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Journal ArticleDOI

Measurements of global and local effects of wave impact on a fixed platform deck

01 Feb 2017-Vol. 231, Iss: 1, pp 212-233
TL;DR: In this paper, a series of model tests were conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure, which represents a simplified topside structure of a tension leg platform.
Abstract: This article describes a series of model tests conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure. Extreme waves of a representative cyclonic sea state were examined in a towing tank within long-crested irregular wave trains. Experimental results presented include global forces and localised slamming pressures acting on a rigidly mounted box-shaped deck, which represents a simplified topside structure of a tension leg platform. The effect of static set-down on the still-water air gap was investigated by applying an equivalent reduction for the deck clearance. It was found that a small reduction of 20 mm (2.5 m full scale) in the original deck clearance can lead to a doubling of the magnitude of the horizontal force and the vertical upward-directed force components, as well as significantly increased slamming pressures in many locations on the deck underside.

Summary (4 min read)

Introduction

  • Current regulations used in the design of an offshore platform for a specific site 1, 2 require a minimum air gap of 1.5 m between the expected magnitude of a 100-year wave crest (including tide and storm surge) and the underside of the lowest deck of the platform.
  • Extreme wave heights were recorded at North Rankin platform off the Western Australia (WA) coast in 1989 during tropical cyclone Orson 11; examination of the damage sustained by the base of the platform indicated that the platform had experienced impacts from waves with a height in excess of 20 m.
  • The air gap for an offshore fixed or floating structure may be smaller in reality than it has been designed to be.
  • In order to mitigate the potential effects of wave-in-deck impacts, a thorough understanding of wave induced loads is required.
  • Besides, measurement repeatability were analysed and the observed variations in forces and pressures were discussed.

Experimental investigation

  • A series of model tests was conducted at the Australian Maritime College towing tank, which is 100 m long, 3.55 m wide and 1.5 m deep.
  • To minimise this undesired effect the stiffness and rigidity of the system was improved by attaching the model to a 4500 mm long steel Hbeam (334 mm x 170 mm x 6/11 mm) mounted on the tank rails and placed 15 m away from the wavemaker as shown in Figure 2.
  • The remaining 85 m of towing tank allowed for sufficiently long run times without interference from reflected waves travelling back up the tank 26.
  • Three deck clearances were nominated based on the platform’s loading conditions, as shown in Figure 3 at model scale with the z-coordinate vertical and positive upward.
  • The wave height of incoming/incident waves, travelling in positive x-direction along the tank, was measured by WP1 and WP2.

Measurements of wave-in-deck loads

  • Both global and local effects of wave-in-deck impact loading were investigated.
  • Two load cells were used to measure the global forces generated due to the impact of the wave crest against the deck structure.
  • The layout of the two AMTI MC3A-100 load cells, denoted by LC1 and LC2, is illustrated in Figure 6.
  • As can be seen in Figure 6 and Table 2, the pressure transducers (denoted as PT) were placed along the diagonal of the bottom plate.
  • A sampling frequency of 20 kHz was chosen for all channels (including wave probes) in order to capture the short-duration slamming pressures 29.

Experimental procedure

  • The experiments were conducted using a combination of the following procedures:.
  • Wave calibration tests – carried out to identify the extreme waves within long-crested irregular wave trains without the model in-place.
  • Free oscillation tests – to find the natural frequencies of the complete test system when subjected to free oscillation tests in air and in water.
  • Wave impact tests – to measure the impact wave forces and localised slamming pressures.
  • In addition the deck accelerations were monitored to identify the structural dynamic response and its effect on the force magnitudes by estimating the inertial force contribution in the load cell responses.

