Crew Transfer Vessels Research Articles

Multi-body dynamic assessment of vessel-floater accessibility for floating wind

Abstract The floating wind industry faces several challenges in the road map to full maturity of commercial-scale floating projects. Accessing the floating offshore wind turbines poses larger risks due to the added degrees of freedom of the platform, with respect to bottom-fixed technology. When a vessel approaches, the platform’s motion leads to the introduction of multi-body dynamic effects. This work deals with the modeling of the vessel-platform dynamics for the VolturnUS-S 15MW platform and IEA15MW Reference Wind Turbine and a generic Crew Transfer Vessel (CTV) by means of performing a fully coupled assessment. The hydrodynamic properties are computed by solving the multi-body radiation-difraction problem. The inertial and hydrodynamic properties are included in an OrcaFlex model to perform a time-domain analysis under several sea states. A P-controller represents the helmsman pushing the vessel against the access point. The contact forces are modeled using a linear fender system to represent slip-and-stick events during the CTV’s push-on maneuver. The resulting relative motions are analyzed, and industry-typical limits are considered to quantify accessibility levels. The results show that peak wave period and wave heading play a major role in the accessibility scores.

Stick/Slip Phenomenon of a Crew Transfer Vessel Pushing Its Bow Against an Offshore Wind Tower During a Transfer Operation

Stick/Slip Phenomenon of a Crew Transfer Vessel Pushing Its Bow Against an Offshore Wind Tower During a Transfer Operation

Cost‐benefit assessment framework for robotics‐driven inspection of floating offshore wind farms

AbstractOperations and maintenance (O&M) of floating offshore wind farms (FOWFs) poses various challenges in terms of greater distances from the shore, harsher weather conditions, and restricted mobility options. Robotic systems have the potential to automate some parts of the O&M leading to continuous feature‐rich data acquisition, operational efficiency, along with health and safety improvements. There remains a gap in assessing the techno‐economic feasibility of robotics in the FOWF sector. This paper investigates the costs and benefits of incorporating robotics into the O&M of a FOWF. A bottom‐up cost model is used to estimate the costs for a proposed multi‐robot platform (MRP). The MRP houses unmanned aerial vehicle (UAV) and remotely operated vehicle (ROV) to conduct the inspection of specific FOWF components. Emphasis is laid on the most conducive O&M activities for robotization and the associated technical and cost aspects. The simulation is conducted in Windfarm Operations and Maintenance cost‐Benefit Analysis Tool (WOMBAT), where the metrics of incurred operational expenditure (OPEX) and the inspection time are calculated and compared with those of a baseline case consisting of crew transfer vessels, rope‐access technicians, and divers. Results show that the MRP can reduce the inspection time incurred, but this reduction has dependency on the efficacy of the robotic system and the associated parameterization e.g., cost elements and the inspection rates. Conversely, the increased MRP day rate results in a higher annualized OPEX. Residual risk is calculated to assess the net benefit of incorporating the MRP. Furthermore, sensitivity analysis is conducted to find the key parameters influencing the OPEX and the inspection time variation. A key output of this work is a robust and realistic framework which can be used for the cost‐benefit assessment of future MRP systems for specific FOWF activities.

A machine learning approach to comfort assessment for offshore wind farm technicians

Current maintenance planning strategies in the operations and maintenance of offshore wind farms rarely account for the comfort of technicians during transits. This creates uncertainties as transit from the vessel to structure might be unacceptable to technicians. Here, we model the welfare of technicians using the discomfort from the motions (three-dimensional accelerations) felt on crew transfer vessels (CTVs) during transits from port to wind farm. To explore technician exposure to vibration, acceleration data from vessel motion monitoring systems deployed on CTVs operating in the North Sea was synchronised with sea-state data from an operational ocean model. Processes of dimensionality reduction and machine learning (ML) were used to model the comfort of technicians from operational limits applied to models predicting Composite Weighted RMS Acceleration. Trained models were shown to provide estimations for the comfort variable with an R2 value of 0.67 and an RMSE of 0.06 ms−2. The comfort-based decision-making model is shown to be able to predict sail or not sail decisions for maintenance transits. The proposed model will have applications in maintenance planning for offshore wind farms, able to account for the comfort of technicians once identified limitations have been addressed to improve model predictions.

