Development of a Stepped Line Tensioning Solution for Mitigating VIM Effects in Loop Eddy Currents for the Genesis Spar

A number of spars are being installed in deepwater areas in the Gulf of Mexico (GoM), which are subject to loop / eddy current conditions and must be designed for Vortex-Induced Motion (VIM). This paper shows how recent advances in VIM prediction enabled an efficient and effective mooring design solution for the existing Genesis classic spar, which is installed in Green Canyon Block 205 in the GOM. The solution may also be applicable to new spar designs. During the Gulf of Mexico Millennium Eddy Current event in April 2001, the Genesis spar platform underwent vortex induced motions (VIM) which were greater than anticipated during the design of the mooring & riser systems. Analysis showed that if such large motions were to occur in higher currents in the range of the 100-year event, they could cause significant fatigue damage, and could lead to peak tensions in excess of design allowables. After a comprehensive evaluation of potential solutions, Stepped Line Tensioning (SLT) was determined to be the best approach for restoring the platform’s original mooring capacity on technical, cost and schedule grounds. SLT did not require extensive redesign of the existing mooring system of the spar. Furthermore, SLT provided a means to improve mooring integrity on an interim basis, while completing details for permanent implementation. Under SLT, the pretensions of the mooring lines are adjusted based on forecast currents in order to keep the platform below the VIM lock-in threshold at all times and for all eddy/loop current conditions up to and including the 100-year condition. High Reynolds number model tests conducted with a new test methodology were used to get a reliable prediction of the spar’s VIM response for this evaluation.

High Holding Power Torpedo Pile: Results for the First Long Term Application

Petrobras developed a new kind of anchoring device known as Torpedo. This is a steel pile of appropriate weight and shape that is launched in a free fall procedure to be used as fixed anchoring point by any type of floating unit. There are two Torpedoes, T-43 and T-98 weighing 43 and 98 metric tons respectively. On October 2002 T-43 was tested offshore Brazil in Campos Basin. The successful results approved and certified by Bureau Veritas, and the need for a feasible anchoring system for new Petrobras Units in deep water fields of Campos Basin led to the development of a Torpedo with High Holding Power. Petrobras FPSO P-50, a VLCC that is being converted with a spread-mooring configuration will be installed in Albacora Leste field in the second semester of 2004. Its mooring analysis showed that the required holding power for the mooring system would be very high. Drag embedment anchors option would require four big Anchor Handling Vessels for anchor tensioning operations at 1400 m water depth. For this purpose T-98 was designed and its field tests were completed in April 2003. This paper discusses T-98 design, building, tests and ABS certification for FPSO P-50.

Floatover Deck Installation: Case Study — Structural Efficiency of Longitudinally and Transversely-Recessed Structures

Conventional floatover deck (FOD) installation on a jacket substructure requires the top of the structure to be recessed appropriately, either in the longitudinal or transverse direction to accommodate the vessel transporting the deck. The recess direction is very important to the design of the jacket and influences the outfitting of appurtenances, the shape of the deck and the floatover concept as a whole. This study investigates the structural integrity of typical longitudinal and transverse jackets suitable for FOD installation under both inplace and installation conditions. The study also assesses the structural efficiency of the two options under various installation conditions and benchmarks their structural performance against more conventional jackets associated with lift-installed decks.

Vortex-Induced Vibration of Catenary Moored Cylindrical Structures

Mooring systems utilised for floating structures typically introduce non-linear load-excursion behaviour. This non-linear compliance and the accompanying amplitude dependent natural frequency, influences the Vortex-Induced Vibration (VIV) response of the structure. The application of linear compliance VIV modelling and experimental data has been demonstrated to produce significant uncertainties regarding VIV onset and response prediction of catenary moored cylindrical structures (Bjarke et al. 2003; Dijk et al. 2003). The vortex-induced vibration issues associated with catenary moored cylindrical structures were investigated through non-linearly compliant elastically mounted rigid cylinder experiments. In particular, third order polynomial, hard spring stiffness, (typical of catenary moorings) was considered. The effect on transverse VIV lock-in and vibration amplitudes was examined using a single degree of freedom experimental rig. The experimental rig consisted of a moderately damped, elastically mounted rigid cylinder, restricted in all but the cross-flow direction through use of linear slide mechanism. The linear and cubic compliance components were independently varied over the non-linear compliance ratio of 0 to 0.3. All experimentation was conducted within the stable sub-critical Reynolds number range. The experimental data was compared to numerical results produced by the VIV modelling software package VisFlo. The program utilises a vortex-in-cell discrete vortex numerical method that was modified to allow the inclusion of varying degrees of structural non-linearity.