Wave calibration tests

  • The wave calibration tests were conducted by measuring the wave elevation profile, using five wave probes spread longitudinally down the tank, whilst running long-crested irregular waves without the deck model in place.
  • Three long-crested irregular wave trains, with a duration of 120 s each, were generated.
  • The wave crests of each wave event were identified from the measured wave elevation time histories with and without the deck structure in-place (at different deck clearances, a0).
  • The maximum wave steepness of the identified wave events was found to be approximately 0.10 (WE5) i.e. nonbreaking wave conditions.
  • In addition, since the magnitude of the peak horizontal force depends on the associated wave velocity, u, in x-direction at the wave crest, the later was estimated using the Stokes second order wave kinematics 29, as given in Table 4.

Free oscillation tests

  • Once the instrumented model was attached to the heavy beam mounted on the tank rails, the full testing assembly (deck model, instruments and force supports) was subjected to a series of oscillatory decay tests.
  • By obtaining the lowest natural frequency of the system in x- and z-direction, the force contribution into the load cell signals due to the system’s dynamics may be identified.
  • The lowest natural frequencies (denoted as fn) obtained in the dry and wet free oscillation tests are summarised in Table 5, based on time traces measured by both load cells and accelerometer.
  • The second and the third modal frequencies obtained from the dry test (16.0 Hz and 21.8 Hz) are not present when the bottom plate of the deck is aligned on the water surface, due to the contribution of the relatively small added mass and viscous damping to the system in the longitudinal direction (x-direction).
  • The lowest natural frequency observed while the deck was tested in air (16.0 Hz) was found to reduce to 14.5 Hz.

Wave impact tests

  • It is important to note that, in order to ensure the repeatability of the results each test condition was repeated 3 – 5 times resulting in a total of 138 runs.
  • Each run covered the measurements of force components (Horizontal and vertical) and localised pressures (impact and downward) such that approximately 2620 peaks were analysed and averaged to obtain reliable experimental data.
  • As can be seen in Table 6, the dynamic air gap (denoted by a) was obtained for each condition using (a0 - ηc).
  • Nevertheless, as discussed earlier the associated wave events (WE7 and WE9) caused wave impact nearby the TE for these conditions.

Results and discussion

  • Wave-in-deck forces in the x-direction (Fx) and z-direction (Fz) as well as localised pressures associated with the test conditions shown in Table 6 are presented and discussed in this section.
  • WE1 (H = 253 mm) and WE8 (H = 249 mm), which are considered the most extreme amongst the investigated wave events, have been selected for detailed discussion in terms of repeatability analyses.

Wave-in-deck forces

  • To assess the uncertainty in the experimental data each test condition was repeated between 3 and 5 times.
  • The force peaks in the z-direction were found for both the upward direction, Fz(↑), and the downward direction, Fz(↓).
  • E.g. in condition 1 a relative difference of approximately 12% was obtained between Run 1 and Run 2.
  • Hence, there is a sufficient confidence in the mean values reported in this paper.
  • Nevertheless, a close view was done in regards to the dynamic response of the impacted deck structure and its effect on the wave-in-deck forces.

Force time history

  • In order to investigate the force variation, time histories of the force measurements are presented for multiple runs.
  • Both time histories show an impulse-like impact i.e. force magnitude sharply increased and then followed by a rapid decrease.
  • The vertical force signal, illustrated in Figure 13, has a large downwards component - approximately double the upwards component.
  • The figure clearly shows a substantial amount of downwards flowing water from the LE of the deck after the wave impact.
  • Causes the vertical wave-in-deck force Fz) is controlled by the magnitude of the horizontal force.

Deck clearance effect

  • The effect of deck clearance on the measured forces is discussed in this section.
  • Peak forces from different runs were averaged and listed against each wave event.
  • Likewise, Table 11 shows the effect of deck clearance on the peak values of the vertical upward force component, Fz(↑), due to the most severe wave events observed in this investigation.
  • As can be noted the time history in both runs has a similar trend in the uplift direction i.e. positive cycle.
  • The ratio between the maximum positive pressures (impact pressure, Pi) in both runs was found to be 0.92 (almost an 8% difference).

