One of the
greatest discoveries of 20th century was oil and it has so many applications
that it cannot be separated from mankind. The oil exploration has started as
yearly as 1900 and the oil exploration initially was concentrated on land. As
the demand for oil increased at an exponential rate, need to find new
discoveries was eminent. During the middle of 20th century, oil discovery
started in near shore and medium range of water depth.
design and construction of offshores structures is arguably one of the most
demanding set of tasks faced by the engineering profession. Over and above the
usual conditions and situations met by land based structures offshore
structures have the added complication of being placed in an ocean environment
where hydrodynamic interaction effects and dynamic response become major
consideration in their design.
The need for
qualified offshore structural personnel are rapidly increasing as the oil
industry moves into deeper water in the search for additional supplies of oil
and gas, new technology is emerging at a rapid peace for the development of new
concepts for offshore platforms.
This study gives
brief introduction to offshore engineering with basic concepts of various types
of offshore structures and provides insight into various design issues and
requirements, fabrication and installation techniques.
The design of offshore structure is not a single step design process. The
structural configuration, arrangement, member sizes and its specification
requirements can be arrived after few design cycles. In order to achieve an
optimum design suitable for the installation method proposed and satisfy the
final operating requirements, a design procedure suitable for the project shall
In an offshore project, the design of structural elements cannot be
initiated unless the basic understandings of the needs are identified. The
basic needs are as following:
What is the type of platform?
What is floor area of topsides required?
Expected maximum weight of facilities?
What is basic water depth and environmental parameters
such as wave and current?
Where is it located?
Is it Earthquake prone?
What is type of installation?
The various design stages in an offshore project is listed below:
Front End Engineering Design (FEED) or Concept Selection
The first step in initiating an offshore project is a FEED or concept
selection. This stage of project will involve following steps in all disciplines
such as Process, Mechanical, Electrical and Instrumentation in addition to
Collection Process Data and identifying process needs and
Preliminary equipment sizing and area requirements
Weight estimation based on past projects
Identification of Structural configurations
Preliminary estimation of structural weight
Identification of installation methods
The above activities will define the project to a basic understanding and
will provide enough insight into carrying out further engineering activities.
2. Basic Design
At this stage of the project, the data collected during the FEED stage will
be further verified to make sure the authenticity and reliability of such data
for further use. A detailed weight estimates of all items involved in the
project will be carried out. The process and mechanical requirements will be
further defined and identified. A Design Basis (DB) will be developed for the
proposed facility containing following information. Process information containing
type of well fluid (oil or gas) and its characteristics, safety requirements
and kind of process technology to be adopted.
Mechanical requirement such as type of facility and basic
equipment required for the process, and material handling and safety
Electrical requirement such power generation equipment,
lighting and switch gears etc.
Instrumentation requirement such as basic control system,
feedback requirement etc.
Piping information such as pressures, pipe sizes required
Meta-Ocean information such as water depth, wave,
current, wind and tidal information at the site.
Structural requirement such as materials proposed or
available for use in the country, design method to be adopted, codes and
specifications to be used etc.
Installation information such as type of barge, lifting
crane, loadout-method, piling, hammer etc.
At the basic design stage, the deck area required, number deck levels etc.
will be defined. This will lead to identification of number of legs required to
support the deck. Normally the spacing between deck legs for a typical platform
can vary from 10m to 20m beyond which it may become uneconomical to design a
braced deck truss structure. Basic weight estimates for various disciplines
such as structural, mechanical, electrical, instrumentation and piping will be
carried out. Based on the weight of total deck, it may then be decided to
fabricate the deck in one piece or in various modules. These kind decisions can
only be taken together with the viable installation options such
“Availability of Heavy Lift Vessels in the region” or use of
float-over technique. In case such methods are not possible, then the total
topsides shall be divided in to various functional modules such as compression
module, process module, utility module, quarters module, etc. These modules are
self-contained units with structure, piping, equipment etc. fabricated and
transported to the site. These modules are then installed on top of the
“module Support Frame”, which transfers the loads to the jacket.
Sometimes this module support frame may not need, if the modules are organised
properly over the legs. This kind of basic ideas shall be made at the basic
Detailed design of offshore platform will be initiated once the basic
design confirms the economic viability and technical feasibility.
combinations shall consider ULS-a, ULS-b and ALS load conditions with
contribution from relevant load types as defined. Load combinations are
established to give maximum footing reactions at the interface between the
offshore module structure and the existing production platform structure, and
resulting stresses in the structure.
loads, wind and earthquake, shall be considered acting from eight different
directions at 45 degrees interval covering the complete rosette, but in this
wind action has been considered for five directions during in place
The structure is
analyzed for wind with average recurrence period of 100 years. Considering the
structure height above water level, Ice load is neglected in these analyses.
Considering the small load magnitude of 0.5 KN/m2 it is concluded that the snow
load can be neglected in the global analyses.
for inplace analyses are performed in Staad.ProV8i.
A design approach
that minimizes static load effects through judicious selection of the platform
geometry is one of the keys to increased capability of platform. Static loads
could be reduced through decreasing member sizes and through adjustment of the
jacket geometry at the top load action zone, so that force cancelation could be
used to advantage. The type of loads considered in the analysis (API RP 2A)
1, includes the following:
loads (E) (wind /wave/buoyancy)
The load combination
used for the analysis purpose includes:
G + 1 D = U
G + 1 E = V
G + 1.3 Q + 1 E + 1D = X
+ 1 Q + 1 E + 1 D = Z.
The load calculations are done based on the
standard DNV, ISO19902, API RP 2A.
The wave load is calculated based on the
Morrison’s equation which is shown in equation (1) below:
F = 0.5 ? |u| u Cd D + ?Cm(d24)? (1)
Where, Cm, Cd =
hydrodynamic inertia & drag coefficient, ? = water density, D = pile
diameter, u = water particle velocity, ? = water particle acceleration.