A Case Study Comparison of Surface Extrapolation and Battelle Structural Stress Methodologies

A case study comparison of the surface extrapolation and Battelle structural stress methodologies has been performed on a side shell connection detail typical of a representative FPSO or tanker vessel. Computations of hot spot stress via either method are consistent with current recommended practice. Convergence analysis to determine the hot spot stresses that best serve as fatigue parameters and a fatigue comparison that employs hot spot stresses from the convergence analysis have been executed at various locations around the detail. Results are reported and discussed.

An Assessment of Structural Details in Lightweight Marine Structures

This paper presents the results of investigations into the mechanics and failure modes of structural details usually encountered in lightweight marine structures. The structural analyses are performed using non-linear finite element analysis. The stress concentration factors and expected fatigue lives of the as designed and the as built structural details are evaluated and alternative configurations are discussed with the aim of improving the designs for production.

Evaluation of a Dynamic Positioning Thruster Wake Using Laser Doppler Velocimetry

To provide information on the wake characteristics of Dynamic Positioning (DP) thrusters when operating with and without a nozzle under varied operating conditions, experiments were done on a ducted propeller model in a cavitation tunnel. The propeller flow field was measured with a two-component Laser Doppler Velocimetry (LDV) system, up to 1.5 diameters downstream. These experiments were carried out with and without the nozzle, and the results are presented here for a pitch/diameter ratio of 1.2 and an advance coefficient of 0.4. This paper shows and compares the results of the DP thruster near wake hydrodynamic characteristics obtained by the LDV system when operating with and without a nozzle.

Water Waves and Marine Structure Interaction

The purpose of this paper is to combine and extend existing potential flow theory in order to analyze the linear free surface problem of an Oscillating Water Column (OWC) device and apply it to moonpool design. Analytical results were obtained implementing the previously derived theories, and later compared to experimental results conducted at the University of New Orleans Towing Tank. The model tests consisted of a study of a cylindrical OWC. The theoretical and experimental results of the free surface for the OWC tests agree for the resonant frequency estimation response but they disagree on the amplitude of the response.

Optimal Vibration Control Strategy for Offshore Platforms Accounting for AMD Constraints

In order to control the excessive vibration of offshore platforms under wave excitations, an H2 control algorithm was presented in this paper. In the present study, noise terms for generating filtered wave loading and accounting for model uncertainty are separated. In addition, in H2 optimization problem, AMD’s capacities are considered by setting the limits of AMD stroke and maximum input control force. And the formulations of such algorithm are described. In order to investigate the feasibility and effectiveness of the proposed method, a numerical example applied to an offshore platform is presented in this paper. The numerical results demonstrate that the proposed algorithm is effective in reducing the vibration of offshore structure when there are some uncertainties in building the mathematical model of the structure. In addition, AMD designed by the proposed method can keep its operation by choosing appropriate feedback gain among several gain candidates based on the AMD limits.

Sensitivity of Mooring Line Pretension in the Design Cycle for Floating Caissons

The new parallel Tacoma Narrows Bridge being constructed by Tacoma Narrows Constructors will be mounted on two towers and these towers in turn will be supported by reinforced concrete caissons referred to as East Caisson (Tacoma side) and West Caisson (Gig Harbor side). Each Caisson is towed to the location and several stages of construction will take place at the actual site. During construction, the floating caissons will be moored in place to hold it against the flood and ebb currents in the Narrows. During the mooring system design, a desired pretension is established for the lines at each draft. However, due to practical limitations in the field some variations to this design pretension value may be expected. It is important to study the effect of this variation on the overall performance of the mooring system. In this paper, the sensitivity of the mooring line pretension on the overall performance of the mooring system for the above caisson is presented. During this study, all the variables that affect the mooring system design such as mooring system layout, mooring line makeup, anchor positions, fairlead departure angles, and fairlead locations are kept constant. The only variable changed is the pretension of the mooring lines. Two approaches for defining the variations in the pretension have been studied in this paper. In the first approach, the pretension is changed in a systematic way (predicted approach). In the second method the pretension is changed randomly. The latter is considered more likely to occur in the field for this type of complex mooring system. Both sets of results are presented for some selected drafts attained by the caisson during its construction. The difference in the results from the two methods is discussed.