Pressure repeatability

  • Repeatability of pressure measurements is introduced in this section by demonstrating the impact pressures associated with WE1 measured in multiple runs.
  • Impact pressures measured by sixteen PTs in multiple runs for condition 1 [WE1 at a0 = 120 mm]. ), the maximum peak pressure was also captured by PT#1 with a mean value of approximately 2.48 kPa, whilst for condition 18 ( Figure 20.
  • The averaged impact pressure measured by each transducer is normalised by the dynamic pressure (0.5𝜌C2) associated with the wave event.
  • The markers represent the number of pressure transducers along the deck (PT#1 – PT#16).
  • A consistent finding can be reached based upon these graphs such that the results confirm that the location of the maximum impact pressure(s) moves towards the trailing edge (TE) as the deck clearance is reduced.

Conclusions

  • This paper described a series of model tests conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure.
  • Extreme waves of a representative cyclonic sea state were observed in a towing tank within long-crested irregular wave trains.
  • This finding should be considered conclusive provided that there are not successive large waves impacting the structure; in which case, the structural and viscous damping appear to strongly influence the force signals, particularly in the direction of wave propagation.
  • Most of the test conditions demonstrated that the magnitude of the impact pressure varies considerably among repeated runs, even if identical wave condition was used.
  • By investigating the effect of deck clearance on the localised impact pressures, it was found that the reduction in the original deck clearance (10 mm or 20 mm) can increase the impact pressure magnitude at many locations along and across the deck underside.

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Measurements of global and local effects of wave impact on a fixed
platform deck
Nagi Abdussamie
1
, Roberto Ojeda
1
, Giles Thomas
2
, Walid Amin
1
1
National Centre for Maritime Engineering and Hydrodynamics
Australian Maritime College, University of Tasmania, Launceston, TAS 7250, Australia
2
University College London, WC1E 6BT, UK
Abstract
This paper describes a series of model tests conducted to examine extreme wave events associated with tropical
cyclonic conditions and their impacts on an offshore deck structure. Extreme waves of a representative cyclonic
sea state were observed in a towing tank within long-crested irregular wave trains. Experimental results
presented include global forces and localised slamming pressures acting on a rigidly mounted box-shaped deck,
which represents a simplified topside structure of a tension leg platform (TLP). The effect of static set-down
on the still-water air gap was investigated by applying an equivalent reduction for the deck clearance. It was
found that a small reduction of 20 mm (2.5 m full scale) in the original deck clearance can lead to a doubly of
the magnitude of the horizontal force and the vertical upward-directed force components, as well as
significantly increase slamming pressures in many locations on the deck underside.
Keywords
Tropical cyclones; offshore platforms; wave-in-deck impact loads.

Introduction
Current regulations used in the design of an offshore platform for a specific site
1, 2
require a minimum air gap
of 1.5 m between the expected magnitude of a 100-year wave crest (including tide and storm surge) and the
underside of the lowest deck of the platform. However, many reports have been published over the past decade
detailing damage of the deck structure of offshore platforms owing to wave impacts. It is important to note
that in many cases insufficient air gap has been reported to be one of the major reasons for damage sustained
by offshore structures, e.g. the North Sea
3
, and the Gulf of Mexico
4
.
Damage to the structure or equipment can have costly economic and safety implications, as highlighted recently
by McBride (2012). Consequently, there is a requirement by classification societies to ensure that an offshore
facility can survive in extreme wave conditions caused by tropical cyclones or hurricanes with long return
periods. Therefore slam events and the associated forces need to be accurately accounted for in the design stage
5
. In addition a large proportion of Australian offshore petroleum installations have been in operation for 10
or more years
6
, and will therefore be soon subject to assessment for recertification and/or lifetime extension.
These assessments need to demonstrate that the structures will be able to withstand the environmental loads,
including wave-in-deck loading, and be safe to remain in operation
7, 8
.
It has been found that deck impacts occur more frequently than have been predicted using theoretical
techniques
9
. This may be predominantly due to wave magnitudes being larger than expected, the actual air gap
being smaller than anticipated and the resulting wave-in-deck impact forces being greater than predicted. These
three factors are now discussed in more detail.
One of the key areas of interest for current and proposed offshore development is the North West Shelf (NWS)
in Australia. This region is susceptible to tropical cyclones which can generate severe wave conditions
10
. For
example, extreme wave heights were recorded at North Rankin platform off the Western Australia (WA) coast
in 1989 during tropical cyclone Orson
11
; examination of the damage sustained by the base of the platform
indicated that the platform had experienced impacts from waves with a height in excess of 20 m. Buchan, Black
12
reported on the intensity of tropical cyclone Olivia which caused significant damage to oil and gas facilities in

