Chapter 1

INTRODUCTION

1.1 General

One of the greatest discovery of 20th century was oil and it has so many applications that
it cannot be separated from mankind. The oil exploration has started as early as —– and
the oil exploration initially was concentrated on on land. As the need for oil expands in an
explosive rate, need for find new discoveries was eminent. During the middle of 20th century,
oil discovery started in near shore and medium range of water depth.

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 book gives brief introduction to offshore engineering with basic concepts of various
types of offshore structures and provide insight into various design issues and requirements,
fabrication and installation techniques.

Chapter 2 gives introduction in to types of offshore platforms based on water depth require-
ments, geometry and installation concepts.

Chapter 3 gives basic loads applied on offshore structures and techniques of calculations of
such loading.

Chapter 4 gives introduction to material requirement for offshore structures including cor-
rosion.

Chapter 5 describes various structural analysis techniques required for the offshore structures
before it can be designed.

Chapter 6 gives introduction to design methodology of offshore structures, various design

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1.2. Types of Offshore Structures Chapter 1. INTRODUCTION

methods such as Working Stress Design (WSD) and Load and Resistance factor Design
(LRFD) and code provisions.

Chapter 7 gives basic principles involved in the design of a tubular member in the offshore
structure including code provisions and optimum member design.

Chapter 8 gives tubular joint design including various types and configurations. This chapter
provides some insight into the design of tubular joints for fatigue loading including Stress
Concentration Factors (SCF) of stiffened and un-stiffened joints.

Chapter 9 gives some insight in to different type of foundations for offshore structures and
its design method

Chapter 10 give some basic principles of fabrication and installation of offshore platform
components.

1.2 Types of Offshore Structures

The offshore structures built in the ocean to explore oil and gas are located in depths from
very shallow water to the deep ocean. Depending on the water depth and environmental
conditions, the structural arrangement and need for new ideas required. Based on geometry
and behaviour, the offshore structures for oil and gas development has been divided into
following categories.

1. Fixed Platforms

• Steel template Structures

2. Compliant tower

• Compliant Tower
• Guyed Tower
• Articulated Tower
• Tension Leg Platform

3. Floating Structures

• Floating Production System

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1.3. Fixed Platforms Chapter 1. INTRODUCTION

1.3 Fixed Platforms

The fixed type of platform shall exhibit a low natural period and deflection again environ-
mental loads.

1.3.1 Steel template Structures

The steel template type structure consists of a tall vertical section made of tubular steel
members supported by piles driven into the sea be with a deck placed on top, providing space
for crew quarters, a drilling rig, and production facilities. The fixed platform is economically
feasible for installation in water depths up to 500m.

These template type structures will be fixed to seabed by means of tubular piles either driven
through legs of the jacket (main piles) or through skirt sleeves attached to the bottom of the
jacket.

The principle behind the fixed platform design is to minimize the natural period of the
structure below 4 seconds to avoid resonant behaviour with the waves (period in the order
of 4 to 25 seconds. The structural and foundation configuration shall be selected to achieve
this concept.

1.3.2 Concrete Gravity Platforms

Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is
remote. These platforms are very common in areas with strong seabed geological conditions
either with rock outcrop or sandy formation.

Some part of north sea oil fields and Australian coast, these kind of platforms are located.
The concrete gravity platform by its name derive its horizontal stability against environmen-
tal forces by means of its weight. These structures are basically concrete shells assembled in
circular array with stem columns projecting to above water to support the deck and facilities.

Concrete gravity platforms have been constructed in water depths as much as 350m.

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