Offshore floating platforms are complex engineering systems, with numerous design challenges for the engineer, from the perspective of safety, reliability and longevity. Among their various applications, floating platforms are the lifeline of offshore oil and gas production. These platforms are subject to extreme environments, ranging from harsh waves to hurricane-strength winds, over a long period of time; ensuring platform and occupant safety is of paramount concern to the designer. This article details the deployment of numerical simulation at Technip, in the design spiral for offshore platforms for cost-effective, faster, efficient design.
OFFSHORE PLATFORM DESIGN: THE CHALLENGE
Of the 21 spars operating or under development, Technip claims delivery of 17. These platforms range in water depths from 590 to 2,382 m, using both dry and wet tree completions. A spar is the only inherently stable platform with a center of buoyancy above the center of gravity—it cannot flip over. There are three different spars—classic spar, truss spar and cell spar, Fig. 1. Spars are typically moored with a taut or semi-taut mooring system, supporting risers for fluid flowing from the seabed to the platform. Classic spars are fully cylindrical; truss spars have cylinders at the top and a truss at the bottom, to minimize heave; and cell spars consist of a number of vertical cylinders. Technip is now extending its floater design portfolio to other platform types, such as tension leg platforms (TLPs) and semisubmersibles.
The design challenges for offshore spar platforms include accurate knowledge of the environmental loads to be experienced by the platform, plus estimates of structural loads and dynamic motions on the platform from extreme wind, current and non-linear/random waves.
SHIFTING DESIGN CYCLE
A typical design process at Technip involves hull sizing to satisfy operating, installation and transportation conditions. The design process starts with global performance in extreme operating conditions. Global performance refers to motion in water, and is typically carried out by using semi-empirical potential, flow-based motion solvers that analytically combine gravitational and inertial forces, and empirically handle viscous forces. Scaled model tests are done to calibrate global performance analysis tools, even though, typically, these tests properly model only the gravitational and inertial forces (Froude & Euler number) and not the viscous forces (Reynolds number).
If the requirements of performance are not met, the entire process is carried out again before new model tests, until the final performance criteria are met. Even with the long history of model tests for spars, there are always uncertainties in model testing. And the model tests still would not completely answer questions regarding wave slamming on structures, structural resonance from wave loading, wave run-up on columns and green water on the deck (air gap), and vortex-induced motion (VIM). Note that many of these design issues deal with the air-sea interface (the free surface).
The traditional design process at Technip involves the semi-empirical (in-house) tool called MLTSIM for the hull model to obtain hydrodynamic coefficients. An in-house catenary modeling tool, FMOOR, is used for mooring modeling, and as a screening tool through a quasi-static analysis. Finally, model tests are performed to calibrate the empirical tools.
Recently, CFD has been included, to augment the design cycle and model tests, removing much a-priori uncertainty in testing results, and posteriori extension of modeling results, once the CFD model is validated (i.e., model the model with CFD). However, due to the cost of using CFD, the semi-empirical tools, with more than 20 years of data correlation, will continue to play the main role in design iteration. CFD will grow in acceptance, doing those design simulations that model tests cannot do well, or at all.
NUMERICAL SIMULATION IN DESIGN
To implement numerical simulation in its design process, Technip uses CD-adapco’s STAR-CCM+, a fully-integrated simulation package. Key differentiating features of STAR-CCM+, compared to other simulation tools, are the accurate capturing of the free surface, to model breaking and impact waves; motion models, including dynamic fluid body interaction (DFBI), embedded DFBI and overset mesh; and powerful pre/post processors.
One of the prohibitive factors of using CFD engineering simulation is the computational cost that is dictated by the hardware resources available and computing time. The in-house hardware resources at Technip included a dedicated CFD cluster (144 computer cores), which can simulate 30 sec to 1 min of real-time platform motion in less than a day. Advanced computing resources available at Texas Advanced Computing Center (TACC) included more than 10,000 cores, which is 10% of the total number of cores in Stampede cluster available through TACC’s industry partner program (STAR). Access to TACC enables multiple simulations of 3 hr of real-time motion in roughly a day, compared to single simulation of 30 sec in the same amount of time. The 3-hr period is important offshore, because that is the average length for a storm to pass over a given location.
