Hutton TLP Marine Operations
Authors: Hart H.J. , White G.J. , von Fischer E.L. 
Published: December 2007

Summary.
This paper reviews the planning and execution of the marine operations for the Hutton tension leg platform (TLP), including well template installation, site preparation, foundation installation, temporary anchorages, mating, major tows, and TLP installation.
Planning the work to meet project objectives resulted in the use of a number of new techniques and advanced equipment applications. These included underwater pile hammers and special acoustic equipment to provide positioning information during underwater operations. However, the detailed planning that preceded each operation enhanced the effectiveness of the equipment significantly. Further, it contributed materially to the attainment of first oil only 22 days after TLP installation.

Introduction
The towing, mating, and installation of the TLP involved extensive marine operations, ranging from conventional sea tows to the first installation of a TLP. To ensure the success of some of the more complex marine operations, special methods and unique equipment applications were required.
This paper provides an overview of all the principal marine operations associated with the TLP, highlighting those that require more than a conventional approach. Fig. 1 shows the relative locations of the operations covered, including Hutton field offshore operations and inshore operations (Inverness Firth anchorage, MoraFirth anchorage, hull tow, deck tow, and mating).
We stress the importance of effective management in the success of marine operations, including the specific areas of advanced planning, contingency planning, coordination with regulatory agencies, and supervision of contractors. The overall marine operations schedule is shown in Fig. 2. Marine operations on the project's critical path were the hull tow. mating, and TLP tow and installation.

Well Template Installation
A 32-slot template for predrilling wells (Fig. 3) was the first structure to be placed at Hutton. The template, measuring 33.2 × 11.6 × and 6 m [109 × 38 × 22 ft and weighing 400 Mg [400 tonnes], was installed in two phases.
Phase 1. In April 1981, the template was transported to the field and was placed on the seabed within 24 hours of arrival at Hutton. The template's final position was within 5 m [ 16 ft] of the geographic target. This accuracy was achieved with a newly developed subsurface acoustic positioning system integrated with surface electronic navigation equipment. The subsurface equipment was developed from an existing transponder that operated in a short baseline mode and transmitted data on position, orientation, and levelness. This system became an integral part of the systems developed for foundation and TLP tether leg installation.
Phase 2. A semisubmersible drill rig was used to complete the template installation. Four of the template slots were used for 76-cm [30-in.] -diameter leveling piles, which were drilled and groutedinto position. The leveling piles were not required because the landed template was level within 0.0043 rad [0.25degrees] and well within the 0.0087-rad [0.50 degrees] tolerance specification. The drill rig then in-stalled two bumper piles for future use during foundation installation, using the well template outriggers (which can be seen attached to the side of the template in Fig. 3) as guides.
Both phases of the template installation were successfully completed in June 1981.

Site Preparation and Foundation Installation

Site Preparation.
From Summer 1981 through Winter 1983, 10 wells were predrilled through the well template. The drill cuttings were returned to the seabed and formed a hill with a peak of 2.5 to 3.0 m [8.2 to 10 ft] centered on the template. These cuttings interfered with placement of the foundation templates, which were designed to be placed on a reasonably flat and level seabed. Approximately 1600 m2 [17,223 ft2] of the foundation area require clearing. Working through late winter and early spring, we successfully cleared the site before the arrival of the foundations.

Foundation Description.
The four foundation templates, measuring 21.8x19.6x9.2m [71.5x64x30 ft] weighed 985 Mg [985 tonnes] each in air. The templates were positioned on the seabed at the corners of a 78x74-m [256x243-ft] rectangle. Each template was fixed to the seabed by eight piles, 1.83 m [6 ft] in diameter and 63.5 mm [2.5 in.] wall thickness. These piles were driven to a depth of 59.15 m [194 ft].
The correct placement of the foundations in both position and elevation was highly critical to the operational capability of the TLP. Therefore, to achieve installation within the specified tolerances, the following special equipment and installation aids (Fig-4) were developed.

Guideframe.
This structure was used to position the foundation templates accurately. The guideframe was a tubular structure 90.5 m [297 ft] long and 50.5 m [166 ft] wide, weighing 920 Mg [920 tonnes]. The height of the guideframe above the seabed was sufficient to avoid the debris. The frame was equipped with mud mats connected to jacks for leveling adjustments.

Pin Pile and Mud Mats.
Each foundation template was supported by four mud mats connected to hydraulic jacks for leveling, A central pin pile, measuring 30 m [100 ft] long and 1.83 m [6 ft] in diameter was preloaded into each template. In its final grouted position, this pile provided the required stability to support the 67-m [220-ft] main piles before driving.

Acoustics.
To solve the more complex problem of foundation template and guideframe installation, the well template acoustic system was further developed (1) to track the guideframe and foundation templates as they were lowered from the surface to the seabed, and (2) to check independently the position, orientation, and elevation of each foundation template after installation.
This system was also successfully used to track the piles and the underwater hammer during main pile stabbing.

Foundation Placement and Positioning Tolerances.
The principal method for foundation placement was by use of bumper piles and a guideframe. The bumper piles were accurately installed with respect to the well template with the positioning outriggers. The guideframe was fabricated to +/-25 mm [+/-0.98 in.] at key points used to index the guideframe and foundations to the preinstalled well template.
JPT
P. 1426^