Dry Tree Semisubmersible System Solutions for Non-Hurricane/Non-Cyclonic Environments
Authors: Rijken Oriol 
Published: June 2014

Dry tree solutions, i.e. the well heads located at deck level, can be an attractive solution to sustain or improve production from one or more offshore oil or gas fields. Existing dry tree solutions have been implemented on Tension Leg Platforms (TLP's) and Spar Platforms. The TLP reaches an economical and thus feasible limit at a water depth exceeding 6000 ft to 7000 ft; Spar technology has been successfully applied at water depths exceeding 6000 ft.

A key element for dry tree technology is the riser tensioner, for both production risers and drilling riser(s). TLP's use relatively short stroke tensioners while spar platforms use relatively long stroke tensioners. Spar riser technology is field proven.

Production semisubmersibles have been successfully installed in water depths up to 9000 ft and have proven to be robust systems. Quayside integration and quayside commissioning are two of the economic advantages of the semisubmersible. A semisubmersible requires typically significantly more tensioner stroke than a spar under the same environmental conditions. However, stroke requirements for semisubmersibles in the more benign regions of the world, such as of the African or Brazilian coasts, are on the order of typical spar tensioners requirements. This indicates that a dry tree semisubmersible is technically possible for those more benign locations.

There are several distinct variants of the dry tree semisubmersible. They include a well head semisubmersible, i.e. a work-over rig with minimal facilities; a drilling semisubmersible, i.e. full drilling capability; and a drilling & production semisubmersible, i.e. the hydrocarbons are processed on the facility before offloading.

The sizing of a dry tree semisubmersible requires a careful balance between the various requirements; some of those requirements are documented in a Basis of Design document, while other requirements are not documented outright but arise from the planned construction yard, e.g. quayside draft and crane capacity, the planned transportation methodology, or the country's jurisdiction, e.g. local content.

The focus of this paper is to present a drilling and production dry tree semisubmersible and to identify some of the major variations in size which are the result of variants on the design requirements and/or construction requirements.

Introduction

Dry tree solutions are proposed as a floating production system solution which may improve or sustain the economic feasibility of deep water offshore or gas fields. The advantage of having the production trees at deck level of the floating vessel is ease of access to the well itself by means of a work-over rig; the wells are initially predrilled and may or may not be completed. A variation on this concept is to drill entire wells from the floating production system. Here the BOP can be positioned at deck level. The floating production unit moves as a result of the environment and as such there will be relative motion between the top of the production risers and the vessel and the BOP and the vessel. The stroke of the tensioners, one tensioner (system) per riser, allow for the relative motion while keeping each riser in tension.

A Tension Leg Platform (TLP) is one of the floating production systems which can be used to support the trees and BOP at deck level. The TLP is vertically moored to the sea floor by means of tendons. These tendons significantly reduce the relative motion (stroke) between the tree / BOP and the vessel. An example of the stroke range in a TLP is shown in Figure 1. The scuff marks on the production riser are less than 30?? tall, and are the result of the relative motion between the production riser and the TLP during a hurricane of near Survival conditions. The TLP, both hull, deck and in particular the tensioners, can be commissioned inshore or quayside. This provides major advantages to cost, risk reduction and schedule. However, a TLP is no longer an economically attractive solution when the water depth exceeds 6000 to 7000 ft.