Buoyancy Modules Research Articles

Estimation of hydrodynamic parameters for underwater systems using a simple off-line regression method: a case study

Estimation of hydrodynamic model parameters such as added mass parameter and drag coefficients is very crucial while mathematically modelling any underwater system. Owing to the nature of the model being coupled and its high non-linearity, estimation of these parameters becomes complicated. Various methods of parameter estimation have been currently employed, involving experiments, computational fluid analysis, and simulations, each having its own advantages and disadvantages. Most of the existing methods use sophisticated external instrumentation and experimentally estimate the parameters using computationally expensive adaptive algorithms, which may not be required or cannot be generalised to all the systems. In this paper, a simple off-line estimation method for estimating crucial hydrodynamic parameters using onboard sensors is presented. The error between the data from numerical simulations (using a mathematical model) and experiments is iteratively minimised using a gradient descent-based optimisation algorithm, by having the unknown model parameters as design variables of the error minimisation process. The method combines the properties of least squares estimation and the free decay tests, where the system can be excited with any known external inputs. A mathematical model with the unknown parameters, fully defining the behaviour of the system, is required and open loop experimental data from onboard sensors for a known input is sufficient for the estimation process, thereby eliminating the requirement of additional instrumentation. Non-linear mathematical models can be directly used in estimation, unlike few other methods which require linearisation and approximation. This method can be generalised to any system, provided sufficient information on experimental input and output, and the equivalent mathematical model of the system are available. The proposed method has been successfully implemented to estimate the added mass and drag coefficients of a standalone, single degree of freedom variable buoyancy module, ‘vBuoy’. A mathematical model defining the dynamics of the heave motion of vBuoy has been derived and the parameters involved in the model have been estimated with the proposed method. The proposed method, as well as the results, are validated by comparing the experiments and simulations at different conditions. The results showed that the proposed method was well suited for the estimation of hydrodynamic parameters of underwater systems.

Performance comparison of stress-objective and fatigue-objective optimisation for steel lazy wave risers

ABSTRACTThe distribution of buoyancy modules is a combined factor defined by multiple design variables for steel lazy wave risers, such as the length of the buoyancy section and the module diameter. Understanding the influence trend under different optimal objectives is significant for riser design. The aim of this study was to investigate riser performance through a comparison of the optimum configurations with the initial design. Genetic algorithms were implemented to minimise the maximum stress and fatigue damage along the risers. The comparison results show that the stress-objective and fatigue-objective can lead to conflicting criteria regarding the choice of the best overall riser configuration. The buoyancy section should be as close to the seabed as possible for a lower fatigue damage. Moreover, the arch height in a nominal position can be an alternative optimal objective for minimising the fatigue damage, which can save considerable computation time.

Hydrodynamics of Flexible Pipe With Staggered Buoyancy Elements Undergoing Vortex-Induced Vibrations

Flexible pipe with staggered buoyancy elements such as lazy wave riser and drilling riser has been widely used in ocean engineering. Under the influence of sea current, both of the buoyancy elements and the riser may experience vortex induced vibrations (VIV). However, when VIV occurs, hydrodynamic characteristics of the buoyancy elements and its influence on hydrodynamic force of the bare pipe still need investigation. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible pipe with staggered buoyancy elements undergoing VIV. The crossflow (CF) hydrodynamic coefficients of the flexible pipe with 25%, 50%, and 100% coverage of staggered buoyancy are obtained from model tests, using hydrodynamic forces and coefficients identification method. Then, the characteristics of added mass coefficients and excitation coefficients in CF direction are analyzed. The results show that the added-mass coefficients of bare pipe are relatively larger than those of buoyancy module, while the total mass per unit length (sum of structural mass and added mass) is consistent along the pipe. Similarly, the range of excitation coefficient on the buoyancy elements is smaller than that on the bare pipe, and their ratio is equal to the reciprocal of diameter ratio 2.5.

Drag Reduction and Vortex-Induced Vibration Suppression Behavior of Longitudinally Grooved Suppression Technology Integral to Drilling Riser Buoyancy Units

Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser. Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability. An innovative solution has been developed to address this gap: longitudinally grooved suppression (LGS). Two model testing campaigns were undertaken: small scale (subcritical Reynolds number flow), and large scale (postcritical Reynolds number flow) to test and confirm the performance benefits of LGS. The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.

