The first issue of the DNV Offshore Standard, DNV-OS-J103 Design of Floating Wind Turbine Structures , was published in June 2013. The standard was based on a joint industry effort with representatives from manufacturers, developers, utility companies and certifying bodies from Europe, Asia and the US. The standard represented a condensation of all relevant requirements for floaters in existing DNV standards for the offshore oil and gas industry which were considered relevant also for offshore floating structures for support of wind turbines, supplemented by necessary adaptation to the wind turbine application. The development of the standard capitalized much on experience from development projects going on at the time, in particular the Hywind spar off the coast of western Norway, the WindFloat off the coast of Portugal and the Pelastar TLP concept. In July 2018, DNV GL published a revision of DNV-OS-J103 as a part of the harmonization of the DNV GL codes for the wind turbine industry after the merger between Det Norske Veritas (DNV) and Germanischer Lloyd (GL) in the fall of 2013. The standard was re-issued as DNVGL-ST-0119 Floating wind turbine structures . This new revision reflects the experience gained after the first issue in 2013 as well as the current trends within the industry. Since 2013, numerous guidelines addressing the design of floating structures for offshore wind turbines have been published by various certifying bodies, and an IEC technical specification on the subject is under way. In addition, several prototypes have been installed and the first small array of floating wind turbines, Hywind Scotland pilot park, are currently in operation. The most important updates in the revision of the standard include formulation of floater-specific load cases, requirements to be fulfilled to support the exemption for design of unmanned floaters with damage stability, and replacement of current consequence-class based requirements for design fatigue factors with low-consequence based factors dependent on the accessibility for inspection and repair, the aim being a safety level against fatigue similar to that which is currently targeted for bottom-fixed structures. Other topics which have been considered in the revision are the floater motion control system and its possible integration with the control and protection system for the wind turbine, the issue of how to deal with slack in tendons in the station keeping system, corrosion, anchor design and power cable design. In parallel to the revision of the standard, a new service specification for certification of floating wind turbines has been developed by DNV GL, identified as DNVGL-SE-0422 Certification of floating wind turbines . For technical requirements, the service specification refers to the revised standard, DNVGL-ST-0119. The technical paper summarizes the updates and changes in the revised standard, in addition to the content of the new service specification.

A Tension Leg Platform supporting a wind turbine was tested in combined waves, wind and current in the Offshore Basin of MARIN. The wind turbine loads were simulated in two ways for these tests. Firstly, the wind turbine was modelled by a physical wind turbine following the approach of performance scaling. This approach intends to scale down the wind, the wind turbine inertia, the rotation speed of the rotor according to Froude’s scaling laws while it keeps the thrust that is representative of the considered wind turbine. The Marin Stock Wind Turbine (MSWT) was used in these tests. Secondly, the wind turbine was replaced by a system actuating the wind turbine loads in which the aerodynamic loads are calculated by a numerical code and applied through a system of winches to a frame mounted on the floater. Lift and drag coefficients of the blades of the MSWT were used by the computer program during these tests. The TLP was tested with both techniques for the same environmental conditions. Tests were done in waves, wind and current with the two techniques in the same basin, enabling an objective comparison of these two methods. The experimental results of both methods are analyzed and compared to each other in this study.

