Tension Leg Platform Design Research Articles

CFD Study of the Non-Linear Physical Phenomena of the TLP of a 15-MW-Class FOWT under Extreme Waves

In this study, a numerical simulation was conducted to investigate the non-linear physical phenomena of a tension leg platform (TLP) of a 15-MW-class floating offshore wind turbine (FOWT). Computational fluid dynamics was employed as the numerical tool, and a deforming mesh technique was used to describe the moving body. To examine the non-linear physical phenomena, an irregular wave was generated with a focus on head sea conditions. The springing and ringing responses were calculated from the numerical simulation results, and the relations between the motions and dynamic tensions of the 15-MW-class FOWT TLP were investigated. From the irregular wave impact simulation, it was found that the springing response via the wave sum frequencies and the ringing response occurred at approximately three times the wave peak frequency. Additionally, whipping simulations were conducted under a focused wave. The results show that the response in pitch resonance frequency was caused by the wave impact. The numerical results of this study could be used as fundamental data for FOWT TLP design.

An experimental investigation on vortex-induced motion (VIM) of a tension leg platform in irregular waves combined with a uniform flow

Vortex-induced motion (VIM) of a tension leg platform (TLP) is critical to avoid structural and fatigue damages to the tendons and risers. Many previous studies have conducted model tests for TLPs in towing tanks. However, the models were usually established using linear springs to replicate the underwater moorings. In this study, a TLP model with full-water-depth tendons and top tension risers (TTRs) was developed. A series of model tests were conducted in a large-scale wave tank at three incidence angles (θ=0°, 22.5°, and 45°) for the reduced velocity ranging from 4 to 11.5 and two extra cases of flow combined with irregular waves. The maximum transverse amplitude of 0.32D was observed at a reduced velocity of 7.0 when θ=0°. On the other hand, the in-line and yaw motion experienced increasing amplitudes with reduced velocities and no lock-in region was identified. The most significant responses of transverse and yaw motion appeared at 0° and 22.5° headings, respectively, while no critical heading was recognized for surge motion. The competing mechanics between VIM and the wave-induced motion were revealed in the cases of combined flows and irregular waves. The tensions in tendons and TTRs that result from VIM were found as significant as those induced by wave loads, which highlights the necessity of considering VIM in the design of TLP. Furthermore, the result showed that the VIM amplitudes were considerably suppressed under the nonlinear stiffness and larger damping provided by the tendons and TTRs.

Performance based design of Tension Leg Platforms under seismic loading and seabed liquefaction: A feasibility study

The response of pile supported Tension Leg Platforms during seismic loading and seabed liquefaction is analyzed numerically, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo-static factors of safety against pullout failure of the pile well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking due to subsoil liquefaction, but fully recovers during the following excess pore pressure dissipation phase and may even exceed the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the finite time period when the static pullout factor of safety of the pile remains less than unity. Thus, final pile head displacements may increase considerably when this time period is elongated. Due to the generally high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout is mostly transmitted to the platform, with only a small percentage corresponding to reduction of tendon elongation. For the cases examined herein, the potential loss of platform buoyancy and tendon pretension may prove detrimental but is unlikely to threaten the safety and functionality of the platform. On the other hand, initially minor tilting of the platform (e.g. due to long period lateral loading and differential tendon pretension) may be magnified during seismic loading above allowable limits.

A system for the hydrodynamic design of tension leg platforms of floating wind turbines

A system that allows testing new concepts quickly and with higher precision is developed and demonstrated through an example of a tension leg platform. The process goes through the mass matrix estimations, hydrostatics, mooring system design, and dynamic analysis. It starts from a multi-purpose 3D model and does not require user intervention or moving data between software. Instead, the same quadrilateral mesh is used for all calculations after modifying a format used by panel method codes. The need to use simplified tables and spreadsheets is eliminated. The frequency domain wave responses are obtained with a potential flow solver and assessed to approximate the platform's performance with the wind turbine. The assessment includes the detection of slack mooring occurrences. The setup allows completing the initial design with minimal amount of input data. A time domain model for simulations is also built within the system. The level of agreement between these two solutions is examined. The platform is assumed to operate in Galicia, Spain, to discuss sets of environmental conditions considering the mooring line responses. Two primary variables are used to illustrate a sizing algorithm on 231 hull variations. A measure of performance is given to clarify the required time for the design space assessment with this method.

