Steel Lazy Wave Riser Research Articles

Extreme Response Prediction of Steel Risers Using a Four Parameter Distribution

Short-term extreme response estimates are required in many areas of ocean and offshore engineering, such as steel risers design. As in many cases, the response in non-Gaussian, a theoretical solution, is usually not readily available for this purpose. Hermite transformation and Weibull-based models, among others, are some alternatives that have been used in connection with sampled response time series. In this work, a new approach is investigated. Recently, a four-parameter distribution known as the shifted generalized lognormal distribution (SGLD) has been presented in the literature. One of its main advantages is that it covers regions of skewness–kurtosis not covered by other distributions of common use in engineering. In this paper, the performance of this distribution is evaluated in the extreme values' estimation of the utilization ratios of steel riser sections. Three alternatives for using SGLD are investigated in two case studies of different dynamic behavior. The first one is a steel-lazy wave riser (SLWR) connected to a turret-moored FPSO (floating, production, storage and offloading unit) in 914 m water depth, and the second is a SLWR connected to a spread-mooring FPSO in a water depth of 1400 m. The results obtained by the SGLD-based analysis, which considered several simulation lengths, are compared to those obtained by means of an extreme value distribution fitted to episodical extremes obtained from many distinct realizations. The results of a traditional Weibull-fitting approach to the response peaks and those obtained with a Hermite transformation-based model are also presented for comparison.

Performance comparison of stress-objective and fatigue-objective optimisation for steel lazy wave risers

ABSTRACTThe distribution of buoyancy modules is a combined factor defined by multiple design variables for steel lazy wave risers, such as the length of the buoyancy section and the module diameter. Understanding the influence trend under different optimal objectives is significant for riser design. The aim of this study was to investigate riser performance through a comparison of the optimum configurations with the initial design. Genetic algorithms were implemented to minimise the maximum stress and fatigue damage along the risers. The comparison results show that the stress-objective and fatigue-objective can lead to conflicting criteria regarding the choice of the best overall riser configuration. The buoyancy section should be as close to the seabed as possible for a lower fatigue damage. Moreover, the arch height in a nominal position can be an alternative optimal objective for minimising the fatigue damage, which can save considerable computation time.

Analytical model for transfer process of deepwater steel lazy-wave riser on elastic seabed

The steel lazy-wave riser (SLWR) is considered as an appropriate approach for deepwater riser installation due to its advantages in service performance. However, the introduction of buoyancy modules system brings about the great nonlinearity of lazy wave configuration in installation, especially during transfer process. This paper presents a comprehensive analytical model to analyze the behavior of SLWR during transfer process which is of great importance to the installation feasibility and analysis. Taking the elastic seabed and boundary-layer phenomenon into consideration, this analytical model is able to deal with the three different stages of the actual transfer process. The reliability of the proposed analytical model is verified and a typical SLWR transfer process analysis is conducted to investigate the riser configurations and some important mechanical parameters which exert influences on the riser’s performance. The proposed method provides a practical reference for the design and analysis of actual SLWR installation.

A nonlinear dynamic model for 2D deepwater steel lazy-wave riser subjected to top–end imposed excitations

ABSTRACTDeepwater steel lazy-wave riser (SLWR) has won more and more applications in deepwater oil and gas resources development. The dynamic response induced by the wave, ocean current, ship motion, internal flow and other factors in operation phase has been highly concerned by researchers and engineers. A 2D nonlinear dynamic model for SLWR based on Euler–Bernoulli beam theory with initial conditions and boundary conditions under top excitations considering the effect of ocean current and internal flow is proposed. Keller Box finite difference scheme is adopted to solve the partial differential equations. Numerical analysis is conducted to investigate the influences of tangential and normal excitations on the riser's dynamic response. The proposed model can be a basic reference for the design and analysis for deepwater SLWR.

Review of Steel Lazy Wave Riser Concepts for the North Sea

A steel riser has benefits over a flexible riser in terms of pressure rating and cost. A steel riser in lazy wave configuration (steel lazy wave riser (SLWR)) is often considered as a good alternative solution for harsh environments where large floater excursions take place. The SLWR configuration is achieved by introducing buoyancy modules into a steel catenary riser (SCR). The buoyancy modules act as a damper and isolate the floater motions from the critical touchdown area. Hence, the SLWR generally has better overall performance than an SCR configuration. This paper attempts to analyze the correlation between the geometric shapes of the SLWR configuration with its capability to absorb the dynamic loadings. For deepwater cases, the behavior of the bottom part of the riser is correlated with the velocity experiences at the riser's hang-off location. Hence, the riser's performance is analyzed by comparing the velocity at the riser's hang-off with the velocity at the sag, hog, and near touchdown. The geometric shape of an SLWR is represented by its arch height, which is the vertical distance between the lowest point at the sag and the highest point at the hog bend of a riser. The results show that there is a correlation between the arch height of an SLWR with the riser's strength and wave-induced fatigue performance. SLWR configurations with higher arch generally have greater capability to absorb the dynamic loadings, as indicated by the lower velocities along the riser, which leads to lower stress utilizations and lower fatigue damage.

