Design of Offshore Structures

Prof. Dr. S. Nallayarasu

Department of Ocean Engineering
Indian Institute of Technology, Madras

Module - 5
Lecture - 4
Jackup RIGS-Analysis and design – 4

(Refer Slide Time: 00:12)

So, we will continue with bearing capacity evaluation for the spudcan as you can see it is
a very similar to circular footing only thing is the shape and you know the
penetration is too large unlike shallow footing normally most of the footing foundations
are on the ground or may be one or two meter below whereas, in this particular case we
just going to penetrate deeper into the soil.
So, you may get better bearing capacity, but we need to see the what type of soil that we
can penetrate because if it is a hot soil you may not be able to achieve a penetration. So,
it is balanced between the large size under soil which is slightly better than the surface
soil and the size that is why you see that in this picture if you look at the right side you
know typical spudpans used in the recent times you know this around 15 to 20 diameter
15 to 20 diameter when it is in number you might look very small, but actually if you
look at the physical size its reasonably large and the the vertical size are.
So, called the cone size on either bottom and top you can see around half of it you know
if you look at the 20 meter diameter scan you have about 10 meter approximately. So, it
is quite quite large in size typically made of steal stiff and plates inside. So, that during
the process of penetration it does not bent too much because you want to have a rigid
foundation system. So, only bother about the soil behavior rather than a complex soil
structure behavior. So, that is why we want to make this spud as rigid as possible. So,
that you know you are only going to look at this geotechnical aspect rather than a
complex behavior there is.
So, we will assume as a rigid foundation and that is how you need to make sure that you
designed the spud as a rigid; that means, you have to steepen it inside with plates and
primarily this is made of steel structure you donot make out of concrete because handling
becomes every time you have to lift it of go over from one place to another place. So,
you never think of making it out of concrete always it is made of steel plates and steep
and sufficiently. So, that during the process of going down you get a the reverse soil
pressure which needs to be designed form and most of time we try to do a static loading
conditions because you remember when the leg is going down it is a forward process and
at the achievement of the particular penetration it is going to, and almost it could
consider the loading as a static, but during the process of penetration you could see that
the load is slightly changing every time when it goes down and you do a So, at that time,
but it the time variation is not very fast.
So, it could as well consider as a static loading that is the idea behind. So, you simplify
the process of doing analysis and spudcans are assumed to be rigid this is what I was
trying to do the reason is we do not want to create a further complication of flexible
foundation and flexible soil interaction becomes too difficult to evaluate and also the
behavior needs to be worried about because if we have such a complexity. So, the rigid
foundation means sufficiently stiff and enough short term loading is considered only
because most of our jackups are not going to be stationed for very long time. So, you just
look at only the local and short term effects and not the long term effects like cyclic and
otherwise soil above spudcan will be assumed to be flowing back when it is going
forward and backward.
(Refer Slide Time: 03:54)

So, you can see here the failure may be due to various reasons, which I was showing here
earlier what type of you know behavior. So, failure may be due to punch through this is
one of the major problem of. So, many failures ninety percent of the failure of jackups
are due to punch through we will just learn little bit more about what is punch through
punch through is the phenomena either during installation or during operation the leg is
suddenly experiencing large displacement in vertical direction due to insufficient
capacity from the soil not from structure.
For example, you have installed already it is in operating condition you have estimated
the operating loads may be several thousand tons and also estimated the loading could be
few times higher than that and you have come up with design foundation is perfectly
alright during operation suddenly a storm comes the storm condition slightly exceeding
the design condition then the loads become slightly larger depending on the type of soil
below you may see that whether it may punch through or not because if you have relied
on a layer for example, if you have stopped your spudcan at a stand layer.
So, ten meter deep something like that at say twenty meter below and if the sand layer is
that you are believing that can take all the load that is what you have decided that there is
a very good sand layer. Suddenly the load requirements becomes larger than the capacity
of the sand layer there will a punch through of a sand layer because the sand layer is not
sufficiently thick if it is very large for example, thirty meter forty meter may be difficult
to punch through, but if it is very less what happens you know that is why when you stop
a foundation not only the jackup any type of foundation for structure you should make
sure that this punch through does not happen means the layer must be sufficiently bigger
otherwise what you need to do is ignore the capacity of that thicker or stronger layer you
know basically that is the idea.

So, this punch through is basically a problem in case of other foundation design we can
ignore her e in spudcan we cannot ignore because even if I want to ignore I cannot take
the spudcan below the stronger layer during the time of installation because it is too
strong for me just now I cannot preload. So, much because, capacity is not available. So,
I forcefully ended up in terminating the spudcan at the top of the sand layer and I could
not achieve higher capacity at the time of installation, but during the storm condition I
am exceeding because huge storm creates bigger reaction and leg punch through the
jackup failure.