General Electric, the National Renewable Energy Laboratory (NREL), the University of Massachusetts Amherst (UMass), and Glosten have recently completed a US Department of Energy (DOE)-funded research program to study technologies for mitigating loads on floating offshore wind turbines through the use of advanced turbine controls and tuned mass dampers (TMDs). The analysis was based upon the Glosten PelaStar tension leg platform (TLP) with GE Haliade 150 turbine, a system developed in a previous front end engineering design (FEED) study funded by the Energy Technology Institute (ETI) in the UK. The platform was designed for the WaveHub wave energy research site, with a mean water depth of 59-m. Loads were analyzed by running time-domain simulations in four 50-year return period (50-YRP) ultimate load state (ULS) conditions and 77 fatigue load state (FLS) environmental conditions. In 50-YRP conditions advanced controls are not active. The influence of TMDs on ULS loads have been reported previously (Park et al. [2]). In FLS conditions advanced controls and TMDs afford dramatic reductions in fatigue damage, offering the potential of significant savings in tower structural requirements. Simulations in turbine idling conditions were run in OrcaFlex , and simulations in operating conditions were run in FAST . Simulations were run with a baseline turbine controller, representative of the current state of the art, and an advanced controller developed by NREL to use collective and individual blade pitch control to maintain rotor speed and reduce tower loads. UMass developed a number of TMD types, with varying system configurations, including passive nonlinear dampers and semi-actively controlled dampers with an inverse velocity groundhook control algorithm. Loads and accelerations in FLS conditions were evaluated on the basis of damage equivalent loads (DELs), and fatigue damage was computed by Miner’s summations of stress cycles at the tower base. To study sensitivity to water depth, loads were analyzed at both the 59-m WaveHub depth and a more commercially realistic depth of 100 m. TMDs reduce fatigue damage at the tower-column interface flange by up to 52% in 59-m water depth and up to 28% in 100 m water depth. Advanced controls reduce fatigue damage at the tower-column flange by up to 22% in 59-m water depth and up to 40% in 100 m water depth. The most effective load-mitigation strategy is combining advanced controls with TMDs. This strategy affords a 71% reduction in fatigue damage in both 59-m and 100-m water depths.

Waves have impacted decks on Norwegian semi-submersibles (semis) and TLPs several times. However, it did not get necessary attention before two accidents in 2015. In 2015, a wave impact caused a fatality and four injured on the semi-submersible drilling rig COSLInnovator. The same year, a wave washed away gratings on Scarabeo 8, causing a man to fall 13.5m to the sea. When examining our records, we found 29 reported incidents related to waves in deck on 17 platforms from 2000–2017. 27 of the wave actions were mainly directed upward and the remaining two were mainly horizontal wave hits. We describe each incident and the circumstances. We discuss common causes related to weather conditions and the physical appearance of the platforms. Further, we summarise design and operational precautions taken by the industry.

There is currently an increasing interest in developing devices that can be used to exploit renewable energy in general and marine renewable energy in particular. Several sources of energy can be obtained from oceans, such as the energy produced by waves, tides, oceanic currents or thermal gradients, etc. It is important to stress that one of the most predictable sources of renewable energy is that derived from tidal and marine currents. Several manufactures are developing devices with which to harness energy from ocean currents, and there is an extended trend towards the development of horizontal axis rotor turbines (HAT-TEC). There is, however, also a considerable amount of debate regarding both the use of fixed or controllable pitch rotors and the use of one or several rotors per device. The decisions made in this respect have a great effect on the behaviour of the devices and the viability of the park as regards whether or not it will be technically and economically feasible. It is, therefore, necessary to be able to compare models and tools in order to design and choose the best and most feasible option. The tools most commonly used for metric comparison are: LCOE (Levelized Cost Of Energy), TRL (Technology Readiness Level) and TLP (Technology Performance Level). This paper presents a formal procedure with which to compare various alternatives: On the one hand, previous results obtained by the GITERM-UPM Research Group as regards mono-rotor and multi-rotor fixed-pitch devices will be shown, together with their general characteristics. The possibilities of controlling them, which is achieved with the group’s own design of a ballast control system that is employed to carry out automatic emersion and immersion maneuvers to reduce the high economic cost of maintenance tasks and repairs, will also be explained. On the other hand, the challenges involved in improving the design of the multi rotor will be listed, including the increase in the power of the rotor, the simplification of the structure, the improvement of the ability to control the device, etc. A new design for a multi-rotor device will also be presented, which takes advantage of the strengths of previous models and manages to substantially improve the amount of annual energy produced. This new device uses controllable pitch rotors, and its size and the amount of power produced by each generator have been improved. Finally, an economic comparison is carried out by means of a series of cost-benefit indicators, and a formal procedure with which to compare their benefits through the use of LCOE is also proposed.