Sensor Guided Robot Path Generation for Surface Repair Tasks on a Large-Scale Buoyancy Module

In the offshore oil and gas industry, the repair of damaged drilling riser buoyancy modules (DRBMs) currently involves the removal of defective regions using hand-held abrasive tools. This operation is not only time consuming, but also exposes the worker to hazardous materials generated by the grinding process. In order to increase productivity and minimize occupational health hazards, an automated robotic repair system is developed. The DRBM has a large and almost featureless surface making it particularly challenging to accurately and efficiently detect and localize all the defective regions on the surface. To address these challenges, an eye-in-hand monocular vision system is used to generate the DRBM's global surface map. The defective regions can then be identified from the map using ellipse detection algorithms, and subsequently removed with a robotic manipulator. Experimental results have shown that the proposed solution is able to accurately and efficiently remove defective regions on the surface of DRBMs.

Analysis of Buoyancy Module Auxiliary Installation Technology Based on Numerical Simulation

To reduce the requirement for lifting capacity and decrease the hoist cable force during the descending and laying process of a subsea production system (SPS), a buoyancy module auxiliary installation technology was proposed by loading buoyancy modules on the SPS to reduce the lifting weight. Two models are established, namely, the SPS lowering-down model and the buoyancy module floating-up model. The main study results are the following: 1) When the buoyancy module enters the water under wave condition, the amplitude of tension fluctuation is twice that when SPS enters water; 2) Under current condition, the displacement of SPS becomes three times larger because of the existence of the buoyancy module; 3) After being released, the velocity of the buoyancy module increases to a large speed rapidly and then reaches a balancing speed gradually. The buoyancy module floats up at a balancing speed and rushes out from the water at a pop-up distance; 4) In deep water, the floating-up velocity of the buoyancy module is related to its mass density and shape, and it is not related to water depth; 5) A drag parachute can reduce floating-up velocity and pop-up distance effectively. Good agreement was found between the simulation and experiment results.

Analytical model for transfer process of deepwater steel lazy-wave riser on elastic seabed

The steel lazy-wave riser (SLWR) is considered as an appropriate approach for deepwater riser installation due to its advantages in service performance. However, the introduction of buoyancy modules system brings about the great nonlinearity of lazy wave configuration in installation, especially during transfer process. This paper presents a comprehensive analytical model to analyze the behavior of SLWR during transfer process which is of great importance to the installation feasibility and analysis. Taking the elastic seabed and boundary-layer phenomenon into consideration, this analytical model is able to deal with the three different stages of the actual transfer process. The reliability of the proposed analytical model is verified and a typical SLWR transfer process analysis is conducted to investigate the riser configurations and some important mechanical parameters which exert influences on the riser’s performance. The proposed method provides a practical reference for the design and analysis of actual SLWR installation.

Dynamic Analysis of a Drillstring-Riser System Drilling in Deep Water

A mathematical model of a drillstring-riser system is developed to analyze the system dynamics when drilling in deep water. For the proposed model, a riser and a drillstring are combined as a pipe-in-pipe structure, along which the interactions between the riser and the drillstring are introduced by a series of spring-friction units. Numerical simulations of the developed mathematical model are carried out by Abaqus in which the finite element method is adopted. Comparing with the riser deflection calculated by previously published models which exclude the influences of the drillstring-riser interactions, the maximal riser deflection obtained by using the proposed drillstring-riser system is relatively small. Namely, the drillstring-riser interactions restrict the riser deflection. This finding indicates that, in a given safety margin of the riser deflection, the drillstring-riser system has stronger capacity of anti-deflection. Additionally, the influence of sea depth is investigated; the dynamic response of a drillstring-riser system drilling in deep water is obviously different from the corresponding condition drilling in shallow water. In order to maintain the drillstring-riser system work in a safe condition, the operations, such as thickening the riser wall, enhancing the control of the top drift, removing a part of buoyancy modules, decreasing WOB, and increasing the top tension of the riser, are suggested to be applied.

