INSPECTION AND QUALITY CONTROL

1. INSPECTION AND QUALITY CONTROL MSc in Oil & Gas and Offshore Engineering Spyros Volonakis PhD Cand., MEng, CEng, CMarEng

3. EFFECTS OF MARINE GROWTH ON HYDRODYNAMIC LOADING AND ON THE DYNAMIC RESPONSE OF OFFSHORE PLATFORMS Marine growth is known to give adverse effects on the performance of offshore structures. It presents will roughened the surfaces of the structure hence increase its drag coefficients. Structures with the best protection scheme from marine organisms would after few years start to be covered by various types of growth. Generally, it was also recognized that the most important source of loading exerted on offshore structures comes from hydrodynamic action which are influenced by CD and CM values. The coefficients Cd and Cm are determined by structure shape, the Keulegan–Carpenter number, Reynolds number and surface roughness.

4. Significant Wave Height In physical oceanography, the significant wave height (SWH or Hs) is defined traditionally as the mean wave height (trough to crest) of the highest third of the waves (H1/3). Nowadays it is usually defined as four times the standard deviation of the surface elevation – or equivalently as four times the square root of the zeroth-order moment (area) of the wave spectrum. The symbol Hm0 is usually used for that latter definition. The significant wave height may thus refer to Hm0 or H1/3; the difference in magnitude between the two definitions is only a few percent.

5. This is the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. This is measured because the larger waves are usually more significant than the smaller waves. For instance, the larger waves in a storm cause the most beach erosion, or the larger waves can cause navigation problems for mariners. Since the Significant Wave Height (Seas) is an average of the largest waves, you should be aware that many individual waves will probably be higher. If we take a sample forecast of Seas Beyond the Reef of 2 to 4 feet, this implies that the average of the highest one-third waves will have a Significant Wave Height of 2 to 4 feet. But mariners need to keep in mind that roughly one of every ten waves will be greater than 4 feet; one in every one hundred waves will be greater than 5 feet; and one in every 1000 waves will be greater than 6 feet. As a general rule, the largest individual wave one may encounter is approximately twice as high as the Significant Wave Height (or Seas). Note: Seas can refers to the combination or interaction of wind waves and swells (combined seas) in which the separate components are not distinguished. This includes the case when swells are negligible or are not 
considered in describing sea state.

6. Environmental Conditions & Loads Applied on Marine Structures • Description of Environmental Conditions • Wind field • Sea waves • Current waves • Loads of marine structures • Wind loads • Sea waves loads • Loads applied on small structures (Morison equation) • Loads applied on big structures (theory of dynamic flow) • Current Loads

7. The good understanding and the definition of the mutual interaction of the marine structures with the their operational environment (marine environment), is one of the important factors for safe and economic operation. The hydrodynamic analysis of marine structures undoubtedly aided by methods used in the area of naval hydrodynamics. In many cases, however, offshore structures, different requirements and determination in relation to conventional surface ships as well as new dimensioning methods of them, require further consideration and introduction of new concepts for the hydrodynamic analysis. So the knowledge and methods used in hydraulic theory, port constructions and soil mechanics are necessary.

8. Environmental Conditions Reliable design of a marine construction, requires a clear knowledge of the environmental conditions in which it is installed and running. Major environmental factors apply loads on her and acting directly or indirectly in functionality are: • Wind • Waves • Sea currents • Sea Bottom Conditions • Tides • Ice-snow • Seismicity of the area

9. Wind Field Power Spectral Density nF(n) (m/s)2 For a given measurement point , the wind speed varies continuously in terms of value and direction for all time scales ( minutes, hours , days , months , years) . A first attempt imprinting the energy distribution of the wind field versus frequency, a first form of the spectrum, made ​​by Van der Ηoven, and illustrated in the fig. The spectrum shows two major peaks. With a period of 3 to 5 days and the other at much higher frequencies. The first peak corresponds to changing weather conditions on a large scale is characterized by the long periods required to complete their development, while the second is intertwined turbulence phenomena in the boundary layer. Between the two peaks there is a region of very low energy periods ranging between 10 minutes and 2 hours .

10. The existence of this region is of practical interest since the portion of the spectrum due to the turbulence are by nature separated from that due to long - meteorological changes . Also notice the use of the period of one hour for the formation of the mean wind speed is sufficient to consider all short-time situations of the wind. This time period is used in Europe and America, although as shown by the figure 10 minutes period will be sufficient to form the mean value. The magnitude of the mean wind speed is a function of the height of the interested point and the time period used in the formation. The usual height above sea level for which data were provided the wind is 10 meters.