Wind power generation has been paid much attention over the years as a countermeasure against global warming. Especially in recent years, researches and developments have also been made on Floating Offshore Wind Turbine (FOWT) in relatively deep offshore. Unlike Bottom-mounted Offshore Wind Turbine (BOWT), the motion characteristics of FOWT is complicated owing to coupled response of wind turbine and floating platform motions since the FOWT system is not fixed to the seabed. Due to these complexities, negative damping is one of the major problems reported for SPAR type FOWT moored by catenary chains. Negative Damping, in which the natural periodic motion is excited by blade pitch control employed for keeping the power generation constant, has to be addressed. In this paper, we discuss the negative damping of TLP type FOWT with a series of dedicated experiments. We manufactured a 1/100th-scale model TLP type FOWT model with a primary control system of the blade pitch angle for a geometrically scaled model of the 5MW wind turbine based on the NREL. At first, we formulated the mechanism for occurrence of Negative Damping and derived the conditions under which unstable fluctuations of the floating platform occurs using the motion equation. After that, we conducted scale model tank tests in wind alone and confirmed the phenomenon wherein the fluctuation of the floating platform does not converge. Finally, how dangerous such coupled motion of wind turbine and floating platform would be for real-scale FOWT is discussed.

Structural analysis of floating wind turbines is normally carried out with the hull considered as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible structures. The analysis is based on numerical simulations of free decays, structural response to wave excitation and short-term fatigue damage accumulation at chosen points of the platform. In addition, the importance of considering hydroelasticity effects is evaluated. It is observed that pontoon flexibility can change the platform natural periods significantly, as well as the intensity and peak frequencies of internal structural loads. The adoption of a fully rigid-body is shown to be non-conservative for the fatigue damage analysis. Loads due to hydroelasticity have order of magnitude comparable to those related to rigid-body motions, but still lower enough to be considered of secondary importance.

Ocean environmental conditions, such as waves, winds, and currents, are getting harsher due to climate change. This means that oil and gas production platforms in the ocean may experience unexpectedly large environmental loads bigger than previous design loads. Also, many platforms are reaching the end of their design lives. Ensuring riser integrity is one of the most important issues for platform safety and service-life extension. Currently, monitoring sensors are deployed on risers, and structural evaluation methods are utilized to examine riser integrity. However, there are some limitations to the structural evaluation methods. Furthermore, platform operators continue to seek for more direct and cost-effective riser monitoring method due to the low price of oil. In this study, the MultiSensor Fusion (MSF) system is proposed to surmount technical and economic obstacles in real-time riser-monitoring technology. The MSF system is validated for TLP (tension-leg platform) risers by using numerical sensors and numerical-simulation tools.

In present work, two combined wind turbine (WT) and wave energy converter (WEC) systems have been concerned. One is a fixed-bottom system, referred as “MWWC” (monopile-WT-WEC combination); the other is a floating system, referred as “TWWC” (TLP-WT-WEC-combination). Comparative study of the hydrodynamic responses of the two combined systems has been done by numerical simulations in the time domain. Hydrodynamic loads of the supporting structures and the WEC are calculated by the AQWA code, which is available for modeling multi-body systems including both mechanical and hydrodynamic couplings between the supporting tower and the WEC floater. Firstly, the effect of different power-take-off (PTO) parameters, wave periods and the displacements of the WEC on the performance of the WEC’s wave energy production of the two combined systems under typical wave cases has been investigate, and preliminary optimal values for the PTO damping stiffness of the two combined systems have been obtained and compared; secondly, the effect of the horizontal contact force between the supporting tower and the additional WEC floater of the two combined systems have been further investigated, which is important for both the fatigue and extreme loads design of the supporting tower. Finally, a new strategy for MWWC system by adding horizontal PTO dampers between the supporting tower and the WEC floater has been proposed and investigated, which is helpful for both reducing the horizontal contact force and using the relative horizontal motion to produce power.

An engineering study was performed in 2017 to develop a multi-span suspension bridge on floating foundations across the Bjørnafjorden in Norway. The bridge was approximately five kilometers long and consisted of three main suspension spans supported by four pylons (towers). Two of the pylons were supported on tension-leg platforms (TLP) due to large water depths. The bridge has to be resistant towards collisions from passing ships. However, submarine impacts to the submerged parts of the bridge were also a challenge due to the bridge location being close to an active submarine training field. This paper focus on the response of one such TLP towards collisions from submarines transiting below the bridge. Nonlinear explicit finite element analysis is used to study the possible collision scenarios, and the response of the TLP and the resulting bridge motion is evaluated. Further, transient failure of a tether was investigated to assess possible consequences of rupture of one of the tethers.