Design of a controllable variable buoyancy module and its performance analysis as a cascaded system for selective underwater deployment

Variable buoyancy systems (VBSs) are commonly used in underwater vehicles to achieve efficient heave motion. Most of the time, these systems are used as a part of conventional underwater vehicles with diverse methods to achieve the necessary buoyancy variation. The potential of VBSs as standalone systems for heave motion underwater has not been given due attention and is still an unexplored area. In this paper, the design of a VBS as a standalone module for heave control and the possibility of using multiple such modules in a cascaded structure for selective underwater deployment is presented. Such cascaded systems have a variety of applications ranging from coastal surveillance to selective deployment for ocean sampling, and so on. A metallic-bellows-based variable buoyancy module (VBM), actuated using linear actuators, is proposed here. The mathematical modelling of the VBM and its open-loop performance are analysed for suitability with regards to the system to be cascaded. Two prototypes have been developed based on the optimal design requirements and tested. The developed modules are simple and efficient for shallow water applications, and can be used as add-ons for existing underwater vehicles or can just be used as standalone systems. These two modules are then cascaded and the performance of the cascaded system is analysed with simulations and experiments. The analysis shows the suitability of the system for selective deployment at various depths.

Review of Steel Lazy Wave Riser Concepts for the North Sea

A steel riser has benefits over a flexible riser in terms of pressure rating and cost. A steel riser in lazy wave configuration (steel lazy wave riser (SLWR)) is often considered as a good alternative solution for harsh environments where large floater excursions take place. The SLWR configuration is achieved by introducing buoyancy modules into a steel catenary riser (SCR). The buoyancy modules act as a damper and isolate the floater motions from the critical touchdown area. Hence, the SLWR generally has better overall performance than an SCR configuration. This paper attempts to analyze the correlation between the geometric shapes of the SLWR configuration with its capability to absorb the dynamic loadings. For deepwater cases, the behavior of the bottom part of the riser is correlated with the velocity experiences at the riser's hang-off location. Hence, the riser's performance is analyzed by comparing the velocity at the riser's hang-off with the velocity at the sag, hog, and near touchdown. The geometric shape of an SLWR is represented by its arch height, which is the vertical distance between the lowest point at the sag and the highest point at the hog bend of a riser. The results show that there is a correlation between the arch height of an SLWR with the riser's strength and wave-induced fatigue performance. SLWR configurations with higher arch generally have greater capability to absorb the dynamic loadings, as indicated by the lower velocities along the riser, which leads to lower stress utilizations and lower fatigue damage.

Ceramic Spheres-A Novel Solution to Deep Sea Buoyancy Modules.

Ceramic-based hollow spheres are considered a great driving force for many applications such as offshore buoyancy modules due to their large diameter to wall thickness ratio and uniform wall thickness geometric features. We have developed such thin-walled hollow spheres made of alumina using slip casting and sintering processes. A diameter as large as 50 mm with a wall thickness of 0.5–1.0 mm has been successfully achieved in these spheres. Their material and structural properties were examined by a series of characterization tools. Particularly, the feasibility of these spheres was investigated with respect to its application for deep sea (>3000 m) buoyancy modules. These spheres, sintered at 1600 °C and with 1.0 mm of wall thickness, have achieved buoyancy of more than 54%. As the sphere’s wall thickness was reduced (e.g., 0.5 mm), their buoyancy reached 72%. The mechanical performance of such spheres has shown a hydrostatic failure pressure above 150 MPa, corresponding to a rating depth below sea level of 5000 m considering a safety factor of 3. The developed alumina-based ceramic spheres are feasible for low cost and scaled-up production and show great potential at depths greater than those achievable by the current deep-sea buoyancy module technologies.

Focus on Ancillary Equipment and Fatigue in Coiled-Tubing Deepwater Commissioning

