Steel Lazy Wave Risers From Turret Moored FPSO for Deepwater Harsh Environment

2015 ◽  
Author(s):  
Adekunle Peter Orimolade ◽  
Daniel Karunakaran ◽  
Trond Stokka Meling
Keyword(s):  
Fatigue Damage ◽  
Environmental Conditions ◽  
Fatigue Performance ◽  
Optimum Configuration ◽  
Extreme Response ◽  
Out Of Plane ◽  
Steel Catenary Risers ◽  
Touchdown Point ◽  
Motion Characteristics ◽  
Wave Induced

Steel catenary risers (SCRs) have found greater applications in deep and ultra-deepwater developments. However, the deployment of SCRs in conjunction with a high motion deepwater floater such as the Floating Production Storage and Offloading (FPSO) system faces significant challenges due to their high motion characteristics, especially in harsh environmental conditions. The challenges posed by FPSO’s high motion characteristics include severe dynamic response on the SCRs and poor fatigue performance at the top section and the touchdown point (TDP) area. A number of alternative configurations of the SCR can be employed to decouple the FPSO’s motion from the SCR, thereby improving performance, and this include the steel lazy wave riser (SLWR) configuration. The lazy wave is achieved by introducing buoyancy modules along some lengths of the riser. In this work, a suitable SLWR configuration for deployment in conjunction with a turret moored FPSO was developed for a typical deepwater offshore West of Shetland environmental conditions. The optimum configuration is a low lazy wave configuration; this was achieved after several analyses using ORCAFLEX software program. In determining the optimum configuration, consideration is given to the SLWR sag and hog bend heights, the net buoyancy force, the buoyant section length, and the hang-off angle, among others. The extreme response, considering a combination of 100-year wave with 10-year current was satisfactory; the maximum stress was below the allowable stress level, and the maximum DNV utilization was less than unity, indicating a safe design. The wave-induced fatigue damage was calculated using a total of 216 load cases, resulting from 12-wave directions, and the wave-induced fatigue performance was satisfactory, with the minimum fatigue life observed at the riser’s TPD. Fatigue damage resulting from vortex induced vibration (VIV) was calculated considering currents in the in-plane and the out-of-plane directions to the riser, with a total of 22 load cases. The VIV fatigue performance was not satisfactory, and therefore fairings and strakes will be introduced to some lengths of the SLWR to suppress VIV. Detailed sensitivity studies also showed how the configuration can be further optimized. Overall, the results of this study showed that, the SLWR is a suitable riser concept for deployment from a turret moored FPSO, in deepwater, harsh environmental conditions such as offshore West of Shetland. The riser can be installed using Reeled-Lay installation method. The installation can be performed using pre-lay, abandonment, and recovery, as this offers advantages over the direct transfer approach.

Sensitivity Study of Critical Parameters Influencing the Uncertainty of Fatigue Damage in Steel Catenary Risers

2010 ◽  
Author(s):  
Feng Zi Li ◽  
Ying Min Low
Keyword(s):  
Fatigue Damage ◽  
Cost Effective ◽  
Sensitivity Study ◽  
Structural Safety ◽  
Critical Parameters ◽  
Design Parameters ◽  
Steel Catenary Riser ◽  
Bending Stresses ◽  
Steel Catenary Risers ◽  
Touchdown Point

The most challenging aspect of a deepwater development is the riser system, and a cost-effective choice is the Steel Catenary Riser (SCR). Fatigue is often a governing design consideration, and it is usually most critical at the touchdown point (TDP) where static and dynamic bending stresses are highest. Unfortunately, it is also at this region that uncertainty is the maximum. The increased uncertainty casts doubt on the applicability of generic safety factors recommended by design codes, and the most consistent way of ensuring the structural safety of the SCR is to employ a reliability-based approach, which has so far not received attention in SCR design. As the number of basic random variables affects the complexity of a reliability analysis, these variables should be selected with caution. To this end, the aim of this paper is to draw up a comprehensive list of design parameters that may contribute meaningfully to the uncertainty of the fatigue damage. From this list, several parameters are selected for sensitivity studies using the commercial package Orcaflex. It is found that variations in seabed parameters such as soil stiffness, soil suction and seabed trench can have a pronounced influence on the uncertainty of the fatigue damage at the touchdown point.