Concrete platforms have proven to be a viable offshore solution for deep-water projects according to the documented performance of over 40 years [1]. Since concrete has resistance to seawater corrosion, reduced maintenance costs and motions, concrete structures have become attractive to the offshore oil and gas industry [2, 3]. There are many floating concrete structures such as barge, ship, platform and LNG/LPG terminals. While most concrete offshore platforms are the fixed structures, floating concrete platforms have been used for drilling, extraction /storage and production units for oil and gas including heavy lifter. There are two representative floating concrete facilities on the Norwegian shelf, Troll B and Heidrun A [4, 5]. Troll B is the first concrete semi-submersible product unit and Heidrun A is the floating concrete tension leg platform. The concrete FPSO (Floating Production Storage and Offloading), however, was assessed to be inferior to the steel FPSO in respect of feasibility. In this study, an optimization design of the cross-section for cylinder shaped concrete platform is presented in order to promote the feasibility of a concrete hull for an FPSO. Optimal aspect ratio (depth to diameter) with economic analysis is examined and tangerine-shaped section which can utilize the excellent material properties of the concrete in compressive strength is developed. Exterior wall having a small curvature are applied to transmit the load to interior bulkhead. A parametric sensitivity study was conducted to find the optimal curvature of the exterior wall. Subsequently, an arch-shaped wall is applied to center wall in order to improve the upper and lower tension force caused by bending moment. The tendon was placed in the center of any cross-section, taking into account the tensile force that can be caused by additional asymmetric loads. The effect of tangerine shaped cross-section can be clearly identified by comparing the results with the circular shaped cross-section.

Vortex-induced motions (VIM) of large marine structures have been emerged as hot issues both in engineering and academic fields. Aiming to investigate the effect of the appendages on the VIM phenomenon of the tension leg platforms (TLP), experimental studies were carried out in a towing tank., The tendon pretensions and mass ratio were modeled exactly by using an air-bearing system. Additionally, the motions in vertical plane were almost restricted and the TLP model was allowed to move freely in horizontal plane. The 6-degrees of freedom (6-DOF) motions of the model were recorded by the motion acquisition system, consistent with an acceleration sensor installed at the deck as well as four load cells employed to acquire the restoring forces of the four mooring lines respectively. This paper presents the VIM responses of the TLP model with bare hull and the model with appendages located at the surface of columns with different relative positions to the current (i.e., 0°, 45°, 90° and 135°, respectively). In addition, the dynamic behaviors are also processed. The results show that the amplitudes of VIM response in the transverse direction are mitigated significantly for the configuration of 45°-appendages, lower than that of the bare hull model about 65 percent but slightly subsided at other locations. Moreover, the appendages at all locations can obviously reduce the amplitudes of the yaw motions, with the maximum decrease of 52 percent of the yaw responses for the configuration of 0°-appendages and 45°-appendages, compared with that of the bare hull model.

Typical offshore structures are designed as tension-leg platforms or gravity based structures with cylindrical substructures. The interaction of waves with the vertical cylinders in high sea states can result in a resonant response called ringing. Here, the frequency of the structural response is close to the natural frequency of the structure itself and leads to large amplitude motions. This is a case of extreme wave loading in high sea states. This understanding of higher-order wave forces in extreme sea states is an essential parameter for obtaining a safe, reliable and economical design of an offshore structure. The study of such higher-order effects needs detailed near-field modelling of the wave-structure interaction and the associated flow phenomena. In such cases, a Computational Fluid Dynamics (CFD) model that can accurately represent the free surface and further the wave-structure interaction problem can provide important insights into the wave hydrodynamics and the structural response. In this paper, the open source CFD model REEF3D is used to simulate wave interaction with a vertical cylinder and the wave forces on the cylinder are calculated. The harmonic components of the wave force are analysed. The model employs higher-order discretisation schemes such as a fifth-order WENO scheme for convection discretisation and a third-order Runge-Kutta scheme for time advancement on a staggered Cartesian grid. The level set method is used to obtain the free surface, providing a sharp interface between air and water. The relaxation method is used to generate and absorb the waves at the two ends of the numerical wave tank. This method provides good quality wave generation and also the wave reflected from the cylinder are absorbed at the wave generation zone. In this way, the generated waves are not affected by the wave interaction process in the numerical wave tank. This is very essential in the studies of higher-order wave interaction problems which are very sensitive to the incident wave characteristics. The numerical results are compared to experimental results for higher-order forces on a vertical cylinder to validate the numerical model.

