Two Tier Well Riser Top Tensioning System
Authors: Wanvik Leiv , Koos John 
Published: October 2008

Abstract
The paper presents the results from a feasibility study carried out by Kvaerner for a top tensioned riser system adapted for its deep-water Deep Draft Floater production unit, which often is referred to as a multileg SPAR. The study addressed allimportant aspects of a multi-riser system (array), including effective drilling/ work-over arrangements for a GOM deepwater application.
The presented paper demonstrates the proposed two tier well and work-over riser top tensioning system has several attractive features and advances the capabilities of the mechanically tensioned systems considerably.
Hydralift USA has been the co-operating partner on the hardware side.
The introductory part of the paper addresses the present art systems (as used in the SPARs) as opposed to the principles of the proposed "two-tier well riser tensioning system". The nature of riser strokes is discussed on a general level along with a proposed approach to develop the basis for functional requirements for the two-tier system, being a mechanical top tensioning system. Two case studies, based on 1500 and 5000 feet water depth respectively, are used to illustrate the stroke demand. The paper puts the main emphasis on the justification of the new philosophy of splitting the tensioning system duties into two parts, based on certain observations made in the frequency domain. In short the split duty philosophy involves separation of the low frequency and steady stroke motions from the wave induced stroke frequency (high frequency). It is believed that this philosophy is a step forward for the mechanically tensioned systems. The configuration of the proposed system has inspired the use of the term "two tier tensioning system", as the proposed system is analogue to hydraulic cylinders working in series. It is concluded that the proposed system can be designed and developed from present state of the art mechanical tensioning equipment components. The two-tier system is especially attractive when used in conjunction with dry tree solutions.

Introduction
Traditionally the industry has been capable to follow up with production unit solutions as the exploration rigs moved into new deepwater frontiers. The current deep-water regime is no exception. Looking at the pace of exploration and interest for future deepwater acreage is a clear signal that decision makers are judging the risk for running into technical "show stoppers" sufficiently low. However, speaking of the deep- to ultra- deep water depth range, 2000 to 10 000 feet, some real challenging technical aspects are evoked:Flow assuranceSea bottom and formation stabilityCarrier water depth sensitivityStation keeping /Mooring systemRiser systems

Carrier Concept Selection - Discussion
All concepts have to solve the a) Flow assurance problem, but it is implicit that carriers that offers dry well solutions will be preferred in most cases (temperature control/rapid well intervention)
The b) Sea bottom and formation stability aspects favor surface solutions that are not vulnerable to sea bottom or foundation failures, which disfavors the TLPs
The TLP is further troubled by the c) issue water depth sensitivity.