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.

Large-scale deployment of multiple structures within a Floating Offshore Wind Farm (FOWF) will place many challenges on both the approach to effective integrity management and the demand to reduce through-life operating costs. Additionally, wind farm designers and operators will need to consider the issues related to design robustness in the design of their systems, whereby rational robustness criteria are applied to address possible accidental conditions that are not explicitly addressed in code-specified design basis conditions. This paper will review the significant advances in the approach to integrity management that have been recently made in the mature offshore oil industry and relate them to the nascent offshore wind industry. In particular, the requirement for consideration across the full lifecycle of the potential for threat introduction and the application of controls to prevent or mitigate the evolution of those threats will be described. This includes threats that may be inadvertently introduced during the design, manufacturing and installation phases, in addition to the more traditional rate-based degradation mechanisms such as fatigue, corrosion and wear that occur once the facility is in operation. The challenges that commercial-scale wind farm developments will face relate particularly to integrity issues arising in the design, manufacturing and installation phases, where the focus needs to ensure that degradation threats are not being introduced into the deployment of multiple repeat-copy units. The issues of common cause/common mode degradation threats will have much higher significance for a commercial-scale wind farm, where rectification of a common issue across a large number of floating units could have significant impacts in terms of reliability, operating costs, insurance premiums and power purchase agreements. FOWF mooring system designs are also likely to be more optimized than that for a typical one-off offshore oil facility. This will require the wind farm designer to have a deep understanding of the fundamental dynamic behavior of the overall system and the local dynamic behaviors of the components within the mooring system in order to be able to fully identify the types of threats that may be present. This may also involve robustness considerations where there may be step changes in the dynamic behavior of the system, often termed as cliff-edge effects. The paper will outline issues that wind farm designers will need to consider in building integrity considerations into the design and execution phases of a development, as well as the opportunities that risk-based integrity management processes offer in terms of through-life condition verification and inspection optimization.

The planned operation of natural gas liquefaction units offshore has raised many new safety issues. Cryogenic spillage is among the most critical as load bearing structures cannot be designed to resist such accidents. In order to propose better protection at manageable cost, to enhance structural integrity and to lower the risks associated with cryogenic spillage on marine structures, TechnipFMC as FLNG market leader, has run an extensive physical test program to support the development of tailor made numerical modeling tools. In a first step, numerous protective materials were screened and then tested to assess their performances. We found that all products currently available in the market display limitations. During the test campaign, alternate product designs were investigated to improve structural design and avoid the need for human intervention “Inspection Maintenance and Repair” during service lifetime. In this paper, a summary of the test campaign and a complete introduction of Series of standard of ISO 20088 will be provided.

Cracks caused by cold bending during shipyard construction were found to be longer in mild steel than in high tensile steel. Because mild steel has better elongation, the cause of such cracking was difficult to ascertain. Bending tests of large number of steel pieces to confirm the effects of multiple parameters, nonlinear finite-element analysis to check strain distribution, and fractography to determine the nature of the fracture indicated that ductile fracture initiated where the strain value was highest, as is generally expected. In the bending tests, the fractures were reproducibly longer in alloys with better ductility. One of the reasons for this phenomenon was that the surface was more hardened by plasma-arc cutting in mild steel because of its high carbon content according to hardness tests on the cutting surfaces. We therefore proposed a new equation to estimate crack length, taking into account three factors: strain, elongation and surface hardness. We showed that the relationship between crack length and the values by the equation explains the results of the bending tests. Additionally, we suggested the possibility that inclusions within an alloy affect the crack length. Cold forming is a general procedure used in shipyards. If inspections fail to find a crack, it may propagate to fatal damage after delivery. This study is therefore useful in preventing the initiation of cracks.

Assessment of scour below pipelines and gravity based structures in a submarine system is revisited with evaluation of data from two inspections and results for semi-empirical simplified models. Usually semi-empirical models are based on small scale experimental data that normally consider uniform and noncohesive sediments and have been questioned as to their practical applications. Recent local metocean data is used to revisit predictions of onset of scour and scour depth for the system, which are compared to survey statistics.

