order to achieve a Steel Lazy Wave Riser (SLWR) configuration.This paper presents a parametric study of a SLWR system for application in deepwater and harsh environments. The selected harsh environment is the seas within the Norwegian Continental Shelf. The main geometric variations are the length of the buoyancy section and the dimension of the buoyancy modules. These variations result in riser configurations with different riser’s wave heights. The term ‘riser’s wave height’ refers to the vertical distance between the lowest point at sagbend and the highest point at hogbend of a riser. The analysis works are performed using Orcaflex.The results show that the application of buoyancy modules help to improve the performance of a steel catenary riser for application in a harsh environment. The dimension of a riser’s wave shape proves to be an important factor that controls strength and fatigue performance of a riser. Lazy wave riser configurations with larger and better defined wave shapes have lower stress utilizations and lower fatigue damage, while riser configurations with smaller wave shapes have higher stress utilizations and higher fatigue damage. This is due to lazy wave configurations with higher wave shape have better capability to absorb the dynamic loads.Copyright

Steel Lazy Wave Risers From Turret Moored FPSO for Deepwater Harsh Environment

Steel catenary risers (SCRs) have found greater applications in deep and ultra-deepwater developments. However, the deployment of SCRs in conjunction with a high motion deepwater floater such as the Floating Production Storage and Offloading (FPSO) system faces significant challenges due to their high motion characteristics, especially in harsh environmental conditions. The challenges posed by FPSO’s high motion characteristics include severe dynamic response on the SCRs and poor fatigue performance at the top section and the touchdown point (TDP) area. A number of alternative configurations of the SCR can be employed to decouple the FPSO’s motion from the SCR, thereby improving performance, and this include the steel lazy wave riser (SLWR) configuration. The lazy wave is achieved by introducing buoyancy modules along some lengths of the riser.In this work, a suitable SLWR configuration for deployment in conjunction with a turret moored FPSO was developed for a typical deepwater offshore West of Shetland environmental conditions. The optimum configuration is a low lazy wave configuration; this was achieved after several analyses using ORCAFLEX software program. In determining the optimum configuration, consideration is given to the SLWR sag and hog bend heights, the net buoyancy force, the buoyant section length, and the hang-off angle, among others.The extreme response, considering a combination of 100-year wave with 10-year current was satisfactory; the maximum stress was below the allowable stress level, and the maximum DNV utilization was less than unity, indicating a safe design. The wave-induced fatigue damage was calculated using a total of 216 load cases, resulting from 12-wave directions, and the wave-induced fatigue performance was satisfactory, with the minimum fatigue life observed at the riser’s TPD. Fatigue damage resulting from vortex induced vibration (VIV) was calculated considering currents in the in-plane and the out-of-plane directions to the riser, with a total of 22 load cases. The VIV fatigue performance was not satisfactory, and therefore fairings and strakes will be introduced to some lengths of the SLWR to suppress VIV. Detailed sensitivity studies also showed how the configuration can be further optimized.Overall, the results of this study showed that, the SLWR is a suitable riser concept for deployment from a turret moored FPSO, in deepwater, harsh environmental conditions such as offshore West of Shetland. The riser can be installed using Reeled-Lay installation method. The installation can be performed using pre-lay, abandonment, and recovery, as this offers advantages over the direct transfer approach.Copyright