Multi Vessel Interaction in Shallow Water

A numerical package for simulating vessel motions in the time domain, WAMSIM, is extended to handle multiple moving bodies interconnected through a nonlinear mooring system. WAMSIM relies on the industry standard program WAMIT to calculate the hydrodynamic characteristics and interaction of multiple bodies in the frequency domain. The numerical code is used to simulate the motions and mooring line and fender forces of two LNG tankers moored side-by-side in shallow water. One of the gas tankers is moored to the sea floor through a turret with chain catenaries. Realistic short-crested irregular waves obtained from a Boussinesq wave model are used to force the model. Motion spectra of the simulated motions are compared to measured motions from physical scale model tests. The model shows good agreement with measured motions and mooring line forces.

Assessment of Cryogenic Fatigue Strength of MARK-III LNG Carrier Insulation System

The importance of dynamic strength characteristics of LNG cargo containment system, various research efforts including dry-drop and wet-drop tests as well as sloshing test have been carried out by GTT, shipyards, oil companies, universities as well as classification societies. However, no explicit consideration with regard to the structural performance of insulation system toward the cryogenic environment is yet given. Besides only limited information regarding experimental procedure and analysis results is shared among stakeholders, and it acts as obstacles for the development of consistent CCS (Cargo Containment System) design criteria in an effective manner. Demand for LNG carriers with increased capacity requires further development of the design technology for LNG cargo tanks to meet the safety requirements due to the larger size LNG tanks. In this study, a systematic experimental research is required for the assessment of fatigue strength at cryogenic environment as well as impact strength of insulation system. The different sized mastic support has been adopted to compare the cryogenic fatigue performance. One of the main challenges in this work is to generalize the test results for use in case of other mastic support sizes. The aim of the study is to investigate the typical failure mode and obtain the S-N data under cryogenic fatigue loading. This study will be used as a fundamental study for the cryogenic fatigue assessment for the LNGC insulation system as well as a design guideline. The fatigue tests of Mark-III insulation system are carried out at room temperature and cryogenic temperature, and considered sloshing effect (R = −10). The load levels have been determined based on ultimate strength of reinforced polyurethane foam 12.2bar.

Offshore Platform Turn Around Using the Critical Chain Project Management Method (CCPM)

The present study discusses the adequacy of the Critical Chain Project Management Method (CCPM) — also known as the Critical Chain Method — for scheduling projects involving shutdowns on oil platforms, as such projects involve decision-making processes under risk conditions. The CCPM is based on the Theory of Constraints and aims at providing more precise and more clearly focused control instruments than those traditionally used in the Critical Path Method (CPM). The CCPM also indicates the best moments to act and where and how the action should be directed. The hypothesis underlying the research is that the CCPM portrays, more adequately than the CPM, the uncertainty that exists in a platform shutdown. This characteristic also makes it possible to draw up a schedule that is both more realistic and more challenging, as it addresses the goal of causing less interruption of production. On the basis of this hypothesis, the two main questions that oriented the entire investigation were: 1) Is the CCPM suitable for scheduling the shutdown of an offshore oil platform and, 2) What advantages might it have over the traditional methods in use? To answer these questions the authors reviewed the existing bibliography on the topic and made direct on-site observations during an actual shutdown. In addition, interviews were held with a number of specialists in the area using qualitative approaches, namely, semi-structured interviews, focus groups, and action research.

A Phenomenological Model for Vortex-Induced Motions of the Monocolumn Platform and Comparison With Experiments

Vortex-Induced Motions (VIM) of floating structures is a very relevant subject for the design of mooring and riser systems. In the design phase, Spar VIM behavior as well as Semi Submersible and Tension Leg Platform (TLP) flow-induced motions are studied and evaluated. This paper discusses flow-induced behavior on the Monocolumn concept by presenting a phenomenological model and comparing its results with a set of experiments that took place in the IPT Towing Tank - Brazil (September 2008). The experimental results have shown some fundamental differences from previous VIM tests on other units such as Spars. This numerical model attempts to identify these disparities in order to better understand the mechanics of this phenomenon. The model is based on a time-domain, two degree-of-freedom structural model coupled with a van der Pol type wake oscillator. The comparison was performed in order to calibrate the model, to study and better understand the tests results, and finally to identify important aspects to investigate in further experiments.

Mitigation of Vortex-Induced Motions in a Monocolumn Platform

A great deal of works has been developed on the Spar VIM issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, the present paper presents a meticulous study on VIM for this type of platform concept. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are inline and cross-flow motion amplitudes, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.

