Extreme Metocean Conditions Research Articles

Modeling and observations of North Atlantic cyclones: Implications for U.S. Offshore wind energy

To meet the Biden-Harris administration's goal of deploying 30 GW of offshore wind power by 2030 and 110 GW by 2050, expansion of wind energy into U.S. territorial waters prone to tropical cyclones (TCs) and extratropical cyclones (ETCs) is essential. This requires a deeper understanding of cyclone-related risks and the development of robust, resilient offshore wind energy systems. This paper provides a comprehensive review of state-of-the-science measurement and modeling capabilities for studying TCs and ETCs, and their impacts across various spatial and temporal scales. We explore measurement capabilities for environments influenced by TCs and ETCs, including near-surface and vertical profiles of critical variables that characterize these cyclones. The capabilities and limitations of Earth system and mesoscale models are assessed for their effectiveness in capturing atmosphere–ocean–wave interactions that influence TC/ETC-induced risks under a changing climate. Additionally, we discuss microscale modeling capabilities designed to bridge scale gaps from the weather scale (a few kilometers) to the turbine scale (dozens to a few meters). We also review machine learning (ML)-based, data-driven models for simulating TC/ETC events at both weather and wind turbine scales. Special attention is given to extreme metocean conditions like extreme wind gusts, rapid wind direction changes, and high waves, which pose threats to offshore wind energy infrastructure. Finally, the paper outlines the research challenges and future directions needed to enhance the resilience and design of next-generation offshore wind turbines against extreme weather conditions.

Storm Franz: Societal and energy impacts in northwest Europe on 11–12 January 2007

Abstract. January 2007 was a bad storm month for much of central and northern Europe with a series of extratropical cyclones bringing high winds and precipitation to highly populated areas between Ireland and Russia. Although Storm Kyrill on 18–19 January 2007 was the most serious for its infrastructure damage and insurance costs, Storm Franz from the preceding week on 11–12 January 2007 was actually more serious for its maritime impacts in western Europe. This contribution takes a closer look at Storm Franz with an overview of its wind field and its impact on energy infrastructure, transportation networks and building damage. Maritime casualties are reviewed with respect to met-ocean conditions. The storm was notable for a series of wave-related accidents off southeast Ireland, the English Channel, and German Bight. An analysis is carried out on water level recorders around the North Sea to assess the storm surge and short period oscillations that may reveal harbour seiches or meteotsunamis. The results are compared with wave recorders, which had a fairly good coverage across the North Sea in 2007. The issue of wave damage to offshore infrastructure was highlighted in events associated with Storm Britta on 31 October–1 November 2006. Offshore wind energy in northwest Europe was in a growth phase during this time, and there were questions about the extreme met-ocean conditions that could be expected in the 20 year lifetime of an offshore wind turbine.

A Hierarchical Met-Ocean Data Selection Model for Fast O&M Simulation in Offshore Renewable Energy Systems

In this research, a hierarchical met-ocean data selection model is proposed to reduce the computational cost in stochastic simulation of operation and maintenance (O&M) and enable rapid evaluation of offshore renewable energy systems. The proposed model identifies the most representative data for each calendar month from the long-term historical met-ocean data in two steps, namely the preselection and the refined selection. The preselection incorporates three distinct metrics to evaluate the characteristics of statistical distributions, including the Jensen–Shannon divergence, the encapsulation of extreme met-ocean conditions, as well as the overall vessel accessibility. For the refined selection, a component of temporal synchrony is devised to emulate dynamic changes of met-ocean conditions. As such, a met-ocean reference year comprising twelve representative historical months is subsequently produced and deployed as the input for O&M stochastic simulation. While this research focuses on the development of a generalised methodology for selecting representative met-ocean data, the proposed statistical method is validated empirically using a case study inspired by real-life floating offshore wind installations in Scotland, e.g., Hywind and Kincardine projects. According to the O&M simulation results with five capacity scenarios, the proposed data selection model reduces the computational cost by up to 97.65% while emulating the original results with minor deviations, i.e., within ±5%. The simulation speed is therefore 43 times quicker. Overall, the proposed met-ocean data selection model attains an excellent trade off between computational efficiency and accuracy in O&M stochastic simulation.