Assessing the Welfare of Technicians during Transits to Offshore Wind Farms

Available decision-support tools rarely account for the welfare of technicians in maintenance scheduling for offshore wind farms. This creates uncertainties, especially since current operational limits might make a wind farm accessible but the vibrations from transits might be unacceptable to technicians. We explore technician exposure to vibration in transit based on the levels of discomfort and the likelihood of seasickness occurring on crew transfer vessels (CTVs). Vessel motion monitoring systems deployed on CTVs operating in the North Sea and sea-state data are used in a machine learning (ML) process to model the welfare of technicians based on operational limits applied to modelled proxy variables including composite weighted RMS acceleration (aWRMS) and motion sickness incidence (MSI). The model results revealed poor to moderate performance in predicting the proxies based on selected model evaluation criteria, raising the possibility of more data and relevant variables being needed to improve model performance. Therefore, this research presents a framework for an ML approach towards accounting for the wellbeing of technicians in sailing decisions once the highlighted limitations can be addressed.

Design Evolution of CTV Vessels

When talking about the construction and operations/maintenance (O&M) of an offshore windfarm site it is critical the windfarm developer identify the important role that maritime vessels will play.There are a huge array of marine assets required in the process of developing and maintaining a windfarm, these include but are not limited to cable laying vessels, construction jack-up vessels, accommodation ships, survey vessels and for the purpose of this paper crew transfer vessels (CTVs).Offshore infrastructure including the wind turbines themselves and offshore substations need to be maintained during their lifetime for the system to be successful and for offshore windfarms the sole means of access to the offshore infrastructure is via a vessel. As a result, access to the wind turbines need to be possible throughout the year in a variety of weather and seastate conditions.For this reason, a very specific vessel type, designed specifically for this purpose is required to provide the maximum possible range of access to the wind turbine maintenance technicians.Like any other you need the right tool for the job.The evolution in this industry depends not only on the designers but also engine and machinery manufacturers, classification societies, shipbuilders, operators and charterers to stay in front and to ensure the best solution is being offered in terms of economics, safety, reliability and environmental impact.In addition, in a renewable industry is important that all parts stay consistent with the industry goal, posing additional requirements and challenges to the design of the vessels of the future.This paper has been written to present the evolution and the future of the design of the CTV vessel, from the experience of a designer and the application worldwide of a technology developed in the birthplace of the offshore wind industry.

Analysing the effectiveness of different offshore maintenance base options for floating wind farms

Abstract. With the growth of the floating wind industry, new operation and maintenance (O&M) research has emerged evaluating tow-to-port strategies (Offshore Wind Innovation Hub, 2020), but limited work has been done on analysing other logistical strategies for offshore floating wind farms. In particular, what logistical solutions are the best for farms located far offshore that cannot be reached by crew transfer vessels (CTVs)? Previous studies have looked at the use of surface effect ships (SES) and CTVs during the operation and maintenance (O&M) of bottom-fixed wind farms, but only some of them included service operation vessels (SOVs). This study analyses two strategies that could be used for floating wind farms located far from shore using ORE Catapult's in-house O&M simulation tool. One strategy comprises of having a SOV performing most of the maintenance on the wind farm, and the other strategy uses an offshore maintenance base (OMB) instead, which would be located next to the offshore substation and would accommodate three CTVs. This paper provides an overview of the tool and the inputs used to run it, including failure rates of floating wind turbine subsea components and their replacement costs. In total six types of simulations were run with two strategies, two different weather limits for CTVs and two weather datasets ERA5 and ERA-20C. The results of this study show that the operational expenditure (OPEX) costs for the strategy with an OMB are 5 %–8 % (depending on the inputs) lower than with SOV, but if capital expenditure (CAPEX) costs are included in the analysis and the net present value (NPV) is taken into account then the fixed costs associated with building the offshore maintenance base have a significant impact on selecting a preferred strategy. It was found that for the case study presented in this paper the OMB would have to share the foundation with a substation in order to be cost competitive with the SOV strategy.

