Edgard Borges Malta

Damping Coefficient Analyses for Floating Offshore Structures

The damping evaluation of floating offshore systems is based on the viscous effects that are not considered in numerical models using the potential theory. Usually, different techniques for different systems are used to evaluate these hydrodynamic coefficients. The total damping is separated by potential and viscous damping, the first one is evaluated numerically and the second through experiments at reduced scale model. Common techniques considering linear motion equations cannot be applied to all degrees of freedom. Some methods were compared for results of decay test, such as: exponential and quadratic fit. Fourier transform (FT) spectral analysis and Hilbert Huang transform (HHT) can be used to evaluate the signal natural frequency and with HHT this can be done during the time domain. Also, analysis through the Random Decrement Technique (RDT) is presented to demonstrate the damping evaluation for irregular waves. The method to obtain external damping was presented for the different techniques in an ITTC semi-submersible model. The linear method is not sufficient to predict the damping coefficient for all the cases, because in most of them, the viscous damping was better represented by a quadratic fit. The HHT showed to be a good alternative to evaluate damping in non-linear systems.Copyright

Conceptual Design of Monocolumn Production and Storage With Dry Tree Capability

A new concept and a preliminary study for a monocolumn floating unit are introduced, aimed at exploring and producing oil in ultradeep waters. This platform, which combines two relevant features—great oil storage capacity and dry tree production capability— comprises two bodies with relatively independent heave motions between them. A parametric model is used to define the main design characteristics of the floating units. A set of design alternatives is generated using this procedure. These solutions are evaluated in terms of stability requirements and dynamic response. A mathematical model is developed to estimate the first order heave and pitch motions of the platform. Experimental tests are carried out in order to calibrate this model. The response of each body alone is estimated numerically using the WAMIT ® code. This paper also includes a preliminary study on the platform mooring system and appendages. The study of the heave plates presents the gain, in terms of decreasing the motions, achieved by the introduction of the appropriate appendages to the platform.

The Influence at Vertical First Order Motions Using Appendages in a Monocolumn Platform

Oil discovery in ultra-deep-waters is carrying out the ocean engineering to develop new conceptions of offshore exploration and production systems. A promising alternative is a monocolumn platform with moonpool. Concept design has shown its good stability make feasible the use as offshore solution. In order to solve the problem brings by increase on water deep, the use of steel catenary risers and wellhead dry completion has been shown a good solution because it decreases the maintenance cost. However, this solution needs a very low vertical motion. In general, platform like the monocolumn present resonance on vertical motion (15–20 sec to heave motion, 20–25 sec to roll/pitch motion) close to typical wave energy. This fact is so that designers introduce mechanisms to leave the natural periods out of wave energy. The moonpool as passive absorber motion was a solution found by designers. The water inside moonpool acts out-of-phase with platform motion. This difference on phases can attenuate the platform dynamic response. Other solution is water-line-area (AWL) reduction that reduced the restoring forces and consequently increased the value of vertical motions natural periods. These two alternatives showed very efficient to improve the monocolumn performance. This paper presents the gain, in terms of motions, achieved as the appendages introduction on the platform. These appendages consist of two plates, one in form of a disk and other around it. It can be located on the bottom at platform and inserted inside at the moonpool. Model test conduced on the scaled platform shows that heave and pitch natural period increased approximately 4 seconds when this appendages is allocated on the bottom. Practically any change is noted when it is allocated only inside of the moonpool.Copyright

Numerical Moonpool Modeling

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to protect drilling equipment from environmental forces, and its design aims the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies have been carried out to predict the water dynamics inside the moonpool. At most, analytical tools have been used with experimental results, to obtain a good evaluation of viscous effects. Another line of development uses the moonpools as a device to reduce motions of ships or oil platforms. In his context, the use of moonpools in monocolumn type platforms was studied during the development of the concept, through the partnership between PETROBRAS and University of Sao Paulo–USP. An alternative that became viable in the last years is the use of numerical methods to evaluate potencial parameters, being only necessary simple experiments to obtains viscous data to complete the model. This work, that is a continuation of articles about the issue written before, intends to consolidate the calculation method of moonpool to monocolumn units.Copyright

Study of Numerical Modeling of Moonpool as Minimization Device of Monocolumn Hull

The steel catenary risers (SCR) have been studied to be used in ultra-deep oil fields. In Brazil, the oil field is going to deeper water. The geographical characteristics impose this type of production risers, and the necessity of these risers is real. However, in order to avoid risers fatigue problems, some premises related to low vertical motions have to be respected. To reach the necessary good seakeeping responses, new type of hull shape has been proposed, the monocolumn hull, to which some motion minimization devices have been developed. There are several conceptions of monocolumn type hull, but the concept presented in this paper is different from the existing monocolumn hull. To have good seakeeping capability, three main minimization devices were proposed: the structural skirt, the inclined wall, and the moonpool. The inclined wall, a variation of a section area near the water-plane, works together with the skirt, an enlarged bilge keel at the bottom of the platform, modifying the effect of damping, added mass and inertia. The moonpool is the old tank usually found in drilling units, properly modified to work as a motion minimization system. It works as a passive heave motion reduction system, shifting the natural period far from the wave spectrum. Some numerical analyses were made to present the effects obtained from these devices. These numerical results from WAMIT® were compared to experimental showing to be according to them.Copyright