Application and improvement of iterative methodology for response based analysis of an FPSO mooring system in tropical cyclones

Response based analysis (RBA) is an advanced alternative to traditional design approach for mooring systems of floating structures, which reduces uncertainty associated with extreme responses. In the traditional approach, mooring analysis is performed for specified sets of extreme metocean conditions, representing typically a return period of 100 years. In RBA, the dynamic analysis is carried out over a long-term database of metocean conditions and statistics of the responses are used to identify responses associated with a return period of 100 years. Whilst the RBA has a more rigorous probabilistic basis, it is also computationally demanding. An “Iterative methodology” has been developed recently, comprising a combination of time domain (TD), frequency domain (FD) and probabilistic analyses of the structural system into a single iterative process, to minimise the number of time-consuming simulations through distinguishing between “mild” and “severe” response events. In the study, the “iterative methodology” is applied for RBA of the mooring system of a turret moored FPSO in the tropical cyclone environment of the North West Shelf of Australia. In the first stage, the time-efficient FD analysis is utilised to identify the contribution of all the storms to the 100-year response. In the second stage, an iteration process is set up, in which only the most contributing response events (tropical cyclones) and sea states are analysed by TD simulations. The iterations proceed by selecting the next most contributing storm until the convergence of the 100-year response is achieved. Application of the iterative methodology leads to significant reduction in computational time and makes RBA feasible as a part of the usual design process by using fully coupled TD analysis. Further optimisation can be achieved through a new method of applying thresholds to metocean parameters, which can reduce the analysis effort further. In the case study considered, it is found that 61% of metocean time history could be excluded from any dynamic analysis by applying a variable storm-dependent threshold. Among the remaining intervals, only 5.2% need to be re-analysed by TD simulations. Several methods for assessing errors in the convergence process are compared, and a sensitivity analysis on the number of storms that should be selected at each iteration step is performed.

Directional dependence of extreme metocean conditions for analysis and design of marine structures

Marine structures are typically sensitive to the direction of wind and waves, especially in extreme metocean conditions. The extreme metocean conditions and their associated predicted directions are not easily reachable from traditional design methodologies. In this research, the most probable combinations of different extreme metocean conditions along with their associated direction are predicted for the HyWind Scotland wind farm, Scotland. To achieve this, the Hierarchical Bayesian Modeling approach is applied to define the Joint Probability Distribution Function (JPDF) of four combinations of metocean parameters, including wave direction, wind direction and wind-wave misalignment. The data is provided by the ERA-Interim dataset in 40 years (1979–2018). The JPDFs are composed of a marginal PDF of directional variables (a mixture of von-Mises Fischer distributions) and two conditional JPDFs which are defined to satisfy the periodicity and positivity of distribution parameters. Then, applying the Inverse First-Order Reliability Method (IFORM) to the JPDFs, the Environmental Contours (ECs) for four sets of metocean data are developed. The results show that extreme values obtained from ECs, including directional variables, are higher than the values of traditional linear ECs. The maximum 50-year extreme value of wind speed from the JPDF of wind direction, wind speed and wave height is 2 m/s higher than the same extreme extracted from the JPDF of wind speed, wave height and period. Another important observed point is that the direction at which the extreme of metocean parameters occurs is quite different from their dominant direction of wind rose or the most probable direction of their probability density function. According to the results, it seems for direction-dependent structures; the application of this method may lead to a more realistic presentation of joint occurrence of linear and directional metocean parameters.