the NWS region. Metocean measurements taken during the storm indicate that the maximum wave heights
were in the order of 15 to 20 m. Such large (and steep) waves are greater in magnitude than the waves that
these structures were designed for and would exceed the still-water air gap of many existing offshore platforms
in the region of NWS.
It has been suggested that the 1.5 m air-gap safety margin recommended by the 21
st
edition of API-RP-2A
2
has provided an inconsistent level of reliability for structures
13
. As a result, the recommended crest values for
the North Sea and Norwegian Sea have recently been increased
14
. New platforms will be designed with an air
gap sufficient to avoid impacts with a 10
-4
annual probability crest, or equivalent to 10,000-year return period.
The air gap for an offshore fixed or floating structure may be smaller in reality than it has been designed to be.
For fixed offshore structures seabed subsidence or platform settlement due to reservoir compaction can over
time reduce the original designed air gap exposing the structure to more severe wave impacts, as exemplified
by the Ekofisk platform
15, 16
. Likewise, for floating structures, a growth in operational weight or the flooding
of compartments due to damage or sea level rise may lead to reduced air gaps
17
. Tension leg platforms (TLPs)
can move downward “set-down” and are subject to rising sea levels, as well as subsidence of the bottom
foundations. Any deck impact will therefore increase the tether tension due to an uplift force and then decrease
the amount of tension as a result of downward force. In both directions, the tethers may experience an
oscillatory sequence of snap loads and slackness. Consequently, the snap load that one or more tethers may
experience is followed by a considerable negative force, i.e. suction force, which may exceed the initial
pretension “zero-tension”, thus imposing a high risk level on the whole system.
It is also important not to over design the offshore structure by obligating an excessive air gap since this can
have severe implications for build and operational costs, and for floating structures in particular it will raise the
centre of gravity and impair the payload performance.
In order to mitigate the potential effects of wave-in-deck impacts, a thorough understanding of wave induced
loads is required. However, even though there is currently a large amount of research effort towards the
computation of wave induced loads on ships and offshore structures, only few studies concentrate on wave

loads from abnormal waves
18
. Several authors have conducted experimental investigations to estimate the
wave-in-deck loading due to regular waves
19, 20
, irregular waves
21, 22
, and a combination of regular and irregular
waves
14
. Despite this research, there is still considerable uncertainty about determining the magnitude of wave
loads acting on structures located above the sea surface.
Particular questions remain. For example many researchers
5, 23
have concluded that wave impact pressure is a
highly localised phenomenon in time and space”. Therefore further investigation is required to assess the repeatability
in the model experiment measurements of global forces and slamming pressures obtained by multiple test runs
and test whether the mean value of pressure maxima obtained from multiple experimental runs can be used in
the design of an offshore deck structure. Furthermore, more effort is required to determine appropriate signal
processing procedures to identify the magnitude of the peak slamming force from the force measurement
signals.
The objective of this work was to analyse the characteristics of extreme long-crested irregular waves and their
impacts on a three-dimensional fixed deck structure using a series of model experiments. The model tests were
conducted at the towing tank of the Australian Maritime College (AMC) to measure both the global and local
force effects of extreme wave events on the deck structure. The horizontal and vertical wave-in-deck forces
due to a number of extreme waves were simultaneously measured with localised pressures along and across the
deck underside and the wave elevation in the vicinity of the model. The role of the dynamic response of the
deck structure was identified by monitoring the acceleration components during wave impact tests. Besides,
measurement repeatability were analysed and the observed variations in forces and pressures were discussed.
The effect of the deck clearance reduction on the peak forces and impact pressures was also examined.
Experimental investigation
A series of model tests was conducted at the Australian Maritime College towing tank, which is 100 m long,
3.55 m wide and 1.5 m deep. It is equipped at one end with a hydraulically driven flap-type wavemaker and has
an artificial beach located at the opposite end of the tank to minimise wave reflections.

