In a riser design, the traditionally assessment adopted by industry for attainment of platform motions applied to the top of riser consists in the use of de-coupled methodologies. These 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 are especially significant and a reduction of the amplification of platform motions if compared to the platform motions obtained from de-coupled analysis is observed. 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. Numerical application is presented in order to assess the comparison of the two presented methods in terms of SCR analysis results and to confirm the reduction of platform motions due to coupling effects. The coupled model here studied is composed of approximately 80 lines connected to the platform which requires an excessive computational effort. In order to reduce the computational 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 where the required accuracy is achieved and the computational time consuming is the minimum possible. The computational time consuming and platform motions are also presented. The conclusion of this work is that coupled analysis should be adopted by industry as usual practice of SCR design in deep waters, where more realistic and optimum results are presented, without an excessive computational effort.
In South America, the recent fields discovered in the pre-salt area are located more than 300km away from the coast, which makes exploring and especially the transportation of associated gas a major challenge in the coming years. Several issues are related to this problem: • distance to shore (more than 300 km) and ultra-deepwater (more than 2000 m); • vessel time and capacity (reduced number of boats/offshore units with capacity to install pipelines in deepwater); • increasing demand for these vessels worldwide; • flaring restrictions in accordance with Brazilian National Petroleum Agency (ANP). Deepwater pipelines have traditionally been built using very thick pipe walls, requiring large quantities of steel and special equipment for milling and pipe laying. This paper presents an innovative gas transportation concept for long distance in ultra-deep waters (X-Stream Pipeline Concept). A way to potentially reduce the required wall thickness of deep water pipelines by controlling in the differential pressure across the pipe wall (and thus allowing for a reduction in the pipeline wall thickness). This concept is also applicable in other deepwater exploratory frontiers, for instance: Gulf of Mexico and West Africa.
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.
With Pre-salt area being a massive new offshore oil frontier, there is a high demand for deepwater floaters for both drilling and production. There are some unique challenges related to the ultra-deepwater (water depth beyond 1500m) and the Brazilian environment for the pre-salt area. Various floater concepts have been evaluated with specific focus for Offshore Brazil applications.This paper provides an overview of the current technology related to dry tree solutions as well as discussion of the pros and cons of various concepts. The paper also discusses some new concepts emerging in the industry attractive for Offshore Brazil.A dry tree solution has the advantage of direct access into reservoirs from the floaters. This allows the operators to drill, complete and workover the well directly from the same hosting unit. The result could be increased reserves and productions in Offshore Brazil as well as significant cost reduction. Proper validation of the concepts and technology qualification of all the systems/components associated with novel technology, establish the basis for a safe and successful project implementation while reducing potential risk to personnel, environment and property.DNV has established processes to systematically evaluate new concepts and new technologies, e.g. Approval in Principle (AiP) and Technology Qualification (TQ). A brief introduction to these processes is included in the paper and how they support the development of these new concepts.
Free span assessment has more and more become an important part of modern pipeline design. The reason for this is partly that the remaining hydrocarbon reservoirs are located in more challenging places, e.g. with very uneven seabed. Another explanation is that the pipeline design codes a few decades ago did not allow for vibrating free spans, while the modern, state-of-the-art pipeline codes, such as DNV-OS-F101 “Submarine Pipeline Systems” (2013) and its Recommended Practices, opens for long spans that are allowed to vibrate as long as the structural integrity is ensured. In presence of non-cohesive soils and high on-bottom flow velocities significant free span development may occur over the design life, e.g. due to scouring. Such spans may be associated with a fatigue life capacity less than the design life if the spans are assumed stationary. For non-stationary spans with occasional long span lengths this may not be true since the criticality is strongly linked to the persistence of long spans and the prevailing environmental condition. A realistic fatigue assessment must account for the history of the span (i.e. stress cycles encountered for a critical weld) including predictions into the future development. High costs related to span intervention puts focus on minimizing these costs while still ensuring integrity of the pipeline with respect to vortex induced vibrations (VIV) and associated fatigue damage. On the other hand the potential costs related to fatigue failure of a pipeline (recovery costs, economical loss and environmental consequences) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits. One frequent challenge faced with old pipelines in operations survey reports are that they report several free spans. Old pipelines were not designed to allow any vibration and usually there is scanty information about different parameters such as soil conditions, operational parameters, lay tension, environmental data, etc., thus it’s difficult to determine whether it’s necessary to intervene the span or not. State-of-the-art free span codes are deterministic in their nature. If the new codes are used to evaluate such old pipeline spans, considering all the before mentioned uncertainties in the input parameters, this would eventually lead to over conservative very low time to failures. The outcome will be that many spans need to be fixed immediately or should have failed already. Such a situation leads to a mistaken conclusion about the conservatism of the codes and not on the way they were applied. This paper discusses some of the challenges often seen with free spans during the operational phase. The objective of the paper is to demonstrate that for in-service pipelines the lack of reliable information about the free spans is the main source of commonly low life encountered and not the methodology used to evaluate the free span. Some of these challenges are discussed in detail and potential ways forward are outlined.
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