Recently the offshore industry has shown an increased interest in developing a competitive Dry Tree Semi (DTS) concept. Several DTS designs have been proposed by floater designers and many of them have been evaluated by Operators and Classification Societies. So far, no DTS has been selected as host platform for a deepwater field development project. A Dry Tree Semi can be utilized for both drilling and production, eliminating the cost of the MODU which can run at approximately USD 500,000 per day and higher for ultra deepwater fields. Once a discovery is made, the field is no longer dependent on the MODU market for drilling access. The DTS concept increases the potential for deepwater development, especially for marginal fields. The result is increased reserves and productions in the Gulf of Mexico. It is essential to ensure that proper qualification of the concepts and all systems/components with novel technology has been carried out, in order to establish the foundation for a safe and successful project implementation, and reduces potential risk on personnel, environment and property. This paper provides an overview of the current technology and the concepts needed to be matured in order to make the project ready from operators and Classification Societies perspectives. The paper will discuss the key technical challenges of the DTS concept in general and how to make this technology ready to implement. Key technical issues discussed in this paper are global performance of the floater, riser system and responses, tensioner capabilities as well as technology qualifications. The paper will make relevant comparisons with other competing floating systems like TLPs, FPSOs and spar units. The methodology recommended to be utilized for the evaluation of the DTS technology is based on Chevron's Project Development & Execution Process. Within this process there is a Technology Selection workshop where the necessary technologies for the project are identified and determined as, enabling or enhancing, engineering or technology, and " proven technology?? or " new technology??. The " new technologies?? are then evaluated through the Technology Qualification Process. The Chevron's TQ Process was co-developed with DNV and is based on DNV-RP-A203 " Qualification Procedures for New Technology??.
In a riser design, the assessment traditionally adopted by the industry for the attainment of platform motions applied to the top of the riser consists in the use of de-coupled methodologies. Those formulations consider the static environmental loads over the platform (current and wind) through a static offset and the dynamic environmental loads due to wave through imposition of top riser displacement calculated from cross spectral response of sea spectrum and vessel's RAOs (Response Amplitude Operators).Nowadays due to shift of oil and gas exploitation to deeper waters more accurate methodologies, based on coupled analysis, have been introduced. The coupled analysis considers the interaction between the hydrodynamic behavior of the hull and the structural behavior of mooring lines and risers submitted to environmental loads. For deep waters the coupling effects of lines over platform motions can be especially significant and it is expected a reduction of the amplification of platform motions if compared to the platform motions obtained from de-coupled analysis.This paper presents a typical Steel Catenary Riser design, connected to a semi-submersible platform, where the motions applied to the top of riser are obtained from the "traditional" way (de-coupled) and from the coupled analysis. A numerical application will be presented in order to assess the comparison of the two presented methods in terms of SCR results.The coupled model studied herein is composed of approximately 80 lines connected to the platform, which requires an excessive computational effort. In order to reduce the CPU time some additional studies are performed considering the variation of line mesh discretization and time step size. The objective of this study is the adoption of an optimized model keeping the required accuracy of platform motions results to be applied on top of SCR. The CPU time consuming and platform motions are presented.The significance of this work is the conclusion that a Steel Catenary Riser design adopting prescribed displacements from coupled analyses will provide more realistic and optimum results.
The paper describes activities and critical issues in relation to the preparations, qualifications, reeling and installation of two 10" Steel Catenary Risers (SCR's) on behalf of Petrobras on the Roncador field, offshore Brazil, in 1360 m of water. A combined installation technique with J-lay of the critical touchdown area and traditional reeling for the remaining flowline/riser part was applied. The operations related to reeling the pipe sections onto the reel, reeling off, welding on site (J-lay sections) and transfer to P-36 have been outlined. The paper describes in more detail the analyses performed to evaluate the effects of using the reeling method for installation of the risers. The following issues are addressed: riser torsional effects due to residual curvature, effects of residual stresses, ovalisation, strain concentrations, variability in yield strength, riser analyses, reeling analyses, fatigue aspects, hoop buckling and propagating buckling.
Within Deepstar (DS), a Guideline (ref. /1/) representing an industry consensus on deepwater design analysis methods and model test verification has been prepared. As the offshore industry move into deeper water, the capacity to analyze and model test deepwater floating systems are challenged. The designers need efficient tools and procedures on how to determine dimensioning responses. For some of the floating systems installed/planned, coupled analyses is considered a must, both with respect to safety and cost. The paper focuses on how to perform coupled analyses efficiently for deepwater floater and riser/mooring systems. 1. Introduction During the last years, the offshore industry has gained increased experience and knowledge with respect to how integrated, coupled analyses should be performed on floater types like FPSO, TLP, Spar, Semi, with associated riser and mooring systems. The introduction of new floater solutions/combinations, new materials and the fact that the riser/mooring system introduce more non-linearities in deeper water emphasise the need of a unified approach for coupled analyses. Model testing alone can not be regarded as solely suited for final verification/validation of complex floating systems in deep water when e.g. truncations have to be included in the tests. Some years ago, DS invited the offshore industry to perform analyses on some selected floating systems, with FPSO, TLP and Spar as the 3 'Theme Structures'. The relative large scatter in the calculated results led to the execution of model testing these 3 floating systems (ref. /2/). Following these model tests, a smaller group of vendors (3 basins and 4 designers) with DNV and OTRC as the coordinators were established. New analyses were conducted with the purpose to explain and reduce the scatter in results (refs. /3/, /4/ & /5/). This exercise typically involved 'modeling the model' used in the model tests and necessary calibrations (model test results vs simulations). Further details on these calibrations and correlations are given in ref. /6/. In parallel with these analyses and calibrations, a Guideline on how to perform coupled analyses of deepwater floating systems was prepared. The main objectives of the Guideline (ref. /1/) are to provide guidance on:when it is deemed necessary to perform coupled analyses.how to efficiently perform coupled analyses.riser and mooring line analyses.relevant load models for floater and slender structures.important coupling effects from slender structures to the floater.the interface between model tests and coupled analyses.which input parameters are of key importance in simulations.obtaining confidence in critical design and validation cycles. The 10 sections of the Guideline deal with the following issues, respectively:Presents objectives, scope and background.Gives an outline description of different floating concepts, riser and mooring systems and an evaluation of coupling effects.Outlines a typical design process for deepwater developments.Provides guidance on how to identify design conditions.Provides guidance on how to perform de-coupled analysis, covering load models, static analysis, frequency domain and time domain analyses.Global slender structure analysis with focus on slender structure load models and FE analyses.Provides guidance on the methodology and procedures of coupled analysis.Model testing and which effects and parameters are of importance.
Floating offshore wind turbines is a field undergoing major development. Several companies and research institutes worldwide are engaged in research programs, pilot projects and even planning of commercial floating wind farms. Developing standards for design of floating wind turbine structures and a framework for prevailing rules are crucial and necessary for the industry to continue to grow. Det Norske Veritas (DNV) is an international provider of offshore standards for both the oil and gas industry and the wind energy industry. The standard DNV-OS-J101 “Design of Offshore Wind Turbine Structures” provides principles, technical requirements and guidance for design, construction and in-service inspection of offshore wind turbine structures. As a first step towards updating this standard to fully cover floating wind turbine structures, a DNV Guideline for Offshore Floating Wind Turbines has been established. This development is based on identification of current floating wind turbine concepts and the guideline includes an evaluation of what is required to make DNV-OS-J101 suitable for floating wind turbine structures. This paper presents the highlights of the new DNV Guideline for Offshore Floating Wind Turbine Structures.
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