Tandem Offloading Research Articles

Assessment of a Full-Electric Floating Liquefied Natural Gas Concept for Harsh Sea State Conditions at Remote Ultra-Deepwater Locations

_ This work describes an offshore gas monetization solution for harsh environmental conditions at ultra-deepwater and remote locations, as an alternative to conventional onshore plants associated with long subsea pipelines for Greenfield projects. Although gas-to-liquids and marine compressed natural gas are initially evaluated, associated gas is supposed to be exported from deepwater floating production storage and offloading units to a central floating liquefied natural gas (FLNG) facility for additional treatment, fractionation, and liquefaction, producing liquefied natural gas (LNG) and liquefied petroleum gas (LPG) for further tandem offloading. To demonstrate the technical feasibility of the proposed FLNG, its key technologies are reviewed, including liquefaction process selection, cargo containment system, station-keeping, offloading, and power generation. A full-electric FLNG is proposed, replacing conventional feed gas power generation systems with inner hull nuclear power modules, envisaging neutral CO2 emissions and earlier first LNG drop milestone achievement. The FLNG general arrangement is developed, along with its internal cargo tanks layout, for preliminary LNG and LPG production rundown and feed gas throughput. Numerical analyses are carried out to show that static and dynamic tilt criteria can be met, even for harsh sea states. Gas import riser configurations for the challenging ultra-deepwater location are outlined, after appropriate FLNG station-keeping analyses, in addition to a feasibility assessment of seawater intake risers, aiming for an overall gas plant efficiency increase. Keywords FLNG; FPSO; LNG/LPG; associated gas; design (general)

Armored steel wire stress monitoring strategy of a flexible hose in LNG tandem offloading operation

Cryogenic flexible hose is the key equipment for tandem offloading operation of Floating Liquefied Natural Gas (FLNG). The harsh marine environment leads hulls motion responses and complex hose motion characteristics during the tandem offloading, thus causing various failure behaviors of the entire hose. In this paper, a combined global-local monitoring strategy for armored steel wire stress distributions in cryogenic flexible hose is proposed. High-precision flexible hose governing equations are established by using the function to fit the relationship between the vertical force and the hose length. A reasonable monitoring strategy is given based on the pipeline configuration variations and sensor accuracy. Real-time monitoring of the flexible hose from the global (hose) to the local (armored steel wire) during the tandem offloading is carried out by installing sensors in specified locations on hulls and hose segments. The tandem offloading operation under different (static, calm and harsh) sea states is modelled by Orcaflex software. An armored steel wire model is developed using ABAQUS software. Comparisons of the pipeline configurations, curvatures, axial forces and maximum wire stresses are made with the simulation results to verify the feasibility of the monitoring strategy. The results show that the proposed monitoring strategy in this paper can accurately and effectively obtain the hose motion characteristics and maximum stress states of armored steel wires under different sea states, and can evaluate the hose structural safety in real time during the tandem offloading operation.

Human-automation interaction centered approach based on FRAM for systemic safety analysis of dynamic positioning operations for offshore tandem offloading

The offshore and maritime industry comprises technologically complex vessels and facilities with increasing automation. However, automation may introduce human-automation interaction problems with possibly catastrophic consequences. Dynamic positioning (DP) operations are typical of high exposure to such threats. This study develops a human-automation interaction centered approach for systemic DP operational safety analysis in offshore tandem offloading. The proposed methodology utilizes a prevailing systemic method, i.e., the functional resonance analysis method (FRAM), to resolve holistic modeling of human-automation interaction. Therein, a novel semi-quantitative framework of FRAM based on Monte Carlo simulation is integrated and improved to facilitate traditional FRAM analysis. A case application of the methodology is presented to demonstrate the usefulness for unraveling human interactions with the DP system and enhancing operational safety. An integral FRAM model is established to investigate the performance variability of human and DP automation and their various manners of aggregation in the interactive context of tandem offloading. The results reveal critical functions, couplings, and interrelated resonant paths for systemic analysis of the emergence of tanker drive-off and collision scenarios. The FRAM analysis embraces Safety-II and resilience engineering perspective to devise more proactive safety strategies through managing variability and augment DP safety management.

