Marine Growth

Question 1

What is the first effect of marine growth on subsea structures

Answer 1

The profile area of any component presented to the water flow will be increased & this will increase the force on the structure overall

Question 2

Name the second effect

Answer 2

 Marine Growth will change the texture of the surface from a smooth, round surface, to a much rougherone.
 The effect of this is an increase in the drag coefficient.
 Data, on the types & the amounts of marine growth,is required to confirm or to modify the design-predicted loads on the structure.

Question 3

What are the knock on effects of marine growth

Answer 3

These two effects have a knock-on effect:
 Increases mass without changing stiffness, causing a reduction in the structure’s natural frequency.
 Increasing the drag forces on the structure.
 Affecting the corrosion rate.
 Reducing the effective area of inlets &
outlets.
 Obscuring important features on the
structure.
 Making inspection impossible before cleaning.

Question 4

What are the main categories of growth

Answer 4

From an engineering standpoint, there are two main categories of marine growth fouling:
Soft
 Organisms that have a density approx. the same as seawater cause soft fouling, important because of their bulk but are, generally, easy to remove.
Hard
 Organisms causing hard fouling are much denser (1.4 times > seawater) & more firmly attached so, more difficult to remove.

Question 5

Soft fouling 1

Answer 5

Algae:
 Often referred to as slime & is generally the first organism to inhabit an offshore structure.
 As it is very light sensitive, it is seldom seen in any quantity below 20m.
 This is a very large family of plants & even includes kelp so, it goes from the very small to the very large.

Question 6

Soft fouling 2

Answer 6

Bacteria:

 Amongst the first inhabitants of an offshore structure & will be present at depths over 1000m

Question 7

Soft 3

Answer 7

Sponges Often found on offshore platforms down to depths of >1000m

Question 8

Soft 4

Answer 8

Sea Squirts These are soft-bodied animals & sometimes grow in large colonies down to depths of 1000m

Question 9

Soft 4

Answer 9

Hydroids Grow in colonies, can be mistaken for seaweed, but are in fact animals related to Sea Anemones. They can produce dense coverage down to 1000m.

Question 10

Soft 5

Answer 10

Seaweeds There are many types of seaweed that attach themselves to structures, but of these kelp produces the longest fronds, which in the North Sea, can grow up to 6m in length under favourable conditions.

Question 11

Soft 6

Answer 11

Bryozoans Have a moss-like appearance but are animals with tentacles, found to depths of 1000m

Question 12

Soft 7

Answer 12

Anemones
 Sometimes called Anthozoans, which means flowering animals.
 The cylindrical body is surmounted by a radial pattern of tentacles & looks a bit like broccoli.
 It attaches itself to the structure by a basal disc, which is so firm, that attempts to remove it often result in tearing the body of the animal.

The colours & shapes are extremely variable even within the same species. Found at depths down to 120m.

Question 13

Soft 8

Answer 13

Dead Man’s Fingers
 Colonies grow to 150mm in length, at depths down to 120m.
 When submerged, many small
polyps arise from the finger- shaped, fleshy main body, each having eight feathery tentacles.
 It is white to yellow or pink to
orange in colour, but when out of the water, it is flesh coloured & the similarity to the human hand gives it its common name.

Question 14

Hard 1

Answer 14

Barnacles:
 The common species is Balanus balanoides.
 These grow in dense colonies to a depth of 15-20m but, are observed to depths of 120m.

Question 15

Hard 2

Answer 15

Mussels.
 This hard-shelled mollusc attaches itself to the structure
with very strong byssal threads at the hinge of the shell.
 Mussels generally form dense colonies to depths of around 20m, but are found down to depths of about 50m.

Question 16

Hard 3

Answer 16

Tu b e w o r m
Often form on flat surfaces. It is white in colour, very firmly attached & difficult to remove - power cleaning is required.
 They grow in colonies &
have been known to fill a warm water outlet, arranging themselves parallel to the flow, in order to obtain the maximum nutriments.
 Although the main growth occurs down to 50m, they are found to 100m.

Question 17

Summary of depths soft growth

Answer 17

Algae 0 to 20
Seaweed 0 to 20
Anemones 0 to 120
Dead man's fingers 0 to 120
Bacteria 0 to 1000
Sponges 0 to 1000
Sea squirts 0 to 1000
Hydroxide 0 to 1000

Question 18

Depths for hard growth

Answer 18

Mussels 0 to 50
Tubeworms 0 to 100
Barnacles 0 to 120
Bryozoans 0 to 1000

Question 19

Factors Affecting Rate of Marine

Growth 1

Answer 19

If no steps are taken to prevent growth (anti-fouling paint) the formation of Bacterial slime occurs in 2 to 3 weeks.
 Marine growth can mature very rapidly.
 Barnacles & soft fouling having been known to attach themselves & reach maturity within 3 to 6 months.
 Generally, it takes 2 seasons for Mussel colonies to develop, often on top of the dead earlier fouling.
 The type of organism, its development & growth-rate will depend on several factors, including………

Question 20

What role does depth play in marine growth

Answer 20

Depth
 The most weight of marine growth is added in in the
vicinity of the surface, which is also the region of the
highest wind & wave-induced loading.
 Light intensity reduces with depth, which reduces the
ability of organisms, such as Algae, to photosynthesise.
 Plants, such as seaweed, also photosynthesise (convert
light) to enable growth.
 The area between the surface & 20m is known as the
Photic Zone. Below this depth, plants will not flourish.

Question 21

What role does temp play

Answer 21

Temperature:
 In general, a rise in water temperature will increase the growth rate of a colony.
 There is, of course, a limit & most organisms cease growth at between 300-350C.

Question 22

What role does current play

Answer 22

Water Current:
 The speed at which the water flows over the surface plays an important part in the type of fouling colony that develops.
 There are two aspects to consider.
 First being that of the larvae attaching themselves to the
structure. A one knot current may be too much at first but,
once established, six knots can be tolerated.
 Second is that in general, once the organism is established, a
strong current brings more food & growth is accelerated.

Question 23

Salinity

Answer 23

Salinity (amount of salt)
 In nearly fresh water, fouling is usually confined to
Algae slime.
 As the salinity increases, so the amount & type of
fouling increases.
 First Hydroids & Barnacles & finally Mussels occur.
 The normal salinity of seawater is 3% - 3.5% & the size
of mussels, for example, increases five-fold from a
salinity of 0.6% to 3.5%.

Question 24

Food supply

Answer 24

Food Supply Growth of fouling is obviously dependent on the quantity of nutriment available.
The slow moving currents, circulating around platforms,
become enriched with nutriments from sewage & other
waste that will increase growth rate.

Question 25

Cathodic Protection

Answer 25

Cathodic Protection Currently, it is not understood why, but ICCP systems seem to have the effect of increasing the marine growth.