INSTALLATION OF OFFSHORE STRUCTURE

( UNIT -6 )

NAVAL ARCHITECTURE & OFFSHORE ENGINEERING

Introduction:-Offshore structures are built onshore and transported to the offshore installation site. The process of moving a structure to the installation site involves three distinct operations referred to as the loadout, transportation and installation operations.structure is subjected to loads that are different in magnitude and direction from the in-place loads. Different types of structures require different methods of transportation and installation. Different installation methods can also be used for the same type of offshore structure.

A fixed substructure is that part of an offshore platform which sits on the seabed and is rigidly connected to it by means of foundation piles (e.g. jackets) or under the effect of itsweight (e.g. gravity base structure).Jackets :-The jacket is a space frame structure made of tubular steel members. The jacket legs and braces transmit environmental and topsides loads into the piles and subsequently into the seabed. Jackets typically have three, four legs. Jackets with three legs are known as tripods. Jackets with a single caisson type leg also exist. These are also known asmonopods.

Piles made of tubular steel are installed through the legs of the jacket or through the pile sleeves connected to the jacket legs at its base The method of installation depends on the weight and the physical dimensions of the jacket and on the capacity of the installation equipment. The following methods are the most common for a jacket installation.1 > Lift and Lower in Water :-This method is used for small jackets, in very shallow water, which are transported on barges in the upright position already pre-rigged for offshore lift and installation by a cranevessel. Once offshore, the jacket is lifted off the deck of the barge and lowered down to the seabed. Jackets installed in such a configuration are typically less than 50 m tall.

The foundation piles for this size of jacket structure are typically transported together with the jacket on the same cargo barge. Once the jacket is set on the seabed, the piles Are installed using the same crane vessel and a pile hammer of an adequate size.

2 > Lift and Upend :-

As the size of a jacket structure increases, it is built and transported on a cargo barge in the horizontal position. The jacket is lifted off the cargo barge using one or two cranes.

There are several methods of upending jackets: -Two-block upending :-upending in air or partially in water using two crane blocks.In this method, the jacket does not have sufficient buoyancy to float without crane assistance. Instead, the upending is achieved by hoisting down the block of one of the two cranes while the other is hoisted up.

Single-block upending :-A jacket installed using this method needs to have sufficientbuoyancy to float in the horizontal position by itself. In this method In this method, the jacket is pre rigged with two sets of four slings. The first set of slings - the lifting slings - are attachedsomewhere along the top jacket frame, while in the horizontal position. The second set of slings - the upending slings are attached to padeyes at the top of the legs when the jacket is in the upright position. The jacket is lifted off the cargo barge with the Lifting slings and lowered into the water until its buoyancy balances its weight. The lifting slings are then disconnected from the crane hook and the upending slings are connected

to the hook.The jacket legs need to be made buoyant by installing rubber diaphragms at the bottom of the legs.Launching :-Jacket structures that are too heavy to be lifted can be launched into the sea off a launch barge. A launch barge is a flat top cargo barge equipped with skid beams, launch winches and a suitable ballasting system. Jackets are designed to be either self-upending or upended with the assistance of a crane vessel. Launched jackets need to have sufficient reserve buoyancy in order to ensure they float at the end of the launch sequence The jacket legs are made buoyant by the use of rubber diaphragms at their bottom ends and steel caps at the top.

A jacket launch naval analysis is required in order to:-

• ensure that an adequate sliding velocity is maintained during the rocker arm rotation;

• determine the jacket behavior during launch; • define operational requirements during launch, including

ballast configuration; • check the stability of the jacket during launch and when

free floating.