Fatigue is one of the key design considerations for moorings, risers, and umbilicals in offshore floating systems. Due to differences in materials, components, configurations, and functionality, these slender structures have distinct drivers for fatigue performance. On the other hand, they also share a number of the same challenges. Typical slender structure design configurations include taut and catenary mooring systems; steel catenary riser, steel lazy wave riser; and catenary, lazy wave, and pliant wave configurations for umbilicals. This paper introduces the fatigue performance drivers for mooring lines, risers, and umbilicals. The paper also compares the fatigue performance of moorings, risers, and umbilicals, and provide cross-disciplinary guidance to the integrated design of floating systems, thereby helping improve the overall system reliability. Moorings, risers, and umbilicals demonstrate quite distinct fatigue characteristics but share some similar challenges. Mooring fatigue performance is influenced by tension and corrosion. Chains in fairlead locations and on the bottom are the most critical fatigue locations for mooring fatigue. Recent fatigue test data show that mean tension significantly impacts chain fatigue and thus whole mooring system fatigue life. Fatigue is a significant challenge for riser design due to tension and bending. The hangoff and touchdown zones are usually the most fatigue critical areas for risers as well. Similar to the mooring design, corrosion contributes to riser fatigue. Riser configuration, internal fluid slugging, as well as seabed stiffness significantly affect riser fatigue performance. Besides tension and bending, one of the specific features for umbilical design is the interaction between the different elements within an umbilical cross-section, which significantly impacts its life. Since this effect is more significant in regions with higher tension and curvature, the hang off region is usually the critical area for umbilical fatigue. The offshore industry is moving towards integrated design and systematic approaches. Although there is abundant literature discussing the fatigue of mooring, risers, and umbilicals separately, there is very limited published literature comparing the performance of these three types of slender structures. This paper can serve as a useful reference for a floating system integrated design. Cross disciplines can also benefit by leveraging the design philosophy and mitigation strategy of different types of slender structures.

This paper describes developments in the fatigue counter methodology and how digitalization is used to deliver valuable online technical service. The fatigue counter methodology consists of using a combination of autonomous and online motion response sensors directly installed on the riser and interfacing Floating Production Storage and Offloading vessel (FPSO) structures. The measured environmental data, FPSO and riser response data are utilized in a machine learning (ML) environment to build more realistic riser response and fatigue prediction models. The results of this is significant reduction in uncertainties, enabling live riser fatigue predictions and providing a basis for life extension and improved accuracy of riser and vessel response analysis. As there is a need to run some risers at higher pressures, the optimum time periods for such high-pressure service can be found without compromising the flexible riser service life. A recent field case is presented whereby the fatigue counter ingest, process and present data on a modern digital infrastructure. The full service was setup based on available onboard sensors including a 6 degree of freedom (DOF) vessel motion response unit (MRU), temperature and pressure transmitters and forecast model for weather. Input and output data are shared through well documented and safe application program interfaces (APIs) between the operating company and the fatigue counter 3rd party service. The operating company receives live updates of accumulated fatigue damage and remaining service life. This enables the operating company to build contextualized and high value dashboards presented on their organizational front end. The paper illustrates the power and applicability of combining modern numerical methods with digital techniques, made possible by streaming input and output data on safe and well documented APIs.

The concept of macro-element modelling – which was first introduced almost 30 years ago – has proven to be a convenient and accurate technique for modelling offshore foundations, but historically these models have mainly been used for academic purposes. Recent developments in foundation modelling now allow for application of such models in engineering practise and design. One such example is the family of new macro-element models that have been developed in the research project REDWIN to represent the foundation behaviour in dynamic analyses of Offshore Wind Turbines (OWTs). These models exhibit characteristic foundation behaviour such as nonlinearity, coupling of the load from different load components and hysteretic load dependent damping. This paper presents two of the REDWIN models, one applicable for monopile foundations and one for skirted suction caisson foundations. Use of the models are demonstrated through two practical problems that reflect typical design analyses of OWTs: the first example shows a fatigue damage assessment for a monopile, and the second considers an extreme load event for a suction bucket jacket. The structural response is computed using the REDWIN foundation models and compared with the response based on distributed API p-y springs for the monopile and clamped legs at seabed for the jacket. Special emphasis is devoted to how the model input is obtained to guide readers on practical use of the models.