A typical hydrodynamic simulation of an offshore platform would require a large number of meshes to capture the free surface. This is accentuated when simulating for extreme environments involving violent, non-linear waves, leading to higher computing time and cost. There are other gaps in the simulation methodology that also need to be addressed. Technip set out to address all the technology gaps in the existing simulation methodology, with the aim of a final design tool that has fully integrated CFD methodology in the design cycle.
STAR-CCM+’s Volume of Fluid (VOF) method has numerous wave models for different scenarios that have been well-validated for free surface capturing. With respect to floating offshore platforms, the package’s fifth-order Stokes Wave model is suited for deepwater simulations, which is the environment in which the majority of spar floating platforms operate. In the event of shallow-water extreme waves, this fifth-order model is not the proper physics model. To overcome this, a fully non-linear wave model was developed in-house for shallow-water extreme waves. In addition, simulation of spar platforms required a very large domain for wave-absorbing, upstream of the platform.
To minimize the computational cost from a very large domain, Technip developed an Euler-Overlay method, where a Euler solution is used in the far-field without the hull structure and a Reynolds Averaged Navier Stokes (RANS) method with DFBI is used near the platform. An overlay method using the momentum and volume fraction sources is used at the intersection of RANS and Euler regions to blend the two solutions smoothly. This method reduces the domain size greatly, thereby reducing the computational time and hardware resources required, by eliminating the need to solve RANS equations over a wider area.
THE EULER OVERLAY METHOD (EOM)
Technip has used EOM with STAR-CCM+ successfully to provide extreme design loads on structures for a variety of offshore platforms. A proper validation of the numerical model, with experimental data, is the key to deciding on the appropriate numerical analysis. To validate the EOM, Technip simulated model tests from Chaplin et al. (1997), involving a long-crested wave and a vertical column (Kim et al., 2012). The moments on the column from the EOM matched well with the data from model tests, thereby validating the methodology.
This method was introduced into the design process with excellent results. A sample of how the EOM helped in the design cycle of various projects is given below.
Wave run-up and air gap analysis. Offshore platforms, like spars, are designed to protect crews and equipment from extreme waves, with their crest height close to the topsides. Waves, with steep fronts hitting the vertical side of the platform, cause wave run-up that further amplifies the crest height, and could potentially damage structure and equipment under topsides. During model testing, the air gap between the water surface and topsides is closely measured, to identify negative air gap area on topsides. However, the physical model test has limitations with regard to providing enough air gap information for topsides design. Air gap information from the model test is limited to wave probe locations, and it could be contaminated by water spray that has to be corrected after the model test. With CFD simulation, all these issues can be resolved easily.
Technip uses a hybrid method utilizing MLTSIM and STAR-CCM+ to calculate air gap on spar platforms, and to design hull appurtenances to prevent run-up and air gap. The air gap estimated from full 3-hr MLTSIM simulation is first used to identify wave events that may cause negative air gap. The screened extreme events are simulated in STAR-CCM+, with moving mesh, with prescribed input wave and spar motion from MLTSIM simulation. Air gap is monitored in the whole horizontal surface, including topsides area. Air gap, due to water spray, can be filtered out easily by inspecting volume fraction in the prescribed area below the top free surface. Figure 2a illustrates wave run up and air gap contour from a spar air gap simulation.
Ringing analysis for gravity-based structure (GBS). Ringing is a phenomenon experienced by tension-leg- and steel-gravity-based platforms, when responses of considerable amplitude are generated by these structures at their resonance period and higher harmonics, potentially causing hull strength and fatigue issues during extreme wave events. EOM was applied to a ringing analysis of a new gravity-based platform subjected to short-crested, irregular waves. Details of the trimmed hexahedral mesh around the GBS, free surface profile and pressure profile on the structure, are seen in Fig. 3.
