Joel S. Sales

Ultra-Deepwater Model Testing of a Semisubmersible and Hybrid Verification

Model test verification of floater systems in ultra-deep water meets limitations when it comes to available laboratory sizes. Systems in depths beyond 1000–1500 m cannot be tested at reasonable scales without the truncation of the mooring and riser system. The development of methods and procedures to overcome this problem has been addressed through extensive research programs at MARINTEK (VERIDEEP, VERIDEEP Extension, NDP, DEMO2000). This led to a hybrid verification procedure which combines reasonable truncation principles, model tests of the truncated system, and numerical simulations, to estimate the system’s response at full depth. There is, however, still a need to address the actual influence from the truncation procedure, and from the integration with simulations, on the final extrapolated full depth results. This paper presents a case study for the validation of the procedure, that compares full depth model test results of a semisubmersible in water depth 1250m against the extrapolated full depth results obtained from a truncated system of 500m. Results are presented for line tension and vessel responses in 3 seastates. In general the extrapolated full depth results were found to be in good agreement with the full depth model tests. However, the results confirmed expectation that the low frequency response has the greater uncertainties and presents the greatest challenge for the procedure.Copyright

Directional stability of the torpedo anchor pile during its installation

The so-called torpedo anchoring system is a novel and yet already intensively field-proven by Petrobras (the Brazilian oil company) offshore, Brazil. It is a pile with a specific elongated form that is buried in the sea bottom to hold mooring lines that are connected to floating production units. This is so even for very deep water (typically 1800 m). The methodology of installation of the torpedo pile so far consists of a vertical launching, starting with the torpedo above and close (typically 100 m) to the sea bottom. This installation path then occurs from the pile at the starting position with zero velocity until it reaches the bottom. During this installation, the pile travels almost freely, only dragging the mooring line. It is obvious that the object has to have a minimum directional stability to arrive vertically at the bottom. The pile becomes useless when it gets a vertical angle that is outside certain limits (typically three degrees). The present article addresses the directional stability of the torpedo pile during the installation.

Worst Sea–Best Sea Wave Group Spectra From Random Sea States

The paper presents a proposition called the worst sea-best sea (WS-BS) method that intends to identify wave group spectra that yield time series that lead, respectively, to the highest wave group excitation (worst sea) and the smallest wave group excitation (best sea) to the second order slowly varying response for a floating structure model submitted to random waves in a wave basin. The methodology is based on the estimation theory. Based on the fact that even with the same wave spectrum it is possible to devise different wave group spectrum, the paper proposes a method to identify the WS-BS time series. Both simulations and concrete model testing are shown indicating the feasibility of the approach.© 2008 ASME

Experimental and Numerical Analysis of the Behavior of a Monocolumn With a Moonpool

Although in most vessels the usual function of a moonpool is a passage for production lines, equipments and divers, the moonpool may be used as a device to reduce the vertical motion of floating structures as it can be observed in published studies. From these published works one may say that changes of the moonpool’s internal geometry modify the behavior of vertical oscillation of the water inside the moonpool. Looking for a “hydrodynamic design of the monocolumn-moonpool”, Petrobras has carried out a set of numerical and experimental tests with monocolumn hulls provided with moonpool. Some of these results have been published by the authors in the literature. To increase the comprehension of the dynamics of the joint motion, fluid and structure, numerical results were obtained using a commercial code based on the potential theory and a commercial code based on the solution of Navier-Stokes equations. The paper presents results of these experiments and analyzes the behavior of the coupled motions of mono-columns and the vertical motion of the water inside the moonpool, comparing the experimental and the numerical results.Copyright

