Method and Apparatus for Subsea Installations

Abstract

There is provided a method and apparatus for lowering and/or raising a load or structure to or from the bed of a body of water. The apparatus comprises a buoyancy apparatus configured to be coupled to a load, and having positive buoyancy sufficient to lift the load. At least one receptacle is provided on the apparatus for receiving a control weight lowered from a vessel to lower or raise the assembly. The lowering method includes forming an assembly from a buoyancy apparatus and a load and submerging the assembly to a position at a first height above the bed. In a preferred embodiment the assembly is submerged by a clump weight tow system. A control weight is deployed from a vessel to the assembly to overcome the positive buoyancy of the assembly and thereby lower the load from the first height to the bed. The raising method reverses the steps of the lowering method.

1 . A method of raising a payload from a bed of a body of water, the method comprising:
 coupling the payload on a bed to a buoyancy apparatus to form an assembly, wherein the buoyancy apparatus has sufficient buoyancy to lift the payload;   retaining the assembly on the bed using a control weight;   using a vessel to retrieve the control weight from the assembly to render the assembly positively buoyant, thereby raising the assembly from the bed.   
     
     
         2 . The method as claimed in  claim 1  wherein the assembly comprises a ballast weight corresponding to the weight of the payload coupled to the assembly, wherein the ballast weight is decoupled from the assembly subsequent to the coupling of the buoyancy apparatus to the payload. 
     
     
         3 . The method as claimed in  claim 2  comprising recovering the ballast weight to surface. 
     
     
         4 . The method as claimed in  claim 1  wherein the control weight is a control chain. 
     
     
         5 . The method as claimed in  claim 4  comprising supporting a first portion of the control chain on a lower surface of a receptacle of the apparatus. 
     
     
         6 . The method as claimed in  claim 5  comprising suspending a second portion of the control chain above the first portion within the receptacle. 
     
     
         7 . The method as claimed in  claim 6  comprising suspending a third portion of the control chain between the control vessel and an opening to the receptacle. 
     
     
         8 . The method as claimed in  claim 1  comprising retaining the assembly at a first height above the bed by a clump weight line. 
     
     
         9 . The method as claimed in  claim 1  comprising towing the assembly away from an installation site using a surface vessel via a clump weight line. 
     
     
         10 . An apparatus for lowering or raising a load to or from a bed of a body of water, the apparatus comprising:
 a buoyancy apparatus configured to be coupled to a payload, the buoyancy apparatus having positive buoyancy sufficient to lift the load; and   at least one receptacle on the buoyancy apparatus for receiving a control weight lowered from a vessel to lower or raise the assembly, wherein the receptacle comprises an opening and a lower surface spatially separated from the opening;   wherein the opening is configured for passage of the control weight into or from the receptacle; and   wherein the lower surface is configured to support at least a portion of the control weight.   
     
     
         11 . The apparatus as claimed in  claim 10 , wherein the receptacle is configured to receive a control weight in the form of a control chain. 
     
     
         12 . The apparatus as claimed in  claim 11 , wherein the receptacle comprises a lower surface for supporting a first portion of the control chain wherein the receptacle is configured for suspension of a second portion of the control chain above the first portion within the receptacle. 
     
     
         13 . The apparatus as claimed in  claim 10 , wherein the receptacle is an elongate tower oriented substantially vertically on the buoyancy apparatus. 
     
     
         14 . The apparatus as claimed in  claim 10 , comprising a plurality of receptacles for receiving multiple control weights from a vessel. 
     
     
         15 . The apparatus as claimed in  claim 10 , comprising a ballast chamber for retaining a ballast weight on the apparatus. 
     
     
         16 . The apparatus as claimed in  claim 15 , wherein the ballast chamber is a chain locker for receiving a ballast weight from a surface vessel. 
     
     
         17 . The apparatus as claimed in  claim 15 , wherein the ballast chamber is a tank for receiving a ballast weight fluid. 
     
     
         18 . An installation system used in an installation or deployment method in a body of water, the system comprising:
 an assembly comprising a payload to be conveyed to or from a bed of the body of water and a buoyancy apparatus coupled to the load, the buoyancy apparatus rendering the assembly positively buoyant; and   a control vessel for deploying a control weight to the assembly;   wherein the buoyancy apparatus comprises at least one receptacle for receiving a control weight lowered from a vessel to lower or raise the assembly;   wherein the at least one receptacle comprises an opening and a lower surface spatially separated from the opening;   wherein the opening is configured for passage of the control weight into or from the receptacle; and   wherein the lower surface which is configured to support at least a portion of the control weight.   
     
     
         19 . The installation system as claimed in  claim 18 , wherein the control weight is a control chain and is operable to be coupled to the assembly. 
     
     
         20 . The installation system as claimed in  claim 19 , wherein the control chain is configured to be located in the at least one receptacle to couple the control chain to the assembly. 
     
     
         21 . The installation system as claimed in  claim 18  further comprising a towing vessel for the assembly and a towing clump weight.