A fully non-linear solution from the EOM is obtained for an incoming wave, with a peak period of 15 sec to capture higher-harmonic wave loading, and the period being around 3 sec. A conventional analytic approach, based on potential theory, fails to predict this short-duration load. Model tests for this GBS were problematic, as regards providing pressure distribution over the hull for the dynamic structural analysis. CFD analysis with EOM enabled proper study of this GBS at higher-harmonic loading by providing a time history of pressure distribution over the hull, which is fed into the dynamic structural analysis model. Comparisons of the structural response on the GBS from analyses with, and without, consideration of dynamic structural responses are shown in Fig. 3c. Numerical computations show the dynamic amplification of the structural response, due to the resonant dynamic response of the structure to higher harmonic loads.
Tendon analysis of a TLP. The TLP tendon is a vertical mooring system that provides high vertical stiffness to eliminate vertical motion in waves. The natural period of TLP vertical motion is roughly 3 to 4 sec, which is at least two times shorter than ambient wave periods. As a result, a significant portion of the tendon tension responses is coming from the second- and higher-harmonic wave loads, which cannot be captured by conventional analysis methods, based on linear wave theory. The resonant tendon response, due to the second harmonics (springing), typically occurs at operational sea states, and affects fatigue life of the tendon significantly. At severe sea states, with higher and longer waves, the tendons resonate to a higher harmonic load induced by non-linear wave diffraction and wave impact (ringing).
There were no reliable analytic tools available to provide springing and ringing loads accurately. Technip is utilizing EOM-based numerical wave tanks to estimate the tendon loads from springing and ringing at the early phases of TLP design. For fatigue sea states, springing load is obtained from EOM simulation with a fixed-hull model. For ringing analysis in extreme sea states, more detailed models must consider all six degrees of freedom concerning hull motion, with mooring and riser systems. Catenary models built in STAR-CCM+ were used to simulate the tendons. The numerically-predicted tendon tension agrees well with model tests. Comparisons of air gap in the time domain with model tests also shows that CFD agrees well with model tests in predicting the air gaps and relative wave elevations.
Semisubmersible motion simulation. The EOM was used for motion analysis of a semisubmersible platform in the design phase. A mooring and riser model was used to calculate the motion of the moorings and riser. Model tests offered data on heave response amplitude operators (RAO), an engineering statistic used to determine the behavior and response of the platform in waves. Numerical analysis with the EOM model shows excellent prediction of heave RAO for the semisubmersible, Fig. 4.
Dry-tree semi-submersible hull optimization. The oil and gas industry has devoted substantial efforts to find a dry-tree solution for the semisubmersible in harsh deepwater environments. The key design aspect of a dry-tree semisubmersible is the minimization of heave motion, to accommodate design limits of topsides equipment in de-coupling platform motion and riser systems. Technip has been developing new hull forms, suited for industry demands in worldwide design environments. EOM-based CFD simulation has been used to provide heave-motion performance of the trial hull forms for the optimal, dry-tree semisubmersible design.
The above examples show the value of simulation as an effective replacement for model tests, early in the design phase, to identify the optimal offshore platform designs, before moving to model tests; with this, time and costs related to model tests and design are reduced. CFD can be used after model testing to extend the design into variations that shorten the overall optimization process. In addition, simulation provides more information on the physics involved, compared to model tests.
An example of the savings can be gathered from the total computational cost of simulations for TLP and semisubmersible analysis, costing $550 and $750, respectively, on 640 cores for real-time motion simulation of 5 min and 1 hr, respectively. This is a very negligible fraction of the model testing costs, and overall project costs, with potential savings in design time and cost running into millions of dollars. The return on investment for simulation is extremely high for offshore platform design. With improved wave models and mooring/riser modeling, Technip intends to reap greater benefits from using numerical simulation in the design cycle.
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