The wind load is
calculated for the critical wind direction for the whole topside module using
the following API recommended formula:
Mumbai basic wind
speed (zone III), Vb = 50.6 m/s (API)
Observed critical angle at Mumbai High = 34º
F = q (h/10)0.22 c.A.Sin ? (2)
Q = 0.613 Vb²
C = shape coefficient, h =datum height, A=
exposed area, q = basic wind pressure or suction,
? = angle between the direction of the wind
and the axis of the exposed member,
Vb = basic wind speed (m/s).
above equations, the calculation of the wave forces in each member is generated
both manually and theoretically. The lateral forces are resolved and the total
axial force in the structure is found out, which is later compared with the
result obtained through analysis. The obtained value is checked and compared to
determine the stability of the structure.
3. SEISMIC LOAD
Mumbai is located
in Seismic Zone III as per IS:1893-2002 (BIS, 2002) signifying that the city
may be subjected to intensity VII damage as per MSK64 Intensity Scale. A review
of the historical as well as the recent earthquake activity in peninsular India
indicates that different parts of the region are characterized by low to
moderate level of seismic activity (Jaiswal and Sinha, 2007). Occasionally some
large and damaging earthquakes, such as the Koyna (1967), Killari (1993),
Jabalpur (1997), and Kachchh (2001) earthquakes have occurred in the region.
Unlike the earthquakes occurring on plate boundaries, demarcated by mid-oceanic
ridges, transform faults and island arcs, these are intraplate earthquakes and
are thus rarer. Mumbai is located near the Panvel seismic source zone, which is
known to be seismically active.
can be defined as fires and explosions, impact from ships, dropped object and
helicopter crash. Impacts loads from ships and helicopter crash have not been
considered in these analyses. The accidental loads have been considered in
these thesis are dropped object accidental load which is defined as a 7.0 tons
container falling from a height of 3.0 meters, explosion load and fire loads.
The module structure must withstand the impact force and prevent damaging of
instruments which are located inside of the module structure. The initial
plastic design of module structure is based on the impact effect of a dropped
object, plastic hinge development and local damage due to the plastic
structure analysed and designed is as follows as:
The main objective
was to do design and analysis of an offshore structure to obtain a proper
weighed structure that has sufficient capacity and strength with respect to
normal operation, transportation and installation phases. Apart from these
factors the goal is to achieve the high safety with respect to life,
environment and economic risk.
analysis and design of offshore structure is to ensure the required safety and
serviceability requirements against different load and load combinations (i.e. explosion load, fire load, live load, wind
load and earthquake) by considering all phases such as inplace, transport and
lifting condition, were done to obtain the main goals.
The structure was
designed and analyzed by using the Staad. ProV8i. Offshore structure designed
and analysed inplace condition. In inplace the
structure has been designed and modelled to withstand against all loads
and load combination assumed to occur during the estimated life period for
normal operation. Global structural analysis is done in Staad.Pro.V8i and
results show that the designed offshore module structure has sufficient
capacity to withstand normal operating loads, such as wind, laydown loads,
earthquake loads. Highest utilization factor from the Staad.Pro analyses is
0.941 which is less than the design limit criteria, UF?1.00.
In inplace the structure
is going to be subjected dropped object impact load scenario, explosion loads
and fire loads. The calculation of affected beams in case of dropped object
impact load based on fully plastic criteria were done to show that the structure has enough capacity to withstand
dropped object impact load without damaging the instruments which are going to
be installed under the offshore module structure. Resulting UF from hand
calculations is 1.00.
are the second accidental loads that have been considered that might be happen
in inplace condition. Structural analysis was done by Staad Pro and results
obtained by analysis shows that the UF in this cases are within the acceptance
limit criteria set in design basis and highest UF = 0.984 which is less than
Fire action is the
last accidental loads which have been considered for inplace condition.The most
effected beams with the highest bending moment and results shows that the new
offshore module structure must be protected against fire loads to fulfill the
design limit criteria basis.
1. Dr.N.Nallayarasu, Offshore Structures Analysis and
Design, Saipem India Projects Limited, Chennai, during 11-15 December 2006.
2. Dr.N.Nallayarasu, Training course for L
engineers on offshore structures, L Valdel office, Bangalore, during 14
3. IS-800:2007 (2007),”Code Practice for Steel
Structure” Bureau of Indian Standards, New Delhi, India.
4. BS ISO 19902-2007(LRFD),” Specifican for fixed
5. ASCE7 –”Minimum Design Loads for Buildings and
6. AISC 360-10 2005, – LRFD Method of design.
7. Arya-Ajmani (1964) “Design of Steel Structures”.
NEM CHAND & BROS; ROORKEE (U.P).
8. BS-5950:2000, “Structural Use of Steel Work in
Buildings” Part-1 Code of Practice for Design in Simple and Continuous
9. S.O. Degertekin (2004) “Design of non-linear
semi-rigid steel frames with semi-rigid column bases”. Electronic Journal of
10. Prof. Ravindra Bhimarao kulkarni, Vikas Arjun
Patil “Comparative Study of Steel Angles as Tension Members Designed by Working
Stress Method and Limit State method”.
11. Dr.Subramanian (2008) “Design of Steel Structures”
Oxford University Press, New Delhi.
12. Swapnil B.Kharmale (2007) “Comparative Study of IS
800(Draft) Code and Eurocode 3 ENV:1933”. M.Tech thesis, Dept. of Structural
Engineering, VeeramataJijabai Technological Institute, Mumbai.
13. API RP2A-American Petroleum Institution
Recommended practice for Planning, Design, fixed offshore platform, API
Publishing Services, 2008.