The Effect of Whipping/Springing on Fatigue Damage and Extreme Response of Ship Structures

2010 ◽  
Author(s):  
Wengang Mao ◽  
Jonas W. Ringsberg ◽  
Igor Rychlik
Keyword(s):  
Fatigue Damage ◽  
North Atlantic Ocean ◽  
Gaussian Model ◽  
Container Ship ◽  
Level Crossing ◽  
Ship Structures ◽  
High Frequency Response ◽  
The North ◽  
Extreme Response ◽  
Wave Induced

Wave-induced vibrations, also known as whipping and springing, are defined as the high frequency response of ship structures. In this paper, the fatigue damage caused by whipping and springing is presented by investigating the amidships section of a 2800 TEU container ship that operates in the North Atlantic Ocean. A simplified fatigue model, originally from the generalized narrow-band approximation for Gaussian load, is employed to include the damage contribution from wave-induced vibrations. In this model, the significant response range hs and the mean stress up-crossing frequency fz are simplified using only the wave-induced loading and encountered wave frequency, respectively. The capacity and accuracy of the model is illustrated by application on the measurements of the 2800 TEU container ship for different voyages during 2008. The whipping-induced contribution to the extreme response is investigated by means of the level crossing approach. It shows that the level crossing model for Gaussian load cannot be used for the prediction of extreme responses, such as the 100-year stress, based on a half-year full-scale measurement. It is found that a more complicated non-Gaussian model is required to consider the contribution from whipping.

Fatigue Design of the Atlantis Export SCRS

2007 ◽  
Author(s):  
Mike Vandenbossche ◽  
Will McDonald ◽  
Dingwu Xia ◽  
Craig Masson ◽  
Jie Fang ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Wave Climate ◽  
Fatigue Performance ◽  
Mardi Gras ◽  
Fatigue Design ◽  
Fatigue Damage Accumulation ◽  
Steel Catenary Risers ◽  
Sigsbee Escarpment ◽  
Girth Welds ◽  
First Time

The Atlantis semi-submersible platform is located in 7040 feet water depth in southern Green Canyon, in the Gulf of Mexico. It supports the Mardi Gras 24-inch oil and 16-inch gas export steel catenary risers (SCRs), the deepest and largest diameter SCRs in the world. Fatigue performance was one of the critical and challenging aspects of the design due to the severe wave climate coupled with the large vessel motions and strong bottom currents below the Sigsbee Escarpment. Preliminary design showed that the fatigue life at the girth welds in the touch down point (TDP) region did not satisfy the design criteria. This paper presents the fatigue design measures that were adopted to improve the fatigue performance of the Atlantis export SCRs. These include the first-time use of manually relocating the host platform to reduce fatigue damage accumulation in the TDP region, and removing the weld caps over the most critical part of the TDP region and grind flush the caps and roots of the welds in the flexible joint assembly to achieve practical defect acceptance criteria. It is demonstrated that the narrow fatigue damage peak in the TDP region can be decreased significantly by spreading it over a wider region, and all the design requirements are satisfied.

Design Features of Risers for the Extendable Draft Platform (EDP)

2004 ◽  
Author(s):  
Lixin Xu ◽  
Qi Xu ◽  
Colin Hough ◽  
John Murray
Keyword(s):  
West Africa ◽  
Gulf Of Mexico ◽  
Environmental Conditions ◽  
Floating Platform ◽  
Design Features ◽  
Field Development ◽  
Wide Range ◽  
Steel Catenary Risers ◽  
Wave Induced

