Investigation Into the Sensitivity of the Dynamic Hook Load During Subsea Deployment of a Suction Can

2007 ◽  
Author(s):  
J. Ireland ◽  
G. Macfarlane ◽  
Y. Drobyshevski
Keyword(s):  
Nonlinear Damping ◽  
Added Mass ◽  
Offshore Structures ◽  
Hydrodynamic Properties ◽  
Deep Basin ◽  
Mooring Lines ◽  
Number Range ◽  
North West ◽  
Order Of Magnitude ◽  
Decay Tests

Suction cans are commonly used as foundations of fixed offshore structures, subsea equipment, and anchors of mooring lines. During the offshore installation phase, when a suction can is submerged, it attracts large heave added mass, which may be an order of magnitude higher than the mass of the can in air. Due to motions of an installation vessel the dynamic hook load may significantly exceed the submerged weight of the can. The dynamic hook load must be accurately predicted, as it governs selection of the vessel, lifting gear and rigging, and defines the allowable installation sea state. The objective of this paper is to examine the sensitivity of the dynamic hook load to hydrodynamic properties of the suction can, in particular its heave added mass and damping. This research is motivated by the lack of data on such properties, which are usually estimated by simplified methods with some engineering judgement and assumptions. A single degree of freedom system is considered and the frequency domain spectral analysis is used, which employs the stochastic linearization of the nonlinear damping component. The added mass and damping of a 6-meter diameter suction can of dimensions typical for Australian North West Shelf developments have been determined by testing a 1:10 model in the 4.1 m deep basin of the Australian Maritime College. Free decay tests were conducted at several frequencies and the added mass, linear and nonlinear damping components determined. The effect of open hatches on the hydrodynamic properties was examined by fitting the model with hatches of various diameters, with up to 4.8% of the relative area open. Results of the tests demonstrate that the added mass and damping are higher, when compared with estimates based on empirical data for non-oscillatory flow. Within the Keulegan-Carpenter number range of 0.1–1.0, open hatches impact significantly on the added mass and produce additional damping, which is found to be linear with the heave velocity. Results of the tests and their interpretation are discussed. Sensitivity analysis shows that if the model test results are used in the dynamic lift analysis for an installation vessel and sea states considered, the predicted hook load is generally less than its values obtained by using simplified estimates. In particular, the increase in linear damping due to open hatches is responsible for up to 20% reduction in the dynamic hook load, with 2.4% of the relative top area open.

Hydrodynamic Properties of a Suction Can Oscillating Near the Free Surface

2009 ◽  
Author(s):  
C. Plummer ◽  
G. Macfarlane ◽  
Y. Drobyshevski
Keyword(s):  
Free Surface ◽  
Added Mass ◽  
Hydrodynamic Properties ◽  
Noticeable Effect ◽  
Sea Floor ◽  
Notable Effect ◽  
Free Decay ◽  
Decay Tests ◽  
Quadratic Damping ◽  
Motion Amplitude

Offshore operations often require heavy subsea equipment, such as suction piles or cans, to be lowered by a support vessel into the sea. A lifting device must have adequate capacity to withstand the dynamic loads generated by the motions of the vessel and the heave response of the structure. The objective of this study is to determine the added mass and damping of a suction can oscillating in heave near the free surface; knowledge of these hydrodynamic properties is required for the accurate prediction of the dynamic lift forces during the deployment. This project is a logical progression following two similar studies, which investigated these hydrodynamic properties for the suction can in the mid-water position and when approaching the seabed. All three studies involved the conduct of model tests to determine the hydrodynamic properties. Free decay tests were conducted at several heave frequencies, and the added mass, linear and quadratic damping components were determined. In addition, the effect of varying the percentage of open hatch area has been investigated. Test data demonstrates that the heave added mass is strongly dependent on the frequency of motion, and its values are significantly smaller than those measured in the unrestricted flow. From observations, there was no dependency on the motion amplitude, nor did the size of open hatches have notable effect on the added mass. It was observed that when the top plate of the structure was in contact with the free surface a mean “pull down” force appeared. This force is caused by the suction underneath the top plate when the can moves upwards. As opposed to the mid-water position and near the sea floor, the study indicates that the area of open hatches has no noticeable effect on the heave damping when the suction can is oscillating near the free surface.