A multi-layer FRAM based approach to short-term human-automation resilience assessment: A case study on dynamic positioning system in offshore tandem offloading process

Resilience engineering brings a new perspective for managing safety against unexpected and unpredictable disturbances. Many engineering systems require coordinating automation and human elements for achieving safe and resilient operations. This paper emphasizes the distinct essence of short-term human-automation resilience and proposes a systemic assessment methodology based on the functional resonance analysis method (FRAM). In this methodology, a multi-layer FRAM based modeling approach is developed to capture the emergent property of short-term resilience manifested from human-automation interaction and cooperation. An improved bidirectional algorithm is integrated with Monte Carlo simulation for numerical modeling of performance variability across the multi-layer FRAM model and thus quantifying resilience using a probabilistic variability-based metric. A case study is presented for applying the proposed methodology to the dynamic positioning system in the offshore tandem offloading process. The results show that it is useful to assess short-term human-automation resilience in different scenarios and guide decision-making and countermeasures for enhancing system resilience.

The Study of Tandem Offloading Performance and Operability on The Cylindrical Hull FPSO Sevan Stabilized Platform with Variation in Mooring System Configuration

This study has been carried out to evaluate the dynamic behavior of the Cylindrical FPSO Sevan Stabilized Platform (SSP) and the LNG Carrier (LNGC) during the process of tandem offloading. The study includes hydrodynamics modellings, computations, and simulations of both cases SSP and LNGC operated individually and in combination for offloading operations. The SSP is moored with two variations of mooring, namely taut and catenary. Environmental loads are waves with the incorporated winds and currents propagating 90o, 210o, and 330o relative to the SSP headings. Excitation of random waves up to Hs = 4.50 m instigates the relatively low SSP motions in standalone condition. In offloading condition, when LNGC is connected, the SSP motion could magnify as much as 2.0 up to 5.0 times higher than that in standalone condition, but still considered in an acceptable level. The motion quality of LNGC in offloading operation is comparable with the SSP. For various random wave headings with Hs = 4.50 m during offloading operation may generate maximum tensions between 1,600 kN up to 2,600 kN in the casse of catenary mooring, and between 4,700 kN up to 7,000 kN in the case of taut mooring. Even then, this largest tension preserves a safety factor of 2.05 which is well above the limit of 1.67 as required by the governing standards. Finally, the study conclude an operability of as much as 90% could be achieved on SSP and LNGC offloading operation in the Masela Block of the Abadi Gas Field.

Development of a weighted probabilistic risk assessment method for offshore engineering systems using fuzzy rule-based Bayesian reasoning approach

This study presents a new safety methodology that is capable of transforming qualitative expert judgement into probabilistic risk outcomes for offshore engineering systems. In the framework, fuzzy set theory is applied to describe variables. Fuzzy data of each input is expressed in terms of the belief degree format representing the extent to which the fuzzy data belongs to the associated fuzzy set. Such data is subsequently combined to derive appropriate consequents using a fuzzy rule approach considering the weights of each input. The information generated in the antecedent and consequent of each rule are then synthesized to reach the fuzzy conclusions using the Bayesian reasoning approach. Such fuzzy conclusions are defuzzified and consequently transformed into the probabilistic nature. The framework is validated using two axioms and demonstrated by a risk study of the propulsion malfunction of an offshore Floating Production, Storage and Offloading (FPSO) during tandem offloading operations. The results are consistent with the axioms since the outcomes are sensitive to the minor alterations of input data and weights. It is concluded that the new approach produces reasonable results considering input weights and the logicality between inputs and outputs without losing too much useful information in the inference process.