Test model and instrumentation details
The topside platform deck structure of a TLP was modelled using a flat horizontal box-shaped deck with
external dimensions of length (L) = 608 mm, breadth (B) = 608 mm and depth (h) = 210 mm. The box
dimensions were selected to represent, at a scale of 1:125, the 76 m × 76 m centre to centre spacing between
columns of the SNORRE-A tension leg platform (TLP) installed in 1992 at a water depth of 310 m in the
Norwegian North Sea
24
.
The model deck was fabricated using a 10 mm thick aluminium plate for the bottom and 100 mm x 25 mm x
2.5 mm rectangular hollow sections (RHS) aluminium extrusions for the sides. Since the purpose of the testing
was to measure wave slamming loads on the front and bottom faces of the deck structure without overtopping,
a 100 mm high acrylic sheet was installed on top of the RHS to prevent water from splashing onto the internal
deck space as shown in Figure 1. The deck was elevated above the water surface at a distance representing the
still-water air gap i.e. deck clearance. The effect of sit-down was examined by reducing the original deck
clearance.
Figure 1. Photograph of the deck model positioned above the water surface.
Recent experimental studies
14, 25
have shown that a considerable dynamic response in force measurements is
introduced when a fixed deck model is directly attached to a towing tank carriage. To minimise this undesired
effect the stiffness and rigidity of the system was improved by attaching the model to a 4500 mm long steel H-
beam (334 mm x 170 mm x 6/11 mm) mounted on the tank rails and placed 15 m away from the wavemaker
as shown in Figure 2. The remaining 85 m of towing tank allowed for sufficiently long run times without

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Abstract: Historical Development of Offshore Structures Novel and Marginal Offshore Structures Ocean Environment Loads and Responses Statistical Design of Offshore Structure Fixed Structure Design Floating Structure Design Mooring, Cables & Anchoring Drilling & Production Risers & Tendon Systems Topside Structure Pipeline Design Design for Reliability: Human and Organizational Factors Physical Modeling of Offshore Structures Installation & Field Operations Materials for Offshore Applications Geophysical and Geotechnical Design

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TL;DR: In this paper, the authors present a review of the recent advances in the assessment of loads for ships and offshore structures with the aim to draw the overall technological landscape available for further understanding, validation and implementation by the academic and industrial communities.

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01 Jan 2013

209 citations


"Measurements of global and local ef..." refers background in this paper

  • ...Although there is currently a large amount of research effort towards the computation of wave-induced loads on ships and offshore structures, only few studies concentrate on wave loads from abnormal waves.(18) Several authors have conducted experimental and/or numerical investigations to estimate the wave-in-deck loading due to regular waves,(19,20) irregular waves(21,22) and a combination of regular and irregular waves....

    [...]

31 Dec 1995
TL;DR: In this article, a description of the theoretical analysis procedures used to predict the wave impact forces acting on offshore platform deck structures in large incident waves is given, in terms of the different type elements that make up such structures and the type of hydrodynamic force mathematical models used to represent the basic forces.
Abstract: A description is given of the theoretical analysis procedures used to predict the wave impact forces acting on offshore platform deck structures in large incident waves. Both vertical and horizontal plane forces are considered, in terms of the different type elements that make up such structures and the type of hydrodynamic force mathematical models used to represent the basic forces. Effects of wave surface nonlinearity (including kinematics), deck material porosity, and velocity blockage and shielding are considered in the analysis, which also includes a physical explanation of various observed phenomena. Results of comparison and correlation with experimental model test data are presented, including description of procedures used in data analysis to eliminate extraneous dynamic effects that often contaminate such data. The influence of wave heading angle relative to different structural elements (and overall structures) is also described, including both analytical representations and physical interpretations.