With pile installation being an important part of the total project cost, improving the efficiency of the offshore operations would require resolving the uncertainties in how different installation methods influence pile capacity. This paper describes a programme of testing involving static loading tests on eight 165 mm diameter open-ended steel tubes driven into medium dense siliceous sand at a well-characterized test site in Perth, Western Australia. Six piles were installed by impact driving under two different driving frequencies, and two piles were installed by vibration with the aim of identifying the effects of impact frequency on the response of driven piles to static axial and lateral loading. It was found that the tension capacity of piles measured 1 week after installation was not significantly dependent on the impact frequency and that these piles had approximately the same capacity as the piles installed by vibration. First time load tests performed 4 months after the initial tests indicated that all pile types had a similar set-up characteristic. Inspection of static tests on lateral behavior of vibrated and impact driven piles suggests that installation method has no impact on the lateral capacity.

Production efficiency is a key factor for oil & gas companies to have a good performance in a low oil prices environment, and planned shutdowns are events that impact crucially in this factor. This technical paper objective is to explain how, starting from a point of high planned losses due to shutdowns bad performance (13%), the application during only one year of best practices compiled from oil & gas and other business experience is able to change things up to achieve an impressive outcome (3%). Long term planning, meticulous preparation and efficient execution are achieved when best practices are applied with accountability, rigor and common sense. Maintenance and Integrity shutdown plans compliance is also improved as they are considered inside a big picture that allows us to schedule them in the more efficient way to assure our HSE standards, and helped by some new technologies as the so called NII (Non-Intrusive Inspections). Standards, procedures, deliverables, communication channels, materials control, organigrams… are the specific tools to be used and refed to implement the lessons we learn at the end of each shutdown. Sharing regularly our experience with other operators is also a source of best practices and cross-pollination. Maintaining this quality standard along the time and improving to the extent possible will drive us to excellence in the middle term. As our teams become more and more involved on this new method, they start to feel more comfortable and propose improvements, being more and more committed as they see good results arriving.

Risk-based in-service inspection enhances structural integrity and minimizes the number of potential unwanted incidents in relation to offshore concrete wind turbine structures. Risk-based in-service inspection enables in-situ inspections, evaluation of inspection results, condition assessment of the offshore wind turbine structures and recommends optimal future inspections. The vagueness, uncertainties and variability present in the risk-based in-service planning process jeopardizes the final inspection recommendations and structural integrity of the structure under consideration. Hence, it is vital to develop methodologies for the systematic utilization of expert knowledge to minimize variability in risk-based in-service assessments. This manuscript suggests a fuzzy set theory-based approach to minimize the risk assessments’ variability and unwanted activities. The suggested fuzzy set theory-based approach enables the identification of the potential hot points that encompass high-risk regions needing priority in future inspections with optimal effort and higher productivity. This manuscript also discusses the use of available inspection methods to enhance the structural integrity of offshore concrete wind turbines subject to deterioration, as well as the use of fuzzy theory-based risk-based in-service inspection assessments for making optimal in-service inspection recommendations.

Based on relevant accident experiences with oil and gas platforms, a brief overview of structural integrity management of offshore structures is given; including an account of adequate design criteria, inspection, repair and maintenance as well as quality assurance and control of the engineering processes. The focus is on developing research based design standards for Accidental Collapse Limit States to ensure robustness or damage tolerance in view damage caused by accidental loads due to operational errors and to some extent abnormal structural damage due to fabrication errors. Moreover, it is suggested to provide robustness in cases where the structural performance is sensitive to uncertain parameters. The use of risk assessment to aid decisions in lieu of uncertainties affecting the performance of novel and existing offshore structures, is briefly addressed.

Nowadays different types of unmanned underwater vehicles (UUVs), such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), are widely used for sub-sea inspection, maintenance, and repair (IMR) operations in the oil and gas industry, archaeology, oceanography and marine biology. Also, lately, the development of underwater snake robots (USRs) shows promising results towards extending the capabilities of conventional UUVs. The slender and multi-articulated body of USRs allows for operation in tight spaces where other traditional UUVs are incapable of operating. However, the mathematical model of USRs is more challenging compared to models of ROVs and AUVs, because of its multi-articulated body. It is important to develop accurate models for control design and analysis, to ensure the desired behaviour and to precisely investigate the locomotion efficiency. Modelling the hydrodynamics poses the major challenge since it includes complex and non-linear hydrodynamic effects. The existing analytical models for USRs consider theoretical values for the fluid coefficients and thus they only provide a rough prediction of the effects of hydrodynamics on swimming robots. In order to obtain an accurate prediction of the hydrodynamic forces acting on the links of the USRs, it is necessary to obtain the fluid coefficients experimentally. This paper determines the drag and added mass co-efficients of a general planar model of USRs. In particular, this paper presents methods for identifying fluid parameters based on both computational fluid dynamic (CFD) simulations and several experimental approaches. Additionally, in this paper, we investigate variations of the drag force modelling, providing more accurate representations of the hydrodynamic drag forces. The obtained fluid coefficients are compared to the existing estimates of fluid coefficients for a general model of USRs.