Modelling of Flow Around a Square Cylinder of Different Roughness Using a Lattice Boltzmann Model

A Lattice Boltzmann model is developed to simulate flow around a square cylinder of different roughness heights. It is evident from this study that the influences of pipe roughness on the flow parameters are quite noteworthy. It is revealed that the circumferential pipe roughness has significant influences on the hydrodynamic forces, circumferential pressure distribution, vortex shedding frequency and the formation of vortices in the vicinity of cylinder. In addition, it is seen that the orientation of the roughness elements or the density of the roughness elements does not have significant influence on the flow parameters.

Linearization of Quadratic Drag to Estimate CALM Buoy Pitch Motion in Frequency-Domain and Experimental Validation

Estimate of the pitch motion of an oil offloading Catenary Anchor Leg Mooring (CALM) buoy is presented. Linearization of the quadratic drag/damping term is investigated by the frequency-domain analysis. The radiation problem is solved to estimate the added mass and radiation damping coefficients, and the diffraction problem is solved for the linear wave exciting loading. The equation of motion is solved by considering the linearized nonlinear drag/damping. The pitch motion response is evaluated at each wave frequency by iterative and various linearization methods of the nonlinear drag term. Comparisons between the linear and nonlinear damping effects are presented. Time-domain simulations of the buoy pitch motion were also compared with results from the frequency-domain analysis. Various linearization methods resulted in good estimate of the peak pitch response. However, only the stochastic linearization method shows a good agreement for the period range of the incident wave where typical pitch response estimate has not been correctly estimated.

CFD Simulation of Wave Run-Up on a Semi-Submersible and Comparison With Experiment

Use of CFD tools for industrial offshore applications is a common practice nowadays. So is the need for validation of such tools against experimental results. This paper presents one of the CFD tools, ComFLOW, which solves Navier-Stokes equations and employs an improved Volume of Fluid (iVOF) method to find temporary location of fluid’s free surface. The code is used to simulate flow around a semi-submersible offshore platform due to an incoming regular wave. In particular, wave run-up on the semi’s columns and under-deck fluid impact phenomena are investigated on high-accuracy computational grids with number of cells being in range of 10 millions. Results of numerical simulations are compared with experimental data and focus is on local fluid flow details in immediate vicinity of the platform. Wave run-up on the platform’s columns and fluid pressures at various locations, including under-deck impact, are reported and verified against the experiment for a range of incoming wave heights.

FPSO and TLWP Interacting at a Reduced Distance for Dry Tree Completion System

Nowadays, an offshore industry challenge is developing a floating system with production, storage and offloading capabilities together with a dry tree system reducing costs and maintenance. A solution could be two offshore units working at the same area. The system consists of a unit with dry completion, like a well head platform with drilling and workover facilities, and other capable to produce the oil and gas, and storage them. A better option is to have both units coupled making them work as one. By doing this, it could be reduced the radius of the mooring line footprint and the risk of clashing between the lines and shuttle tankers. The dimensions of the unit with the dry tree could be decreased because some facilities could be allocated at the large unit with storage capacity. The main goal of this paper is show the viability of this innovative system composed by two offshore units. A Tension Leg Wellhead Platform (TLWP) and a Floating Production Storage and Offloading (FPSO) coupled by synthetic ropes and non-symmetrical mooring system. Some comparisons were done between the numerical results from Numerical Offshore Tank (TPN) and physical tests carried out in NMRI (National Maritime Research Institute - JAPAN). The model scale of 1:100 and the numerical model was set up with similar main properties and equivalent lines stiffness. The models were exposed to extreme wave conditions for some incidences. Besides that, regular wave and current analyses were generated as well.

Dry Wellhead Completion for Mono-Column FPSO Units Operating in Ultradeep Waters

A floating structure is proposed to act as a dry wellhead completion support for a mono-column design developed by Petrobras: the MONOBR FPSO. After the discussion of the design issues involved, the MONOBR design is introduced and the peculiarities affecting the insertion of a floating structure into its moonpool are analyzed. In what follows, the main characteristics of the proposed floating structure layout are presented in the context of its purpose of allowing for the effortless vertical motion relatively to the housing mono-column unit, followed by the considerations pertinent to the ballast system devised to allowing for the adequate operation, by the characteristics of the roller mechanisms proposed to ensure the aligned motion desired and by the analysis of the structural issues and the proposed structural layout.