극한 해양환경에서 생성된 파 에너지 성분이 부유식 해상풍력 구조물 거동에 미치는 영향

극한 해양환경에서 생성된 파 에너지 성분이 부유식 해상풍력 구조물 거동에 미치는 영향

Site-specific ultimate limit state fragility of offshore wind turbines on monopile substructures

Assessing the risk posed by severe storms to offshore wind turbines (OWTs) is a challenging task. Stochastic environmental conditions represent the main source of variable loading; consequently, a high level of uncertainty is associated with assessing structural demands on OWT structures. Failure of any of the primary structural components implies both complete loss of the OWT and loss of earnings associated with production stoppage. In this paper, we propose the use of a probabilistic risk modelling framework to assess the structural risk posed by extreme weather conditions to OWTs. To achieve this, fragility functions are developed for OWTs on monopile foundations exposed to extreme metocean conditions using dynamic aero-elastic simulations. Structural fragility represents a key component of any probabilistic risk model and expresses the likelihood of different levels of damage experienced by an OWT over a range of wind and wave hazard intensities. We compare the effect of various modelling and analysis choices on the obtained fragility functions and investigate potential interdependencies between failure modes of OWT structural components. Results from this study highlight how different assumptions affect the estimated structural performance and the resulting structural fragility of a case-study OWT. We apply the proposed framework to two case-study sites, one in the USA East Coast and one in the North Sea, discussing possible outcomes of the proposed framework.

Brief review on the limit state function of dynamic scour protections

Offshore foundations, namely for offshore wind, wave and tidal applications, often require the use of scour protections. Rip-rap scour protections are an important element of the foundation to ensure that the natural frequency stays within the design limits. Scour protection design still presents a remarkable empirical nature, which typically leads to uncertainty on their behaviour under extreme met-ocean conditions. Therefore, reliability assessment of scour protections has been seen as a possibility to account for design uncertainty and to optimise the scour protections. However, the definition of a suitable limit state function is still a matter of research focus, namely, regarding the proper definition of the acceptable damage level for dynamic scour protections. This research provides a brief review on the recent studies related to both the limit state function and the calculation of damage numbers through bathymetric data. A discussion is raised on how the methodologies for calculating the damage number may influence the limit state function and a theoretical example is given to assess the effects on the probability of failure. Results have shown that the acceptable damage number requires a clearer definition, which should be based on the number of layers of rock material and the area of filter exposure. In addition, this research highlights the need for alternative ways to assess damage.

Innovative Design and Execution Lead to Successful Grand Banks Platform Operations

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28803, “Hebron Platform: Innovative Design and Efficient Execution,” by Widianto, Justin Chichester, Adel Younan, and Jameel Khalifa, ExxonMobil; Krishna Komperla, WorleyParsons; and Knut Bidne, Kvaerner, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission. The Hebron platform was successfully installed on the Grand Banks offshore Newfoundland and Labrador in June 2017. It consists of a single-shaft concrete gravity-based structure (GBS) supporting an integrated drilling and production topsides. The design of the platform was challenged by sub-Arctic and extreme metocean conditions that required innovative design and layout approaches for many elements considered routine for typical platforms. This complete paper highlights the underlying technologies, analytical and design methods, and capital-efficient execution strategies employed. Introduction The Hebron platform comprises a GBS and topsides installed in 93-m water depth. Produced crude oil is stored in storage cells and pumped to shuttle tankers by an offshore loading system. The topsides facilities (operating weight 65,000 tonnes) include the following major modules: Utilities and process module (UPM) Derrick equipment set (DES) Drilling support module (DSM) Ancillaries (flare boom, east and west liftboat stations) Living quarters (LQ) designed to accommodate 220 personnel during steady-state operations The total height of the platform with GBS is approximately 235 m, topsides length is approximately 183 m, and the width is approximately 75 m. The GBS concept includes a single shaft supporting the topsides and encompasses all wells in the initial development. It is designed to withstand sea ice, icebergs, and other meteorological and oceanographic conditions at the offshore Hebron site. The GBS is designed for an in-service life of 50 or more years to support future developments. The lower portion of the GBS up to an elevation of approximately 27 m was constructed in a drydock created by building a bund wall and dewatering the site behind it. Subsequently, the dock was flooded, the bund wall was removed, and the GBS base was towed approximately 3 km to a deepwater site. The floating GBS was held in place with mooring lines while the remaining construction was completed. All walls were constructed using the slip-forming technique. The topsides structure was fabricated in modules at various locations in Newfoundland and Labrador and South Korea. Two of the topsides modules, UPM and DES, were fabricated in South Korea using block/pancake construction methods (Fig. 1). After UPM and DES fabrication was completed, the modules were loaded out separately to heavy vessels for transportation to Bull Arm, Newfoundland and Labrador. All other topsides modules were integrated with the main UPM module at the finger pier in Bull Arm and the completed topsides structure was mated with the GBS while floating at the deepwater site. The mated platform then was towed offshore and installed at the final offshore location.