Delivering full service life performance for mooring systems of Floating Production Storage & Offloading assets (FPSO) has been a frustrating challenge to operators across the industry. Remaining strength and fatigue assessment on degraded top mooring chains of the Bonga FPSO and Single Point Mooring (SPM) loading Buoy has been investigated as part of an in-house Bonga Asset Preservation Program. Both facilities are located approximately 120 km off the coast of Nigeria in the Gulf of Guinea operating in tropical waters just North of the Equator, where top chain links have been subjected to accelerated deterioration from Sun Corals and other forms of Microbiologically Induced Corrosion (MIC). These phenomena have led to overall corrosion rates being slightly above general design requirements, but more importantly to formations of large pitting on several sections of the top chain links. Remotely Operated Vehicles (ROV) assisted inspections, chain link cleaning and underwater 3D photogrammetry have allowed capturing the surface geometry of representative degraded chain links of the mooring lines to provide detailed input data for further analyses. Reverse engineering has been performed via Finite Element Analysis and fracture mechanics methodologies using the scanned geometry of selected highly exposed critical links to estimate the residual strength and fatigue life performance of the degraded links relative to their original design criteria. To evaluate the potential impact of cracks on the capacity of degraded chains relative to a reference link, crack tip Stress Intensity Factors have been computed at worst case stress-raising pits and parametric analyses using varying initial crack sizes have been performed to calculate the number of years for the cracks to propagate to critical sizes. A baseline for benchmarking the strength, fatigue and crack growth behaviour of the degraded links investigated has been provided by analysing non-degraded and uniformly corroded links after 12 years of service with projection to end of service life capacity. The paper provides a comprehensive application of numerical methods for assessing the fitness-for-service of the chains and recommendations on in-situ performance integrity management by circumventing the need to retrieve chain samples for testing.

Offshore Structures operate for decades in extremely hostile environments. It is important during this period that the structural integrity is efficiently managed to ensure continuous and safe operation. Increased use of enhanced oil and gas recovery means it is likely that many existing installations will remain operational for a significant period beyond the original design life. The operator needs to capture, evaluate and, if necessary, mitigate design premise changes which inevitably occur during the life of a structure. Further, advances in knowledge and technology may imply changes in codes and standards as well as in analysis methodologies. Changes in corporate structures, transfer of operator responsibility and retirement of experienced engineers call for reliable means to transfer historical data and experience to new stakeholders. Effective emergency preparedness capabilities, structural integrity assessments and inspection planning presuppose that as-is analysis models and corresponding information are easily accessible. This paper presents an implementation of the in-service integrity management process described in the new revision of NORSOK standard N-005 [1] for a large fleet of jackets at the Norwegian Continental Shelf. The process, comprising management of design premise changes as well as state-of-the-art technical solutions over a range of disciplines, has enabled the operator to prolong the service life with decades at minimum investments. A structure integrity management system (SIMS) has been developed and digitized over years and streamlined to meet the needs and challenges in the operation and management of the jacket platforms. SIMS enables a rather lean organization to control the structural integrity status of all load-bearing structures at any time. Platform reinforcements and modifications along with other operational risk reducing measures like unman the platforms in severe storms enable continued use with the same level of safety as for new manned platforms. Advanced analyses are used to document regulatory compliance. Modern fatigue and reliability based inspection planning analyses have reduced the costs needed for inspection of fatigue cracks significantly. The benefits from the SIMS system are substantial and the resulting safety and productivity gains are apparent. The continuity of knowledge and experience is maintained, reducing risk to safety and regularity. The digital transformation related to management of structural integrity status as described in NORSOK standard N-005 is realized through SIMS.

