Larger MW-Class Floater Designs Without Upscaling?: A Direct Optimization Approach

The trend towards larger offshore wind turbines (WTs) implies the need for bigger support structures. These are commonly derived from existing structures through upscaling and subsequent optimization. To reduce the number of design steps, this work proposes a direct optimization approach, by which means a support structure for a larger WT is obtained through an automated optimization procedure based on a smaller existing system. Due to the suitability of floating platforms for large MW-class WTs, this study is based on the OC3 spar-buoy designed for the NREL 5 MW WT. Using a Python-Modelica framework, developed at Fraunhofer IWES, the spar-buoy geometry is adjusted through iterative optimization steps to finally support a 7.5 MW WT. The optimization procedure focuses on the global system performance in a design-relevant load case. This study shows that larger support structures, appropriate to meet the objective of the hydrodynamic system behavior, can be obtained through automated optimization of existing designs without the intermediate step of upscaling.

Effect of Wind Turbulence on Extreme Load Analysis of an Offshore Wind Turbine

Evaluation of dynamic responses under extreme environmental conditions is important for the structural design of offshore wind turbines. Previously, a modified environmental contour method has been proposed to estimate extreme responses. In the method, the joint distribution of environmental variables near the cut-out wind speed is used to derive the critical environmental conditions for a specified return period, and the turbulence intensity (TI) of wind is assumed to be a deterministic value. To address more realistic wind conditions, this paper considers the turbulence intensity as a stochastic variable and investigates the impact on the modified environmental contour. Aerodynamic simulations are run over a range of mean wind speeds at the hub height from 9–25 m/s and turbulence levels between 9%–15%. Dynamic responses of a monopile offshore wind turbine under extreme conditions were studied, and the importance of considering the uncertainties associated with wind turbulence is highlighted. A case of evaluating the extreme response for 50-year environmental contour is given as an example of including TI as an extra variant in environmental contour method. The result is compared with traditional method in which TI is set as a constant of 15%. It shows that taking TI into consideration based on probabilistic method produces a lower extreme response prediction.

Use of 3D Scan of Weld Joint in Finite Element Analysis and Stochastic Analysis of Hot-Spot Stresses in Tubular Joint for Fatigue Life Estimation

Several methods for modelling and finite element analysis of tubular welded joints are described in various design codes. These codes provide specific recommendations for modelling of the welded joints, using simple weld geometries. In this paper, experimental hot-spot strain range results from a full-scale automatically welded K-node test are compared to corresponding finite element models. As part of investigating the automatically welded K-joint, 3D scans of the weld surfaces have been made. These scans are included in the FE models to determine the accuracy of the FE models. The results are compared to an FE model with a simple weld geometry based on common offshore design codes and a model without any modelled weld. The results show that the FE model with 3D scanned welds is more accurate than the two simple FE models. As the weld toe location of the 3D scanned weld is difficult to locate precisely in the FE model and as misplacement of strain gauges are possible, stochastic finite element modelling is performed to analyse the resulting probabilistic hot-spot stresses. The results show large standard deviations, showing the necessity to evaluate the hot-spot stress method when using 3D scanned welds.

The Effect of Aspect Ratio on the Drag of Bare Cylinders

The drag coefficient for long-slender structures that is typically provided in design guidance has been determined from test specimens of sufficient length that they are unaffected by the aspect ratio (L/D), whereby they are considered to be of “infinite” length. However, it is apparent from the literature that aspect ratio does have a significant non-linear effect at short L/D ratios. DNV provides guidance on the aspect ratio effect on the drag coefficient of a cylinder, for which no experimental source data has been cited. The DNV design guidance has wide usage in the offshore industry and merits critical evaluation. This paper critically reviews the literature and presents the results of a series of tow tank experiments performed by the authors. A series of tow tank tests of a surface-piercing cylinder has been undertaken using a range of aspect ratios as well as testing the effect of various end conditions, where the effects of VIV and ventilation has been deemed insignificant. Correlation of the various data sets of the literature and the experimental test programme provides the basis for developing an alternate design guidance curve for the effect of aspect ratio on the drag coefficient of cylinders.

