Caisson Breakwater for LNG and Bulk Terminals: A Study on Limiting Wave Conditions for Caisson Installation

As the worldwide oil and gas market continues to grow and environmental concerns with respect to in-port offloading of gas have increased, there has been a boom of interest in new liquefied natural gas LNG terminals in the past years. Loading - offloading operations at LNG and bulk terminals are generally protected by a breakwater to ensure high operability. For these terminals, caisson breakwaters are generally a preferred solution in water depth larger than 15 m due to its advantages compared to rubble mound breakwaters. The caisson installation is generally planned to be carried out in the period where sea conditions are relatively calm. However, many of these terminal locations are exposed to swell conditions, making the installation particularly challenging and subject to large downtime. There is no clear guidance on the caisson installation process rather than contractors’ experiences from different projects/sites. Therefore, studies are required in order to provide general guidance on the range of acceptable wave conditions for the installation operations and to have a better understanding of the influence of the caisson geometry. This paper presents a numerical study to determine the limiting wave conditions for caisson installing operations at larger water depth of 30–35 m for a confidential project along the African coast. Three caisson sizes/geometries are considered in order to assess and compare the wave-structure hydrodynamic interaction. The linear frequency-domain hydrodynamic analysis is performed for various seastates to determine the limiting wave conditions. Viscous effects due to flow separation at the sharp edges of the caisson are considered by using a stochastic linearization approach, where empirical drag coefficients are used as inputs. Parametric studies on caisson size and mooring stiffness are also presented, which can be used as a basis for future optimization. The uncertainty in the applied empirical viscous drag coefficients taken from the literature is examined by using a range of different drag coefficients. Further, the use of clearance-independent hydrodynamic coefficients (e.g. added mass and damping) may be questionable when the caisson is very close to the seabed, due to a possible strong interaction between caisson bottom and seabed. This effect is also checked quantitatively by a simplified approach. The findings of the study are presented in the form of curves and generalized to be used by designers and contractors for general guidance in future projects.

Effect of Spudcan Sliding on Structure and RPD of Jack-Up Platform

In the same sea area, with the increase of the number of operations, the situation of jack up offshore drilling platform carrying out secondary or multiple Jack-up leg embedment operations in the vicinity, or even in the same location will increase year by year. Therefore, the operation of jack up offshore drilling platform makes the sea area “step on the footprints” The problems become more and more frequent, and have a significant impact on the operation safety of offshore platforms. During the “footprint” process of jack-up drilling platform, the spudcan will slide transversely along the direction of Jack-up leg pits to varying degrees. If the sliding distance is too large, the leg or platform structure will be damaged. Based on the symmetrical structure of the three-leg jack-up offshore platform, the bow and the starboard spudcan are slided along x-axis at different angles, respectively. According to the critical bearing capacity of the platform legs, the critical distance of spudcans sliding is calculated. Finally, the relationship between the sliding distance of spudcan in different directions and RPD (Rack phase difference) is obtained by monitoring the RPD value of legs. It provides meaningful guidance for platform designers and operators.

Continuous Drilling Sensor Data Reconstruction and Prediction via Recurrent Neural Networks

There is an ever-increasing amount of data being recorded in oilfield operations. During drilling a well a large number of parameters is being monitored and saved, often reaching several hundreds. We are seemingly monitoring everything, from basic parameters such as Weight on Bit, Torque, and Rate of Penetration (ROP), to the exhaust temperature of engine no. 3. Unfortunately, the quality of collected data does not match the quantity. Critical sensors, such as gamma and inclination, are often lagging many meters behind the bit. Despite best efforts, sensors stop working, hard drives corrupt files, and data mud pulse telemetry uplinks fail. Methods of infilling data spanning many meters or minutes are necessary. We present a novel approach that enables reliable prediction of data lagging behind the bit through deep neural networks by merging trend-based prediction with traditional neural network approach. We were able to predict continuous inclination data in a curved section of a well with an average absolute error of only 0.4 degrees up to 20 meters from last known value.