Heave Added Mass and Damping of a Suction Can in Proximity to the Sea Floor

2008 ◽  
Author(s):  
T. F. Roe ◽  
G. Macfarlane ◽  
Y. Drobyshevski
Keyword(s):  
Deep Water ◽  
Target Location ◽  
Added Mass ◽  
Water Tank ◽  
Sea Floor ◽  
North West ◽  
Hydrodynamic Damping ◽  
Water Value ◽  
Decay Tests ◽  
The Impact

Suction cans are usually deployed by the crane of a construction vessel, which must have adequate capacity to withstand the dynamic hook loads generated by motions of the vessel and heave response of the suction can. Before the structure is placed on the sea floor, it must be positioned above the target location; in this phase the suction can is manoeuvred into position being suspended in proximity to the sea floor. Hydrodynamic properties of the structure in the positioning phase are different from those experienced during the decent, due to the effect of the bottom proximity. As a result, the dynamic hook loads experienced in this phase may be also different from the deep water condition. The objective of this study is to quantify these effects; in particular the impact of the bottom proximity on the heave added mass and hydrodynamic damping. The added mass and damping of a 6-metre diameter suction can, of dimensions typical for Australian North West Shelf conditions, have been determined by testing a 1:10 model in the 4.2 m deep water tank of the Australian Maritime College. Free decay tests were conducted at several heave frequencies, and the heave added mass and damping determined. Four clearances of the model from the sea floor were investigated ranging from 1.20 to 0.20 of the can height. For each clearance, several sizes of open hatches were examined, by testing the model with 3 pairs of hatches of various diameters, with up to 4.8% of the relative area open. Model tests demonstrate that the heave added mass and damping increase as the suction can approaches the sea floor. Increase in added mass is found to be within 20% of its deep water value, and is made less pronounced by opening hatches of larger area. Linear (proportional to velocity) hydrodynamic damping also increases moderately as the under-bottom clearance reduces. Quadratic (proportional to velocity squared) damping is strongly affected, especially at very small clearances, with drag coefficient reaching unusually high values; this is attributed to substantial constraining effect of the bottom, which causes increasing flow velocities past the lower edge of the can. Results of the tests are presented, and their application for the installation lift analysis is discussed.

Overview of Tacoma Narrows Bridge Floating Caisson Design

2004 ◽  
Author(s):  
M. Sri Krishna ◽  
Partha Chakrabarti ◽  
Subrata K. Chakrabarti ◽  
Adinarayana Mukkamala ◽  
Nagaraj Anavekar
Keyword(s):  
Design Procedure ◽  
Suspension Bridge ◽  
Offshore Structures ◽  
Mooring Lines ◽  
Critical Areas ◽  
Dynamic Forces ◽  
Order Of Magnitude ◽  
Associated Design ◽  
Tacoma Narrows Bridge ◽  
Full Height

Tacoma Narrows Constructors is building a new suspension bridge in Tacoma, close to Seattle, Washington State, USA next to an existing bridge at the location. The new bridge is being built just south of the existing bridge. This new bridge will be built on towers mounted on two caissons. The caissons are towed to the site from the harbor with the cutting edge, first full lift, and the second and third exterior lifts. The piers are constructed on site up to their full height as floating caissons at varying drafts. During the construction, the floating caissons on both ends of the new bridge are moored in place with 32 catenary mooring lines. The current flow due to ebb and flood tide in the narrows is very high. This high current and the consequent vortex-induced dynamic forces provided a technical challenge in the design of the caisson and its mooring system whose dimensions are of similar order of magnitude as typical offshore structures exposed to severe environment. This paper provides an overview of this challenge, and describes the steps taken in overcoming these difficulties. The design procedure adopted of the moored caisson system and the piers in the overall scheme of the Tacoma Narrows Bridge are summarized. This overview stresses the practical side of towing, mooring and in place construction of the caissons. Some of the critical areas of associated design challenges and their solution techniques are highlighted.

scholarly journals Effect of Roughness of Mussels on Cylinder Forces from a Realistic Shape Modelling

2021 ◽  
Vol 9 (6) ◽  
pp. 598
Author(s):  
Antoine Marty ◽  
Franck Schoefs ◽  
Thomas Soulard ◽  
Christian Berhault ◽  
Jean-Valery Facq ◽  
...  
Keyword(s):  
Marine Species ◽  
Offshore Structures ◽  
Offshore Wind ◽  
Mooring Lines ◽  
Shape Modelling ◽  
Offshore Wind Turbines ◽  
Specific Effects ◽  
Hydrodynamic Loading ◽  
Floating Offshore Wind Turbines ◽  
Realistic Shape

After a few weeks, underwater components of offshore structures are colonized by marine species and after few years this marine growth can be significant. It has been shown that it affects the hydrodynamic loading of cylinder components such as legs and braces for jackets, risers and mooring lines for floating units. Over a decade, the development of Floating Offshore Wind Turbines highlighted specific effects due to the smaller size of their components. The effect of the roughness of hard marine growth on cylinders with smaller diameter increased and the shape should be representative of a real pattern. This paper first describes the two realistic shapes of a mature colonization by mussels and then presents the tests of these roughnesses in a hydrodynamic tank where three conditions are analyzed: current, wave and current with wave. Results are compared to the literature with a similar roughness and other shapes. The results highlight the fact that, for these realistic roughnesses, the behavior of the rough cylinders is mainly governed by the flow and not by their motions.