Project integrated LNG offloading availability assessment for FLNG

This extended abstract explains a combined heuristic, analytical, and probabilistic process to evaluate LNG offshore offloading availability in combination with facility uptime and commercial drivers such as LNG sales/supply contracts. The heuristic assessment is informed by facility operators’, LNGC masters’, and tug operators’ experiences in offshore offloading and berthing operations. The analytical process includes assessment of met-ocean, mooring, manoeuvrability simulation, model testing and event forecasting methods. Gaps about uncertainties for future predictions are filled by probabilistic Monte-Carlo simulations. The heuristic, analytical, and probabilistic approach, combined with commercial drivers, is put together into uptime assessment to forecast the techno-commercial performance of the facility. The uptime assessment enables: confidence on achievable LNG throughput, the best for facility configuration and size, the best for facility location and facility’s operational expenditures; contractual viability—for LNG supplier and gas off-taker; and, key to terminal performance guarantee to gas off-takers. This process has been developed within INTECSEA during the past six years and has been applied to more than 15 LNG offshore offloading facilities at varying geographical locations. This extended abstract explores the key drivers and describes the effect on those key drivers due to varying location, varying technology, or LNG sales/supply contracting strategy. The key drivers include: achievable LNG throughput, uptime, downtime, demurrage, cargo cancellation, facility downturn, and partial LNG offloading. The process described is specific to side-by-side offloading operations; however, it can also be adapted to standard jetty offloading operations and tandem offloading operations.

Hydrodynamics of an FLNG system in tandem offloading operation

Floating liquefied natural gas (FLNG) is a new type of offshore platform that consists of a ship-type floating production storage and offloading (FPSO) hull equipped with LNG storage tanks and liquefaction plants. In LNG offloading operation, tandem configuration where LNG carrier connects to the stern of an FLNG, shows its prospects. In this study, the hydrodynamic characteristics of a single point turret-moored FLNG system in tandem offloading operation are investigated by using a time-domain coupled dynamic analysis program and a set of comparative model tests. The numerical simulation model features well the hydrodynamic performance of the coupled system obtained from the experiments. The influence of distance between the two vessels on their hydrodynamic performance has been investigated. Furthermore, two different ways of connecting FLNG vessel and LNG carrier have also been comparatively studied. The numerical results show that there is significant difference at the hydrodynamic performance in the two ways of tandem offloading operations, which means that the connection between FLNG vessel and LNG carrier plays an important role. The outcome of this study would offer better understanding on the hydrodynamics of multi-bodies, which can further lead to more practical applications for the design and operation of FLNG.

Time-domain analysis of FPSO-tanker responses in tandem offloading operation

Problems experienced during Floating Production, Storage and Offloading (FPSO) tandem offloading operations were investigated. The aim of this research was to improve the reliability of such systems, and it needed a means to assess them. Time-domain simulation and analysis of offloading systems was performed using the multi-body mooring software ARIANE 7.0. Hydrodynamic interaction between the vessels was considered. The responses of the offloading system in different loading cases, different parameters of offloading hawsers and the effects of challenging environmental conditions were calculated. There was a focus on the problems of relative motion between the two bodies and its effects on the intensity of hawser forces. Minimum relative distance, maximum relative headings and maximum tension in the hawsers of offloading systems were obtained by time-domain analysis. The time-domain analysis was effective and comparative study can be used to optimize parameters of the system and extend operating limits.

NGH-FPSO用タンデムオフローディング装置の開発

NGH (Natural Gas Hydrate)-FPSO is deemed as one of the attractive solutions for relatively small gas fields. NGH process plant is installed on board FPSO and it produces NGH from raw gas transferred directly from subsea wells through flowlines and risers. For 3 years since 2004, we have been investigating tandem offloading system for NGH-FPSO and a shuttle tanker in the south east Asia and Oceania. We evaluated workability of the tandem offloading system for NGH-FPSO under combined environmental conditions. As a result, we confirmed that workability under the offloading conditions attained 100% by using a mooring unit that has large restitution force. Based on the aforementioned results, we carried out a safety assessment of the offloading system according to HAZID (Hazard Identification Study) and were awarded AIP (Approval in Principle) by NK. Furthermore, comparison of economical efficiency between NGH-Chain and LNG-Chain, under the conditions that gas production was between 48 to 275MMscfd, the production period was 30 years, and the destination was Japan, indicated that NGH was advantageous when gas production was 80MMscfd or less. The results of investigation on the tandem offloading system for NGH-FPSO are reported in this paper.