Second-Order Difference-Frequency Loads on FPSOs by Full QTF and Relevant Approximations

This paper investigates three different approximations of the full Quadratic Transfer Function (QTF) for calculating horizontal plane second-order difference-frequency loads on FPSOs, namely Newman’s approximation, full QTF without free surface integral and the white-noise approximation. Second-order excitation loads obtained from approximated QTFs are compared in frequency-domain with those obtained by the full QTFs computed from second-order diffraction/radiation analysis in WADAM. The comparison is performed for a new-build FPSO in a range of water depths and environmental combinations. The full QTFs from second-order diffraction/radiation analysis are further compared to empirical QTFs as identified from cross bi-spectral analysis of model test results in irregular waves. A mesh convergence study is presented for calculating full QTFs by the near-field approach in a second-order diffraction/radiation analysis. The importance of including viscous damping in heave, roll and pitch is illustrated for the mean wave-drift force in surge and sway. FPSO motions and mooring line tensions from fully-coupled time-domain analysis in OrcaFlex is compared when using approximated QTFs and full QTFs from second-order diffraction/radiation analysis.

A Method for the Frequency Domain Seakeeping Analysis of Offshore Structures in the Early Design Stage

The motion analysis of floating offshore structures is a major design aspect which has to be considered in the early design stage. The existing design environment E4 is an open software framework, which is being developed by the Institute of Ship Design and Ship Safety, comprising various methods for design and analysis of mainly ship-type structures. In context of the development to enhance the design environment E4 for offshore applications this paper presents a method to calculate the response motions of semi-submersibles in regular waves. The linearised equations of motion are set up in frequency domain in six degrees of freedom and the seakeeping behaviour is calculated in terms of the amplitudes of the harmonic responses. The hydrodynamic forces onto the slender elements of the semi-submersible are accounted for by a Morison approach. As the drag and damping forces depend quadratically on the amplitudes, these forces are linearised by an energy-equivalence principle. The resulting response amplitude operators of the semi-submersible are validated by comparison with model tests. The method represents a fast computational tool for the analysis of the seakeeping behaviour of floating offshore structures consisting of slender elements with circular cross sections in the early design stage.

Coupled Dynamic Analyses of Deep-Water Semi-Submersible With New Spread Mooring System

Offshore complaint structures dominate the deepwater oil exploration and production due to their adaptive geometric form and well-established construction practices. Semi-submersible is one of the widely preferred, floating production systems due to its form-dominant ability, better stability characteristics, and best constructional features. It is usually position-restrained using a dynamic-positioning system (active-restraining) or mooring system (passive-restraining); being less-sensitive to freak ocean environment is an added advantage. The Semi-submersible, chosen for the present study is based on a similar configuration of a 6th generation deep-water Hai Yang Shi You (HYSY) – 981 platforms, commissioned by the China National Offshore Oil Corporation (CNOOC) in 2012. A sixteen-point, spread catenary-mooring without submerged buoy (case-1) in the form of chain-wire-chain type configuration is used for position-restraining. Response behavior of the semi-submersible with a conventional spread catenary-mooring system with a submerged buoy (case-2) is compared. API spectrum is used for computing wind loads, while the JONSWAP spectrum is used to represent irregular waves for various directions of wave heading. The effect of non-linearly varying current is considered up to 10% of water depth. Numerical analyses of the semi-submersible are carried out under 10-years, and 100-years return period events using Ansys Aqwa. Under wind, wave, and current loads, motion responses of the Semi-submersible at 1500 m and 2000 m water depths are investigated for both the cases in time-domain. Dynamic mooring tension variations arise from the environmental loads are further investigated for a fatigue failure using the S-N curve approach. It is found that the fatigue life of the mooring lines after the inclusion of the buoy is enhanced. It was also observed that, during failure of mooring lines there is an increase in tension of the mooring lines which are adjacent to the failed mooring lines and this is due to the transfer of mooring load and hence reducing their fatigue life.

Simulating High Mode Vortex Induced Vibration of Riser in Linear Sheared Current Using an Empirical Time Domain Model