11. Sea Waves The waves come primarily from the effect of wind. The process of forming the wave have not fully explained , there are several theories . When the wind starts blowing over the calm sea , it has been observed that high-frequency instabilities generated thereon . This means that due to the turbulence of the wind shown small ripples on its surface located near the surface tension waves , moving quickly and have a short shelf life . The appearance of these ripples is crucial to the further development of the phenomenon . The sea surface is no longer flat and thereby significantly increasing the resistive force due to the mean wind speed . Thereby creating a non- linear energy transfer mechanism by high at lower frequencies causing waves of progressively increasing length . This is the crucial point in the process , the energy flowing from high to low frequencies .

12. For this reason, in the case of developing seas (those which wind blows for a short time) the peak of the spectrum is at high prices. Conversely when the velocity of the wind (case decaying sea), the peak of the spectrum is at low frequency values due to the tenancy of the high frequency components of the spectrum rapidly dampened. When the wind blows over the surface of the sea for a long time, with constant speed , then the induced ripples are characterized by spectra , which has been restored stable energy distribution as a function of their frequency . In this case we are talking about fully developed seas. The main mechanism of energy transfer is the mean wind speed. The higher the speed is , the greater the frequency of the peak of the spectrum and the higher the significant wave height

13. DRAG • The drag equation may be derived to within a multiplicative constant by the method of dimensional analysis. If a moving fluid meets an object, it exerts a force on the object. Suppose that the variables involved – under some conditions – are the: • speed u, • fluid density ρ, • viscosity ν of the fluid, • size of the body, expressed in terms of its frontal area A, and • drag force FD. • Using the algorithm of the Buckingham π theorem, these five variables can be reduced to two dimensionless parameters: • drag coefficient CD and • Reynolds number Re. • Alternatively, the dimensionless parameters via direct manipulation of the underlying differential equations.

14. That this is so becomes apparent when the drag force FD is expressed as part of a function of the other variables in the problem: This rather odd form of expression is used because it does not assume a one-to-one relationship. Here, fa is some (as-yet-unknown) function that takes five arguments. Now the right-hand side is zero in any system of units; so it should be possible to express the relationship described by fa in terms of only dimensionless groups.

15. There are many ways of combining the five arguments of fa to form dimensionless groups, but the Buckingham π theorem states that there will be two such groups. The most appropriate are the Reynolds number, given by and the drag coefficient, given by Thus the function of five variables may be replaced by another function of only two variables: where fb is some function of two arguments. The original law is then reduced to a law involving only these two numbers.

16. Because the only unknown in the above equation is the drag force FD, it is possible to express it as Or and with Thus the force is simply ½ ρAu2 times some (as-yet-unknown) function fc of the Reynolds number Re – a considerably simpler system than the original five-argument function given above.

17. Dimensional analysis thus makes a very complex problem (trying to determine the behavior of a function of five variables) a much simpler one: the determination of the drag as a function of only one variable, the Reynolds number. The analysis also gives other information for free, so to speak. The analysis shows that, other things being equal, the drag force will be proportional to the density of the fluid. This kind of information often proves to be extremely valuable, especially in the early stages of a research project. To empirically determine the Reynolds number dependence, instead of experimenting on huge bodies with fast-flowing fluids (such as real-size airplanes in wind-tunnels), one may just as well experiment on small models with more viscous and higher velocity fluids, because these two systems are similar.

18. HYDRODYNAMIC LOADS 1. Waves Loads Wave load is one of the most important effects we should consider for offshore structural analysis. Wave can be represented analytically using different theories. Several wave theories are available in ocean engineering, such as Airy, Stream Function, and Stokes, cnoidal, Solitary, trochoidaltheory depending on three dimensional parameters, d, H, and T. Or the validities of these theories could be described in terms of two dimensional parameters, H/T2 and d/T2. To calculate wave forces, one must select a proper wave theory first to compute the water particle velocities and acceleration. .

