TLP Operational Experience Enhancing Margins In-field as well as New Design
Authors: Engebretsen Knut , Hannus Henrik , Sandvik Arild , Husvik Jørgen 
Published: October 2008

The TLP concept offers proven production capability and a design methodology that is well known to the technical community.

The paper is based on a range of studies and projects performed by Aker Maritime, primarily work associated with the Snorre and Heidrun TLPs The application of the results is intended for new developments that involve TLPs.

Following several years of operation, existing designs have been modified and upgraded significantly. This has enhanced the payload capacity. Further, operational experiences have contributed to better predicting environmental loads and the hydrodynamic performance.

This paper describes how the payload of the Snorre TLP was increased, so as to carry a module to tie-in a subsea development (Vigdis Field). This was achieved by reviewing and updating design criteria, improved analysis methods and benefit of the as-installed condition versus the design margins.

Alternatives for platform weight management (tendon tension measurement versus weight tracking) are described and the experiences of the operations crew are considered.

The measurements of the platform response in operations are compared with analyses and model tests, to verify and calibrate the analysis tools. Experiences of both the Snorre and the Heidrun TLPs are discussed.

At completion of the field life, the TLP may be considered for re-deployment at another field. Case study findings of requirements for re-deployment are summarized. The TLP is well documented in water depths up to 4,000ft. When considering re-deployment in deeper applications, the tendon and riser systems need to be enhanced. The concepts and maturity of these elements are described.

The operational influences must be captured to identify the opportunities for platforms in the field as well as for enhancement of new designs.

Introduction

An overview of Tension Leg Platforms (TLPs) currently in operation is shown in Table 1.

The main focus of this paper is to address some important aspects related to TLPs in operation to identify the opportunities for platforms in the field as well as for enhancement of new designs.

Installation of Future Equipment on the Snorre TLP

Two additional fields (Vigdis and Snorre North) were discovered and developed using subsea tie-backs to the Snorre TLP and that required expansion of the facilities on the TLP.

The Snorre TLP was originally designed with layout space and "theoretical" payload reserves for approx. 3,400 tonnes of future equipment. This reserve was related to the most favourable position of horizontal center of gravity.

During the development of the Vigdis and Snorre North facilities, the operating weight increased beyond the future load capacity since the the actual layout caused a weight eccentricity resulting in increased ballast requirement. Consequently the Snorre TLP did not have sufficient margins for the new development based on the original design criteria (1989). Based on recent experience with the Heidrun TLP, a comprehensive re-evaluation of the Snorre TLP design criteria was performed.