Waves and Swells in High Wind and Extreme Fetches, Measurements in the Southern Ocean

The generation and evolution of ocean waves by wind is one of the most complex phenomena in geophysics, and is of great practical significance. Predictive capabilities of respective wave models, however, are impaired by lack of field in situ observations, particularly in extreme Metocean conditions. The paper outlines and highlights important gaps in understanding the Metocean processes and suggests a major observational program in the Southern Ocean. This large, but poorly investigated part of the World Ocean is home to extreme weather around the year. The observational network would include distributed system of buoys (drifting and stationary) and autonomous surface vehicles (ASV), intended for measurements of waves and air-sea fluxes in the Southern Ocean. It would help to resolve the issues of limiting fetches, extreme Extra-Tropical cyclones, swell propagation and attenuation, wave-current interactions, and address the topics of wave-induced dispersal of floating objects, wave-ice interactions in the Marginal Ice Zone, Metocean climatology and its connection with the global climate.

Multivariate analysis of extreme metocean conditions for offshore wind turbines

Most offshore wind turbines (OWTs) are designed according to the international standard IEC 61400-3 which requires consideration of several design load cases under 50-year extreme storm conditions during which the wind turbine is not operational (i.e. the rotor is parked and blades are feathered). Each of these load cases depends on combinations of at least three jointly distributed metocean parameters, the mean wind speed, the significant wave height, and the peak spectral period. In practice, these variables are commonly estimated for the 50-year extreme storm using a simple but coarse method, wherein 50-year values of wind speed and wave height are calculated independently and combined with a range of peak spectral period conditioned on the 50-year wave height. The IEC Standard does not provide detailed guidance on how to calculate the appropriate range of peak spectral period. Given the varying correlation of these parameters from site-to-site, this approach is clearly an approximation which is assumed to overestimate structural loads since wind and wave are combined without regard to their correlation. In this paper, we introduce an alternative multivariate method for assessing extreme storm conditions. The method is based on the Nataf model and the Inverse First Order Reliability Method (IFORM) and uses measurements or hindcasts of wind speed, wave height and peak spectral period to estimate an environmental surface which defines combinations of these parameters with a particular recurrence period. The method is illustrated using three sites along the U.S. Atlantic coast near Maine, Delaware and Georgia. Mudline moments are calculated using this new multivariate method for a hypothetical 5MW OWT supported by a monopile and compared with mudline moments calculated using simpler univariate approaches. The results of the comparison highlight the importance of selecting an appropriate range of the peak spectral period when using the simpler univariate approaches.

Finite Element Analysis of Guyed Offshore Monotower Subjected to Extreme Environment in Malaysian Water

This paper presents the finite element structural sensitivity analysis of a cable guyed monotower known as the Tarpon Monopod when subjected to extreme environments in Malaysian waters. A hydrodynamic loading and static platform response analysis is performed in SACS v5.3 to gauge the structural robustness in extreme Malaysian metocean conditions. A Stokes Fifth Order Wave Theory was employed to obtain wave kinematics and dynamics for load computation. The Tarpon Monopod design is reviewed generically. An actual platform located in 60m water depth within Malaysian waters is modelled for analysis. Four different guying cable scenarios are considered which are the fully guyed condition (three guy cables pinned), two guy cables condition (one wire loss), one guy cable condition (two wire loss) and free standing condition (total loss of guy wires) are presented. The environmental load sets are simulated at different headings using 45 degree steps. The results suggest that the structural caisson contributes little to the lateral stiffness of the platform. The Tarpon Monopod has little structural redundancy and its integrity is highly dependent on guy wire condition and environmental load headings.

