Influence From Helical Strakes on Vortex Induced Vibrations and Static Deflection of Drilling Risers

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex induced vibrations (VIV). This reduction will increase fatigue life, and also reduce drag magnification from cross-flow vibrations. But sections with strakes will also have a larger drag coefficient than the bare riser. Hence, the extension of a section with strakes along a riser should be large enough to reduce oscillations, but not too long in order to limit drag forces from current and waves. The optimum length and position for a given riser will therefore vary with current profile. Dynamic response from waves should also be taken into account. The purpose of the present paper is to illustrate the influence from strakes on VIV, as well as on static and dynamic response for a drilling riser. Hydrodynamic coefficients for a cylinder with helical strakes are found from experiments and applied in an empirical model for the analysis of VIV. The result from the VIV analysis is used for a second calculation of drag forces that are applied in an updated static analysis. Dynamic stresses from regular waves are also presented, but VIV are not considered for these cases. A simple study of length and position of the section with strakes is carried out for some standard current profiles. Results are presented in terms of oscillation amplitudes, fatigue damage, bending stresses and riser angles at ends. The study is based on test data for one particular strake geometry, but the analysis method as such is general, and the computer programs used in the study can easily apply other test data.

A Method for Fatigue Analysis of Pipelines on Topsides of FPSO Systems

Pipelines located on the decks of FPSO systems are exposed to damage due to sea waves induced random loading. In this context, a methodology for estimating the fatigue life of conveying-fluid pipelines is presented. The pipeline is subjected to a random support motion which simulates the effect of the FPSO heaving. The equation of motion of the fluid-carrying pipeline is derived by assuming small amplitude displacements, modeling the empty pipeline as a Bernoulli-Euler beam, and adopting the so-called “plug-flow” approximation for the fluid (Pai¨doussis, 1998). Random vibration analysis is carried out by the Galerkin method selecting as basis functions the natural modes of a beam with the same boundary conditions as the pipeline. The discretized equations of motion are used in conjunction with linear random vibration theory to compute the stress spectrum for a generic section of the pipeline. For this purpose, the power spectrum of the acceleration at the deck level is determined by using the Response Amplitude Operator of the FPSO hull. Finally, the computed stress spectrum is used to estimate the pipeline fatigue life employing an appropriate S-N fatigue curve of the material. An illustrative example concerning a pipeline simply-supported at both ends is included in the paper.

Simulation Program for Berthing and Mooring of Two Vessels in Consideration of Exact Hydrodynamic Interactions

A numerical simulation system has been developed for safety evaluation of berthing and side-by-side transfer offshore operations between two vessels. Water-tank experiment has been conducted on behaviors of connected two vessels in waves, providing accuracy of the system. Also behavior of two vessels during ship-to-ship transfer offshore operation of crude oil has been simulated by the present system to demonstrate usefulness of the system.

Parametric Analysis of a Lazy-Wave Steel Riser

Lazy-wave steel risers appear as a possible solution for ultra deepwater oil fields in Campos Basin. The design of such a solution, however, is a very time consuming task as several configurations must be studied, including static, dynamic and fatigue analysis. In the first cycle of design, simplified models can be used to speed up the selection process of feasible configurations. This paper presents a parametric analysis that was implemented on a computer tool, aiming to select the feasible geometric configurations for a lazy-wave steel riser in the first cycle of design.

Improved Strake Design for Vortex Induced Motions of Spar Platforms

An extensive model test program was conducted to explore the effectiveness of alternate strake designs to reduce Truss Spar VIV response. Different strake configurations were tested to minimize VIV response. The paper presents results of the model tests. The model test set-up is described, important parameters that are modeled (including hull and truss geometry, strake configuration, mass and mooring properties) and considerations of instrumentation and test methodology are discussed. The paper also describes the analysis of the test results and shows the effectiveness of new strake design. The present results are compared with VIV response of existing Truss Spars with conventional strake design.

An Analytical/Numerical Procedure for Structural Analysis of Hybrid Riser Systems

In deep and ultra-deep water petroleum exploitation activities, floating production systems such as semi submersible platforms and FPSO (Floating Production, Storage and Offloading) units have been commonly employed. However, the utilization of flexible risers in ultra-deep waters has been hindered by technical and economical reasons. On the other hand, first order motions from the floating unit due to environmental loads are not favorable to the use of Steel Catenary Risers (SCR) in a free-hanging configuration. This fact has motivated several studies on hybrid riser systems, including the system studied in this work, which is based on a sub-surface buoy with large dimensions, moored to the seabed by tethers. This system employs flexible lines connecting the floating unit to the buoy, in the region where dynamic effects are more relevant due to the floating unit motions, and also SCRs that extend from the buoy to the seabed, in the region where dynamic motions are not so significant. The objective of this work is to describe a solution procedure for the analysis of such a hybrid riser system. This procedure is based on an analytical formulation that is solved numerically. One of the main features of this procedure is the fact that it takes into account the effects of current loads acting on the lines. Current profiles can be considered, with direction and velocities varying with depth, therefore configuring a full three-dimensional solution. This procedure can be employed either as a preliminary static analysis tool, to be used in parametric studies in order to assess the feasibility of candidate configurations of hybrid riser systems, or else for the generation of finite-element meshes for a full time-domain nonlinear dynamic simulation. It is important to start the dynamic simulation from a statically balanced configuration, since the transient effects can be dramatically shortened and the total simulation time can be reduced. The results obtained from this procedure are compared with a discrete solution obtained using a nonlinear finite-element based solver. The strategy considered here is intended to be an approach that will speed up the tasks involved in the design of hybrid risers systems based on the subsurface buoy concept.