Riser-soil interactions have significant influence on the fatigue assessment of steel catenary risers (SCRs) near the touch down zones (TDZs), and can be critical in realizing a competitive SCR design and in extending a SCR design life. The motivation of the paper is to share the proprietary model test data and research/ design experience accumulated over the past three decades on the riser-soil interactions with the riser engineering community in order to deepen the understanding of this complex problem and to realize economical SCR designs. The objectives are (i) to present a fully nonlinear hysteresis model that captures the loading history of the riser-soil interactions at TDZs in the vertical direction for SCRs, and (ii) to propose a simplified equivalent linear elastic vertical stiffness for SCRs at TDZs for quick fatigue assessments. The paper begins with the mathematical formulation of the nonlinear model and the equivalent linear model for riser-soil interactions. Then the proposed model is verified through the consistent predictions from time-domain and frequency-domain simulations of an SCR from a Tension Leg Platform (TLP), the close agreement between the proposed model predictions and small scale pipe-soil interaction model test results, reduced scale sectional SCR model test results, and the latest study in the literature.

This paper discusses steel catenary riser (SCR) design challenges subject to complex bi-modal and bi-directional seastates offshore West Africa in ultra-deepwater (2,600m). Metocean data is key to determining the design of SCRs. As offshore exploration continues in remote regions and deeper water depths, it is very important to obtain accurate and comprehensive environmental data for the field. Different locations around the world have distinct offshore environments such as hurricanes, typhoons, swells and squalls with seastates exhibiting a varying range of frequency content. The work in this paper describes effects of multiple seastate modes on the riser’s performance. The risers evaluated are 10 inch production and 18 inch export SCRs connected to a spread moored FPSO. Riser fatigue was particularly challenging given the unique environment consisting of up to four wave modes in a single sea-state. Several sensitivities were performed to improve the fatigue performance of the risers. Finally a feasible riser concept was selected which met all of the project design requirements. Work showcased in this paper will address the challenges of placing steel catenary risers in remote deep water fields with relatively unfamiliar metocean conditions. Several sensitivities are performed with the riser properties, platform motions and environmental data which help to reduce design conservatism and better understand the fatigue response of SCRs connected to a spread moored FPSO.

This paper investigates life extension of the risers used for the Hoover DDCV (a classical Spar) in GoM, based on the latest technology and monitored data. Deepwater development in Gulf of Mexico (GoM) started about two decades ago. It is the time to evaluate the global integrity of the riser systems and explore the possibility of life extension in a structured and systematic way. A Deep Draft Caisson Vessel (DDCV), has been keeping up its class services for the past 17 years since it was installed in 2000. It was expected to extend the riser design life by 10 more years. This paper showed the life extension verification process starting from the initial planning, reviewing of the original design document, gathering and analyzing production and offshore measured data and finally to offshore focused underwater inspection. It introduces the methodology to assess the integrity of Steel Catenary Risers (SCRs) and Top Tensioned Risers (TTRs) used for the DDCV. An independent global performance was analyzed by utilizing latest environmental data, as-built information, measured VIM motion data and the latest analytical tools. It computes the riser global performance including dynamic strength and fatigue with the contributions from wave fatigue, VIM and VIV. The factored riser fatigue life is provided with the original safety factors to meet the intended 10 more years of service life.

