Docking and Undocking Considerations for Floatover Analyses and Operations

An alternative to lifted installation of topsides by a derrick barge is installation of single, integrated offshore platform topsides by floatover method. Floatover installation reduces hook-up and commissioning, which results in overall schedule and cost savings. Numerous papers were written recently to describe many aspects of the floatover operations. Nature of the floatover is such that it requires detailed engineering analyses, numerical simulations, model testing, and planning to evaluate all phases of the operation [Ref 5], [Ref 6]. Proper analysis of floatover requires numerical simulations using time-domain methodology to evaluate the system non-linearities inherent in the floatover hardware, fendering, mooring lines. Normally, weight transfer stages are given a high profile however it is found that the docking and undocking stages are equally as important. These sensitive stages of the floatover operation occur when the barge is entering the jacket slot prior to the floatover and exiting the jacket slot afterwards. The operation is sensitive to the prevailing weather and the number of simulations to make sure the operations can be performed safely is significant. Results of the docking and undocking analyses usually determine the weather standby and thus workability. This paper will address the docking and undocking stages of floatover for a barge that does not have its own propulsion. The paper shall include a concurrent investigation on effects of weather criteria. Stiffness of the hardware, mating lines/cross lines, mooring lines and the effect they have on the system will be discussed.

A Frequency-Domain, Finite-Order Representation of a Catenary Riser’s Dominant Nonlinear Dynamics

A numerical simulation and system identification approach to the dynamic equilibrium of a catenary riser has been developed. A finite DOF representation of the dominant dynamics is constructed using frequency domain identification by applying nonlinear signal theory techniques on response data series when exciting the structure with sinusoidal motions at the top. Data series are obtained through numerical integration of a finite differences simulation model on the basis of the six nonlinear partial differential equations describing the riser dynamics. Dynamic equilibrium is mathematically formulated by the very same equations that implicate both geometric and hydrodynamic nonlinearities; the latter are depicted by Morison’s formula. Thus, spatio-temporal series are generated for riser bending moments induced by sinusoidal heave motions of various amplitudes and frequencies. These data are consequently transformed to the frequency domain where complex Singular Value Decomposition is applied in order to derive the full nonlinear spectrum. The significant harmonics of the riser’s spectrum for the bending moment on the 2D plane of reference are demonstrated to be the three lower odd harmonics and a set of orthogonal modes for these three significant harmonics is derived. The methodology proposed is finally applied to a typical test case for validation.

Experimental Validation of a New Shallow Water CALM Buoy Design

This paper presents an overview of the model testing of a new turret-type CALM buoy concept developed by WISON for shallow water (20m–80m) applications. In the WISON design the outer body of the buoy is hexagonal, a geometry that allows for ease of fabrication while retaining hydrodynamic efficiency. The overall objective of the model tests was to demonstrate the performance of this new design for a typical shallow water environment under both operating and survival conditions. Additionally, the model tests were intended to provide data to calibrate the numerical models for buoy motions and line tensions used in the design, and to give guidance regarding the suitability of the buoy freeboard and deckhouse arrangement.

Design Advantages on Umbilical Systems Brought by a New High Performance SDSS Tube

Subsea umbilical systems developed for deep offshore applications become more and more demanding regarding injection capacity, number of functionalities and water depth. Some applications, such as subsea boosting, subsea separation or gas lift are even more severe, leading to tube temperature, which can exceed, in some cases 70°C. These operating conditions and requirements are significantly impacting the performance of the main umbilical. The most common solution, to avoid such issues, is to design thicker tubes to improve the strength of the umbilical cross section. The positive effect of the wall thickness increase has to be opposed to major drawbacks, such as weight increase and fatigue performance degradation generating more issues than providing solutions. To face these challenges, Vallourec Umbilicals, with the technical support of TOTAL SA headquarter Technology Division, has developed a new manufacturing process for seam welded stainless steel tubes (SAF 2507), with higher mechanical properties and tighter wall thickness tolerances. The benefit of this innovation is to provide for a given application (i. e. pressure, water depth and temperature) thinner tubes able to meet severe operating conditions without impacting performances of the umbilical structure. This paper, after a description of the manufacturing process and product qualification protocol (that led to a Type Approval Certificate from Bureau Veritas in October 2012), presents the technical advantages brought by seam welded solution, compared to seamless super duplex tubes.

Experimental Analysis of Higher-Order Sliding Mode Control Applied to Dynamic Positioning Systems

The control algorithm normally used in Dynamic Positioning (DP) Systems is based on linear control theory (proportional-derivative or linear quadratic MIMO controller), coupled to an Extended Kalman Filter (EKF) to estimate the environmental forces and wave filtering. Such controllers and estimators have problems of performance and stability related to large variations of loading (for tankers for example) or environmental conditions. The adjustment of controller gains and parameters of EKF is a complex process. Therefore, other techniques are being applied. An investigation into the area of control of mechanical systems was made, carrying out theoretical and experimental studies involving nonlinear robust control techniques applied to dynamic positioning of floating vessels. Two robust control techniques were applied and compared: first order sliding mode control (SMC) and higher order sliding mode control (HOSM). It is known that the main drawback of SMC is the presence of high-frequency oscillations called chattering. This undesirable effect can be eliminated by using HOSM. In order to ascertain the performance of the controller under the DP system, time-domain simulations were done. Furthermore, the technique of sliding mode requires higher order derivatives of the vessel’s position signal. Therefore was developed an exact real-time differentiator, a mathematical technique used to obtain the signal derived from the position signal in real time. To validate the simulated controller, experimental tests were performed considering a small-scale model of a DP tanker. The results confirmed the robustness of the HOSM controller, the good performance of the differentiator and the elimination of the chattering problem.