Development planning of deepwater gas fields: the application of floating production platforms

Producing North West Australia (NWA) deepwater hydrocarbon reserves, particularly gas reserves to LNG plants, poses unique challenges. These include extreme metocean conditions, unique geotechnical conditions, long distances to infrastructure and LNG plants, as well as high reliability/availability of supply. This extended abstract addresses important technical, commercial, and regulatory factors that drive the field development planning, including the selection of suitable production facilities for these deepwater hydrocarbon developments off NWA. While all-subsea developments have been an inspiration for offshore engineers for a few decades, subsea gas compression, dehydration, power supply, and control are still technically and commercially demanding, especially for long distance tie-backs. Subsea well intervention and facility maintenance requirements also favour the application of dedicated floating platforms. A wet or dry-tree floating production platform, therefore, is required in most cases. Whereas Semisubmersible, TLP, Spar, FPSO, and FLNG (or LNG FPSO) designs all have the attributes to be a host gas production facility or a part of a production system, only oil FPSOs have been installed in this region to date. Linkages between key reservoir and fluid characteristics and surface facility functionalities are discussed in this extended abstract. Advantages and disadvantages of various platform designs are compared. A focus is on the influence of regional drivers and site characteristics, in particular, metocean and geotechnical conditions and remoteness of the NWA fields. The differentiation between oil and gas developments are addressed. It is emphasised that platform applicability and compatibility should be assessed in the context of field development planning for individual projects to achieve optimum risked life cycle financial values.

Techbits: Mumbai Workshop Explores Deepwater Strategies

Techbits Recognizing the growing potential of deepwater developments off the coast of India, SPE hosted its second deepwater-focused applied technology workshop (ATW) in Mumbai in less than 2 years. The ATW, “Unlocking Deepwater Potential: Strategies and Technologies,” brought together 131 delegates from 11 countries and 40 companies to share information and help Indian companies assimilate the knowledge they will need to monetize India’s deepwater assets. Sudhir Vasudeva, director (Offshore)-ONGC and ATW Steering Committee chair, opened the ATW with an address describing India’s deepwater potential. The country’s deepwater regions hold approximately 51 billion bbl of oil and gas equivalent in a sedimentary area spanning 1.35 million km2. Thus far 31 hydrocarbon discoveries have been made—26 gas discoveries and the remaining 5 oil and gas discoveries. The discoveries are spread over Mahanadi, Krishna-Godavari, and Cauvery basins off the east coast of India, which is poised to be a gas hub of Asia. Vasudeva projected that given India’s gross-domestic-product growth rate of 8–9%, hydrocarbon requirements will increase from the current level of 125 million tonnes of oil equivalent (toe) to 435 million toe in the next 25 years. Global energy consumption is expected to grow by 50% over the same period, putting the onus on the oil and gas industry to find more reserves and curb production declines in existing assets. After 2012, deep water is the only sector which is expected to grow continuously to meet burgeoning global demand. R.S. Sharma of ONGC next stressed that because 43% of India’s sedimentary basins lie in deep and ultradeep water, the E&P industry has ample exploration opportunities here. However, he acknowledged that the pace of technological progress is sometimes too rapid for operators, who need to have a great deal of confidence and assurance in technologies prior to bidding for deepwater blocks. He stated that critical areas for study going forward include drilling and completions; subsea facilities; and health, safety, and environmental factors. Pushing Boundaries The first technical session provided case histories highlighting two recent deepwater developments from different parts of the world. K.V.K. Prasad of Shell Technology India first presented the development strategy for Shell’s Perdido oil field in the US Gulf of Mexico (GOM). Operational and subsurface challenges included extreme water depth (8,000 ft at the platform), extreme metocean conditions, rugged sea floor, flow assurance, a high-risk reservoir (the first Paleogene production in the GOM), low pressure, and high gas-to-oil ratio.