Hydrodynamic Loading on Macro-Roughened Cylinders of Various Aspect Ratios

1989 ◽  
Vol 111 (3) ◽  
pp. 214-222 ◽  
Author(s):  
A. Theophanatos ◽  
J. Wolfram
Keyword(s):  
Offshore Structures ◽  
Water Tank ◽  
Fluid Loading ◽  
Test Rig ◽  
Test Program ◽  
Number Range ◽  
Marine Fouling ◽  
Aspect Ratios ◽  
The Uk ◽  
Regular Arrays

This paper describes experiments which comprise part of the UK joint SERC/industry-sponsored program on fluid loading. The experiments have been undertaken in a novel test rig which accelerates a cylinder from rest to a constant velocity in a still water tank and cover the Reynolds number range 105 to 106. Results are presented for 30 cylinders ranging in diameter from 150 mm to 400 mm. The test program comprised: (a) cylinders with different distributions of marine growth (mussels and kelp) and artificial roughness at low surface cover; (b) fully sand/gravel-roughened cylinders with aspect ratios (L/D) from 3.75 to 10 and relative roughness (k/D) up to 0.025; (c) cylinders covered in regular arrays of pyramids at (k/D) up to 0.1. Based on the results, some tentative conclusions are drawn about the estimation of the effect of marine fouling on the fluid loading of offshore structures.

Sedimentary Differentiation as a Factor in the Moine-Torridonian Correlation

1951 ◽  
Vol 88 (4) ◽  
pp. 257-266 ◽  
Author(s):  
W. Q. Kennedy
Keyword(s):  
North West ◽  
Scottish Highlands ◽  
Order Of Magnitude ◽  
End Members

AbstractThe chemical differences which exist between the Moine and Torridonian rocks of the Scottish Highlands are of the same nature and order of magnitude as those between extreme end members of the Norwegian Sparagmite Formation. It is concluded that in Scotland, as in Norway, a unilateral trend of sedimentary differentiation operated from north-west to south-east across the direction of the Caledonian geosyncline and led to chemical grading of the arenaceous sediments transverse to the trend of the late Pre-Cambrian depositional basin.

A Joint-Industry Effort to Develop and Verify CFD Modeling Practice for Vortex-Induced Motion of a Deep-Draft Semi-Submersible

2021 ◽  
Author(s):  
Hyunchul Jang ◽  
Dae-Hyun Kim ◽  
Madhusuden Agrawal ◽  
Sebastien Loubeyre ◽  
Dongwhan Lee ◽  
...  
Keyword(s):  
Test Data ◽  
Model Test ◽  
Working Group ◽  
Cfd Modeling ◽  
Physical Model Test ◽  
Mooring Lines ◽  
Cfd Models ◽  
Induced Motion ◽  
Modeling Practice ◽  
Decay Tests

Abstract Platform Vortex Induced Motion (VIM) is an important cause of fatigue damage on risers and mooring lines connected to deep-draft semi-submersible floating platforms. The VIM design criteria have been typically obtained from towing tank model testing. Recently, computational fluid dynamics (CFD) analysis has been used to assess the VIM response and to augment the understanding of physical model test results. A joint industry effort has been conducted for developing and verifying a CFD modeling practice for the semi-submersible VIM through a working group of the Reproducible Offshore CFD JIP. The objectives of the working group are to write a CFD modeling practice document based on existing practices validated for model test data, and to verify the written practice by blind calculations with five CFD practitioners acting as verifiers. This paper presents the working group’s verification process, consisting of two stages. In the initial verification stage, the verifiers independently performed free-decay tests for 3-DOF motions (surge, sway, yaw) to check if the mechanical system in the CFD model is the same as in the benchmark test. Additionally, VIM simulations were conducted at two current headings with a reduced velocity within the lock-in range, where large sway motion responses are expected,. In the final verification stage, the verifiers performed a complete set of test cases with small revisions of their CFD models based on the results from the initial verification. The VIM responses from these blind calculations are presented, showing close agreement with the model test data.