Module and Vessel Interaction Analysis for Module Ocean Transportation

Bechtel has been contracted for and is in the process of executing multiple onshore Liquefied Natural Gas (LNG) Engineering, Procurement, and Construction (EPC) projects utilizing the modular construction strategy. Modules and associated pieces of equipment have to be shipped to the job site from various manufacturing and fabrication facilities across open oceans. Naval Architects play a key role to assure safe and effective module ocean transportation. Primary naval architectural work consists of a routing study, module design criteria definition, ballasting and stability analysis, grillage and seafastening design, transportation vessel selection and support for module load out and load in. The main challenge is to make the modules, which are originally designed for onshore assembly, sound for ocean transportation. Therefore, module design criteria related to ocean transportation become crucial. Among these criteria, the wave induced inertia loads and vessel deflection have great impact on designed module structure integrity. In order to define inertia loads and deflection appropriately, the interface between vessel and module becomes a main concern. It raises the question of whether the transport vessel and module should be fully integrated. It also increases complexity of the hydrodynamic interaction, which has been demonstrated by widely divergent methods used to address the interface issue in offshore industry. More importantly, whether or not the interface is thoroughly taken into account is critical to successful module design and fully meeting the Marine Warranty requirements. In order to achieve safe and economic module design, a direct method of integrating vessel and module is considered preferable in analysis and design when the inertia effects and structure hydrodynamic response are significant. This paper will provide an overview of integrated vessel and module interaction analysis for the module ocean transportation and focus on the method and procedure of how Bechtel performs analyses: i) spectral motion analysis with a fully coupled constitutive model and ii) vessel and module interaction analysis utilizing an integrated 3D model with fully hydrodynamic loads transferred. In order to determine realistic extreme load case, the equivalent design wave selection will be addressed as well.

Improving 3-D Variational Stereo Reconstruction of Oceanic Sea States by Camera Calibration Refinement

Studies of wave climate, extreme ocean events, turbulence, and the energy dissipation of breaking and non-breaking waves are closely related to the measurements of the ocean surface. To gauge and analyze ocean waves on a computer, we reconstruct their 3-D model by utilizing the concepts of stereoscopic reconstruction and variational optimization. This technique requires a pair of calibrated cameras — cameras whose parameters are estimated for the mathematical projection model from space to an image plane — to take videos of the ocean surface as input. However, the accuracy of camera parameters, including the orientations and the positions of cameras as well as the internal specifications of optics elements, are subject to environmental factors and manual calibration errors. Because the errors of camera parameters magnify the errors of the 3-D reconstruction after projection, we propose a novel algorithm that refines camera parameters, thereby improving the accuracy of variational 3-D reconstruction. We design a multivariate error function that represents discrepancies between captured images and the reprojection of the reconstruction onto the images. As a result of the iteratively diminished error function, the camera parameters and the reconstruction of ocean waves evolve to optimal values. We demonstrate the success of our algorithm by comparing the reconstruction results with the refinement procedure to those without it and show improvements in the statistics and spectrum of the wave reconstruction after the refinement procedure.

Calculation of Wave Height Dependent Force RAO’s on Submerged Bodies in Close Proximity to the Free Surface

Diffraction calculations overpredict motion RAO’s and force RAO’s in cases where a small layer of water is present on top of a submerged body. This was observed after conducting model tests on a free floating SSCV Thialf and a captive submerged cylinder. A parameter study is done to get a better understanding of why diffraction calculations overpredict the forces in heave direction. From this study it was observed that unrealistically high water elevations existed on top of the cylinder causing the heave forces to be overestimated. A damping lid is therefore implemented to decrease this water elevation. On top of that, a new method is developed to be able to capture the dependency of the force RAO on the wave height. This method uses the instantaneous submergence height (the height of water on top of the submerged body) to determine the time averaged force RAO for a given wave height and wave frequency.

A Comparison of Different Approximations for Computation of Second Order Roll Motions for a FLNG

The use of FLNG units for gas exploration and production offshore is a subject in study by some oil companies. More complex and sophisticated than a FPSO production plant, a gas production plant has strict motion criteria in order to have an optimal operational performance. Due to this, designers have been trying hull concepts with small initial stability and higher roll motion periods in order to reduce the unit motions and improve the plant performance. Indeed, the increase of roll natural period dramatically reduces the first order roll motions. However, the unit still responds at its resonance due to second order excitation. These kinds of loads are also more complex and require a great computational power to be evaluated. Due to its complexity, which would involve the solution of a non-homogeneous free surface boundary condition, some approximations are used in order to assess the second order loads and motions. In this paper, the different formulations for the first part of QTF, contributed by first order quantities, are revisited and the differences are highlighted. Furthermore the approximations for the computation of the second part of the QTF, contributed by the second order potential, are benchmarked for the case of a FLNG operating in deep water depth.

Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain: Semisubmersible Case

This paper is a continuation of a series of investigation for the dual stiffness approach for polyester mooring lines. Tahar et. al. (2012) has presented the global performance comparison between the dual stiffness method and the traditional method for the Spar platform. As shown in that study, there are appreciable differences between the former and the later methods especially in lateral motions, which, however, result in little difference in SCR strength response. Is it because the Spar has better motion characteristics than other wet tree floating platforms such as the semisubmersible and FPSO? This paper will investigate the effect of the dual stiffness method and the traditional method to SCR response for a Semisubmersible platform. The fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the dual stiffness approach. Two axial stiffnesses (EA) of polyester line, post installation (static) stiffness and storm (dynamic) stiffness have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. The SCR will be modeled and analyzed using ABAQUS software.