Experiments were carried out in a towing tank regarding the flow around a stationary array of four cylinders with low aspect ratio, H/L = 1.5, piercing the water free surface. One isolated cylinder and one array of 4 cylinders were tested; this number of cylinders corresponds to the number of columns of a typical multi-column offshore floater system, such as semi-submersibles and TLP platforms. Three different geometries of cylinder sections were studied: circular, square and diamond. The aims are to better understand the influence of the relative position of the cylinders on the flow around this type of structure; to create a reliable database for CFD studies and afterward to use this research in VIM calculations for free multi-columns systems. Two parameters were investigated for a distance between column centers S/L = 3: current incidence angles 0, 22.5 and 45 degrees and column section geometries. The Reynolds number range covers from 50,000 up to 250,000. Forces were measured in each column by using a three degree-of-freedom load cell. Results of lift and drag forces are presented for each column separately and for the whole system. This paper is a piece of an extensive experimental study on flow around an array of multi-cylinders.

The simulation of free-surface flow around moored or floating objects faces a series of challenges, concerning the flow modelling and the numerical solution method. One of the challenges is the simulation of objects whose dynamics is determined by a two-way interaction with the incoming waves. The ‘traditional’ way of numerically coupling the flow dynamics with the dynamics of a floating object becomes unstable (or requires severe underrelaxation) when the added mass is larger than the mass of the object. To deal with this two-way interaction, a more simultaneous type of numerical coupling is being developed. The paper will focus on this issue. To demonstrate the quasi-simultaneous method, a number of simulation results for engineering applications from the offshore industry will be presented, such as the motion of a moored TLP platform in extreme waves, and a free-fall life boat dropping into wavy water.

Dry tree top-tensioned risers (TTRs) are widely used on floating production systems such as TLPs and Spars for drilling, completion, workover and production. The interference between neighboring TTRs is an important consideration which has a direct impact on the total TTR payload budget and the wellbay size for floater sizing and cost. Since the realistic sizing of a floater is essential towards the concept selection process for a field development, TTR interference should be addressed at the early stages of an offshore oilfield development. If the floater is a tension leg platform (TLP) and the field has strong current with associated extreme waves, riser interference may be very challenging and can have direct impact on riser design and the sizing and layout of the TLP. The waves and the oscillating motions of the TLP will have effects on riser interference. The oscillating motion of the TLP can excite the vibrational motion of the risers, and the wave-induced velocity of water particles and the motions of the risers with the movement of the TLP increases the relative flow acting on each riser. The combined effects will increase the deflection of the risers and thus the likelihood of riser interference. The industry has not seen an acceptable interference analysis approach yet which can account for the combined effects of current, waves, and TLP motions. This paper proposes two engineering approaches for the interference analysis of top tensioned risers for tension leg platforms with the combined effects of current, surface waves, and associated floater motions being addressed.

The offshore environment contains many sources of cyclic loading. Standard design S-N curves, such as those in DNVGL-RP-C203, are usually assigned to ensure a particular design life can be achieved for a particular set of anticipated loading conditions. Girth welds are often the ‘weak link’ in terms of fatigue strength and so it is important to show that girth welds made using new procedures for new projects that are intended to be used in fatigue sensitive risers or flowlines do indeed have the required fatigue performance. Alternatively, designers of new subsea connectors, used for example in tendons for tension leg platforms, mooring applications or well-heads which will experience cyclic loading in service, also wish to verify the fatigue performance of their new designs. Often operators require contractors to carry out resonance fatigue tests on representative girth welds in order to show that girth welds made using new procedures qualify to the required design S-N curve. Operators and contractors must then interpret the results, which is not necessarily straightforward if the fatigue lives are lower than expected. Many factors influence a component’s fatigue strength so there is usually scatter in results obtained when a number of fatigue tests are carried out on real, production standard components. This scatter means that it is important first to carry out the right number of tests in order to obtain a reasonable understanding of the component’s fatigue strength, and then to interpret the fatigue test results properly. A working knowledge of statistics is necessary for both specifying the test programme and interpreting the test results and there is often confusion over various aspects of test specification and interpretation. This paper describes relevant statistical concepts in a way that is accessible to non-experts and that can be used, practically, by designers. The paper illustrates the statistical analysis of test data with examples of the ‘target life’ approach (that is now included in BS7608:2014 + A1) and the equivalent approach in DNVGL-RP-C203, which uses the stress modification factor. It gives practical examples to designers of a pragmatic method that can be used when specifying test programmes and interpreting the results obtained from tests carried out during qualification programmes, which for example, aim to determine whether girth welds made using a new procedure qualify to a particular design curve. It will help designers who are tasked with specifying test programmes to choose a reasonable number of test specimens and stress ranges, and to understand the outcome when results have been obtained.