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25665, “Adaptation of Coiled Tubing for Deep- and Ultradeepwater Commissioning: Focus on Fatigue and Ancillary Equipment,” by François Lirola and François-Régis Pionetti, Saipem, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. The installation of flowlines in ever-deeper and -more-remote areas requires specific technologies for precommissioning. Coiled tubing can be a solution; however, offshore precommissioning can require coiled tubing to be deployed several times for durations sometimes exceeding a month and requires larger diameters. Therefore, a campaign was initiated to characterize the behavior of coiled tubing under combined plastic and elastic fatigue. In addition, an innovative bend-stiffener design was developed to control the stress levels in the coiled tubing at the hangoff location. Introduction Precommissioning is a critical part of flowline installation and operation. This process occurs at the end of the installation to validate the integrity and performance of any system before commissioning. When no direct access is available from a surface facility, precommissioning is, most of the time, performed from a multiservice vessel (MSV) using a flexible line. However, as water depth and the diameter and length of the flowline to be precommissioned increase, the submerged weight of the line becomes an issue, requiring the installation of buoyancy modules to limit the loads transferred to the vessel. To simplify offshore precommissioning activities, the company decided to use coiled tubing. Coiled tubing offers the advantage of having a higher diameter/weight ratio than flexible lines, allowing for large-diameter tubing to be deployed from an MSV while keeping loads transferred to the vessel at an acceptable level. In addition, coiled tubing has an extensive track record onshore and most of the required components are available off the shelf. However, onshore applications of coiled tubing are essentially static while offshore precommissioning activities are dynamic. The coiled tubing hangs from the vessel and, therefore, is subject to loads from the waves, currents, and vessel motions. This means that the coiled tubing and the associated equipment should be designed to withstand extreme loads and elastic fatigue.

Topology optimization for submerged buoyant structures

ABSTRACTThis paper presents an evolutionary structural topology optimization method for the design of completely submerged buoyant modules with design-dependent fluid pressure loading. This type of structure is used to support offshore rig installation and pipeline transportation at all water depths. The proposed optimization method seeks to identify the buoy design that has the highest stiffness, allowing it to withstand deepwater pressure, uses the least material and has a minimum prescribed buoyancy. Laplace's equation is used to simulate underwater fluid pressure, and a polymer buoyancy module is considered to be linearly elastic. Both domains are solved with the finite element method. Using an extended bi-directional evolutionary structural optimization (BESO) method, the design-dependent pressure loads are modelled in a straightforward manner without any need for pressure surface parametrization. A new buoyancy inequality constraint sets a minimum required buoyancy effect, measured by the joint volume of the structure and its interior voids. Solid elements with low strain energy are iteratively removed from the initial design domain until a certain prescribed volume fraction. A test case is described to validate the optimization problem, and a buoy design problem is used to explore the features of the proposed method.

해양 라이저의 부력재 최적 배치를 위한 시뮬레이션 기반 설계 기법에 관한 연구

*** 한국해양과학기술원 부설 선박해양플랜트연구소 해양플랜트산업기술센터 ABSTRACT: This paper reports a simulation-based design method for the optimized arrangement design of buoyancy modules in a marine riser system. A buoyancy module is used for the safe operation and structural stability of the riser. Engineers design buoyancy modules based on experience and experimental data. However, they are difficult to design because of the difficulty of conducting real sea experiments and quantifying the data. Therefore, a simulation-based design method is needed to tackle this problem. In this study, we developed a simulation-based design algorithm using a multi-body dynamic simulation and genetic algorithm to perform optimization arrangement design of a buoyancy module. The design results are discussed in this paper.

심해저 채광 시스템에서 유연관의 안정적인 운용을 위한 부력재 배치 설계

This paper focuses on the efficient arrangement plan of buoyancy modules, which plan is used to secure the safe operation and structural stability of a marine riser. The marine riser is connected between a vessel and seabed devices. The movement of the vessel and the seabed devices are affected by the motion of the riser. The riser of a deep-seabed integrated mining system exerts a strong influence on the healthy transfer of minerals. So, buoyancy modules must be equipped to compensate for the problem which is the structure stability and the dynamic motion. Installation locations and quantities of the buoyancy modules are determined by real sea experiments. But this is not easy to do because in real sea experimental conditions the cost is expensive as well as being, time-consuming and dangerous. Therefore, the locations and quantities should be determined by numerical simulation. This method is called simulation-based design. The dynamic analysis models of the riser and the buoyancy modules are built into the commercial software of DAFUL.

해저 파이프라인의 전체 좌굴 제어 방법 비교

Global buckling is a bending of pipeline and it occurs when the stability of pipeline is distributed by excessive axial force. Subesea pipeline is subjected to axial force induced by temperature and pressure from well and resulting phenomena should be controlled in appropriate manner. Global buckling of subsea pipeline is still ongoing research subject and is studied various organization. In this study, various control methods such as buoyancy module, sleeper, and snake lay for global buckling of subsea pipeline were numerically investigated with various design parameters. From the numerical simulation results, the global buckling control method using sleepers shows better results than buoyancy module and snake lay control methods in the sense of combined stress after buckling. Furthermore, the global buckling of full scale pipeline of 80km with uneven seabed profile were successfully managed when the sleeper was installed.