The paper presents a true digital twin concept, which is a general and novel methodology that significantly improves the fatigue prediction models of existing marine structures. The actual structural condition of existing marine platforms can often change after several years in operation due to degradation mechanisms and/or other structural changes. It is within this context, the true digital twin concept has been developed and the general idea is to create a coupling between the digital twin and measurements. The measurements are performed by Structural Health Monitoring Systems (SHMS). This coupling facilitates a direct performance evaluation of the digital twin against measurements and most importantly creates the basis for improving the performance of the digital twin to accurately capture the actual condition of the structure, and thus become a true digital twin. The full concept of creating a true digital twin encompass novel advanced analysis methods ranging from linear system identification, expansion processes, Bayesian FE model updating, wave load calibration, quantification of uncertainties from measured data, and Risk- and Reliability Based Inspection Planning (RBI) analysis, [1]. This paper presents the first 3 levels for establishing a true digital twin. The levels are illustrated by 3 case stories.

In-line inspection (ILI) data processing is a crucial process on assessing the remaining capacity of corroded pipelines. It is used to generate the RBP (river bottom profile) or M AV (moving average) file which could be further used in the capacity calculations. There are large uncertainties regarding the qualities of ILI data itself, which are dependent on the accuracies of the ILI tool, the reporting method and the condition of the inspected pipelines et. al. It is thus required that the post-process of the ILI data should not add more uncertainties to the capacity check. Challenges exist on generating reliable RBP/MAV from the ILI data on general base. This paper originates from work during the in-house software development for corroded pipeline tool. Three different algorithms on filtering the ILI data are examined. They are the method recommended by DNV-RP-F101 (with modification), the median filter algorithm and the gaussian filter algorithm. The latter two algorithms are from standard methods for image denoising. A comparison study has been performed with several actual ILI data sets. Finally, conclusions and suggestions have been made, which may provide useful hints for industry application.

There are many failure mechanisms that may occur in flexible pipes during their service life, such as external sheath damage, damage of tensile armours due to fatigue, kinks due to corrosion, and over-bending. The above mentioned defects are very common and can lead to the failure of flexible pipes. Therefore, huge operational costs have to be spent to ensure the integrity of the flexible pipeline system. Risk-Based inspection (RBI) methodology can establish a cost-effective strategy for inspection and monitoring, while maintaining the expected level of safety. However, there are some factors that can hamper the development of an optimized RBI for the integrity management of flexible pipelines, such as the complexity and variety of flexible pipes’ multi-layer structures with their interacting failure mechanisms, the somewhat poor defect capability of the existing inspection techniques and the lack of broad [1]. In this paper, RBI planning is described thoroughly to ensure the integrity management of flexible pipelines. All risk terms have been described in detail with respect to integrity management strategy. The working process of RBI, including initial assessment and detailed assessment, is introduced. Estimation and evaluation of risk level, including the development of optimized inspection planning, have been described completely.

In order to ensure the integrity of subsea pipeline in the whole design life, it is very important to carry out quality evaluation for the girth welding since severe ocean environment and installation conditions. In this paper, based on the principle of “suitable for use”, the level 2B, British BS7910 standard fracture assessment criterion is adopted. According to the mechanical properties of the pipe joint, welding method and installation load conditions, the Engineering Critical Assessment (ECA) has been carried out for an export subsea pipeline in a gas field in deep water of South China Sea. Larger allowable initial weld flaws can be accepted through ECA evaluation, the acceptable criterion for AUT inspection in the process of pipeline laying is obtained, which effectively reduce the welding repair rate with significant cost saving and improve installation efficiency.

In this study, the authors developed the dynamic routing algorithm combining an image detection technique to support the optimal route plan of Autonomous Underwater Vehicle (AUV) inspecting an offshore wind farm affected by ocean currents. A modular structure is applied to program design by the graphical language, LabVIEW (Laboratory Virtual Instrument Engineering Workbench). The modular structure is composed of 6-DOF (Six Degrees-of-Freedom) motion module, a self-tuning fuzzy control module, a stereo-vision detection module, and a dynamic routing module. In terms of path planning for inspection, several Pareto frontiers are solved iteratively according to two objectives, namely, cruise time and energy consumption. Performances obtained from MOPSO (Multi-Objective Particle Swarm Optimization) -based dynamic routing algorithm would be in comparison with those from SOPSO (Single-Objective Particle Swarm Optimization) -based dynamic routing algorithm. In addition, selections of fixed weight and dynamic weight of MOPSO-based dynamic routing algorithms would be discussed in the environment with or without ocean currents. Eventually, the image inspection mode is not only beneficial for optimizing feasible routes but it can also identify features of obstacles for positioning.