The objective of the STREAM (Steel Riser Enhanced Analytics using Measurements) JIP is to provide a measurement based foundation for SCR and lazy wave riser modelling to ensure that the fatigue response is assessed with adequate but not overly conservative parameters. To achieve this objective, the JIP utilizes field measurements from 4 in-service SCRs and 1 SLWR in water depths from 3,000 ft to 5,000 ft. The field measurements correspond to a range of environments including hurricanes and loop currents, riser functions, sizes, VIV suppression coverages and host vessels. The processing commences with data QA, error assessment and data filtration. Riser response is categorized into wave dominated events, VIV events and others such as MIV events. As-built finite element riser models are developed and simulations are conducted using measured motions. The resulting analytical responses are compared with the measured motion and strain data to determine the level of conservatism or otherwise in typical riser wave fatigue analysis. SHEAR7 models driven by measured current profiles are used to compare predicted VIV response to observed VIV amplitudes and frequencies. Analysis results indicate that industry standard fatigue assessment is indeed conservative. Sensitivities are conducted and presented on key design parameters that are known to be conservatively used in design such as hydrodynamic coefficients and SHEAR7 inputs. A set of parameters is derived that not only reduces fatigue damage bias but also improves the reliability in predictions. Recommendations are made with regards to further refinement of analysis parameters and understanding of atypical riser responses. Measured riser response that does not conform to typical wave and VIV spectra are presented and discussed. The combined assessment of full scale field data from multiple catenary risers is an industry first. The results from this JIP offer valuable insight into riser response characterization with potential applications for SCR/SLWR life extension and more efficient new designs. Note that in this paper, the term SCR is often used a generic term to describe both the regular SCR as well as the lazy wave implementation of it, the SLWR.

Fatigue damage due to structural stress is a common problem for equipment used in offshore drilling such subsea connectors and subsea riser joints. Fatigue damage is characterized by the weakening of a structure or component of the equipment due to cyclic loading. Consistently operating the equipment above the operational parameters can lead to premature failure of the equipment causing unplanned downtime and posing a safety risk to nearby workers. There are several methods currently being used to determine cumulative fatigue damage as a way of assessing the operational life of machines used for drilling. Linear cumulative fatigue damage analysis is one of the most used methods for life prediction of a structure and components of equipment subjected to cyclic loading. The model involves examining the operational stress ranges caused by cyclic loading and comparing them to an established fatigue curve to estimate the total utilization and predict equipment failure. However, the linear damage rule (Miner's rule) has several limitations namely: The damage model often depends on complex, and time consuming, stress analysis depicting exact geometry and operating conditions. Damage can only be assessed subsequently, making it difficult to forecast use and plan scheduled maintenance. This document presents the development of a mechanical stress soft sensing algorithm for determining real-time cumulative fatigue damage using finite element analysis with response surface methodology. The results in this document show that the new real-time cumulative damage determination approach could effectively help address the limitations of the current models by providing a means of determining real-time cumulative damage with little computational power.

Steel Catenary Riser (SCR) is the most cost effective option for numerous deep water field developments. Despite its structural simplicity, the design and analysis of SCR's can be complex especially due to the hydrodynamic forces. This paper selects three of the unexplained observations in SCR analyses, solves the mysteries analytically, and provides recommendations accordingly for improving riser design. The three selected observations that are not well understood occur at three distinct locations along the SCR: the hang-off, the middle section, and the Touch Down Zone (TDZ). The riser maximum stress at the hang-off can be lower when the wave particle motion is considered in the dynamic simulation compared with using vessel motion time trace only. The TDZ fatigue damage is observed to be lower when background current is applied along riser in the motion fatigue analysis, even if the current direction is perpendicular to the riser motion direction. Finally, the motion fatigue response is observed to be better with a shorter strake length compared with that of the configuration wherein the strake coverage is extended to the seabed. The first two phenomena are generally explained by the "damping" effect, however, how the damping works was never clearly demonstrated. The third observation is actually contradictive to the damping effect as less strake length results in smaller drag forces and thus less damping. All of the above phenomena are related to the hydrodynamic forces that are exerted onto the riser by the surrounding sea water. Those forces are calculated by Morison's equation which includes drag force and inertia forces. In this paper, a detailed analytical study using Morison's equation illustrates how the drag force or inertia forces will affect the riser motion, and which force is the dominating one, for each of the three observations aforementioned. The understanding of these riser hydrodynamic behaviors leads to improved riser design. The applications of wave hydrodynamics and background currents present more realistic riser responses and reduce conservatism and thus cost. A riser with shorter strake length but more fatigue life will result in reduced cost too provided that adequate VIV fatigue life is preserved.

