Application of Advanced Composite Materials for Offshore Jacket Mudmats
Authors: Fryett G.N. 
Published: October 2008

"Mudmats" are large structures used to temporarily prevent offshore platforms or "jackets" from sinking into soft unconsolidated soil on the seabed. The functional life span of these mudmats is limited, as they are essentially redundant after installation of the foundation piles.

Conventional steel mudmats are subject to high corrosion rates and require large numbers of sacrificial anodes to slow down the rate of corrosion. Advanced composite materials would offer significant advantages as alternative means of construction as they are corrosion resistant, lightweight and cost effective.

This paper describes the industrial process from the initial concept of Glass Reinforced Polymer (GRP) mudmats through to manufacture, large scale mechanical testing and on site installation of a GRP mudmat system for a major offshore development in the Caspian Sea.

Introduction

To extract hydrocarbons from beneath the seabed requires drilling platforms to be deployed at strategic offshore locations. Multi-legged steel tubular framed substructures or "jackets" are used to support the topsides, risers and caissons. These structures are generally constructed onshore, loaded and towed out on a barge, and then launched over the side. The jacket is designed to be self-supporting during installation, where it is lowered over the seabed template. Located at the bottom of each leg is the mudmat structure, which, having a large surface area, distributes the weight of the jacket over the topsoil layer of the ocean floor. The mudmat also provides the foundations for the jacket until completion of the pile grouting operation, normally completed with a few weeks of installation. Piles are driven through sleeves in the mudmat, situated adjacent to the jacket legs - Figure 1. The piles, which are installed in sections using a pile-driving hammer, are grouted in place once the design penetration depth is reached. The effectiveness of the mudmats is therefore limited and they become functionally useless after completion of the pile grouting operation.

Conventional steel mudmats are typically fabricated from large stiffened plates, which are welded to a framework of beams attached to the jacket leg - Figure 1. However, as is the case with all metallic structures, they corrode when immersed in an electrolyte, such as seawater. The rate of corrosion can be suppressed by attaching sacrificial anodes. Connecting a metal of higher potential to a metallic structure creates an electrochemical cell in which the metal with lower potential becomes a cathode and is protected. This technique is called Cathodic Protection (CP).

Steel mudmats, when welded to the jacket legs, form an electrical connection to the main jacket structure. This contributes to the drain on the sacrificial anodes throughout the functional life of the offshore platform, typically thirty years. Consequently, this increases the required number of anodes used as part of the cathodic protection system to slow down the rate of corrosion. Depleted anodes must also be replaced for continued protection of the steel jacket.

Figure 1. Steel Mudmat Frame (Available in full paper)