This paper presents 3D path tracking simulations and path-following by experimental testing with a small inspection class ROV. The ROV is underactuated, but moves freely in surge, heave and yaw (4 DOF). Paths used are a spiral for path-tracking and a lawnmower pattern for path-following, where both paths are related to a proposed structural setting and a mission. The structure is a cylindrical fishing cage used for fish farming in the aquaculture, and the task is to perform inspection with equipped cameras and sensors while tracking or following the path.

The metocean conditions in the Barents Sea entail lower temperatures, and more snow and icing on offshore structures than what can be expected further south on the Norwegian Continental Shelf. The conditions are not considered extreme compared with other Arctic waters, but low temperatures, accumulation of snow and icing on structures and equipment may affect operations and equipment functionality. To manage these issues and ensure a safe working environment, winterization of installations is necessary. This article describes some results from a winterization study executed for the Petroleum Safety Authority Norway. The study objective was to provide an overview of known solutions and measures for winterization related to low temperatures, snow and icing for drilling rigs and production platforms operating in the Norwegian part of the Barents Sea. The study also describes solutions and measures that may be applicable at different locations in the Barents Sea, and available standards and guidelines related to winterization. Issues related to sea ice, bergy bits and icebergs have not been addressed. As a part of the study, winterization standards and manuals for drilling rigs and production platforms were reviewed, along with available experience from previous and ongoing projects. The information was systematized, and a high level summary produced. The results showed that the main issues related to low temperatures, snow and icing for rigs and platforms operating in the Barents Sea can be simplified and summarized as follow: 1) Snow and icing on exposed surfaces. 2) Freezing of fluids in piping/equipment/tanks. 3) Personnel exposure to low temperatures and weather. These issues were further categorized to include typical areas and equipment types. Several solutions and measures were identified for each of them. The study showed that most of the identified solutions and measures are applicable for most locations in the area open for petroleum activities. However, more permanent and robust solutions should be selected for the coldest areas, as icing rates increases, and wind chill temperature decreases with lower temperatures. The review of winterization manuals also showed the importance of implementing organizational and operational measures related to winterization. Education and training in proper use of winter technology, and defensive behaviour with respect to falling ice and slippery surfaces were identified as some of the most important measures to avoid incidents. Checklists for inspections, before and during the winter season, are important measures to secure areas and equipment when temperatures fall towards freezing point. The study results show a large variety in available solutions and measures for each issue. These shall, combined with organizational and operational measures, ensure safe operations in the Barents Sea. The study shows that the challenge is not whether solutions and measures exist, but in finding the right combination of them for different conditions at specific locations.

Marine structures like jacket structures are often highly utilized structures operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is to a large extent governing the structural dimensions and the amount of resources utilized in inspection and maintenance of members and joints. There is a considerable degree of uncertainty related to the parameters determining the fatigue damage. The models applied, both for describing the fatigue driving mechanisms e.g. the wave-description and load modeling and the deterioration mechanism, are always compromises between the ability to accurately describe the nature and computationally efficiency. The main focus in this paper is to show how sensitive the calculated fatigue damage of a jacket is to different models for the short term variability of wave heights. To obtain consistent basis for comparison a deterministic fatigue analysis is considered and a potential structural dynamic amplification is not included in the comparison study. Sensitivity to selection of wave spectra will not be addressed. In a deterministic approach the long term distribution of individual wave heights is used to calculate the stress ranges occurring in the joints and butt welds. Typically, the long term variability of sea state conditions is given by a scatter diagram of significant wave height (H s ) and the peak period (T p ). When converting the scatter diagram of sea states to the long term distribution of wave heights, it is common to assume that the individual waves in the sea states are Rayleigh distributed. Later developments indicate that a Forristall distribution may be a more accurate assumption. The following cases have been considered: 1. Assuming that the individual waves in each sea state are Rayleigh distributed. 2. Assuming that the individual waves in each sea state follows a Forristall distribution. 3. Calculating the long term wave height distribution from time domain simulations. In the third method, second order wave theory was used to simulate all sea states in the H s /T p scatter diagram. I.e. extensive time domain simulations were carried out to cover the complete scatter diagram of possible sea states. The study is performed for an 8-legged jacket. The analyses are performed for a typical North Sea wave environment for water depth about 110 m. The objective of this study is to investigate the robustness in the current design practice for jacket structures where the individual waves in the sea states are Rayleigh distributed. The paper documents the calculated fatigue lives for main joints along the height of the jacket for the three wave height distributions. Further, the paper gives advice on application of wave distribution models for design of new structures and reassessment of existing structures.