Numerical vs. Experimental Predictions of Yaw Motion of Single Point Moored Vessel

The accurate prediction of SPM vessel yaw motion is important to its mooring system design. Inconsistencies have been observed between the numerical and model test predictions of offloading responses. In some cases, the numerical simulation predicted unstable yaw behavior of the vessel (fishtailing) while the model tests did not show such instability. This discrepancy between experiment and theory casts doubt as to whether the numerical simulation predicts correctly the vessel yaw motion. The work presented in this paper investigates the following two hypotheses to possibly explain the non-expected fishtailing in the numerical simulations: The mooring software may not accurately integrate non-linear differential equations that describe the yaw motion of the SPM vessel. Some damping terms may be under-estimated in the software (user input issue). To validate the integration scheme of the system of non-linear differential equations as implemented in the mooring software, a stability analysis has been conducted on a shuttle tanker moored to a West Africa deep water buoy. Variations of parameters like the hawser length, its axial stiffness and the vessel’s drag coefficients have been studied to explore their impacts on the vessel yaw stability. The approach is to identify without performing any time domain simulations, the domains of stability by linearizing the differential equations of SPM vessel’s yaw motion around its equilibrium point. The validity of the developed approach is then confirmed by performing time domain simulations of the same case. The second conjecture which may explain the non-expected fishtailing in numerical simulations was that some damping terms may be under-estimated. A semi empirical formula for the drag moment can be derived from rotation tests and comparisons were performed with the engineering model implemented in the mooring analysis software. The results show that by calibrating this damping term with the one derived from the experiments, the numerical simulations would match the stable yaw motion behavior as predicted during model tests. Following the above findings, a tool has been developed to fit the yaw drag moment engineering model based on experimental measurements, for any case of mooring analysis.

Riser Response Based Optimal Positioning of Deep-Water Vessels

The present paper discusses the mathematical modeling of risers and riser-like structures applied in a positioning context for deep-water floating vessels. The main purpose of the paper is to show that an estimate for the optimal vessel position, sufficient for most practical applications, is obtained from measurements of the riser inclinations or related parameters at lower end, and optionally upper end, through a solution based on the variably tensioned beam differential equation. Due to the ease of implementation this solution is well suited for direct application in on-line riser monitoring systems. The method is an attractive alternative to on-line FE-analyses, application of pre-computed regression curves based on idealized loading or black-box neural networks, which has been proposed by others to be applied as basis for interpretation of the measured riser responses. The basic idea behind the method is based on the observation that the riser inclinations or stress-joint moments at upper and lower end have mainly two causes. Firstly an effect caused by the position of the riser top end relative to the wellhead due to permanent vessel offset and slow drift vessel motions, and secondly the effects of transverse current down the riser. The general theory behind the method will be outlined. It will then be shown how the method adapts to drilling-risers with flex-joints, risers with stress-joints and also to the special case of well intervention with coiled tubing in open sea without applying a work-over or marine riser. The performance of the method is illustrated using simulated vessel and riser dynamic response data. The simulations are performed for selected vessel types both for deep-water and shallower waters applying state-of-the-art software for simulation of the riser and vessel dynamic response in random sea states.

Dimensional Study for Brazilian FPSO

After the boom of converted floating, production, storage and offloading systems, based on the old VLCC design, many engineering institutes started thinking about optimum dimension for new units. However, these new FPSOs designs carried out worldwide concerns about good seakeeping behavior when considering general weather conditions, in order to apply their project to different locations around the globe. Analyzing the Brazilian specific conditions, it was verified that, considering waves, current and wind characteristics, the dimension proportions found in the projected units were not the best options, mostly because of the swell waves influence. Thus, in a cooperative project between University of Sao Paulo and PETROBRAS, the best dimensions for a specific case were studied, based on real premises from Campos Basin. During the study, the roll motion, which usually creates operational limits during hard environmental conditions, was focused. It was possible not only to evaluate the best breath and draught relations, but also the inclusion of a structured skirt in the ship bilge. The influence of different shapes in the ship’s bow and stern were also evaluated, showing interesting results regarding the forces applied on the vessel. All the analyses were conducted considering numerical analysis, and the final dimensions were applied to a scaled model, which allowed to verify the real behavior of the projected unit in a test basin. As a conclusion, it was possible to define an optimized hull for the PETROBRAS premises, giving them a real design to be used in future explorations.

Design Drivers of Hybrid Riser Towers

DORIS Engineering and Stolt Offshore have jointly designed the three Girassol hybrid riser towers installed offshore Angola. They were the first riser towers designed to provide very efficient insulation properties in very deepwater conditions. Some towers are under development or construction for other fields but in similar design conditions. Based on the experience of Girassol, DORIS engineering and Stolt Offshore have developed the concept to adapt to much deeper waters (around 2500 m) and more severe environments such as the Gulf of Mexico loop current or hurricane waves. This paper will present the key drivers for design of riser towers in these environments addressing for example the impact on the bundle cross section, buoyancy requirements, top tank size, flexible jumper lengths. Limitations will be defined and explained both in terms of design, fabrication and installation. As FPSO units are likely to be part of the development of the Gulf of Mexico deepwater areas in the not-so-distant future, the benefit of riser towers will become obvious, given the existing facilities for fabrication along the coast and spreads for installation. Other areas such as the Mediterranean sea, or even the Northern North Sea are potential candidates for this type of riser arrangement which provides multiple benefits over steel catenary risers and flexible pipes.