Extreme Response Prediction of a Multi-Column Tension-Leg-Type Floating Wind Turbine under Operating Conditions

The extreme response of a multi-column tension-leg platform (TLP) floating wind turbine (the WindStar TLP system) is analyzed here under operating conditions. Coupled dynamic response simulations are performed using FAST, a fully coupled aero-hydro-servo-elastic time-domain simulation tool. A land-based system (a U.S. National Renewable Energy Laboratory offshore 5 MW baseline wind turbine installed on land) is quantitatively compared with the WindStar TLP system. Extreme loads with different return periods are obtained by statistical load extrapolation. Peaks are extracted using peak-over-threshold analysis with time separation between peaks. The WindStar TLP system shows almost similar mean generator power output to the land-based system under operating conditions owing to its small dynamic motion responses to environmental loads. It shows greater characteristic load (blade 1 out-of-plane moment at the blade root and tower base fore-aft bending moment) under operating conditions than does the land-based system, with the characteristic load ratio reaching approximately 1.2. These results will help identify the extreme response differences between the two systems, and thus aid the development of relevant modifications to the initial WindStar TLP design.

Storm surge prediction with management information systems: A case study of estimating value and observations system

A natural disaster is a major adverse event resulting from the earth’s natural forces. We can now take advantage of information technology (IT) devices to make migration decisions and hazard analysis. The range of IT applications is very wide, and could include robot manufacturing, remote exploration, fuzzy Lyapunov techniques, artificial intelligence, intelligent devices, tension leg platform design, consumer and service quality, and management information systems. Management information systems are especially helpful to medical personnel when carrying out the treatment and medication of patients. Sometimes, though, hospital staff might not have immediate access to a patient’s records such as response to treatments, medication status, or examination results because the computer equipment is not easily portable. In such situations, it is only possible to keep written records, with the data being cataloged or referred to only after returning to the nursing station or office. If the hospital could implement a computerized medical service cart for use in the wards, the above problems would be solved. These computers could use a wireless network system, linked with the central server to access information. The technology now exists and the era of the smart phone has now been entered. Smart phones/tablet PCs could be applied for clinical medical care. For the elderly and patients with chronic diseases, smart health devices designed for home care service are necessary and should be applied as soon as possible. This study designs a wireless physiology signal monitoring system that uses a smart phone with a wireless network apparatus to provide convenient monitoring for patients in the home care service system. The wireless care system is designed for patients who need long-term home care services or assistance with chronic diseases. This paper provides important suggestions to develop ways to connect patients to medical care institutions through the internet.

Initial Design of Tension Leg Platform for Offshore Wind Farm

As a floating foundation for wind turbine, dynamic response of offshore structure to external forces should be as small as possible because the motion of the structure imposes inertial force to a wind turbine system. Generally, a TLP (Tension Leg Platform) has favorable characteristics that its dynamic response in waves is negligibly small compared to other type of floating foundation. Our conceptual design of TLP for wind turbine was carried out in consideration of mooring force and easy maintenance. Numerical analyses were performed on dynamic response and tension deviation of leg in waves, natural frequency of vibration, and dynamic response to seismic load. As a result, it is found that natural periods of heave and pitch are rather short and amplitudes of motions are very small. It is also found that it keeps sufficient safety factor to extreme environmental conditions including earthquake, and is free from resonance with a wind turbine system.

Influence of the setdown of a tension leg platform on the extreme airgap response

One distinguishing characteristic of a tension leg platform (TLP) is the setdown of the hull when the platform moves in its compliant modes (surge, sway and yaw). The nonlinear setdown has profound implications in various aspects of TLP design, but this paper focuses on its impact on the available airgap. Although there is general consensus that setdown should be included in the airgap assessment, to date there is no systematic procedure of analysis, and related literature is scarce. This paper aims to develop a simple method for incorporating setdown in the extreme response prediction of the airgap. The proposed method requires only the covariances of the platform motions, and these are available from a frequency domain analysis. From a case study, the crossing rates calculated by the proposed method is found to be in good agreement with Monte Carlo simulation, with only a slight disparity at high threshold levels. This work has also afforded physical insight; for example, it is discovered that the wave-frequency (WF) motions are more critical to the extreme airgap compared to the low-frequency (LF) motions, because the surface elevation is correlated with the WF motions, but not with the LF motions.