The Extendable Draft Platform (EDP) is a deep draft semisubmersible with an extendable heave suppression pontoon, designed as a drilling and production floating platform for deepwater field development. The EDP supports both top tensioned risers (TTRs) for dry-trees and steel catenary risers (SCRs) for export risers and subsea wells. The TTRs can be supported at their tops by tensioners mounted on the main deck, and the SCRs can be connected to the EDP either by porches external to hull or by pull-tubes internal to the hull columns. This paper discusses design features of the EDP risers, including TTR tensioning system and coupled effects on hull motion performances, riser keel guide design and wear allowances, and SCR hang-off options. Also investigated are the effects of the EDP’s low motions on the riser systems, for instance, the wave induced fatigue of SCRs is significantly improved for the EDP, in comparison with a conventional semisubmersible of similar payload. Furthermore, designs of the EDP as well as riser systems can be optimized for different prevailing environmental conditions, such as those of West Africa, Brazil, West of Shetland, and the Gulf of Mexico. The EDP provides an effective and reliable way to support dry-trees and SCRs for a wide range of deepwater applications.

Evaluating the Coupledness of the Aerodynamics and Hydrodynamics on the Estimation of Fatigue Damage Equivalent Load for a Floating Offshore Wind Platform

2018 ◽  
Author(s):  
Samuel Kanner ◽  
Bingbin Yu
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Environmental Conditions ◽  
Offshore Wind ◽  
Coupled Analysis ◽  
Offshore Wind Turbine ◽  
Wave Conditions ◽  
Equivalent Load ◽  
The Waves

In this research, the estimation of the fatigue life of a semi-submersible floating offshore wind platform is considered. In order to accurately estimate the fatigue life of a platform, coupled aerodynamic-hydrodynamic simulations are performed to obtain dynamic stress values. The simulations are performed at a multitude of representative environmental states, or “bins,” which can mimic the conditions the structure may endure at a given site, per ABS Floating Offshore Wind Turbine Installation guidelines. To accurately represent the variety of wind and wave conditions, the number of environmental states can be of the order of 103. Unlike other offshore structures, both the wind and wave conditions must be accounted for, which are generally considered independent parameters, drastically increasing the number of states. The stress timeseries from these simulations can be used to estimate the damage at a particular location on the structure by using commonly accepted methods, such as the rainflow counting algorithm. The damage due to either the winds or the waves can be estimated by using a frequency decomposition of the stress timeseries. In this paper, a similar decoupled approach is used to attempt to recover the damages induced from these coupled simulations. Although it is well-known that a coupled, aero-hydro analysis is necessary in order to accurately simulate the nonlinear rigid-body motions of the platform, it is less clear if the same statement could be made about the fatigue properties of the platform. In one approach, the fatigue damage equivalent load is calculated independently from both scatter diagrams of the waves and a rose diagram of the wind. De-coupled simulations are performed to estimate the response at an all-encompassing range of environmental conditions. A database of responses based on these environmental conditions is constructed. The likelihood of occurrence at a case-study site is used to compare the damage equivalent from the coupled simulations. The OC5 platform in the Borssele wind farm zone is used as a case-study and the damage equivalent load from the de-coupled methods are compared to those from the coupled analysis in order to assess these methodologies.

First Ocean Going Ships With Springing and Whipping Included in the Ship Design

2011 ◽  
Author(s):  
Gaute Storhaug ◽  
Erlend Moe ◽  
Ricardo Barreto Portella ◽  
Tomazo Garzia Neto ◽  
Nelson Luiz Coelho Alves ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Current Model ◽  
Life Time ◽  
Model Tests ◽  
Full Scale ◽  
Development Projects ◽  
Transient Vibration ◽  
Hull Girder ◽  
Fatigue Evaluation ◽  
Wave Induced

