Concept Selection and Global Sizing of the Mars TLP
Authors: Larrabee R.D. , Hodges S.B. , Gonzalez R.D. 
Published: October 2008

The Mars Tension Leg Platform (TLP) was developed under tight constraints on cost, schedule and payload, while accommodating the highest peak production and deepest water (2933 feet) of any platform in the Gulf of Mexico when installed in 1996. The major break though was the use of a dual concentric, high pressure drilling riser, allowing a lighter, skiddable, conventional platform drilling rig.

The Mars TLP that resulted is a progression from the Auger TLP, and drawing on two separate initiatives - theTension Leg Well Jacket, and the matrix well bay TLP. The number of wells needed to develop the field and the projected drilling demand, as well as the need to reduce capital cost, drove the concept selection.

The TLP's dimensions and tendon size were set through a comprehensive global sizing procedure. The key parameters were column spacing, draft, and column to pontoon volume ratio. The procedure minimized total estimated cost (hull +deck + foundation + tendons), subject to constraints such as target payload, minimum column and pontoon diameters, towout draft and stability, and total column height for construction lifts.

As structural design and time progressed, design dimensions like deck span and column diameter became new global sizing constraints, and cost benefits of payload weight savings declined. Receipt of the Model Test results and the topsides weight reduction program allowed setting the last hull parameters - the draft and the under-deck air gap.

Introduction

Global sizing is a key engineering design process in both the concept selection phase and design phase of a floatingstructure. The sizing of a TLP (like other moored floating structures) involves strong relationships between the payload, the size of the hull, and the cost of the mooring. Sizing starts during the concept selection phase as the basis for costestimates used to compare concepts. Sizing then continues through determining the dimensions and proportions of the hull and the design of the moorings, in this case tendons.

This paper will emphasize the engineering disciplines of Global Dynamics and Naval Architecture, Drilling, and weight and cost estimating. We show how the concept selected evolved from other real and proposed structures -withthe sizing process used to put costs on the options. Then we go on to the actual sizing process that lead to the final dimensions of hull, deck and tendons, and how it relates to the overall schedule and other concurrent design activities.

Concept Selection

The selection of the concept for Mars started in late 1991, while Shell's first TLP Auger was under construction. Thesearch was already on for lower cost solutions. Maintaining well accessibility and the capability to re-drill wells continued to be an important requirement for the Mars field.

In this paper, we'll pick up the Mars story in mid-1992, about the time that a tension leg platform was preferred over two other concepts: a tanker-based FPSO and a Semisubmersible production unit. These other concepts were early production systems, and made sense when considering that they would provide early cash flow and at the same timeprovide reservoir information.