Deepwater jacket design method

Abstract

A new jacket design method is disclosed, especially for deepwater water heavy jacket applications. The method utilizes a special type of air bags, called Ship Launching Air Bags (SLAB), to provide low cost temporary buoyancy used for jacket installation purpose only. A designer only needs to satisfy the jacket stiffness for the resistance of environmental and gravity loads without the consideration of jacket reserve buoyancy. The required jacket reserve buoyancy could be increased with the utilization of temporally attached SLABs during the jacket installation.

What is claimed is: 
     
         1 . A method for designing a jacket of an offshore fixed platform, the jacket having a plurality of jacket members including main leg members, diagonal members and horizontal members, the method comprising:
 preparing a plurality of non-steel buoyancy tank groups, wherein each buoyancy tank group comprises a plurality of air bags; and   installing the prepared plurality of non-steel buoyancy tank groups on the jacket;   wherein more than 80 percent of jacket members between water surface and 40 meters (about 130 ft) below water surface are slender members with D (member diameter)/t (member wall thickness) less than or equal to 30.   
     
     
         2 . The method according to  claim 1 , wherein each of the air bag is made of nature rubber and multiple layers of polyester nets bonded together through a vulcanized process. 
     
     
         3 . The method according to  claim 1 , wherein each of the air bag comprises a middle tubular section and two cone sections at each end. 
     
     
         4 . The method according to  claim 3 , wherein each of the air bag comprises a plurality of rows of side ring pairs longitudinally attached to the sides of the air bag middle section. 
     
     
         5 . The method according to  claim 4 , wherein the act of preparing a plurality of non-steel buoyancy tank groups comprises:
 dividing a plurality of air bags into a plurality of groups, each group comprising two or more air bags; and   forming a sheet of air bags for each group by connecting adjacent air bags within the group laterally.   
     
     
         6 . The method according to  claim 5 , wherein the act of forming a sheet of air bags comprises connecting the plurality of rows of side rings of each air bag with the corresponding plurality of rows of side rings of an adjacent air bag by a plurality of wires. 
     
     
         7 . The method according to  claim 5 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around the jacket main legs near water surface in a near vertical orientation. 
     
     
         8 . The method according to  claim 5 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around some jacket horizontal members near water surface. 
     
     
         9 . The method according to  claim 5 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on a jacket member comprises connecting a steel ring attached at the each end of an air bag to a wire circulating around the jacket member, wherein the wire circulates around the jacket member through a plurality of padeyes welded at the jacket member surface. 
     
     
         10 . The method according to  claim 1 , wherein the jacket does not employ any conventional ballast tanks inside jacket main leg bottoms. 
     
     
         11 . A method for installing a jacket at an offshore installation site, comprising:
 preparing a plurality of non-steel buoyancy tank groups, wherein each buoyancy tank group comprises a plurality of air bags; and   installing the prepared plurality of non-steel buoyancy tank groups on the jacket;   injecting air into each of the plurality of non-steel buoyancy tanks to achieve a first predetermined internal air pressure level;   transporting the jacket to the installation site with a transportation apparatus;   removing the jacket from the transportation apparatus, wherein the jacket becomes self afloat maintaining positive reserve buoyancy after removing from the transportation apparatus;   releasing air from each of the plurality of non-steel buoyancy tanks; and   removing the plurality of non-steel buoyancy tanks air bags from the jacket after the jacket installation is complete;   wherein more than 20 percent of the jacket total reserve buoyancy is contributed by attached non-steel air bags.   
     
     
         12 . The method according to  claim 11  further comprising reinjecting air into one or more air bags to achieve the first predetermined internal air pressure level after arrival at the installation site and before the jacket is off a transportation apparatus; . 
     
     
         13 . The method according to  claim 11 , wherein each of the air bag comprises a middle tubular section having a plurality of rows of side ring pairs longitudinally attached to the sides and two cone sections at each end. 
     
     
         14 . The method according to  claim 13 , wherein the act of preparing a plurality of non-steel buoyancy tank groups comprises:
 dividing a plurality of air bags into a plurality of groups, each group comprising two or more air bags; and   forming a sheet of air bags for each group by connecting adjacent air bags within the group laterally.   
     
     
         15 . The method according to  claim 14 , wherein the act of forming a sheet of air bags comprises connecting the plurality of rows of side rings of each air bag with the corresponding plurality of rows of side rings of an adjacent air bag by a plurality of wires. 
     
     
         16 . The method according to  claim 14 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around the jacket main legs near water surface in a near vertical orientation. 
     
     
         17 . The method according to  claim 14 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around some jacket horizontal members near water surface. 
     
     
         18 . The method according to  claim 14 , wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises connecting a steel ring attached at the lower end cone section of an air bag to a wire circulating around a jacket member, wherein the wire circulates around the jacket member through a plurality of padeyes welded at the jacket member surface. 
     
     
         19 . The method according to  claim 18 , wherein the act of removing the plurality of non-steel buoyancy tank air bags from the jacket comprises cutting the connecting wire between each air bag and the wire circulating the jacket member by a ROV. 
     
     
         20 . The method according to  claim 11 , wherein the act of injecting and releasing air from air bags is conducted through a main control center above water surface. 
     
     
         21 . The method according to  claim 11 , wherein the installation site is a deepwater water location with a water depth greater than 60 meters (200 ft). 
     
     
         22 . The method according to  claim 11 , wherein the jacket does not employ any conventional ballast tanks inside jacket main leg bottoms. 
     
     
         23 . The method according to  claim 11 , wherein each of the air bag is made of nature rubber and multiple layers of polyester nets bonded together through a vulcanized process.