This paper addresses the performance evaluation of an empirical time domain Vortex Induced Vibrations (VIV) model which has been developed for several years at NTNU. Unlike the frequency domain which is the existing VIV analysis method, the time domain model introduces new vortex shedding force terms to the well known Morison equation. The extra load terms are based on the relative velocity, a synchronization model and additional empirical coefficients that describe the hydrodynamic forces due to cross-flow (CF) and In-line (IL) vortex shedding. These hydrodynamic coefficients have been tuned to fit experimental data and by considering the results from the one of existing frequency domain VIV programs, VIVANA, which is widely used for industrial design. The feature of the time domain model is that it enables to include the structural non-linearity, such as variable tension, and time-varying flow. The robustness of the new model’s features has been validated by comparing the test results in previous researches. However, the riser used in experiments has a relatively small length/diameter (L/D) ratio. It implies that there is a need for more validation to make it applicable to real riser design. In this study, the time domain VIV model is applied to perform correlation studies against the Hanøytangen experiment data for the case of linear sheared current at a large L/D ratio. The main comparison has been made with respect to the maximum fatigue damage and dominating frequency for each test condition. The results show the time domain model showed reasonable accuracy with respect to the experimental and VIVANA. The discrepancy with regard to experiment results needs to be further studied with a non-linear structural model.

Software Application Based on Subsea Engineering Design Codes

Development of software application for subsea engineering design and analysis is to a large extent based on codes and standards used in the offshore industry when considering subsea pipelines. In this paper a software is described which main purpose is to facilitate the design and analysis process and such that results and documentation are automatically generated to increase quality of documentation. Current scope is a standard calculation tool covering different aspects of design in compliance with relevant offshore codes. A modularization technique is used to divide the software system into multiple discrete and independent modules based on offshore codes, which are capable of carrying out task(s) independently. All modules in range operate from a project model that is accessed directly by other modules for analysis and performance prediction and allows design changes to flow through automatically to facilitate smooth communication and coordination between different design activities. All the modules have a number of common design features. The quality of an implementation of each offshore code in independent software modules is measured by defining the level of inter-dependability among modules and their interaction among them, and by defining the degree of intra-dependability within elements of a module. This modularization technique also includes other benefits, such as ease of maintenance and updates. The improvements are related to the objectives of a state-of-the-art procedure of performing engineering, design and analysis by use of offshore codes implemented in a software application. The application is developed in .NET C# language with MS Visual Studio Technology that provides a powerful graphical user interface well integrated in windows environment.

Design of Water Hammer Protection for FPSO Domestic Water System Based on AFT-Impulse

Due to the limitation of cabin space, FPSO domestic water pipe network system has the characteristics of long water delivery distance, more bending of pipeline and frequent opening and closing of valves, etc. The above characteristics are very likely to cause water hammer in the pipeline, resulting in increased risk of safe operation of pipe network system. In this paper, a FPSO domestic water system was taken as the research object. In view of the water hammer problem in the pipe network system, the model of FPSO domestic water system was established by using dynamic fluid analysis software AFT-Impulse, combined with the factors affecting the water hammer phenomenon (such as pipe diameter, velocity of wave, pipe length, pipeline roughness and valve closing time), the steady state analysis and transient analysis of multi-working conditions and multi-scenarios were realized to determine the main control factors. Based on the influence of main control factors, a comparison scheme of water hammer protection in FPSO domestic water system was proposed. Through the transient analysis of AFT-Impulse software under multi-working conditions, the optimal scheme of water hammer protection for FPSO domestic water system was obtained, which provided guarantee for the safe operation of the system.

Detailed Finite Element Modeling of a High Capacity Mooring Steel Wire Rope: Calculation of the Stress Concentration Near the Connection

The mooring of floating platforms is an important challenge for the offshore industry. It is an important part of the design engineering and, often, a critical point for the fatigue life assessment. A solution that could improve the fatigue life is to directly connect the mooring rope to the platform, without an intermediate chain. However this solution is not widespread and the behavior of a rope near such a connection is little known. The present paper proposes to better understand this behavior, thanks to a detailed finite element model of the rope. The study case is a steel wire rope directly connected to a floating wind turbine. A local finite element model of the rope has been built, where the wires are individually modeled with beam elements. One end of the rope is clamped, simulating the connection, while tension and cyclic bending oscillations are applied to the other end. A localized bending takes place near the connection, leading to stress concentration in the wires. The stress concentration and the local contact forces are calculated for each wire. These data are important entry parameters for a local failure or fatigue analysis. This latter is however not presented here. Despite IFPEN experience in the development of local finite element models of steel wire ropes, it is the first time that such a high capacity rope (MBL = 12 500 kN) is modeled. This is challenging because of the large diameter of the rope and the large number of wires. However this modeling approach is very valuable for such ropes, because the experimental tests are rare and very expensive.