Probabilistic fatigue analysis of marine structures using the univariate dimension-reduction method

Probabilistic fatigue analysis of offshore structures requires the numerical simulation of a huge number of loading cases to compute the long-term multi-dimensional integral associated to the fatigue damage assessment. This paper proposes the implementation of the univariate dimension-reduction method developed by Rahman and Xu [1] in order to compute the long-term fatigue damage more efficiently. This method is particularly attractive because it reduces significantly the number of simulations by decomposing the N-dimensional integral associated to expected long-term fatigue damage assessment into the sum of N one-dimensional integrals. In addition, this paper compares the univariate-dimension reduction method with the brute force direct integration methodology and other methods based on Taylor expansions, such as perturbation approach and asymptotic expansion method discussed by Low and Cheung [2]. Two comprehensive examples are included to show the effectiveness of the method. At first, the performance of the univariate dimension-reduction method is evaluated by assessing the fatigue damage of a theoretical structure represented by a single stress response amplitude operator (RAO). Then, in order to show a case of practical application, the fatigue damage is evaluated for a Steel Lazy Wave Riser (SLWR) connected to an FPSO in a water depth of 2200 m.

A nonlinear mechanical model for deepwater steel lazy-wave riser transfer process during installation

To meet the increasing applications of the deepwater steel lazy-wave riser (SLWR), its installation is a great challenge for SLWR which affects its service performance. This paper is focused on the transfer process during the SLWR installation which is of great importance to the installation feasibility and analysis. A comprehensive mechanical model based on the nonlinear large deflection beam theory for the deepwater SLWR transfer process is developed. The presented model is able to deal with three different stages analysis of the transfer process and the length and tension of the two cables used to lift the riser's pull-head: A&R cables of installation vessel and pull-in cable of production platform. Numerical analysis is conducted to investigate the riser configurations and some important mechanical parameters which exert influences on the riser's performance. The proposed model can be a basic reference to the design and dynamic analysis of deepwater SLWR installation.

Mathematical model of steel lazy-wave riser abandonment and recovery in deepwater

To meet the increasing applications of the deepwater steel lazy-wave riser (SLWR), the riser abandonment and recovery (A&R) is a great challenge for SLWR installation and its mechanical analysis is necessary and significant. This paper is focused on the abandonment and recovery process which is of great importance to the installation feasibility and analysis. A comprehensive mechanical model based on the nonlinear large deformation beam theory for simulating the deepwater SLWR is proposed. And a simple and suitable model for analyzing the A&R cable is developed. The presented model with detailed boundary conditions is able to deal with three different A&R methods. Numerical analysis is conducted by the finite difference method to investigate the riser configurations and somev important mechanical parameters of the pipeline and cable which exert influences on the riser's performance and the A&R cable's requirement. Parametric comparison study is conducted to analyze the advantage of different method. The proposed model can help on development of deepwater SLWR A&R design and analysis.

A nonlinear model for deepwater steel lazy-wave riser configuration with ocean current and internal flow

Steel lazy-wave riser (SLWR) has gained a viable solution as a kind of advanced steel catenary riser (SCR) to meet the increasing challenges of simple catenary riser on large hang-off tension and fatigue damage of touchdown point (TDP) in deepwater applications. The configuration and static performance of SLWR are essential during its design stage and varies a lot when subjected to loading from ocean current and internal flow. The governing equations which consist of conventional small deformation beam theory for the portion of pipeline lying on the seabed, coupled with a large deformation beam theory for the suspended section are established. The proposed model with appropriate boundary conditions can simulate the nonlinear mechanical behavior of SLWR with the effect of pipe–soil interaction, ocean current and internal flow. The finite difference method is adopted to obtain the numerical results, and comprehensive parametric analysis is carried out to investigate the influences of ocean current and internal flow on the riser’s static performance. The proposed model can be an important reference for the design and analysis of deepwater SLWR.