SPOWTT: Improving the safety and productivity of offshore wind technician transit

AbstractThis paper describes the SPOWTT project. The intention of this project was to understand how sailing by crew transfer vessel (CTVs) to offshore wind farms affects the mental and physical wellbeing of individuals on board. The focus was on quantifying this impact, understanding the key drivers, with an aim to ensuring personnel can arrive to the wind turbines in a fit state to work safely and effectively. Impacts looked at subjective state beyond simply vomiting. Key results include the ability now to predict vessel motions from given Metocean conditions and vessel designs. We also discovered that the impact of vessel motions on seasickness is different for different symptoms and is driven not only by vertical z‐axis accelerations but also by certain frequencies of motion in the y‐axis. Frequencies other than 0.16 Hz were found to be impactful, and x‐axis movements appeared to have a longer‐lasting effect on the day's work. Through the formulation of a new, evidence‐based understanding of seasickness, we have created an operational planning tool, designed to have a direct benefit on the safety and productivity of offshore wind farm operations.

An optimization framework for daily route planning and scheduling of maintenance vessel activities in offshore wind farms

To increase energy production, offshore wind farms are currently installed far from shore, providing a challenge for vessels to undertake maintenance tasks from the designated hub port. Service Operation Vessels (SOVs) are utilized to carry out the offshore wind turbines maintenance tasks, which act as a servicing station having required technicians and daughter crafts (i.e. Crew Transfer Vessels (CTV)) onboard to facilitate on-time and on-demand servicing of wind turbines. This paper proposes an optimization framework, called OptiRoute, for daily or short-term maintenance operations based on route planning and scheduling while minimizing the cost under different operational constraints. Different heuristic and clustering techniques are developed and integrated to make the framework computationally effective. OptiRoute considers climate data, vessels specifications, failure information, wind farm attributes and cost-related specifics. The series of the overall operational tasks are divided into sequential sessions, including maintenance crew pick-up and drop-off tasks while the vessel routing optimization is performed for all sessions separately. OptiRoute reliability is tested by employing different Case studies while a user-friendly Graphical User Interface (GUI) is also developed to depict the various maintenance scheduling scenarios. Experimental results reveal that OptiRoute can efficiently increase the operational window especially when SOV and CTVs are used together.

Analysis of the Use of Electric Drive Systems for Crew Transfer Vessels Servicing Offshore Wind Farms

The article presents issues related to the possibility of using electric propulsion systems in units used to transport crews servicing wind towers at sea. Offshore wind energy issues are discussed. Proposals for electric propulsion systems that could be used on units for transporting crews servicing offshore wind farms are presented. The possibility of using purely electrical drive systems or hybrid drive systems operating in a diesel-electric configuration is analyzed. By observing the motion of real CTV units, based on the data from the MarineTraffic service, a mathematical simulation model was developed, for which a number of simulations were carried out in the Modelica environment. The developed mathematical model takes into account the dynamic loads acting on the ship’s hull, hydrodynamic resistances, electric and diesel propulsion systems’ properties together with their individual elements’ characteristics. The tests of the electric propulsion system showed reduced fuel consumption (approx. 60%) and harmful gas emissions to the atmosphere (approximately 70%) in relation to conventional, internal combustion engine propulsion.

Simulation-based optimisation for stochastic maintenance routing in an offshore wind farm

Scheduling maintenance routing for an offshore wind farm is a challenging and complex task. The problem is to find the best routes for the Crew Transfer Vessels to maintain the turbines in order to minimise the total cost. This paper primarily proposes an efficient solution method to solve the deterministic maintenance routing problem in an offshore wind farm. The proposed solution method is based on the Large Neighbourhood Search metaheuristic. The efficiency of the proposed metaheuristic is validated against state of the art algorithms. The results obtained from the computational experiments validate the effectiveness of the proposed method. In addition, as the maintenance activities are affected by uncertain conditions, a simulation-based optimisation algorithm is developed to tackle these uncertainties. This algorithm benefits from the fast computational time and solution quality of the proposed metaheuristic, combined with Monte Carlo simulation. The uncertain factors considered include the travel time for a vessel to visit turbines, the required time to maintain a turbine, and the transfer time for technicians and equipment to a turbine. Moreover, the proposed simulation-based optimisation algorithm is devised to tackle unpredictable broken-down turbines. The performance of this algorithm is evaluated using a case study based on a reference wind farm scenario developed in the EU FP7 LEANWIND project.