Dynamic Positioning System: Systematic Weight Assignment for DP Sub-Systems Using Multi-Criteria Evaluation Technique Analytic Hierarchy Process and Validation Using DP-RI Tool With Deep Learning Algorithm

The Dynamic Positioning (DP) System of a vessel involves complex interactions between a large number of sub-systems. Each sub-system plays a unique role in the continuous overall DP function for safe and reliable operation of the vessel. Rating the significance or assigning weightings to the DP sub-systems in different operating conditions is a complex task that requires input from many stakeholders. The weighting assignment is a critical step in determining the reliability of the DP system during complex marine and offshore operations. Thus, an accurate weighting assignment is crucial as it, in turn, influences the decision-making of the operator concerning the DP system functionality execution. Often DP operators prefer to rely on intuition in assigning the weightings. However, it introduces an inherent uncertainty and level of inconsistency in the decision making. The systematic assignment of weightings requires a clear definition of criteria and objectives and data collection with the DP system operating continuously in different environmental conditions. The sub-systems of the overall DP system are characterized by multi-attributes resulting in a high number of comparisons thereby making weighting distribution complicated. If the weighting distribution was performed by simplifying the attributes, making the decision by excluding part of them or compromising the cognitive efforts, then this could lead to inaccurate decision making. Multi-Criteria Decision Making (MCDM) methods have evolved over several decades and have been used in various applications within the Maritime and Oil and Gas industries. DP, being a complex system, naturally lends itself to the implementation of MCDM techniques to assign weight distribution among its sub-systems. In this paper, the Analytic Hierarchy Process (AHP) methodology is used for weight assignment among the DP sub-systems. An AHP model is effective in obtaining the domain knowledge from numerous experts and representing knowledge-guided indexing. The approach involved examination of several criteria in terms of both quantitative and qualitative variables. A state-of-the-art advisory decision-making tool, Dynamic Positioning Reliability Index (DP-RI), is used to validate the results from AHP. The weighting assignments from AHP are close to the reality and verified using the tool through real-life scenarios.

Combined Wave-Current Induced Instantaneous Liquefaction of a Sandy Seabed

Accurate prediction for the liquefaction depth of a porous seabed is crucial to the design of shallow foundations; nevertheless most previous studies are predominantly limited to wave-only conditions. In this study, the combined wave-current induced instantaneous liquefaction of a sandy seabed is investigated analytically. The explicit expression of liquefaction depth under combined wave-current loading condition is derived, which can converge to that under the linear wave condition when the current velocity approaches zero. Parametric study indicates that the effects of imposing a current onto progressive waves on the distribution of excess pore pressures and the corresponding liquefaction depth are unneglectable, especially for the opposite current conditions.

Direct Time Domain Simulations for a FPSO Tandem Offloading Operation

In this study, direct time domain offloading simulations are conducted without condensing the metocean data using High Performance Computing (HPC). With rapidly growing computing power, from increased CPU speeds and parallel processing capability, the direct time domain simulation for offloading analyses has become a practical option. For instance, 3-hour time domain simulations, covering the entire service life (e.g. 100,000 simulations for 35 years) of a floating platform, can now be conducted within a day. The simulation results provide realistic offloading operational time windows which consider both offloading operation sequence (i.e. berthing, connection, offloading duration and disconnection) and required criteria (i.e. relative responses, loads on hawser and flow line, etc.). The direct time domain offloading analyses improve the prediction of offloading operability, the sizing of the FPSO tank capacity, and the shuttle tanker selection. In addition, this method enables accurate evaluations of the economic feasibility for field development using FPSOs.

Fundamental Engineering Characteristics of Cohesive Sediments in the Northern Region of South China Sea

The northern region of South China Sea is the important strategic region for the offshore oil and gas resources development in China. The main shallow sediment of the region is the cohesive soil with diverse engineering characteristics difficult to be determined. The paper collects the comprehensive geotechnical data obtained from the laboratory test and the in-situ Cone Penetration Test (CPT) for the offshore oil and gas projects in the northern region of South China Sea, and presents the fundamental engineering characteristics of the cohesive soil. Results indicate that the cohesive soil with the low plasticity index and the low clay particle content in shallow water is obviously different from that in deep water in the northern region of South China Sea. The physical properties of the clay soil with the high plasticity index and high clay particle content in the northern deep water region of South China Sea are similar to those found in the Gulf of Mexico and West Africa. Moreover, there are two different deposit modes for the sediment in the northern region of South China Sea, which are the fine-grained and coarse-grained govern deposit modes in deep and shallow water respectively. It is found that the sleeve friction ratio of the cohesive sediment is very low in shallow water. The normalized values of the clay soil in deep water are consistent with those from the Gulf of Mexico and West Africa.