Deepwater gas fields usuallyrequire large diameter risers to provide high flowrate. Steel catenary risers (SCRs) are a feasible solutionwhere the requirement for large diametersrules out the use of flexible risers. To improve fatigue performance and platform payload, SCRsare sometimes fitted withpermanent buoyancy modules, forming a wave shape, and are called Steel Lazy Wave Risers (SLWRs). SLWRs are typically installed by large pipelay vessels using the J-lay method. Some challenges for SLWR installation are the high top tension requirement, as well as offshore welding of large diameter (and often CRA clad) lines on critical path. These factors contribute to the cost and risk of installation. Utilising a towed installation method canreduce these risks, and remove the need for a J-lay vessel. This paper describesthe methodology for towing a SLWR in asingle length using previously established methods for pipeline bundles. A case study based on a large deepwater gas field offshore Australia has been used to show the feasibility of the towed method for large diameter risers. Simulation of the tow was performed with OrcaFlex™ to confirmthat various outputs are within allowable limits of design stress and marine operability. The paper also lists advantages and disadvantages of the towed installation of a SLWR when compared to J-lay.

The paper presents a proposed methodology to evaluate wind-induced fatigue of onshore slender frame structures with lattice-type design. The methodology for fatigue evaluation is based on a similar offshore industry design practice and has been repurposed for large onshore structures fabricated from tubular pipes, square tubes, and angle bars exposed to wind loading, including wind buffeting loads and wind vortex shedding. Multiple cracks resulting from wind-induced fatigue have been experienced on offshore installed flare booms and flare towers. Consequently, design codes were introduced, and it has been a standard design practice to evaluate potential fatigue damage of slender frame structures due to vibrations induced by vortex shedding, as well as buffeting loads due to gusting. This is, however, not yet standard design practice for land-based slender frame structures despite obvious similarities in terms of structural configuration, size, wind loads, and consequences of failure. Some operators have internal requirements to assess these operational risks. In this study, wind-induced fatigue design and analysis methodologies have been developed and calibrated based on offshore design practices. The proposed methodology uses a spectral analysis method that considers wind data, and includes simplified design checks as well as advanced analysis approaches. Further comparisons are presented for structural configuration and various end connection types with relevant examples. Both welded and gusset-type bolted connection of tubular members are investigated. This paper provides designers with methodology guidance to calculate structural fatigue damage due to both wind buffeting loads and vortex shedding vibrations. Key design considerations for wind fatigue analysis, typical design solutions and recommendations for improvements of fatigue life, and practical ways for the global fatigue screening are also provided.

An effective Structural Integrity Management (SIM) program allows operators to manage structural risks and reduce the need for hazardous and costly underwater interventions. Inspection and operating decisions and costs are significantly affected by the SIM program definition and execution. This is particularly true for unique, deepwater assets like the Baldpate compliant tower, which features an innovative structural system and is among the tallest structures ever built. This valuable deepwater asset requires a well-executed SIM program to provide confidence that continued service and potentially expanded operations can be safely achieved. Earlier studies had shown that fatigue sensitivities at critical locations could potentially limit Baldpate's useful life beyond the original 20 year target. In order to better understand the characteristics of this unique structure, a series of reassessment analyses were performed. These showed that fatigue sensitivities are not limiting factors for continued service of Baldpate, and the areas of fatigue sensitivity were incorporated into a long term SIM plan to focus future inspections and monitoring efforts. This paper provides details on the challenges facing the execution of the SIM plan, including benchmarking and verification performed for the Baldpate structure using field measurements, implementation of a real-time monitoring system, and targeted underwater inspections. The dynamic behavior of axial tube guides, modeled using gap elements and surface friction, was identified as one of the more critical objectives to be addressed in the SIM plan. This posed a challenge since previous design and assessment efforts did not consider these components critical for global response. SIM options for these components were included in the plan, such as underwater inspections and on-site dynamic motion recording to examine the potentially undesirable behavior of these components. Undertaking these tasks led to affirmation of the structural reassessment results and facilitated development of new SIM resources. In particular, benchmarking the monitoring system provided Baldpate with constant surveillance of its condition. This real-time monitoring plan measures the dynamic characteristics of the tower under different load conditions to indicate when certain unique structural characteristics (e.g., axial tube motion within the guides) would affect the fatigue life of critical joints in the Baldpate tower.