So, this is. So, basically irrespective of what you can achieve still you are having
problem with punch through type of problem. So, we have to just make sure if it is not
suitable do not bring this jackup to this particular site may be you have to come up with a
bigger spudcan or bigger type of jackup. So, all those things needs to be. So, this punch
through is basically a serious issue the department of energy in uk they have done a
study and found that almost several jackups failed only because of punch through not
because of any other design failures.

So, you will see that the most of the jackups soil structure interaction is actually the basic
study required for any jackup to be relocated from one location to other location other
than mechanical or portion which is slightly a less difficult to do whereas, this one the
site dependant seafloor consist of slightly granular material also can cause scoury
because you know very well when you install a foundation next semester you will learn
more about estimation of is a symbol phenomena when you put an abstraction in the flow
conditions in the sea bed and if the material is ((Refer Time: 07:49)) granular material
started to remove from the location.

Where the structure is is could easily a when you go to the beach once upon to a beach
granular material starts to move away from the abstraction because of the turbulence and
particles get the lifted up and then moved away due to the current. So, this exactly the
phenomena will happened to the this is very large size and trying to go and settle down
you have got a large size which you can make. So, due to this you could see that failure
could be happening. So, one of this problem is progressive loss capacity is keep on
reducing because covering material can be…

(Refer Slide Time: 08:36)

So, typically what we want to understand in this ((Refer Time: 08:39)) two slides how
the behavior can be. So, if you look at the first picture uniform clay layer uniform stand
layer most of the will look very similar those we are studied soil mechanics then easily
see the the idea of bearing capacity derived by you know basically four or five segments
of soil behaving in a slightly different manner. So, if you see this the first one under the
under the spud scan you see a portion of a soil becoming the part of foundation itself
which is very similar when you drive a, you will be using that as the part of the
foundation.

Similarly on the sand layer you will see the behavior it almost similar except whether its
shear or you know depending on type of soil clay or sand the shape and size of this say
mm the the the behavior of the soil will be slightly different depending on the soil type
that is why when the deriving equation will see whether it is a c equal to zero or phi
equal to zero type of soil basically sandy material or claying material will find out the
equations whenever you have a layers soil then this became slightly complex problem
you see here stronger clay over weaker clay or weaker clay over stronger clay.

The behavior could be different. So, if you look at this second one we will easily
understand you have a good material below, but there is a material on the top is trying to
just get depressed away when you are trying to press or increasing a loading because of
the spud can Whereas in the case of stronger clay at the top, but the weaker clay at the
bottom is trying to away on coming on the side to the top you know as much as possible
because there is a very good cover layer So; that means, this might give you better
bearing capacity then this one is it not? Because this unable to escape because you got a
depends on the how much is thickness of the layer If the thickness of layer is smaller
than the loose clay can come out. So, the behavior is going to be slightly complex many
times we do not use equation because that is only applicable for single layer type of
situation when you have multiple layers when you have multiple deeper and deeper those
derivation may not be useful.

And now a days, in fact, that several years back we to use several empirical formulas
inaccurate and because of that you might seen seventy’s and eighty’s several numbers of
failed because of differential you know leg moment Now, a day’s what we do is we have
finite element softwares where you can model all layers of soil including the spud and try
and evaluating them reasonably correct So, that is one of the advantage in the recent time
we have computational devices like this there are several softwares available to do such
kind of whereas, ten twenty years back you might even we not able to model using basic
soil mechanics equation, and also enable to predict based on the empirical formula’s
available from research that is where you will…

(Refer Slide Time: 11:52)

How we do the bearing capacity evaluation there are several ideas. So, one of them
basically assuming pin condition spud can because if you look at the overall global
behavior what is going to happening is the going to have lateral resistance only coming
from the spud of the bottom with the reaction and also on the legs is a leg is not a
circular pipe you know. So, the resistance will be where is small.

So, you will be almost looking like all the three legs are pined at the bottom and trying to
rotate. So, the stability is going to coming from the distance between the legs the larger
the distance what will happen The overturn stability will be better of Stability analysis of
on individual footings can be assumed with also boundary condition called this and then
displacement check based on non-linear soil structure interaction this is where we need
to go and get the help of the computational softwares simplified method will be very
difficult So, we normally do one of the software called we have got several other
softwares which we can use the software we also have a license in our department there
is a time.

So, the first step is getting the bearing capacity and sliding resistance is the important
idea how do we get it? Model the spud can as an equivalent circular for plate; that
means, we trying to approximate you can do that first exercise just equivalent circular
disk evaluate using simplified formula as long as your layers are reasonably you know
constant not varying to much and then you can draw bearing capacity chart something
which I show in the next slide basically a variation of the capacity versus the depth
penetration just like what we are doing for pipes I think next time we will be doing that
and using the curve find out what is the required preload to achieve a particular depth
penetration.
(Refer Slide Time: 13:51)

So, if you look at that basically something like this you know the green line is the
capacity chart; that means, every meter of penetration of the spud can what will be
capacity achieved because every time when you go down the soil slightly getting better
and depth of penetration you get a over button pressure on the side So, the larger over
button pressure you will discuss detail in the next course basically larger the soil pressure
you will get better capacity because it is not a living the soil to move way and move out.
So, what we are achieving from this graph is basically trying to find out what it is load at
the time of operation.