99 citations


"Measurements of global and local ef..." refers result in this paper

  • ...This finding is in contrast with the theoretical models which assume that the deck structure is transparent to the impacting waves.(33)...

    [...]

Journal ArticleDOI
01 Sep 2009-Energy
TL;DR: In this article, a meta-model analytic framework is applied to perform sensitivity analysis and explore the interactions of assumptions on model output to derive functional relations that describe the likely contribution the collection of destroyed assets would have made to future production in the Gulf of Mexico.

36 citations


"Measurements of global and local ef..." refers background in this paper

  • ...In many cases, insufficient air gap has been reported to be one of the major reasons for damage sustained by offshore structures, for example, in the Gulf of Mexico.(3) Damage to the structure or equipment can have costly economic and safety implications, as highlighted recently by McBride....

    [...]

  • ...For instance, similar steep waves were measured by three wave radars on the Marco Polo TLP during hurricane Rita in the Gulf of Mexico.32 The generated waves had good repeatability using multiple runs at different values of a0, with only a very minimal disturbance on crest height of WE#1 seen in Figure 7(a) due to the presence of the deck structure....

    [...]

  • ...In many cases, insufficient air gap has been reported to be one of the major reasons for damage sustained by offshore structures, for example, in the Gulf of Mexico.3 Damage to the structure or equipment can have costly economic and safety implications, as highlighted recently by McBride.4 Consequently, there is a requirement by classification societies to ensure that an offshore facility can survive in extreme wave conditions caused by tropical cyclones or hurricanes with long return periods....

    [...]

Frequently Asked Questions (11)
Q1. What are the contributions mentioned in the paper "Measurements of global and local effects of wave impact on a fixed platform deck" ?

This paper describes a series of model tests conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure. The effect of static set-down on the still-water air gap was investigated by applying an equivalent reduction for the deck clearance. 

To overcome this issue, a sufficient number of repeated runs (seems to be more than five per condition) are required during tanks experiments. 

Current regulations used in the design of an offshore platform for a specific site 1, 2 require a minimum air gap of 1.5 m between the expected magnitude of a 100-year wave crest (including tide and storm surge) and the underside of the lowest deck of the platform. 

By investigating the effect of deck clearance on the localised impact pressures, it was found that thereduction in the original deck clearance (10 mm or 20 mm) can increase the impact pressure magnitudeat many locations along and across the deck underside. 

The second and the third modal frequencies obtained from the dry test (16.0 Hz and 21.8 Hz) are not present when the bottom plate of the deck is aligned on the water surface, due to the contribution of the relatively small added mass and viscous damping to the system in the longitudinal direction (x-direction). 

This may be predominantly due to wave magnitudes being larger than expected, the actual air gap being smaller than anticipated and the resulting wave-in-deck impact forces being greater than predicted. 

It is important to note that, in order to ensure the repeatability of the results each test condition was repeated 3 – 5 times resulting in a total of 138 runs. 

The JONSWAP spectrum with a peak shape parameter γ = 1.0, which in this case is identical to the Pierson-Moskowitz (PM) spectrum 29, was used to synthesise short-time wave trains using the towing tank wavemaker. 

The x-position of the sixteen pressure transducers was usedas x-axis to represent the length of the deck structure (Table 2) where the LE and TE are denoted by vertical dashed lines at x = -304 mm and x = 304 mm, respectively. 

The phase celerity, C, was estimated by λ/Tz for each WE so that the resulting impact pressure could be related to the associated dynamic pressure (0.5𝜌C2). 

The role of the dynamic response of the deck structure was identified by monitoring the acceleration components during wave impact tests.