Tacking in the Wind Tunnel

2009 ◽  
Author(s):  
Frederik C. Gerhardt ◽  
David Le Pelley ◽  
Richard G. J. Flay ◽  
Peter Richards
Keyword(s):  
Wind Tunnel ◽  
Aerodynamic Force ◽  
Three Dimensional ◽  
Added Mass ◽  
Mass Moment ◽  
Strip Theory ◽  
Dynamic Velocity ◽  
Order Of Magnitude ◽  
Velocity Prediction ◽  
Unsteady Effects

In recent years a number of Dynamic Velocity Prediction Programs (DVPPs), which allow studying the behaviour of a yacht while tacking, have been developed. The aerodynamic models used in DVPPs usually suffer from a lack of available data on the behaviour of the sail forces at very low apparent wind angles where the sails are flogging. In this paper measured aerodynamic force and moment coefficients for apparent wind angles between 0° and 30° are presented. Tests were carried out in the University of Auckland’s Twisted Flow Wind Tunnel in a quasi-steady manner for stepwise changes of the apparent wind angle. Test results for different tacking scenarios (genoa flogging or backed) are presented and discussed and it is found that a backed headsail does not necessarily produce more drag than a flogging headsail but increases the beneficial yawing moment significantly. The quasisteady approach used in the wind tunnel tests does not account for unsteady effects like the aerodynamic inertia in roll due to the “added mass” of the sails. In the second part of paper the added mass moment of inertia of a mainsail is estimated by “strip theory” and found to be significant. Using expressions from the literature the order of magnitude of three-dimensional effects neglected in strip theory is then assessed. To further quantify the added inertia experiments with a mainsail model were carried out. Results from those tests are presented at the end of the paper and indicate that the added inertia is about 76 % of what strip theory predicts.

Mooring Analysis of a Dual-Spar Floating Wind Farm with a Shared Line

2021 ◽  
pp. 1-37
Author(s):  
Guodong Liang ◽  
Zhiyu Jiang ◽  
Karl O. Merz
Keyword(s):  
Wind Turbines ◽  
Wind Farm ◽  
Wind Farms ◽  
Sensitivity Study ◽  
Design Stage ◽  
Mooring Lines ◽  
Natural Period ◽  
Preliminary Design Stage ◽  
Decay Tests ◽  
Line Design

Abstract Wind farms with shared mooring lines have the potential to reduce mooring costs. However, such wind farms may encounter complex system dynamics because adjacent wind turbines are coupled. This paper presents an analysis of the shared mooring system with a focus on the system natural periods. We first apply Irvine's method to model both the shared line and the two-segment single lines. The response surface method is proposed to replace iterations of the catenary equations of the single lines, and a realistic single line design is presented for OC3 Hywind. Then, system linearization and eigenvalue analysis are performed for the wind farm consisting of two spar floating wind turbines, one shared line, and four single lines. The obtained natural periods and natural modes are verified by numerical free decay tests. Finally, a sensitivity study is carried out to investigate the influence of mooring properties. It is found that the shared line has a significant influence on the natural periods in the surge and sway modes. The natural periods in the surge and sway modes are also most sensitive to the mooring property variations. Two sway eigenmodes are identified, and the lower sway natural period varies between 23 s and 88 s in the sensitivity study. The present analysis method can be used to identify critical natural periods at the preliminary design stage of shared mooring systems.

Palio dubia (Nudibranchia: Doridina) from the north-west Atlantic Ocean: is its morphology at hatching consistent with settlement one day later?

2010 ◽  
Vol 91 (8) ◽  
pp. 1651-1654 ◽  
Author(s):  
Jeffrey H.R. Goddard
Keyword(s):  
Embryonic Development ◽  
Atlantic Ocean ◽  
Larval Period ◽  
North West ◽  
The North ◽  
Order Of Magnitude ◽  
Size And Morphology ◽  
Mode Of Development ◽  
Veliger Larvae

Hamel et al. (2008) reported that veliger larvae of the nudibranch gastropod Palio dubia settled 1 to 3 days after hatching and that metamorphosis commenced shortly thereafter. This is an anomalously short larval period for a nudibranch described as having planktotrophic development. I examined the embryonic development and hatching larvae of P. dubia collected intertidally from Maine, USA. Veliger larvae with shells 120 μm long and lacking eyespots and propodia developed in 7 days at 20°C from eggs averaging 69 μm in diameter. Their size and morphology were typical of planktotrophic nudibranchs known to have minimum larval periods of weeks to months, and the available evidence does not suggest any other mode of development exists in P. dubia. The larval period of P. dubia is likely an order of magnitude longer than reported by Hamel et al. (2008).

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