Dry Tree Semisubmersible Research Articles

Numerical Analysis on Mathieu Instability of a Top-Tensioned Riser in a Dry-Tree Semisubmersible

AbstractThe development of a dry-tree semisubmersible (DTS), a new type offshore hydrocarbon production system, is facing unconventional challenges in the issues of dynamic stability, structural integrity, and parametric resonance. The Mathieu equation is used to assess the dynamic stability of a top-tensioned riser (TTR) in order to prevent the parametric resonance which leads to detrimental effects on structural integrity. The objectives of this paper are to (i) study a Mathieu stability diagram and its coefficients for an assessment of the stability of a TTR, (ii) identify the effects of the dynamic tension variations in the Mathieu stability assessment, and (iii) analyze the stability of the TTR on a DTS, which is equipped with a long-stroke tensioner, by using numerical simulation. The dynamic tension variation in the DTS was identified to induce instability in the TTR. Hence, the Mathieu stability assessment is recommended to be included in an analysis of TTR behaviors in a dry-tree interface of semisubmersibles.

Tension variations of hydro-pneumatic riser tensioner and implications for dry-tree interface in semisubmersible

In real sea environments, excessive dynamic axial tension variations can be exerted on the top-tensioned risers (TTRs) and lead to structural integrity issues. The traditional riser-tension-variation analysis, however, by using parametric formulation is only conditionally valid under certain strict limits and potentially underestimates the total magnitudes of tension variations. This phenomenon is especially important for the long stroke tensioner in dry-tree semisubmersible with larger global heave motion and longer stroke. In this paper, the hydro-pneumatic tensioner (HPT) is modeled in detailed component-level which includes a set of hydraulic and pneumatic components. The viscous fluid frictional effect in the HPT is considered. The main objectives are (i) to develop a detailed tension variation model of the HPT; (ii) to identify the deviations between the conventional parametric formulation and component-level formulation; (iii) to numerically analyze the tension variation of long stroke tensioner in a dry-tree semisubmersible (DTS). The results demonstrate the necessity of component-level formulation for long stroke tensioner in the development of DTS.

Transportation Analysis of Dry Tree Semisubmersible

With the advancement of oil & gas into ultra deepwater, the need for drilling and production platforms has becomes more acute. The dry tree semisubmersible can be used for direct vertical access into reservoirs from deepwater since it offers small in-place motions, large open deck areas, dockside commissioning and minimum offshore hookup. The direct access allows the operator to drill, complete and work over the well directly from the same platform. The drytree semisubmersible consists of a keel tank which can be telescoped up and down based on the requirement of the platform. The keel tank is fully telescoped down while in operating condition and telescoped up while in transportation condition. The transportation analysis of a drytree semisubmersible using the linear radiation, diffraction panel program WAMIT is presented in this paper. Parametric studies were carried out in WAMIT by varying the size of the keel tank by considering the keel tank size as 80X80m, 91.5X91.5m and 100X100m. The Case 2 in which the keel tank dimensions were 91.5×91.5 m was found to be most appropriate of all, as the response for it was lower compared to other cases and also the structure was good from stability point of view. It was found that the heave and response were high only beyond 15 seconds wave period thus making transportation operations safe, as transportation is carried out in sea states 5 and 6 which have wave periods less than 14 seconds. Also the pitch RAO was found to be very low thus posing no threat in transportation mode.

Dry Tree Semisubmersibles: The Next Deepwater Option

Dry Tree Semisubmersibles To commercialize some of the world’s most challenging deepwater fields, offshore engineering companies are developing a new class of drilling and production platform known as the dry tree semisubmersible (DTS). The challenge involves taking existing technologies and lessons learned from previous floater designs to create an alternative platform for waters too deep to cost-justify the use of tension-leg platforms (TLPs) and fields too large for spar platforms. Dry trees, also known as Christmas trees, are wellhead devices installed during the completion stage of a well’s life and give the operator control over production. Used on onshore wells, dry trees have been used extensively on shallow water fixed platforms, TLPs, and spars, but never on a deepwater semisubmersible because that platform’s motions are too extreme to support a dry tree system. As the DTS concept awaits introduction into the deepwater market, multiple designs are under evaluation by major off-shore oil companies and the Norwegian classification society Det Norske Veritas (DNV). The chief advantages of using a DTS are that opera-tors can drill, complete, and carry out intervention operations on multiple wells from the same platform in depths below 6,000 ft. This saves the operator significant resources that otherwise would be spent over the life of the field on contracting mobile offshore drilling units or purpose-built, well-intervention semisubmersibles. Going Deeper With a DTS A TLP typically uses four columns to support a large topside facility and is secured to the seafloor with mooring lines that allow the floating platform to move from side to side, but not up and down. Operating at a water depth of 4,674 ft, ConocoPhillips’ Magnolia TLP in the US Gulf of Mexico (GOM) is the deepest structure of its kind in the world. Next year, Chevron hopes to begin first production from its Big Foot TLP, also to be located in the GOM, at a depth of approximately 5,200 ft. Beyond 6,000 ft, TLPs become impractical because of the amount of steel needed for the tendons that moor the platform to the ocean bottom.

Technology Focus: Offshore Facilities (February 2012)

Technology Focus Last year, we reviewed some of the more-prominent examples of how the industry continues to respond to the need for safe and cost-effective production facilities in ever-more-challenging environments. We also highlighted the increasingly important role that constructive collaboration can play in facilitating the desired outcomes for all parties. This year, we illustrate how this same theme of constructive collaboration has been applied effectively at the other end of an offshore facility’s life span, in the major decommissioning program for the Frigg field. Most of us are very familiar with the term offshore hookup, but soon we may become equally familiar with what offshore “hookdown” really involves. We also take a look at an approach for safely extending the useful life of aging offshore-production infrastructure, in locations where the subsea tieback of new fields warrants the associated investment. Our focus is not entirely on end-of-life scenarios though. New offshore-platform concepts continue to evolve to suit the changing needs of operators. Some of these aim to offer the reduced well cost of fixed structures with the redeployment advantages offered by floating structures. One of the most challenging new frontiers for the offshore industry and for society at large is the Arctic region. More specifically, it is development of underwater hydrocarbons where the presence of ice affects the nature of the development. Moving into any new frontier first requires gathering sufficient environmental data to be able to predict quantitatively the character and envelope of conditions at that location throughout the field’s production lifetime. We take a look at how these issues are being addressed in the specific case of the proposed Shtokman-field floating production facility. As we return for this annual JPT Focus on the technology associated with offshore oil/gas facilities, it is timely to mention the launch of the new SPE magazine Oil and Gas Facilities, primarily geared toward the whole offshore-facilities sector. This effort represents a significant initiative to broaden the appeal and relevance of SPE to the wider oil/gas community, and any feedback that you may have in this regard would be welcomed warmly. Recommended additional reading at OnePetro: www.onepetro.org. OTC 21708 Decommissioning of Frigg and MCP01—A Contractor View by T. Gram, Aker Solutions, et al. OTC 22505 The First Dry-Tree Semisubmersible Drilling and Production Platform Sold by Roberto Noce, Moss Maritime, et al. OTC 22078 Design Considerations for an Arctic Intervention Vessel by Tor Einar Berg, Marintek, et al.