A Comparative Assessment of Crew Transfer Vessel Motions Between Inclined and Vertical Boat Landing Arrangements

Crew Transfer Vessels (CTVs) are an essential part of operations and maintenance activities for offshore wind farms. CTVs, which are generally multihull vessels transport crew from shore to offshore wind farm sites to carry out the scheduled maintenance and repair activities. CTVs thrust onto turbine structures from the rendered bow and push against the structure creating frictional thrust to prevent the bow from relative motions. This study presents and discusses the results from a set of model experiments and numerical calculations to allow a comparative assessment of a CTV’s landing-manoeuvre performance between inclined and vertical boat landing arrangements. The hydrodynamic performance of a CTV in open water and the “thrust-in” condition using inclined and vertical landing arrangements are presented for different vessel thrust force and different fender properties.

A data-driven method for optimal control of ship motions for safe crew transfer to offshore wind turbines

Due to uncertainties and random behavior of sea loads, presenting an accurate hydrodynamic analysis for motion control of offshore ships is a big challenge. This paper aims to propose a novel method for motion control of a crew transfer vessel (CTV) in order to ensure safe crew transfer to an offshore wind turbine (OWT). For this purpose, we propose a novel neural network observer-based optimal control (NOPC) scheme to tackle unknown dynamics and disturbances, nonlinear effects, and non-symmetric control input saturation constraints. Accordingly, the neural network (NN) structure addresses the Hamilton-Jacobi-Bellman (HJB) equation and forms an optimal control signal remaining in the saturation bounds. The Lyapunov theory guarantees the Ultimately Uniformly Boundedness (UUB) of all signals of the closed-loop system. The high performance of the presented method is demonstrated in regular waves with high frequency in comparison with the previous studies. It is worth mentioning that there are not any limitations to implement the adopted strategy for other offshore applications.

Research on a cost-effective measure dedicated to stabilizing offshore wind farm crew transfer vessels

Abstract The rapidly growing offshore wind industry is calling for more crew transfer vessels to deliver increasing number of minor maintenance tasks as about 75% of onshore wind turbine failures are related to the minor errors occurring in the electrical and power electronic systems of the turbines. The situation in offshore wind farms may be worse due to the wet, salty and corrosive air in offshore environments. Due to the limitations of small hull and deck spaces, there is difficulty to apply the proven motion stabilization techniques to wind farm crew transfer vessels. Consequently, the present crew transfer vessels have limited capability in providing safe transfer between the vessel and wind turbines, particularly in rough sea waves. To tackle this issue, a new motion stabilization technique is studied in this paper by using both numerical analysis and experimental testing approaches. Through investigating the vessel's motions under different wave conditions before and after applying the proposed technique, it is found that the heave, roll and pitch motions of the vessel, especially in its resonant frequency regions, have been successfully constrained after applying the proposed stabilizing technique. Moreover, the amount of motion reduction can be further improved through optimizing the size of stabilizers and their underwater distance.

Offshore monopile in the southern North Sea: part II, simulated hydrodynamics and loading

This paper considers two methods for determining the local wave particle kinematics and hydrodynamic forces on an idealised wind turbine monopile in the southern North Sea using sea-state data from Teesside Offshore Wind Farm. An assessment of local flow hydrodynamics is important with regard to safe access of personnel from a crew transfer vessel to a monopile. The hydrodynamic behaviour was calculated using an analytical solution from linear diffraction theory and numerical predictions using OpenFoam, with both slip and no-slip cylinder boundary conditions. Provided the underlying sea state is unidirectional, it was found that close agreement is obtained between analytical and numerical spectra derived from the time series of local free surface elevation, water particle velocity components and in-line wave force loading on the monopile. Less satisfactory agreement is achieved with sea states possessing a bimodal spectrum, which suggests that bimodal spectra may not be unidirectional.

Improving global accessibility to offshore wind power through decreased operations and maintenance costs: a hydrodynamic analysis