Reliability-based design criterion for TLP tendons

This paper presents a methodology to perform a Load and Resistance Factor Design (LRFD) criterion for the design of Tension Leg Platform (TLP) tendons in their intact condition. The proposed design criterion considers the Ultimate Limit State (ULS) for the tendon sections, expressed in terms of the expected value of the extreme Interaction Ratio (IR), considering long-term storm sea states, and takes into account the dynamic load effects interaction and the statistics of its associated extreme response. The partial safety factors are calibrated through a long-term reliability-based methodology for the storm environmental conditions in deep waters of the Campeche Bay, Mexico. Different target reliability values are considered in order to evaluate the effect of this key parameter on safety factors. The results show that the partial safety factors reflect both the uncertainty content and the importance of the random variables in structural reliability analysis. When tendons are designed according to the developed LRFD criterion, a less scattered variation of reliability indexes is obtained for different tendon sections across a single or a variety of TLP designs. Finally, it is observed that the target reliability value has a strong influence over the safety factor values and thus over the final size of the structural elements.

Dynamic behaviour of square and triangular offshore tension leg platforms under regular wave loads

Among compliant platforms, the tension leg platform (TLP) is a hybrid structure. With respect to the horizontal degrees of freedom, it is compliant and behaves like to a floating structure, whereas with respect to the vertical degrees of freedom, it is stiff and resembles a fixed structure and is not allowed to float freely. The greatest potential for reducing costs of a TLP in the short term is to go through previously applied design approaches, to simplify the design and reduce the conservatism that so far has been incorporated in the TLP design to accommodate for the unproven nature of this type of platform. Dynamic analysis of a triangular model TLP to regular waves is presented, considering the coupling between surge, sway, heave, roll, pitch and yaw degrees of freedom. The analysis considers various nonlinearities produced due to change in the tether tension and nonlinear hydrodynamic drag force. The wave forces on the elements of the pontoon structure are calculated using Airy's wave theory and Morison's equation, ignoring the diffraction effects. The nonlinear equation of motion is solved in the time domain using Newmark's beta integration scheme. Numerical studies are conducted to compare the coupled response of a triangular TLP with that of a square TLP and the effects of different parameters that influence the response are then investigated.

Heave-Restrained Platform Reduces Costs and Eases Operations

Heave-Restrained Platform Reduces Costs and Eases Operations

TLP Rigid Riser: A Case Study

Summary This case summarizes tension-leg platform (TLP) rigid riser designconsiderations and presents results of TLP riser analysis for aproduction/injection riser in the North production/injection riser in the North Sea. Analysis methods, design criteria, and design optimization are addressed. The riser is designed for 300-m water depth in compliance with Norwegianregulations. Emphasis is placed on application of the regulations andquantitative comparison of alternative methods for analysis of fatigue andextremes. Introduction This paper summarizes TLP rigid riser design considerations and presentsresults of the TLP riser analysis. TLP functions include drilling, production/injection, and export through rigid risers. TLP design is mostsensitive to the many production/injection risers. Such parameters as riserspacing, top tension, stroke, and pontoon clearance strongly influence globaldimensions and deck spacing/layout. Parameters used in this case study aresummarized in Table 1. Fig. 1 is a schematic of the TLP riser system. Thedesign is based on Norwegian Petroleum Directorate regulations and guidelines. Criteria for local design are summarized in Ref. 3. Analysis Methods Both frequency domain and time do analysis methods were used. Fatigueanalysis is primarily based on frequency domain and extremes on time domain. The riser is a nonlinear dynamic system, and the inaccuracy in using frequencydomain (assuming constant top tension and using stochastic linearization) isdiscussed below. The approach adopted for extreme responses is regular waveanalysis. This method was used to overcome uncertainty in statisticalextrapolation from an irregular time simulation and, for efficiency, to enablemany design iterations. Dynamic analysis was carried out without introducingload factors. Load factors are applied to responses when carrying out codechecks and when preparing design values for component specifications (e.g., stroke results). Boundary conditions include TLP motion, tensionercharacteristics (see Stroke section) and template/subsea wellhead interface(see Components and Interfaces). The riser analysis included TLP offsets in therange corresponding to TLP extreme motions. First-order response is includeddirectly as a transfer function in the riser analysis, and all other TLP motioncomponents are taken into account by defining a range of riser mean positionsupon which the first-order response is superimposed. TLP setdown is included ateach timestep in the time domain analysis. Sensitivity and Optimization As usual for deterministic design, an extensive sensitivity study must becarried out. The following were investigated:wave periods;currentvelocity;riser periods;current velocity;riser location andgeometric phase;hydrodynamic parameters, variation with depth, Reynoldsparameters, variation with depth, Reynolds number, Keulegan-Carpenter's number, and roughness;annulus and tubing fluids;top tension and tensionercharacteristics;TLP offset;extent of marine growth; andTLP huginfluence on wave kinematics. Quantitative sensitivity results vary widely forthe various responses along the riser, and the resulting priority is influencedby those responses that govern the design. The rigid risers tend to be inconflict with the overall TLP optimization on such parameters as top tensionand riser spacing. parameters as top tension and riser spacing. Optimization, therefore, is an iterative process carried out on the basis of the sensitivityresults for both the risers and the TLP. Handling and operational weatherlimitations are most sensitive to riser spacing and wellbay layout. Theserequire early detailed consideration because they are important to TLPefficiency in service. An alternative analytical approach to sensitivityanalysis and optimization with probabilistic methods could be used whereprobabilistic methods could be used where quantitative sensitivity results aregenerated directly in one analysis. This method has been applied successfullyto TLP tethers. Extreme Stresses Allowable tensile stresses in the riser wall were checked against values in Ref. 2. The Von Mises (combined) stresses were solved with a minimum wallthickness, taking into account corrosion/wear allowance and wall-thicknesstolerances. Ref. 9 is used for sum calculations. Compressive stresses also werechecked for load where axial stresses were low. Local buckling checks did notlead to additional design requirements. Extreme results vary considerably withanalysis method.