Three Semisubmersible Floating Production System Options With Steel Catenary Risers

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25265, ’Field Development Using Semisubmersible Floating Production System With Steel Catenary Risers in Western Australia Harsh Environment,’ by Alaa M. Mansour, Shankar Bhat, Dharma Pasala, and Dhiraj Kumar, Intecsea, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. Semisubmersible floating production systems (SFPSs) with steel catenary risers (SCRs) are attractive field-development solutions, especially in deeper water of Western Australia. However, SCR design is challenging in the offshore Western Australia environment because of severe cyclonic environments and persistent swells. This paper presents a comparison of three different field-development alternatives that use SFPSs with SCRs in deepwater Western Australia. These alternative solutions are a conventional semisubmersible hull design with steel lazy wave riser (SLWR), a deep-draft semisubmersible hull design with conventional SCRs, and a novel semisubmersible hull design with conventional SCRs. Introduction Building robustness in an SFPS, especially when personnel stay onboard during cyclonic events, requires the system to be designed to survive events with return periods larger than the typical 1,000 years adopted for survival events in the Gulf of Mexico. Events with 10,000-year return periods or greater need be adopted for Western Australia. Events with 10,000- year return periods have higher significant wave height and longer peak periods, both of which increase the SFPS motion response, especially in the heave direction. This, coupled with a design location in moderately deep water (<5,000 ft), makes SCR strength design very challenging. In addition, the persistent swell in the Western Australia area makes SCR fatigue design more challenging. There are potentially three alternatives to overcome the SCR strength and fatigue challenges. One option is to improve the SFPS motion by using a very deep- draft semisubmersible (VDDS) while maintaining a conventional SCR design. Another option is to use conventional semisubmersible design and adopt an SLWR. The third alternative is to use conventional SCRs with a novel semisubmersible hull design that does not require a deep draft but offers benign motion response, especially in the heave direction. In this paper, the three different hull/riser combinations are investigated and compared for a presumed wet-tree gas field with a host facility in 4,000 ft of water. The free-hanging-solid-ballast (FHSB) semisubmersible design fundamentally consists of a conventional semisubmersible hull with four columns connected by a ring pontoon at the keel level. The semisubmersible’s motions and overall performance are enhanced by use of a robust, low-tech feature: namely the FHSB tanks (Fig. 1). The FHSB tank is attached to the hull below keel level by means of chains, which pass through guide fairleads at the keel level and connect to deployment winches at the top of the columns. The addition of an FHSB tank to the semisubmersible results in a significant increase in the semisubmersible heave natural period, mainly through its own weight and added mass from its large surface area, while controlling the heave response in the wave-frequency range below 16 seconds.

Numerical solutions for nonlinear large deformation behaviour of deepwater steel lazy-wave riser

To meet the increasing challenges of simple steel catenary riser on large hang-off tension and fatigue damage of the touchdown point (TDP) in deepwater or ultra deepwater applications, the steel lazy-wave risers (SLWR) have gained a viable solution. This paper is focused on the static equilibrium of SLWR which is of great importance to the basic configuration design of the riser. A more accurate model based on nonlinear large deformation beam theory is proposed to simulate the suspended part of SLWR, having evident advantage over previously applied small deformation beam or catenary theory which ignores the riser's bending stiffness and is hard to simulate the rapid change of the inclination angle. The model also deals with the pipe–soil interaction for simulating the TDP. The finite difference method is applied to obtain the numerical solutions. The results are comparatively in agreement with OrcaFlex. As an application, parametric analysis of the SLWR is carried out to investigate the influences of arc length of buoyancy section, arc length of upper section and soil stiffness on the static performance. The proposed model can be a basic reference for the dynamic analysis of the deepwater SLWR.

On the Extreme Value Analysis of the Response of a Turret Moored FPSO

One of the standardized procedures used in the design of floating systems and their mooring and production lines is the so-called short-term design approach where the system is analyzed for some specific extreme environmental conditions. Along with this procedure, a nonlinear time-domain coupled dynamic analysis, considering the floater and its risers and mooring lines, can nowadays be incorporated as a feasible part of the design practice. One very important and challenging aspect of this process is concerned with the estimation of the characteristic short-term extreme values of the system response parameters based on the sampled time-series. In this paper a common procedure used to establish these extreme values for floater system response parameters, which is based upon a Weibull distribution model for the peaks of the time-series, is reviewed in the light of a recently proposed approach based on a general parametric model for the average conditional exceedance rate of peaks. It is shown that the former model corresponds to a particular case of the latter one. Numerical results are presented for the response parameters of a turret-moored Floating, Production, Storage and Offloading (FPSO) unit considering a short-term coupled analysis of the whole system under an extreme environmental condition of wind, wave, and current. Specifically, the extreme response of surge motion, top tension of the most loaded mooring line, and Det norske Veritas (DnV) codes utilization factor for the most critical section of an 0.20 m outer diameter SLWR (steel lazy wave riser) are investigated.