The oil and gas industry has long sought a semisubmersible design with low hull motion response suitable for a dry tree application in deepwater. The top tensioned riser (TTR) makes it possible to support production risers for direct vertical access (DVA) to the producing wells, without the need for disconnection during severe environmental events. However, the application of TTRs requires a hull with both minimal heave response and lateral motion due to the limitation of tensioner stroke in extreme conditions. Motion responses of the conventional semisubmersible to extreme environmental design events exceed current generation TTR stroke limits.
Over the past few years, Technip has been developing a low-motion deep draft semisubmersible for use as a wet-tree floating platform solution. The HVS (Heave and VIM Suppressed) class semisubmersible achieves significantly improved heave and vortex-induced motion (VIM) performance, as compared to other conventional deep draft semisubmersibles in the industry, through the use of hull form optimization.
To satisfy demands for a dry tree application, Technip has now adapted the HVS to provide a longer heave natural period while maintaining reduced heave motion and VIM response. The new design is a modification to the conventional deep draft semisubmersible, and provides a cost effective, dry tree application based on well-established and proven technologies.
Design convergence for dry tree semi
|Evolution of the HVS class semisubmersible.|
The main characteristic feature of the HVS class semisubmersible is the redistribution of displacement from the pontoon to the column, through the use of a "column step." This feature has the appearance of a blister, partially around the base of the column. The redistribution reduces the vertical (heave) hydrodynamic excitation and, consequently, the heave response. Also, the sharp-edged column steps break up the vortex coherence along the column length and, in concert with the narrower pontoons, reduces the VIM response of the hull.
Technip's new dry-tree design includes a minor modification to the initial wet tree HVS class hull form in order to extend its heave natural period characteristics. This is achieved via added hydrodynamic mass from upper and lower "paired-pontoon plates" located in the corners of each column to pontoon connection.
The convergence of the design can be seen as an evolution from the wet tree semisubmersible hull shape to the dry tree application. The hull shape for the dry tree application has a slightly increased draft. The location and shape of the paired-pontoon plates can be optimized for an improved hull motion performance according to the design requirements and environmental conditions.
The pontoon plates also provide additional damping to reduce the heave motion at the heave period, and increase stiffness at the high stress point of the column to pontoon connection by adding rigidity at that location. Finally, the paired-pontoon plates improve the hull motion response, with only a minor change in the hull design, and a minimum increase in fabrication costs.
Analysis tools, model testing
Technip has applied the following analysis tools in the development and validation of its dry tree semisubmersible solution:
- MLTSIM – an in-house non-linear time domain 6-DOF motion analysis program
- WAMIT – a frequency domain program (used as input to MLTSIM)
- CFD – Computational Fluid Dynamics analysis technology
- NWB – Technip's proprietary Numerical Wave Basin.
MLTSM was used for the global performance analysis. This is a time domain simulation program that employs a large-amplitude rigid-body formulation. It calculates the six-degree-of-freedom motions of a floating body (or multiple inter-linked bodies) at each time step under time-varying external loads. The external loads include wind load, hydrostatic load, hydrodynamic loads from waves and currents, loads from the mooring and riser system, and any other user-defined loads.
The program accounts for Froude-Krylov forces, finite wave amplitude, relative velocity/acceleration, wave drift, and viscous damping effects. Diffraction forces can be included for large-diameter members, along with linear radiation damping and drift forces from frequency-domain analysis programs, such as WAMIT.
CFD technology is a maturing tool which gives accurate hull motion response in irregular seas and VIM response in loop current conditions. Traditionally, the analysis tools used for initial platform design are based on classical potential flow theory, and the viscous effects on the hull are calculated through semi-empirical approaches (Morrison equation). In this approach, the empirical constants (drag and inertia coefficients) are estimated from model test data. Validations of the semi-empirical tools have been performed with results from model tests and field measurements. However, in screening the response of a new platform, the CFD tools are more reliable than the semi-empirical tools because they account accurately for the viscous effects and do not require conservative estimates of empirical coefficients.
|CFD analysis for hull performance in wave and GIV response.|
An extensive CFD analysis was performed on the present dry tree semisubmersible design with Technip's proprietary Numerical Wave Basin (NWB) to optimize the hull, including the column steps, pontoons, and pontoon plates.
Technip's wet-tree semisubmersible design has been extensively model tested for global hull motion performance at the Maritime Research Institute Netherlands (MARIN) in 2011, at the Offshore Technology Research Center (OTRC) in 2012, and for VIM response – with various hull shapes in the towing tank – at Force Technology in 2010.
In addition, the semisubmersible for the dry tree application has been model tested in 2012 at the OTRC test basin. The model was based on a topside payload of 25,000 metric tons (27,558 tons), and a riser configuration assuming one drilling riser and three production risers, with a full rig system. The tested environments included the 100-yr and 1,000-yr central Gulf of Mexico hurricanes in 6,000 ft (1,829 m) water depth. Through the model tests, the numerical tools have been validated and correlated with the measured data.
|Comparison motion RAO based on irregular wave tests.|
A short riser tensioner stroke range is a key parameter to making the semisubmersible suitable for the dry tree application. The maximum riser tensioner stroke range is estimated from 6-DOF motion considering intact and damaged conditions, wellbay layout, non-linear stiffness of tensioner, storm surge, and subsidence.
Based on existing, proven tensioner systems, spars have required relative riser tensioner strokes of 20-28 ft (6.1-8.5 m) for the GoM, while TLPs have a stroke range of around 6-10 ft (1.8-3 m). For the 100-yr extreme condition, the estimated maximum stroke range for Technip's dry tree design is 26.1 ft (7.9 m).
Hence, Technip's dry tree design achieves a maximum stroke range comparable to that of the spar for the 100-yr condition. In the robustness check for the 1,000-yr survival condition, the maximum stroke range is 36.9 ft (11.2 m), still within the technically-feasible range.
|Estimated maximum riser tensioner stroke for 100-year central Gulf of Mexico environment.|
This design provides various other benefits. For example, in the shallow draft case during quayside integration, the water plane area is redistributed from the narrower pontoons to the larger outboard diameter of the lower columns, creating greater stability. The column step also provides added stability in the case of a wet tow, and during transition from towing draft to operational draft at the final site.
Technip's semisubmersible provides other advantages, including ease of construction and installation:
- It does not require a complicated tendon installation and tether foundation
- It does not have fixed ballast for stability
- It can be transported to the site with all the topsides installed – there is no on-site requirement for upending, topside installation by heavy-lift vessel and mating of deck to structure.
Flexibility is another key advantage:
- It is not effectively limited by water depth
- Capability to include drilling and workover
- Flexibility on the number and arrangement of risers
- Easy to ballast for various operational configurations.
The design offers commercial advantages as well:
- It has the same fabrication unit costs as a conventional deep draft semisubmersible
- Enough work space, and low cost and light structural support steel in the processing topside and auxiliary systems.
The Technip dry tree semisubmersible design meets the oil and gas industry's criteria for a cost-effective dry tree system, and can support a TTR system with stroke well within the allowable ranges of the conventional production riser tensioning systems. Although the design incorporates a novel hull shape, it also shares common ground with conventional semisubmersibles in fabrication and constructability, including quayside integration and complete commissioning of hull and topside without requiring offshore operation for topside mating, upending and complex installation of stationary keeping systems and, consequently, has minimum innovation risks.