For offshore structures in harsh environments, horizontal wave impact loads should be taken into account in design. Shafts on GBS structures, and columns on semisubmersibles and TLPs are exposed to impact loads. Furthermore, if the crest height exceeds the available freeboard, the deck may also be exposed to wave impact loads. Horizontal loads due to waves impacting on the structure are difficult to quantify. The loads are highly intermittent, difficult to reproduce in model tests, have a very short duration and can be very large. It is difficult to calculate these loads accurately and the statistical challenges associated with estimating a value with a prescribed annual probability of occurrence are formidable. Although the accurate calculation of crest elevation in front of the structure is a significant challenge, industry has considerable experience in handling this problem and the analysis results are usually in good agreement with model test results. The present paper presents a statistical model for the distribution of horizontal slamming pressures conditional on the incident crest height upwave of the structure. The impact load distribution is found empirically from a large database of model test results where the wave impact load was measured simultaneously at a large number of panels together with the incident crest elevation. The model test was carried out on a circular surface piercing column using long simulations of longcrested, irregular waves with a variety of seastate parameters. By analyzing the physics of the process and using the measured crest elevation and the seastate parameters, the impact load distribution model is made seastate independent. The impact model separates the wave impact problem in three parts: – Given an incident crest in a specified seastate, calculate the probability of the crest giving a wave impact load above a threshold. – Given a wave impact event above a threshold, calculate the distribution of the resulting peak load. – Given a peak load, calculate the distribution of slamming pressures at one spatial location. The development of the statistical model is described and it is shown that the model is appropriate for fixed and floating structures and for wave impact with both columns and the deck box.

This paper presents a full long term analysis of a TLP extreme tendon tensions using the all seas approach, and its comparison to the results estimated by the contour line approach. The analysis of the TLP tendon tension response is performed in the time domain using a coupled model where the floater is modelled in the software SIMO, while the tendon system is represented by a Finite Element Model in RIFLEX, including therefore the effects of non-linear restoring from the tendon system and bending and deformations of the tendons. The characteristic tendon tensions with q-annual probability of exceedance are estimated from a full long term analysis where both the short and long term variability are considered. These results are then compared to those obtained through the long term estimate from the contour line approach when assuming the 90 th percentile for the worst sea state with q-annual probability of exceedance. The results from the full long term analysis will allow us to verify the adequate percentile level to be used with a contour line approach when estimating extreme TLP tendon tensions.

This paper represents the condensed version of an original Master Thesis written in 2016 [1]. The main aim lays in the structural analysis of Tension Leg Platforms (TLP) as support structures for Floating Wind Turbines (FWT). Special focus is given to a methodology study of different static analysis methods for components of a FWT support structure by Finite Element Method (FEM). The analysis considers Ultimate Limit States (ULS) only. Based on a given open source TLP design [2], the support structure is fragmented in its components. Each component is categorized, based on whether it can be analyzed by parametric equations or by computational methods. For the components, verifiable by parametric equations, the relevant standards are examined. The analysis of the decomposed support structure shows that existing standards can be applied for many of the components. One specific tubular joint of the support structure is selected and analyzed by computational methods. Considering the complexity of Finite Element Analysis (FEA) for FWT, the performed methodology study compares and evaluates various simulation methods on how loads are applied onto the structure. The methodology study results show that for ULS consideration, all load driven simulation methods are generally applicable. In the prospect of fatigue analyses further developments and improvements are considered crucial. This especially includes a better coherence of the stiffness representation between global load simulation model and local structural analysis model.