A stability analysis of tandem offloading systems at sea

In this article, we analyze the linear stability of tandem offloading systems in wind, current, and waves. The wind and current forces are evaluated with the help of published experimental data, while the hydrodynamic coefficients and wave drift forces are rigorously estimated by using a three-dimensional singularity distribution method based on potential theory. The bow hawser and mooring lines are described quasistatically by elastic catenary equations. In order to examine the linear static and dynamic stability of the system, the equations for surge, sway, and yaw are linearized. The effect of design parameters such as turret position, mooring stiffness, and hawser length and stiffness on stability is investigated based on linearized model. The stability analysis clarifies the mechanism of the limit cycle for tandem offloading systems, which is known as fishtailing motion. The theoretical results of the shape and amplitude of the limit cycle are found to be in good agreement with those of simulations and experiments.

FPSO-Shuttle Tanker Collision Risk Reduction

To reduce the occurrence of tanker drive-off, and to reduce the probability of recovery failure should drive-off happen, are two basic principles to reduce FPSO and DP shuttle tanker collision in tandem offloading. In this paper we focus on measures to reduce recovery failure. Previous analyses revealed that recovery failure probability is high due to a lack of time for the DP operator to initiate the recovery action in a tanker drive-off scenario. To increase the time window and to reduce the operator reaction time are therefore two main principles of recovery failure reduction. A substantial increase of the separation distance between FPSO and the tanker, and a possible reduction of the main propeller(s) forward thrust that can be used by the tanker DP system are two measures to increase the time window. The effective reduction of operator reaction time can be achieved by a number of measures, i.e., selection, training, procedure, crew resource management, and automation support, which are designed to improve early detection as well as effectively reduce the DP operator’s time in diagnosis and situation awareness.

Probabilistic modeling and evaluation of collision between shuttle tanker and FPSO in tandem offloading

DP shuttle tanker drive-off in tandem offloading poses a threat of collision to Floating Production, Storage, and Offloading unit (FPSO) with potential serious consequences. A two-stage probabilistic model of FPSO and tanker collision in the tandem offloading operation is developed. The first part of analyses based on this model deals with tanker drive-off scenarios. Findings reveal that the tanker drive-off frequency in tandem offloading is high. Based on analyses of incidents, failure prone situations, termed as excessive surging and yawing, under which tanker drive-off frequently happened, are identified. Measures to eliminate excessive surging and yawing, which ultimately reduce tanker drive-off occurrence, are proposed. The second part of analyses deals with recovery action initiated by tanker DP operator in drive-off scenario. Given the favored recovery strategy by the majority of operators, analyses revealed that recovery failure probability is still high due to lack of time for DP operator to initiate the recovery action in tanker drive-off scenario. Measures to increase available time, and to reduce operator reaction time, are proposed to reduce the recovery failure.

Prediction of Relative Motion and Probability of Contact Between FPSO and Shuttle Tanker in Tandem Offloading Operation

Excessive relative motions between Floating Production Storage Offloading Unit (FPSO), and tanker, which are termed as excessive surging and yawing events, are identified as the “failure prone situation” in tandem offloading. These events have contributed to the initiation of tanker drive-off in most collision incidents that happened in the North Sea in recent years. To estimate and reduce the probability of excessive surging and yawing events in tandem offloading, a simulation-based approach, which is based on a state-of-the-art time-domain simulation code SIMO, is presented in this paper. A typical North Sea FPSO and a DP shuttle tanker simulation models are setup in SIMO and calibrated by full-scale measurements. The simulated relative distance and relative heading between FPSO and tanker are analyzed by fitting their extreme values into statistical models which give out probabilities of excessive surging and yawing events. Sensitivity studies are performed to pinpoint contributions from various technical and operational factors. Measures to minimize the occurrence of excessive surging and yawing events are identified in design and operational perspectives.

Development of Dynamic Positioning System of Coupled Floating Bodies by Neural Networks

Recently, it is in a tendency that the ocean oil field development is expanded to the deep sea and/or severe external force areas asking for new petroleum resources. To maintain high operating rate of the floating oil production systems, which are promising in the deep-sea area, it is important to reduce relative horizontal distance and hawser tension between FSPO and the shuttle tanker at tandem offloading configuration even in the severe sea conditions. In this study, a neural network control technology, which enables to reduce hawser tension by controlling FPSO's yaw angle has been developed. The control algorithm has been verified to raise wave height limit of work from 3.5m to 4.5m through computer runs and model tests in an experimental tank.