Improved access to renewable energy in developing economies will be a major factor in future global efforts to reduce CO2 emissions, while simultaneously raising living conditions in areas presently without or with only limited access to electricity. Coastal populations stand to benefit greatly from reduced costs of offshore wind farms, which are one of the fastest growing and most economical sources of marine renewable energy. A considerable drawback of offshore wind power is the high cost of operations and maintenance (O&M), which can account for 25-50% of total energy production costs. Present-day maintenance procedures, using crew transfer vessels, rely on the significant wave height (HS) as the limiting factor by which to decide whether or not it is safe to access the offshore turbines. In practice, HS has to be applied conservatively, thus raising the costs through increased downtime. A method is proposed here with the objective of reducing overall costs through improved analysis of the motion of the crew transfer vessels (CTVs) used to transport repair technicians onto offshore wind turbine structures. CTV motion depends on the hydrodynamic forces incident on the vessel under operating conditions and the effect that the presence of the turbine has on the flow field. A change in the hydrodynamic field caused by the turbine monopile can cause a vessel abutted against the turbine support column to lose frictional contact and slip. Using the open-source computational fluid dynamics software, OpenFOAM, and in situ experimental results, the diffracted surface elevation and a wave kinematics model for the near-wake of a turbine monopile are presented. More accurate estimates of significant wave height and wave kinematics incident on a vessel close to a turbine monopile will facilitate much improved analysis of vessel motions under operational conditions.

Numerical investigation of the landing manoeuvre of a crew transfer vessel to an offshore wind turbine

ABSTRACTCrew transfer vessels are a fast and cost-effective means of transportation to bring service personnel and technical equipment to an offshore wind turbine. The landing manoeuvre of such service vessels is a complex coupled process involving the ship's motion in the seaway and the frictional contact between the fender and the boat landing. With the help of numerical simulations, we aim to identify the limit conditions under which a safe crew transfer can still be ensured, and to investigate innovative transfer concepts which serve to avoid severe accidents. In our research, we draw on a partitioned solution strategy which has already been successfully applied to other sophisticated multi-field phenomena and which allows to reuse specialised existing solvers for the solution of the involved subproblems.

Investigating Key Decision Problems to Optimize the Operation and Maintenance Strategy of Offshore Wind Farms

This paper investigates three decision problems with potential to optimize operation and maintenance and logistics strategies for offshore wind farms: the timing of pre-determined jack-up vessel campaigns; selection of crew transfer vessel fleet; and timing of annual services. These problems are compared both in terms of potential cost reduction and the stochastic variability and associated uncertainty of the outcome. Pre-determined jack-up vessel campaigns appear to have a high cost reduction potential but also a higher stochastic variability than the other decision problems. The paper also demonstrates the benefits and difficulties of considering problems together rather than solving them in isolation.

Optimisation of maintenance routing and scheduling for offshore wind farms

An optimisation model and a solution method for maintenance routing and scheduling at offshore wind farms are proposed. The model finds the optimal schedule for maintaining the turbines and the optimal routes for the crew transfer vessels to service the turbines along with the number of technicians required for each vessel. The model takes into account multiple vessels, multiple periods (days), multiple Operation & Maintenance (O&M) bases, and multiple wind farms. We develop an algorithm based on the Dantzig–Wolfe decomposition method, where a mixed integer linear program is solved for each subset of turbines to generate all feasible routes and maintenance schedules for the vessels for each period. The routes have to consider several constraints such as weather conditions, the availability of vessels, and the number of technicians available at the O&M base. An integer linear program model is then proposed to find the optimal route configuration along with the maintenance schedules that minimise maintenance costs, including travel, technician and penalty costs. The computational experiments show that the proposed optimisation model and solution method find optimal solutions to the problem in reasonable computing times.

Advanced logistics planning for offshore wind farm operation and maintenance activities

Offshore wind turbine technology is moving forward as a cleaner alternative to the fossil fuelled power production. However, there are a number of challenges in offshore; wind turbines are subject to different loads that are not often experienced onshore and more importantly challenging wind and wave conditions limit the operability of the vessels needed to access offshore wind farms. As the power generation capacity improves constantly, advanced planning of Operation and Maintenance (O&M) activities, which supports the developers in achieving reduced downtime, optimised availability and maximised revenue, has gained vital importance. In this context, the focus of this research is the investigation of the most cost-effective approach to allocate O&M resources which may include helicopter, crew transfer vessels, offshore access vessels, and jack-up vessels. This target is achieved through the implementation of a time domain Monte-Carlo simulation approach which includes analysis of environmental conditions (wind speed, wave height, and wave period), operational analysis of transportation systems, investigation of failures (type and frequency), and simulation of repairs. The developed methodology highlights how the O&M fleets can be operated in a cost-effective manner in order to support associated day-to-day O&M activities and sustain continuous power production.