Regulatory Considerations in the Design of Tension-Leg Platforms

Summary The tension-leg platform (TLP) is an accepted development option indeepwater Gulf of Mexico projects. The first TLP installed in projects. Thefirst TLP installed in U.S. coastal waters is now in operation. Thecertification of TLP's under current U.S. regulations is a new experience forthe platform operators accustomed to operating in the Gulf of Mexico and forthe regulatory bodies with platform-design jurisdiction. Introduction Operators in the Gulf of Mexico introduced two unique deepwater floatingproduction developments in 1987 and 1988. The Placid Green Canyon floatingproduction unit is based on a converted semisubmersible drilling unit in1,450-ft [440-m] -deep water. Conoco's Jolliet tension-leg well platform(TLWP), in 1,760-ft [535-m] -deep water, is based on the original TLP conceptdeveloped in the North Sea (Fig. 1). Other operators have proposed TLP's forGulf of Mexico sites because TLP technology is now established and the economicadvantages of TLP's are comparable to those of fixed or floating platforms. Of particular interest to operators are the hidden costs of design, construction, and installation for these new deepwater developmentalternatives. This paper addresses a fairly narrow aspect of the overalleconomics of the TLP concept: the design certification process and its impacton costs. Factors unique to TLP certification are discussed. This paper alsobriefly explains the background of TLP regulations in U.S. coastal waters andcompares the basic philosophies of maritime and oilfield philosophies ofmaritime and oilfield practices on which current regulations are based.practices on which current regulations are based. OCS Regulatory Approach Regulatory requirements for TLP's evolved from existing regulations forfixed platforms and mobile offshore drilling units (MODU's). The currentregulatory approach is the result of combining fixed-production platformrequirements developed by platform requirements developed by the MineralsManagement Service (MMS) and MODU requirements developed by the U.S. CoastGuard. Ref. 1 provides background on the evolution of the rules; Refs. 2through 4 give information on actual regulations. Two main areas of interest to TLP operators are the division of authoritybetween the Coast Guard and the MMS concerning process facilities and technicalstandards to process facilities and technical standards to be applied for theremaining TLP systems. Specific technical requirements for TLP's do not existtoday within Coast Guard or MMS regulations. This paper explains thesignificant differences in philosophy between these two agencies. Background of Coast Guard Regulations. The Coast Guard's main interest ispreserving the safety of life at sea. Its requirements derive from 150 years'experience in the marine environment. The Coast Guard's extensive regulations, found in the Code of Federal Regulations, cover all types of vessels, includingpassenger, cargo, and fishing vessels and MODU'S. While based largely onconventional merchant-vessel practices, Coast Guard requirements for MODU'S, developed with the assistance of the oil industry, are "in tune" withthe offshore workplace environment and are supported by the offshoreindustry. In formulating its strategy for reviewing TLP designs, the Coast Guarddetermined that MODU regulations would form a good basis for TLP safetyregulations. It recognized the need to remain flexible in setting specificrequirements for the TLP. Rather than generating specific requirements forTLP's, the Coast Guard chose to work with the industry and the API indeveloping design standards for TLP'S. API RP2T provides general guidance forthe design, provides general guidance for the design, construction, andoperation of TLP'S, liberally incorporating the practices and requirements ofthe Coast Guard and the American Bureau of Shipping. The initial application ofRP2T was on the Jolliet TLWP project. Background of MMS Regulations. The MMS is responsible for E and P on theU.S. Outer Continental Shelf (OCS). MMS requirements are based onfixed-platform practices in the Gulf of Mexico, southern practices in the Gulfof Mexico, southern California, and Atlantic coast. MMS' interest is preserving and protecting the environment and geology inareas protecting the environment and geology in areas of petroleum E and P. Itis interested mainly in drilling and production systems, including well-baysafety and fire protection, hazardous-area classification, blowout prevention, subsea equipment design, prevention, subsea equipment design, emergencyshutdown procedures, and platform fire protection. JPT June 1990 P. 764