Implementation of vision based 2-DOF underwater Manipulator

Manipulator is of vital importance to the remotely operated vehicle (ROV), especially when it works in the nuclear reactor pool. Two degrees of freedom (2-DOF) underwater manipulator is designed to the ROV, which is composed of control cabinet, buoyancy module, propellers, depth gauge, sonar, a monocular camera and other attitude sensors. The manipulator can be used to salvage small parts like bolts and nuts to accelerate the progress of the overhaul. It can move in the vertical direction alone through the control of the second joint, and can grab object using its unique designed gripper. A monocular vision based localization algorithm is applied to help the manipulator work independently and intelligently. Eventually, field experiment is conducted in the swimming pool to verify the effectiveness of the manipulator and the monocular vision based algorithm.

Vortex induced vibration excitation competition between bare and buoyant segments of flexible cylinders

This paper addresses a practical problem: “Under what fractional coverage of buoyancy modules, would the vortex induced vibration (VIV) excitation on buoyant segments dominate the response?” The source of data is a recent model test on a 38m long flexible cylinder, densely instrumented with fiber optic strain gauges and accelerometers. A pipe model with five staggered buoyancy configurations was tested. The paper emphasis is on exploring the winner of excitation competition between bare and buoyant segments and offering useful insights on the design of a pipe with staggered buoyancy modules in uniform flow. Four particular topics are covered: (i) the identification of VIV excitation regions, (ii) the experimental determination of the winner of excitation competition between bare and buoyant segments, (iii) the prediction of the winner of excitation competition, and (iv) the factors affecting fatigue damage rate for pipes with staggered buoyancy modules. Potential areas for promising additional research and a general guidance in designing staggered buoyancy pipes are proposed.

Coupled fluid–structure simulations for evaluating a performance of full-scale deepwater composite riser

A global–local analysis methodology based on fluid–structure coupling is used to investigate the mechanical responses of both composite and steel risers. Since the design of the riser system can be a daunting task, involving hundreds of load cases for global analysis, semi-empirical fluid load models are considered for the reduced order computations of full-scale riser models. The structural performance of composite risers under real sea current conditions is investigated systematically and discussed with regard to the practical concerns in full-scale settings. The failure envelops of internal liners are found to be within that of the composite layers, which reveals that the liner is the weakest link for composite riser design. Results show that the composite risers can be more prone to vortex-induced vibration (VIV) due to their lower structural frequencies. In the present study, the composite riser yields 25.5% higher RMS strains than the steel riser. Placement of buoyancy modules along the riser may be critical for the design against VIV, and our results show that the modules are not recommended at the top region of the riser, especially if a top-sheared current is expected. Instead, it is preferable to implement them at the bottom-half portion of the riser and as a continuously buoyed region rather than short discrete buoys separated with gap spaces. Composite risers with different metallic liners are studied, and the titanium liner riser is found to be favourable over the steel and aluminum liner risers.

Static analysis of deepwater lazy-wave umbilical on elastic seabed

Lazy wave configuration is considered as an appropriate approach for deepwater umbilical installation due to its advantages of accommodating large vessel offsets, decoupling the vessel motions from the touchdown point of the umbilical and reducing the hang-off tension. However, the introduction of buoyancy modules system brings about the great nonlinearity of lazy wave configuration, and the configuration is also significantly affected by the change of buoyancy modules. This paper presents an analytical model to analyze the static behavior of deepwater lazy-wave umbilical on the elastic seabed. Taking the environmental loads, elastic seabed and boundary-layer phenomenon into consideration, this lazy-wave model is divided into three parts: cable segment suspended in the water, boundary-layer segment in the neighborhood of the touchdown point and touchdown segment laid on the seabed. FEM by OrcaFlex was employed to verify the accuracy of the analytical model. The results from the two methods show excellent consistency. As well, a series of sensitivity analyses are also presented to highlight the influencing parameters in lazy wave configuration. The proposed method will provide a practical reference for deepwater lazy-wave umbilical installation.