19. Linear Airy theory, the sea surface elevation, water particle velocity and accretion for the regular wave, which could be expressed as:

21. The natural sea state is a stochastic process. Several wave spectrum functions are proposed to describe the sea state. The most frequently used spectra for wind generated seas are the Pierson- Moskowitz (PM) spectrum for a fully developed sea, and the JONSWAP spectrum for a developing sea. The formula for the JONSWAP spectrum is written as follows: (10) Where the significant wave height, Hs, and the peak period, TPare the required parameters to define a wave spectrum, γis the Peak-shape parameter. The Pierson-Moskowitz spectral density function may be regarded as a special case of the JONSWAP spectrum with γ =1.

22. Given the significant wave height and peak spectral period for a single simulation, the wave spectrum is calculated first and then a random, stationary sea surface elevation process composed of irregular, long-crested waves is generated. Irregular random waves, representing a real sea state, can be modeled as a summation of sinusoidal wave components. The wave profile is computed as: with Hydrodynamic loading resulting from the interaction between waves and structural members is known as a key factor in the design of offshore structures.

23. A vertical cylinder representing an offshore substructure can be considered as a slender structure in waves. For the slender structure, the diameter D of the cylinder is small compared with the wavelength λ, or the diffraction parameter D/λ is less than 0.2 (figure). In this case, the forces on the structure can be calculated from the drag and inertia components using Morison’s equation. The drag and inertia components are calculated from the water particle kinematics aforementioned. The force per unit length of member is: Where uwand uware the water particle velocity and acceleration, respectivelyusis the structure acceleration, the first term in Eq. is referred to inertia force, second one is the water added mass force, and the third one is drag force.

24. The total shear force on a slender member elevating from z1 to z2 could be found as: The coefficients Cd and Cm are determined by structure shape, the Keulegan–Carpenter number, Reynolds number and surface roughness. From Morison’s equation, hydrodynamic loads depend on the forms of the structure and the current, and inertia and drag forces. The marine growth increases the member’s diameter, surface roughness and mass of the structure, and therefore affects the hydrodynamic loads.

25. For the offshore space frames, which do mainly consist of tubular elements jointed together with different members, such as jacket structure, Morison’s equation could be also used. For the cylinder member which is oriented along a unit vector I withdirectional cosines (l, m, n), the force per unit length on the elementmay in general be written as a vector sum of the inertia or massforce Fm the drag force Fd and a transversal lift force Fl, that is:

26. HYDRODYNAMIC LOADS 2. Currents Loads When a structure is not only subjected to wave particle velocity and acceleration, but also to a current, the current velocity must be incorporated in the calculation of the total hydrodynamic load in the Morison equation. Current velocities could be calculated based on three current profiles as follow:

27. The force per unit length of member will be modified as below eq. to take the current effect into account.

28. HYDRODYNAMIC LOADS 2. Hydrodynamic mass Hydrodynamic mass (added mass) is defined as the mass of fluid around an object which is accelerated with the acceleration of the object. It is caused due to relative acceleration between the object and the fluid. It can be determined by the integration of pressure around the object and is often expressed by: Where Ca is the added mass coefficient and V is the volume of the object. It can be proved that for a circular cylinder, Ca = 1. The hydrodynamic mass has been assumed to equal the mass of the displaced water volume, which is used for wave load determination on offshore structures.

29. Marine Growth 1. Types of marine growth Marine growth refers to all the different types of plants and animals that find a habitat on submerged surfaces of structures installed offshore, such as boats, docks or rocks. “Fouling” is a term used to describe barnacles, algae and other marine plants and animals that attach themselves to submerged surfaces of offshore structures. The variety and severity of marine growth vary according to geographic location, water depth, water temperature, salinity, season, ocean currents, food supply and oxygen content of the water, as well as platform design and operation. Basically, marine growth can be classified into two categories: hard and soft growth. Hard growth includes mussels, oysters, barnacles and tube worms that create relatively thin but hard encrustations. Soft growth includes seaweeds, soft corals, sponges, anemones, hydroids, sea squirts and algae. The typical shapes and sizes of the two kinds of marine growth can be found in Msut’s work Msut, I. J. and Frina, J. W., “Effects of marine growth and hydrodynamic loading on offshore structures,” JurnalMekanikal, Vol. 1, No. 1, pp. 77-96, 1996

30. Marine Growth 2. Distribution of marine growth All marine organisms mentioned earlier are actually in direct competition for space, food and light and in most cases each established communities appear at distinct depth zones. Next Fig. shows depth/thickness profile of marine growth on a typical North Sea jacket platform Chakrabarti, S. K., “Hydrodynamics of offshore structures,” Computational Mechanics, 1987.