Methodology for Assessing Reliability of Tension Leg Platform Tethers

The tension leg platform (TLP) is a large, floating offshore platform for producing oil and gas in deep water. The TLP is moored to the ocean floor by vertical tethers. Loads in the tethers have several components that are timevariant and non‐Gaussian. Therefore, conventional methods of reliability analysis typically applied to fixed offshore platforms are not applicable to TLP tethers. This paper presents a method for probabilistic combination of time‐variant (Gaussian and non‐Gaussian) and time‐invariant tether load components. The procedure is applied to a Gulf of Mexico TLP design where the environmental hazard is due to hurricane events. Two modes of failure are considered for the TLP tethers: (1) The maximum lifetime tension exceeds the yield load; and (2) the minimum lifetime tension is less than zero, i.e., the tethers experience the loss of tension. The resulting TLP tether reliability is compared to the calculated reliability of conventional fixed offshore platforms, which form the basis of industry experience.

The Design of Tension Leg Platforms by a Constrained Optimization Method

A method is developed for optimizing the size of a tension leg platform (TLP) and its tethers so as to minimize the weight-based cost of the TLP system subject to various design constraints. The method is intended for preliminary design studies of feasibility and cost sensitivity. Design constraints include minimum tension and maximum tensile stress in the tethers, maximum platform offset under current and waves, and minimum fatigue life of the tethers due to resonant vibration. Other given quantities include deck load, riser pretension, tether properties, and basic results from structural design studies and hydrodynamic analysis or model tests for a specific class of platforms. Example in TLP design studies are presented to illustrate application of the constrained optimization method.

Selection and Evaluation of High-Strength Steel for Hutton TLP Tension Leg Elements

The development of the tension leg platform (TLP) for the Hutton Field in the North Sea represents the first application of this deepwater concept. The use of vertical tension legs as the mooring system is the most novel part of the TLP design. One of the unique features of the tension legs is the use of 92.5-mm (3.64-in.) thick tubulars forged from 3 1/2 percent Ni-Cr-Mo-V high-strength steel (795 MPa (115.3 ksi) minimum yield strength) as mooring elements. Because of the importance of these elements to the survival of the TLP, the chemical composition and the steelmaking procedure were critically examined and optimized. In addition, extensive evaluation of the corrosion fatigue behavior of the steel was undertaken. This paper discusses the basis for the steel selection and specification, the results of the rigorous corrosion fatigue test program, and the assessment of the variation of properties along the length and through the thickness of full-size components. These results illustrate the suitability of the proposed high-strength steel for the mooring system application and establish confidence in both steelmaking and quality control procedures.

Offshore structure design and development

The kinds of technology currently being applied to the design, construction, installation and operation of offshore structures for oil and gas exploration and production are quite sophisticated and include many examples of innovative configurations and approaches. The decade of the 1990s should see further evolution, reinterpretation and improvements of concepts that are already in service or being readied for service. The importance of offshore oil and gas may be judged by the projection that over half of overall exploration investments will go to offshore prospects in future years. This paper surveys some expected evolutions, with particular emphasis on the challenging area of deep-water applications. Some features of a tension leg platform design are discussed as an example of a deep-water oil production system. An attempt is made to recognize the problems of applying advanced engineering and analytical capabilities, when many specialists must interact, to producing a thoroughly engineered design, which is also balanced and economical, for such innovative systems.

Tension-Leg Platform Design: Interaction of Naval Architectural and Structural Design Considerations

As oil exploration and production move into water depths over 1000 ft (305 m), several companies are investigating compliant structures as possible solutions to the economic and engineering problems involved. This paper focuses on the tension-leg type of platform, a positively buoyant structure kept on location by a pretensioned anchoring system. The basic approach to design of a tension-leg platform system, the various components of the system and the interaction between naval architectural and structural design considerations are presented. The author points out that planning for a successful design must include:preparation of good design criteria,determination of variables,evaluation of variables for sensitivity and interaction,accurate and timely engineering decisions in selection of a set of parameters, andparallel studies covering fabrication, transportation and installation requirements. Appendices give the mathematical bases for computing motions of the tenslon-leg platform and graphic illustrations of platform characteristics and the effects of typical exciting forces.