Design And Fatigue Analysis Of Reel-pipe Underwater Drilling System

As a new fossil energy,deep-water natural gas hydrate shows great potential and will become an important target of commercial exploitation in the future.At present,it seems that the main way to exploit gas hydrate is to build wells on the seabed of enriched areas.However,it is obviously difficult to meet the needs of commercial exploitation economically by using existing deep-water drilling platforms to construct a kilometer-deep riser group to exploit gas hydrate solid deposits buried less than 500 meters.In view of the characteristics of "deep water and shallow well" in gas hydrate well construction and development,a new type of reel pipe underwater drilling system specially used for gas hydrate well drilling has been put forward.In this paper,the specific design of the system is given by referring to the gas hydrate production test technology in Shenhu sea area,which mainly includes the following work:(1)Layout design of coiled tubular underwater drilling systemReferring to the wellbore structure of natural gas hydrate production test in Shenhu sea area,this paper presents the basic structure and layout design of main devices such as underwater coiler,coiler and its anti-sinking plate,underwater wellhead injection head,and puts forward feasible design specifications in engineering,and elaborates the function and performance index of coiled tube underwater drilling system in detail.(2)Calibration calculation of coiled tubular underwater drilling systemBy analogy with API SPEC 5ST-2010(R2015)Code for onshore coiled tubing,the paper designs the underwater reel in detail and realizes the verification of coiled tubing based on Abaqus finite element simulation.The effects of the diameter of coiled tubing,the diameter of coiled tubing and the wall thickness of coiled tubing are discussed.The stress and strain of the coiled pipe in the process of "coiling-unwinding-coiling" underwater are analyzed in detail,and the stress cycle hysteresis curve of the pipe is obtained,which provides a basis for material design.(3)Fatigue analysis of deep-water underwater Welded reel-pipeThe underwater coiled pipe bears the external pressure load in deep water,the internal drilling fluid load and the periodic load of coiling-unwinding on the reel.The fatigue cracking at the welding position will be the most likely failure mode.In this paper,the structural strain method with grid insensitivity is applied to fatigue analysis of the circumferential weld of the reel-pipe.The feasibility of this method under complex load conditions is verified,which provides support for establishing the design criterion of the underwater welded reel-pipe with fatigue endurance as an important index.