If the relationship between the bearing capacity and depth is slightly like this we should
very happy Because it comes increasing in a on linear there is no big zig jack Now, if
you have a load we need to find out what is the preload require for achieving this
penetration from this graph then you go back to your jack up and that you know the
thanks to get the preload and you will achieve the penetration So, basically these two
will be the most important numbers to remember one is the operating load and the other
one is preload. So, at both condition s you should achieve a whatever required by the
code when we were designing foundation systems we normally design of the at least two
or two and half is not it. So, similarly here we need to find out what is the capacity and
then design it here.
So, basically this V p is the preload V naught is the load at the time of normal operations
like drilling and positioning. So, basically that is the idea. So, the approximations what
we have done is is made equivalent as a circular disk which is penetration into the
ground to the depth of x meters t that depth what could be the capacity.

(Refer Slide Time: 15:51)

So, what we need is every penetration we try to evaluate the capacity by this equation
basically I think those who have studied soil mechanics will be able to understand the
idea is basically if it is a clayey type of soil is related to undrained shear strength and
bearing capacity factors which could be easily derived this equations hardly takes five
minutes to derives by based on the this type of analysis which we will be doing it later in
the next course. So, basically the ultimate bearing capacity at the time of failure can be
computed and we could prorate it to working capacity by means of a factor of safety.

So, this is the generic formula for the footing of size b and l whereas, if it is a circular
footing we could substitute the shape parameter and the depth parameter and ultimately
we will get a equation which is six times s u with a depth parameter the deeper you go
you are going to get a better capacity because the soil will be better. So, this is just for
the idea of how the bearing capacity can be related to soil parameter the importance what
i wanted to conclude here is it all depends on the the shear strength of the soil here which
is basically the clay characteristic if it is a sandy material it would be slightly different
formula which you do not want disposing this particular course. So, basically soil
settlement, which I think some of the failure mechanism we saw yesterday also. You
know it could be a small displacement at one location,, but subsequently the jack up
starts tilting, because of the large moment generated by that effect because the leg
reaction changes. And then there will be a overall over turning stability, which could
cause more reaction to the soil on one side which then becomes higher than the original
capacity and gets penetrated then punch-through can happen. So, it basically a sequential
operation of one problem can cumulatively add and create a final punch-through. Several
reason which I have mentioned basically, we have seen this one in the picture hard layer,
soft layer, existence of sand over soft clay strata which is one of the scenario there. Firm
clay with sand and silt pockets, you know basically can punch-through very soft clay
where the rate of increase of capacity does not correspond to the loading rate, basically
the along the depth.

(Refer Slide Time: 24:13)

What the mitigation measure we can do is basic idea is, evaluate during the pre
installation process. You know, if required the preload water, the legs are reduced while
penetrating through the soft layers, basically you just make sure that able to control the
preloading process. Even if it is going to happen during the installation stage,; that
means, during the process of lowering the legs and penetration you are able to pump out
and pump in water. So, that in case of problems, you can remove some water from one
side. So, that you can slow down the process of the punch-through, if you are not having
control, just you can only see and watch, this is going to just keep on sinking. So, there
are several activities that we need to keep in our mind that we can prevent the punch-
through and follow several guidelines like distance great from the disturbed soil, to
reduce the brace reactions during overload, basically the punch-through locations.

(Refer Slide Time: 25:29)

I have given some of the guidelines, which I will just show you. These are four
guidelines, which I have taken some of the information. I have I think most of them are
available in our library – three of them are available in library only this is not available,
because this is recent one. The first one is you know the Society of Naval Architects and
Marine Engineers, which is downloadable, I think you can download, it is free, so which
gives reasonable guidelines on engineering and installation of Jack-ups.

The second one is basically only the structural part which is a similar to any fixtures
offshore structures, Structural Design of Self Elevating Units, which is like Jack-ups and
other systems. The third one is only a guidance, it is not mandatory, it has got reasonable
information regarding all the hazards like what I have discussed for last three-four
classes. Possible hazards, possible mitigation methods which is basically a health and
safety executive of UK department of energy, which is give us a guidance only. And the
last one is compiling all the information together is a very good document, which I am
trying to get a copy, old version I have – Site-specific assessment of mobile offshore
units, this is basically including the floating as well as fixtures.
You know in case of semisubmersible or drill ships, you have moiré system and their
safety, whereas, in case of jack-ups is basically the jack-up legs penetration and
associated. Here you will not find anything related to the hull, whereas, in this you will
find lot of information and the design and the process regarding hull itself. So, you can
see the distinctive paths, this DNV will not talk about hull or soil, it will talk about the
structural design of the legs and structural design of the hull as the plated structure.
Whereas the HSE-UK department of the energy guidance will give you all the possible
the ideas what can happen during installation and operations. So, it is actually a
comprehensive collection which you could use it for the purpose of learning more. I
think will stop now.