Estimation of Damping Coefficients of Moonpools for Monocolumn Type Units

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to have easy installation process and protect drilling equipment from environmental forces, and it is desirable the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies, refs. /7,10,11,12,13/, have been carried out to predict the water dynamics inside the moonpool using analytical and experimental tools, this last due to the high viscous influence in the responses, and the study of coupled dynamics of floating units with moonpools have been presented in previous works (refs. /1,2,3,8,9,17/), with especial focus on FPSO’s and Monocolumn type units. In the present study, the objective is to verify the reliability of numerical approach in heave dynamic evaluation of monocolumns. With this approach the objective is to tune the natural period the floater and moonpool. Two monocolumn example units were numerically calculated using WAMIT software, making possible to verify the 6 degree of freedom of the unit and the 3 of the moonpool. As the software does not evaluate viscous forces, decay tests were performed in order to verify the external viscous damping to be considered in the numerical calculations. The calibrated numerical model transfer functions (RAO) were compared with regular and transient wave tests, where heave and moonpool heave amplitudes could be compared.© 2005 ASME

Effects of Coupled Hydrodynamic in the Performance of a DP Barge Operating Close to a FPSO

In ocean systems composed by two or more closing floating bodies, coupled hydrodynamics effects must be considered. Dynamic positioned systems (DP), for example, need an accurate determination of environmental forces to guarantee a safe operation. This work presents a numerical methodology, using the WAMIT code, to evaluate both first order motions and mean drift forces of a system composed by a DP Crane Barge operating close to a turret-moored FPSO. The first order wave forces were evaluated using the code standard method. The second order forces (or mean drift forces) were obtained using the alternative control surface method. The work discussions are centered on the effects of FPSO presence on DP Barge hydrodynamics. Two relative positions between vessels were evaluated as well as three FPSO drafts (full, intermediate and ballasted). The effects of wave incidence angle were also discussed.Copyright

Semi-Submersible Hub Platform With an Internal Dock Model Testing

The recent discoveries and development of the Pre-salt reservoir in Brazilian coast require a new logistical model for crew transportation and transhipment to the drilling and oil rigs due to the large distance from coast, harsh environment conditions and large amount of workers to be transported against the actual model adopted considering only transportation by helicopters in order to reduce overall costs. The adoption of a logistic model with maritime transportation in these scenarios could provide several advantages, however there are several challenges from the technical point of view in transhipment between ship-shaped vessels, that could represent a great limitation in terms of operational window. Previous works showed the feasibility of monocolumn platforms with an internal moonpool as a Logistic HUB [1], allowing the boat docking in sheltered conditions. This work shows an overview of the model testing of a semi-submersible with an internal dock and the comparison of the free-surface elevation and RAOs (Response Amplitude Operators) between experimental results and potential flow computations. The tests were performed for 5 headings considering 10 regular, 5 irregular and 1 transient waves under a single draft and 5 different devices to reduce wave energy in the interior region.Copyright

Wave Absorption Improvements Inside the Moonpool of a Monocolumn Platform With an Internal Dock

Monocolumn platforms are usually considered an alternative concept for drilling and offshore production unit designs. This concept could provide high stability, small motions and also storage capacity. More recently, it was considered for a logistic hub platform design. The use of this concept with an opening for boat docking seems to provide a feasible solution for the people transfer in harsh environments, especially in areas like pre-salt Brazilian offshore fields, which are also very far from the coast. Although the helicopter technology has been improved in recent years to attend transport requirements from shore to the platforms, the use of a hub platform at sea still shows some economical and operational advantages, related to the conventional operations.Although the sheltered area provided by the interior of the monocolumn platform seems to solve the transfer problem in harsh environments, the hydrodynamic behavior of the moonpool became a problem to be solved during the feasibility studies of the concept, since the moonpool water column elevations tends to be resonant in special cases. To deal with this problem, some attempts were done to absorb the wave energy inside the moonpool and eliminate its resonance, achieving the desired level of absorption. This work shows the attempts and the results obtained, which has proven the feasibility of the concept.Copyright

MPSO Design: Part 1 — Wave Excitation Forces and Moments

The MPSO is characterized by the use of hydrodynamics appendages, such as moonpool, beach and skirts, which improve the hydrodynamic behavior of the unit in waves. This type of platform may be designed for different offshore scenarios as, for example: the possibility of oil and gas storage, dry tree completion system and the use of steel catenary risers (SCR). An optimization procedure to choose the geometric dimensions of the MPSO becomes important in order to achieve the optimum hydrodynamic behavior to operate in harsh environmental conditions for each scenario. The optimization procedure might be useful in the preliminary design phases to reduce the verification time of the solution evaluated with model tests; for that reason it is necessary to create a database with experimental results to make the optimization procedure possible. The main idea of the study is to carry out an extensive experimental model test aimed at obtaining the parameters not well predicted using numerical codes. With this intent, the work is subdivided into three parts: Part 1 – Wave Excitation Forces and Moments; Part 2 – Damping and Added Mass Forces and Part 3 – Optimization Process. Results will be presented in different papers. The first one presents the experimental results for captive tests, the second one the experimental results for forced oscillation tests and the last one the methodology to use the experimental results as input in an optimization tool. The first paper presents the methodology in which nondimensional variables based on MPSO geometric characteristics were defined. These variables were related to a fixed moonpool diameter and they were determined in terms of four geometric dimensions: external diameter; height and diameter of the beach and platform draft. As a consequence, 21 different MPSO model geometries could be defined and experimentally tested in order to obtain the wave excitation forces and moments in 6 DOF. The experiments included transient waves so as to better understand the hydrodynamic behavior of the hull, such as, the response amplitude operator (RAO), cancelation points, the beach/bottom/moonpool effects for the different dimensions. The wave forces and moments obtained experimentally were compared to the results of a numerical code based on potential wave theory.Copyright