It is well known that ships vibrate due to waves. The wave induced vibrations of the hull girder are referred to as springing (resonance) and whipping (transient vibration from impacts). These vibrations contribute to the fatigue damage of fatigue sensitive details. An Ore Carrier of 400 000 dwt is currently being built by DSME, and at time of delivery, it will be the world’s largest bulk (ore) carrier. The scantlings of large ships must be carefully designed with respect to global loading, and when extending the design beyond experience, it is also wise to consider all aspects that may affect operation and the life time costs. The vessel will also enter a long term contract and is therefore to be evaluated for 30 year Brazil-China operation. In order to minimize the risk of fatigue damage, the vessel is designed according to DNV’s class notation CSA-2 requiring direct calculations of the loading and strength. Further it has been requested to include the effect of springing and whipping in the design. Reliable numerical tools for assessing the additional fatigue effect of vibrations are non-existing. DNV has, however, developed an empirical guidance on how the additional effect may be taken into account based on previous development projects related to the effect of vibrations on large ore carriers Due to the size and route of operation of the new design, it has, however, been required by the owner to carry out model tests in both ballast and cargo condition in order to quantify the contribution from vibration. The results from this project have been used for verification and further calibration of DNV’s existing empirical guidance. A test program has been designed for the purpose of evaluating the consequence in head seas for the Brazil to China trade. Full scale measurements from previous development projects of ore carriers and model tests have been utilized to convert the current model tests results into estimated full scale results for the 400 000 dwt vessels. It is further important to carefully consider how the vibrations are to be included in the design verification, and to develop a procedure for taking into account the vibrations which results in reasonable scantlings based on in-service experience with similar designs and trades. This procedure has been developed, and a structural verification has been carried out for the design. The final outcome of the model test was in line with previous experience and in overall agreement with DNV’s empirical guidance, showing a significant contribution from vibrations to the fatigue damage. The springing/whipping vibrations more than doubled the fatigue damage compared to fatigue evaluation of the isolated wave induced loading. The cargo condition vibrated relatively more than experienced on smaller vessels. Various sources to establish the wave conditions for the Brazil to China ore trade were used, and the different sources resulted in significant differences in the predicted fatigue life of the design.

The Effect of Bow Shape on the Springing/Whipping Response of a Large Ocean-Going Vessel: Investigated by an Experimental Method

2007 ◽  
Author(s):  
Gaute Storhaug ◽  
Torgeir Moan
Keyword(s):  
Fatigue Damage ◽  
Transfer Functions ◽  
Vertical Vibration ◽  
Full Scale ◽  
Relative Importance ◽  
High Frequency Response ◽  
Bow Flare ◽  
Excitation Mechanisms ◽  
The Difference ◽  
Wave Induced

Wave induced vibrations often referred to as springing and/or whipping increase the fatigue and extreme loading in ship hull girders. Both effects are disregarded in current ship rules. Various numerical codes exist for predicting the wave induced vibrations, but so far they are not considered reliable. Another means to investigate the importance of the high frequency response, although more resource demanding, is to carry out full scale measurements and/or model tests. Recently, full scale measurements of blunt ships have been carried out by DNV, and in this paper one of these ships was considered and tested in a towing tank to evaluate the additional fatigue damage due to the wave induced vibrations. Different excitation sources may excite the 2-node vertical vibration mode depending on ship design, and it is not straight forward to determine which is more important. The relative importance of the excitation mechanisms are investigated by two approaches in this paper. The first approach separates the whipping from springing to illustrate their relative importance based on basic theory in combination with model test results. The linear and second order transfer functions are utilized in this procedure. The second approach deals with the effect of the bow design on the additional fatigue damage. Three different bows were tested. The first bow design is identical to the real ship. The second bow design is a simplified version of the first one, by removing the bulb and flare. The third bow is fundamentally different from the two former blunt bows. Bow three is sharp pointed with a vertical sharp stem and vertical ship sides. The results indicate that the importance of whipping depends on the sea state, but that it is of similar importance as springing for the sea states that contributes most to the fatigue damage. Moreover, the difference in the additional fatigue damage due to wave induced vibrations for different bow shapes is moderate. This indicates that vessels with pointed bows and without pronounced bow flare, such as LNG vessels, may have a similar contribution from wave induced vibrations. Modern container vessels, which are more slender, but with pronounced bow flares should be further investigated.

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