Support of Operational Decisions for Prelude’s Side-by-Side LNG Offloading

Prelude floating liquefied natural gas (FLNG) facility reached a significant milestone in June 2018 when gas was introduced onboard for the first time as part of the facility startup process, loaded from an LNG Carrier (LNGC) moored in side-by-side (SBS) configuration. This first offshore LNG SBS operation allowed Prelude’s utilities to switch from running on diesel to running on gas. SBS mooring is the base case configuration for offloading both LNG and Liquefied Petroleum Gas (LPG) into product carriers using Marine Loading Arms (MLA) once the Prelude FLNG facility is fully operational. These complex and weather sensitive operations are expected to take place on a weekly basis. This means critical decisions about weather-window and timing should be supported as much as possible by predictive analysis and modelling of forecast environment to reduce the risks. There are multiple criteria for evaluating the operability of LNGC or LPGC SBS offloading. These criteria cover the various phases of an operation, such as personnel transfer to the visiting carrier, Terminal Team Leader (TTL) transfer, spool fitting and fender lowering, approach and mooring, connection and testing of the loading arms, ramp-up of product transfer, full rate cargo transfer, loading arm purging and recovery, de-berthing, and people and hardware recovery. The criteria have been tailored to be appropriate to the phase of the operation. They comprise both environment-based criteria (maximum acceptable wind and waves conditions), and criteria related to motion or mooring (carrier roll, MLA envelope, mooring line tension, fender deflection). Motion and mooring criteria are evaluated through dynamic time-domain simulations. This allows an accurate modelling of non-linear effects, including mooring characteristics and partially filled cargo tanks. Thrusters can be used to control Prelude FLNG facility position if needed. The required thruster force to maintain the selected heading is calculated with frequency-domain calculation for all possible headings. This paper presents a visual reporting tool, developed by TechnipFMC in partnership with Shell. This tool has been used to support operational decisions during commissioning and startup, for SBS LNG and LPG import to Prelude FLNG facility. The daily reports used weather forecasts, in combination with numerical simulations, to predict the maximum motion and mooring criteria which contribute to determine both the timing and the decision to proceed with the operation. The format of the report has been designed to be user friendly for offshore operational staff, summarizing efficiently and in a visual manner the usage factors for each criterion separately. An overall operability is also presented for a quick overview. This paper also presents the details of numerical simulations, summarizes the different studies carried out to ensure the reliability of these simulations and discusses the possibilities for future development.

Model Tests and Numerical Simulation on Effect of Spudcan Penetration on P-Delta of an Adjacent Pile

Model tests under 1g condition and numerical simulations of spudcan penetration in the silty fine sand were conducted to study effects of spudcan penetration on the p-delta effect of adjacent free-head and elastic-head piles subjected to lateral and vertical head loads. The p-delta factors were defined and determined based on the maximum pile shaft bending moments and pile head deflections before and after spudcan penetration, which were used to analyze the variations of the p-delta effect with the spudcan penetration depth. The conclusions were obtained as follows. The p-delta effect depends on the pile head constraint condition, the direction of the lateral pile head load and the spudcan penetration depth. For a free-head pile, the p-delta factors decrease with increasing the spudcan penetration depth. For an elastic-head pile, the p-delta factors also decrease during spudcan penetration and the decreases depend on the spudcan penetration depth and the lateral pile head load direction. The decrease associated with the lateral load direction facing back to the spudcan is larger than that associated with the lateral load direction facing to the spudcan. Therefore, the spudcan penetration does not increase the p-delta effect of adjacent piles subjected to lateral and vertical head loads. But the maximum bending moment and its occurring position change during spudcan penetration.