An Experimental Investigation of Roll Motions of an FPSO

FPSO roll motions can be major contributor to riser fatigue. This is especially true in regions where wind, waves and currents are non-collinear. Roll motions as high as 23 degrees have been reported in the Campos Basin. The most common roll mitigation strategy consists of adding bilge keels to the FPSO. Motivation for this work came from a need to develop a better understanding of roll motions as a function of bilge keel width. In addition to roll motions, the hydrodynamic forces on the bilge keels were measured. A series of tests were conducted at the LabOceano offshore basin. This new facility has a length of 4 0 m, a width of 30 m, a depth of 15 m and is equipped with a multi-flap wave generator on one side. A ship-shaped FPSO design with sponsons for a deepwater offshore development in Brazil was tested. It has a length of 316 m, a breadth of 57.2 m and a draft of 28.3 m. A 1:70 scale model was constructed. A horizontal soft mooring system consisting of four lines with springs was used. Regular waves of different amplitudes as well as random waves were generated in the basin. Two different loading conditions, ballast (draft = 6.7 m) and loaded (draft = 21.7 m), as well as three wave headings, beam seas (90°), and quartering seas (22.5°, 45°) were considered. Tests were undertaken for four bilge keel configurations, corresponding to a case without bilge keels, as well as bilge keels of 3 different widths (1 m, 2 m and 3 m). In all cases, the bilge keels had a length of 200 m. An optical system was used to measure ship motions in all six degrees of freedom. The hydrodynamic loads on the bilge keels were measured using strain gages.© 2008 ASME

Seakeeping Tests With Gaussian Wave Packets

An important property used in the design stage of floating systems is the RAO (Response Amplitude Operator), the transfer function, for motions, forces and so on. This importance has motivated the development of several analytical, numerical and experimental tools to obtain the hydrodynamic behavior of platforms and ships. Experimental model tests in wave tanks are advisable for the accurate evaluation of the body movements. Three known techniques are used to obtain the RAO curves: tests with regular, irregular and transient waves. In the present work, special attention is given to the technique used to perform model testing with one type of transient wave: the Gaussian Wave Packet. The advantages of using such technique are discussed and results are also presented for a semi-submersible model during tests carried out at Laboratorio de Tecnologia Oceanica - LabOceano/COPPE/UFRJ, Brazil. Numerical calculations and tests with regular and irregular waves are used for validation and comparison.Copyright

Calibration of Mooring Line Damping for Taut-Leg FPSOs Model Testing

The paper discusses alternatives to represent the MLD (Mooring Line Damping) in models tests with truncated mooring lines. The work has performed both numerical experiments and reduced model tests. The results for stiffness and damping have been compared. This allows further considerations for future designs.© 2008 ASME

Consequences of a Non-Linear Asymmetric Restoring Force on the Offshore Barge Rolling Motion

A certain barge widely used for offshore operations in Brazil has a ramp to allow riser and pipeline installations. This ramp crosses the free surface, leading to a non-linear restoring force. The restoring force is also asymmetric, since the ramp is placed in starboard side. Because of the irregular geometry, the predicted hydrodynamic behavior obtained considering potential theory is affected. It is usual consider using duffing equation (inclusion of a cubic term in restoring force) to reproduce GZ curve for analysis at large angles. Although non-linear, the behavior of Duffing model (limits of stability) has been stressed by scientific community and can be considered well-known. Unfortunately, for this analysis, duffing model is not the most appropriated to be used, because of the asymmetry. A quadratic model of restoring force may present better results describing roll evolution and should be investigated. Several studies have been done to improve the dynamic behavior of the barge. As a part of this effort, model tests have been conducted to allow the installation of bilge keels to control the rolling behavior. This was a good opportunity to access the non-linear behavior what will permit comparison with theoretical analysis.© 2008 ASME

Development of a Training Simulator for Oil Spill Response

The paper describes the development of a training simulator for boats used on Oil Spill response at sea. The simulator models the dynamics of tug boats pulling an oil boom under waves, wind and current. The boom is modeled as a flexible line connected by lumped masses and its flotation characteristics and loads are calculated by Morison equation. A simplified model is used to simulate the oil itself and its interface with the boom line. The verification process was applied and its outcomes are discussed. Also, some case study scenarios are presented and the results are used to evaluate the applicability of the simulator as a training system.Copyright

Uncertainty Analysis for Inclining Tests

In this paper, we addressed the qualitative consequences on uncertainty for the execution of a inclining test of a Semi-submergible platform with mooring system and risers at production site and compared the results to the ones taken from typical inclining test procedures at sheltered waters, as defined by ASTM F1321. To accomplish that, we applied Uncertainty Analysis according to ISO procedures, by evaluating propagation of uncertainties from the measurements until the final calculations. We discussed the use of measurement devices for the variables of concern and performed some numerical simulations to address the mooring system restoring effects and its implications on uncertainty analysis. We used data from two semi-subs with different displacements and found small influence on final uncertainty from the addition of mooring system and risers. At end, we discussed how new technologies on data acquisition and filtering of signals can become an important tool to help in the safety of the offshore floating production units, by facilitating the verification of the updated center of gravity of them after important interventions.Copyright