31. Depth/Thickness Profile for a Typical Jacket in the North Sea The thickness was assessed based on relevant local experience and existing measurements. The depth/thickness profile of marine growth recommended by DNV (DET NORSKE VERITAS) guidelines is shown in Fig. 5. According to the definition of the effective element diameter, marine growth affects the mass, geometry and surface texture of the support structure of offshore platform.

32. Cross-section of Tubular Member with Marine Growth Layers of marine growth

33. For the purpose of quantifying marine growth and its effects the measurements defined graphically in previous Fig. are used. Deis the effective diameter, k is the roughness height and t is the thickness of marine growth. Both k and t are average values for the whole tubular. In practice the thickness and distribution of marine growth on the structure are measures by divers or alternatively by interpreting the recorded scaled videotapes and/or scaled photographs taken by diver's or ROV's. Ideally the type, density, thickness and pattern of surface cover are recorded for all types of marine growth; as are the extent and order of the overlapping of the various layers of the fouling. Survey of steel platforms usually performed with considerations of the orientation of the members. Particularly, observations of outer, inner, upper and lower surfaces of the members are usually noted as such. Wolfram, 1. and Theophanatos, A., Marine Fouling and Fluid Loading: Environmental Forces on Offshore Structures and Their Prediction, Society of Underwater Technology, Vo1.26, London, 1990.

34. HYDRODYNAMIC LOADS

35. HYDRODYNAMIC LOADS • Wave Theory Waves are the most important source of hydrodynamic loading affecting the structure, inducing maximum response. The selection of wave theories is also very important in obtaining reliable response in offshore structural analysis. This is due to the different pattern in water particle kinematics with respects to certain water depth and wave length ratio. The selection of wave theories may be done by referring to next Fig. while Norwegian Petroleum Directorate suggested the selection as shown in the next Table I. Department of Energy, Offshore Installations; Guidance on Design, Construction and Certification, 4th Edition, London, 1990. NPD (Norwegian Petroleum Directorate), Guideline for the Determination of Loads and Load Effects. Regulation for the Structural Design of Load Bearing Structures Intended for Exploitation of Petroleum Resources, Acts, Regulations and Provisions for the Petroleum Activity, Vol. 2 (Updated 1.1.90),Norway, 1984.

36. Regular Wave Theory Selection Diagram

37. HYDRODYNAMIC LOADS • Current Currents that are associated with hydrodynamic loading on offshore structure may be categorised into three main groups; Tidal currents (associated with astronomical tides), (2) Circulational currents (associated with oceanic-scale circulation patterns), and (3) Storm-generated currents. The vector sum of these currents gives the total current magnitude and the current profile deduces the speed and direction at certain elevation of water depth. API (American Petroleum Institute), Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms, API Recommended Practice 2A (RP 2A). 19th Edition, Washington, 1991.

38. Tidal currents are regular in nature and predictable since they are associated with the highest or lowest astronomical tide. The other components, circulational and storm surge are irregular in nature and unpredictable. Currents can affect the height and length of periodic surface waves, that is when currents travels in the same direction, wave length become longer and when it is travels in opposite direction results in shorter wave length and higher wave amplitude. However, there are some uncertainties in the effects of current on the loading of offshore structures. They are among others; directional uncertainty corresponds to the direction of wave incident as well as sheilding effects that reduces the strength of current. experienced by structural member. In offshore design and analysis, generally, currents are important loading component, especially in the case of fixed offshore structures where currents effect the total force exerted on the structure. It also affects the orientation of the structures concerning location and orientation ofboat-landing bumper.

39. HYDRODYNAMIC LOADS • Estimation of Fluid Loading Estimation of hydrodynamic loading on offshore structure may generally be done using Morison's equation to estimate the hydrodynamic force, F. Drag + Inertia per unit length Where ρ is water density, CDis drag coefficient, CMis inertia coefficient, De is member's effective diameter (including marine growth), U is water particle velocity in direction of force and U is water particle acceleration in direction of force. Here will comprise the sum of the water particle velocity and the current velocity.

