Innovative Drilling Rig for Deepwater Dry Tree Floating Platform
Authors: Srinivasan Nagan , Sridhar Meena 
Published: April 2012

The deep water development in the Gulf of Mexico (GOM) harsh environment couldpotentially reduce the capital cost with the self-contained drilling unit onthe production floater. This paper studies the technical feasibility of acomplete drilling package; an independent pipe deck combined drilling supportstructure on top of the ultra deepwater production floater. A dry-tree supportsemi-submersible is used for this study for water depths from 7000 to 9000 ft. The objective of the paper is to reduce the drilling costs with the use of anonboard platform rigs over dry trees. In this design, a 2.5 million hook loaddrilling rig in association with the drilling support modules on a box-on-boxconcept structure type is considered.

The substructure skids are over the strong back in the transverse direction andthe strong back skids are over the platform capping beam in the longitudinaldirection. The substructure contains a trip tank and an independent mudseparate system. The larger mud process system and the power units are isolatedfrom the substructure that allows the substructure to be lighter and easy tomove both longitudinal and transverse to cover the matrix of the well pattern.A bridge type structure communicates between the drill floor and the drillingsupport structure by transporting the pipe to and fro from the pipe deck to thedrill floor. The bridge is also used to make up and break up the pipes inaddition to the pipe-conveyer work. The substructure is designed as a box-on-box type thus easing the fabrication, transportation and rig-up. This paperconceptually engineers the drilling structure with the selection of majorequipment accordingly, to meet the proposed platform requirement for thedry-tree support semi-submersible with production unit.

The merits are the reduction in the overall drilling, operating and wellenhancement cost. Additionally, the advantages are early production withaccelerated production revenues leading to a reduction in the overalldevelopment capital requirement. Also the ease of well intervention fieldrequirements to enhance the well performance is feasible. This paper alsodefines and illustrates a suitable dry tree vessel for ultra deepwater harshenvironment to make the GOM ultra deepwater development reliable, feasible andcost effective.

Introduction

In general, the future of the oil and gas is very bright for internationalplayers. In the USA, the GOM deepwater development has a significant role withseveral prospective fields in water depths over 5000 ft. Some of these fieldsare over 7000 ft and close to 10,000 ft range and will be developed in the merefuture. The GOM environment in general is called harsh environment with themaximum wave height of 91.5 ft in a 1000 year return period. For such largewave height the free board required for the floating vessel is about 100 ft. Several dry tree support concepts are available. This paper presents one of themost suitable dry tree supported semi-submersible platform concepts toaccommodate the conceptually engineered drilling rig. The technical advantagesand the features of the selected dry-tree support semi-submersible is alsopresented in middle part of the paper after presenting the drilling concept. The selected dry-tree vessel with minimum heave acceleration would make designof the drilling rig as light weight. A cost and weight effective drilling rigstructure that is suitable for the dry tree support platform is one of theprimary tasks required for the ultra deepwater development to be successful. Aninnovative drilling rig with a safe, light weight, reliable, efficient, easy toconstruct, easy to transport and easy to install (utilizing modern drillingequipment) will become necessary for the dry tree support ultra waterdevelopments.