In-field riser strain or motion monitoring provides direct indication of riser fatigue and strength performance for prognostic integrity management. However, direct riser monitoring in the field has been limited in offshore developments to date with respect to its use as a standard on every floating platform and on every riser system attached to the vessel. This is due to the high perceived capital or operational costs of direct subsea monitoring when deployed on every riser to reliably monitor over the entire service life. Furthermore, monitoring devices are typically installed at practical locations, which may not be at fatigue critical areas. A lack of known response in service can result in qualitative inspection planning, conservative fatigue predictions and reduced asset utilization. Data-driven virtual sensors provide an innovative solution for life of field riser monitoring and predictive inspection planning. The virtual sensor is a machine learning model that calculates stresses in the time domain at the fatigue hot spots and at high stress locations along the riser. The sensor is driven by platform motions and environmental loads that are typically measured in the field. It allows calculation of fatigue in near real-time using time synchronous data. The virtual sensor system can be integrated with a topside or centralized data platform. The vision for the virtual sensor is to train a machine learning model initially with finite element global riser analysis data and subsequently with full scale field data from the riser in service. Data aggregation from a term and deployment limited riser monitoring system is employed to capture system-specific response. This hybrid strategy results in low cost virtual sensors for life of field riser monitoring. This paper describes the motivation, methodology, validation and applications for riser virtual sensors. A framework for the development of a riser system machine learning model is described. Global response data is obtained from a vertical top tensioned riser operating in a water depth of 5,000 ft. Fatigue predictions are compared against the fatigue damage calculated from finite element analysis data.

Piping systems having flow discontinuities like bends, tees, reducers and valves can experience acoustic induced vibration (AIV) or flow induced vibration (FIV) due to internal pressure fluctuations (acoustic or turbulent flow sources of excitation). The excitation may range from 20 Hz to 5000 Hz depending on the pipe geometry and flow characteristics. At such frequencies, excessive vibration can cause fatigue-induced failure of the piping system if not designed properly. In some extreme cases, failure may occur in a matter of days or even hours. This paper proposes a practical frequency domain based finite element analysis (FEA) approach/design-tool to calculate the fatigue damage of piping systems due to FIV or AIV. For a FIV analysis, the two key steps that can be identified in this approach are: (1) the development of a transfer function, and (2) the generation of a pressure fluctuation spectrum over the frequency range of interest. The transfer function defines the relationship between the stress range (∆σ) and a unit pressure fluctuation (∆p) that is used to calculate the fatigue damage. It is assumed that the stress range reaches a maximum when the internal acoustic pressure spatial distribution completely coincides with the natural mode shape of the pipe system. Harmonic analysis is performed to obtain the maximum stress range at each natural frequency of the piping system. From the transfer function and pressure fluctuation spectrum, a stress range spectrum can be calculated. Assuming a Gaussian fluctuating pressure distribution, a Rayleigh distribution is expected for the stress range. The fatigue damage can be calculated using a closed-form solution with an associated S-N curve. A typical tee-joint in a pipe spool will be examined to illustrate the procedure. The proposed procedure can be modified for an AIV analysis specifically, which is not presented here.

Wellhead fatigue monitoring was performed during Plug and Abandonment (P&A) activities of a well in the Gulf of Mexico (GOM). Fatigue monitoring was deemed necessary for several reasons: an older vintage of wellhead system with uncertain fatigue life consumption from previous drilling activities, excessive wellhead stick-up, potentially large rig motions from a moored drilling rig for P&A. Fatigue monitoring was performed to ensure the integrity of the wellhead during P&A operations. The wellhead fatigue monitoring methods previously developed by several of the coauthors was applied in the wave-dominated GOM environment. Motions of the Blow Out Preventer(BOP) stack were measured and combined with a model of the riser/BOP/wellhead/casing system to reconstruct fatigue damage in the wellhead, conductor and surface casing. To measure wellhead motions, Subsea Vibration Data Loggers (SVDLs) were run with the riser and retrieved via Remotely Operated Vehicle (ROV). Fatigue damage reconstruction in the wellhead, conductor and surface casing was performed directly using the measured motion data and an analytical transfer functions obtained from the calibrated Finite Element (FE) model. Results were provided with fast turnaround time to support operations. Results demonstrated that fatigue damage rates compared well with pre-deployment predictions, though measured fatigue demand was slightly higher. It was also demonstrated that the primary cause of fatigue damage was due to wave activity at the site. Analysis of low frequency response at the riser natural frequencies indicated that the first few riser modes may have been excited by currents. It was also demonstrated that the low frequency response did not contribute significantly to fatigue life. The wellhead monitoring methods discussed result in rapid turn-around of valuable fatigue life consumption information, enabling informed decisions to be made in challenging conditions. The monitoring instrumentation and fatigue analysis methods increase the safety and efficiency of drilling, workover and P&A activities. In a larger sense, measured data also serves as a benchmark for analytical model calibration activities, reducing the known conservatism in stress and fatigue in future deployments.