40. On small diameter structure such as steel jacket structure having tubular members, the forces exerted by waves and current may basically be represented by simple vertical cylinder extending above the free surface as shown in Fig.. Steady flow parallel to x direction passed the cylinder will results in in-line force and transverse (lift) force. Forces on Vertical Cylinder

41. The inline force, FDmay be represented by the drag term in the above equation. The shedding of vortices at certain flow velocities, give rises to transverse or lift force (e.g. in the case of marine riser). The transverse force, FL, may be expressed in a similar form as the drag force. Where: CLis lift coefficient, and r, De, and U as previously defined.

42. However, current practice in offshore industry is not to consider transverse forces in the loading assessment of jacket and other nominally rigid tubular structures. Whilst it is true that the absence of coherence of these force between members means they will have negligible effect on the overall structural loading the same may not be true for individual member loads in the presence of marine roughness. There-is evidence that roughness not only increase tranverse forces but also increases their spanwise coherence on vertical cylinders in oscillatory and regular wave flows. This may significantly affect the loading. Wolfram, 1, The Effect of Marine Growth on Vortex Shedding and Fatigue Life of Tubular Members: Results from a Case Study, Proceeding of the 1st International Offshore and Polar Engineering Conference, Edinburgh, U.K., 1991.

44. EFFECTS OF MARINE GROWTH ON LOADING Marine growth has number of effects on loading of offshore structure that may among others be listed as the following: (a) increase in structural diameter and displace volume, (b) increase in force coefficients, (c) increase in structural weight, (d) increase in mass and hydrodynamic added mass, (e) increase flow instability, (f) conceal the member's outer surface and (g) cause physical obstruction. Generally, these effects cannot be overlooked if accurate estimation of the response of the structure is required.

45. Structural Diameter and Displace Volume • The present of marine growth on the outer surface of a submerged member will increase its effective diameter hence displaced volume of the structure. This change at certain levels will increase the overall loading substantially, reference the inertia term in Eqn. • especially if the growth is abundant on a relatively small structure. • Increase in Force Coefficients • A member's surface will become roughened with the attachment of the fouling organism. The increase in surface roughness gives rise to changes in both the drag and inertia coefficients in Morison's equation. In general the drag coefficient increases with the increase of surface roughness and the inertia coefficient decreases with increasing surface roughness. Next Fig. clearly shows the relationship between surface roughness and drag coefficients where at high KC value CDincrease with surface roughness. • Barltrop, ,N.D.P. and Adams, A.I., Dynamics of Fixed Marine Structures, Third Edition. Butterworth Heinemann Ltd & Marine Technology Directorate Ltd., London, 1991.

46. Effects of Surface Roughness on CD

47. Increase in Structural Weight • Marine growth will also increase the total weight of the structure. However, the increase is found to be insignificant compared to the total weight of the structure and the variable deck loading. This is due to the low specific gravity of marine growth. • Increase in Mass • The increase in displaced volume due to the present of marine growth will increase the mass, mr, and hydrodynamic added mass, ma, of the structure. This increment in mass will in turn decrease the natural frequency of the structure as represented by Eqn where k is stiffness. This can be significant for small diameter members and may move the structural response closer to resonance.

48. Range of measured values of CL, (rms) Vs Re for a smooth circular cylinder which does not oscillate.

49. Increase Flow Instability • The accumulations of marine growth cause the surface profile become irregular. Marine growth also increases the size of member's diameter to an effective diameter, De. This change will affect the formation of vortex shedding that usually occurs at Strouhall number, Sn> of 0.2. where Deis effective diameter, f is vortex shedding frequency and v is flow velocity. This will increase the strength of vortices and their spanwise coherence thus increasing the cyclic lift forces that may significantly reduce the estimated fatigue life, particularly of a small diameter member Wolfram, J., Javidan, P. and Theophanatos, A, Vortex Shedding and Lift Forces on Heavily Roughened Cylinders of Various Aspect Ratios in Planar Oscillatory Flow, Proceeding of the 8th International Conference on Offshore Mechanics and Arctic Engineering, The Hague, Holland, 1989.

50. Conceal the Member's Outer Surface • The structure in its life time will undergo a routine inspection to ensure the integrity of structural members particularly at the welded joints. The natures of marine growth attaching themselves on the structure and spreading, tend to cover the member's outer surface. This coverage has to be removed before inspection can be carried out. • Cause Physical Obstruction • The size and accumulation of marine growth can physically block or restricting the function of some system on the structure. For example the seawater inlet manifold may be covered by fouling thus to some extent reduce the overall performance of the structure.