For the Benguela-Belize Compliant Piled Tower (CPT), the interim construction phases during the installation played an important role in the structural design. Because of susceptibility to fatigue of the unfinished tower, in combination with weather sensitive installation phases, it was crucial to find the most accurate way to predict weather downtime during construction and hence assess the interim construction fatigue exposure.
Workability assessments using detailed hindcast data, accurate wave forecasting and offshore decision support assisted to control the fatigue design aspect both in the engineering and installation phase.
Due to various delays, the installation of the CPT was pushed beyond the favorable installation period, resulting in potential high interim construction fatigue exposure. By using ballast tanks, temporarily replacing the topsides weight during construction, the allowable interim period before the topsides were set could be increased.
During the offshore installation, a combination of accurate weather forecasts and the ability to predict the SSCV's motion behavior during critical installation activities has proven to be successful, as the necessary installation windows were identified in time with the best possible accuracy and no critical installation weather windows were missed.
On April 1st 2003, Cabinda Gulf Oil Company Ltd. awarded the EPCI Contract for the Benguela-Belize Compliant tower to Daewoo Shipbuilding and Marine Engineering Company Ltd (DSME) and Heerema Marine Contractors Nederland B.V (HMC) as the EPCI installation Subcontractor.
The detailed design of the Compliant Piled Tower and the Topsides was subcontracted to Mustang Engineering Inc (MILP) and Kvaerner Brown and Root (KBR) respectively.
The CABGOC Benguela-Belize Compliant Tower was the first Compliant Tower installed in West Africa and outside the Gulf of Mexico (GoM).
MILP pioneered the CPT concept and designed all the existing towers in the GoM (Lena, Petronius and Baldpate). With the benefit of this experience, MILP performed an early FEED study for CABGOC in which not all installation related issues associated with the swell-dominated West African waters were addressed.
HMC previously installed the Baldpate CPT in the GoM, and has performed various heavy lifts in West African waters but all in the period mid November to early March. Never before had a 10,000 tons lift been performed in April - May in Angola.
The Benguela-Belize CPT platform comprised a 400 m high substructure and topsides facilities (Fig. 1). The substructure included a base section, foundation piles and two tower sections, with a total weight of 50,000 tons. The topsides included a Module Support Frame (MSF) with various topside modules weighing 35,000 tons.
Installation of the Benguela-Belize Compliant Tower was split into two phases. The first phase, installation of the foundation section of the substructure, included the installation of the Levelling Piles Template (400t, dual crane), the Levelling Piles (310t each, dual crane upend), the Tower Base Template (3,100t, dual crane) and the Foundation Piles (1,000t each, dual crane upend).