Steel catenary risers (SCRs) are a cost effective option for deepwater developments. Throughout industry hundreds of SCRs have been deployed and have proven to be highly reliable. Nevertheless, as the inventory of SCRs across industry ages and as operators seek to extend the lives of deepwater platforms beyond their originally intended service life, strategies are needed to ensure that SCRs can continue to operate reliably. Since touchdown point fatigue damage is a strong factor determining the service life of an SCR, strategies to mitigate fatigue damage in this region may be required for service life extension. One such strategy commonly adopted is platform relocation in order to shift the SCR touch down point throughout the life of the field and hence spreading the fatigue damage along the riser touch down region; however, this typically needs to be planned for in the original concept development. In this paper, two additional strategies based on modifying the shape of an existing SCR are presented. These strategies reduce the fatigue damage rate in the original touchdown point (TDP) region by managing the shape of an existing SCR either through adding buoyancy or by changing the length of the riser such that the original segment of riser pipe in the TDP region is moved to a low stress region. The proposed techniques also accommodate riser response during extreme environmental events. The techniques have been examined based on case studies of a production SCR and an export SCR. Trade-offs in placement of buoyancy and the impact that shape change has on meeting riser strength requirements are presented. It is shown that the potential exists to roughly double the computed fatigue life in the touchdown region.

Awareness of the challenge with lower available Wellhead fatigue life has increased as the Oil and gas industry has gained more knowledge through advanced analysis and tests. Loads on Wellhead are influenced by movements from drilling rig transferred through Marine Riser and BOP. These loads cause cyclical bending moments and Wellhead fatigue degradation. The intention of Reactive Flex-Joint (RFJ) is to lower the fatigue damage by reducing cyclical bending moment, which is a main contributor for Wellhead fatigue. It's was built a technology demonstrator (scale 1:25) in 2006 and patent of the system was approved. Later a full-scale workshop test model was constructed including a Flex-Joint (FJ) for simulation of environmental conditions. In addition there were preformed thousands of transient dynamic simulations with irregular sea conditions. An independent company duplicated these analyses and all results are verified by 3rd party. The 3rd party company also approved product qualification and use before offshore deployment. An environmental test through a complete drilling campaign of a new well was preformed spring 2016, verified the technology and efficiency covering a wide variety of environmental conditions. The BOP was instrumented to monitor live bending moments and inclination with and without the RFJ. More than 50 subsea tests proved 55 - 65% reduction of cyclical bending moment meaning 10 - 15 times extension of the remaining Wellhead fatigue life. The conclusion is that a modified FJ can reduce dynamic bending moments transferred from Riser to Wellhead during drilling and Work-Over operations. Reduced fatigue exposure gives increased Well access, improved Well Integrity and significant extended Wellhead fatigue life. The reduction of cyclical bending moments on Wellhead give extension of available days for connection, enabling additional Workover and/or side track drilling resulting in Increased Oil Recovery.

The role of soil response on fatigue life of Steel Catenary Risers (SCRs) has been investigated through a scaled sectional SCR (cut-out section) in a geotechnical centrifuge. The SCR model simulated a section of the SCR starting at 16 ft (4.9 m) above the seafloor and extending approximately 360 ft (~110 m) through the touchdown region to the anchor point. The centrifuge tests were successfully performed using soils from the Gulf of Mexico (GoM) and offshore Angola. Past studies have demonstrated that fatigue damage in a full-scale SCR at touchdown zone can accurately be modeled using a scaled sectional SCR in a geotechnical centrifuge. The centrifuge results were compared to finite element simulations with the seabed response modeled using nonlinear springs derived based on unload-reload stiffness (secant stiffness) of degraded soil at steady-state conditions. This resulted in development of a simplified approach involving the nonlinear soil springs. Fatigue analyses indicated that this simplified approach can predict fatigue damage as measured in the SCR centrifuge tests. Centrifuge tests were also performed to determine the impact of trench depth on SCR fatigue. The tests involved a flat seabed and seabed with an initial prescribed trench. SCR fatigue damage in the test with an initial prescribed trench was similar to that of the flat seabed; however, the critical fatigue point had significantly shifted.

Drilling risers are regularly deployed in depths beyond 5000 ft with large sections covered in buoyancy. The smooth cylindrical shape of buoyancy modules can result in significant VIV response, causing overall amplification of drag experienced by the riser. In turn, operations can be suspended due to drag effects on top and bottom angles; high current speeds can lead to a halt in operations, even complete disconnection and retrieval of the riser string. Although suppression technologies exist to reduce VIV response (helical strakes/fairings), these have not been integrated into the manufacture of buoyancy modules for practical reasons (stackability, ease of manufacturing/assembly/installation). Innovative Longitudinally Grooved Suppression (LGS) technology has recently been developed and tested with two main bodies of work undertaken; model testing and full scale numerical modelling. This paper covers the methodology and results of full-scale OrcaFlex/SHEAR7 simulation work performed on drilling risers in realistic current profiles for the Gulf of Mexico (GoM), concluding with the impact this technology could have in reducing costs by increased operability for drilling risers. LGS technology provides significant reduction in VIV response whilst maintaining similar buoyancy to conventional modules. Significant reduction in VIV response has multiple benefits: less drag, reduced fatigue damage, and an increase in maximum currents drilling risers can be operated in - without any moving parts or extra deployment installation time. High Reynolds Number model testing results showed LGS technology behaved favorably in observable repeatable conditions, whilst the full scale numerical modeling demonstrated how these parameters measured in model testing can be used to perform numerical modelling of full scale offshore drilling risers in a variety of water depths and conditions; improving operability and riser fatigue life. Operability during GoM eddy current events was calculated to increase 30% annually, equating to estimated annual savings of over $10M, with potential to save similar amounts within a short period in the case of extreme current events. By applying LGS technology, significant financial savings can be achieved through increased operability and increased fatigue lifetimes.

Subsea equipment design verification and validation are to be performed by analysis and testing, respectively. Two of the most important aspects of this process for high-pressure, high-temperature (HPHT) applications are load estimation and fatigue assessment. This paper presents dynamic loading scenarios typically considered in the design of subsea wellhead in HPHT applications during service life as well as load estimation of these scenarios, particularly on riser loads. Fatigue damage to the subsea wellhead is also included to examine the significance and effect of the loads from each of the loading scenarios. Dynamic loading scenarios in drilling and production phases are evaluated in the paper. In the drilling phase, typical loading scenarios for the riser in connected conditions with wellhead are evaluated. A typical drilling system, generic sea state scatter diagram, soil condition and operational profile are selected to estimate the dynamic loads for each of the scenarios. In the production phase, typical loading scenarios are also evaluated. A typical subsea production system with the same sea and soil condition as drilling but with different operational profile is selected to estimate the dynamic loads for each of these scenarios. A typical wellhead system including wellhead housing, conductor, and casing system is selected for the riser load estimation and the wellhead fatigue assessment using fracture mechanics method is carried out in accordance with API 17TR8. Dynamic loads estimated from the scenarios in drilling and production phases are compared at the top of the wellhead in terms of maximum load and load histogram. The significance and effect of these loading scenarios are also evaluated from the fatigue standpoint at select hotspots on the wellhead system. From the study, it is observed that loading from drilling and production phases are equally important and both should be considered to verify the subsea wellhead's structural capability. In the paper, an analysis procedure to evaluate the riser load to the wellhead and fatigue assessment of the wellhead is proposed, which may be used for the design verification of subsea wellhead in HPHT applications.